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    Laser Instruments and Applications

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  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Introduction to Laser Instruments and Applications

    When the laser was invented in 1960, it was amazingly, a solution looking for a problem. While the laser's weapons potential was clear, most of the uses of lasers that have changed the World were not foreseen even by the so-called experts of the time. In this chapter, we touch on perhaps one tenth of one percent of those where lasers are now indispensable, or at least have the potential to be in the future.

    But lasers are not the solution to every problem. There are applications where lasers are not useful and probably never will be. Among the short list of idiotic proposals for lasers are (in no particular order): grass and tree trimming, insect extermination, and advertising on the moon. For more details and a few chuckles, see the section: Laser Humor.



  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Rangefinders

    Using a Laser to Measure Distance, Position, or Speed

    There are a variety of ways of using lasers to measure distance. The precise 3-D shape or profile of solid objects can be determined using laser scanning techniques. Common approaches include: Laser Atlanta Optics is an example of a company that specializes in laser based distance and speed measureing technology.

    Manufactures/suppliers of devices used in laser rangefinders include: E-O Devices and Analog Modules.

    Optical Rangefinders

    This is the basic principle used in 35 mm rangefinder cameras and other devices where you view the distance scene and turn a knob to line up two images that are either superimposed or split top/bottom half. In the case of the camera, turning the lens focus ring adjusts the angle of mirror A below.

    
              To distant scene.
              ^               ^
              |               |
              |       C/------/D    
              |A       |      
              \--------\       (B is partially silvered or a half mirror to
             adjust   B|        permit viewing of both sides from the scene.)
             angle     ^
                   view here
              |               |
              |<- baseline -->|
    
    

    The further apart the mirrors are (size of baseline), the greater the useful range. Adjust the angle of mirror A or D until the images are superimposed. Calibrate the angular setting to distance.

    The distance from A to the scene is then: tan(angle A) * baseline.

    For long distances, C and D can be eliminated - they compensate for the difference in path lengths of the two views - else the sizes would not be the same. (Even this doesn't work perfectly in any case. Can you figure out why?)

    You can add telescopes and other optics if you like - this is just the basics.

    Look Ma, no electronics. :-)

    Note that SLR cameras do NOT use this approach as they are entirely optical (meaning that adjusting the focus only controls the lens - nothing else!). With SLRs, a pair of shallow prisms oriented in opposite directions (or many in the case of a 'microscreen' type) are cemented onto a clear area of the ground glass. When the image is precisely focused onto the ground glass, the prisms have no effect. However, when the image is in front or behind, they divert the rays such that the two halves of the image move apart (or the image breaks up in the case of the 'microscreen').

    There were some "Amateur Scientist" articles in Scientific American a few decades ago on constructing several types of optical range finders. These were included in the book, "Light and Its Uses". See the section: A HREF="laserclt.htm#cltsi">Scientific American Articles on Lasers and Related Topics.

    Simple Laser Rangefinder Based on Triangulation

    (Portions from: Mike Cimorosi (mcimoros@hopi.dtcc.edu).)

    My students construct a simple laser rangefinder using a few basic parts:

    Equipment:

    Basic procedure:

    1. Place the laser to the left of the optical bench. Follow standard safety procedures for using 1/2 mW lasers.

    2. About 3 inches to the right of the laser aperture (opening), place the beam splitter at an angle of 45 degrees with respect to (wrt) the incident beam. This will split the beam into two different paths. Most of the beam will pass through the splitter. Some will be reflected at a right angle wrt the incident beam.

    3. About 6 feet to the right of the splitter, place the rotary table with the mirror on it and face it toward the beam that passes through the splitter.

    4. Now, before you turn on the laser, make sure you have a safe place to aim the beam for the distance you want to determine.

    5. Now fire up the laser. Note where the first reflected beam strikes the target (a wall maybe?). Now, slowly and carefully rotate the rotary table until the beam reflected from the mirror coincides with first reflected beam. You now have formed a right triangle made of laser light! Pretty neat! Remember to respect the beam, especially with respect to your eyes!!!

    6. Finally, you can use the trig relation: distance = 6 ft x tan(angle) to determine the distance. How's your trig? :-)

    7. It's not the most precise rangefinder - i.e., the equation is pretty sensitive to the angular precision of the rotary table. However, it does demonstrate the basic principle. Maybe the diagram below will help with setting up the laser rangefinder.

    Rough diagram of rangefinder setup:

    
                   To wall                    To wall
                     ^                           ^
                     |                             \ 
            distance | first reflected beam          \ second reflected beam
                     |                                 \
                     |                             angle \
        Laser --3"---/------------------------------------/
                Beamsplitter                    Rotary table with mirror
                     |<------------- 6 feet ------------->|
    
    

    Of course, you can make the non-laser version of this type of rangefinder (but this is a laser FAQ! --- sam). My students also make that one as well. Both are pretty neat and demonstrate the power of trig to determine distances!

    Comments on Laser Rangefinders

    (From: Andrzej Hanczak).

    I am just finishing the development of a range finder based on the TOF (pulse-Time-Of-Flight) measurement method. There are also different methods like phase-shift method which compares the phase shift between outgoing modulated beam and reflected light.

    The Pulse TOF method has some advantages which make it very useful: you can use relatively high pulse power and still be in the Class I safety range.

    While building such a range finder there are two crucial components which have influence on its accuracy: the time measurement circuits and the receiver. Our aim was to build a laser scanner with the resolution of 1 cm which means that you have to be able to measure the time with the resolution of 67 ps. The range of the scanner should be approx. 30m. We are not ready yet but there are some results.

    For the first prototype we used a 1.25 GHz oscillator and special microstrip design to get the resolution of 70 ps. In the current prototype we use a special prototype IC which should deliver 50 ps resolution.

    The problems are on the receiver side, a relatively large jitter (which I'm fighting now) destroys my high time measurement precision. The jitter on the input results in the distance differences of approximately 10 cm). This can be filtered out by averaging of a number of measurements and that is what we are doing now. Our measurement frequency is at present 100 kHz, but we will probably perform the averaging over 10 measurements so that effective measurement rate will be 10 kHz.

    (From: jfd (jezebel@snet.net).)

    The problem is getting simultaneous long standoff range and extremely accurate range. You can phase detect with accuracies in the sub-inch range using direct detected RF modulated LIDARS or you can use an interferometric technique with a reference to get sub-micron distances.

    (From: Robert (romapa@earthlink.net).)

    For much better resolution than would be possible with simple sampling while still maintaining low cost, digital TOF rangefinders can combine a precision analog temporal interpolator with say a CMOS system running at 100 MHz. The analog circuitry to accomplish this is in many production units (for different applications) - but 5 ps resolution has been achieved with low-cost components and in production for 15 years from at least one manufacturer. The idea is interpolate between the digital count periods with a precision time-to-voltage converter which is then sampled by microcontroller and combined with the digital counter results.

    (From: Bill Sloman (bill_sloman@my-deja.com).)

    You may be able to achieve this at low unit cost, but getting a precision analog temporal interpolator to work well next to CMOS running at 100 MHz isn't something I'd describe as easy.

    We developed a system of this sort at Cambridge Instruments between 1988 and 1991 using a mixture of 100K ECL and GigaBit Logic's GaAs for the digital logic. Any digital signal going to or from the analog temporal interpolator was routed as a balanced pair on adjacent tracks, and we were very careful about the layout, but we still had to work at getting the noise on the interpolator output down to the 60 picosecond jitter on our 800 MHz master clock (getting a better master clock was the next priority).

    Current-steering logic (like ECL and GaAs) is a lot quieter than voltage-steering logic (like TTL and CMOS), which is why very fast DACs and ADCs use ECL interfaces. Precision analog interpolators are no less sensitive.

    Do you know who has actually achieved that 5 ps resolution and for what application? Tektronix and time domain reflectometers come to mind, though Tektronix isn't exactly cheap. IIRR Triquint was originally their in-house analog foundry and I think Tektronix has been using GaAs ASICs in their faster gear for quite some time now.

    The hybrid approach certainly isn't new, but getting it to work is a fair test of one's analog skills.

    Of course, using phase-shift not only makes for easier circuit design, but also lets you run your LED at a 50% duty cycle, giving you a lot more reflected photons to work with than the 0.01% you get with TOF.

    (From: Lou Boyd (boyd@fairborn.dakotacom.net).)

    The Texas Instruments book "Optoelectronics: Theory and Practice" published by McGraw-Hill had a chapter (23) on the design of an LED/Si Diode rangefinder with schematics of the transmitter, receiver, and timing section. This was a phase modulated design but obsolete by todays standards. Low cost modern rangefinders like those by Leica or even Bushnell are far more advanced in the detection circuit than that in the TI book. Most eye-safe commercial rangefinders use phase modulated techniques. This gives good accuracy but limited range, usually less than 1 kilometer with measurement times typically 1/10 second.

    Most military rangefinders use a much higher power transmitter with a time of flight method. A time of flight rangefinder just sends a single pulse and receives it. Some use multiple pulses for improved resolution and range but that typically isn't necessary. A counter is started on the rising edge of the transmitted pulse and stopped when the rising edge of the receive pulse is detected. If the counter is measuring a 150 MHz (approx) clock the range will be displayed in meters. Unfortunately that fast of counter requires at least a few high speed chips beyond the capability of standard CMOS or TTL logic. Since the round trip takes only 6.667 microseconds per kilometer you don't even need blanking on the displays. They can be attached directly to the counters or just read by a computer. A four or five digit counter suffices for most purposes. There is a little added complexity on sophisticated units for making the sensitivity of the receiver increase with time after the pulse is transmitted. This is sometimes done by charging a capacitor attached to a gain control which increases the gain with the square of time out to the maximum the unit is capable of. These rangefinders tend to be expensive because of the technology but the electronics is simple in concept. Ranges are limited only by the transmit power which can be extremely high using solid state Q switched lasers.

    Surplus lasers and the associated electronics from military rangefinders have been showing up on the surplus market in the $300 range. Unfortunately the receivers have not.

    For some insight on the level of complexity involved look at the boards sold by E-O Devices These are time of flight pulsed laser rangefinder components designed for use primarily with LED's or diode lasers. Also check Analog Modules for examples of state of the art variable gain rangefinder receivers. If you want one of their modules plan on spending between $1,000 and $2,000. :-(

    Phase shift methods allow achieving high precision in distance resolution with lower power and lower speed circuitry. That equates to lower cost and higher precision. Which type is best depends on what properties are needed.

     Parameter      Single Pulse           Phase Shift
    -------------------------------------------------------------------
     Range          100 m to 100 km        1 m to 10 km
     Resolution     1 m any target         1 mm corner cube to 1 m any
     Cost           $5000 and up           $100 and up
     Power level    10 w to 1 MW           1 mW to 1 W
     Time to read   sub-ms                 0.01 to 10 seconds
     Applications   artillery, navigation  surveying, hunting
    

    Single pulse rangefinders typically use YAG or erbium lasers while most of the phase shift type use diode lasers.

    (From: Don Stauffer

    Which type to use depends a bit on what range resolution you are looking for. If you want high resolution, you will be working with a high modulation frequency. Then you may find many circuits designed for receiving audio modulation may not provide enough bandwidth.

    Also, there is the range ambiguity problem. If you go high enough in frequency, you may find some range ambiguity.

    You will also likely be needing very accurate phase measurement circuits if you are using moderate modulation frequency, so study carefully high accuracy phase detectors. These are not trivial circuits. In order for them to work well, you need a pretty good SNR.

    (From: A. E. Siegman (siegman@stanford.edu).)

    Adding to what others have said, hand-held laser rangefinders using low-power RF-modulated CW lasers (a.k.a. diode lasers) together with phase-detection techniques are simpler, cheaper, smaller, *much* more battery efficient, and much safer; and are more or less replacing the pulsed hand-held versions of yore.

    These techniques are also moderately old. Coherent (maybe Spectra also) were making widely used laser surveying instruments ("Geodolite"?) that worked this way a couple decades or more ago (and there may have been incoherent light source versions even further back).

    I suppose that compared to TOF, one disadvantage is that it takes longer to integrate up the signal to get a range finding, and if you're in a tank battle and want to get off the first shot before alerting the enemy that you're illuminating him and giving him a chance to duck, the pulsed type may still be better.

    Do some web searching: You can buy binoculars with a built-in diode laser rangefinder from Amazon, and use it to measure the distance to the pin on your next golf outing.

    (From: Louis Boyd (boyd@apt0.sao.arizona.edu).)

    Prior to laser diodes (1960's) there were optical geodimeters which used a tungsten lamp, a Kerr shutter (which modulates light at multi-megahertz rates using polarizers and high voltage rf driven nitrobenzene), and photomultiplier receivers. These could measure distances to a few centimeters at ranges of several kilometers. They were large, expensive, and a bi*ch to calibrate. They used phase shift techniques similar to modern diode rangefinders, but without the aid of microprocessors. They switched modulation frequencies to resolve phase ambiguities.

    Modern rangefinders often use pseudorandom modulation and cross-correlation computation to give the round-trip delay which is proportional to distance. Distance resolution can be much finer than the length of the shortest pulse.

    With modern geodimeters the distance accuracy is primarily limited by uncertainty of light propagation velocity in the air since it's not practical to measure the pressure and humidity at all points along the path, but can be accurate to better than 1 part in 10^6 with care. Tape and chain is difficult to get better than 1 part in 10^3 which is the typical accuracy of $200 pocket laser rangefinders.

    (From: Mike Poulton (mpoulton@mtptech.com).)

    Using pulses is not very practicable - if you want to achieve a resolution of a few mm over a distance of 100 m or so, you find that you'd need extremely short pulses (recall that 1 ns corresponds to 30 cm or 12 inches, approximately, so you's need pulses of a few ps); you could do this with a W-switched SS laser, but those little hand-held devices, who do have a resolution in this order of magnitude, cannot work in this way. They use a RF-modulated CW signal from a laser diode, say with 100 MHz, and measure the phase shift of the 100 MHz signal between outgoing and incoming beams. This phase shift can be very accurately measured by first converting the 100 MHz down to a few 100 kHz (like a superheterodyne receiver).

    Some while ago I had been interested in such a circuit myself (for measuring optical path lengths) but didn't find anything useful on the web.

    (From: Repeating Rifle (SalmonEgg@sbcglobal.net).)

    Equipment of this ilk is called *distance measuring equipment* or DME and has all but replaced the use of chains in surveying practice. Various implementations have been used. Some use high frequencies to obtain precision and lower frequencies for range ambiguity resolution. Others use inconmensurate frequencies that are not all that different from one another. I you match the filtering to the transmission, you pretty much get the same signal to noise ration for all kinds of devices. The broad-band pulses mentioned above use short pulses. The CW devices use narrow band filters.

    The first items of this nature used RF directly without light.

    Trade names that come to mind quickly are tellurometer and geodimeter.

    For the military rangefinders that use high power pulses, signal processing is less than optimum. An error of 5 meters will usually not be a big deal. For surveying, that kind of error will usually be unacceptable. In both cases extended (in range) targets will introduce error.

    Almost all of the inexpensive hand-held rangefinders on the market use a simplified form of phase detection with relatively low modulation rates. Phase sensing rangefinders uses a variable pulse width modulated laser diode. It would use use thousands of on/off transitions in determining each distance measurement by comparing the modulation pattern to the returned signal using cross-correlation techniques. Resolution is a function of measurement time, speed and size of the registers, and instrument stability. Single pulse TOF rangefinders on the other hand are generally used for very long ranges (several km and up) with very high pulse power (kilowatts to megawatts peak) and range resolution rarely better than a meter. Low power single pulse rangefinders are rare as the expense of the detection circuits isn't justified for the low resolution.

    The accuracy of quality surveying distance meters is limited primarily by the uncertainty of the velocity of propagation of light through the atmosphere. That varies of with air pressure and humidity which can't easily be determined over the entire path. Still, they're orders of magnitude better than a tape or chain.

    (From: Phil Hobbs (pcdh@us.ibm.com).)

    Modulated CW measurements also allow you to use very narrow measurement bandwidths very easily (e.g. with a PLL), which helps the SNR very much. In shorter range units, sinusoidal modulation can also be used to prevent back-reflections from causing mode hopping. You choose delta-f so that the phase modulation of the back-reflection (in radians) is at a null of the zero-order Bessel function J0. This can make a huge difference (3 orders of magnitude) in the back-reflection sensitivity.

    Building a Time-of-Flight Laser Rangefinder

    The following is what I would suggest for a relatively low cost approach achieving 15 to 50 cm resolution and 100 meter or more range. However, also see the next section for a much simpler approach that may be adequate.

    A Q-switched solid state laser will give you short pulses with minimal fuss. A unit like the small surplus Nd:YAG laser (SSY1) described in chapter: Solid State Lasers was originally part of the M-1 tank rangefinders and thus should be ideal. It is quite trivial to build a suitable power supply these laser heads since a passive Q-switch is used and this doesn't require any electrical control.

    A few mJ should be sufficient. (SSY1 is probably in the 10 to 30 mJ range using the recommended pulse forming network.) With a Q-switched laser, the required short pulse if created automagically eliminating much of the complexity of the laser itself.

    Diode laser assemblies from the Chieftain tank rangefinder are also available on the surplus market but you probably would have to build a pulsed driver for them which would be more work.

    For the detector, a PIN photodiode or avalanche photodiode (APD) would be suitable. The preamp is the critical component to get the required ns response time. You need to sample both the pulse going out and the return since the delay from firing the flashlamp (if you are using a solid state laser) to its output pulse is not known or constant.

    15 cm resolution requires a time resolution of about 1 ns (twice what you might think because the pulse goes out and back). GHz class counters are no big deal these days.

    However, approaches that are partially analog (ramp and A/D) which don't require such high speed counters are also possible. In fact, if your digital design skills aren't so great, this is probably the easiest way to get decent resolution, if possibly not the greatest accuracy/consistency. All you need is a constant current source and an A/D (Analog to Digital converter). This can be as simple as a FF driving a transistor buffer to turn the voltage to charge the capacitor on and off with a transistor set up with emitter feedback for as a constant current source. Or, it can just be an exponential charge with non-linear correction done in software. The A/D doesn't need to be fast as long as its output word has enough bits for your desired resolution. For a typical exponential charging waveform, add 1 bit to the required A/D word size. For example, determining distance over 100 meters to to 5 cm resolution would require that the full voltage ramp be about 700 ns in duration (a bit over maximum round trip time, cut off sooner if there is a return pulse) and then sampled with a 12 bit A/D.

    Another even simpler way of doing this is to charge the capacitor as above but then discharge it with a much longer time constant and determine how long it takes to reach a fixed voltage. By making the discharge time constant sufficiently large, any vanilla flavored microprocessor could be used for control and timing.

    All in all, these are non-trivial but doable projects.

    See the previous sections on laser rangefinders for more info.

    Here is a Web site that appears to go into some detail on the design of TOF laser rangefinders:

    (From: Anonymous.)

    A laser phase shift distance meter can be constructed by analog modulation of the laser and measuring the phase shift of the return signal. With some filtering you can do multiple frequencies at the same time. Also, the feedback diode in a semiconductor laser can be used as the sensor (in which case the circuitry gets interesting). High precision can be accomplished relatively easily. I'm trying to get better than 0.1 mm (preferably better than 0.01 mm) over short distances (a couple of meters).

    Resonant Time-of-Flight Laser Rangefinder

    This is a slightly modified approach and may be made to work with relatively simple inexpensive circuitry. The idea is to use a normal IR or visible laser diode (e.g., such as from a CD or DVD player) in conjunction with a common photodiode to form an oscillator whose frequency will depend on the path delay between them - i.e., the distance to the "target". Basically, the laser diode is turned on which sends out a leading edge of a light pulse. The light hits the target and is reflected back into the photodiode, which turns the laser diode off. The loss of signal then turns the laser diode on and the cycle repeats continuously. The oscillating frequency is then equal to 1 over (4 times the distance to the target plus 2 times the internal circuit delay). A simple frequency to voltage converter drives an analog meter. No really high speed components are needed.

    This was seen as a project in a Dutch book: "Lasers in Theorie en Praktijk: Experimenten - Meten - Holografie", by Dirk R. Baur, Uitgeverij Elektuur/Segment B.V., Postbus 75, 6190 AB, Beek (L) The Netherlands.

    I'm not convinced that the circuit as presented works - there is at least one part value (C4, 100 uF) which would appear to be much larger than desired inside the feedback loop. The principle appears valid though.

    Time-of-Flight Laser Rangefinder using CCD Camera

    Each pixel of a CCD-based image sensor accumulates charge proportional to the light intensity and shutter open or "gate time". For normal video, the electronic shutter is open for a duration which is a large fraction of a video frame to maximize sensitivity and minimize aliasing in moving images. For stop motion photography, much shorter shutter open times are used. If it were possible to synchronize the electronic shutter with the generation of a light pulse illuminating the scene, then the amount of charge in each CCD cell would also depend on how long it takes for the light to reach the CCD (since the shutter would close before the light from more distant points returned). One problem, of course, is that this is possible only under very special conditions. A way to get around this would be to do the measurement in two steps:

    In order for this to be implemented with a normal CCD camera, either direct control of the electronic shutter is needed, bypassing any synchronous logic, or a "sync" output from the camera must be available. Also note that the charge integration times involved - 10s or 100s of ns - are orders of magnitude smaller than those normally used on all but very specialized CCD cameras, even with a fast shutter. So, sensitivity is going to be very low. A high power pulsed laser may be needed to generate adequate photons and even then, the CCD may not be able to supply enough charge.

    However, there are CCD image sensors that have been designed specifically for this application. They include logic on each pixel to enable the arrival time to be determined and stored. This permits an entire depth map to be captured with a single TOF pulse. See, for example: CSEM Optical Time-Of-Flight Imaging - A Technology for Multiple Applications.

    Using a CD or DVD Optical Pickup for Distance Measurements

    The simplist way of doing this may be to use the existing focusing mechanism of the pickup. Focus in a CD or DVD device depends on a reflection from a relatively flat smooth surface (the metalized information layer of the disc/k) to produce an elliptical spot back at the photodiode array. The major axis of the ellipse lies on a diagonal (45 or 135 degrees) and depends on the distance above or below optimal focus - at that point, it is a perfect circle. A four quadrant photodetector takes the difference of the amplitude of the return signals from the two pairs of diagonally opposed quadrants to determine the focus error. See the document: Notes on the Troubleshooting and Repair of Compact Disc Players and CDROM Drives for more on how optical pickups actually work.

    If the surface is smooth and flat over a scale of 5 to 10 um, this could work as a way of determining distance to the pickup. In other words, the dominant return from the surface has to be a specular reflection back to the source in order for the focus servo to lock properly. (The width and depth of the pits/lands of the CD or DVD disc is small compared to the beam so they are mostly ignored by the focus servo.) I don't know how much angular deviation could be tolerated.

    The output would be an analog voltage roughly proportional to focus error which could be mapped to lens height (assuming the device is in a fixed orientation with respect to gravity - more complex if you want to do this while on a roller coaster or in microgravity!). The total range would be 1 to 2 mm with an accuracy of a few um.

    Also see the section: Can I Use the Pickup from a CD/DVD Player or CD/DVDROM Drive for Interferometry?, which would be even more precise but more complex. The practical issues of using the guts of these devices are also discussed there.

    Using a CD or DVD Optical Pickup in a Precision Position or Angle Encoder

    Conventional optical encoders - whether they are the dirt-cheap variety inside your computer mouse or the precision type found in industrial robots and other machine tools - consist of a light source or sources, some means of interrupting or varying the light intensity based on linear position or rotation angle, and photodetectors to convert the light to an electrical signals. By using various patterns on film or glass strips or discs, relative (2 bits) or absolute (many bits) measurements can be made with a computer or dedicated logic calculating position or angle, speed or rotation rate, acceleration, and so forth from this data. Through clever design and careful manufacturing, extremely high resolution is possible using conventional LEDs or incandescent lamps for the light source(s). However, lasers can be used as well with some potential advantages - even higher precision and stand-off (some distance between the moving parts) operation.

    Since the 'stylus' of a CD player has an effective size of around 1 um (DVD would be even less), it could in principle be used to implement a very high resolution optical encoder for use in linear, rotary, or other sensing application. The stand-off distance (from objective lens to focal point) can be a couple of mm which may be an advantage as well. While this is probably somewhat less difficult than turning a CD player into an interferometer (see below), it still is far from trivial. You will have to create an encoder disc or strip with a suitable reflective pattern with microscopic dimensions. Without access to something like a CD/DVD mastering unit or semiconductor wafer fab, this may be next to impossible. Your servo systems will need to maintain focus (at least, possibly some sort of tracking as well) to the precision of the pattern's feature size. To obtain direction information, the 'track' would need to have a gray code pattern similar to that of a normal optical encoder - but laid down with um accuracy in such a way that the photodiode array output would pick it up. (Implementing an absolute encoding scheme would probably require so many changes to the pickup as to make it extremely unlikely to be worth the effort.) Of course, you also need laser diode driver circuitry and the front-end electronics to extract the data signal. Not to mention the need for a suitable enclosure to prevent contamination (like lathe turnings) from gumming up the works. And, with your device in operation, any sort of vibration or mechanical shock could cause a momentarily or longer term loss of focus and thus loss of your position or angle reference.

    If you are still interested, see the section: Can I Use the Pickup from a CD/DVD Player or CD/DVDROM Drive for Interferometry? since some of the practical issues of using the guts of these devices are discussed there.

    Measuring Speed with a Laser

    Speed is just the rate of change of position so any of the approaches that measure position can be adapted for speed measurements by simply taking a pair of readings and computing their difference with respect to time. More direct methods using CW lasers depend on using some form of the doppler shift of the reflected beam, usually of a subcarrier imposed on the the laser beam by amplitude modulation.

    For example, if the outgoing laser beam is modulated at 1 GHz and the reflected beam is combined with this same reference 1 GHz in the sensor photodiode or a mixer, for relative speeds small compared to c (the velocity of light), the difference frequency will be approximately 1 Hz per 0.5 foot/second.



  • Back to Laser Instruments and Applications Sub-Table of Contents.

    General Interferometers

    Basics of Interferometry and Interferometers

    The dictionary definition goes something like:
    "INTERFEROMETER: An instrument designed to produce optical interference fringes for measuring wavelengths, testing flat surfaces, measuring small distances, etc."
    As an example of an interferometer for making precise physical measurements, split a beam of monochromatic coherent light from a laser into two parts, bounce the beams around a bit and then recombine them at a screen, optical viewer, or sensor array. The beams will constructively or destructively interfere with each-other on a point-by-point basis depending on the net path-length difference between them. This will result in a pattern of light and dark fringes. If one of the beams is reflected from a mirror or corner reflector mounted on something whose position you need to monitor extremely precisely (like a multi-axis machine tool), then as it moves, the pattern will change. Counting the passage of the fringes can provide measurements accurate to a few nanometers!

    A simple version of a Michelson interferometer is shown below:

    
                                    _____ Mirror 1 (Moving)
                                      ^ 
                                      |
                                      |  Beam
                                      |  Splitter
                   +-------+          | /          |
                   | Laser |=========>/<---------->| Mirror 2 (Fixed)
                   +-------+        / |            |
                                      |
                                      |
                                      |
                                      v    Screen (or optical viewer,
                                   -------    magnifier, sensor, etc.)
    
    

    1. The laser produces a coherent monochromatic beam which is expanded and collimated by a pair of positive lenses (not shown).

    2. Part of the laser beam is reflected up by the Beamsplitter (half silvered mirror), reflects off of Mirror 1 and back down. A portion of this passes through the Beamsplitter to the Screen.

    3. The remainder of the laser beam passes through the Beamsplitter and is reflected from Mirror 2. Part of this is reflected down by the Beam Splitter to the Screen.

    4. The two beams combine at the Screen resulting in an interference pattern of light and dark fringes or a full field varying between light and dark as the path length is changed. A magnifier, microscope, or other optical system imaging to a human observer or electronic sensor may be provided in place of the screen to view the fringe pattern in more detail or provide input to an electronic measurement system.

      In a perfectly symmetric Michelson interferometer, the fringe pattern should uniformly vary between bright and dark (rather than stripes or concentric circles of light) depending on the phase difference between the two beams that return from the two arms. A circular pattern is expected if the two curvatures of the wavefront are not identical due to a difference in arm-lengths or differently curved optics. Stripes (straight or curved) in any direction) would be an indication of a misalignment of some part of the interferometer (i.e. the beams do not perfectly overlap or one is tilted with respect to the other).

    5. A microscopic shift in position or orientation of either mirror will result in a change to the pattern. Presumably, the mirror designated as 'Moving' is mounted on some equipment such as a disk drive head positioner that is being tested or calibrated. For these applications, setting up the interferometer is set up to produce a fringe pattern with at least two sensors to determine direction and velocity in a sophisticated version of the A-B quadrature decoder used in your typical computer mouse. :)

    (Yes, about 50 percent of the light gets reflected back toward the laser and is wasted with this particular configuration. This light may also destabilize laser action if it enters the resonator. Both of these problems can be easily dealt with using slightly different optics than what are shown.)

    A long coherence length laser producing a TEM00 beam is generally used for this application. HeNe lasers have excellent beam characteristics especially when frequency stabilized to operate in a single longitudinal mode. However, some types of diode lasers (which are normally not thought of as having respectable coherence lengths or stability) may also work. See the section: Interferometers Using Inexpensive Laser Diodes. Even conventional light sources (e.g., gas discharge lamps producing distinct emission lines with narrow band optical filters) have acceptable performance for some types of interferometry.

    Such a setup is exceedingly sensitive to EVERYTHING since positional shifts of a small fraction of a wavelength of the laser light (10s of nm - that's nanometers!) will result in a noticeable change in the fringe pattern. This can be used to advantage in making extremely precise position or speed measurements. However, it also means that setting up such an instrument in a stable manner requires great care and isolated mountings. Walking across the room or a bus going by down the street will show up as a fringe shift!

    Interferometry techniques can be used to measure vibrational modes of solid bodies, the quality (shape, flattness, etc.) of optical surfaces, shifts in ground position or tilt which may signal the precursor to an earthquake, long term continental drift, shift in position of large suspended masses in the search for gravitational waves, and much much more. Very long base-line interferometry can even be applied at cosmic distances (with radio telescopes a continent or even an earth orbit diameter apart, and using radio emitting stars or galaxies instead of lasers). And, holography is just a variation on this technique where the interference pattern (the hologram) stores complex 3-D information.

    NASA has some information on interferometry oriented toward cosmic measurements at the NASA Interferometry Page. And you can try your hands at aligning a Michelson interferometer at the NASA Interactive Interferometer Page.

    This isn't something that can be explained in a couple of paragraphs. You need to find a good book on optics or lasers. Here are some suggestions for further study:

  • Gordon McComb's: "The Laser Cookbook [1} and the Scientific American collection: "Light and its Uses [5]" include various type of interferometers which can be built with (relatively) readily available parts.

  • Agilent (among others) manufacture 'Laser Interferometry Measurement Systems' based on these techniques. Information and application notes are available by searching for the key words: "Laser" or "Dimensional Measurement". For Agilent in particular, searching for "5501" or "5517" will find information on their specific systems.

  • The Astroweb Internet Resources for Astronomy Web site (and others). There are links to people interested in designing, building, and operating various types of laser interferometers. Much of the information relates directly to the testing of optical components for astronomical telescopes but there should be much of general interest as well.

    Where Does All the Energy Go?

    Suppose we have a Michelson interferometer (see the section: Basics of Interferometry and Interferometers) set up with a perfectly collimated (plane wave source) and perfectly plane mirrors adjusted so that they are perfectly perpendicular to the optical axis (for each mirror) and the beamsplitter is also of perfect construction and oriented perfectly. In this case, there won't be multiple fringes but just a broad area whose intensity will be determined by the path-length difference between the two beams. Where this is exactly 1/2 wavelength (180 degrees), the result will be nothing at all and the screen will be absolutely dark! So, where is all the energy going? No, it doesn't simply vanish into thin air or the ether, vacuum, the local dump, or anywhere else. :-)

    Your initial response might be: "Well, no system is ideal and the beams won't really be perfectly planar so, perhaps the energy will appear around the edges or this situation simply cannot exist - period". Sorry, this would be incorrect. The behavior will still be true for the ideal case of perfect non-diverging plane wave beams with perfect optics.

    Perhaps, it is easier to think of this in terms of an RF or microwave, acoustic, or other source:

    Hint: From the perspective of either of the two signals, how is this different (if at all) than imposing a node (fixed point) on a transmission line? Or at the screen of the interferometer? After all, a nodal point is just an enforced location where the intensity of the signal MUST be 0 but here it is already exactly 0. For the organ pipe, such a nodal point is a closed end; for the string, just an eye-hook or a pair of fingers!

    OK, I know the anticipation is unbearable at this point. The answer is that the light is reflected back to the source (the laser) and the entire optical path of the interferometer acts like a high-Q resonator in which the energy can build up as a standing wave. Light energy is being pumped into the resonator and has nowhere to go. In practice, unavoidable imperfections of the entire system aside, the reflected light can result in laser instability and possibly even damage to the laser itself. So, there is at least a chance that such an experiment could lead to smoke!

    (From: Art Kotz (alkotz@mmm.com).)

    We don't have to to think all that hard to figure out where all the energy is dissipated in a Michelson interferometer. Nor do we have to refer to imperfect components either. The thought experiment of perfect non-absorbing components still renders a physically correct solution.

    To summarize a (correct) previous statement, in a Michelson interferometer with flat surfaces, you can get a uniform dark transmissive exit beam. The power is not dissipated as heat. There is an alternate path that light can follow, and in this case, it exits the way it came in (reflected back out to the light source).

    In fact, with a good flat Fabry-Perot interferometer, you can actually observe this (transmission and reflection from the interferometer alternate as you scan mirror spacing).

    In the electrical case, imagine a transmitter with the antenna improperly sized so that most of the energy is not emitted. It is reflected back to the output stage of the transmitter. If the transmitter can't handle dissipating all that energy, then it will go up in smoke. Any Ham radio operators out there should be familiar with this.

    (From: Don Stauffer (stauffer@htc.honeywell.com).)

    Many of the devices mentioned have been at least in part optical resonators. It may be instructive to look at what happens in an acoustic resonator like an organ pipe or a Helmholtz resonator.

    Let's start with a source of sound inside a perfect, infinite Q resonator. The energy density begins to build up with a value directly proportional to time. So we can store, theoretically, an infinite amount of acoustic energy within the resonator.

    Of course, it is impossible to build an infinite Q resonator, but bear with me a little longer. It is hard to get an audio sound source inside the resonator without hurting the Q of the resonator. So lets cut a little hole in the resonator so we can beam acoustic energy in. Guess what, even theoretically, this hole prevents the resonator from being perfect. It WILL resonate.

    No optical resonator can be perfect. Just like in nature there IS no perfectly reflecting surface (FTIR is about the closest thing we have). Every time an EM wave impinges on any real surface, energy is lost to heat. With any source of light beamed at any surface, light will be turned into heat. In fact, MOST of the energy is immediately turned to heat. By the laws of thermodynamics, even that that is not converted instantaneously into heat, but goes into some other form of energy, will eventually turn up as heat. You pay now, or you pay later, but you always pay the entropy tax.

    (From: Bill Vareka (billv@srsys.com).)

    And, something else to ponder:

    If you combine light in a beamsplitter there is a unavoidable phase relation between the light leaving one port and the light leaving the other.

    So, if you have a perfect Mach-Zehnder interferometer like the following

    
                +-------+      BS          M
                | Laser |=====>[\]---------\
                +-------+       |          |         M = Mirror
                                |          |        BS = Beamsplitter
                                |       BS |
                              M \---------[\]---->A
                                           |
                                           |
                                           V
                                           B
    
    
    If you set it up so that there is total cancellation out of, say, port A, then Port B will have constructive interference and the intensity coming out port B will equal the combined intensity coming in the two input ports of that final beamsplitter. This is due to the phase relation between the light which is reflected at the beamsplitter. That which is reflected and goes out port A will be 180 degrees out of phase with that which is reflected and goes out port B. The transmitted part of port A and port B are the same. Hence the strict phase relationship between the light from the two output ports. This is an unavoidable result of the time-reversal symmetry of the propagation of light.

    (From: A. Nowatzyk (agn@acm.org).)

    A beam-splitter (say a half silvered mirror) is fundamentally a 4 port device. Say you direct the laser at a 45 degree angle at an ideal, 50% transparent mirror. Half of the light passes through straight, the rest is reflected at a 90 degree angle. However, the same would happen if you beam the light from the other side, which is the other input port here. If you reverse the direction of light (as long as you stay within the bounds of linear optics, the direction of light can always be reversed), you will see that light entering either output branch will come out 50/50 on the two input ports. An optical beam-splitter is the same as a directional coupler in the RF or microwave realm. Upon close inspection, you will find that the two beams of a beam-splitter are actually 90deg. out of phase, just like in an 1:1 directional RF coupler.

    In an experiment where you split a laser beam in two with one splitter and then combine the two beams with another splitter, all light will either come out from one of the two ports of the second splitter, depending on the phase. It is called a Mach-Zehnder interferometer.

    Ideal beam-splitters do not absorb any energy, whatever light enters will come out one of the two output ports.

    Interference between E/M Radiation of Different Wavelengths

    We all know that light from a single coherent source can create interference patterns and such. What about arbitrary uncorrelated sources?

    There will be interference but you won't see any visible patterns unless the two sources are phase locked to each-other since even the tiny differences in wavelength between supposedly identical lasers (HeNe, for example) translate into beat frequencies of MHz or GHz!

    (From: Charles Bloom (cbloom@caltech.edu).)

    The short answer is yes.

    Let's just do the math. For a wave-number k (2pi over wavelength), ordinary interference from two point-like apertures goes like:

    Psi = (e^(ik(L+a).) + e^(ik(L-a).))/2
        = e^(ikL) * cos(ka)
    
    I = Psi^* Psi = cos^2(ka)
    
    (a is actually like (x-d)^2/L where 2d is the slit separation, and x is the position along the screen; L is the distance from the center of the slits to our point on the screen).

    Now for different wavenumbers:

    Psi = ( e^(ik(L+a).)+ e^(iK(L-a).))/2
    
    I = Psi^* Psi = 1/2 [ 1 + Re{ e^(i ( k(L+a) - K(L-a) ).)} ]
    	      = 1/2 [ 1 + cos( L(k-K) + a(k+K) ) ]
    	      = cos^2[ 1/2( L(k-K) + a(k+K) ) ]
    
    This is almost a nice interference pattern as we vary 'a', but we've got some nasty L dependence, and in the regime L >> a where our approximations are valid, the L dependence will dominate the a dependence (unless (k-K) is very small; in particular, we'll get interference roughly when a(k+K) ~ 10 and L(k-K) ~ 1 , and L >> a , which implies |k-K| << |k+K| , nearly equal wavelengths.)

    The L dependence is the usual phenomenon of "beats" which is also a type of interference, but not the nice "fringes" we get with equal wavelengths (the L dependence is like a Michelson-Morely experiment to compare wavelengths of light, by varying L (the distance between the screen and the sources) I can count the frequency of light and dark flashes to determine k-K.

    What about Hobbyist Interferometry?

    Building something that demonstrates the principles of interferometry may not be all *that* difficult (see the comments below). However, constructing a useful interferometer based measurement system is likely to be another matter.

    So you would like to add a precision measurement system to that CNC machining center you picked up at a garage sale or rewrite the servo tracks on all your dead hard drives. :) If you have looked at Agilent's products - megabucks (well 10s of K dollars at least), it isn't surprising that doing this may be a bit of a challenge. As noted in the section: Basics of Interferometry and Interferometers, a high quality (and expensive) frequency stabilized single mode HeNe laser is often used. For home use without one of these, a short HeNe laser with a short random polarized tube (e.g., 5 or 6 inches) will probably be better than a high power long one because it's possible only 2 longitudinal modes will be active and they will be orthogonally polarized with stable orientation fixed by the slight birefringence in the mirror coatings. As the tube heats up, the polarization will go back and forth between the two orientations but should remain constant for a fair amount of time after the tube warms up and stabilizes. Also see the section: Inexpensive Home-Built Frequency or Intensity Stabilized HeNe Laser.

    The problem with cheap laser diodes is that most have a coherence length that is in the few mm range - not the several cm or meters needed for many applications (but see the section: Can I Use the Pickup from a CD Player or CDROM Drive for Interferometry?). There may be exceptions (see the section: Interferometers Using Inexpensive Laser Diodes) and apparently the newer shorter wavelength (e.g., 640 to 650 nm) laser pointers are much better than the older ones but I don't know that you can count on finding inexpensive long coherence length laser diodes. Even if you find that a common laser diode has adequate beam quality when you test it, the required stability with changes in temperature and use isn't likely to be there.

    The detectors, front-end electronics, and processing, needed for an interferometer based measurement system are non-trivial but aren't likely to be the major stumbling block both technically and with respect to cost. But the laser, optics, and mounts could easily drive your cost way up. And, while it may be possible to use that $10 HeNe laser tube, by the time you get done stabilizing it, the effort and expense may be considerable.

    Note that bits and pieces of commercial interferometric measurings systems like those from HP do show up on eBay and other auction sites from time to time as well as from laser surplus dealers. The average selling prices are far below original list but complete guaranteed functional systems or rare.

    (From: Randy Johnson (randyj@nwlink.com).)

    I'm an amateur telescope maker and optician and interferometry is a technique and method that can be used to quantify error in the quality of a wavefront. The methods used vary but essentially the task becomes one of reflecting a monochromatic light source, (one that is supplied from narrow spectral band source i.e., laser light) off of, or transmitting the light through a reference element, having the reference wavefront meet the wavefront from the test element and then observing the interference pattern (fringes) that are formed. Nice straight, unwavering fringe patterns indicate a matched surface quality, curved patterns indicate a variation from the reference element. By plotting the variation and feeding the plot into wavefront analysis software (i.e., E -Z Fringe by Peter Ceravolo and Doug George), one can assign a wavefront rating to the optic under test.

    The simplest interference test would involve two similar optical surfaces in contact with each other, shining a monocromatic light source off the two and observing the faint fringe pattern that forms. This is known as a Newton contact interferometer and the fringe pattern that forms is known as Newton's rings or Newton's fringes, named for its discoverer, you guessed it, Sir Issac Newton. If you would like to demonstrate the principle for yourself, try a couple of pieces of ordinary plate glass in contact with each other, placed under a fluorescent light. Though not perfectly monochromatic, if you observe carefully you should be able to observe a fringe pattern.

    Non-contact interferometry is much tougher as it involves the need to get a concentrated amount of monochromatic light through or reflected off of the reference, positioning it so it can be reflected off of the test piece, and then positioning the eye or imaging device so that the fringe pattern can be observed, all this while remaining perfectly still, for the slightest vibration will render the fringe pattern useless.

    (From: Bill Sloman (sloman@sci.kun.nl).)

    An interferometer is a high precision and expensive beast ($50,000?). You use a carefully stabilized mono-mode laser to launch a beam of light into a cavity defined by a fixed beamsplitter and a moving mirror. As the length of the cavity changes, the round-trip length changes from an integral number of wavelengths of light - giving you constructive interference and plenty of light - to a half integral number of wavelengths - giving you destructive interference and no light.

    This fluctuation in your light output is the measured signal. Practical systems produce two frequency-modulated outputs in quadrature, and let you resolve the length of a cavity to about 10 nm while the length is changing at a couple of meters per second. The precision is high enough that you have to correct for the changes in speed of light in air caused by the changes temperature and pressure in an air-conditioned laboratory.

    Hewlett-Packard invented the modern interferometer. When I was last involved with interferometers, Zygo was busy trying to grab a chunk of the market from them with what looked liked a technically superior product. Both manufacturers offered good applications literature.

    (From: Mark Kinsler (kinsler@froggy.frognet.net).)

    You can get interferometer kits from several scientific supply houses. They are not theoretically difficult to build since they consist mostly of about five mirrors and a lens or two. But it's not so easy to get them to work right since they measure distances in terms of wavelengths of light, and that's *real* sensitive. You can't just build one on a table and have it work right. One possible source is: Central Scientific Company.

    (From: Bill Wainwright (billmw@isomedia.com).)

    Yes, you can build one on a table top. I have done it. I was told it could not be done but tried it anyway. The info I read said you should have an isolation table to get rid of vibrations I did not, and even used modeling clay to hold the mirrors. The main problem I had was that the image was very dark and I think I will use a beamsplitter in place of one of the mirrors next time. The setup I had was so sensitive that lightly placing your finger on the table top would make the fringes just fly. To be accurate you need to take into account barometric presure and humidity.

    Interferometers Using Inexpensive Laser Diodes

    The party line has tended to be that the coherence length of diode lasers is too short for interferometry or holography. (See the sections beginning with: General Interferometers.) While I was aware of CD laser optics being used with varying degrees of success for relatively short range interferometry (a few mm or cm - see the section: Can I Use the Pickup from a CD Player or CDROM Drive for Interferometry?), the comments below are the first I have seen to suggest that performance using some common laser diodes may be at least on par with that of a system based on a typical HeNe laser (though not a high quality and expensive frequency stabilized single mode HeNe laser).

    While I don't know how to select a laser diode to guarantee an adequate coherence length, it certainly must be a single spatial (transverse) mode type which is usually the case for lower power diodes but those above 50 to 100 mW are generally multimode. So, forget about trying to using a 1 W laser diode of any wavelength for interferometry or holography. However, single spatial mode doesn't guarantee that the diode operates with a single longitudinal mode or has the needed stability for these applications. And, any particular diode may operate with the desired mode structure only over a range of current/output power and/or when maintained within a particular temperature range.

    (From: Steve Rogers (scrogers@pacbell.net).)

    I have been involved with laser diodes for the last 15 years or so. My first was a pulsed (only ones available at that time) monster that peaked 35 watts at 2 kHz with 40 A pulses! It was a happy day when they could operate CW and visible to say the least. Anyway, in the course of my working travels, I have built numerous Twymann-Green double pass interferometers for the wave front distortion analysis of laser rods, i.e., Nd:Yag, Ruby, Alexandrite, etc. The standard reference light source for this instrument has always been the 632.8 nm HeNe laser. Good coherence length and relatively stable frequency was its strong suit.

    When visible diode lasers came out I often wondered aloud about their suitability as a replacement for the HeNe. I despise HeNe lasers. They are bulky and I have been shocked too many times from their power supplies.

    I assumed that since CD player laser diodes at 780 nm could have coherence lengths on the order of tens of centimeters or into the meters (!!, see, for example: Katherine Creath, "Interferometric Investigation of a Diode Laser Source", Applied Optics (24 1-May-1985) pp. 1291-1293), Visible Laser Diodes (VLDs) could make excellent replacements. As it turned out, VLDs tend to have coherence lengths which are considerably shorter according to the latest technical literature and I held off on experimenting with them. Last week, I went through my shop and found enough mirrors, beamsplitter, assorted optics to throw together my own double-pass interferometer for home use. This coincided with my acquisition of a 635 nm 5 mw diode module - a good one from Laserex.

    To make a longer story shorter, I assembled said equipment with the VLD and WOW! excellent fringe contrast (a test cavity of four inches using a .250" x 4.0" Nd:Yag rod as the test sample.) When a HeNe laser was substituted for the VLD, virtually no difference in the manual calculation of wave front distortion (WFD) and fringe curvature/fringe spacing. The only drawback with the VLD is that it produces a rectangular output beam. When collimated you have a LARGE rectangular beam rather than a nice round HeNe style beam. My interferometer now occupies a space of 10" x 10" and is fully self contained. It probably could even be made smaller. Not only that, but it runs on less than 3 V!!!

    I am just as surprised as you are with the results that I achieved. This is one reason why it took me so long to attempt this experiment (something like 4 to 5 years). I have always assumed that a HeNe laser would be FAR superior in this configuration than a VLD would be. Perhaps others may know more about the physics than I do. One thing is certain, these are "single mode" index guided laser diodes and typically exhibit the classic gaussian intensity distribution which is not so evident with the "gain guided" diodes. This in turn implies a predominant lasing mode which in turn would imply a (somewhat) stable frequency output. Purists would note that this VLD has a nominal wavelength of 635 nm +/- 10 nm while the HeNe laser is pretty much fixed at 632.8 nm. This variable could account for extremely minor WFD differences.

    (From: W. Letendre (wjlservo@my-dejanews.com).)

    There's an outfit in Israel selling a diode based laser interferometer enough cheaper than Zeeman split HeNe units to suggest that they are using a laser diode in the 'CD player' class, or perhaps a little better. They are able to measure, 'single pass' (retro rather than plane mirror) over lengths of up to about 0.5 m, suggesting that as an upper limit for coherence length.

    Can I Use the Optical Pickup from a CD/DVD Player or CD/DVDROM for Interferometry?

    With the nice precision optics, electromechanical actuators, laser diode, and photodiode array present in the mass produced pickup of a CD/DVD player, CD/DVDROM drive, or other optical disc/k drive, one would think that alternative uses could be found for this assembly after it has served for many years performing its intended functions - or perhaps, much earlier, depending on your relative priorities. :-) (Also see the section: Using a CD or DVD Optical Pickup in a Precision Position or Angle Encoder.

    People sometimes ask about using the focused laser beam for for scanning or interferometry. This requires among other things convincing the logic in the CD/DVD player or CD/DVDROM drive to turn the laser on and leave it on despite the possible inability to focus, track, or read data. The alternative is to remove the optical pickup entirely and drive it externally.

    If you keep the pickup installed in the CD player (or other equipment), what you want to do isn't going to be easy since the microcontroller will probably abort operation and turn off the laser based on a failure of the focus as well as inability to return valid data after some period of time.

    However, you may be able to cheat:

    Where such a feature is not provided:

    CAUTION: Take care around the lens since the laser will be on even when there is no disc in place and its beam is essentially invisible. See the section: Diode Laser Safety before attempting to power a naked CD player or simlar device.

    It may be easier to just remove the pickup entirely and drive it directly. Of course you need to provide a proper laser diode power supply to avoid damaging it. See the chapter: Diode Laser Power Supplies for details. You will then have to provide the focus and/or tracking servo front-end electronics (if you need to process their signals or drive their actuators) but these should not be that complex.

    Some people have used intact CD player, CDROM, and other optical disc/k drive pickup assemblies to construct short range interferometers. While they have had some success, the 'instruments' constructed in this manner have proven to be noisy and finicky. I suspect this is due more to the construction of the optical block which doesn't usually take great care in suppressing stray and unwanted reflections (which may not matter that much for the original optical pickup application but can be very significant for interferometry) rather than a fundamental limitation with the coherence length or other properties of the diode laser light source itself as is generally assumed.

    In any case, some of the components from the optical block of that dead CD/DVD player may be useful even if you will be substituting a nice HeNe laser for the original laser diode in your experiments. Although CD optics are optimized for the IR wavelength (generally 780 nm), parts like lenses, diffraction grating (if present and should you need it), and the photodiode array, will work fine for visible light. However, the mirrors and beamsplitter (if present) may not be much better than pieces of clear glass! (DVDs lasers are 635 to 650 nm red, so the optics will be fine in any case.)

    Unfortunately, everything in a modern pickup is quite small and may be a bit a challenge to extract from the optical block should this be required since they are usually glued in place.

    If what you want is basic distance measurements, see the section: Using a CD or DVD Optical Pickup for Distance Measurements which discusses the use of the existing focusing mechanism for this purpose - which could be a considerably simpler approach.

    Also see the section: Basics of Interferometry and Interferometers.



  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Interferometers Using Two-Frequency Lasers

    Introduction - Why Two-Frequency Lasers?

    Interferometer-based techniques are used in all types of systems requiring precision measurement of position, velocity, angle, straightness, and many other parameters. These are part of a class of what are called metrology applications. Examples include semiconductor wafer steppers in photolithography systems, hard drive and CD/DVD/Blu-ray disc mastering, optical diamound turning and other high performance CNC machines, general machine tool calibration, and many more. Measurements can be made over 10s of meters with resolution down to nanometers using the wavelength of a known wavelength of laser light as the meter (or yard) stick. Before discussing systems using two-frequency lasers, we need to back up.

    Note that all the techniques being discussed are for measuring displacement (or position change), not absolute position. Absolute measurements are possible using lasers but require additional techniques that are beyond the scope of this discussion.

    There are two classes of measurement interferometers called "homodyne" and "heterodyne". The homodyne approach uses a single-frequency laser and compares the phase of a reference and a measurement beam directly to compute displacement (or position change). The laser is typically a stabilized HeNe laser and the optics are in a configuration like a Michelson interferometer. These systems are probably best where the change in position (for example) is relatively small (a few cm) and keeping cost to a minimum is important. One suitable application would be in a hard drive servo writer. A typical configuration is shown in Interferometer Using Single Frequency HeNe Laser. The laser can be any of the frequency stabilized HeNe lasers described in the chapter: Commercial Stabilized HeNe Lasers, except that the linearly polarized beam must be oriented at a 45 degree angle with respect to the Polarizing Beam-Splitter (PBS). Half of it then gets polarized horizontally (into the plane of the diagram) by the PBS and is returned from the retroreflector of the "Fixed Arm" as the reference beam (REF) while the other half gets polarized vertically passing through the PBS and is returned from the retroreflector of the "Test Arm" as the measurement beam (MEAS). They are recombined in the PBS as a single beam that has two components whose relative phase depends on the relative position of the two retroreflectors, and this changes as the Test Arm is moved. Some fraction of the combined beam goes to an "Intensity" photodiode that produces an output proportional to the beam power. This is needed to keep track of the actual signal level. The remainder is split into two parts which go to separate photodiodes (PDs). One PD has a polarizer in front of it which results in a sinusoidally varying output based on the relative phase of the REF and MEAS beams. Call this the "cos" signal. The other PD has a Quarter-Wave Plate (QWP) and polarizer in front of it. The QwP shifts the relative phase of the REF and MEAS beams by 90 degrees so that the PD now produces a signal that is shifted by 90 degrees in phase. Call this the "sin" signal. (An alternative implementation is to use a "special optic" before the photodiodes to convert the phase difference between the reference and measurement beams to a rotation in polarization. Then, only polarizers would be needed in front of the photodiodes and it would seem to be much more elegant! I assume that every first-year physics student knows what the "Special Optic" consists of in their sleep.) The sin and cos signals being in quadrature (offset from each other by 90 degrees) are sufficient to determine displacement (consisting of both distance change and direction) using digital hardware only slightly more complex than a common up-down counter. This is the same type of hardware used with optical encoders based on parallel lines or gratings, but with the interferometer approach, using the wavelength of light itself.

    The benefit of the homodyne approach is simplicity and low cost (at least in a relative sort of way as none of these systems is exactly inexpensive!). And for some applications, it's more than adequate. The basic measurement processing is little more than what keeps track of the position of a computer mouse. The laser can be built very inexpensively (despite what it probably costs the end-user!) and the optics and optical receiver are quite rudimentary.

    However, there are several deficiencies that make these systems undesirable (or at least much more difficult to implement) for more sophisticated applications. Since they are comparing the phases of the REF and MEAS beams directly, the result at any given time is a DC level that depends not only on the relative phase, but also on the actual output power of the laser and optical losses elsewhere in the system, drift in the electronics, and even very slight changes in optical alignment. But the electronics does tend to be simpler and unlike the two-frequency systems, the only upper limit on velocity is one of optical detection and processing speed, not the value of the "split" frequency of the laser.

    Interferometers Using Two-Frequency Lasers

    The interferometers described in the previous sections and found in physics labs (assuming such topics are even taught with hands-on experience!) all use CW lasers and look at the fringe shifts as the relative path lengths of the two arms is changed. While this works in principle and has been used widely, modern commercial measurement systems based on interferometry often use more sophisticated techniques to reduce susceptibility to signal amplitude changes and noise and improve measurement accuracy, stability, and convenience. These are called "heterodyne" systems in which the laser beams are in essense carriers for a lower "split" frequency in the MHz range provided by the two-frequency laser. The split frequency is detected optically, but then can be manipulated using straightforward electronics totally in the AC domain. If you're totally confused by now, never fear. There is much more below. ;)

    The microchips in virtually all modern electronics (including the CPU and memory inside the PC or MAC you're reading this on) were likely produced on photolithography systems incorporating wafer steppers using two-frequency interferometers for multiple axes of ultra-precise motion control.

    Interferometer-based measurements systems typically use some type of low power stabilized helium-neon laser to produce the "yardstick" beam of light. By stabilizing the laser with reference to the neon gain curve, the accuracy of the optical frqeuency/wavelength can easily be known to better than +/-0.1 ppm. As noted above, a basic system may use such a laser in a Michelson or similar interferometer, with a quadrature (sin/cos) detector to count fringes representing changes in path length as described above. Problems with such a system are that changes in light intensity will result in measurement errors, alignment is very critical to obtain adequate fringe contrast, and they are more susceptible to noise.

    In two-frequency interferometers, a special stabilized HeNe laser is used that produces a beam consisting of two very slightly different frequencies (wavelengths) of light simultaneously. This may be achieved by various techniques. HP/Agilent lasers employ a special tube which uses a magnet to perform Zeeman splitting while Zygo uses an external acousto-optic modulator. As above, both types of lasers are locked in such a way that the optical frequency is very precisely known.

    A diagram of the general approach is shown in Interferometer Using Two Frequency HeNe Laser.

    The following description applies to the HP/Agilent approach using Zeeman splitting. With Zygo, the method of generating the two frequencies differ, but their use in the interferometer is the same.

    In the Zeeman split approach, the two-frequency laser consists of a HeNe laser tube surrounded by permanent magnets which produce a constant axial magnetic field. The laser tube is short enough that without a magnetic field, only a single longitudinal mode will normally oscillate if it is located near the center of the neon gain curve. (Those on either side will not see enough gain.) The net result of the magnetic field is that instead of a single longitudinal mode, two modes are produced that differ very slightly in frequency and have right and left circular polarization. The difference between the two frequencies is typically in the 1.5 to 4 MHz range (though some go up to 6 MHz or more), which makes the resulting signals extremely easy to process electronically. The actual difference frequency is determined by the strength of the magnetic field, length of the internal laser cavity, and other physical details, as well as the exact place on the Zeeman-split neon gain curve where the laser has been locked.

    To stabilize the laser, there is a piezo element and/or heater to precisely adjust cavity length. A feedback control system is used to adjust the cavity length to maintain the position of the Zeeman-split frequencies - and thus the wavelengths - constant. The feedback is generally based on the simple approach of forcing the orthogonally polarized outputs to be equal, which results in the highest beat frequency and most stable optical frequency.

    The wavelength of the laser is the measurement increment ("yardstick") and will remain essentially unchanged for the life of the instrument. For example, with the doppler broadened gain curve for the HeNe laser being about 1.5 GHz FWHM (1 part in about 300,000 with respect to the 474 THz optical frequency at 633 nm) and a 1 percent accuracy within the gain curve, the absolute wavelength accuracy will then be better than 1 part in 30 million! Not too shabby for what is basically a very simple system. In practice it's even better. :)

    Note that the exact value of the difference frequency does not need to be very precisely controlled over the long term. Rather, it is the difference between the reference difference frequency and the measurement difference frequency that matters, and the latter only depends on the motion of the target reflector - and the speed of light. Thus, the exact beat frequency of each laser need not be precisely controlled or even precisely measured and recorded or used anywhere in the calculations.

    Since the output of the laser is a beam consisting of a pair of circularly polarized components, a Quarter-Wave Plate (QWP) and Half-Wave Pate (HWP) are used to separate these into two orthogonal linearly polarized components called F1 and F2, and to orient them such that they are parallel to the horizontal or vertical axes.

    The beam consisting of F1 and F2 is split into two parts with a non-polarizing beam-splitter: One part goes through a polarizer at 45 degrees (to recover a signal with both F1 and F2 linearly polarized in the same direction) to a photodiode which generates a local copy of the reference frequency (REF, the difference between F1 and F2) for the measurement electronics; the second is the measurement beam which exits the laser. The return beam is called MEAS.

    The purpose of the remainder of the interferometer is essentially to measure the path length change between two points. In a typical installation, the beam consisting of F1 and F2 is sent through a polarizing beamsplitter. F1 goes to a cube-corner (retro-reflector) on the tool whose position is being measured and F2 goes to a cube-corner fixed with respect to the beamsplitter and laser. However, differential measurements could be made as well using F2 in some other manner. Various "widgets" are available for making measurements of rotary position, monitoring multi-axis machine tools, etc.

    The return from the object corner reflector is F1+ΔF1 which is recombined with F2 and sent to an "optical receiver" module - a photodiode behind a polarizer at 45 degrees and preamp which generates a new difference frequency, F2-(F1+ΔF1). This signal, called "MEAS" is compared with REF to produce an output which is then simply ΔF1. (A mixer is shown in the diagram but in practice, it might be implemented with digital logic like counters and subtractors.) A change in the position of the object by 316 nm (1/2 the laser wavelength) results in ΔF1 going through a whole cycle. By simply keeping track of the number of complete cycles of ΔF1, this provides measurements of object position down to a resolution of a few hundred nm with an accuracy of +/-0.02 ppm! And the typical implementation will multiply the REF and MEAS frequencies by 16 or 32 or more using a pair of phase-locked loops to produce a corresponding improvement in resolution down to a few nanometers or better!

    The primary disadvantage of heterodyne systems is that the maximum velocity is limited in the direction that would reduce MEAS since going through 0 Hz would be confusing at best. So, one of the key specifications for these lasers is the (minimum) split frequency. For example, the HP-5517B has a split frequency range of 1.9 to 2.4 MHz with typical samples being 2.20 MHz. But the minimum is the critical value and for 1.9 MHz, the maximum velocity will be around 0.5 m/s using the simplest (linear) interferometer. Zygo lasers have a 20 MHz split frequency so the velocity can be over 10 times higher.

    More information on the two-frequency HeNe laser can be found in the sections: Hewlett-Packard/Agilent Stabilized HeNe Lasers and Two Frequency HeNe Lasers Based on Zeeman Splitting. Searching on the Agilent Web site will yield product specific information and application notes on two frequency interferometers. A comprehesive but not too hairy description of the two frequency approach can be found in the Hewlett-Packard`Journal, February, 1976. Yes, this is an old technique (actually much older)! Searching at HP Archive for "Interferometer" and similar terms will turn up many more interesting articles.

    Zygo, another manufacturer of interferometer measurement systesms using two-frequency lasers had an excellent tutorial called "A Primer on Displacement Measuring Interferometers" but it seems to have disappeared from their Web site. But never fear, I archived it at Sam's Copy of Zygo's Primer on Displacement Measuring Interferometers. And, an even more extensive introduction can now be found in the PowerPoint presentation at Introduction to Displacement Measuring Interferometry and Sam's Copy of Introduction to Displacement Measuring Interferometry. (The actual title found by Google is the same as the previous one, "A Primer on Displacement Measuring Interferometers".)

    Lasers for Interferometers Using Two-Frequency Lasers

    There are generally low power HeNe lasers with either specially designed (and expensive) laser tubes or an external acouto-optic modulator (also expensive) to produce the two (relatively) closely spaced optical frequencies with orthogonal polarization. Depending on technique, the difference frequency can be anywhere from a few hundred kHz to 20 MHz or more. Since the beat frequency between the reference and measurement signals decreases with one direction of motion and can't go below 0 Hz (or at least becomes confusing as it passes through 0 Hz), a higher difference frequency translates into higher maximum speed of position change in the measurement system. Therefore, depending on the specific application, a higher difference frequency may be essential.

    Optics for Interferometers Using Two-Frequency Lasers

    The following discusses the various types of optical components supplied by Hewlett Packard (now Agilent) for measurement of position (or more accurately, displacement or change of position) or velocity (rate of change of position). There are also other optical configurations for measurement of angle, flatness, straightness, squareness, and more. But in essense, all of these convert a change in the measurement parameter into a change in position. So, the basic principles of operation are the same. Optics from other companies like Excel Precision and Zygo are similar.

    While the description below deals with "AC" or "heterodyne" systems using a two-frequency laser, the same optical configurations are also applicable to "DC" or "homodyne" systems using a single-frequency laser. Aside from the type of laser, the optical receivers (and subsequent processing) will also differ. Teletrac (now Axsys) was one manufacturer of these generally lower performance (and lower cost) systems.

    The most basic application (for a single axis measurement) will consist of the following optical components:

    Since the optical frequency/wavelength is being used as the "measuring stick" in these systems, it must be known to a high degree of precision and anything that affects it must also be taken into account. In particular, the temperature, pressure, and humidity of the air must be factored into the measurement calculations. Or, if part or all of the measurement setup is in a vacuum, this will affect it. These corrections can be done at least partially automatically with sensors, or by manually entering parameters into the measurement computer. See Refractive Index of Air and Wavelength Correction Calculator (NIST).

    These systems generally allow a single laser to be used with installations where the motion of multiple access needs to be measured. So in addition to the actual measurement optics, there will be components to split and redirect the original beam from the two-frequency laser to each axis.

    Interferometers:

    The heart of all of these systems are the interferometers. The three most configurations used most often are shown in Most Common Hewlett Packard/Agilent Interferometers. (This diagram applies directly to two-frequency lasers like the 5517 where F1 (the lower optical frequency) is oriented horizontally. Where F2 (the higher optical frequency) is oriented horizontaally as with the 5501A/B, simply swap F1 and F2 in the diagram.) There can be various permutations of the individual components that are optically and functionally equivalent. Combinations of multiple interferometers mounted on a common platform are also available for compact multi-axis applications. What HP calls the "interferometer" consists of all the components in the center of each diagram - the Polarizing Beam Splitter (PBS), 1 or 2 Retro Reflectors (RRs), and 0, 1, or 2 Quarter-Wave Plates (QWPs). There will also be a RR or Plane Mirror (PM) on the "tool" whose position is to be measured, and an Optical Receiver (OR) for the return beam. The Two-Frequency Laser (TWL) can be shared among all the axes of the machine by distributing its beam using non-polarizing beam splitters and 45 degree mirrors ("Beam Benders", but all beam orientations must be a multiple of 90 degrees to the original TFL).

    As noted, all of these interferometers contain a high quality Polarizing Beam Splitter (PBS) cube as their central component. What gets added on to the PBS depends on the specific type and may include Retro-Reflectors (RRs, which are solid cube-corners) and/or Quarter-Wave Plates (QWPs). Please refer to the diagram, above. For HP-5517 lasers, F1 is the lower frequency component and is horizontally polarized, while F2 is the higher frequency component and is vertically polarized. (HP-5501 lasers are the opposite, but the only effect is a sign change in the measurement calculation.)

    Two other relatively commonly used configurations are the Angular and Straightness Interferometers, shown in Hewlett Packard/Agilent Angular and Straightness Interferometers. The Angular Interferometer measures small changes in orientation while the Straightness Interferometer measures small displacements perpendicular to the direction of travel (not angle over distance as some other techniques use). Compared to the normal displacement/distance and velocity, the sensitivity to small changes is much higher for these interferometers. For example, the factor is around 360 for long range straightness! Therefore, it doesn't take much vibration or other environmental factors, or sloppy setup and alignment to produce huge uncertainty or systematic errors in the measurements.

    The Agilent prices are also interesting in that they are at least 5 times what similar optical components would cost from a supplier like Melles Griot or Newport. I do not know how much - if any - of this is due to the required optical quality compared to less demanding applications. Most of the cost is likely due to the specilized precision mounting and the relatively low production volume of interferometer optics. Fortunately for hobbyists and experimenters, the common HP/Agilent interferometers are readily available surplus, often at very reasonable prices. And non-HP/Agilent optics should be just fine for non-critical applications.

    See Links to Agilent Laser and Optics User's Manual for general information on the current Agilent lasers and interferometer optics (including the more exotic configurations for angle, flatness, straightness, and others). For much more on these systems, go to Agilent and search for "Interferometers".

    Another clever configuration is the Differential Plane Mirror Interferometer (DPMI). With minor modifications, the DPMI provides either displacement or angular measurements that are extremely precise because the optical paths for both the reference and measurement beams are virtually identical. In addition, the reference and tool mirrors themselves can be located in close proximity. Thus the effects of environmental changes will be the same and not affect the measurement accuracy. A diagram of the Zygo implementation is shown in Zygo Differential Plane Mirror Interferometer (DPMI). The Polarization Shear Plate (PSP) splits the F1/F2 beam into a pair of parallel beams separate by about 9 mm. This is equivalent to a Polarizing Beam Splitter (PBS) and 45 degree mirror or prism. The Half-Wave Plates (HWPs) rotate the polarization axis of one of these beams to be the same as the other. The remaining components - Polarizing Beam Splitter cube, Retro-Reflector (RR), and Quarter-Wave Plate (QWP) handle each of these beams in the same manner as the measurement (MEAS) beam in the Plane Mirror Interferometer (PMI) described above. Each of the beams pass through the PMI and gets reflected by either the Reference Mirror (RM), or the Target or Tool Mirror (TM). But due to the QWP, the polarization axis is rotated 90 degrees on the way back, so it reflects off the diagonal of the PBS down to the RR and back up reflecting once again off the PBS to the TM. Finally, due to a second polarization rotation of 90 degrees, it passes through the PBS cube back to the PSP where the beams are combined in reverse, and then out to the Optical Receiver (OR) typically via a fiber-optic cable. In short, the purpose of all this complexity is to split the F1/F2 components apart so that their path lengths can change independently and then combine them back into a single beam.

    The DPMI is the key component of the Zygo Compact Wavelength Compensator (CWC), shown in Photo of Zygo Compact Wavelength Compensator (CWC). (The cover protecting the delicate mirrors was removed for the photo.) This device provides a separate measurement whose value is precisely proportional to the change in wavelength from a reference point due to changes in temperature, pressure, and humidity. The resulting value can then be used by the processing electronics to greatly improve the accuracy of all measurements. The HP/Agilent 10717A Wavelength Tracker, which is functionally equivalent to the Zygo CWC, is based on their version of a DPMI which has an extra RR to configure the input beam rather than a PSP. Excel Precision apparently offered a true refractometer rather than a compensator or tracker. This provided an absolute wavelength reference and would thus be far superior if it was accurate.

    It should be noted that the exact same optical configurations are used with single frequency (homodyne) interferometers. With those, f1=f2, usually produced by orienting the linearly polarized input beam at 45 degrees.

    Electronics for Interferometers Using Two-Frequency Lasers

    The most basic requirement is to convert the phase shift between REF and MEAS to a count or position. Conceptually, this is nearly trivial, being simply the different between the total cycles of the REF and MEAS signals. And in fact, the original HP-5505A display unit did this by brute force with a pair of counters and a subtractor. As will be shown below, this turns out to be a clean, but rather hardware intensive solution. In general, care needs to be taken in the design of the processing hardware and/or software to avoid possible errors ultimately resulting from the Uncertainty Principle.

    Possible approaches

    Counting cycles of the phase difference is fine for education and demos, but real systems almost always implement some type of resolution enhancement scheme like frequency multiplication using a Phase-Locked Loop (PLL) or ultra-high speed sampling. The spec sheets can then claim a resolution that is not simply a fraction of the wavelength of the HeNe laser, but down a few nanometers or even better. It is not known to what extent these spectacular specs are realizable and repeatable in practice over the life of the system taking into consideration the normal decline in laser power, accumulated dust on the optics, changes in alignment, and other factors which result in increased noise in the optical signal.

    Sam's Measurement Display for Two-Frequency Interferometers

    I was tired of searching for something inexpensive on eBay and anyhow, wanted something interesting to do. I also miss the days of 0s and 1s a bit (no pun...) and my drawer of ancient TTL chips was getting rather lonely. :)

    I decided on the approach using the up/down counter due to its simplicity. The schematic except for the up/down counter is shown in Sam's Pulse Converter for Two-Frequency Interferometer. It consists of:

    While the laser is warming up and there is no REF signal, only the green LED is lit. Once REF appears but if the MEAS beam is misaligned or blocked, only the RED LED is lit. Once everything is stable and aligned, neither is lit if there is no movement. I also breadboarded a version using a PAL to generate the control signals. This eliminates most of the discrete logic chips.

    Before I added the display, I had to be content watching the green and red LEDs. Any vibration - including moderately loud sounds - result in flickering. I'm not sure what type of music they prefer, but a sustained tone can result in quite impressive activity. :-)

    The multi-digit up/down counter ICs I had found tended to be marginal with respect to their maximum frequency count capability. So I decided to go with the separate chip "brute force" approach for the first version. The prototype shown in Photo of Sam's Measurement Display 1 in Action has been tested and appears to work as expected. This measurement display may be avilable as a kit or fully assembled along with other metrology components for educational purposes in the future. See Installation and Operation Manual for Sam's Interferometer Measurement Display System 1 (SG-MD1) for more info including complete specifications.

    The next version of a measurement display will probably be based on an FPGA for the front-end logic with an on-board microprocessor to handle the units/format conversion, user interface, and communications with a PC. But I don't expect that to happen any time soon, if ever. :)

    Of course, this isn't exactly rocket science. Besides HP/Agilent, Zygo, Excel, and the other major players, these things have been stuffed into LSI ICs for a long time. One example of an almost single chip solution is from Laser Metric Systems, Inc.. They have a much more limited line of metrology systems but do have a PC ISA or PCI bus card (PRM-004 series) whose brains is a single Altera FLEX FPGA that can handle the performance requirements of just about any two-frequency laserinterferometer with a claimed resolution down to 0.1 nm. Well, maybe. :) See the product info on their Web site.

    And check out this (open access) paper: "FPGA-Based Smart Sensor for Online Displacement Measurements Using a Heterodyne Interferometer", Sensors 2011, 11, 7710-7723. They digitize the analog REF and MEAS signals in dual 14 bit 20 MHz flash A/Ds. So, in addition to the basic computation taking the difference of (wrap-corrected) accumulators for REF and MEAS, they use the analog waveforms to refine the measurement and estimate the actual phase difference (partial wavelength) claiming a resolution of 3.4 nm over a range of 3 m.

    Sources of Measurement Error in HP/Agilent Metrology Systems

    While it sounds really impressive to be basing precision measurement on the wavelength of light, and HP/Agilent lists the nominal wavelength of the 5501 and 5517 lasers to 9 significant figures, there are many environmental and installation factors that impact what is actually useful. The following is the briefest of summaries of accuracy issues. A large portion of the operation manuals for these systems is devoted to this topic. More than you could ever hope to know can be found in the links in the section: Agilent Laser and Optics User's Manual. The following is a brief summary.

    Laser wavelength accuracy

    A common question that comes up with respect to these systems is: "Since these are based on two-frequency lasers, which frequency is used as the wavelength specification?". The quick answer is: It's not clear. :)

    A phase change of 360 degrees of the difference frequency of the measurement beam with respect to the reference beam represents a change in position of of the moving part of the measurement system (e.g., the "tool") by 1/2 wavelength (linear or single beam interferometer) or 1/4 wavelength (plane mirror interferometer). Thus the component (F1 or F2) that goes to the tool is the actual "yardstick" wavelength. If both F1 and F2 are used in a differential measurement, then each contributes to the measurement based on its wavelength. So, strictly speaking, one should use those wavelengths in the calculation. But as a practical matter, it really doesn't matter as the difference in frequency between the two components F1 and F2 is so small compared to the optical frequency, that the error introduced by using one or the other is way below the accuracy specification for even the military calibrated versions of these systems.

    It's not clear (at least to me) where the value of 632.991372 nm for the 5501B and 5517A/B or 632.991354 nm for the 5517C/D comes from. My assumption would be that it's the theoretical lasing line center of the Zeeman-split neon gain curve. Various sources list other slightly different values for HeNe lasers. Wikipedia has a page on the Meter Measuring Unit that gives a value of λHeNe=632.99139822 nm. And an HP Journal article on the 5528A gives yet another value of 632.991393 nm for the 5518A, but that tube should be essentially identical to the tube in the 5517A! The actual fill pressure, ratio of He:Ne and their isotopic mix, temperature, and other factors will affect the exact wavelength. But why the different values even for essentially similar lasers from HP/Agilent including lasers that are still in production? The corresponding difference in optical frequency between 632.991372 nm and 632.991354 nm is about 13.5 MHz, so it's way beyond the error due to whether line center or either F1 or F2 is used. Perhaps, it has something to do with the newer tubes being filled with pure Ne20 or Ne22 and the older ones having a mixture to guarantee compatibility in legacy applications. How's that for a wild guess? :) The difference is still under 0.03 ppm, so it should generally not be a huge issue in any case.

    The normal HP/Agilent lasers have a long term stability specification of +/-0.1 ppm. The laser stabilization depends on the condition of the electronics and may result in a small variation in this wavelength. Even one power cycle to the next does not result in a precise return to the exact same conditions due to the particular temperature, and thus cavity mode number, at which lock occurs. But any variation will only be a few MHz at most, well below the +/-0.1 ppm specification.

    The Agilent info indicates that they will certify a laser to MIL STD-45662 for long term stability of +/-0.02 ppm. MIL STD-45662 requires traceability to a (wavelength/frequency) reference, which I would assume to be something like an iodine-stabilized laser. However, this doesn't sound like the laser design is necessarily any different, just that the optical frequency of the specific laser is measured precisely, and the exact value to even more significant digits than the those given above, is included in the calibration report. With that number known, +/-0.02 ppm corresponding to about +/-9.4 MHz, really shouldn't be hard to maintain.

    Velocity of light

    Ambient temperature, air pressure, and humidity all have a very significant effect on the measurement. Approximately a 1 part-per-million change in the velocity of light and thus measurement wavelength will result from:

    Since 1 ppm is 10X of the basic measurement specification for accuracy, it is clear that these factors must be taken into account. The HP/Agilent measurement systems have sensor options to allow this to be done automatically, or the corrections can be entered manually.

    And, if the "tool" is in a vacuum, that's roughly a 3 part in 10,000 error due to the difference in the index of refraction of a vacuum compared to air!

    See Refractive Index of Air and Wavelength Correction Calculator (NIST).

    Material effects

    Depending on the construction of the equipment on which the interferometer optics are mounted, the change due to thermal expansion and other effects can be very significant resulting in serious errors if not taken into consideration.

    Determining the Exact Laser Wavelength or Frequency

    So, how accurately can the wavelength or frequency of one of these lasers be determined outside of a NIST calibration laboratory or Agilent test facility? It all really comes down to having a standard of reference. Commercial instruments called wavelength meters available from companies like TOPTICA Photonics AG can have very high accuracy, some down to 2 MHz (0.0000027 nm) or better. But that is only if a more accurate reference is used for calibration not to long before making measurements of the unknown laser. 2 MHz isn't very much when it comes to optical frequency! So, even if you could afford one of these expensive instruments or could borrow one, it would still need to be calibrated against some reference!

    The most common reference of relevance for this testing would be a mode stabilized HeNe laser like a Spectra-Physics 117A. Its frequency will have a long term stability of 10 MHz or less, but whether the absolute accuracy can be nailed down to better than 50 or 100 MHz - about 0.1 to 0.2 parts-per-million (ppm) - is questionable due to factors like the exact neon isotope(s) in the gas fill and the precise point on the neon gain curve where the laser is locked. So, a higher precision reference would be needed to calibrate that!

    However, some of the HP/Agilent lasers themselves may have their frequency known to extremely high accuracy. For the normal commercial versions of the 5517 lasers, the spec'd accuracy is *only* +/-0.1 ppm, which is equivalent to +/-0.0000633 nm (+/-47.4 MHz in optical frequency) with a nominal wavelength of 632.991354 nm (for the 5517C/D; 632.991372 nm for the 5501B and 5517A/B and some others even though the tube is virtually identical in all of these lasers).

    But there are some 5517 lasers that come with a "pedigree" - a report including the wavelength of the specific laser measured to very high accuracy under controlled environmental conditions.

    The report for one particular 5517D laser included the following: Temperature and humidity test conditions of 22.8 °C and 36.2%, respectively, a locked output power of 409 uW (spec is 180 uW minimum), a split (REF) frequency of 2.75 MHz (spec'd range is 2.4 to 3.0 MHz), and the actual vacuum wavelength of 632.9913662 nm (nominal is 632.9913540 nm).

    That difference of 0.0000122 nm is actually a rather large error, equivalent to about 9.4 Mhz in optical frequency or about 0.02 ppm.

    I don't know if it is even remotely possible to obtain information for a specific serial number laser from Agilent unless you're the original buyer, and I rather doubt it. After all, your good fortune on eBay is hardly something that Agilent is likely to care much about! :) But, it may be possible to obtain the information from the original buyer if they can be tracked down. I was able to have access to one of these lasers that I was repairing, and they provided the above data from the calibration report. With this data, I was hoping to be able to measure the exact wavelength/frequence of one of my HP/Agilent lasers and then use it as the reference when I return the other one. However, it turned out that the wavelength report didn't apply to this particular sample but a similar one. Oh well.

    However, age and use of the laser will affect the optical frequency by enough to matter. So unless the laser is relatively new, any data may be unreliable. The drift may be predominantly due to changes in the He and Ne partial pressure. These both alter the width of the Doppler broadened Ne split gain curves, and shift the center optical frequency. End-of-life lasers - the type often found on eBay - would be most prone to such effects. But even relatively young lasers will see a drop in pressure that may be significant. One tip-off to a potential discrepancy could be the REF frequency. From my observations and the comment by an engineer who tests these lasers, the REF frequency tends to increase slightly with use. This is consistent with a broadening of the gain curves which will increase the mode pulling effect. (But, at least the change is in the beneficial direction - the one that increases the maximum measurement velocity specification, assuming the processing electronics can handle the higher REF frequency!)

    One reference is: "Frequency stability measurements on polarization-stabilized He-Ne lasers", T. M. Niebauer, James E. Faller, H. M. Godwin, John L. Hall, and R. L. Barger, Applied Optics, vol. 27, no. 7, 1 April 1988, pp. 1285-1289.

    However, assuming you found a new-in-box Agilent laser with complete documentation of optical frquency:

    Some relevant numbers: 633 nm is about 474 THz based on the speed of light of 299,792,458 m/s. So, 1 nm is 0.749 THz or 749 GHz and 0.0000001 nm is 74.9 kHz. 100 kHz is 0.000000134 nm or 1 MHz is 0.00000134 nm. The difference between 632.9913540 nm and 632.9913662 nm is then around: 9.137 MHz.

    Comparing the Optical Frequencies

    With one of these lasers in hand, it's really quite straightforward at least in principle to determine the exact frequency offset of another similar laser by beating (heterodyning) the two outputs in a high speed photodiode and measuring the difference frequency. Since frequency doesn't depend on environmental conditions, nothing that happens outside the laser will affect it. The resulting optical frequency value can then be divided into the speed of light in the relevant medium (e.g., air at STP or vacuum) to compute the exact wavelength.

    My Two-Frequency Interferometer Laser Tester (See Diagram of Two-Frequency Interferometer Laser Tester and Photo of Two-Frequency Interferometer Laser Tester) was modified to permit the beam from a second HP/Agilent laser to be combined and sent to an optical receiver or biased photodiode. A diagram is shown in Diagram of Test Setup for HP/Agilent Laser Optical Frequency Comparison. A Polarizing Beam-Splitter (PBS) was added, at first only on a somewhat adjustable mount (normally used for Beam Splitters or Beam Benders in these systems) and could easily be installed and removed without tools. However, this proved to have nowhere near the estimated 1/10th of a mR precision needed to align the beams to obtain a beat signal. So, a Newport MM1 kinematic mount with the PBS attached to it was installed in its place. The second laser itself is on a fully adjustable platform (3 screws), so the beams can be lined up precisely.

    The distance from each laser to the PBS is relatively closely matched, so if they have the same optics (e.g., 6 mm), the wavefront curvature should be similar resulting in minimal alignment issues and decent signal signal amplitude. How much mismatched optics (e.g., 6 mm with 9 mm) will affect this is not known though. But in the far field, the more significant issue would probably be the loss of available optical power from the size mismatch since the wavefronts should be quite close to planar.

    A Half-Wave Plate (HWP) which may be installed in front of either laser will rotate the polarization if needed to pass it through the PBS, with the PBS itself serving to eliminate the unwanted F1 or F2 component. Then, F1 or F2 from Laser 1 can be beat with F1 or F2 from Laser 2. However, not knowing the range of possible difference frequencies likely to exist for any given pair of lasers, the highest bandwidth HP/Agilent 10780 may not be adequate to capture the difference frequency without a lot of luck, especially if comparing lasers with different spec'd nominal optical frequencies like the 5517A and 5517C (12 MHz difference). So my original intent was to replace the HP optical receiver with a Zygo 7080 which is used with their lasers having a REF frequency of 20 MHz. Assuming a maximum of a +/-0.1 ppm frequency offset with respect to nominal for each laser, the difference could be up to 95 MHz, though I highly doubt even half of this is at all likely. Nonetheless, some higher speed photodiode detector may be needed in general. And my poor old 10 MHz oscilloscope which had been dedicated to monitoring of HP/Agilent REF and MEAS frequency signals was augmented with another equally old 50 MHz scope. :) (As it turned out, for the tests I actually performed on two specific 5517B lasers, the original 10780A was quite adequate.)

    Since F1 (the lower optical frequency) and F2 (the higher optical frequency) have known orientations, being able to select each one will make it possible to unambiguously determine whether the difference frequency between the two lasers represents Laser 1 or Laser 2 being the one that is higher in frequency.

    Another approach would be to monitor the difference frequency as the cube-corner or plane mirror ("tool") in the interferometer is moved; whether it decreases or increases for a given direction of motion will also unambiguously determine which laser's frequency is the higher one.

    For maximum accuracy, both lasers need to warm up from a cold start (not restarted) for several hours in an environment with a fairly constant temperature. But of course, the activity during warmup is in itself quite exciting. :)

    Note that the stray magnetic field from one laser will change the REF frequency of the other laser if they are close together, especially if they are parallel to each-other. The frequency offset that is introduced is only order of 1 or 2 percent of the reference frequency at most. It may in fact not affect the optical frequency very much, if at all. And even if it does, the magnitude should be of negligible consequence. But any disturbance is still worth minimizing. A bit more on this below.

    There may also be second-order effects of external magnetic fields that are not aligned with the laser's magnetic field. This will not only change the strength of the axial magnetic field, but will also introduce a transverse component to the magnetic field with unknown consequences.

    Although it turned out that I don't have a laser with a known optical frequency and had to return the one that was probably close, I used a pair of healthy 5517Bs to perform the experiment. But nothing is perfect! :) A few of the issues:

    Using a Half-Wave Plate (HWP), it was possible to select F1 or F2 of Laser 1 (L1) to beat with F2 of Laser 2 (L2). The difference in optical frequency changed from 5.0 Mhz (no HWP, L2F2-L1F1) to 2.7 MHz (HWP oriented at 45 degrees for a 90 degree rotation, L2F2-L1F2), proving that Laser 1 has the lower absolute optical frequency. (Laser 1's REF or split frequency is 2.3 MHz.) It sure feels good when the physics cooperates! :) (For convenience in trying to keep track of things, the difference frequencies here are referenced to F1 of both lasers, rather than the average of F1 and F2, which strictly speaking may be more accurate. In most cases, this results in a shift of less than 1 MHz.)

    The difference frequency between F1 of Laser 1 and F1 of Laser 2 (L2F1-L1F1) after coming Ready can be anywhere from approximately -2 MHz to +6 MHz depending on whether one or both lasers was started from being cold. (Of course, before locking, the difference frequency can be up to 1.6 GHz or so - the FWHM width of the neon gain curve!) The beat then drifts by 4 or 5 MHz over the next hour or so. Typically, if both lasers are started at the same time, having been off for more than an hour, the difference frequency (L2F1-L1F1) just after locking is around -1 MHz, goes down to -2 MHz, and then climbs to a final difference frequency of around +2.6 MHz over the course of an hour or so. Pretty impressive for systems running at 474 THz. That 2.6 MHz is less than 1 part in 100,000,000!

    The temperature of the laser - or more likely the temperature of the control electronics - also affects the optical frequency by slightly shifting the lock point on the split HeNe gain curve. The data above was with the lasers undressed. Installing their covers resulted in the difference frequency dropping to under 1 MHz after an hour but then it went through 0 Hz and seems to have finally settled at around -2.3 MHz after 5 hours. Removing the cover over the PCB on Laser 1 caused the difference frequency to climb back up to +2.6 MHz, so a change of almost 5 MHz. I swapped in the Control PCB from another 5517 laser just for grins and giggles, The optical frequency after 10 hours from a cold start moved to about -1 MHz with at least as wide a variation during warmup as with the original Control PCB. I don't presently have a 5517B with a newer digital Control PCB so I can't compare its stability to that of the common analog Control PCB.

    Finally, I swapped the beam sampler assemblies including the LCD switch on Laser 2 and this resulted in a significant change in the difference frequency - almost 5 MHz. It's possible that an LCD panel that is starting to delaminate or degrade in some other way could do this, but I have no reason to suspect that one of these was bad.

    Given how small the difference frequency between two randomly selected 5517Bs is, could it be that these 5517B lasers really are close to the nominal spec'd value of 632.991372 nm or 473.612234 THz? Possibly, since the least significant digit of 632.991372 or 0.000001 nm is about 0.75 MHz. Where's a NIST calibration lab when you need one?

    I also did some very basic experiments with magnetic fields applied to one of the lasers. Placing another similar laser along-side Laser 1, the difference in optical frequency could be dramatically changed as the distance or orientation of the two lasers was altered. But, most - but not all - of the frequency change was eliminated in a few seconds as the control loop readjusted the locked position. The initial response didn't seem to be instantaneous, so perhaps it was something related to a slight change in laser tube current or something else affecting the temperature of the tube. Given that both the strength and symmetry of the magnetic field was being affected by repositioning, it's not at all clear what was actually changing. But everything else should be intuitively obvious. :-)

    Not content to permit one of these lasers to rest wherever it pleases, I've built a simple network to introduce a small offset into the error signal driving the laser tube heater power amp to fine tune the optical frequency. The circuit consists of a pair of 1K ohm resistors feeding 4.7 V zener diodes from +15 VDC (TP8) and -15 VDC (TP10) to ground (TP1). (These testpoints represent convenient locations to attach the circuit.) The regulated +/-4.7 VDC are probably not really needed but won't hurt. A 25 turn 25K ohm pot connects between +/-4.7 VDC with its wiper feeding a "gain control resistor", whose other end is the offset output. The "Power Amp" test jumper on the Control PCB was removed and replaced with a connector having a 2K ohm resistor to partially isolate the driving op-amp (test header Pins 1 and 2, assuming pin 1 is on the left), and the offest is introduced to pin 4 (which is shorted to pin 2 on the PCB). With a 33K ohm gain control resistor, one turn of the pot changes the optical frequency by about 1 MHz, providing a range of more than +/-10 MHz. A positive offset reduces the optical frequency on the 5517 laser. (It would probably be the opposite for the 5501B.)

    There's no need to bother with this for the 5501A as it has a "Photodiode Offset" pot (R4) on the Lock Reference PCB, which essentially performs the same function. (It's the square pot in the corner.) Normally, the pot is adjusted to maximize the REF/split frequency, which automagically centers the lasing position on the split neon gain curve. But it has quite a wide range - at least +/-50 MHz, possibly much more.

    It would be a simple extension to lock two of these lasers together at 0 Hz with a PLL using the REF frequency of Laser 1 (L1F2-L1F1) as the reference input to a phase/frequency detector (a flip-flip and some simple logic), and the difference frequency between the lasers (L1F2-L2F1) as the "VCO" input. The phase/frequency detector output would be the offset error signal fed into the power amp. The locked state would then have L1F1 equal to L2F1.

    Even simpler and actually more flexible would be to use a monostable and RC filter to convert the L1F2-L2F1 difference frequency to a voltage, and compare that with a set-point value to generate the offset error signal. This would allow the difference frequency to be adjusted over a wide range with closed loop control, including the case where L1F1 equals L2F1 (or with trivial modification, where L1F2 equals L2F2).

    Both of these schemes are left as exercises for the student. :)

    There is still another annoyance. It is a slow variation in the difference frequency with a period of a few seconds. It looks like sort of a dance with the control loops of one or both lasers hunting back and forth a few hundred kHz to and much as +/-1 MHz. (The values for the difference frequencies above were averages.) The period is between 2 and 3 seconds and appears to remain relatively constant. When I first noticed this hunting behavior, I didn't know if was a peculiarity of the locking electronics or from some external influence like the DC power supplies, vibrations, drafts, or aliens attempting to communicate with Earth. :) There is a digital clock with a period of 2.56 seconds on the laser Control PCB but I didn't think it should be doing anything once the laser locks. And it wasn't something associated with the original Control PCBs in either laser since swapping them with Control PCBs from other lasers didn't noticeably change the behavior. Nor did swapping one of the HeNe laser power supplies on the off chance that the switching frequencies were interacting in some peculiar way.

    I also substituted a 5501B for Laser 2, so call it Laser 3. (The final resting place for the frequency difference was around 2.5 MHz for L3F1-L1F1.) The variation in difference frequency was still present, but its deviation seems to be much lower, perhaps +/-100 kHz. The other thing that changed was the power supply since I had to use a different one for the 5501B. So it was possible that power supply fluctuations, origin unknown, could be the cause. However, then I swapped in the power supply originally used for Laser 2 to power Laser 1 and that made no difference. Thus, not the power supply. I then substituted Laser 2 for Laser 1 and the large fluctuations returned. So, they must be either associated with the tube itself or the LCD switch, since everything else of relevance (Control PCB and HeNe laser power supply) had already been swapped with no effect on the frequency fluctuations. And note that the combination of Laser 1 and Laser 3 did have some of this, just not nearly as much with Laser 2. But, even though it isn't pretty, this could still very likely be considered normal since even the much greater variations of Laser 2 are below the allowable specifications for these lasers. However, I then swapped the tube from Laser 2 into another case which meant the HeNe laser power supply and Connector PCB were different. At first, it looked like the difference frequency variation was way down, below +/-50 kHz, but over time as both lasers reached equilibrium, it climbed back up to around +/-300 kHz. Possibly somewhat lower than before but nothing conclusive. The difference frequency had also shifted down by about 5 MHz (L3F1-L2F1 of 6 MHz). Swapping the control PCB made little difference, but swapping in the original beam sampler assembly restored the difference frequency to its previous value of 1.5 MHz but also restored the large fluctuations! So, then yet another beam sampler assembly was installed and now the difference freuquency is around 0.5 MHz, but the fluctuations are also way down at +/-50 kHz and seems to be staying that way. So, it may be that the beam sampler has the most impact on both of these issues. Seriously strange.....

    In frustration, I finally did some of the things I should have done originally - changing the digital clock speed, looking at some control PCB signals and - gasp! - actually reading the service manual in more detail! :) It's easy to select clock speed on 5517B/C/D lasers - there is a convenient jumper for "Normal" and "High". Switching to High immediately caused the hunting to be much faster. Parallelling some key resistors to change the oscillator speed also had a similar effect. So, the hunting was related to the digital circuitry but how? Checking some key test points immediately revealed that my assumptions were totally wrong and that the cause is a direct result of the HP/Agilent implementation of the feedback loop using the LCD switch to alternately select each of the two polarized modes, rather than incorporating a polarizing beam sampler with separate photodiodes. Before realizing the cause, I had assumed that the LCD switch was alternating polarization states at 50 Hz. It's not. The two states are (1) when there is no drive to the LCD and both sides are at the same potential (passive state, polarization rotated 90 degrees) and (2) when the LCD is driven by a 50 Hz squarewave of opposite polarity on each side (active state, polarization unchanged). Apparently, the response of the LCD is slow enough that the active state is essentially DC - it doesn't see the 100 Hz ripple. (At least that's the theory. Given that the behavior of these lasers in terms of exact locked optical frequency and slow speed oscillation in optical frequency is affected by the specific beam sampler with its LCD that is installed, I wonder if there is enough variation in the residual response to be the cause.) The LCD switches from the passive to the active state with a period of 2.56 seconds! Just before each state change, the appropriate sample-and-hold is latched with the photodiode voltage corresponding to that state. This happens every 1.28 seconds. So the hunting is a direct result of this digital artifact in an otherwise analog system. It should be possible to eliminate or greatly reduce the effect. A few of the possibilities are to simply run the system at a higher speed once it's locked, to improve the sample-and-hold circuits (e.g., bigger caps!), or to replace the LCD and S&H circuits entirely with a normal polarizing beam sampler and dual photodiodes.

    At least one mystery remains and it has to do with the REF (or split) frequency behavior after locking. Starting at that time there is a fairly long period of an hour or more where the REF frequency oscillates by 1 or 2 percent up and down (period of several minutes), with a smaller oscillation of a few tenths of a percent period of 20 or 30 seconds). For example, on Laser 1, it starts between 2.279 MHz and 2.308 MHz (change of about 1.25 percent) and the deviation of the faster oscillation is 0.002 MHz (change of about 0.09 percent). The deviation of these oscillations gradually declines until it becomes small or non-existent, or the periods become very long so the changes are not easily seen. The mystery is why there is no obvious corresponding oscillation of the difference frequency between two lasers! One might expect that the origin of the REF frequency oscillations is the optical frequency varying about its nominal value causing the REF frequency to change based on the lasing position on the split gain curve. But while the difference frequency drifts during warmup, there is no obbious correspondance, by eye at least, with the REF frequency variations.

    So, both of these will probably remain unresolved for now.

    See the section: HP/Agilent Laser Wavelength/Optical Frequency for measurements of other lasers and additional comments.

    I then ran a pair of these lasers for many hours to get an idea of the longer term stability. In all cases they are Lasers 2 and 3 from the above link, a healthy 5517B and 5501B. The first set of data is for both from a cold start:

      Lasers      Balanced
      Time on     Frequency
      (hh:mm)    Difference
     -----------------------
        0:00     -8.50 MHz*
        0:15     -6.75 MHz
        0:30     -6.65 MHz*
        0:45     -6.55 MHz
        1:00     -3.95 MHz
        1:15     -2.55 MHz
        1:30     -2.05 MHz
        1:45     -1.75 MHz
        2:00     -1.75 MHz
        2:15     -1.69 MHz
        2:30     -1.67 MHz
        2:45     -1.66 MHz
        3:00     -1.61 MHz
        3:15     -1.57 MHz
        3:30     -1.55 MHz
        3:45     -1.57 MHz
    
        4:00     -1.45 MHz
        5:00     -1.44 MHz
        6:00     -1.32 MHz
        7:00     -1.44 MHz*
        8:00     -1.45 MHz
    

    The "Balanced Frequency Difference" is the mean optical frequency for Laser 2 [(L2F1+L2F2)/2] minus the mean optical frequency for Laser 3 [(L3F1+L3F2)/2]. * Denotes interpolated values, not measured. Read: Guessed, because I missed recording the value for that time slot! :)

    So, approximately 2 hours was required to get within 0.5 MHz of the final value. However, these are the average of several 10 second measurements and there is an uncertainty of several hundred kHz for each one. There is high frequency jitter at the switching frequencies of the HeNe laser power supplies (40 kHz tyical), as well as at the PWM frequency of the 5501B controller (50 kHz typical), and slow hunting back and forth from the control loops of both lasers (a few seconds or more).

    Next, I turned off Laser 2 for 2 hours to let it return to room temperature without touching Laser 3, and then started Laser 2 up again:

      Laser 2     Balanced
      Time on     Frequency
      (hh:mm)    Difference
     -----------------------
        0:00     -8.15 MHz
        0:15     -6.60 MHz*
        0:30     -5.05 MHz
        0:45     -3.52 MHz
        1.00     -2.48 MHz
        1:15     -1.84 MHz
        1:30     -1.61 MHz
        1:45     -1.44 MHz
        2:00     -1.35 MHz
        2:15     -1.20 MHz
        2:30     -1.21 MHz
        2:45     -1.26 MHz*
        3:00     -1.32 MHz
        3:15     -1.39 MHz
        3:30     -1.39 MHz
        3:45     -1.39 MHz
    

    Finally, I turned Laser 3 off and let Laser 2 run all night:

      Laser 3    Balanced
      Time on    Frequency
      (hh:mm)    Difference
     -----------------------
        0:00     -3.25 MHz
        0:05     -1.45 MHz
        0:15     -1.65 MHz
        0:30     -1.55 MHz
        0:45     -1.20 MHz
        1:00     -0.85 MHz
        1:15     -0.97 MHz*
        1:30     -1.10 MHz
        1:45     -1.15 MHz*
        2:00     -1.20 MHz*
        2:15     -1.26 MHz
        2:30     -1.31 MHz*
        2:45     -1.37 MHz
        3:00     -1.46 MHz
        3:15     -1.52 MHz
        3:30     -1.51 MHz
        3:45     -1.46 MHz
    
        4:00     -1.36 MHz
        8:00     -1.56 MHz
       10:00     -1.95 MHz
       12:00     -1.85 MHz
       24:00     -2.02 MHz
       25:00     -2.05 MHz
       26:00     -1.75 MHz
       29:00     -1.69 MHz
       30:00     -1.94 MHz
       32:00     -1.68 MHz
       33:00     -1.79 MHz
       34:00     -1.79 MHz
       36:00     -1.76 MHz
    

    So, it would appear that most of the frequency drift with warmup is due to Laser 2, and Laser 3 has very little drift after the first few minutes and for several hours after that. I don't know if this is a characteristic of 5501Bs in general, or whether Laser 2 is simply particularly slow to reach equilibrium. But based on previous testing, other 5517s were not much better. The 5501B control scheme does differ in subtle ways from that of the 5517s, though not in anything fundamental. The cause of the increase in optical frequency difference after about 8 hours is not clear but it does seem to have settled in at the new location. It's unlikely to be anything related to the lasers themselves reaching equilibrium as that should have happened well before this time. The ambient temperature may be changing as I have little control over that in my, um, lab. :) The limit cycle seems to have a total deviation of +/-150 kHz or about 0.0003 ppm.

    Recall that these are averages over multiple 10 second intervals. My next objective was to reduce the short term variations, initially those due to the switching noise from the HeNe laser power supplies in the lasers. So, I fabricated adapters so that the tubes in both lasers could be powered from Spectra-Physics 248 exciters, which are AC line powered HeNe laser power supplies with linear regulators. The residual ripple in the tube current is below the scope noise floor and undetectable, compared to 3 percent p-p from the VMI power supplies HP used. Based on the previous test where I substituted an SP-248 in one of the lasers, I assumed this would clean it up. Well, not quite.

    The result was slightly less messy, but there was still serious frequency modulation with deviation in the MHz range, now seeming to be at a bit over 50 kHz. Since I was beating a 5517B with a 5501B, the last possibility for switching noise of this type was from the 5501B, which uses Pulse Width Modulation (PWM) to drive the heater rather than a linear power amplifier. It runs at around 50 kHz nominal, but that's not very precise as it is simply determined by an RC network.

    So, I swapped in another 5517B (Laser 1) and Voila! The beat is now a really nice sinusoid with only a hint of fuzz. This is what a heterodyne signal should look like! No wonder HP did away with the PWM driven heater in the 5517 lasers! Exactly how the ~50 kHz noise makes its way into the optical frequency is not at all obvious. It would seem to be too high a frequency to be a mechanical effect but the heater coil is bifilar-wound so there should only be a very small magnetic field from it. But perhaps, very small is still enough!

    As further insurance, I have modifed a pair of SP-248s to reduce the already essentially undetectable power line ripple by at least another order of magnitude. For details, see the section: Reducing SP-248 Current Ripple I also plan to add a protection circuit to the SP-248s that will shut the supply down if either the current exceeds 4 mA due to a fault or the current drops out. I don't want a power supply failure to fry a tube!

    Photo of Test Setup for HP/Agilent Laser Optical Frequency Comparison is really ugly but quite functional. The scope on the right shows the actual beat frequency of about 3 MHz between F1 of one 5517B facing left-right and F2 of the 5517B pointing toward the back of the photo, in this case with their covers off. The Variac is powering the two SP-248s sitting under it to reduce the AC input voltage until I add the addtional RC filter. (Although they work at normal line voltage, the voltage across the regulator pass treansistors is pushing their limits at the low current.) The Thorlabs DET-110 can just barely be made out in place of the HP interferometer. In front of that also sitting on the butcher block is a polarizer wheel that may be installed in either beam. And the HWP to select F1 or F2, and bounce mirror for alignment, are on the table in front of the butcher block. The HP-5508A is displaying "PR Error" because it isn't getting the signals it normally expects.

    Most of the high frequency FM modulation is gone but the slow variation persists with the frequency difference still varying by at least 300 kHz and sometimes much more over the course of a few seconds. This is very puzzling. I have substituted a healthy 5517A for each of the 5517Bs with no change and even put the AC power line to the entire setup on a Sola constant voltage transformer, again with no change. The control loop is not quite totally linear due to the 100 Hz sampling of the LCD polarization selector, but this should be a high enough frequency not to matter. But we do know that the specific LCD has at least some effect on this behavior so some interaction there is not out of the question. It sort of looks like a limit cycle in a chaotic system, but part of this is probably because both lasers are doing it at an independent rate. If not due to the control loop impementation, what else can it be besides the aliens attempting to contact us? :)

    Causes of Variation of the Optical Frequency

    These lasers were apparently designed to be good enough for the metrology applications, but are far from what would be considered to have low noise or a super narrow line-width. The following are only some of the possible causes resulting in changes to or frequency modulation of the optical frequency by as much as 1 MHz or even more, over various time scales. The following applies directly to HP/Agilent 5517 and 5501B lasers except as noted, but much of it also applies to other HeNe Metrology lasers:

    Not included here are normal variations in the optical frequency that are a result of the lasing process itself.



  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Scanning Fabry-Perot Interferometers

    Introduction

    While the interferometers described in the previous sections have many applications in diverse areas, the Scanning Fabry-Perot Interferometer (SFPI) is specifically designed to make measurements of the longitudinal (axial) mode structure of CW lasers. The SFPI rates it's own set of sections both due to its importance and because it is possible to construct a practical SFPI at low cost without the need for a granite slab or optical table for stability.

    The longitudinal mode structure of a laser is one of those concepts that is often explained but not so often demonstrated. There are a number of indirect ways of showing that it exists including monitoring the beat frequencies between modes and looking at the fringe patterns in a Michelson or other conventional interferometer. But one of the clever ways of actually being able to display the modes as they would appear in a textbook is to use an instrument called a "Scanning Fabry-Perot Interferometer" (SFPI). The term "Laser Spectrum Analyzer" (LSA) may also be used for these instruments, but an LSA should not be confused with an "Optical Spectrum Analyzer" (OSA), which is generally - but not always - based on a very different technology, the scanning monochromator. Most SFPIs accept a free-space laser beam. However, some - mostly designed for telecom applications - may use a fiber-optic connector for input - a fiber collimator inside the instrument generates a free-space beam for the actual SFPI.

    While an SFPI is conceptually simple and actually quite straightforward to construct (at least in principle), even a basic system can display detail in the longitudinal mode structure of a laser that represents about 1 part in 50,000,000 compared to the optical frequency of oscillation or wavelength of the laser. For a red HeNe laser, the "resolvance" of such an instrument would be on the order of 10 MHz (out of 474 THz) or 0.013 picometers (0.000013 nm, out of 633 nm)! It's all done with mirrors! :-)

    However, an SFPI can only show the spectrum of a laser's output over a limited range of wavelengths (determined by the SFPI's mirrors) modulo a much smaller value called the "Free Spectral Range" (FSR). The FSR is determined by the mirror spacing and is typically not more than a few hundred times the resolution. An SFPI cannot measure the absolute optical frequency or wavelength of a laser. That requires an instrument like an Optical Spectrum Analyzer (OSA) or optical wavemeter, or comparison (heterodyning) with a known reference laser. So, an SFPI is like a microscope that can display a very small region of a laser's spectrum at very high resolution. And depending on the specific application, the SFPI display may only make sense if there is already at least some idea of what to expect. :)

    An SFPI can be used to view the mode structure of lasers where the gain bandwidth is less than its FSR such as a HeNe laser (~1.6 GHz) or ion laser (~5 GHz). A typical SFPI display of a 5 mW HeNe laser is shown in SFPI Mode Display of Melles Griot 05-LHR-151. Two sets of 3 modes are visible due to the 1.75 GHz FSR of the SFPI. For the red (633 nm) HeNe laser, 1.75 GHz is sufficient to cover all lasing modes.

    However, an SFPI can usually be used to determine if a laser is Single Longitudinal Mode (SLM) regardless of its gain bandwidth since the chances of multiple modes being both stable in height and frequency, and falling on top of one-another (modulo the FSR) so that the display shows a single peak is very small. And if more than one SFPI is available with different FSRs, and the displayed frequency offset of a laser with multiple longitudinal modes is the same on both, then that's an indication - though no guarantee - that the distance between modes is less than the smaller FSR.

    A number of companies currently (as of 2010) offer SFPIs including Coherent, Thorlabs, and Toptika. For only a few thousand dollars, one of these instruments can be yours. Alternatively, it's possible to build something with very respectable performance for a couple bucks. Companies like Spectra-Physics, and TecOptics no longer manufacturer SFPIs but their instruments (as well as those that are still in current production) may show up surplus at very affordable prices. SFPIs may also be called "Laser Spectrum Analyzers". But they should not be confused with even more expensive "Optical Spectrum Analyzers" which are generally scanning monochromator-based instruments that do not even come close to the resolving power of a typical SFPI.

    Principles of Operation

    An SFPI uses the optical transmission characteristics of a specially designed Fabry-Perot (F-P) resonator as a very selective filter to scan across the optical spectrum of the laser. Any F-P resonator will have a transmission behavior that has peaks and valleys based on optical frequency (or wavelength). The peaks will be located where the distance between mirrors is an integer multiple of one half the laser wavelength. As the reflectivity of the mirrors approaches 100 percent, the peaks become increasingly narrow and the valleys increasingly flat and close to zero transmission. This characteristic looks like that of a "comb" filter which is very selective.

    An SFPI consists of a pair of mirrors with relatively high reflectivity (90% to 99.9% or more is typical) mounted in a rigid frame. In most SFPIs, the laser under test (LUT) is aimed into one end and a photosensor is mounted beyond the other end. Depending on the type of SFPI, the coarse spacing and alignment of the mirrors may be adjusted by micrometer screws, by some other means, or set precisely and fixed at the time of manufacture. The axial position of one of the mirrors can also be varied very slightly (order of a few half-wavelengths of the LUT) by a linear PieZo Transducer (PZT). (Other methods of moving the mirror can and have been used but the PZT is most popular.) By driving the PZT with a ramp waveform and watching the response of the photosensor on an oscilloscope, the longitudinal modes of the LUT can be displayed in real time. In essence, the comb response of the SFPI is used as a tunable filter (by the PZT) to analyze the fine detail of the optical spectrum of the LUT. As long as the FSR (c/2*L except under certain (but very useful) conditions, described below) of the SFPI is larger than the extent of the lasing mode structure of the LUT, the mode display will be unambiguous. Where this condition isn't satisfied, the mode display will wrap around and may be very confusing. For example, the common helium-neon (HeNe) laser has a gain bandwidth of about 1.5 GHz and longer HeNe laser tubes will generally operate with multiple longitudinal modes covering much of this range. Thus the FSR of an SFPI to be used with such a laser must be greater than 1.5 GHz, corresponding to a SFPI cavity length of less than about 100 mm (assuming c/2*L for a planar cavity), as in the photo, above. For Nd:YAG, the gain bandwidth is about 150 GHz, which results in a required SFPI cavity length of less than 1 mm! However, in practice, lasers don't necessarily lase over their entire gain bandwidth, especially if specific steps have been taken to assure single or dual mode operation (also called single or dual frequency operation). For those - which include many useful lasers - the requirement can be relaxed such that the FSR of the SFPI only needs to be larger than the width of the expected mode structure. And for a single mode laser, this would be only the width of the lasing line itself. Therefore, in these cases, a long cavity low FSR SFPI will result in the highest resolution.

    Commercial scanning Fabry-Perot interferometers usually cost thousands of dollars - or more! But it's possible to construct an SFPI that demonstrates the basic principles - and can be even quite useful - for next to nothing, and one that rivals commercial instruments for less than $100.

    The resolution ("resolvence") of a Fabry-Perot (FP) interferometer is determined by the wavelength, mirror reflectance, mirror spacing, and incidence angle of the input beam. For the following, we assume normal incidence (which will be satisfied in most practical situations).

    Consider a plane-plane mirror FP cavity with a mirror spacing (d) of 80 mm and reflectance (R) of 99 percent at a wavelength (λ) of 632.8 nm (red HeNe laser):

                  λ2 * (1-R)         4*10-13 * 0.01
     Delta-λ = --------------- = --------------------- =
                2*d*π*sqrt(R)     0.16 * 3.14 * 0.995
    
    
      ~8*10-15 m = 0.000008 nm or about 6 MHz.
    
    (A wavelength of 633 nm corresponds to an optical frequency of 474 THz, so 0.000008/633*474 THz is approximately 6 MHz.)

    Another measure of the performance of an interferometer or laser cavity is the "finesse". This dimensionless quantity is the ratio of the FSR to the resolution. In essence, for the SFPI, finesse determines the how much fine detail is possible within one FSR. The reflectance finesse for a plane-plane cavity is equal to π*sqrt(R)/(1-R) where R is the reflectance of each mirror (which are assumed to be equal). For R near 1 as would be the case in a useful SFPI, this reduces to π/(1-R). For most discussions of finesse that follow, high R mirrors are assumed so this equation is valid and finesse is then inversely proportional to (1-R). (But if you're a stickler for precision, feel free to use the exact equation!) So, with a reflectivity of 99 percent for both mirrors, the theoretical maximum finesse will be roughly 300. If the FSR is 1.875 GHz as in the example above, the resolution will be approximately 6 MHz.

    However, to simplify setup and improve usability, practical SFPIs are often built with cavity configurations other than plane-plane known as "mode degenerate", which typically have a theoretical finesse that is lower by a factor of 2 or 4. Even for these, alignment and mode matching of the input beam to the SFPI cavity still impact finesse. And various aspects of the mirrors themselves such as coating losses, surface finish, dust, and contamination can further reduce the realizable finesse and limit resolution, especially for high performance instruments. Much more below.

    Transmission of Fabry-Perot Resonator versus Optical Frequency is a composite plot that shows how finesse affects F-P behavior. The Transmission of Fabry-Perot Resonator Slide Show has a separate plot for each value of finesse, which is less confusing when finesse is high. In an SFPI, the cavity spacing rather than optical frequency is varied, but the SFPI display of a single frequency (single longitudinal mode) laser for a given value of finesse will look similar to these plots. Where the laser has more than one frequency as is typical of common (unstabilized) HeNe lasers, the display will essentially be a summation of shifted and scaled versions of these plots (much more below). Low finesse F-P cavities (often even lower than a value of 1!), usually called etalons, may be used to select specific lasing lines due to their effect on intra-cavity gain. However, to be useful for an SFPI, a high finesse is desired to be able to resolve lasing lines that are close together. The finesse of a general purpose SFPI will typically range from 100 to 500. Higher values are possible but require better quality more expensive mirrors. As they say, it's all done with mirrors. :)

    A few other relevant equations can be found at the bottom of the Vintage Spectra-Physics Model 450 SFPI Brochure Page.

    Other factors will conspire to reduce the useful resolution of a practical SFPI. At modestly high mirror reflectivity (e.g., R=99%), these include alignment, input beam diameter, and input beam collimation. As R is pushed closer to 100%, the quality of the mirrors, their cleanliness, and internal losses become increasingly important. But for the example above, even if the actual finesse is worse by an order of magnitude compared to the theory, it will still be possible to easily resolve the individual modes of any common HeNe laser and probably even the nearly 2 meter long Spectra-Physics model 125 (177 cm resonator, mode spacing of 85 MHz). This is a factor of better than 1 part in 10,000,000 comparing resolution to optical frequency!

    However, note that while textbooks will tell you that the peaks should get through with little attenuation, this is probably not going to be true with practical high finesse SFPIs. (At least not those you're likely to see!) The amplitude of the peaks will depend critically on the quality of the mirrors and of course, on the alignment. For "laser quality" dielectric mirrors, I've gotten as high as 5 to 10 percent peak transmission for a high finesse SFPI using mirrors with a reflectivity of 99.8%. I'm sure this can be improved upon but even so, for a 1 mW laser, there is still more than enough optical power at the output of the SFPI to produce a nice display on most scopes using a 1:1 probe without a preamp.

    An on-line calculator of Fabry-Perot behavior can be found at Light Machinary Etalon Designer. However, the finesse is extremely sensitive to the entry for "SUrface Finish". Put in a value of "0.01 nm" to get textbook results. :)

    (From: A. E. Siegman (siegman@stanford.edu).)

    In evaluating the effect of losses in Fabry-Perot mirrors you really have to distinguish between internal losses (or loss-equivalent effects, like scattering) that are physically located "inside" the mirrors (i.e., inside the effective reflection plane of each end mirror), and external losses that are physically located "outside" the effective reflection plane, but still within the physical layer of the mirror.

    Losses that are outside the mirrors are effectively just additional external transfer losses in the system - they have the same effect as if they were separate from the FP, so that they don't affect the FP itself but just weaken the light before or after the FP.

    Losses inside the mirrors (aka "internal" losses) are more serious because they are exposed to the higher-intensity resonant fields inside the F-P and therefore can significantly affect the finesse and peak transmission of the FP.

    Just measuring the net reflectivity and net transmission of the mirror itself won't clearly distinguish between these internal and external losses. Also, how you'd describe a situation where the losses are distributed through a moderately thick mirror layer is something I've never thought through; doing this would require a slightly more sophisticated wave calculation of forward and backwave wave propagation inside the finite-thickness partially absorbing mirror layer itself.

    (Too bad I'm no longer actively teaching laser courses; this calculation would make a nice homework problem to torment -- sorry, educate -- students.)

    Mode Degenerate Fabry-Perot Interferometers

    A major disadvantage of the general spherical F-P cavity is that super precise alignment and control of the input beam size and collimation, along with an intracavity aperture, may be needed to suppress higher order transverse modes in the SFPI resonator. Though not present in a TEM00 laser, higher order modes are almost unavoidable in the SFPI cavity and may in fact dominate the display and render it completely useless. Even if the time consuming steps required to eliminate the higher order modes are taken, there will always be uncertainty as to what is actually being seen. The flat-flat cavity doesn't have this problem but suffers from disadvantages of its own, mainly in the need for a well collimated input and very precise mirror alignment to achieve high finesse and as a result, reflection of the input back directly back into the laser, which may destabilize many types of lasers.

    One way to eliminate the transverse mode problem is to use a cavity configuration called a Mode Degenerate Interferometer (MDI) in which the higher order transverse modes have the same frequency/wavelength as the TEM00 (longitudinal) modes and thus simply fall on top of them in the display. Even though each peak in the display representing a longitudinal mode of the input laser may actually be built up of contributions from multiple transverse modes excited in the resonator of the interferometer, the characteristics of the individual longitudinal mode components in each of these transverse mode are the same so the accuracy of the resulting display isn't affected. (This should not be confused with the very different situation of a laser having multiple transverse modes in its output where the frequencies, phases, amplitudes, and polarizations of the corresponding longitudinal modes in each transverse mode may differ.)

    Two practical arrangements that satisfy this condition are the (1) spherical cavity (d=2*r) and (2) confocal cavity (d=r). The confocal cavity has the larger finesse and is thus usually employed in SFPIs since the finesse is a measure of Q-factor with respect to the FSR or mode spacing, and thus higher finesse results in better resolution. A planar cavity (r of infinity) doesn't support higher order modes at all and its theoretical finesse is double that of the confocal cavity, but is generally a less desirable configuration due to alignment and other issues (see below).

    Note that the term "confocal" actually refers to any cavity where the focal points of the two mirrors are coincident. However, only the case where d=r is stable and thus generally useful for the MDI SFPI. This means that the two mirrors must have an identical Radius of Curvature (RoC) for optimal performance, and thus should be from the same manufacturer and production run if possible. (SFPIs constructed using mirrors with different RoCs may not be totally useless, but should be avoided for most applications.)

    The frequencies of the transverse modes of a symmetric cavity Fabry-Perot resonator are given by the following equation:

              c            1                              d
      fmn = ------ * [q + ---- * (1 + m + n) * cos-1(1 - ----)]
            2 * d          π                              r
    

    where:

    So, the equation can be rewritten as a base frequency plus an offset:

             c       c                      1               d
      fmn = --- + {------ * (1 + m + n) * [--- * cos-1(1 - ----)]}
             λ     2 * d                    π               r
    

    Thus the first term is simply the optical frequency of the laser while the second term consists of three parts: the longitudinal mode spacing or FSR, the integer mode numbers, and a correction factor (<1) that depends on the mirror RoC and spacing.

    The interferometer will be mode degenerate when there are TEM00 modes that have the same frequency/wavelength as some of the transverse modes. The requirement for this to be satisfied is for the inverse cosine term in the equation above to be equal to π divided by an integer, l. Then there will be "l" types of modes with one type - where (1+m+n) is equal to 1, modulo(l) - having the same frequencies/wavelengths as some TEM00 modes. When (1+m+n) is not equal to 1, modulo(l), that mode will fall in between the TEM00 modes in locations depending on (m+n)/l, modulo(l). So, the SFPI display will be similar to that of a non-MDI setup where only the TEM00 modes are excited except that the FSR will be reduced by a factor of l. But the resonances will actually be mostly for higher order transverse modes (of the interferometer) unless the alignment of the input beam is near perfect - and that usually doesn't happen by accident. (More on this below.) And for the user, good performance is achieved with non-critical alignment.

    For the following, "d" is the mirror spacing and "r" is the RoC of the mirrors as above:

    A diagram comparing the most common configurations is shown in Three Scanning Fabry-Perot Cavities with Equal FSR. The mirror reflectance is assumed to be high enough that the finesse approximation is valid. But other factors will reduce the realizable finesse.

    For the confocal cavity, half of the transverse modes are not mode degenerate when an on-axis input beam is used as there are two types of modes depending on whether the quantity (1+m+n) is even or odd:

    This seems a bit strange that the TEM00 modes (m+n=0) have non-integer mode numbers but the equation has been confirmed from at least two different sources.

    As noted, with two sets of peaks, the FSR is effectively cut in half to c/(4*d). Rearranging the equation above with the new FSR of c/(4*d) out in front, one sees that the various transverse modes (those that differ in m+n) result in a frequency difference of c/(4*d). However, integer differences in q corresponding to the longitudinal modes, still have an FSR of c/(2*d). Where a paraxial beam (one parallel to the optical axis) enters the confocal cavity off-center, the beam path repeats itself after two traversals of the cavity (in a zigzag pattern) and the FSR is easily seen to be c/(4*d) rather than c/(2*d). However, if the beam is very well aligned and centered, the FSR will be c/(2*d) since only some symmetric modes will be excited. However, the finesse is still the same (with respect to the 4 pass round trip cavity).

    When adjusting the mirror distance of a confocal cavity SFPI to be precisely confocal as it needs to be, there will be many positions where the SFPI may appear to work but which aren't quite confocal. This is especially true of short confocal cavities - the type most commonly found in commercial instruments. Depending on the specific distance, non-degenerate higher order modes will result in ghost peaks and/or a variation in the amplitude of the lasing modes depending on their position on the voltage ramp drive signal. The amplitude will also be lower overall. However, when the correct distance is approached, all of these ghosts will collapse into the desired high amplitude display. Don't be fooled! Thus it's best to know or determine the exact RoC for the mirrors before installing them in the SFPI so the initial distance can be set reasonably precisely. However, those other bogus resonances have been exploited to investigate a variable FSR mode degenerate SFPI - it's a feature, not a bug! :) See the next section.

    Planar mirrors may also be used since a true flat-flat cavity does not support any stable higher order modes, degenerate or otherwise, but it is the most difficult to align. And, although the theoretical finesse is double that of the confocal cavity, the realizable finesse is usually lower and is also much more dependant on the alignment than with the confocal or with other non-planar configurations. Also, with optimal alignment, the incident beam is reflected directly back into the laser which may result in instability for many types of lasers. So, it's often always necessary to use an optical isolator of some type (Faraday or polarizing beam-splitter with Quarter-Wave Plate (QWP), or at least an optical filter to reduce the intensity of the back-reflected beam (by the square of the transmission coefficient). However, where the distance between the mirrors of the SFPI is adjustable as in some general purpose instruments like the TecOptics FPI-25 or the (likely) custom Burleigh Triple-Pass Scanning Fabry-Perot Interferometer described below, there is no choice. Both of these enable the distance between the mirrors to be varied from almost touching (for an FSR of 100 GHz or more) to 15 cm or more (for an FSR of of 1 GHz or less). (Intracavity etalons also usually use planar mirrors but the finesse of these does not generally need to be very high so the alignment is not nearly as critical.)

    Some useful things to keep in mind:

    Selectable FSR Mode Degenerate Fabry-Perot Interferometers

    While the confocal and spherical MDI configurations are the best known and the confocal SFPI is the most widely used, it's possible to make use of symmetric cavities having values of d/r other than 1 or 2 and they may be useful for certain applications. Check out the short paper: K. Kernera, S. M. Rochestera, V. V. Yashchuka, and D. Budkera, "Variable Free Spectral Range Spherical Mirror Fabry-Perot Interferometer". However, the term "variable FSR" is not strictly accurate: Unlike a plane-plane cavity, the FSRs here are restricted to specific values determined by the RoC of the mirrors. Thus "selectable FSR" is a more accurate description. :)

    The equation that must be satisfied for resonance in a cavity with spherical mirrors having identical RoCs is:

          d             k*π
         --- = 1 - cos(-----)
          r              N
    

    Where:

    (My apologies for changing the names of variables. I've maintained my own if possible. But "k" replaces "l" both because l may be similar to the number "1" in many type fonts like Courier, and so as not to be mistaken for l in the MDI equation, above, which now becomes "N". Are you confused yet?)

    The FSR will be equal to c/(2*N*d) and the amplitude of each peak will scale as 1/N. Plugging in N=2 and k=1 results in the confocal cavity; N=1 and k=1 is the normal spherical cavity. The number of spots on each mirror (if not perfectly aligned on-axis) will be equal to N.

    Here are four examples (including the true spherical and confocal):

    The following table lists all valid cavity configurations for N from 1 to 10, as well as a planar cavity with a spacing of d=r (the RoC of the mirrors for the spherical cavities). The values of FSR, FWHM, and Finesse are all relative to those of the planar cavity.

                    <---- Relative ---->  <----- Example (5) ----->
                    (2)     (3)    (4)      d     FSR    FWHM  Fin-
      N  k   d/r    FSR    FWHM  Finesse  (cm)   (GHz)   (MHz) esse  Notes (1)
    -------------------------------------------------------------------------------
      1  0  0.000  1.000   1.000  1.000   3.331  4.500   15.00  300  Planar 1-0
    -------------------------------------------------------------------------------
      1  1  2.000  0.500   0.500  1.000   6.662  2.250    7.50  300  Spherical 1-1
      2  1  1.000  0.500   1.000  0.500   3.331  2.250   15.00  150  Confocal 2-1
      3  1  0.500  0.667   2.000  0.333   4.995  3.000   30.00  100  Spherical 3-1
      3  2  1.500  0.222   0.667  0.333   1.665  1.000   10.00  100
      4  1  0.294  0.854   3.414  0.250   0.975  3.841   51.21   75
      4  3  1.707  0.146   0.586  0.250   5.685  0.659    8.79   75  Spherical 4-3
      5  1  0.191  1.047   5.236  0.200   0.636  4.712   78.54   60
      5  2  0.691  0.289   1.447  0.200   2.301  1.302   21.71   60
      5  3  1.309  0.153   0.765  0.200   4.395  0.688   78.54   60
      5  4  1.809  0.111   0.553  0.200   6.024  0.498    8.29   60
      6  1  0.134  1.244   7.474  0.167   0.446  5.598  111.96   50
      6  5  1.866  0.089   0.536  0.167   6.214  0.402    8.03   50
      7  1  0.099  1.443  10.098  0.143   0.330  6.491  151.47   43
      7  2  0.377  0.379   2.656  0.143   1.254  1.707   39.84   43
      7  3  0.777  0.184   1.286  0.143   2.589  0.827   19.29   43
      7  4  1.223  0.117   0.818  0.143   4.071  0.526   12.27   43
      7  5  1.623  0.088   0.616  0.143   5.406  0.396    9.24   43
      7  6  1.901  0.075   0.526  0.143   6.330  0.338    7.89   43
      8  1  0.076  1.642  13.137  0.125   0.253  7.390  197.06   38
      8  3  0.617  0.202   1.620  0.125   2.056  0.911   24.30   38
      8  5  1.383  0.090   0.723  0.125   4.604  0.407   10.85   38
      8  7  1.924  0.065   0.520  0.125   6.407  0.292    7.80   38
      9  1  0.060  1.842  16.582  0.111   0.201  8.291  248.73   33
      9  2  0.234  0.475   4.274  0.111   0.779  2.137   64.11   33
      9  4  0.826  0.134   1.210  0.111   2.752  0.605   18.15   33
      9  5  1.174  0.095   0.852  0.111   3.908  0.426   12.78   33
      9  7  1.766  0.063   0.566  0.111   5.881  0.283    8.49   33
      9  8  1.940  0.057   0.516  0.111   6.459  0.258    7.73   33
     10  1  0.049  2.043  20.432  0.100   0.163  9.194  306.48   30  Max FSR/FWHM
     10  3  0.412  0.243   2.426  0.100   1.373  1.092   36.39   30
     10  7  1.588  0.063   0.630  0.100   5.287  0.283    9.45   30
     10  9  1.951  0.051   0.513  0.100   6.497  0.231    7.69   30  Min FSR
    

    Notes:

    1. These are the named cavities, and the two described in the text, above. The planar cavity isn't strictly part of this series but is included for completeness, and as a baseline for comparison. Its mirror RoCs would be infinite but its mirror spacing (d) is equal to the RoC (r) used for all the spherical cavities. Configurations for N above 10 would have larger maximum values for the FSR and FWHM (k=1), and smaller minimum values for the FSR (k=N-1), with the FWHM approaching that of the normal spherical cavity (1-1).

    2. FSR takes into account the actual value of d as the cavity spacing is varied. The values shown are relative to that of the planar cavity (with mirror spacing equal to RoC=r of the mirrors used in the spherical cavities). Then, FSRr(N)*d/r=1/N, or FSRr(N)=r/(d*N).

    3. FWHM is the width of the spectral peaks relative to that of the planar cavity based on the mirror reflectivity (R) and mirror spacing (d).

    4. Finesse is with respect to the effective FSR, relative to that of the planar cavity and scales as 1/N.

    5. This example shows the resulting mirror spacing (d), FSR, FWHM of the spectral peaks, and finesse for each spherical cavity with mirror RoC=r compared to a planar cavity with a mirror spacing of d=r=3.331 cm for an FSR of 4.5 GHz with a finesse of 300 (mirror reflectivity of approximately 99 percent).

    So, most of these are actually the resonances that one avoids when setting the spacing of a normal confocal SFPI! :) Since the spectral width of each peak is determined only by the mirror reflectivity (R) and mirror spacing (d), and r (RoC) would be fixed for a selectable FSR spherical SFPI like this, d (and thus FSR and width) can vary quite dramatically for any given value of N. For example, spherical cavities 4-1 and 4-3, these differ by a factor of almost 6! But the width is never narrower than that of the normal spherical cavity (1-1), so the resolution can be no better as well. If a very small FSR is selected, the finesse is also lower. For example, when N=10, the finesse within 1 FSR is only 1/10th of what it would be for the normal spherical cavity or 1/5th that of the normal confocal cavity! The main benefit seems to be the capability of being able to select a large FSR using mirrors that would result in a much smaller confocal FSR. In the example above, it would be possible to have an FSR of over four times the confocal FSR with cavity 10-1 (9.194 GHz), but the finesse then would only be 1/5th as large (30). Though cavity 4-1 with almost double the confocal FSR, would be less than 1/3rd as long and still 1/2 the finesse (75). There could be other special situations where a selectable FSR and/or non-confocal SFPI might be useful, for example if the RoC of the only mirrors available would result in a confocal cavity too long to be practical. Note that commercial confocal SFPI heads cannot readily be used as their adjustment range is quite limited. Thus, either a custom-built cavity or a one of the totally general and very expensive commercial SFPIs normally fitted with planar mirrors would be required. So it's not clear when being able to actually select among the unusual spacings would be generally desirable enough to warrant the added mechanical complexity, effort, and cost involved in setting and fine tuning them beyond the intellectual challenge of an academic exercise. ;-)

    Hemispherical versions of all of the above - half the length with a planar mirror substituted for one of the curved mirrors - should also be valid, but with a further halving of the finesse if using mirrors with the same reflectivity, so I couldn't resist building one. :-) See the section: Sam's Selectable FSR Hemispherical and Spherical Mode Degenerate Scanning Fabry-Perot Interferometers.

    But note that one mirror really does need to be planar - close is probably not useful since the cavity will no longer be mode degenerate. Even mating a 1 meter RoC (rather than planar) front mirror with a 4.3 cm RoC rear mirror (a ratio of over 20:1) will result in compromised performance with ghost or double peaks showing up at times even for the most basic (almost) hemispherical confocal mirror spacing. Perhaps with a 5 or 10 meter RoC, the degradation would be small enough to be acceptable but planar is still best.

    Other approaches have been investigated using what might be termed "semi-mode degenerate" cavities, where the first few undesirable transverse modes merge with the TEM00 mode, but others (which have much lower amplitudes) may not. Some examples can be found in U.S. Patent #5,418,641: Fabry-Perot Optical Resonant Cavity Systems. granted to Newport Corporation in 1995. This patent (which is definitely a great cure for insomnia!) covers just about every imaginable cavity configuration (including all those above). But with one exception, I am not aware of any commercial SFPIs employing a cavity other than the planar or confocal, even from Newport (who now has none at all), and none other than planar where the FSR can be changed with a knob. That one exception is the very old Tropel 2440 using the hemispherical confocal cavity. See the section: Tropel 2440 Scanning Fabry-Perot Interferometer.

    Further investigation of these special cases is left as an exercise for the determined student. :)

    More Information on SFPI Theory and Practice

    In addition to what is present in the sections below, check out the following links:

    SFPI Types and Capabilities

    Any electrically adjustable fabry-perot resonator can be used in a variety of ways including as a tunable filter or etalon. However, our interest here is mostly with respect to their application as a measurement instrument: The SFPI used as a laser spectrum analyzer. Nowadays, there are even F-P devices based on MEMS (Micro Electro-Mechanical Systems) technology. A complete device may be extremely small, but as such, they will be restricted to larger FSRs. Their flexibility will also be limited since all adjustments *must* be done electrically. Excluding these presently more exotic techniques, there are three common types of SFPIs that one is likely to find in an optics lab. These have been around and substantially unchanged for several decades:

    1. General purpose Fabry-Perot interferometers.

    2. Reconfigurable confocal cavity Fabry-Perot interferometers.

    3. Fixed configuration confocal cavity Fabry-Perot interferometers.

    In all cases, a separate ramp generator is used to drive the PZT and an oscilloscope is used for the display. Commercial units will typically include the ramp generator since the PZTs in most SFPIs require several hundred volts of drive to cover a few FSRs. The 'scope must be provided by the user. The ramp generators can be of various levels of sophistication, some including fine tuning of alignment for the general purposes SFPIs with triple-PZTs. Some drivers include temperature controllers for those inteferometers with heaters to maintain constant (average) cavity length. A few SFPIs like those from Thorlabs have a requirement of only 5 V/FSR. Home-built SFPIs can be of any of the three types, but (2) is easiest to construct if appropriate mirrors are available. When using a beeper element as the PZT, a function generator (with at most a simple op-amp circuit to boost the voltage) can serve as the ramp generator since their drive requirements are modest.

    Subsequent sections cover home-built as well as common commercial SFPIs.

    Using an SFPI to test for SLM

    Testing a laser to confirm that it is running Single Longitudinal Mode (SLM) - sometimes referred to as single frequency - is one of the common uses for an SFPI. For the following, the use of a somewhat generic commercial confocal cavity SFPI is assumed. However, the same approach applies to a home-built SFPI except that the controls and settings will differ.

    Where the laser it putting out more than 10 or 20 mW, attenuation must be added to limit power to the SFPI. Since testing a laser for SLM is usually done up to full power, only a small portion of the original beam must be sampled for the SFPI. This may be done with a combination of one or more beam samplers (beam-splitters) and optical filters. For this purpose, an uncoated glass plate such as a microscope slide will suffice as a beam sampler. Each reflection will cut the power by around a factor of 10. An optical filter can then be used instead of another beam sampler to attenuate the beam further. The use of at least one beam-splitter will enable the original optical setup to remain essentially undisturbed.

    CAUTION: Though the SFPI specifications may allow for more, limiting the input to 10 or 20 mW will minimize the chance of any damage to the mirrors. 2 or 3 mW would be optimal.

    Note that the output from the laser is assumed to be more or less collimated. Even if the SFPI head has an adjustable input lens, it probably doesn't have enough range to accommodate a widely diverging beam. Thus the beam should be collimated at the laser.

    The basic connections for a typical setup are:

    The SFPI controller should be set approximately as follows:

    The scope should be set approximately as follows:

    Now comes the fun part. ;-)

    1. Set up the SFPI head so the center of its aperture is at the same height as the laser output. Power up the laser.

    2. Use a beam-splitter like a microscope slide in a "third hand" to pick off a small portion of the beam. If the power is still over 20 mW, add either a second beam-splitter or optical filter to reduce it to a few mW.

    3. The SFPI should be aligned so that the beam hits the center of the aperture and reflects back almost but not quite into the source.

    4. With the controller and scope powered, there should be some sort of display on the screen. Adjust the the SFPI alignment to maximize the height of the peaks and minimize their width. Adjust the SFPI controls for the best scope display.

    5. The initial goal is to display 2 or 3 FSRs on the screen. If the laser is is SLM, there will then be only 2 or 3 peaks.

    The initial goal is to have the SFPI scanning over 2 or 3 Free Spectral Ranges (FSRs), which will result in the pattern to repeating 2 or 3 times. Adjusting the variable dispersion or expansion knob will change this. Adjusting the Centering control will move the peaks left and right. controls the ramp repetition rate (keep fully CCW). The dispersion or expansion switch can be used to zoom in on what's located near the center of the screen.

    The peaks from a well behaved laser should be fairly stable in amplitude and position. A small amount of jitter is to be expected, due to vibrations and the SFPI itself. However, multiple sets of peaks (usually of greatly differing amplitude) and/or a noisy pattern would be an indication that the laser is running multi-mode. A mode hop would be a sudden shift in the position of the peaks. (There will always be a slow drift due to thermal changes, but this would be an instantaneous change.)

    Problems with Confocal Cavity Scanning Fabry-Perot Interferometers

    (This section deals with the interferometer itself. Electronic problems with ramp generators are not common, and when they do occur, relatively easy to diagnose and repair.)

    Commercial SFPI heads are generally quite reliable but problems can arise from misadjustment, abuse, or even poor environmental conditions over time. Symptoms can range from "no output from photodetector" to "rattles when shaken". :( :) The following assumes the system is set up correctly, the driver and scope are functional, and their controls are set properly. Refer to operation manual(s). :)

    Constructing Inexpensive Scanning Fabry-Perot Interferometers

    I have used commercial general purpose Scanning Fabry-Perot Interferometers (SFPIs). For example, the TecOptics FPI-25 is an example of a very solidly constructed precision instruments with adjustments for just about everything. However, being so general, in some sense it is not optimal for anything! (See the section: The TecOptics FPI-25 Scanning Fabry-Perot Interferometer.) There are somewhat less flexible but easier to use SFPIs from companies like Coherent, Thorlabs, and Toptica Photonics. These provide the following:

    They also have a price tag to match - those from Thorlabs start at around only $2,400 not including the driver box (around $800), others are even more expensive. (Several are covered in subsequent sections, below.) You don't want to ask about the prices of the very flexible SFPIs. :)

    My challenge was to prove that I could construct an SFPI that would at least demonstrate the basic principles and possibly even be useful. The results are described in this and the following sections. All of mine cost me absolutely nothing (except time) but that wouldn't sound as credible as $1.00 or $2.00 or $3.00. :) Yet many aspects of their performance are comparable to the multi-$k commercial SFPIs. And in some cases, far superior.

    The heart of the SFPI is its two mirrors. For longer visible wavelengths (i.e., 600 to 700 nm), the mirrors can be the OCs salvaged from a pair of dead red (632.8 nm) HeNe laser tubes. For other wavelength ranges, mirrors from green (532 nm) DPSS lasers, green or blue ion lasers, HeCd, and other lasers may be useful. While some of these mirrors may have a relatively broad band reflectance, this cannot be counted on. More often than not, the reflectance falls off dramatically beyond 10 or 20 nm from the spec'd wavelength. And, obtaining proper single mode performance of the SFPI without great pain may require that mirrors with specific reflectances and RoCs not normally found in common lasers be used. Of course (gasp!), suitable mirrors can be also be purchased. For common wavelengths, they may be available from companies like CASIX at very reasonable prices. But in general, obtaining the optimum mirror might require ordering a set of custom mirrors. It's not the ground and polished mirror glass itself that will cost a lot of money. They can often be standard concave lenses with suitable curvature available from places like Edmund Industrial Optics or Melles Griot. It's the custom coating, which can easily exceed $1,000, and it doesn't matter that much whether the lot is 2 mirrors or 200 mirrors as what counts is the coating machine time. So, find 99 friends who want to build the same SFPI and the per-mirror cost could still be quite low. :)

    For a short RoC confocal cavity SFPI (more below), the only readily available mirrors I know of are either the misfits I'm using in my $3 SFPI for HeNe lasers (also more below) or mirrors from flowing dye lasers. Unfortunately, the latter tend to have ground, but not polished, outer surfaces. However, since the outer surfaces aren't critical, simply using some index-matching fluid, optical cement, or even common oil or water, between the ground surface and a piece of glass like a microscope slide or cover slip is know to work well enough. It's the coated mirror surface that's important.

    As far as attempting to coat your own mirrors - in two words: Forget it. :) Unless you have access to a dielectric mirror coating machine and know how to use it (and are permitted to use it!), there is no way to produce coatings that will do anything more than provide a hint of what's possible. Metal (aluminum, silver, gold) coated mirrors do not work well since their maximum reflection coefficient is around 94 to 97 percent and they have high absorption losses. Thus finesse will be poor and the photodetector signal will be very small. And except for gold, the coatings degrade (tarnish, oxidize) in air without a protective layer, with silver being the worst. For good quality dielectric mirrors, absorption losses only become a major concern for very high reflectivities (perhaps above 99.9%) and modern coatings do not degrade significantly under normal conditions as long as they are not subject to physical abuse or improper cleaning techniques.

    When specifying the mirror RoC (r) for a particular application, it usually makes sense to base it on the maximum frequency range over which there will be action, not simply on the gain bandwidth of the laser(s) being observed. Not only will this result in the best resolution, but doing otherwise may simply not be practical. For common gas lasers like the HeNe and argon ion which have longitudinal modes filling most of their gain bandwidth, (1.5 GHz and 5 GHz, respectively) there's no choice if the display is to be unambiguous. But where the modes have already been limited by an etalon or some other means, only the range of the modes that are present need to fit into the SFPI's FSR. For example:

    I have a variety of inexpensive mirrors suitable for 633 nm SFPIs available on Sam's Classified Page. These include short cavity mirrors that result in a truly spectacular finesse at 633 nm. (FSR of 1.7 GHz, finesse may exceed 500!)

    The other major components of the SFPI include the PieZo Transducer (PZT) to move one of the mirrors a few microns, and a photodiode to monitor the output beam.

    High quality PZTs can be purchased at exorbitant cost. But the beeper from a digital watch or similar device will work nearly as well and has the advantage that it runs on much lower voltage than most other types. You never did like that alarm anyhow. :) But there is no need to discombobulate your watch as these piezo elements can be purchased from electronics distributors or surplus places for about $1.00. :) While they aren't quite as linear or have as good a frequency response as the high priced units, these deficiencies don't really matter much for an SFPI. And since they will move several microns on only 50 V, a high voltage amplifier isn't needed as with many commercial SFPIs. The 20 or 30 V p-p output of a typical function generator or simple op-amp circuit is quite adequate.

    The photodiode can be almost anything since it needs neither a large area or high frequency response. The salvaged photodiode from a barcode scanner with a 10K ohm resistor load and 10:1 or 1:1 scope probe is often adequate. Where more sensitivity is needed as with very high-R mirrors or low power lasers, a trans-impedance amplifier with high gain can be added since frequency response isn't that critical. Almost any common op-amp will suffice, expecially if multiple stages having modest gain (e.g., 5 to 10) are used.

    Everything else is hardware. The structure and mirror mounts are easily home-built. However, one area where it may be hard to compete with commercial SFPIs is in minimizing the effects of temperature. They typically construct the main support as a cylinder or set of rods made from Invar, a low coefficient of thermal expansion alloy. Some designs further compensate for residual effects by balancing them against those of the PZT resulting a near zero net change in FSR with respect to temperature and/or may include a heater in a closed-loop temperature stablization system. Invar stock is available or can be salvaged from various dead lasers. Some people build SFPIs by mounting the back mirror and PZT in an Invar tube, positioning the front mirror using a 5-axis lab stage, and then gluing it in place permanently when the optimal mirror spacing and alignment has been determined. But glue tends to be too permanent for my taste. :) Constructing the SFPI using Invar rods is nearly as good. But simply enclosing a non-Invar based SFPI in an insulating box will go a long way in reducing temperature effects.

    And if the objective is to achieve a high finesse and maintain it, then enclosing the entire SFPI to prevent mirror contamination is essential, if not during operation at least for storage. While testing my high resolution SFPI, the finesse had been steadily declining over a few days which turned out to be dust collecting on the mirrors even though their coated surfaces were vertical. Any cleaning of high quality mirrors is to be avoided. Even when using the proper techniques for cleaning of laser mirrors, some permanent degradation of the dielectric coatings is virtually unavoidable. With many cleanings - or only one if the proper techniques are NOT followed - the damage will be enough to result in a noticeable decline in performance. (For information on laser mirror cleaning techniques, see the section: Cleaning of Laser Optics.)

    A triangle (or sawtooth) wave source (it can be a simple circuit constructed for this purpose or a general purpose function generator) and oscilloscope (preferably dual trace and/or with an X-Y display mode) will be required to view the scan but needn't be dedicated to the SFPI, so they don't count toward the cost!

    The next few sections include general descriptions and photos of several home-built SFPIs. Schematics for both a photodiode preamp and simple function generator are provided later in this chapter.

    (From: A. E. Siegman (siegman@stanford.edu).)

    When thinking about producing small and not too fast mechanical motions or pressures, consider also magnetic methods.

    After University Labs in Berkeley introduced the first really low-cost lasers in the early 1970s (priced at circa $300 each rather than the prevailing several thousand dollars and up), it also produced a really neat and equally inexpensive little scanning F-P interferometer with plastic end plates and the scanning mirror driven by what was in essence a miniature loudspeaker coil.

    One of the advantages of the magnetic versus piezoelectric approach is low voltage, higher current drive circuitry, perfectly adapted to IC or semiconductor electronics. Another advantage is wider range of motion.

    Sam's $1.00 Scanning Fabry-Perot Interferometer

    This is the first of several SFPIs I've constructed, differing mostly in the mirrors and their spacing. It uses curved mirrors but is not mode-degenerate, having been built before I knew about such things. :)

    The basic design is shown in Home-Built Scanning Fabry-Perot Interferometer 1. My prototype uses the OC mirrors from a couple of dead Aerotech 1 mW HeNe laser tubes. The PZT is the beeper from some sort of musical greeting card with a 4 mm hole drilled in the center. The photodiode is from a barcode scanner. The frame and mounts are a bit different than those shown in the diagram, above. They were made from the platter clamping plates from some ancient 5-1/4" harddrives, hex spacers, and miscellaneous scrap metal. The circular plates are nice because they have predrilled holes with 6-fold symmetry thus simplifying construction. See Photo of Sam's $1.00 Scanning Fabry-Perot Interferometer. (For the mirrors, /V denotes Concave, /P denotes Planar.) Here is a summary:

    The front mirror is removable so other reflectances or RoCs can be tried. The rear mirror is glued to the PZT. The hole was made by placing the PZT on a hard surface (e.g., an aluminum plate) and drilling through it slowly with modest pressure using a normal metal bit in a drill press. The piezo material is more of a compressed powder than a true ceramic so it's possible to grind it away (using the metal drill) with minimal chipping. Thin flexible wires were already attached but if they aren't, solder the top lead near the edge to leave room for the mirror and to minimize any change in elasticity of the top surface. Once soldered, Secure the wires mechanically with a drop of flexible adhesive like 5-minute Epoxy. Also note that the metallization tends to disappear with even modest heat or stress so solder quickly. Conductive paint or silver Epoxy can be used to touch up bare spots if needed but use as thin a layer as possible as it may increase stiffness and reduce response sensitivity in that area. For this reason, DO NOT coat the entire surface with adhesive of any type!

    To perform initial alignment, I used a yellow-orange HeNe laser thinking it would be easier since the mirrors are less reflective away from the 632.8 nm design wavelength. The scatter off of the mirror surfaces was used as the initial means of setting alignment, by minimizing the size of the line or blob formed by the multiple reflections. With a pair of concave mirrors, not only do they have to be aligned with respect to the input beam, they also have to be aligned with respect to each other. In other words, their optical axes must coincide which requires walking them until the scatter pattern is minimized. When misaligned, it will be a line or circle and no amount of adjustment of only one mirror may improve it. Once the initial alignment was done, the PZT could be driven and the output of the photodiode used to fine tune it. In retrospect, using the funny color HeNe laser wasn't necessary as enough red light gets through to be easily seen for alignment purposes. And the display of the modes of that multi-wavelength and multi-transverse mode laser was definitely strange.

    The preliminary results using a Melles Griot 05-LHR-911 HeNe laser were also confusing. This is a 2 mW laser using a tube with about 165 mm between mirrors, corresponding to a mode spacing of 883 MHz. The scope trace in Sam's SFPI Display of Melles Griot 05-LHR-911 HeNe Laser - Initial Attempt shows a jumbled mess due to many transverse modes being excited in the SFPI. The trace on the left should cover a span of approximately three FSRs of the SFPI - about 19.5 GHz. Three clumps that look about the same are clearly visible but the complexity isn't real. The trace on the right is an expanded region of the one on the left. A hint of the modes of the laser can be seen but only a hint. The 05-LHR-911 should have 2 or 3 longitudinal modes at most but the short cavity of the SFPI using long radius mirrors is resonating with multiple transverse modes.

    There is also some hysteresis in the PZT response. It's barely visible on the display as the pattern differs slightly on the positive and negative slopes of the triangle driving function. Using X-Y mode on the scope would show up the hysteresis more clearly. Reducing the sweep speed slightly virtually eliminates the hysteresis. (A 20 trace/second display has minimal hysteresis and is still quite usable. Of course, this wouldn't be an issue with a digital scope

    The overall linearity of the PZT is around 5 to 10 percent over a range of +/-20 V, corresponding to 5 or 6 FSRs of the SFPI. I've actually tested several PZTs (another one was from a digital clock for which the alarm was more of a nuisance than useful!). The response of one is compressed more toward the upper end of the voltage range; the other is slightly compressed at both ends. Within a single FSR, the linearity is probably better than 2 percent and a range of a single FSR provides all the information usually needed. For a system of this type where qualitative information is most important, perfect linearity, especially over multiple FSRs, really isn't a major issue in any case as long as it is known and doesn't change over time. A third PZT was quite linear but had a range of only around 1 FSR of the SFPI - probably due to the excessively thick layer of silver Epoxy I used to cover some bald spots on the piezo disk.

    To confirm that transverse modes were the cause of the complex display and to partially remedy the situation, I aligned the SFPI more carefully by adjusting the front mirror so that the 05-LHR-911 beam bounced directly back to the source with dancing interference patterns, then aligned the rear mirror for maximum amplitude of the displayed signal, and added an aperture about 0.3 mm in diameter (a pin hole in a piece of aluminum foil) inside the SFPI cavity. The aperture was mounted on a micropositioner but could be installed permanently so that doesn't blow my budget. :) The results are shown in Sam's SFPI Display of Melles Griot 05-LHR-911 HeNe Laser. The sequence of the six traces show the modes of the 05-LHR-911 cycling over time as they move under the HeNe gain curve. The horizontal scale is the same as in the jumbled mess trace, above, but the transverse modes have been almost entirely eliminated. The distance between similar peaks (2.2 boxes on the screen) is the FSR of the SFPI - about 6.5 GHz. The distance between longitudinal modes (0.3 boxes) is the 883 MHz FSR of the 05-LHR-911. The math even works. :) So, this represents success of sorts but alignment of everything is super critical and any vibrations - even the audio from a radio - create havoc with the display. There is also a quasi-periodic fluctuation in amplitude of all the displayed modes with no corresponding power fluctuations in the laser. I suspect this to be due to residual mode competition in the SFPI as the frequency of the modes changes relative to the SFPI cavity, possibly a side effect of the aperture.

    Sam's SFPI Display of a Melles Griot 05-LHR-151 HeNe Laser shows the result using the same setup for a longer laser with more closely spaced modes - 436 MHz compared to 833 MHz for the 05-LHR-911. With this higher power laser, there are still some non-TEM00 modes just visible in the display but they are fairly low level. Sam's SFPI Display of Vertically Polarized Modes of Melles Griot 05-LHR-151 HeNe Laser shows the effect of inserting a polarizing filter into the beam. Since adjacent modes tend to be of orthogonal polarization in randomly polarized HeNe lasers, every other mode on the display has disappeared.

    Finally, I tried a Spectra-Physics model 117A HeNe laser head, which when used with its mating controller is a frequency or intensity stabilized (single longitudinal mode) laser. I'm running it on an SP-248 so it's not stabilized but the modes are a bit interesting. The mode spacing is around 600 MHz which is consistent with a 2 to 3 mW HeNe laser. However, as the modes cycle, there isn't a smooth progression through the gain curve. It almost seems as though having exactly 2 modes is enhanced somehow and that it's very unlikely to see 1 or 3 modes. When 1 or 3 modes would be expected to pop up, they might appear very briefly, or be skipped entirely in favor of the 2 modes one of which is on the opposite side of the gain curve. The polarizations of the modes also appear to be of the "flipper" variety, changing suddenly rather than staying with a particular mode. I don't know if this behavior is by design. However, since orthogonally polarized modes are sensed by a pair of photodiodes in the laser head and used for stabilization, strong mode pairs could be beneficial.

    After determining experimentally that an aperture helped but didn't totally eliminate the transverse mode problem, a Post Doc in our lab wrote a simple Matlab program to calculate Hermite Gaussian transverse mode profiles given the mirror RoCs and the distance between mirrors. Plugging in the long radius SFPI cavity configuration revealed that the TEM00 and TEM10/01/11 modes have a high degree of overlap regardless of axial position. So, any aperture that suppresses them very effectively would also result in unacceptable attenuation of the TEM00 mode. So, on to plan B. :) I hope to have a compiled version of this program available in the near future as it appears to be quite useful for visualizing cavity modes in general.

    Here is a summary of the configurations I've tried so far on the $1.00 SFPI:

    Of these, the first is probably the best choice unless super high resolution is needed. All except the flat-flat required an aperture inside the SFPI cavity to suppress non-TEM00 (transverse) modes.

    Sam's $2.00 Scanning Fabry-Perot Interferometer

    Well, it wasn't actually $2.00. :) I found some small radius mirrors originally intended for a research project that is now defunct. These should work well in a confocal configuraion in the green region of the spectrum free of those annoying transverse modes!

    The mirrors were actually Melles Griot plano concave lenses custom coated (along with a batch of microchip laser crystals) for 1,540 nm. Now, it's perhaps a not so well known fact that a dielectric mirror coated for a wavelength of X nm will also perform reasonably well at a wavelength around X/3 nm (think of a stack of 3/4λ layers instead of 1/4λ layers). The actual reflectance function will depend on the design of the original mirror including the number of layers and uniformity of the layer thickness. The reflectance at the new wavelength will almost certainly be lower and the losses may be slightly higher. But with luck, these mirrors will be useful in a wavelength range centered around 513 nm (1,540/3).

    I had two types available: Those that were supposed to be 98 percent as OC mirrors and those that were supposed to be HR mirrors, both at 1,540 nm. Here are how they performed at the two green wavelengths of interest:

                             Reflectivity at       Reflectivity at
         Mirror Type       532 nm (Green DPSS)   543.5 nm (Green HeNe)
      -----------------------------------------------------------------
       OC (98%@1,540nm)          97.8%                    88%
       HR  (HR@1,540nm)          99.8%                    99%
    

    For 532 nm, neither is really ideal. The "OC" is a bit low - I would have preferred around 99% to achieve a higher finesse. However, 97.8% is still decent. The reflectance of the "HR" - which could be even higher than the measured 99.8% since the 0.02% transmission measurement was not very accurate - might be too high to get a decent signal but could result in a very high finesse. But at 543.5 nm, the "HR" mirror seems to be perfect.

    The only thing not wonderful about these mirrors is that the planar side isn't AR coated. (Since they were intended only for some tests, we saved money by not having AR coating!) But, if they are slightly tilted, hopefully, this won't be a major problem.

    There are also several radii to choose from. For the first version, I used the longest RoC which is a Melles Griot 01-LPK-01. This is a 10 mm diameter BK7 lens with a focal length of -20 mm which has a RoC of about 10.3 mm. (For BK7, the RoC of a plano-concave lens is -0.517 of the focal length.) This results in an FSR of about 7.8 GHz. Note that the FSR is c/(4*d) for the confocal cavity, one half that of the long radius or planar SFPI cavities. See the previous section. So, these will be good for all green HeNe lasers and longer cavity single mode green DPSS lasers like the C315M and C532, as well as that Far East disaster described in the section: Reconstruction of an 80 mW Green DPSSFD Laser. However, short cavity DPSS lasers including green laser pointers, the Uniphase uGreens, MCA based DPSS lasers, and possibly the Transverse TIM622 will require a shorter SFPI cavity. The other sets of mirrors go down to around a 5 mm RoC so another version may be built with a set of these.

    However, note that since the gain bandwidth of Nd:YAG and Nd:YVO4 is over 150 GHz and the SHG green conversion also doubles the frequency between modes, multimode solid state lasers may have frequencies which greatly exceed the FSR of these medium length SFPI cavities. Unambiguous display of their modes may require an SFPI with an FSR of more than 300 GHz - a cavity length of 0.25 mm for the confocal configuration! It's simply not impractical to grind and polish mirrors with very small RoCs. The limit is about 2.5 mm for an FSR of 30 GHz. Fortunately, what's often most important is to confirm single or maybe dual longitudinal mode performance so a much smaller FSR is adequate and desirable for maximum resolvance. With a bit of care in interpretation, almost any FSR will be fine for this purpose.

    The mechanical configuration is similar to the $1.00 SFPI except that the rear mirror mount can be moved along the optical axis on threaded rods to match the mirror distance to the RoC of the mirrors. A diagram along the lines of the simple design of the $1.00 SFPI is shown in Home-Built Scanning Fabry-Perot Interferometer 2. Again, mine was constructed of cast off disk drive parts and other miscellaneous junk. :) The first photodiode I used for this SFPI was a $2.00 part from Digikey - which would have been my total cost if it hadn't already been in one of my random stuff drawers. :) And, the frame is a bit shorter since the RoC of all of these mirrors is so small. Please see: Photo of Sam's $2.00 Scanning Fabry-Perot Interferometer.

    For the initial test, I am using the HR mirror set with an 05-LGR-151 green HeNe laser head. Since this is a less than 0.5 mW output laser and the sensitivity of silicon photodiodes at 543.5 nm is somewhat lower than at 632.8 nm, detection is more difficult.

    Furthermore, in order for the SFPI to be mode degenerate, the mirror spacing really has to be quite close to the RoC for the confocal configuration. Since these were originally lenses and not mirrors, the exact RoC is not really known. OK, the real story is that I didn't locate the part numbers of the lenses until after I did the initial construction and wrote this paragraph! There are many ways to determine the actual RoC of the mirrors. A collimated beam can be reflected from the mirror at a slight angle. The focal point will be at a distance of one half the RoC. Alternatively, a point source like a bare visible laser diode can be imaged back onto itself from the mirror. Then, the RoC is the distance to the mirror. However, any such measured RoC is only approximate. For the SFPI to be mode degenerate, it needs to be quite precise and this can only be determined experimentally.

    The mirror alignment itself isn't super critical. It's best to have a way of changing mirror distance without affecting alignment very much but simple three-screws adjusters work just fine. The laser used for the alignment should have a known spectrum if possible, preferably a single longitudinal mode. As the correct distance is approached, the little peaks from all the modes of the not quite confocal cavity - which may indeed be very small or undetectable - will gradually merge into one peak whose amplitude will increase and width will decrease dramatically.

    Note that the MDI doesn't eliminate higher order transverse modes. It only assures that they will appear at the same locations on the display as the TEM00 modes. If the distance between the mirrors isn't close to the RoC, there will be higher order modes at essentially random frequencies relative to the TEM00 modes. The result will be very low fringe contrast in the output as the PZT voltage is varied and lumps all over the place in the display. However, as the correct distance is approached, these will approach the TEM00 modes. Visually, if the distance between the mirrors is moved slowly with the PZT around the optimal distance, the output beam from from the SFPI (going to the photodiode) will flash on and off uniformly across its entire width, while on either side there will be concentric rings of light and dark sweeping from center to edge or vice-versa. It's actually quite remarkable that varying the PZT voltage by hand (ramp turned off), the output of the SFPI can be tuned to all light or all dark very precisely when the distance is just right. In addition, alignment of the SFPI relative to the laser is very easy. The reference I am using is to adjust the the reflection from the planar surface of the front mirror to be just below the output aperture of the laser, then adjust the position of the beam (without changing its angle) to center the reflected blob from the curved rear surface of the front mirror.

    After some fiddling, I am able to see the modes of the 05-LGR-151, though the signal is extremely low level and the finesse is poor. In addition, the modes appear to be somewhat distorted - possibly due to the distance between the mirrors not being quite correct. Switching the function generator to DC output mode and adjusting the voltage through the modes of the HeNe laser shows a very complex transverse mode pattern which is clearly not degenerate even when the mirror distance is very close to optimal. I don't know if this is due to the distance still not being perfect (commercial SFPIs are set to within a few um) or due to poor accuracy in the spherical shape of the mirrors. Focusing the beam improves the resolution and amplitude of the signal somewhat or just due to the nonuniformity of the coating which results in the reflectance decreasing from center to edge. A modest size aperture (perhaps 1 mm) will probably help to eliminate many of the higher order mode since they are quite spread out.

    Up to this point, my conclusions were mixed. Yes, the jumbled peaks were gone. And, alignment is definitely much less critical - once the distance of r is found, any two of the three rear mirror mount nuts or mirror adjusters can easily peak the output in no time flat. But, the resolution is lower than my $1.00 SFPI - between 50 and 100 MHz, compared to better than 25 MHz. Whlte the larger FSR means that the resolution will not as fine for the same finesse, another factor may be the quality of the mirrors (or lack thereof, actual specs unknown). A focusing lens (see below) and modest size intracavity aperture will help somewhat. And a photodiode preamp will help make alignment easier. As long as the reflections from the various front optics don't return to the HeNe laser, the modes are quite stable. However, very obvious instability results if a major portion of the reflected HeNe beam hits the laser's output mirror. Then, wild mode fluctuations appear in the SFPI display - some modes may momentarily double in amplitude or disappear entirely. And visible power fluctuations are also visible in the beam and interference patterns.

    The next step will be to add a proper focusing lens as shown in the $2.00 SFPI diagram (there is none in the one in the photo). Presently, the curved surface of the front mirror results in a large diverging effect on the input beam. Using a long focal length lens helps somewhat. But in a test using a short focal length positive lens mounted in a spring clothspin on a micropositioner helps even more. This cancels out the negative curvature of the front mirror and adds some additional focusing to match the TEM00 mode of the confocal cavity. The signal amplitude increases by at least a factor of 2 and the resolution also improves.

    Eventually, I will probably construct a preamp for the photodiode to provide an adjustable gain of up to 1,000 using a couple of op-amps. This will greatly ease alignment since the height of the signal on the scope on its most sensitive setting with a 10X probe now is only about 1/2 cm at best using the low power green HeNe laser. A possible design is shown in Adjustable Gain Photodiode Preamp. (Frequency compensation capacitors which may be needed for stability are not shown.) The gain is variable from 0.1 to 1,000 compared to the bare phododiode feeding a 10K ohm load. A gain of 10 would be sufficient so this should have enough headroom for other lower output power lasers and/or higher reflectance mirrors.

    However, for now, I just replaced the 10K phododiode load resistor with a 100K pot and substituted a 1X probe for the 10X probe. This resulted in more than enough sensitivity even for the low power green laser while maintaining adequate frequency response.

    Finally, I installed a 9 mm focal length focusing lens as shown in the diagram. This results in a collimated input beam coming to a focus inside the cavity (the focal length of the lenses being used for the mirrors is -20 mm).

    And then it was perfect. :) Well not quite perfect - the finesse isn't much better but it is quite stable, there is no evidence of unwanted ghost frequencies, it is easy to align, and all in all, works quite well. With careful alignment and centering of the input beam, I was even able to achieve the situation where the FSR became c/(2*d) or 14.6 GHz. In this case, every other mode display per sweep of the SFPI nearly disappeared with the remaining ones almost doubling in amplitude.

    The finesse is probably not as terrible as I'm implying. For my 99 percent mirrors, the theoretical finesse is a bit over 150. So, 14.6 GHz divided by 150 is about 100 MHz which is close to what I've measured. And, as noted, it's quite possible the mirrors are actually somewhat less reflective than the 99 percent being used for the finesse calculation.

    This SFPI can be used to easily test most DPSS green (532 nm) CW lasers for single frequency operation. It's easier to set up than a commercial SFPI with a separate ramp generator/preamp box as my Wavetek function generator is always there on top of the scope. :) The high reflectivity of the mirrors for 532 nm turned out to not be a problem. The ~14.6 GHz FSR is large enough to display unique modes for the C215M, C315M, C532. While the cavities of the uGreen and LWE-142 lasers are very short and have a higher FSR, it's still possible to detect spurious non-single frequency operation since the extra modes will not be stable or have a fixed relationship to the primary mode.

    As a free bonus, the same SFPI can also be used for 1,5XX nm lasers by swapping the photodiode. When I became obsessed with the desire to look at the longitudinal modes of a Melles Griot 05-LIR-150 1,523 nm HeNe laser, there didn't seem to be too many options. None of my other SFPIs (home-built or commercial) would work beyond 900 nm. But then it occurred to me that I already had this SFPI using mirrors coated for 1,540 nm. Being HR at that wavelength (and probably close to HR at 1,523 nm), getting a signal might be quite a challenge, but aside from the near impossibility of lining everything up with the <1 mW beam from the IR laser, it was worth a shot.

    I did have an IR photodetector for a Newport power meter, so Simply removing the existing PD board would allow the beam to exit the back of the SFPI. To have the most flexibility, the PD preamp from an SP-476 SFPI driver was used (but not the HV scan output, though that could also have been used in place of the function generator with a resistor divider to reduce the maximum voltage). After a bit of fiddling with room lights out (any bit of fluorescent light overwhelmed the signal and/or added 120 Hz ripple), a really messed up display was obtained with all sorts of ringing and garbage (technical term!). However, it was possible that this was due to the detector being designed for a laser power meter with a smoothing capacitor or something else inside.

    I had just rediscovered the use of cut-open germanium transistors as sensors for 800 to 1,800 nm. With one of those, despite it's somewhat low sensitivity, the modes of the 1,523 nm laser appeared in all their glory. The finesse is only between 100 and 150, but it is more than adequate for displaying IR HeNe laser modes. I don't do Telecom. :) Since I now have a SP-470-03 which works from 532 nm through at least 650 nm, that's what's used for routine laser testing and characterization. So, I'll probably leave the IR PD permanently in the $2 SFPI and dedicate it for IR lasers.

    In 20:20 hindsight, the mechanical design of a confocal SFPI can be considerably simpler than what I created. With careful alignment of the mirrors during mounting and glueing (for the one on the PZT, which has wedge), no adjustable alignment is really necessary. So, it becomes a pair of plates on threaded rods. The front mirror would simply be clamped or glued to the front plate and the back mirror would be glued to the PZT, which is clamped or glued to the back plate. After initial setup setting the precise confocal spacing, alignment is done by X/Y (pitch/yaw) adjustments of the mounting for the SFPI head. This can be a fancy kinematic or spherical mount, or simply a platform with adjustable screws for feet.

    Sam's $3.00 Scanning Fabry-Perot Interferometer

    About a year after building my $2 SFPI, I came across some other short radius mirrors:

    At first I thought these were for some Spectra-Physics dye laser. But thinking about it, I'm now inclined to believe they were a HeNe laser mirror goof. The specifications called for 43 cm RoC mirrors and someone dropped a factor of about 10 between design and manufacturing. (My measurement may be off by a couple of mm, so they could indeed be 45 mm mirrors.) Hey, if NASA can goof up units, so can a laser company! How else to explain that there were literally thousands of these available surplus at one time. SP never sold that many dye lasers, but production runs of thousands of HeNe laser tubes for barcode scanners at the peak of their popularity would not have been unusual. Also, SP's dye laser pump mirrors with short RoC mirrors tended to have the non-reflective side fine ground (not polished and AR coated as with these). Also, the SP dye laser mirrors I've seen have an RoC of about 50 mm, not 43 mm. Regardless of the origin, I'm not complaining. The person I got the mirrors from insists they are HeNe mirrors and will even send me a laser tube that uses them if he can find one. In principle, I suppose that is possible but it would have to be a very peculiar resonator configuration with a focal point inside. I won't hold my breath in anticipation. :) He had been selling them on eBay (sorry, no more available from there!) and had so many that he was using them as decorative stones in his tropical fish tank. Transgressions like that really need to be punished! :-)

    The mirrors were installed in a slightly stretched frame to enable the longer 43 mm resonator length as shown in Home-Built Scanning Fabry-Perot Interferometer 3. It was then a simple matter to get this rig to work with much better finesse. That is, after I realized two things:

    1. The focusing lens from the $2.00 SFPI had too short a focal length for the much longer cavity and was smearing out and reducing the amplitude of the response.

    2. The confocal distance was indeed 43 mm and not 38 or 40 mm as I originally thought. At 38 mm, the SFPI initially appeared to work but the display wasn't stable at all, mode amplitudes varied depending on where they were on the ramp voltage, and the photodiode signal was quite weak. Once it was adjusted at 43 mm, the display looked very much like the one in a textbook. :)

    The only problem with this SFPI for use with HeNe lasers is that the Free Spectral Range (FSR) for the mode degenerate confocal configuration is c/(4*d), which is only about 1.75 GHz for the 43 mm cavity. This is just barely more than the Doppler broadened gain bandwidth of the HeNe laser, about 1.5 GHz. So, there can be some confusion when interpreting lasing lines on the tails of the gain curve, though this is minor. However, a benefit is that the 1.75 GHz FSR provides nearly the largest useful resolution by almost filling the FSR with the HeNe laser modes.

    I have a set of basic parts available for building a similar SFPI. Sorry, it will cost more than $3 though. :) More information can be found at Sam's Classified Page.

    See W's Scanning Fabry-Perot Interferometer Page for an SFPI using these same mirrors (as well as others for other wavelengths). His mechanical setup uses parts that are a bit more professional and several orders of magnitude more expensive than mine though. Yet, he complains about instabilities that my resonator frames constructed from recycled harddrive parts and Home-Depot hardware don't have. :)

    As with the $2 SFPI, simplification of the mounting is indeed possible.

    Sam's Plane Mirror Scanning Fabry-Perot Interferometer

    The planar-planar (P-P) cavity is a configuration that will not support higher order transverse modes at all. However, it is only borderline stable for the TEM00 mode and extremely difficult to set up and align as the two mirrors must be parallel to a very high precision AND the input beam must be orthogonal to the input mirror to get decent performance. The latter requirement is particularly troublesome in that without an optical isolator, that any reflections will go directly back to the source. Since the SFPI mirrors are nearly 100 percent reflecting most of the time (except when in resonance), this means a nearly total return. And most lasers get modestly to really annoyed when any of the output beam is reflected back into their cavity, resulting in a variety of instabilities and changes in mode structure. However, where it is desired to either have an FSR for which confocal mirrors are not available - which is most FSRs without custom mirrors - or to be able to adjust the FSR for various applications, there is no choice.

    I built a P-P SFPI using a similar structure to that my others inexpensive home-built "instruments". Unfortunately, Murphy took no days off and ALL of these problems were present. I selected an FSR of around 5 GHz (25 mm mirror spacing) so it would unambiguously display modes of a two-frequency Zeeman HeNe laser like an HP-5517B. (Due to the Zeeman splitting, these lasers can potentially have lasing modes over a much wider bandwidth than the normal 1.6 GHz or so of the Doppler-broadened neon gain curve. Think of a pair of neon gain curves that are shifted by several hundred MHz with respect to each-other, thus making the available range larger.) None of my other SFPIs would be entirely suitable. My home-built one for 633 nm had an FSR of only 1.7 GHz and the Spectra-Physics 470-03 (see below) has an FSR of 2.0 GHz. A Zeeman laser could easily have lasing modes over a bandwidth of 3 GHz or even more, especially one that has been rebuilt improperly. Where the behavior is not what is expected, the aliasing in a narrow range SFPI would make interpretation of what's actually going on very difficult.

    The planar mirrors used were near-HR (99.8%) at 633 nm with no AR coating but ground with wedge so the reflections from the uncoated surface would not interfere with the SFPI operation. But the home-built mirror adjusters that were perfectly adequate for the confocal SFPI were barely marginal for this one, requiring very careful tweaking for best response. And they didn't want to remain aligned for more than a few minutes. But worst of all, without an optical isolator, the modes of the usually well behaved 05-LHP-151 laser head being used for testing were jumping all over the place due to the back-reflections. And there were never more than 3 modes present, generally only 2, and sometimes only a single mode. Normally, there would be 4 or 5 modes at all times for this 5 mW laser.

    During those periods where it behaved, the performance was quite acceptable easily resolving the 05-LHP-151's longitudinal modes (spacing of 438 MHz) with a factor of 3 or 4 to spare. This would have been more than enough for use with the short laser tubes (mode spacing of 1 GHz or more). But the needs for super-precise alignment and virtually unavoidable back-reflections makes this impractical for my intended application of easily analyzing the modes of a variety of HP/Agilent lasers and home-built equivalents.

    Sam's Hemispherical Confocal Scanning Fabry-Perot Interferometer

    When I first acquired a Tropel 2440 SFPI head with a planar front mirror and curved rear mirror, I had my doubts that such a configuration was valid. All other commercial SFPI heads I'm aware of are built with a pair of identical curved mirrors, nearly always in a normal confocal configuration. Whlie some general purpose instrumentsdesigned for masochists :) use planar mirrors, they don't have separate self-contained SFPI heads. Perhaps it was simply a short hemispherical cavity and precise alignment was necessary to avoid higher order modes in the display. Or, perhaps someone had replaced the curved front mirror with a planar mirror for reasons unknown. But that seemed extremely unlikely. Based on what could be seen of the curved rear mirror after removing the front planar mirror, the cavity length looked about right to be one half its RoC. In fact, the Tropel 2440 is what got me thinking about the Hemispherical Confocal (HC) cavity SFPI in the first place - then having serious doubts about its functionality.

    Although the HC cavity would have a resonance as a result of the reflection from two passes up and back to the curved mirror via the planar mirror, there would also be the normal resonance between the front and rear mirror. So, why wouldn't this one dominate and screw up the behavior? If valid, the FSR should be the same as for a normal confocal configuration with mirrors having an RoC equal to d and spaced d apart (FSR=c/(4*d), but the HC cavity would be half as long and the PZT sensitivity should double, requiring only half as much voltage to cover one FSR.

    As a test, I replaced the front mirror in my home-built plane mirror SFPI (above) with a 4.3 cm RoC mirror like those used in my normal confocal SFPIs (see the sections: Sam's $3.00 Scanning Fabry-Perot Interferometer and Scanning Fabry-Perot Interferometer Head Frame using Commercial Parts), as well as in the SFPI kits available on Sam's Classified Page. (The rear mirror is a salvaged internal mirror HeNe HR.) And this actually works quite beautifully. :) It was quick and easy to set the cavity spacing and obtain a clean display. Alignment is non-critical and similar to that of the normal confocal configuration. As predicted, the FSR remained unchanged at around 1.7 GHz and the PZT sensitivity (V/FSR) appears to have doubled, tuning through up to 8 or 10 FSRs using a Wavetek function generator with a maximum output of around 20 V p-p. Initially, no mode matching lens was used so the finesse wasn't as high as it should be based on theory but if the planar mirror were in front, mode matching would be less critical and a lens would probably not be required. But since this was really only done as a test, replacing the front mirror was much simpler, not being glued to the PZT. And dusting off the planar mirror on the original SFPI assembly that had been sitting in a dusty closet for several years made a dramatic improvement. Can you believe that? :-) With a 2 inch focal length lens positioned near the front mirror, the finesse is at least 100, perhaps over 150, which still may be limited by the recycled planar mirror. See Sam's Hemispherical Confocal Scanning Fabry-Perot Interferometer. Note the spacing of the mirrors of around 0.85 inches (one half of the 43 mm RoC). (As a reference, the distance between the two front plates is approximately 1 inch.) A "third hand" holds the randomly selected mode matching lens. With the rear mirror being HR, the photodiode signal is slighly weaker than when a lower reflectance mirror is used but not by that much. The photodiode preamp of the SP-476 is being used, but on its least sensitive setting, but as with the other open-frame SFPIs, a cover may be necessary to block room light (not shown).

    When alignment is near perfect (close to being centered and aligned with the optical axis), instead of every other FSR dropping out and the others doubling in amplitude as with the normal confocal SFPI, the pattern is over 4 FSRs or pairs of FSRs and even 3 of the 4 dropping out and the remaining one quadrupling in amplitude. Other than that, it would be difficult to tell the two apart from general behavior. All in all, quite fascinating. ;-)

    Sam's Selectable FSR Hemispherical and Spherical Mode Degenerate Scanning Fabry-Perot Interferometers

    I figured it would be interesting to build a system of the type described in the section: Selectable FSR Mode Degenerate Fabry-Perot Interferometers. However, using the usual 4.3 cm (1.7 inch) RoC mirrors would require the spacing to vary over almost a 3.5 inch range, with fine adjustment at any given position. This could be done with a 4 inch linear slide and separate micrometer positioner. I had even dragged out some salvaged LaserDisc player slider bearing rails that could actually be used to build a nice linear slide with a 4 inch travel. An interferometer-based positioning system could be used to set the spacing down to the angstrom, but that's a bit more complexity than I had in mind! ;-) Then it occurred to me that using the hemispherical version of the mode degenerate cavity would only require half the range. This was attractive because: (1) I just happened to have a compact Newport linear positioner with a 2 inch travel and (2) the hemispherical cavity differs from the one presented in the paper and conceivably has not been built before. This would enable me to at least pretend that I'm doing something new! ;-)

    Here are all the resonances up to N=10 for a selectable FSR hemispherical SFPI with a planar mirror and 4.3 cm RoC curved mirror, both having a reflectance of approximately 99% corresponding to a planar finesse of 300:

                      <---- Relative ---->  <--- 4.3 cm RoC (5) ---->
          Num         (2)     (3)    (4)      d     FSR    FWHM  Fin-
      N k Rep  d/r    FSR    FWHM  Finesse  (cm)   (GHz)   (MHz) esse  Notes (1)
    ------------------------------------------------------------------------------
      1 0  1  0.000  1.000   1.000  1.000   4.300  3.486   11.62  300  Planar 1-0
    ------------------------------------------------------------------------------
      1 1  2  1.000  0.500   2.000  0.500   4.300  1.743   11.62  150  HS (1-1)
      2 1  4  0.500  0.500   4.000  0.250   2.150  1.743   23.24   75  HSC (2-1)
      3 2  6  0.750  0.222   2.667  0.167   3.225  0.775   15.49   50
      3 1  6  0.250  0.667   8.000  0.167   1.075  2.324   46.48   50
      4 1  8  0.146  0.854  13.657  0.125   0.630  2.976   79.35   38
      4 3  8  0.854  0.146   2.343  0.125   3.670  0.511   13.61   38
      5 1 10  0.095  1.047  20.944  0.100   0.411  3.561  121.69   30
      5 2 10  0.345  0.289   5.789  0.100   1.486  1.009   33.63   30
      5 3 10  0.655  0.153   3.056  0.100   2.814  0.533   17.75   30
      5 4 10  0.905  0.111   2.211  0.100   3.889  0.385   12.85   30
      6 1 12  0.067  1.244  29.856  0.083   0.288  4.337  173.47   25
      6 5 12  0.933  0.089   2.144  0.083   4.012  0.311   12.45   25
      7 1 14  0.050  1.443  40.391  0.071   0.213  5.029  234.68   21
      7 2 14  0.188  0.379  10.624  0.071   0.809  1.323   61.73   21
      7 3 14  0.389  0.184   5.145  0.071   1.672  0.641   29.89   21
      7 4 14  0.611  0.117   3.272  0.071   2.628  0.407   19.01   21
      7 5 14  0.812  0.088   2.464  0.071   3.491  0.307   14.32   21
      7 6 14  0.950  0.075   2.104  0.071   4.087  0.262   12.23   21
      8 1 16  0.038  1.642  52.548  0.063   0.164  5.725  305.31   19
      8 3 16  0.309  0.202   6.480  0.063   1.327  0.706   37.65   19
      8 5 16  0.691  0.090   2.893  0.063   2.973  0.315   16.81   19
      8 7 16  0.962  0.065   2.079  0.063   4.136  0.226   12.08   19
      9 1 18  0.030  1.842  66.327  0.056   0.130  6.423  385.36   17
      9 2 18  0.117  0.475  17.097  0.056   0.503  1.656   99.34   17
      9 4 18  0.413  0.134   4.841  0.056   1.777  0.469   28.12   17
      9 5 18  0.587  0.095   3.408  0.056   2.523  0.330   19.80   17
      9 7 18  0.883  0.063   2.265  0.056   3.797  0.219   13.16   17
      9 8 18  0.970  0.057   2.062  0.056   4.170  0.200   11.98   17
     10 1 20  0.024  2.043  81.727  0.050   0.105  7.123  474.84   15
     10 3 20  0.206  0.243   9.704  0.050   0.886  0.846   56.38   15
     10 7 20  0.794  0.063   2.519  0.050   3.414  0.220   14.64   15
    

    Notes:

    1. These are the named cavities: Planar, HemiSpherical (HS), and HemiSpherical Confocal (HSC). The planar cavity isn't strictly part of this series but is included for completeness, and as a baseline for comparison. Its mirror RoCs would be infinite but its mirror spacing (d) is equal to the RoC (r) used for all the spherical cavities. Configurations for N above 10 would have larger maximum values for the FSR and FWHM (k=1), and smaller minimum values for the FSR (k=N-1), with the FWHM approaching that of the normal hemispherical cavity (1-1).

    2. FSR takes into account the actual value of d as the cavity spacing is varied. The values shown are relative to that of the planar cavity (with mirror spacing equal to RoC=r of the mirrors used in the spherical cavities). Then, FSR(N)*2*d/r=1/N, or FSR(N)=r/(2*d*N).

    3. FWHM is the width of the spectral peaks relative to that of the planar cavity based on the mirror reflectivity (R) and mirror spacing (d).

    4. Finesse is with respect to the effective FSR, relative to that of the planar cavity and scales as 1/(2*N).

    5. This shows the resulting mirror spacing (d), FSR, FWHM of the spectral peaks, and finesse for each hemispherical cavity with mirror RoC=4.3 cm compared to a planar cavity with a mirror spacing of d=r=4.3 cm for an FSR of around 3.5 GHz and a finesse of 300 (mirror reflectivity of approximately 99 percent).

    The completed unit is shown in: Sam's Selectable FSR Hemispherical Mode Degenerate Scanning Fabry-Perot Interferometer. The 4.3 cm RoC 99.5+%@633 nm rear mirror is glued to a PZT attached to the front of a 1/2" kinematic mount with the photodiode mounted on a piece of plastic on the back. This assembly is fastened to a Newport 422-1S Miniature Ball Bearing Linear Stage, enabling the mirror spacing to be varied over a 2 inch range with a thumbscrew. (The actual continuous range with the thumbscrew is only about 1.5 inches, so a 1/2" block must be placed between the tip of the screw and carriage to achieve the final 1/2". The specifications for the 422-1S actually only lists a 1 inch travel but that's probably for the guaranteed alignment tolerance - it is more than acceptable over the entire 2 inch range for the SFPI.) The planar 99.8%#633nm ("near HR") stationary front mirror is in an adapter in a similar 1/2" kinematic mount. The adjustable mounts aren't absolutely essential but they were available and too ugly to use for most other applications. :) And having fine control of mirror alignment should help to optimize performance. Everything is on a heavy baseplate with 3 adjustment feet. A scale on the baseplate enables the coarse position to be set easily. Space has been reserved in front of the stationary mirror for a mode matching focusing lens, but this will hopefully not be required due to the hemispherical cavity.

    The results are encouraging. Setup was very easy with alignment using the baseplate screws and fine tuned with the mirror mounts, and a shim placed under the rear mirror mount to match the heights of the mirrors. Near optimal alignment can be maintained over the entire travel of the movable mirror.

    However, while quite usable, the finesse is still rather disappointing, being less than one half of the value predicted by theory. The only way that seems to improve it somewhat is to add a lens to focus the beam directly at the surface of the front mirror. For the specific case of the hemispherical confocal cavity length of r/2, the first bounce back from the curved mirror should do exactly that. But for some reason, it's not as good. So, I considered the possibility that there needs to be another factor of two reduction in fesse thrown in that is not being accounted for by the equations. But this doesn't appear all that likely. Consider the following thought experiment starting with the normal confocal cavity: Take one mirror and rotate it 90 degrees so it is at a right angle to the other mirror. Now add an ideal planar mirror at 45 degrees in between them and adjust the total cavity length to be the same as before (r=RoC). Then the logical geometry really hasn't changed and the behavior should be identical. And with the normal off-axis alignment of the input beam, it should be possible to further rotate the curved mirror from 90 to 180 degrees so the two curved mirrors merge into one to create the hemispherical confocal cavity, again without changing the geometry. The only thing the ideal planar mirror does is to flip the "handedness" of the intracavity beam, but it's not clear how this could affect anything related to finesse unless eliminating the independence of the two curved mirrors somehow causes input alignment to be super critical. Now that's really grasping for optical straws! :)

    I even tried a real planar HR to see if that would help. But after fighting with alignment due to the low signal, there was no dramatic improvement in finesse beyond what could be accounted for with 99.9+%R compared to 99.8%R.

    And to confirm that I wasn't just unlucky having selected mediocre mirrors, the front planar mirror was then replaced with one identical to the rear mirror (4.3 cm RoC, 99.5%R@633 nm) thus converting this rig into a selectable FSR spherical SFPI. The range of travel is not sufficient to allow for much above the confocal cavity length, but does permit all the shorter ones to be selected. And as a practical matter, most of the possible FSRs with longer than confocal spacing are not all that useful anyhow (or shall we say are even less useful than the others).

    Here are all the resonances up to N=10 for a selectable FSR (spherical) SFPI with 4.3 cm RoC mirrors having a reflectance of approximately 99% corresponding to a planar finesse of 300:

                      <---- Relative ---->  <---- 4.3 cm RoC (5) --->
          Num         (2)     (3)    (4)      d     FSR    FWHM  Fin-
      N k Rep  d/r    FSR    FWHM  Finesse  (cm)   (GHz)   (MHz) esse  Notes (1)
    ------------------------------------------------------------------------------
      1 0  1  0.000  1.000   1.000  1.000   4.300  3.486   11.62  300  Planar 1-0
    ------------------------------------------------------------------------------
      1 1  1  2.000  0.500   0.500  1.000   8.600  1.743    5.81  300  Sphere 1-1
      2 1  2  1.000  0.500   1.000  0.500   4.300  1.743   11.62  150  Confoc 2-1
      3 1  3  0.500  0.667   2.000  0.333   2.150  2.324   23.24  100
      3 2  3  1.500  0.222   0.667  0.333   6.450  0.775    7.75  100
      4 1  4  0.293  0.854   3.414  0.250   1.259  2.976   39.67   75
      4 3  4  1.707  0.146   0.586  0.250   7.341  0.511    6.81   75
      5 1  5  0.191  1.047   5.236  0.200   0.821  3.651   60.84   60
      5 2  5  0.691  0.289   1.447  0.200   2.971  1.009   16.82   60
      5 3  5  1.309  0.153   0.764  0.200   5.629  0.533    8.88   60
      5 4  5  1.809  0.111   0.553  0.200   7.779  0.385    6.42   60
      6 1  6  0.134  1.244   7.464  0.167   0.576  4.337   86.73   50
      6 5  6  1.866  0.089   0.536  0.167   8.024  0.311    6.23   50
      7 1  7  0.099  1.443  10.098  0.143   0.426  5.029  117.34   43
      7 2  7  0.377  0.379   2.656  0.143   1.619  1.323   30.86   43
      7 3  7  0.777  0.184   1.286  0.143   3.343  0.641   14.95   43
      7 4  7  1.223  0.117   0.818  0.143   5.257  0.407    9.51   43
      7 5  7  1.623  0.088   0.616  0.143   6.981  0.307    7.16   43
      7 6  7  1.901  0.075   0.526  0.143   8.174  0.262    6.11   43
      8 1  8  0.076  1.642  13.137  0.125   0.327  5.725  152.65   38
      8 3  8  0.617  0.202   1.620  0.125   2.654  0.706   18.82   38
      8 5  8  1.383  0.090   0.723  0.125   5.946  0.315    8.40   38
      8 7  8  1.924  0.065   0.520  0.125   8.273  0.226    6.04   38
      9 1  9  0.060  1.842  16.582  0.111   0.259  6.423  192.68   33
      9 2  9  0.234  0.475   4.274  0.111   1.006  1.656   49.67   33
      9 4  9  0.826  0.134   1.210  0.111   3.553  0.469   14.06   33
      9 5  9  1.174  0.095   0.852  0.111   5.047  0.330    9.90   33
      9 7  9  1.766  0.063   0.566  0.111   7.594  0.219    6.58   33
      9 8  9  1.940  0.057   0.516  0.111   8.341  0.200    5.99   33
     10 1 10  0.049  2.043  20.432  0.100   0.210  7.123  237.42   30
     10 3 10  0.412  0.243   2.426  0.100   1.773  0.846   28.19   30
     10 7 10  1.588  0.063   0.630  0.100   6.827  0.220    7.32   30
    

    Notes:

    1. These are the named cavities: Planar, Spherical, and Confocal. The planar cavity isn't strictly part of this series but is included for completeness, and as a baseline for comparison. Its mirror RoCs would be infinite but its mirror spacing (d) is equal to the RoC (r) used for all the spherical cavities. Configurations for N above 10 would have larger maximum values for the FSR and FWHM (k=1), and smaller minimum values for the FSR (k=N-1), with the FWHM approaching that of the normal hemispherical cavity (1-1).

    2. FSR takes into account the actual value of d as the cavity spacing is varied. The values shown are relative to that of the planar cavity (with mirror spacing equal to RoC=r of the mirrors used in the spherical cavities). Then, FSRr(N)*d/r=1/N, or FSRr(N)=r/(d*N).

    3. FWHM is the width of the spectral peaks relative to that of the planar cavity based on the mirror reflectivity (R) and mirror spacing (d).

    4. Finesse is with respect to the effective FSR, relative to that of the planar cavity and scales as 1/N.

    5. This shows the resulting mirror spacing (d), FSR, FWHM of the spectral peaks, and finesse for each hemispherical cavity with mirror RoC=4.3 cm compared to a planar cavity with a mirror spacing of d=r=4.3 cm for an FSR of around 3.5 GHz with a finesse of 300 (mirror reflectivity of approximately 99 percent).

    Here are the available spherical resonances up to N=10 sorted by increasing mirror spacing from 0 to 5 cm. All the useful larger FSRs (compared to the confocal cavity, CFSR = 1.0) fall within this range. The Planar cavity 1-0 at the top with the same mirror spacing as the confocal cavity (d=RoC=4.3 cm) would have an FSR of 2.0 x CFSR, but is much more difficult to use.

                                            <------ 4.3 cm RoC ----->
          Num         <---- Relative ---->    d     FSR    FWHM  Fin-  FSR Rel to
      N k Rep  d/r    FSR    FWHM  Finesse  (cm)   (GHz)   (MHz) esse   Confocal
    -------------------------------------------------------------------------------
      1 0  1  0.000  1.000   1.000  1.000   4.300  3.486   11.62  300  2.00 x CFSR
    -------------------------------------------------------------------------------
     10 1 10  0.049  2.043  20.432  0.100   0.210  7.123  237.42   30  4.09 x CFSR
      9 1  9  0.060  1.842  16.582  0.111   0.259  6.423  192.68   33  3.68 x CFSR
      8 1  8  0.076  1.642  13.137  0.125   0.327  5.725  152.65   38  3.28 x CFSR
      7 1  7  0.099  1.443  10.098  0.143   0.426  5.029  117.34   43  2.89 x CFSR
      6 1  6  0.134  1.244   7.464  0.167   0.576  4.337   86.73   50  2.49 x CFSR
      5 1  5  0.191  1.047   5.236  0.200   0.821  3.651   60.84   60  2.09 x CFSR
      9 2  9  0.234  0.475   4.274  0.111   1.006  1.656   49.67   33  
      4 1  4  0.293  0.854   3.414  0.250   1.259  2.976   39.67   75  1.71 x CFSR
      7 2  7  0.377  0.379   2.656  0.143   1.619  1.323   30.86   43
     10 3 10  0.412  0.243   2.426  0.100   1.773  0.846   28.19   30
      3 1  3  0.500  0.667   2.000  0.333   2.150  2.324   23.24  100  1.33 x CFSR
      8 3  8  0.617  0.202   1.620  0.125   2.654  0.706   18.82   38
      5 2  5  0.691  0.289   1.447  0.200   2.971  1.009   16.82   60
      7 3  7  0.777  0.184   1.286  0.143   3.343  0.641   14.95   43
      9 4  9  0.826  0.134   1.210  0.111   3.553  0.469   14.06   33
      2 1  2  1.000  0.500   1.000  0.500   4.300  1.743   11.62  150  1.00 x CFSR
      9 5  9  1.174  0.095   0.852  0.111   5.047  0.330    9.90   33
    

    With a mode matching lens, the finesse is indeed much higher, but that mode matching becomes annoying as the optimal lens or at least its optimal location depends on the FSR that is selected. Thus, it would be useful to have the lens on a linear slide as well.

    All the same resonances appear to be available as listed in the table for the selectable FSR spherical cavity SFPI, scaled for a confocal FSR of 1.75 GHz instead of 2.25 GHz. Although the hemispherical cavity halves the finesse for mirrors with identical reflectance, the reflectance of these mirrors is somewhat higher so the finesse may end up being similar or even greater.

    However, for some spacings corresponding to higher orders of N, the amplitudes of the peaks corresponding to the various FSRs tend to differ, sometimes quite dramatically, even when alignment is far from on-axis. This was not totally unexpected though.

    A computer-controlled interferometer-based positioning system with smart parameter calculation software would be a definite plus here. :) It's often difficult to determine which exact table entry is being displayed, especially where usable mirror spacings are close together. The mediocre repurposed microscope mechanical stage scale really isn't adequate (aside from it reading from 40 to 105 instead of 0 to 65, which would be more useful). Eventually it will be upgraded. :( :)

    As another test, I installed a 1 meter RoC 99.4%R mirror in place of the planar mirror. This results in a highly asymmetric spherical cavity. As expected, this is not pure MDI so while similar major resonances are present, ghost and double peaks come and with changes in the input laser's longitudinal mode frequencies and SFPI cavity length. Perhaps a 5 meter RoC mirror would be close enough to planar to reduce these artifacts to acceptable levels but I don't have one of those.

    I had originally dismissed this selectable FSR SFPI as an academic exercise. However, there is at least one application where this could prove useful to the kind of experiments on which I spend way too much time: Observing the mode sweep of Zeeman-split HeNe lasers. While the resolution is not high enough to actually see the split Zeeman-split lasing mode, the larger FSRs easily dialed up using this system such as 2.976 GHz (Cavity 4-1) would easily enable the overall mode sweep behavior to be observed, while the higher finesse default FSR of 1.75 GHz would suffice for most everything else. Pretty lame, huh? :)

    While the rig described above is certainly adequate as a "proof of concept", constructing a more robust and stable selectable FSR SFPI would require a more elaborate structure. Even playing a radio (remember those?) at modest volume causes the display to jump around. One possibility for quickly setting coarse mirror spacing might be to use a mechanism similar to that of a zoom lens where an inner element is moved via a knurled ring. Instead of zoom factors, it would be labeled in FSRs! Fine tuning would be via a separate threaded ring similar to the mirror spacing adjustment of some normal confocal SFPIs. Both would be capable of being locked in place. Yes, I know, it's more likely that there will be a sighting of pink flying pigs before anything like this gets built! ;-)

    A Non-Confocal Cavity SFPI Using Transverse Mode Confinement?

    Well, that was the idea anyhow. The rational was that rather than finding a matched set of confocal mirrors for a high finesse SFPI which has proven to be rather challenging (or at least expensive, see the next section), why not build a cavity with concave mirrors to simplify alignment but use the bore of a (defunct) HeNe laser tube to suppress higher order modes. It works in the HeNe laser, right? Then, the exact cavity length wouldn't be as critical, and it could actually be longer than the confocal length (to decrease FSR). In addition, if it could be forced to operate only in the TEM00 mode, there would be no loss of a factor of 2 in finesse as there is with the confocal configuration.

    To test this idea, an already dead (up to air) Spectra-Physics 088 HeNe laser tube was sacrificed by removing the cathode-end (HR) mirror. Actually, the entire end-cap assembly came apart at the glass-to-metal seal when, after scoring the metal tip-off, a pair of pliers was used to try to break it off. But there was no damage to the remainder of the tube including the entire glass envelope. (This might have been a result of a hairline crack already being present at the seal and the reason for the leak.) So, the anode-end mirror attached to its mount, bore, tube envelope, and centering spider was installed in my laser test jig, normally used with one-Brewster HeNe laser tubes. This tube is supported by 4 Nylon screws in two places to permit fine adjustment of centering and alignment. With this scheme, by slightly loosening two pairs of Nylon screws, the tube could be easily moved over a range of FSRs of about 400 to 500 MHz, hopefully without totally losing alignment. A mirror with similar characteristics to that of the SP-088 OC was glued to a PZT beeper element and attached to the adjustable mirror mount.

    Since the OC mirror is known to be properly aligned, its reflection could be used to align the test laser (initially a Melles Griot 05-LHR-911). Then when that was close, the function generator and scope were activated and fine alignment of the adjustable mirror could commence.

    This entire exercise turned out to be easier than I had expected but the first results were somewhat under-whelming: I was able to obtain a finesse of nearly a value of... 2, and just barely recognize what I assumed to be the modes of the 05-LHR-911 laser as lumps. :)

    Now, I was expecting and hoping for a finesse of 200 or 300 to be able to resolve the split modes of HP/Agilent metrology lasers, a few MHz apart. Clearly some more work was called for.

    My initial thought was that the first problem was that the 05-LHR-911 beam diverges as a fast enough rate that it clips the bore so diffraction losses are very high and doesn't even create a stable intracavity mode volume. So, the next step was to at least confirm this as one problem by first moving the test laser closer to the SFPI to reduce the beam diameter. A long focal length positive lens was also added to focus the laser beam into the SFPI cavity.

    These did help a bit but didn't produce any eureka moment. Then something happened. My memory of the exact sequence of events is somewhat fuzzy, but then the finesse jumped to a much more reasonable value. Part of the problem was that the photodiode output wasn't terminated except using the 1M ohm input impedance of the scope. This was both resulting in saturation at higher light levels and seriously low pass filtering the response. A 10K ohm resistor took care of that. However, I don't believe this was the entire problem because I had tried terminating the signal with no significant improvement. But then, fiddling with the alignment resulted in a very dramatic increase in finesse and output level. So, it was probably a combination of factors. Possibly the original response was not even due to a direct path down the bore with a couple reflections off the side-wall of the bore. Not that likely, but possible. The finesse is now consistently at least 50 and likely over 100 at times. Not great, but more respectable than 2! :)

    That's the good news.

    The bad news is that this scheme still has problems. For one, higher order modes are still present. Not as many as with my original short cavity SFPI, but enough to be annoying and confusing. Their amplitude can be anywhere from 10 to 100 percent of the height of what I believed to be the TEM00 modes. But the relative heights of the modes can be varied by any change in alignment, even pressing gently on the mirror mount. With the FSR of the SFPI being much less than the neon gain bandwidth, interpreting what was going on became even more difficult.

    I have attempted to more closely match the input beam to the mode volume of the SFPI, but so far, this is not been very productive. And even if this did work, requiring such painstaking setup for each test would be impractical.

    In short, although not a total failure, this approach has significant difficulties of its own, so I intend now to go back to Plan A, which is to build a long confocal SFPI. :)

    Sam's High Resolution Scanning Fabry-Perot Interferometer

    This would be a nice long confocal SFPI capable of resolving the two lines of Zeeman-split two-frequency HeNe lasers such as those from HP/Agilent and Excel. :) The typical separation of the two frequencies (called F1 and F2) is between 1.6 and 4.0 MHz. So, an SFPI with a resolvance of 1-2 MHz would be required. This will need both a combination of larger mirror spacing and decent finesse. However, the basic design would be similar to that of my other SFPIs.

    One possibility for mirrors would be the OCs from deceased low to medium power HeNe laser tubes. The type that could be satisfactory would be 99%@633nm with an RoC of 60 cm. For the confocal configuration, the FSR would be 125 MHz with a finesse of about 150 producing a resolvance of about 0.83 MHz. And the Gods of Dead Lasers know I have many of these mirrors. :) However, one deficiency is their small diameter. Most are around 7 mm if bare, but only 3 or 4 mm if mounted in the original mirror mount stems, somewhat less than desired. Removing mirrors intact is a hit or miss proposition, especially for those like Melles Griot having a thick bead of glass frit. My success rate has been very low. You don't want to see the results! An alternative would be the OC mirrors from Spectra-Physics 084 tubes which have a similar RoC and reflectance. However, they are in limited supply, though I could probably dig up a pair. But a somewhat higher reflectance would be even better, say 99.5% or even 99.8%, which would boost the finesse. Now it turns out that I have found suitable mirrors in a somewhat strange place - the HR mirrors of tube from Spectra-Physics 117A stabilized HeNe lasers! (This is the same as the Melles Griot 05-STP-901 and Melles Griot has made the tubes for these lasers for many years.) Unlike most modern HeNe laser tubes, these are curved (rather than being planar) with an RoC of 60 cm. They also have an AR coating to minimize back-reflections. In addition, they are probably have a slightly lower reflectance than a true HR (but still much higher than an OC) since the beam sampling is done through the HR mirror. But I've already totally destroyed one trying to get it off the mount - it fractured from scraping the frit, not even particularly vigerously. And my supply is rather limited - 3 or 4 more at most are available from dead SP-117A tubes.

    But the required length of the SFPI using 60 cm RoC mirrors would be somewhat unwieldy, so what I'd really like would be a smaller RoC to make the instrument more compact - say 30 cm - and with a reflectance of 99.5% to 99.8%. Unfortunately, this combination or RoC and reflectance is rather hard to find. Melles Griot does offer some curved HR mirrors, but they cannot guarantee that the reflectance wouldn't be so high as to be useless for an SFPI. And while not that expensive as these things go, they wouldn't be free or cheap enough to try out.

    I also have several New Focus kinematic mounts for 1" optics. These are a bit large, but hey, you use what you have! :) And the nice thing about these is that they have 3 screws (rather than two adjustment screws and a ball bearing for the pivot). So the mirror can be translated precisely along the Z axis to fine tune the spacing. Then, all that's required is a rigid frame. I considered using the L-bar resonator from from a defunct Spectra-Physics 124 laser. Its length would be ideal for use with 60 cm RoC mirrors and one of the mirrors could be installed in the existing mount at one end. But simply disassembling that laser would be a fair amount of work and I'd have to schedule an appointment with the Gods of dead lasers to determine what else might be needed in the way of chants and incantations. :)

    In the end, I decided to build the smaller instrument, at least for now. It would use a pair of 30 cm RoC OC mirrors salvaged from some unknown 1 or 2 mW HeNe laser tubes. I have several of these, already removed from the tubes (no further special approvals required), have tested their reflectance, and selected a pair with the highest, about 99.0 and 99.2 percent. (Their average reflectance, 99.1, will be close enough for calculation purposes.) These mirrors will result in an FSR of around 250 MHz (for the confocal condition) and theoretical finesse (with respect to the confocal condition) of π*sqrt(0.991)/(1-0.991)/2=174 and a theoretical resolvance of about 1.41 MHz - sufficient to view the split modes of any HP/Agilent two-frequency metrology laser.

    I dug up a length of extruded aluminum stock to serve as a rail. The two New Focus mounts were attached directly to it with the relevant surfaces of the mirrors spaced exactly 30 cm apart. The adjustment screws provide approximately a 10 mm total range in distance to allow for a normal tolerance in RoC. My determination of RoC is not very precise, based on the location of the focal point of a collimated HeNe beam reflected from the mirror. So I'm assuming that it is a nice round number of cm! The final determination would be made by adjusting the SFPI for the mode degenerate condition. Should the RoC turn out to be in error by more than 10 mm, the hole by which one of the mounts is attached could easily be elongated or moved. However, I believe the RoC is very close to 30 cm. :)

    The parts went together easily with the two mounts positioned such that the distance between the surfaces of the mirrors was 30 cm. One mount is secured in an elongated hole just in case. :) The 99.2% mirror was glued to a $1 PZT beeper from Digikey and the 99% mirror was sandwiched between a pair of small hard drive platter clamping plates so it could easily be swapped with something else if needed. All the knobs were removed from the New Focus mounts so that a hex driver is needed for adjustment, making inadvertent screwups less likely. (The knobs can be easily put back on, another benefit of the New Focus mounts, but that will probably never be needed.)

    Please see: Photos of Sam's High Resolution Scanning Fabry-Perot Interferometer 1. The frame is a recycled chassis rail. The New Focus mirror mounts are ideal for this application since they have three adjustment screws so that the cavity length can be fine tuned. They also have four 8-32 tapped holes conveniently positioned to hold stuff. :) The front mirror (lower left photo) is gently clamped between a pair of metal plates with a nylon washer for cushioning, and then the entire assembly is clamped to the New Focus mount with 4 screws. The back mirror (lower middle photo) is glued to the PZT beeper, which is clamped to the New Focus mount with 4 Nylon screws over a plastic sheet to insulate it from the chassis. The insulation makes it possible to drive the PZT with my SGSF1 ramp generator, which can be configured for a 50 V p-p signal from a pair of opposite polarity outputs. (As it turned out, this was not necessary as even the 15 or 20 V p-p available from a function generator was more than enough to span several FSRs.) The photodiode has an approximate active area of 5x8 mm (because that's what I had available) plugs into a pair of socket pins pressed into a black plastic sheet (lower right photo). A 10K ohm load resistor provides appropriate termination as long as the maximum transmitted optical power is less than about 0.1 mW. For higher power lasers, either optical filters can be added to drop the power, or a proper transimpedance (zero voltage offset input) amplifier can be used (one of these is part of SGSF1). Filters have the added benefit of reducing back-reflections into the laser, which most lasers do not like very much. Though not as much of a problem with a confocal SFPI as with a plane-plane SFPI, they are still possible. To minimize stray light into the photodiode, a cylindrical shield (removed for the photo) about 2 inches in length normally surrounds the back mirror/PZT assembly. The feet (2 at the front and 1 at the rear) are 4-40 machine screws turned down to a point allowing for fine alignment of the overall SFPI with respect to the laser.

    Of all the SFPIs I've experimented with (both home-built and commercial), there is no doubt that this one is by far the easiest to align. Practically just placing it in the general vicinity of a laser results in modes on the scope. :) The nice New Focus mirror mounts help but their contribution is very minor. Fundamentally, there is a wide range of alignment - both of the mirrors and of the SFPI with respect to the laser - over which a usable signal with decent amplitude and resolution is produced. In fact, there was a credible mode display the first time it was switched on. The only alignment done prior to that was to set up the laser so its beam went approximately down the axis of the SFPI, and then adjusting the mirrors to get a clump of reflected spots on both mirrors. With most optical systems "long" and "easy alignment" are oxymorons. Not so with this configuration, a benefit of the stable confocal singularity and the ease with which the incoming beam can be aligned with the SFPI mirrors. After some minor touch-up of the mirror alignment and distance, the performance is approaching expectations. There's no doubt that it is running at the confocal distance (or very close to it). The display is quite stable with only the two modes of the 05-LHR-911 laser present, changing during mode sweep in the normal manner. Of course with an FSR of only 250 MHz and a longitudinal mode spacing of 883 MHz for the 05-LHR-911, the peaks don't have the normal spacing. It is in fact 883 MHz modulo 250 MHz, so they are actually located (relative to one of the modes) at 0 MHz, 133 MHz (883-3*250 MHz), 250 MHz, 383 MHz, etc. However, their amplitudes vary exactly as they would with a larger FSR, but they move across the screen really quickly due to the FSR (250 MHz) being much smaller than the neon gain bandwidth (~1.6 GHz). The peaks are clean and symmetric, there are no smaller ghost modes, none of the random variations in mode heights that are usually present when the distance is way off, or the peculiar slow mode height variations not correlated with laser modes seen with the non-confocal barcode scanner tube SFPI described above. It's about as close to a textbook display as possible without being in a textbook. :) The finesse may still be a bit lower than what calculations predict - perhaps 100 instead of 174 (with respect to the confocal FSR of 250 MHz). But even with a finesse of 100, it should be possible to resolve the split modes of an HP/Agilent 5517D or 5517E, and probably a 5517C. But there's still room for improvement and fine tuning of alignment and cavity length help somewhat. A long focus positive lens didn't have any dramatic effect one way or the other, though while fiddling with it (stuck on a "third hand") I stumbled on the condition of perfect alignment where the FSR doubles - every other set of modes vanishes and the remaining modes double in height, with the effective finesse then becoming 200 with respect to the 500 MHz FSR. But that was probably a coincidence as it occurred again later without the lens.

    What I hadn't realized initially was that the sensitivity of the display with respect to offset from the confocal cavity length is much lower for a longer SFPI, with more than one turn of the 80-pitch screws being needed to produce a noticeable change in the display. Having just fought with the 30 GHz Coherent SFPI's 5 mm cavity where 1 or 2 degrees of rotation was significant (and difficult to do consistently), this was most welcome. The only annoyance is the frame. Aluminum is not exactly the most thermally stable structural material. But covering the SFPI with a cardboard box helps. :-)

    The next test was to try a Hewlett Packard 5517C, with a split frequency of around 2.4 MHz. (I had performed my "soup can mod" to bring the split frequency down to near the lower end of the HP-5517C range, 2.4 MHz.) The laser was set up on an adjuatable platform with an HP beam expander oriented backwards to reduce the beam diameter to around 1 mm. And sure enough, once everything was arranged on appropriate high tech wood blocks :) to get their height approximately equal within the range of the adjustments of the laser platform and SFPI, it was immediately obvious that when the 5517C was locked, there was only a single peak per FSR since it is single longitudinal mode, but that it was jagged at the top, not a normal mode. (While the laser was warming up, two clean modes could be seen over part of the mode sweep cycle and the jagged one being present during a small portion, mostly by itself.) With careful adjustment of alignment and some fine tuning of cavity length, the existence of the two Zeeman modes was clearly visible. The peaks weren't distinct and totally separate, but a 10 to 20 percent dip could clearly be seen between them. Based on a Matlab simulation, this indicates that the finesse is probably quite close to 100 (relative to the 250 MHz confocal FSR). Not surprisingly, the resolution is often, but not always, highest just about when the optimal alignment is achieved such that the FSR doubles and every other mode disappears (or at least gets much smaller). Interestingly, the actual resolution of the larger and smaller modes are not generally the same (even accounting for their difference in height). The larger peak usually has a more pronounced dip but not always. More below. It was fairly easy to obtain a finesse of about 125 with a dip of around 25 percent. I also tried the laser without the beam reducer (its beam is only 3 mm in diameter), but the results were somewhat worse. Rather than include all the boring details, suffice it to say that in the end (or at least what passes for the end so far), the beam reducer at its narrowest setting along with careful alignment of the laser, beam reducer, and SFPI, as well as positioning the SFPI closer to the laser, and last but not least - cleaning the mirrors - produced the best results with a finesse of around 160 and a dip of around 45 percent. This is very close to the theoretical finesse of 174 shown in Simulation of Sam's High Resolution SFPI 1 Display of HP-5517C Modes which includes plots of more than a full FSR (400 MHz range) and a zoom-in of the Zeeman-split peak (20 MHz range. The plots for the measured finesse of 160 are barely distinguishable from the these except that the dip is about 5 percent less deep. The real-time display looks very similar to the simulation except that every other peak is smaller as noted above. However, it's just much easier to capture the results of the simulation than to photograph a jittery expanded scope trace! Better cleaning would probably recover the missing finesse. The mirrors must collect dust even being vertical surfaces because I know the finesse had been declining over the last few days. I need to make a cover. :)

    Recall how I said this SFPI was easy to align? Well, that was to get some sort of display. To actually approach the theoretical finesse required significant effort and time. And it's still not clear what combination of conditions actually results in the best finesse, but it always occurs near to point of perfect alignment but not precisely at it. One set of peaks is about 50 percent larger than the other set. The larger peaks have the high finesse while the smaller peaks may be half of that or even less even with the cavity length set optimally for the confocal condition. But the finesse is actually lower at perfect alignment when one set of peaks disappears. My speculation is that this would be the optimal condition if the cavity were perfectly mode-matched to the incoming laser. The finesse of that larger peak also seems marginally higher if the cavity is a bit short compared to the confocal length by somewhere around 0.5 mm. Getting all the way to the finesse of 175 predicted by theory (at which point the dip should extend down to approximately half the peak) should still be possible but may require sacrifices to the Gods of Laser Instruments. :) Or, as noted, it may simply be a matter of cleaner mirrors, which would still require those sacrifices given how much I detest cleaning mirrors. But a few nice crunchy dead HeNe laser tubes would probably be acceptable sacrifices. :) However, there's little point in spending yet more time removing every last molecule of contamination from the mirrors unless the SFPI is covered to keep them clean! Of course, it's also possible that my measurements of the mirror reflectances weren't accurate, as hard as this may be to believe. :) But if the reflectance of both mirrors were 0.99 or one was 0.989 and the other was 0.91, then the theoretical finesse is only around 149 - so it's already way past that!

    The Zeeman-split modes were also not quite equal in amplitude, but that's in the laser and due to the modified control PCB in this 5517C. Unlike the standard one, I added a mode balance pot which was not set correctly. It was interesting to watch the relative mode amplitudes change as that pot was adjusted. The relative split mode heights also vary slightly, which may be due to back-reflections even through the ND filter. And, this high mileage laser had significant amplitude and/or frequency ripple due either to a defective HeNe laser power supply or plasma oscillations which were showing up in the display and confusing the interpretation. These were eliminated by running on a different HeNe laser power supply at a slightly higher tube current. I'll have to fix that. :) For more on HP/Agilent metrology lasers, see the section: Hewlett-Packard/Agilent Stabilized HeNe Lasers.

    And it turns out that this home-built SFPI actually has an even better theoretical resolution than the fancy long Coherent SFPI that I have wanted. The Coherent SFPI has an FSR of 300 MHz with a finesse of 200 for a resolution of 1.5 MHz (compared to 1.41 MHz for mine). Of course it does have the advantage of using a proper sealed low expansion cavity with a nice adjustable mount. But then there is the small difference in cost. :)

    Next up: An SFPI with several times the resolution! This wouldn't be as spectacular as the ultra-high resolution SFPI described below, but would be along the lines of something you or I could build with a bit of lucky scrounging or real money. It would have the following specifications:

    All that would be required are a pair of high quality mirrors with an RoC of 50 cm and a reflectance of 99.7 percent. An actual plot of the two modes of an HP-5517A laser (1.65 MHz separation) made with such an instrument can be found in High Resolution Scanning Fabry-Perot Interferometer Display of HP-5517A Modes 1. However, this was not made by me, as suitable mirrors have yet to materialize.

    Scanning Fabry-Perot Interferometer Head Frame using Commercial Parts

    Building these things from scratch can be quite rewarding, but it does become somewhat boring after the 15th or 20th. :) For around $100 in readily available parts, many of the headaches can be eliminated. The following assumes the use of "30 mm Cage" parts from Thorlabs. The parts list for a short RoC (e.g., 43 mm/1.7 GHz) SFPI is then:

     Qty   Part #   Description
    ---------------------------------------------------------------------------
      1    CP02     SM1-Threaded 30 mm Cage Plate, 0.35", 2 Retaining Rings
      1    CP02T    SM1-Threaded 30 mm Cage Plate, 0.50", 2 Retaining Rings
      1    SM1AD8   Externally SM1-Threaded Adapter for 8 mm Optic
      1    CP11     SM05-Threaded 30 mm Cage Plate, 0.35", Two Retaining Rings
      1    ER4-P4   Cage Assembly Rod, 4" Long, 6 mm, 4 Pack
    

    The CP02T and the SM1AD8 is used to mount the front mirror while the CP02 is used to mount the PZT on one side and the photodiode on the other. The CP11 is for mounting a focusing lens. The 4 cage rods allow for quick and easy coarse adjustment of cavity and lens spacing. Rotation of the SM1AD8 is then used for fine cavity spacing, locking in place with one of the retaining rings. This isn't quite "plug and play" as some gluing or drilling and tapping is required, as well as a few other odds and ends to complete the SFPI assembly as shown in SFPI Frame Using Thorlabs Cage Parts and Most Other Components. But the effort will still be an order of magnitude less than building one of these from salvaged harddrives and scrap aluminum. :) Depending on the rod length (4 inches in this case), almost any confocal cavity can be accommodated with the appropriate RoC mirrors and focusing lens.

    To simplify initial alignment, a KC1 cage-compatible 3-screw kinematic mount can be substituted for the CP02, but this does roughly double the cost and increases the size and bulk. The prototype of such a rig is shown in SFPI Head Using Thorlabs Cage Parts with Adjustable Front Mirror. It's kind of a waste though since once the SFPI is set up, the adjustable mount is of little value and possibly a liability as its settings can change. However, being able to precisely align the two mirrors so their optical axes coincide may result in slightly better performance. An alternative to a KC1 would be to mount the PZT on a plate with 3 springs or split washers so that its alignment could be fine tuned. Added cost: $0.00.

    Having said all that, the simpler SFPI head design really does work just fine. The completed unit mounted on a 3-screw adjustable platform is shown in Completed SFPI Head Using Thorlabs Cage Parts. Coarse mirror spacing is set by moving the CP02T with the front mirror. This can be done by first setting its position based on the known RoCs of the mirrors, and then by maximizing the envelope of the SFPI display of a well behaved laser like an 05-LHR-151. Fine tuning is then performed by rotating the SM1AD8 adapter holding the front mirror using an improvised spanner made from a large paper clip. :) A tapped 4-40 hole added on the side of the CP02T for a soft-tipped set-screw that is snug but still allows movement would enable smooth adjustment and lock the setting in place. An opaque cover should be added to prevent ambient light from getting to the photodiode and protect the mirrors from dust and other contamination. The 8-32 tapped hole in the CP02T can be used to attach the SFPI head to an adjustable platform or multi-axis mount as in the setup, above. A photo of an actual scan from this unit is shown in SFPI Mode Display of Melles Griot 05-LHR-151.

    Both of these are quite stable and worth every penny. ;-) The only problem seems to be a ringing in the $1 PZT at the beginning of the scan due to the abrupt change in direction, resulting in distortion of the display over the first 10 percent of the scan when running at 100 Hz, with a proportionally lower percentage at lower scan rates. This is an issue with all of the SFPIs using the thin beeper PZTs. Using a similar PZT but with a higher resonance frequency, a modified waveform that starts more gradually, or applying damping material to the PZT would help. Partially coating the back of the PZT with hardware store acrylic caulk did in fact reduce the duration, though a lossier material would probably be better. Of course, in the grand scheme of things, a bit of distortion over less than 10 percent of the scan is something that is probably tolerable. If not, simply delay the scope trigger by 1 or 2 ms. ;-)

    Sam's Scanning Fabry-Perot Interferometer Driver 1 (SG-SF1)

    I have also now designed a stripped down function generator especially for driving the PZT of these SFPIs. See Sam's Scanning Fabry-Perot Interferometer Driver 1. This unit generates a variable frequency triangle (approximately 5 to 200 Hz) or sawtooth (approximately 10 to 400 Hz) with a full range adjustable offset. The output amplitude may be set from 0 to to over 25 V p-p, with both non-inverted and inverted outputs. Where neither side of the PZT is grounded, it may be connected between the two outputs to provide a voltage range of up to more than 50 V p-p. The output may also be set to DC and adjusted over the full range using the offset control for initial setup of the SFPI. The sawtooth has a slew-rate limited falling edge so there is no risk of damage to the PZT or excessive ringing at the start of the scan. The adjustable amplitude implements sweep expansion to enable examination of the fine detail of the laser spectrum. A photodiode preamp is built in to SG-SF1 completely eliminates the need for anything beyond an oscilloscope and +/-15 VDC power supply.

    Using better op-amps than the jelly bean LM358s might increase the maximum output voltage range slightly but at these frequencies, won't make much difference in any other respect. Of course, it would be trivial to modify this circuit for a different frequency or voltage range. But, as drawn, it will cover the needs of most SFPIs using "drum head" type PZTs, as well as the Thorlabs SA200 and SA210.

    Blank PCBs are now available for the combined PZT driver and photodiode preamp. The PCB for SGSF1 is just under 2x2.5 inches as shown in Photo of Sam's Scanning Fabry-Perot Driver 1. Power requirements are regulated +/-15 VDC at 50 mA. A suitable dual DC power supply would be trivial to construct using a wall adapter putting out 14 to 16 VAC, a pair of diodes, a pair of 1,000 uF, 25 V filter capacitors, and 7815 and 7915 (or similar) IC regulators. SGSF1 may be used as shown, or built into a project box with front panel controls in place of the switch(es) and trimpots. A selector switch and fixed resistors may be substituted for the pot to provide calibrated expansion factors (e.g., 1X, 2X, 5X, etc.). And the offset control could be a 10 turn pot. For more info, please go to Sam's Classified Page or contact me via the Sci.Electronics.Repair FAQ Email Links Page.

    Dual Polarization Detector for Scanning Fabry-Perot Interferometer

    For displaying the longitudinal modes of some types of lasers, it is desirable to be able to view the polarized modes separately. This would probably be most useful for common random polarized HeNe lasers and Zeeman-split HeNe lasers, though it could also be used with other types of lasers that are not polarized. (These's no benefit with linearly polarized lasers.)

    All that's required is to replace the normal sensor with a polarizing beam-splitter and a pair of photodiodes (and possibly separate pre-amps). The polarizing axes of the detector will need to be oriented to be the same as those of the laser, but most SPFIs allow for the sensor to be rotated in its mount. For axial Zeeman lasers producing circularly polarized outputs, a Quarter-Wave Plate (QWP) would be required to convert to linear polarization. A QWP is usually part of a commercial Zeeman laser tube assembly. However, if you're rolling your own, then an external QWP will need to be added.

    This is most dramatic with a fancy digital scope so the polarized modes can be shown in living colors like red and blue in real time. :) However, any old 2 channel scope will work. Or, the SFPI can be swept at low speed if necessary with the PD outputs fed to two channels of a PC data acquisition system. Where the polarizing beam-splitter is wavelength sensitive, a separate unit may be needed for each mirror set/wavelength range of interest.

    As an example, for the SP-470, the normal sensor assembly can be replaced with a short piece of 1/2" PVC pipe filed down to fit into the SPFI housing into which a 4 or 5 mm PBS cube and inexpensive photodiodes are mounted. The PD signals can be fed directly to two scope channels with only 10K load resistors, or through suitable preamps depending on the laser power involved. The one I built worked reasonably well, but there was between 5 and 10 percent crosstalk, due to a combination of the PBS cube not having perfect separation, not being optimally oriented, or reflections off the faces of each photodiode getting into the other channel due to their close proximity. In fact, even the best polarizing beam-splitter cubes may have substantial residual P-polarization in the reflected S-polarized output, though the transmitted P-polarization is generally quite pure. (A non-polarizing beam-splitter with pieces of sheet polarizer mounted in front of each photodiode might actually have better separation.) So, rather than ripping the PD assembly apart and then finding that the crosstalk was still present even after extensive fiddling and frustration, I built a two stage preamp which includes a means to suppress it by subtracting a small (adjustable) amount of channel 1 from channel 2 and vice-versa. The trans-impedance (first) stage needed to be a high speed op-amp, a MC33077 because it was handy. With the original jelly-bean LM358, there was serious overshoot and ringing at all but the lowest gain settings. And with small capacitors (e.g., 10 pF) across the feedback resistor, the waveform still had artifacts if the amplitude was more than a few hundred mV. While a larger (100 pF) capacitor resulted in a clean output, the frequency response was reduced to where the SFPI resolution would be compromised at the normal scan rate. The second (subtracting) unity gain stage was perfectly happy with an LM358. The crosstalk cancellation works quite well when SFPI alignment is close to optimal. Otherwise, there may be a small phase shift between two channels making perfect cancellation impossible.

    A similar crosstalk cancellation technique may be applied elsewhere to allow for the use of less expensive polarizing optics or where alignment cannot be easily optimized. The cross-gain for each channel should be set up with a pure linearly polarized beam aligned with its axis while nulling the output of the other axis assure optimum orthogonality.

    Dual Polarization SPFI Display of HeNe Laser with Higher Order Spatial Modes shows a series of screen shots from my antique Tektronix 465B analog scope (sorry, no color). All the peaks with a height of more than about one screen division are the normal longitudinal modes and they behave, well, normally. However, the smaller peaks appear to belong to a weak non-TEM00 higher order spatial mode. This isn't necessarily a laser where the dual mode display provides a critical benefit, but being able to easily see how the rogue spatial modes behave with respect to polarization is somewhat illuminating. Sorry, pun intended. :-) While the dual polarization display doesn't help to reveal the existence of the higher order spatial modes, it does shed light on how they associate themselves with normal modes, or actually run away from them. Originally (with the normal SFPI), I had assumed that the little blips closest to a larger normal mode were of the same polarization, not orthogonal to them, which is the actual situation.

    And I couldn't resist creating a complete design including the Sam's Dual Polarization Photodiode Preamp 1 Schematic and Sam's Dual Polarization Photodiode Preamp Front Panel Layout. No PCB layout yet, sorry. :) This same design can also be used for general data monitoring or capture using a data acquisition system. For use with an SFPI, power would most conveniently be provided by a 14 to 16 VAC wall adapter feeding the AUX PWR connector, J3. But for other applications, the the power pins on the output connector can be used. A PCB may be available in the future.

    SG-DP1 Specifications

    These assume the component values shown in the schematic:

    Most common random polarized HeNe lasers produce only two sets of orthogonally polarized longitudinal modes, generally fixed with respect to the tube orientation. However, this is not always true and there can be modes at arbitrary angles. One example is of a Zeeman-split laser that was intended to be a clone of an HP-5517D. However, it used an HeNe laser tube that was too long, resulting in a pair of low level rogue longitudinal modes in addition to the normal Zeeman modes. In that case, the resulting modes (after the waveplates) were linearly or elliptically polarized and orthogonal to each-other, but oriented at 30 degrees with respect to the primary modes. (The normal linearly polarized output modes result from circularly polarized tube modes, but for the rogue modes to be off-axis may mean that they were elliptically polarized tube modes.) The dual polarized mode SFPI would have easily identified those since they would show up on both channels and the detector assembly could then be rotated to maximize channel separation. Where pure linearly polarized modes could not be achieved, the degree of elliptical polarization and orientation could be easily determined. Hmmmm, the detector assembly will need to be mounted on a calibrated rotation stage. ;-)

    Note that the success of the dual polarization detector assumes that the SFPI itself is polarization insensitive. An SFPI has no polarizing components, but thin film mirror coatings may have a very slight polarization anisotropy even at normal incidence. This would not be a problem for most applications, but where hundreds or thousands of reflections are involved as in an interferometer, even a small polarization anisotropy in one of both mirrors could add up and result in a polarization preference or other peculiar behavior. So, it's worth checking the SFPI with a linearly polarized input and confirming that the output polarization remains pure and tracks the input as its orientation is changed. I don't know if this is ever an issue with commercial SFPIs though since a polarization asymmetry would also result in a response that depends on the laser polarization or SFPI orientation even with the normal polarization insensitive detector - something that should not generally be present.

    An alternative implementation that would have no (new) issues with mirrors would be to use the normal detector with a switchable polarization rotator in front of the SFPI (like the LCD device in HP/Agilent lasers but possibly faster). The polarization would be oriented horizontal and vertical on alternate scans with appropriate scope triggering.

    While I assume that every patent for SFPI technology has something along these lines of: "Although specifically described with respect to a single detector, anyone skilled in the Art will recognize that this approach can be used with multiple characteristics of laser light such polarization or wavelength each with its own detector.". Nonetheless, should a major company read this section and decide to market a product, I DO expect royalties. OK, right, pigs will need to fly first on both counts. ;-)

    Sam's Proposed Stand-Alone Scanning Fabry-Perot Interferometer Controller

    The basic setup of the SFPI hasn't changed in any significant way in over 40 years: A ramp generator, photodiode preamp, and (user supplied) oscilloscope. While this is certainly adequate, what about alternatives made possible by modern low cost digital technology.

    One possibility would entail converting to a (PC or Mac) USB interface with both the user controls and display being on the computer. However, I would caution against rushing full speed into such a project as it's extremely difficult to implement a software driven user interface on a conventional personal computer that approaches the utility and convenience of real knobs! But this would open up the possibility of providing additional measurement capabilities as well as more advanced functions like closed-loop etalon control. And the full color display would add a touch of class. :)

    Or, how about fully custom hardware? While I rather doubt that the development of such a device could ultimately be justified based on expected profits due to a relatively limited market, it never hurts to dream. See Sam's Proposed Microprocessor-Based Stand-Alone Scanning Fabry-Perot Interferometer Controller. This would use a high resolution color LCD display with 10 soft keys and 4 soft knobs for all user interaction. The objective with the GUI would be to have the most commonly used functions expected in a conventional SFPI always available without descending into endless menus. In addition, capabilities not found in any commercial SFPI like sweep magnification (in addition to sweep expansion) can easily be provided, which zooms in on only a variable portion of the central area of the sweep. And when not being used for an SFPI, your kids will love the color Etch-A-Sketch™ app, included free for a limited time only. ;-)

    A university student has sent me their Senior Project proposal for a microcontroller-based SFPI system based on a commercial confocal SFPI head. Stay tuned for the exciting developments.

    Scientific Connections has apparently developed (or at least developed the specifications for) an advanced SFPI called EagleEye™ which interfaces via USB and in addition to the normal SFPI functions, can compute linewidth well below the resolution limit determined by the finesse and FSR of the confocal cavity. This is done by controlling the drive voltage while measuring the photodetector response to locate the FWHM points on a mode. They claim a raw resolvance of 1 MHz but a computed linewidth limit of 20 kHz. This may all be vaporware though - I have asked about it via their Website.

    In fact, EXFO/Burleigh had developed a similar concenpt, though like the system above, it may never have been fully commercialized, and is now officially discontinued. The NuView FPS-250 consisted of a USB controller and EXFO NuView Laser Spectral Analysis Software. Together, these would provide control of a conventional SFPI, and display and analysis on a PC.

    Here's another possibility: The "DSO Quad™" is a pocket 4 channel digital color storage oscilloscope which is based on open source hardware and software. Search for "DSO Quad" and you'll get more than you ever wanted to know about it, but the features relevant for an SFPI are 2 analog input channels with a combined sample rate of at least 36 ks/sec and a function generator output that can be set for triangle or sawtooth (among others) from 10 Hz to 1 MHz. A driver for the scan voltage will be required if using an SFPI head requiring high voltage, but perhaps not if one of mine or Thorlabs. Add a dual channel photodiode preamp and this could represent a nice compact instrument. Unfortunately, I found that the triangle output of the DSO-Quad™ is quite pathetic with a dozen or so steps :( at best, so it would need to provide a trigger for an external ramp generator or use external trigger. And mastering the rather strange user interface has so far been a challenge for me at lest. But I haven't given up yet as I believe the DSO-Quad™ still has potential.

    Sam's Proposed Tablet PC-Based Scanning Fabry-Perot Laser Spectrum Analyzer

    A tablet PC would be an ideal platform to implement the ultimate SFPI controller and display without the need to design special purpose hardware other than for the ramp driver, photodiode preamp, and their USB interface. An optimized touch-screen-based GUI could enable all functions to be carried out in a natural way while eliminating the costs (as well as retro appearance) of hardware knobs and buttons! And most of the software infrastructure will already be present allowing for natural finger gestures to perform most common functions. For example, spread fingers to control expansion or magnification; touch and drag to change position (centering), or pull on selected mode peak to change preamp gain or use soft arrows. The tablet PC screen layout could look similar to what's in the diagram above, but with all interaction via the touch-screen and thus no need for physical knobs and buttons. What what a great excuse to buy a tablet PC (or better yet, have your company buy one for you!). Who could resist? :)

    Here are proposed specifications for a complete system, tentetively called the LSA100, consisting of an SFPI head with built-in USB interface attached to a touch screen-based tablet computer as depicted very roughly in Sam's Proposed Tablet PC-Based Scanning Fabry-Perot Laser Spectrum Analyzer. No other equipment would be necessary to perform display and analysis of laser mode structure, with export of the resulting data to other apps. The following lists the common specifications for all versions not dependent on FSR or mirror wavelengths.

    The $99 Scanning Fabry-Perot Interferometer

    While my $1 SFPI can be made to work, the choice in the types of mirrors that are typically available surplus or from salvage are severely limited. Alignment becomes extremely critical and an aperture is needed to suppress non-TEM00 modes. In addition, reflections back to the laser under test may be destabilizing. Though this is probably not a major issue with typical HeNe lasers or green DPSS lasers with an IR-blocking filter in their output, It could be significant for stabilized HeNe lasers and IR DPSS and other non-frequency converted lasers.)

    I lucked out for my $2 SFPI in just happening to have short radius mirrors that could be pressed into service, but most people wouldn't have this option.

    For my $3 SFPI, I do have mirrors available but there are really only useful for red HeNe lasers, for which they are nearly ideal.

    Being able to specify the mirror radius of curvature, wavelength, and reflectance, would greatly expand the possibilities and still result in an instrument for under $100. That's really not too bad considering it should have almost the same performance as a $9,999 commercial SFPI.

    (From: Christoph Bollig (laserpower@gmx.net).)

    Just some comments on the SFPI resonator options: The confocal configuration has the big advantage that it can be used at an angle or an offset! Most single-frequency lasers outputting at the fundamental (not frequency converted) don't like it if they get reflections straight back, and especially when those reflections are from a high reflectivity mirror and well aligned to go back into the laser. And that's exactly what you need to do with a plane-plane interferometer or even with most other non-confocal ones.

    With the confocal interferometer, the best choice would probably be to come in along the optical axis but with a slight offset. The back-reflection will then be at an angle. Since such an arrangement will need two round trips to reproduce, the second mirror can be HR and the "transmission" will be through the same mirror as the incoming beam, just at a different angle as shown in Confocal Scanning Fabry-Perot Interferometer. As you can see, there are no reflections back into the laser.

    Another advantage is that since the second mirror is high reflector, no hole is needed in the PZT. :)

    We have considered different options for the mirrors for use with near-IR lasers, but one of the more likely scenarios is to use a 50 mm RoC output coupler from CASIX with either 98 or 94 percent reflectivity (NDO0205, $50). see CASIX Nd Laser Optics. These are also available in other curvatures down to 25 mm). For the high-reflector on the PZT one could use one of the CASIX standard HR mirrors from the DPSS series (quite a few from CASIX Diode Pump Laser Optics Kits would do. For example, the DPO1301 or DPO1302 ($45) (or the green laser output coupler from Roithner, also 50 mm radius). Or the DPO1303 (HR at both 1,064 nm and 532 nm) which would then be useful for green DPSS lasers as well.

    Ultra-High Resolution Scanning Fabry Perot Interferometer

    John Barry at Yale University builds ultra-high resolution SFPIs for his work in laser cooling of molecules. (See, for example: Laser Makes Molecules Super-Cool.) If there's a scientific instrument for laser nerds to drool over, this would be a good candidate as it puts everything else described here to shame:

    (Portions from: John Barry.)

    The cavity is a simple confocal design consisting of a quartz tube, two 1018 steel end-caps, a small ring PZT, and two mirrors (RoC of 500 mm, and reflectivity of ~99.983% at 632.8 nm) based on the specifications from the manufacturer at Layertec Optics HR Mirror (Low Loss, >99.97%, 620-680 nm). While the specifications list 99.97% reflectivity, this is at the wavelength extremes (620 nm and 680 nm) and is lowest there. At 633 nm, the plot shows it to be at least 99.983%, and since they are conservative, 99.99% or even higher is not out of the question. One mirror is glued to a threaded steel piece for rough adjustment of the cavity length. The other mirror sits behind the PZT for fine cavity length adjustment. See Photo of Ultra-High Resolution Scanning Fabry-Perot Interferometer. The closeups at the bottom show the PZT with its mirror hidden inside on the left and other mirror glued to the steel piece on the right.

    The cavity is designed to be athermal. As temperature increases, the quartz expands and lengthens the cavity mirror spacing. Simultaneously the 1018 steel expands relative to the ends of the quartz where it is attached, thereby acting to decrease the mirror spacing. By balancing these two effects and accounting for smaller corrections (expansion of mirrors and PZT), the distance of the cavity can be made to be largely independent of temperature. Here are the specifications:

    The theoretical values are based on a mirror reflectance of 99.983% obtained from the plot on the Layertec Web page, above. Since this is probably a lower bound, potential performance may be much higher.

    Now this isn't exactly the sort of SFPI head you attach to a pan-tilt mount on a table top. It's normally located in a temperature-controlled environment inside a vacuum chamber with feedback to eliminate outside sources of error.

    With this spectacular finesse/resolvance, a plot of the Zeeman-split modes of an HP-5517A laser, separated by 1.65 MHz, looks like the SFPI display of a normal HeNe two mode laser where the modes are several hundred MHz apart! See Ultra-High Resolution Scanning Fabry-Perot Interferometer Display of HP-5517A Laser Modes. The two Zeeman-split longitudinal modes (F1 and F2) are 1.65 MHz apart with the span from 0 to 0.01 seconds being about 3.8 MHz (out of the 150 MHz FSR). The measured performance (based on careful analysis of this plot - counting pixels on a blown up version in MS Paint!) are as follows:

    The two values correspond to the widths of the left and right peaks on the plot. There may be a 5 percent uncertainty. The smaller peaks are most likely higher order transverse modes - evidently the length of the cavity is not exactly confocal. The reason the spacing of those higher modes is not constant relative to the two lowest order modes may be due to the cavity vibrating. This is one reason this 150 MHz cavity is no longer used. The newer design is both shorter and uses quartz that is twice as thick.

    (From: Sam.)

    Here, we have a resolvance described in terms of kHz when most other SFPIs can't even achieve the equivalent number of MHz! While I'm fighting to get a resolvance of a few MHz, this one does more than 100 times better. Of course, it might cost 10,000 times as much to build and definitely lacks something in the portability department! :-)

    So, I emailed John and asked about borrying it - after all he did say they don't use it there anymore! Within an hour, I received the tracking number! As they say, "be careful what you wish for....". This will now require real work. :) My intention being to see how well it performs with an HP/Agilent 5517B laser in air (I'm not going to put everything in a vacuum the way they do for their experiments).

    The test setup is shown in Ultra-High Resolution SFPI Test Setup:

    The SP-476 and DC power supplies for the laser can be seen behind everything else sitting on foam blocks for vibration isolation.

    I originally thought that the most difficult aspect will be initial alignment of the SFPI to the 5517 laser to get any detectable signal. A common SFPI has a finesse measured in the low hundreds with mirror reflectivities of around 99 percent. Enough laser light gets through the mirror (around 1 percent) and there is enough scatter from imperfect mirrors, that the position of the beam can be seen at both ends to provide a rough guide to alignment. With 99.98% reflectivity, it might be possible to see the position of the beam on the input mirror, but *nothing* worth writing home about will get through the second mirror unless the FP cavity is in resonance. In fact, nothing could be seen on the first mirror either.

    However, initial alignment turned out to be relatively easy by simply setting the X and Y position of both ends of the SFPI tube to approximately line up with the input beam. At the front, this is done trivially using the beam itself. For the back, the photodetector is centered on the beam and locked in place, and it then acts as the reference. The, a small amount of searching would produce recognizable blips.

    Originally, the SFPI quartz tube was almost entirely exposed using short pieces of head cylinder. Ahhh, a naked SFPI! :-) The result was that the room had to be completely dark or else the ambient light (and 120 Hz hum from the fluorescent lamps) overwhelmed the photodetector. Since it wasn't possible to see any scatter from the mirrors anyhow, fully enclosing the quartz tube inside a HeNe head cylinder didn't sacrifice anything, and this totally eliminated the problem.

    But the display was bouncing around due to vibrations. Both the DC power supplies for the Agilent laser and the SP-476 (visible in the photo, above) produce line frequency vibration. Since I don't have a vibration-damped optical table with overhead racks for equipment, I put both of the trouble-makers on soft foam blocks and this seemed to calm things down considerably.

    The next issue was that the display had lumps with multiple peaks. This was due to the distance between the mirrors not satisfying the confocal condition of the RoC of 50 cm. The input mirror is mounted on a large threaded cylinder. Some careful adjustment in increments of 360 degrees (since the mirror isn't quite perfectly centered or perpendicular to the optical axis) tuned the distance to be quite close. However, due to the backlash and slop in the machining of the threaded cylinder and the outer cylinder into which it mates, fine adjustments were, to put it mildly, hit or miss. Originally, I was simply moving it by hand but even after building a tool out of a pill bottle, the consistency wasn't much better. And with the high finesse, the confocal distance is extremely critical. I'm contemplating adding some means of apply permanent pressure to the threaded cylinder to both eliminate the free play and keep it from changing position.

    But then the mode spacing appeared to be around 10 MHz, not the 2.2 MHz expected from the laser. At first I thought: "Wow, that finesse isn't too bad...". The problem turned out to be back reflections into the laser forcing it to be tuned offset from where it is supposed to be. I'm not exactly sure how it comes up with a clean spacing of 10 MHz, unless it has something to do with aliasing of multiple longitudinal modes with the 150 MHz FSR. But by adding a pair of ND filters, the Zeeman-split mode spacing now is correct for the 5517B laser based on the ratio of the distance between the twin peaks and the FSR (2.3 MHz to 150 MHz).

    Another problem was that the (temporal) frequency response was not high enough for these narrow peaks to be displayed accurately when run at a convenient repetition rate with a span of a full FSR or more. At first I thought this was limited by the PD preamp in the SP-476 with a Thorlabs DET110 photodiode. The rep rate*span product then needed to be reduced by a factor of 10 or more to get a decent display. Switching to a Thorlabs PDA55 amplified photodiode helped slightly, but then I realized that the high finesse - or equivalently, Q-factor - of the FP cavity could be even more significant by acting like a low pass filter. The response can be modeled as an RC filter with a time constant equal to the cold cavity decay time (Tcc), how long it takes for light inside the cavity to decay to 1/e of its original value with no outside input. Tcc = [1/(Mirror Transmission) * (Cavity Length)/c)] = [1/(1-0.99983) * 1.67 ns] = 9.8 µs. Then the 3 dB bandwidth is f3dB = 1/(2*π*Tcc) = ~16.24 kHz. Clearly, the scan rate needs to be greatly reduced to get decent resolution with a large span. Only if scanning in a narrow range (as in the plot above) can a relatively high scan rate produce an accurate display. Due to both vibrations and temperature variations conspiring to prevent a stable display, the only way to do this using the oscilloscope without some fancy locking scheme is to trigger on the actual peaks rather than the scan ramp or blanking pulse from the HV driver.

    With a fair amount of fiddling of the confocal spacing and alignment, the finesse is now somewhere around 4,000 (!!) based on the Matlab simulations, not quite up to what should be possible but not too shabby either. See Display and Simulation of Ultra-High Resolution SFPI. The multiple peaks in slightly different positions captured while the camera "shutter" was open are probably due to the way the scope was synced and a combination of PZT non-linearity and vibration. It's possible that the response is still limited by the SFPI's temporal bandwidth and slowing down the scan rate would still result in some improvement. The scope time-base is set at roughly 0.4 ms/div which means that the pulse width shown in the photo (about 0.15 divisions) is about 60 µs representing a temporal bandwidth within a factor of 2 of that of the SFPI/SP-476/PDA155 combination. And further testing shows that the resolution does improve slightly at a slower scan rate. There's something rather stramge about a simple physical device whose frequency response is being seriously limited by the speed of light! ;-)

    I doubt the performance now is quite up to what might be possible, but it's probably close. The three areas that might produce some improvement would be (1) setting the confocal distance more precisely, (2) better mode-matching of the input, and (3) putting the entire thing in a vacuum. None of these are easy (especially the last!) or likely to happen so at this point, it's probably as good as it's going to be! :)

    But one relatively simple way to stabilize the display would be to lock the peaks to the scan ramp. Then, a much narrower span and higher repetition rate could be used. This could be done with not much more than three monostables and an op-amp integrator. A retriggerable monostable would be clocked by the PD output to produce a pulse wider than the distance between the pair of peaks (PD Pulse). A second monostable would be triggered by the start of the scan ramp to produce a pulse to position the lock point at the desired location on the scope screen (Delay Pulse), and its trailing edge would then trigger a third monostable to produce a pulse with the same width as the PD Pulse, called the Scan Pulse. Then, if the PD Pulse preceeded the Scan Pulse, the integrator would be incrementally decreased, and if the Scan Pulse preceeded the PD pulse, it would be incrementally increased. The output of the integrator would be added into the PZT voltage. The logical OR of the Delay Pulse and Scan Pulse would act as a gate so that garbage during scan retrace would not confuse the locking scheme. This probably won't handle serious vibration but should compensate for environmental changes in temperature and pressure. Someday, maybe. :-)

    Commercial Scanning Fabry-Perot Interferometers and Drivers

    Most of the following are no longer in production. The only exceptions are some of those from Coherent, and apparently they are being phased out. However, if you're inclined to actually buy an SFPI new, several companies still do offer complete systems and components including Thorlabs and Toptika.

    Spectra-Physics 470 Scanning Fabry-Perot Interferometer

    The SP-470 Scanning Fabry-Perot Interformeter head along with the SP-476 controller provides similar capabilities to my $2 SFPI for only an additional $4,998. :) Actually, I don't know what the selling price was but these are typically $5,000 or more. The SP-470 came in several flavors depending on the wavelength range of the mirror set (450 to 550 nm or 550 to 650 nm) and Free Spectral Range (FSR, 2 GHz or 8 GHz with 20 MHz or 40 MHz resolution). The finesse is 200 for all versions. More info may be found under Vintage Lasers and Accessories Brochures and Manuals at the end of the section for Spectra-Physics in the "High Bandwidth Scanning Interferometer Brochure".

    One that I have is the SP-470-3, 550 to 650 nm with a 2 GHz FSR. This is absolutely ideal for all common visible HeNe lasers including the green HeNe at 543.5 nm. (There was no obvious reduction in resolution at 543.5 nm, though I didn't do any precise measurements. And, even for a 532 nm DPSS laser, the finesse was still at least 50.) When I first acquired this unit, the cavity length was all messed up so I had to set it for the confocal condition. This was done using a low power red (632.8 nm) HeNe laser which has only 2 or 3 longitudinal modes at most. After chasing my tail for quite awhile, I found the sweet spot. The adjustment is by turning the mirror cell at the detector end. Being recessed, a plastic "tool" was needed to get at it without fear of damaging the mirror. It's spring-loaded so should stay put, but there is no way to lock it in place. An external detector (Thorlabs DET-110 was set up beyond the end of the SFPI head, which was on a kinematic pan-tilt mount, and that was clamped down so it would not move relative to the HeNe laser. Once the confocal condition was achieved, it was relatively easy to jog the adjustment one way or the other to fine tune the cavity length. And then it really did work like the diagrams in textbooks, and almost as well as my $2 SFPI. :) OK, it is actually better in certain respects: The solid massive resonator virtually eliminates any drift due to the short term effect of temperature on the cavity length and also results in much reduced sensitivity to vibration.

    In fact, it appears as though the resolution may actually be much better than the 20 MHz listed in the specs.

    For a simple display of the modes of a HeNe laser, this high resolution really doesn't matter and may actually be a distraction. I need to find a laser that will do it justice!

    However, the 2 GHz FSR is too small to display the modes of a Zeeman-split HeNe laser without aliasing. When an axial magnetic field is applied to a HeNe laser tube, the neon gain curve splits into two similar curves, one shifted up and the other shifted down by several hundred MHz. Thus, the effective split gain curve can be much wider than the nominal 1.6 GHz or so of the normal HeNe laser. Only when the alignment between the SFPI and laser is essentially perfect and the FSR doubles is the display unambiguous.

    The destabilizing effect of any back-reflections from the SFPI into the laser is also very evident as random noise superimposed on the mode display. So, either an optical isolator must be used, or the beam aimed at an angle so the none of the reflection of the beam enters the laser aperture.

    The SP-470 allows for interchangeable mirrors and detector, unlike the SP-450 where everything is fixed. There were also optional lenses, polarizers, and apertures that can be screwed into the front of these SFPI heads. The lenses can improve the resolvance under some conditions, but not always. The aperture does generally help and also makes alignment even easier, at the expense of signal level.

    As a practical matter, swapping mirrors still requires precise adjustment of their spacing, so it's not quite as easy as described in the SP brochure. So, it's best to have a separate SFPI head for each wavelength range.

    I've also tested a 470-02, 450 to 550 nm, 8 GHz FSR, ideal for most of the common wavelengths of the argon ion laser as well a the 532 nm DPSS laser.

    The sensor is just a photodiode with a lens to focus the center 3 mm or so of the transmitted beam onto the detector. For a narrow beam like that from a HeNe laser, almost any photodiode will work. However, performance does seem to be better with the lensed photodiode compared to a cheap non-lensed one for large diameter beams, perhaps simply because the capture area of the lensed PD is smaller. I'm not sure exactly what else, if anything, is inside the detector housing beside the PD socket. What I believe to be a genuine SP detector measures 1.5K ohms across the PD socket, but no other components are shown in the SP-476 schematic. And, the 1.5K across the PD results in power line frequency hum, which is particularly annoying on the more sensitive ranges. This is apparently due to ground loops inside the SP-476 as rearranging the wiring inside the supposedly shielded PD preamp section affects it. My home-built detectors using cheap photodiodes mounted inside filed-down 1/2" PCV pipe couplings work fine without the resistor and do not have noticeable hum. (Where the resistor is present, the amplitude of the hum can be reduced by about 75 percent by removing the black ground wire between the PD Input BNC connector and the PD preamp PCB, and rearranging some of the cables. But I wasn't able to get it to go away. I don't believe there is any fault in this SP-476. It's just not built using good analog design practices. Perhaps there's an ECO for later versions. It might also be possible to remove the resistor inside the detector housing, though that appears difficult.)

    Spectra-Physics 450 Scanning Fabry-Perot Interferometer

    The SP-450 Scanning Fabry-Perot Interferometer head along with a suitable controller has similar performance to the SP-470 but is in a smaller package with a single permanently attached cable for PZT drive and photodiode output. It terminates in a strange plug that doesn't mate with the SP-476. There is no mention of that in the SP documentation that I've seen but an adapter cable to BNCs apparently exists. I simply cut off the plug and installed a pair of well marked coaxes for PZT and PD. The pins are as follows:

       Pin   Wire Color   Function
      -----------------------------------
        A      Black      PD Anode
        B       Red       PD Cathode
        C      Bare       Shield/Ground
        D      White      PZT HV+
        E      Green      PZT HV-
    

    PD is simply a silicon photodiode with no other circuitry. Normally, PD Cathode would be grounded with PD Anode being the signal input. The polarity for HV has frequency increasing to the right on the display (PZT shrinks with voltage). Reverse to have wavelength increasing to the right.

    Like the SP-470, the SP-450 is also a confocal design but with a fixed approximately 1 mm hole on the input side. But this makes it very easy to use. Just about any beam that enters the hole roughly on-axis will result in a decent display with at most fine tuning of alignment needed for it to be perfect.

    See Components of SP-450 Scanning Fabry-Perot Interferometer Head. The double convex lens that was glued to the front of the PZT/optics assembly can be seen to the left of it. The photodiode is mounted within a Nylon, just visible behind the white and green wires.

    Access to the internal components is remarkably straightforward. After removing the rear cover and cable (3 itty-bitty set-screws), use a thin tool to carefully pry out the large red O-ring. The entire PZT/optics assembly can then be pulled free. It's only anchored with a similar large red O-ring at the front. There may be a focusing lens glued to the front optics holder. The PZT HV+ (white) wire is fastened with a set-screw at the front whose head is possibly blocked by the focusing lens. It's real easy for this wire to break off since the set-screw tends to mangle the strands. I installed a larger solid wire so the set-screw could be tightened more securely and then soldered to that, with heatshrink for protection over the splice. The PZT HV- (green) wire is secured with a screw into the rear of the PZT/optics assembly.

    The cavity should never need adjustment but if someone before you decided the outer ring at the rear was loose and screwed it down tight, I believe that's what does it. :) (The inner ring secures the mirror.) And while the description of the SP-450 didin't offer the option of swapping mirror sets, it really should be possible.

    Spectra-Physics 476 Scanning Fabry-Perot Interferometer Driver

    While a low speed function generator with a maximum output of 20 to 40 V p-p will work with my PZT beeper-based SFPIs, most commercial instruments like the SP-470 described above require 100 V or more to provide enough sweep span. The SP-476 Scanning Fabry-Perot Interferometer Driver is basically a dedicated high voltage ramp generator designed specifically for this purpose. It produces a variable frequency sawtooth ramp with selectable and adjustable amplitude and offset. The maximum output is switch selectable between 300 and 1,000 V, which really specifies the approximate maximum range including the centering or offset voltage, the actual p-p output of the sawtooth is somewhat less than these values). A photodiode preamp with 5 gain ranges, as well as a temperature controller (for SFPI setups with thermal control capability) are also included.

    Note that the SP-481 and SP-481A Dye Laser Etalon Controllers have most of the functions of an SP-476 and some additional ones including a set of slow speed ranges and a front panel temperature control. (I don't know what the difference is between the SP-481 and SP-481A.) They have a bunch more stuff as well for the dye laser etalon control application including a separate HV output for the cavity. (Either the etalon or cavity HV output may be used for the sweep, but not both at the same time.) None of this interferes with its use for an SFPI. The only relevant difference seems to be that the SP-481/A lacks a selection for 300 V or 1,000 V maximum output - it's always 1,000 V. But since there is a knob to adjust the amplitude from 0 to maximum anyhow, that's no great loss as long as care is taken not to exceed the voltage rating of your PZT.

    Both the SP-476 and SP-481/A show up on eBay quite frequently, generally going for less than $200. And they provide more features than most other SFPI drivers including adjustable outputs for blanking/scope trigger and (scaled) ramp, a heater controller, and the ability to be used as a high voltage amplifier. Although they are quite old, except for the power transformer which is custom (and identical for both models), the circuitry isn't very complex and uses common readily available components should repair be needed:

    However, one nice thing about the SP-476 (and presumably the SP-481/A as well) is that since the high voltage driver is essentially a voltage controlled shunt regulator, even a continuous short circuit of the output to ground will do no harm, apparently unlike in many other designs! :)

    A description and photo of the SP-476 can be found under Vintage Lasers and Accessories Brochures and Manuals at the end of the section for Spectra-Physics on page 5 of the "High Bandwidth Scanning Interferometer Brochure".

    Burleigh/EXFO SA Plus Scanning Fabry-Perot Interferometer

    Burleigh, since taken over by EXFO, manufactured a variety of optical instruments including wavemeters and interferometers. (Though as of 2010, they have discontinued this product line.)

    The SA Plus is a system similar to the Spectra-Physics 476 controller with 450 or 470 SFPI head, but of more modern construction and some nice features compared to those vintage instruments. A brochure/spec sheet can be found at Burleigh/EXFO SA Plus Laser Spectrum Analyzer Brochure. The same head is used for all versions but the FSR can be either 2 GHz or 8 Ghz, determined by the mirrors and mirror holders. Mirror sets are available covering wavelengths from 550 nm to 1,800 nm, with a finesse of either 200 or 300 depending on the wavelength range, higher for IR:

       ID       Wavelength     Comments
     -----------------------------------------
       07     550 -   650 nm   Green - Red
       08     650 -   750 nm   Red - Near-IR
       09     750 -   890 nm   Near-IR
       10     850 -   990 nm    "   "
       11     980 - 1,145 nm    "   "
       12   1,150 - 1,345 nm    "   "
       13   1,300 - 1,550 nm    "   "
       14   1,425 - 1,675 nm    "   "
       15   1,550 - 1,800 nm    "   "
    

    The ID numbers correspond to the suffixes used when ordering. The lack of any standard mirrors below 550 nm is interesting, despite the obvious lack of IDs 01 to 06. :) (The operation manual for the earlier version of the Burleigh confocal SFPI lists mirrors for 450 to 550 nm, 550 to 650 nm, 760 to 850 nm, or 1,010 to 1,110 nm.)

    The standard mount for the SA Plus SFPI head has both pan and tilt, and X-Y adjustments. The X-Y greatly simplifies setup as the laser then doesn't need to be on an adjustable platform. The only problem is that the mount is quite HUGE!

    The cavity length can be easily fine tuned and then locked in position without going inside, unlike the SP-470 which requires removing the photodiode and using a tool to turn the mirror holder, then replacing the PD and checking if the adjustment helped or hurt. Or, mount the PD externally and angle a tool inside to turn the holder without scratching the mirror. There's also no way to lock the mirrors in the SP-470 against vibrations messing up the distance. (The SP-450 is adjusted at the factory and cannot be changed.)

    The ramp generator doesn't have as many bells and whistles as the SP-476 but it is adequate and also adds a nifty little bar-graph display to show approximately the amplitude location of the ramp and relative to the maximum voltage swing available.

    The photodiode preamp is a separate little box powered by a pair of 9 V batteries. The PD itself slips into the back of the SFPI head and is held in place by a pair of magnets.

    The SA Plus I tested has the mirror set for 550 to 650 nm. At least I think it does since there were no markings on the mirror holders and I wasn't about to remove the actual mirror glass to check them. The mirrors had that silvery broad-band appearance in reflection and deep purple in transmission.

    It works well at 633 nm and 543.5 nm, and probably even at 532 nm though I didn't do a complete test there. I believe it will also be useful well beyond 650 nm. Setup is very easy once I mounted the SFPI head assembly on a Newport post holder screwed to a wooden plank. :) It took under a minute from a totally misadjusted condition to find and fine tune the cavity length for optimal confocal response. (I, of course, had disassembled it to see what was inside and try to determine the part numbers on the mirrors!)

    Although the controller only has a single-turn pot for centering, that seems to be enough. The photodiode preamp saturates at around 1 mW in each mode, but that could be partially because the batteries were somewhat weak at 7.5 V instead of 9 V. :) An ND filter takes care of that and also helps to reduce backreflections to the laser.

    I also have the head (only) from what is probably an earlier version - perhaps the "SA" without the "Plus". :) The manufacturer's sticker had been removed so I do not know the precise model. It has a holder with a threaded lock-ring for the back mirror and a plate with the mirror glued into it for the front mirror. But it is otherwise similar with a large locking ring and fits a standard 2 inch mount.

    The mirrors in mine are for an IR wavelength range (as yet to be determined), but it was fairly easy to replace them with 633 nm 1.7 GHz FSR high finesse mirrors. The back mirror dropped right in being the same size as the Burleigh mirror. But rather than attempting to remove the Burleigh mirror from it's glue and possibly ruining it, a quick and dirty adapter plate was fashioned out of the end-cap from a dead 3/4" diameter HeNe laser tube. This also enabled the cavity to be lengthened by approximately 0.2 inches to accommodate the difference in FSR (1.7 versus 2.0 GHz for the Burleigh mirrors). The 633 nm mirrors worked quite well. It was easy to locate the optimal spacing and lock it in place and the achievable finesse was quite decent, tested using an Agilent 5517 laser with a ~2 mm beam. However, the "sweet spot" for alignment was quite small possibly indicating that the two mirrors are not quite aligned with their optical axes coincident. So, I will probably have a proper adapter plate machined eventually, and at the very least, it will look a lot spiffier compared to the re-purposed HeNe laser tube end-cap!

    Coherent/Tropel 240 Scanning Fabry-Perot Interferometers

    These are another line of Scanning Fabry-Perot Interferometers (SFPIs) similar to the instruments from Spectra-Physics and Burleigh, although Coherent calls it a "Laser Spectrum Analyzer". These were originally developed by Tropel, which then became part of Coherent (for this as well as many other products). But unlike the other SFPIs I've come across, this ones from Coherent were a current product (as of 2010) but have now (2014) been "phased out of the product portfolio" as Coherent puts it. I suppose that does sound better than "terminated" :( :). There were four versions with FSRs of 300 MHz, 1.5 GHz, 7.5 GHz, and 30 GHz. I suppose HeNe lasers are no longer of high priority to be analyzed as none of these FSRs is really optimal for a 1.6 GHz gain bandwidth! However, even the original Tropel versions had the same FSRs.

    An old Coherent 240 manual only listed three wavelength ranges - and in Angstroms (A) which really dates it, but this may simply have been edited to change the name from Tropel:

       ID      Wavelength      Comments
     ----------------------------------------------
        1    4,500 -  5,500 A   Blue - Green
        2    6,000 -  7,000 A   Yellow/Orange - Deep Red
        3   10,500 - 11,500 A   Near-IR
    

    Except for the 300 MHz FSR head, they all use the same body (spacer tube and lens assembly, part number 33-2492) so only the mirror sets differ. The one with a 300 MHz FSR has a body that looks more like a telescope or flashlight being about 14.5 inches long (part number 33-2502) that expands at the front. Eventually under the Coherent name, there were 14 standard mirror sets available covering wavelengths from 337 nm to 1,625 nm (except for the 30 GHz FSR version which lacks the 5 shortest wavelength rangess.) But there are some gaps in the IR wavelength coverage. (This may be more a matter of specifications than anything else as it's likely that mirrors on either side would still have decent performance in the gaps. They are listed below.) The finesse is spec'd at 200 for all except the 30 GHz FSR, for which it is only 100. If you'd like to order one from Coherent and have a working time machine the price in 2010 was around $6,500 with the ramp driver. :-) As noted, Coherent has now phased out the entire product line. Go to Coherent and search for "laser spectrum analyzer" and you'll be able to confirm the bad news. I have saved the glossy product brochure at Coherent Laser Spectrum Analyser System. Since this is an obsolete product, I'm hoping Coherent won't mind. :) (The 240 model designation is historical. Coherent used fancier harder to remember model numbers.)

    One example is the 33-6438-001 with the standard Coherent gimbal mount as shown in Typical Coherent Scanning Fabry-Perot Interferometer Head and Mount. This is representative of all units except the stretch version with the 300 MHz FSR.

    The short SFPI heads have an adjustable focusing lens in front which enables the focal point to be optimally positioned in the center of the cavity. The photodiode on a bayonet mount (looks like an oversized BNC) which makes it more secure with alignment that is more precise and consistent. The mount that comes with the SFPI head uses a gimbal design which means that the head pivots about a common point near the center of the mount. (A kinematic mount pivots about a point near one corner. As a practical matter, this doens't really matter very much for an SFPI.) The mount has pan and tilt adjustments and is on a post that slides into a massive base, adjustable in height. However, unlike the Burleigh SA mount, there is no side-to-side adjustment. Fine X and Y adjustment would be highly desirable, especially for the more finicky heads with the 30 GHz FSR (more below). The 300 MHz version fits the same mount with an adapter ring (since it's narrower over most of its length), but it's not at all clear if pan and tilt with its axis near the center of the cavity is really optimal for ease of adjustment with this long head. The ramp driver has a maximum output of 250 V which is sufficient for viewing 2 FSRs, but not with a lot of margin. It appears generally similar to the one for the Burleigh SA, but has the PD preamp built in.

    The first Coherent SFPI head I acquired had the nice mount, but no driver, so I'am using it with an SP-476 driver and that works fine. However, since Coherent specs the maximum voltage to be 500 V, the 1,000 V setting of the SP-476 should not be used; the 300 V setting is enough for much more than 1 FSR. This SFPI head has an FSR of 30 GHz with a wavelength range of 550 to 650 nm. That might be useful for some types of diode lasers but is certainly far from ideal even for short HeNe lasers with a large mode spacing.

    With the mirrors having a Radius of Curvature (RoC) of only about 2.5 mm (1/10th inch) spaced an equal distance apart, alignment to the laser even with the confocal cavity becomes much more tricky. The person who sent it to me couldn't get any response at all from the photodiode, and at first, I had the same problem. But finally, after very careful alignment, it started to behave more like the other confocal SFPIs I've tested, except that the finesse is poor. With a spec'dfinesse of only 100, the resolvance under ideal conditions (adjustment, alignment, and beam focusing) is only about 300 Mhz. So far, I've only been able to achieve a finesse of around 50 (resolvance of 600 MHz) which can barely resolve the longitudinal modes of a Melles Griot 05-LHR-911 HeNe laser (mode spacing of 883 MHz). I have not yet found the cause. It is very likely at least in part due to the beam diameter being too large, as well as the mirror spacing not being precise enough, which seems quite likely since the previous owner may have attempted to adjust it after not being able to obtain a signal. And it is very fincky! But there could conceivably be damage to the mirrors as a result of abuse it may have been subjected to in its previous life. The specs may simply be overly optimistic for real World conditions.

    Disassembly for adjustment or removal of the mirrors is supposed to require special optical spanner wrenches so it's more difficult to do anything inside (or mess it up!) than with the Spectra-Physics or Burleigh SFPI heads. But mirror sets are intended to be replaceable by the user, so it shouldn't be that bad. :) Since my Coherent tool set hasn't arrived, at first I was using an improvised pair of filed-down needle-nose pliers, but this required removing the photodiode assembly and there wasn't enough clearance to mount it externally since it would have to be relatively close due to the small RoC of the mirrors and divergence of the transmitted beam. So, observing the display in real-time was not practical. But this enabled the rear mirror cell to be removed for inspection. Both mirrors appear fine. It's amazing how small they are - a 2.5 mm RoC and nearly a complete concave hemisphere. How do they even grind the substrate for a mirror like that? Finally, threatening the SFPI with a dental pick did the trick. Angling the dental pick into one of the holes in the adjustment ring with an external photodiode permitted very fine movement while observing the SFPI display. It is still VERY finicky. Even with careful adjustment of cavity length - which is still a pain - and perhaps a bit of fudging of the data, the finesse is close to the spec'd value of 100 based on the FWHM, though the lower portion of each peak is more spread out than would be expected based on the textbook plots, and there are still some artifacts in the display. Then again, perhaps all those assumptions made in calculating finesse simply don't work very well with a finite beam size and such a small mirror RoC! Something about serious spherical abberation conspiring to mess it up. Perhaps, the beam really has to be a smaller diameter or better collimated or something. And various other things conspire to make this more difficult than it could have been with better design and more precise machining. The problem with the adjustment is that the required setting must be accurate to well under +/-1 degree of rotation, and even a slight change affects other aspects of alignment due to tolerances in the machining, fit, and polish. :) For example, the two very tiny mirrors are held inside long tubes that protrude from mounts on screw threads at opposite ends of the spacing tube. Expecting their focii to line up within a small fraction of 1 mm is asking a lot. And, there is no backing spring - apparently the special sticky grease is supposed to keep everything in line. So, it's not clear how stable this is over time - just a lot of fine thread area and grease. Too bad they didin't include at least a set-screw for locking.

    I don't really have a good use for a 30 GHz, 550 to 650 nm SFPI, though it's quite possible one could turn up. However, the Coherent model I would really have liked would be their part number 33-6305: 300 MHz FSR, 550 to 650 nm. Then I wouldn't have had to build my own SFPI to resolve the split line of two-frequency HeNe lasers from HP/Agilent and Excel. However, Coherent may not have offered this particular combination anymore since there was no link to it on their Web site (even as of 2010). But I would have settled for a used one. Perhaps, Coherent might have accepted a trade. :) (As it turned out, I did end up building my own with an FSR of 250 MHz and finesse exceeding 150. See the section: Sam's High Resolution Scanning Fabry-Perot Interferometer.)

    Now here's the peculiar effect of the week: I wanted to put a neutral density filter in front of the laser to reduce the maximum possible intensity of any back-reflections. This is desirable to minimize the chance of laser instabiliity that can result from mutually coherent light re-entering its cavity. The transmitted power would be the input power times the filter's transmission coefficient, so the amplitude of the display would be reduced, but there is plenty of gain! However, any back-reflections would be reduced by the square of the transmission coefficient, even if there was 100 percent reflection. For example, with a filter coefficient of 0.25, the transmitted power would be 25 percent of the laser power, and the maximum reflected power would be no more than 6.25 percent. I tried several filters that are basically amber-colored glass. Three out of four behaved as expected: The optical power reaching the SFPI had the expected value and amplitude of the display was reduced. Some adjustment of the SFPI alignment was required to optimize the display if the glass plate was at an angle, but the resulting amplitude was very close to what would be expected based on the transmission coefficient of the filter. However, the forth piece of glass behaved, well, strangely. While it reduced the power as measured by eye and with a laser power meter by the expected amount (75 percent), the SPFI display disappeared entirely regardless of the orientation of the filter or adjustment of the SPFI alignment for an amplitude of exactly zero or 0.0000000 on the 'scope even with the gain controls set at maximum! Not even any tiny bumps. In addition, under normal conditions when optimally aligned, the back-reflection from this SFPI is a large more or less uniform disk of light. That was totally absent as well.

    And to add to the strangeness, there was no similar effect using the same laser and filter with a Spectra-Physics 8 GHz SFPI.

    It turned out that the cause was very simple: The one problematic filter has a rather large wedge - probably 1 or 2 degrees. The others have little or no wedge. It's not enough to detectably divert the beam but must be tilting the wavefront so the required interference cancels out. And in fact with the wedge producing a deflection horizontally, careful realignment including repositioning the SFPI head horizontally finally was able to restore the display to normal. The extremely small cavity of the 30 GHz SPFI is much more finicky about (laser) alignment in general so it must be much more sensitive to the wavefront as well. Interesting..... :)

    As noted, these started out as Tropel so not surprisingly, the construction of the Coherent versions is almost identical. While they look very similar and the heads do fit the same mount, there are some subtle differences. At the very least, the thread diameter for mounting the photodiode assembly differs by just enough on some versions that they are not interchangeable. Go figure. :) I acquired a Tropel 240 with the mirror set for green/blue lasers, which is probably equivalent to Coherent part number 33-6206 which has an FSR of 1.5 GHz. And of course the first thing I did was to attempt to confirm that the confocal mirror spacing was optimal. It probably was and so this became a BIG mistake. The special Tropel grease had congealed over the eons so while the mirror barrel initially seemed to be movable, it jammed solid and required total disassembly to be able to apply enough torque to free it - and that was almost impossible. I nearly broke the official Coherent mirror spacing adjustment tool in the process and had to use filed down snap-ring pliers to rotate the mirror barrel with the threaded metal flange of the inner cylinder clamped in a (cushioned) vice! However, there were two benefits to this: Primarily, it allowed the internal construction to be documented. :) See Tropel Model 240 Scanning Fabry Perot Interferometer Head Components. The only parts not shown are the 4 screws that secure the BNC connector. Disassembly (and reassembly) is straightforward except that the BNC connector needs to be unsoldered from short stubs of the red and black wires before the inner assembly can be removed. That's not so bad. Going the other way is the pain. :( :) The Coherent photodetector assembly is shown; I do not have one from Tropel but assume they looks similar even if the thread size differs by enough to be annoying. And now the mirror spacing adjustment is smooth as silk. I didn't have the special Tropel or Coherent grease, so I used the tiniest bit of high vacuum grease. It may not stay put quite as well but I can add a dab of 5 Minute Epoxy (removable should the need arise) to prevent any drift.

    Another short Coherent head I acquired had 30 GHz FSR mirrors for 900 to 1,070 nm. This would be useful for Nd:YAG and other similar lasers, but they are kind of boring. :) So, it occurred to me that with small adapter rings, the mirrors I provide in my SFPI kit for 633 nm could be made to fit the Coherent SFPI head. The mirrors would need to poke slightly beyond the normal mounting surface since their 43 mm RoC is 7 mm shorter than the 1.5 GHz FSR of the SFPI body. (The 7.5 GHz and 30 GHz Coherent/Tropel mirrors do something similar.) At first, I was going to cobble something together but in the end decided to have them professionally machined. The mirrors are glued to the adapter rings and then they are easily installed like the Coherent/Tropel mirrors. And their performance is actually quite phenomenal. For a 6 mm diameter beam from a Zygo 7701 laser, the finesse exceeds 350; for a 3 mm beam, it is over 550, and probably slightly higher for a smaller beam! (Theory predicts it can exceed 600 based on mesaured mirror reflectances.)

    The performance at 633 nm is considerably better than that of the standard Coherent/Tropel mirrors. The two-frequencies 20 MHz apart are resolved as nearly independent peaks. See Scanning Fabry-Perot Interferometer Display and Simulation of Zygo 7701 Laser Spectrum. The photo on the left is the unretouched screen shot with a span of about 2 FSRs. The photo in the middle is one of the twin peaks expanded by a factor of 10 on the scope. The plot on the right is a Matlab simulation of a Zygo spectrum with an SFPI finesse of 550.

    Tests using a REO tunable HeNe laser show a finesse of between 400 and 450 at 604/612 nm and between 250 and 300 at 594 nm. I need to test at longer wavelengths but don't have a convenient tunable laser for that. If the reflectance function is symmetric, the finesse at 670 nm should be in the 250 to 300 range. However, since the coatings of HeNe laser mirrors are often designed with a reflectivity fumction that is a "cliff" with respect to wavelength, it's possible that the useful range above 633 nm may not extend that far.

    I acquired a Coherent model 216 - the nice long one with an FSR of 300 MHz. (The "216" designation is a carryover from Tropel and seems to have zero correlation with the more recent Coherent part numbers.) Construction is generally similar to the shorter Coherent (and Tropel 240) SFPIs with mirror mounts glued to a PZT spacer tube. For the long SFPI, the PZT is the same length, but a glass cylinder extends it the required distance. I say "glass" but it may be Zerodur or something like that. The SFPI cavity is sandwiched between the front and back end-plates, with only a rubber O-ring as cushioning at the front (to allow for the PZT to do its thing). The end-plates have aluminum-on-aluminum threads, which meant that I once again had to fight with metal-lock. But this did allow me to inspect the inside. Someone may have repaired this unit at some point in the past as the glue jobs on the PZT look suspect and the rear mirror mount actually came off the glass tube, but some 5 minute Epoxy solved that. :)

    Unfortunately, the wavelength range of the mirror set was not labeled. They have a slightly bluish tint in reflection and a slightly yellowish appearance in transmission.

    Here is a list of all the Coherent standard wavelength ranges:

       ID       Wavelength     Comments
     ----------------------------------------------
        1     275 -   305 nm   UV
        2     305 -   337 nm   "
        3     307 -   365 nm   Near-UV
        4     365 -   405 nm   Near-UV - Violet
        5     405 -   450 nm   Violet - Blue
        6     450 -   550 nm   Blue - Green
        7     550 -   650 nm   Green - Red
        8     650 -   775 nm   Red - Deep Red
        9     690 -   830 nm   Deep Red - Near-IR
       10     790 -   930 nm   Near-IR
       11     900 - 1,070 nm    "   "
       12   1,000 - 1,100 nm    "   "
       13   1,250 - 1,450 nm    "   "
       14   1,400 - 1,625 nm    "   "
    

    (The IDs are my arbitrary designation. All of these were available for the 1.5 GHz, 7.5 GHz, and 300 MHz FSR heads. The 30 GHz FSR version lacked the UV/violet options, IDs 1 through 5. More details can be found in the product brochure at Coherent Laser Spectrum Analyser System.) IDs 6 and 7 were ruled out both by their appearance and by testing with HeNe red and DPSS green lasers. The yellowish tint in transmission means that the wavelength for peak reflection must be in the deep blue or 3 times that wavelength in the near-IR.

    Having initially concluded (perhaps incorrectly) that the original mirrors weren't likely to be that useful, even once their wavelength range could be determined, I decided to installed mirrors of my own, at least as a test. Fortunately, swapping mirrors in all of the Tropel/Coherent SFPI heads is relatively easy and low risk, each being held in by a threaded ring with a rubber O-ring for cushioning. The required RoC is 25 cm and the mounts have a diameter of 12 mm diameter. I did have some HeNe mirrors that I thought had an RoC of 25 cm, and they were the right size so I popped them in. But it turned out that their RoC was actually 30 cm, so the results were, well, a bit strange. They did work but with optimal adjustment of the SFPI cavity, the display had an effective FSR of 60 MHz (1/5th of the expected 300 MHz), but with a finesse referenced to 300 MHz. And the reflectance of these mirrors is about 98.5 percent - somewhat lower than optimal. Thus, the resolvance was not very good. But nonetheless, it could be seen that a Zeeman-split HP-5517C laser produced a pair of wavelengths, though their separation could not be measured. (This was partly due to not attempting to optimally mode match the widely diverging beam from an HP laser without the normal collimator to the SFPI. However, this did prove that the SPFI was operational. So the finesse wasn't actually too terrible and in fact, such a system could still be useful to confirm that a laser is single longitudinal mode (SLM). Although the multiple peaks weren't as consistent in amplitude as they would be with the proper mirrors, non-SLM behavior would still be apparent. But this wasn't really what I Wanted. One option would be to make adapters so that standard 6 mm HeNe mirrors can be dropped in, of which I probably have several suitable candidates with a somewhat higher reflectaace, but perhaps not enough of an improvement to warrent the effort.

    Later, I did some more testing of the original mirrors. They passed 405 nm from a diode laser like a sieve so that eliminated IDs 4 and 5 (above). Testing with a 1,064 nm laser tossed ID 11 and 12. Based on the appearance, the most likely mirrors would be IDs 13 or 14. Finally, I dug up a Lightwave Electronics model 120-02 laser that operates at 1,319 nm. Bingo! The mirrors appear to have very low tranmission and high reflectance at 1,319 nm. Using an IR detector card, the transmission was essentially zero. The next step was to reinstall these mirrors and test the SPFI in its original configuration, except using one of my cut open germanium transistor photodetectors (since the original sensor did not come with the 216).

    I expected it to be fairly easy to get going at 1,319 nm based on my experience with this SFPI at 633 nm and my home-built high resolution SFPI with a slightly longer cavity length. However, alignment turned out to be a pain in the you-know-what. This was partially due to my desire to do the initial test before I had a proper adjustable mount for the SFPI (or the laser) and the cavity spacing of the SFPI was almost certainly not optimal for the new mirror set, but also for other reasons that will become obvious.

    The laser I used for testing was the LWE-120-02 (about 4 mW) with a single frequency output at 1,319 nm (or at least that what the specs say). I started with a Newport IR detector intended for a power meter like the 835. I know this to be reliable but with a mediocre frequency response. I could always reduce the scan rate to check for display quality. And once any display was achieved, I would switch to one of my cut-off germanium transistors which have a nice small detector area, low capacitance, and thus decent frequency response. But it took quite awhile to obtain anything resembling an SFPI trace. I was even at the point of suspecting the installation of the mirrors and checked both to be sure they were in the correct way around and not cocked in their mounts. They were fine. Occasionally, there would be some very low level blips that correlated with the SFPI scan but it was very difficult to maintain these. Finally with enough duct tape and modeling clay (just kidding), a trace appeared that was stable for long enough to be able to approximately peak the SFPI cavity length. Even with the Newport detector, it became obvious that this was no ordinary mirror set. And with a stable setup, switching to the better cut-off transistor detector was a snap.

    Not only does this thing work well at 1,319 nm, but the finesse of at least 500 and possibly approaching 1,000 even without worrying about the laser's beam size or divergence. My conclusion is that these are probably not a standard mirror set. And recall that the finesse of a confocal cavity SFPI is cut in half, so the mirrors must be even better. With the scope set so that one FSR spans the complete screen, a peak is essentially a thin vertical line, much narrower than what's typical with the off-the-shelf mirrors or found in the brochure (above). Only when expanded using the SPFI controller or 10X sweep on the scope can the width be resolved. To achieve an effective (confocal) finesse of 500 requires mirrors with a reflectance of 99.7%. Since the finesse here could be 1,000 (or more), the reflectance may be much higher. Now I know why it was more of a pain to get going at 1,319 nm then at 633 nm with my 98.5% mirrors. These are closer to HRs. And of course working with a low power laser at 1,319 nm didn't help.

    Not surprisingly, the LWE-120 must not like back-reflections. There is a second mode that comes and goes but is more likely with the SFPI peaked for maximum resolution. Misalign the SFPI sufficiently and the second mode goes away entirely. On the display it appears ~50 MHz away, but who knows where it actually is due to the aliasing of the SFPI's FSR. Because of the way everything is set up now, I can't really move the laser any distance away - it's nose-to-nose with the SFPI - so back-reflections are inevitable. And I don't have an optical isolator for 1,319 nm.

    The standard mount that is supposed to also be used with this SFPI (that looks more like a HeNe laser head cylinder) seems to be far from ideal. And although I have one for the other Coherent SFPI head, I don't have the required adapter ring as the barrel diameter is smaller than for the short SFPIs. So, I may build a platform mount with three adjustable feet for it.

    In 2014, I finally - FINALLY - was able to test a Coherent 216 with the 550 to 650 nm mirrors. :) As luck would have it, I had to repair it first - the front mirror mount assembly had broken off of the spacer tube. But it was straightforward to glue it back in place. :-) Alignment using an Agilent 1211A two-frequency laser was easy in so far as obtaining a display, but similarly fiddly as my home-built high resolution SFPI to optimize it. Even with the N1211A's narrow beam, a pinhole was needed to maximize resolution. And interestingly, any back-reflections seem to shift the laser's lock point slightly so the amplitude of the two peaks changes. The pinhole reduced that effect as well. (An optical isolator could have been used to prevent back-reflections, with the beam first passed through a polarizer at 45 degrees to include both frequency components and be linearly polarized.) I should have taken a photo but neglected to do so, and I'm not setting it up again simply for that purpose! So, a simulation will have to do. :) See Simulation of Coherent 216 SFPI Display of Agilent N1211A Modes. The difference frequency is around 1.6 MHz and the actual display had at least as much resolution as the simulation using a finesse of 400.

    More to come.

    TecOptics SA Scanning Fabry-Perot Interferometer

    TecOptics is currently a manufacturer of custom optics including (fixed) Fabry-Perot etalons. However, they used to offer a variety of standard products including the FPI-25 general purpose plane mirror SFPI with adjustable FSR, as well as confocal cavity SFPI systems similar to those from Burleigh, Coherent, Spectra-Physics, and others.

    The SA series of confocal SFPIs consists of an SFPI body which includes a fixed lens at the front and fine-thread adjustment ring at the back into which the Si or Ge photodiode module is installed. The adjustment ring is always accessible to enable the mirror spacing to be fine tuned, and may be locked in place so the setting shouldn't change. The mirror sets are mounted on plates mounted via two screws to the PZT assembly inside.

    The HV connection is a miniature coax and the photodiode uses a 3-pin DIN so they can't be accidentally swapped, at least not at the head-end!

    Here are some photos of an SA-7.5 which has an FSR of 7.5 GHz (mirror spacing of about 10 mm):

    The mirror set in the unit I was given has a wavelength range of 850 to 920 nm, which doesn't overlap any of the lasers I really care about. It might be good for use with a Ti:Sapphire laser, which regrettably, I don't happen to have available at the moment. :) It could also be used with single spatial mode diode lasers. But as yet, I have not actually tested this unit, though I see no reason why it shouldn't behave like all the others. I have thought about making suitable mounting plates for a pair of mirrors with the same reflectivity as those in my $2 SFPI, approximately 99 percent at 532 nm, custom coated for a defunct project on substrates that are Melles Griot plano-concave lenses. But since I already have an SP-470-02 (8 GHz, as well as my $2 SFPI), this wouldn't really provide anything fundamentally new. So, I'll simply have to find a suitable laser!

    Thorlabs SA200 and SA210 Scanning Fabry-Perot Interferometer

    The SA200 and SA210 series of SFPI heads from Thorlabs comes in several wavelength ranges: 350 to 535 nm, 535 to 820 nm, 820 to 1,275 nm, 1,275 to 2,000 nm, and 1,800 to 2,500 nm. (The mirror sets are not intended to be swapped by the user, if at all.) The SA200 has an FSR of 1.5 GHz while the tThe SA210 has an FSR of 10 GHz. Google will come up with the relevant Thorlabs info by searching for "Thorlabs Scanning Fabry-Perot" or by searching on their Web site.

    The SA200 and SA210 are of a modern design with a PZT requiring only around 20 V rather than several hundred V to cover approximately 2 FSRs. (This is similar to my PZT beeper-based home-built SFPIs!) So, while Thorlabs does offer the SA201 control box, a normal function generator will suffice to drive the PZT. (The SA201 also include a PD preamp.) CAUTION: The rated input of the PZT is only 150 V, so take care if using a high voltage ramp driver like the SP-476; add a voltage divider or zener or neon lamp clamp to limit the maximum voltage! Another nice feature is iris diaphragms at both the input and output (photodetector) ends of the head. Reducing the apertures aids in alignment and enables the effective resolution to be maximized. A small aperture also tends to reduce the likelihood of back-reflections which may destabilize the source laser. Neither head has an input focusing lens built-in like the SP-470s, so an external lens with its focus approximately at the center of the SFPI cavity may need to be added for best performance.

    The SA201 Spectrum Analyzer Controller includes a ramp generator and photodiode preamp. The ramp driver has a 10 turn pot with a nice large knob for sweep offset, a 7 position switch for 1x, 2x, 5x, 10x, 20x, 50x, or 100x sweep expansion (which actually selects sweep time), a button to select sawtooth or triangle waveform, and a recessed trimpot for sweep amplitude. There is no provision for true sweep expansion, which would maintain the same total sweep time (or equivalently, sweep frequency), but with its amplitude reduced by factors of 1, 2, 5, 10, 20, 50, or 100 around the center of the sweep. Having both sweep expansion and sweep time (as with the SP-476) would be desirable, though hardly essential. The sawtooth has a fixed limited slew rate for its fall (retace) so there is no risk of harming the PZT. The photodiode preamp may be set to a gain of 10k, 100k, or 1M V/A, and there is a recessed trimpot to adjust risetime. The PZT output and scope trigger connectors are on the front panel while the PD input and output connectors are on the rear panel. It has a nice basic set of features and would work well with my home-built SFPIs. :) The design is based on a Lattic CPLD for the waveform generation with virtually no analog timing components, so it should be quite stable over time. My only complaint is with respect to size and weight: With modern components and a wall adapter for power, the entire controller could easily be 1/10th as large and weigh 1/10th as much. And the power button apparently controls a relay producing a resounding CLUNK, as though it's using 10 kW of AC power! :)

    The complete operation manuals for the SA200, SA210, and SA201, may be found on the Thorlabs Web site.

    I wanted an SFPI with the smallest FSR available (from Thorlabs) to check the line-width of a stabilized line-narrowed diode laser at 685 nm. My SP-470-03 is only spec'd from 550 to 650 nm so there was some question as to whether what I was seeing there was accurate. My home-built high resolution SFPI has mirrors that would have been suitable, but the FSR is too small.

    The head I tested was an SA200-5B which has the 535 to 820 nm mirror set. The 1.5 GHz FSR is a bit small for unambiguously monitoring the modes of red (633 nm) HeNe lasers as outlying modes may alias on the display - 2 to 2.5 GHz would be preferred. But for most modern :) applications like checking for single longitudinal (single frequency) performance of diode and DPSS lasers, this isn't a problem. Although only spec'd down to 535 nm, not surprisingly, the performance at 532 nm is still exceptional. In fact, without careful measurements, it would be impossible to tell the difference between the resolvance at the three wavelengths I used for testing: 532 nm (DPSS), 633 nm (HeNe), or 685 nm (diode). I suppose that spec'ing 535 nm may necessary to take into account worst case variations in mirror coating wavelength coverage. However, given the prevalence of 532 nm, it would have been nice to be included within the spec'd wavelength range.

    As it turned out, again not unexpectedly, SP's specs are also conservative so the SP-470 display was very similar to that of the SA200-5B at 685 nm and 532 nm, though the SA200-5B may have had slightly better resolvance. Both these are well outside the 550 to 650 nm range of the SP-470, but 532 nm is very close to the low-end spec of the SA200-5B and 685 nm is near the middle of its spec'd range.

    The lack of a focusing lens also didn't seem to have any detrimental effect on performance with any of these lasers, but they all have a small beam size. The apertures could always be reduced (laser power permitting) for lasers with a larger beam diameter.

    The PD preamp has a wide dynamic range so it's easy to find a combination of gain setting and scope sensitivity that fills the screen without clipping, distortion, or excessive noise. The lack of a sweep voltage knob was not really of any consequence. (There is the recessed trimpot.) It came from the factory set at a full span of approximately 2 FSRs, which would be my default setting anyhow. The sweep expansion switch provides more zoom than could be ever be needed.

    And the Lab Snacks Thorlabs included with the loaner unit were definitely yummy. ;-)

    Of course, even a perfect product can be improved, so here are some suggestions (Thorlabs, are you listening?):

    Burleigh Triple-Pass Scanning Fabry-Perot Interferometer

    This thing showed up on eBay with a listing title of "Burleigh Laser Exciter with Prisms (UNTESTED)". It had what appeared to be cube-corners at both ends and a bunch of wires coming out of it. Even though the seller provided many photos, none was very good at revealing what the device really was. If it was indeed a laser exciter (I have no idea where they got that name), then perhaps there was a fancy laser inside! But the 6 skinny wires hanging out of it terminated with pin-plugs didn't seem right for a laser and seemed more appropriate for a bomb. :) It was listed several times over a couple weeks with no takers and with the "Buy-It-Now" price getting lower and lower and lower until....It finally became cheap enough that I HAD to satisfy my curiosity!

    Well, the "Burleigh Thing" as I've been calling it turned out to be a Fabry-Perot interferometer with a PieZo Transducer (PZT) for fine adjustment of mirror spacing. The mirrors are planar and their separation is adjustable by changing the position of the PZT assembly, so it can be set for a wide range of FSRs. (Free Spectral Range - the extent over which optical frequencies are unique.) This device could certainly serve as a Scanning Fabry-Perot Interferometer (SFPI) for displaying laser modes, though it's more likely to have been used as a tunable etalon or narrow-band optical filter. The normal beam path goes through the mirrors three times, which is like putting three SFPIs, etalons, or filters in series. One thing is for sure: This must have cost someone (probably the U.S. Taxpayer!) a fortune. :)

    Here are a few photos.

    The design wavelength of the mirrors is not known, but they are about 75%@633nm and 90%@532 nm, which would be rather mediocre if used with a single pass interferometer. They appear greenish in reflection and weak pink/purple in transmission. Based on appearance and that the reflectance is decent at 532 nm, I would suspect they may have been intended for 1,5XX nm (roughly 3x532 nm) and have a higher reflectance there. However, the transmission functions through the interferometers multiply when they are used in series. So with three passes, this thing may still be quite impressive at these wavelengths, even if they aren't optimal. If one pass suppresses out-of-band wavelengths by a factor of 10, with three passes, it would be a factor of 1,000.

    (From: John Barry.)

    "I found that the best way to consider this problem involved thinking about the transmission function for a single Fabry Perot, given below from the Wikipedia entry for "Fabry-Perot Interferometer". (Note: I derived these myself just to check that it is applicable.)

                 (1-R)2                  1
      Te = ------------------- = -----------------
            {1+R2-2*R*cos(δ)}     [1+F*sin(δ/2)2]
    

    where δ = 4*π*distance between F-P mirrors/wavelength and F is the coefficient of finesse, which is related to, but not identical to the actual finesse:

                                     π
                 finesse = ---------------------
                            2*arcsin[1/sqrt(F)]
    
                  1            π
     2*arcsin(---------) = ---------
               sqrt(F)      finesse
    
                  1             π
       arcsin(---------) = -----------
               sqrt(F)      2*finesse
    
                   1                π
               --------- = sin(-----------)
                sqrt(F)         2*finesse
    
                                    π
                 sqrt(F) = sin(-----------)
                                2*finesse
    
                                     π
                       F = [sin(-----------)]-2
                                 2*finesse
    
    
    For Three F-P interferometers in series, the transmissions should multiply (as you said) giving the final transmission to be Te3 where Te is given above. Unfortunately this curve of Te3 has a different functional form than Te so the Finesse is not really well defined."

    Compare Transmission of Fabry-Perot Resonator versus Optical Frequency with Transmission of Triple-Pass Fabry-Perot Resonator versus Optical Frequency. Note how much narrower and deeper the peaks are with three passes even for very low finesse. It would appear that while multiple passes doesn't do exactly the same thing as increasing the finesse by using higher reflectance mirrors, the overall effect is similar. For three passes, the resolvance has increased by a factor of between 2 and 3. Comparison of Transmission of Single-Pass and Triple-Pass Fabry-Perot Resonator versus Optical Frequency show specific examples for cavity finesse equal to 1, 5, and 20. Even more dramatic than the improvement in finesse is the change in contrast between the peak and the minimum, which can increase by multiple orders of magnitude. (For a finesse of 20, it will be larger by a factor of more than 1,000.)

    Fabry-Perot Interferomter (Univeritat Osnabruck) and Sam's copy of Fabry-Perot Interferomter (Univeritat Osnabruck) has a discussion of the multipass interferometer technique with a triple-pass example (as well as other useful info). But this lecture note doesn't really address the relative merits of multiple passes versus higher reflectance mirrors.

    Burleigh apparently did have optional CCAs for their standard adjustable Fabry-Perot interferometers. The RC-22 (50 mm) fits the RC-110, RC-140, and RC-150, while the RC-27 (70 mm) fits the RC-170. Masks for 3 or 5 pass operation were included to aid is setting beam position and size. The recommended mirror sets for multipass operation had lower reflectivity, aleast in part because aligning with the normal high-R mirrors would be virtually impossible. They were: 3 pass only - 93%; 5 pass only - 88%, and if alternating between 3 and 5 pass - 90% as a tradeoff. But the Burleigh Thing is almost certainly a custom job.

    While I doubt that anyone will ever see another one of these beasts, for reference, here is the wiring info for the PZTs. The polarity is arbitrary - I don't know whether a positive voltage expands or shrinks the PZT bars:

         Color          Function
     -------------------------------
       Red/Orange      PZT1+/PZT1-
       Black/White     PZT2+/PZT2-
       Blue/Green      PZT3+/PZT3-
    

    The operation manual for a Burleigh RC-44 Ramp Generator lists color codes that agree with these. The "+" is assigned to the HV Ramp, and the "-" to the HV Bias (electrical adjustment for fine tuning of mirror alignment, which I intend to add eventually). So, this color code must be used for other Burleigh SPFIs.

    With the cube-corner assemblies removed (single pass), and set for an FSR of about 6 GHz, I aligned the mirrors using a red HeNe laser and attached the PZTs (in parallel) to my SP-476 controller. And the Burleigh Thing does work, sort of. The finesse for a single pass at 633 nm is truly mediocre, but it is able to resolve the longitudinal modes of a Melles Griot 05-LHR-911 HeNe laser (883 MHz mode spacing) - barely, as overlapping lumps rather than peaks. The finesse is probably optimistically around 8. This poor performance was expected given the low reflectance of the mirrors at 633 nm. With a single frequency 532 nm DPSS laser (Coherent C215M), the display is much better with distint peaks and a finesse of at least 15. :) Theory predicts a finesse of about 30, but the discrepency could be due to the fact that the beam of the C215M laser is slightly diverging. I'm not sure I have the determination to get it working triple-pass but it may come to that! However, driving all three PZTs using the single HV output of the SP-476 works well enough with no evidence of a change in amplitude over several FSRs. The PZT/Mirror assembly does make quite a racket though - a loud ticking sound - with the SP-476's sawtooth drive, much more so than the small SP "normal" SFPI heads. Or the near silence of my home-built SPFIs! But being able to control the drive voltages (or at least the bias voltages) independently would provide fine tuning of mirror alignment, which is difficult with the large clunky adjustment screws. More on this in the next section.

    And no, I didn't attempt to get the Burleigh Thing to work in triple-pass mode and probably never will!

    I had a really fuzzy vague recollection of being shown an SFPI similar or identical to the Burleigh Thing (but without the CCAs) a few years ago at a local college, gathering dust in the corner of an undergrad physics teaching lab. I mentioned to the instructor who pointed it out to me that the mirrors didn't look like they were good for red, but I didn't realize that they also weren't very good for green, and he probably had no clue either. I might have also commented on how much larger and more massive it was than the SFPIs I build. Then again, perhaps all this was just my imagination. Or, as someone else put it: "When I was a child, I had a fleeting glimpse of something out of the corner of my eye. When I turned to look, it was gone and I cannot put my finger on it now." :) Now totally coincidentally, I had the opportunity to visit the teaching lab because the SFPI was a bit sick. And wouldn't you know, it really didn't look anything like the Burleigh thing! See the section: The Tropel 350 Scanning Fabry-Perot Interferometer.

    Installing High Finesse Mirrors in the Burleigh Triple-Pass Scanning Fabry-Perot Interferometer

    The original mirrors in this thing result in mediocre performance for any lasers I care about (and that's being generous!). They were most likely designed for 1.5 µm and produce a display barely recognizable as a spectrum at 532 nm or 633 nm. So, I decided to attempt to replace them with mirrors that would be useful for visible lasers. Now I just happened to acquire a pair of nice planar mirrors. While also likely designed for 1.5XX µm, they were probably intended to be HRs there and the reflectivity at 532 nm is around 99 percent. As we now know based on my $2 SFPI, an HR at 1.5xx µm will have a lower reflectance at one third of 1.5 µm or approximately the 500 to 533 nm range. I figured that 99 percent at 532 nm (and probably slightly less at 543.5 nm but close enough for Government work!) would be ideal, but that was based on a confocal cavity, not plane-plane for the Burleigh thing. I did not realize how difficult alignment would be with the planar mirrors and not so fantastic Burleigh mirror adjusters.

    Removing the original mirrors wasn't going to easy because they were glued into recessed tight fitting holders. And in fact, for various reasons, they did not survive very well. So, there was no going back. I had to make the replacement mirrors work.

    The original mirrors were about 1.5 inches in diameter and the replacements were 2 inches in diameter. My plan was to glue them to the lips of the original holders with 5 minute Epoxy. Hopefully, the resilience of the Epoxy would minimize any stresses on the mirrors that might distort them. And these mirrors were about half the thickness of the originals. This approach worked nicely for the back mirror on the PZT because that had plenty of clearance. But what I had not anticipated was that the front mirror fit through a hole in the end-plate attached to the SFPI body and that hole was only slightly larger than the mirror holder, and was definitely less then 2 inches! So, the plate had to come off. But then the larger diameter mirror would prevent the mirror mount from ever being removed without ungluing the mirror. That might be an annoyance in the future but of more immediate concern was the fact that the mirror mount blocked access to the screws securing the end-plate to the SFPI body when it was installed. It seemed like a catch-22 situation that would require some tiny robots to go inside and install the mirror after the end-plate and mount were screwed back together. But as it turned out, there was just enough leeway to permit a ball-end hex wrench to get in between them to tighten the screws to reattach the end-plate even with the mirror attached.

    So, both replacement planar mirrors were installed without incident. That was the easy part.

    Mirror alignment is always a royal pain with a plane-plane Fabry-Perot (F-P) cavity and even more critical with a high finesse F-P cavity. The mirror adjusters on this thing are to put it mildly, not ideal for fine control. OK, not putting it mildly, they stink. :( :) The construction is simply three screws pulling the mirror mount plate toward the end-plate attached to the SFPI body, and another three screws behind the mirror mount plate holding it back. No springs or even springy washers! So, once the alignment is close, it's a matter of tightening each pair of screws against each-other while attempting to do the fine tuning. Optimizing alignment is a matter of observing the scatter off the mirror from the multiple reflections and forcing them into as tight a splotch as possible with these clunky adjusters, which also tend not to be totally independent of each-other. Admittedly, this was designed to be aligned once and locked in place forever. Then likely fine-tuned by adjusting the bias voltages to the 3 PZTs. (More on this below.) So, fancy mirror adjusters wouldn't be justified. And 1,000 tpi screws or a differential screw scheme would be required to get the sort of resolution needed to really make manual alignment less of a royal pain.

    As if this weren't enough, in a plane-plane SFPI, the laser must be set up so that its beam is perpendicular to the input mirror so that its beam is reflected precisely back on itself. Now, most lasers get mighty unhappy when this is done and make their hurt feelings known by becoming unstable and mode hopping all over the place, even off the optical table. :) But if the beam is at an angle to the input mirror, not only is the finesse achievable much lower, but there is no way to judge when the mirrors are parallel to each-other, and when they aren't, the scatter pattern becomes a curved rather than a straight series of spots, making it much more difficult to even figure out which way to turn the alignment screws.

    So, the laser must be aligned to the input mirror hoping it doesn't complain too much. An optical isolator can be quite effective but is also quite expensive (BIG $$$), although a polarizing beam-splitter and Quarter-Wave Plate (QWP) might work if the laser is polarized. An optical filter can also be used to reduce the intensity of the return beam at the expense of usable output power. At first, I was using a 5 mW green laser pointer that had a real on-off switch and was designed for more or less continuous operation. But the transmission through two ~99 percent mirrors was so low that only in a totally dark room, was it possible to begin to see anything coming through. Even so, that did enable the SFPI to be more or less aligned and to produce a viewable signal. But the pointer was probably multi-mode on its own and really complaining with constant mode hops, oscillation, and noise. However, these mirrors are very narrow band and almost useless for a green HeNe laser at 543.5 nm, which I tried next. So, I switched to a ~20 mW Coherent C215M laser. Its higher power made life a lot easier and for the most part, it seemed immune to the back-reflected beam, though there was some jitter and the occasional mode hop (which may simply have been due to the C215M case not being temperature-controlled).

    The two SPFI mirrors must be aligned parallel to each other to a very small fraction of a mR - much higher precision than even for a large frame (narrow bore) HeNe laser. And with the mirror adjusters locked tight, just pushing on the massive mirror plate still resulted in a detectable change in the signal. I ended up using rubber wedges between the mirror mount plate and backing plate to fine tune it since it was impossible to use the adjusters alone to peak the signal. There was too much friction and backlash to make precise enough tweaks. A ramp generator with individual bias settings for each PZT would have helped with the fine alignment, but I didn't have one handy.

    So, I built a little bias box for use with the SP-476 (or any other single output ramp generator). It has a built-in 200 VDC power supply (salvaged from a particle measuring system, probably for an avalanche photodiode) so it can be used with any ramp generator. Three pots provide independent bias to the normally grounded ends of the PZTs. There are 100K ohm resistors in series with the PZTs for protection against shorts in the PZTs or wiring, and 0.1 uF capacitors between the pot wipers and ground to bypass any feed-through of the drive voltage via the capacitance of the PZTs. (The operation manual for the Burleigh RC-44 Ramp Generator shows the PZTs being composed of separate ramp and bias sections with the center point grounded. But the Burleigh thing only has single PZT elements. Thus the bypass capacitors may be needed.) With the pots more or less centered, the mechanical alignment can be performed so that the display looks decent with tall narrow peaks, but it doesn't have to be the absolute best possible. Then, the three pots provide convenient and repeatable optimization.

    Since even with near perfect alignment, there is inevitable walk-off and spreading after a zillion reflections, a small photodiode or a small aperture in front of the photodiode also helps to improve resolution. In the end, I used a photodiode with an active area of about 1x1mm.

    With careful setup (laser, mirror, and photodiode alignment), the Burleigh thing is now capable of a finesse of 200 to 300 at a wavelength of 532 nm. For some reason, when adjusted for best resolvance the display has a skinny peak sitting on top of a wider pedestal, with the FWHM point being well above the wide section. The pedestal is assymetric though, more spread out on one side than the other. But this is not an electrical issue such as a slow photodiode preamp - the spread out side swaps if the polarity of the PZT drive is reversed. If a positive input to the PZTs decreases cavity spacing (by increasing the length of the PZT element), the wide side is on the slope of the peak going towards smaller mirror spacing. It almost has the appearance of one or more additional lasing modes just at the limit of resolvance, but this laser has been tested with another SFPI and is known to be pure SLM. And multiple lasing modes so close together that remain stationary with respect to the main mode as these do would be extremely unlikely or impossible anyhow. So, this is very likely simply a result of imperfect alignment as the peak can be increased to a decent height relative to the pedestal with enough fiddling. :) The higher ratio of peak to pedestal also translates into better finesses. One of the better results so far (with the replacement mirrors) is shown in Burleigh Thing Plane-Plane SFPI Display of 532 nm SLM DPSS Laser. The unequal heights of the two peaks is likely due to differences in sensitivity of the three PZTs, which has not been corrected since they are driven in parallel. But this does show how sensitive the finesse (and thus peak height) of a plane-plane F-P cavity is to mirror parallelism: A miniscule change in alignment over a movement of less the 1 µm (with a mirror spacing of over 37 mm) is enough to produce a sizable effect. The relative heights of multiple peaks in the display can also be changed at will with the mirror bias pots.

    The main benefit of a plane-plane SFPI is that the FSR can be varied over a wide range by changing the distance between the mirrors. But even with the non-adjustable mirror (and PZT) mounted in cylinder that's a precise fit to the SFPI body, moving it any significant distance will mess up alignment, requiring going through mechanical alignment all over again!

    So the conclusions of this exercise (and my plane mirror SFPI) may be that attempting to make a high finesse plane-plane SFPI is not worth the effort unless it's absolutely needed, usually to achieve a higher FSR than what's practical in a confocal SFPI due to the very small highly curved concave mirrors that are required. The highest FSR for a commercial confocal SFPI I know of is 30 GHz from Coherent. Those mirrors, which have an RoC of only 2.5 mm, look like the interior of a pea that's been cut in half with each being a good portion of a full hemisphere. How are high quality mirrors like that even ground and polished? And confocal SFPIs from most other manufacturers only go to 8 or 10 GHz. A plane-plane SFPI can go up to over 1 THz! But if this is required, start with a precision instrument like the TecOptics FPI-25 or Tropel 350 described below. As a practical matter, though, a monochromator-based Optical Spectrum Analyzer (OSA), while pricey, is probably a better and easier instrument to use with high bandwidth lasers.

    Burleigh TL Series Plane-Plane Scanning Fabry-Perot Interferometer

    This small head has no model number and originally I thought it might have been custom and intended as a tunable etalon though it was in the standard Burleigh 4-axis gimbal mount as shown in Burleigh Plane-Plane Scanning Fabry-Perot Interferomter Head in Gimbal Mount. And at first I assumed it was a normal confocal SFPI head with IR (1,5xx nm) mirrors (based on appearance) but the behavior was strange: It was not possible to even obtain "lumps" for a display using a green (532 nm) DPSS laser (which should have worked at least marginally at 1/3rd the design wavelength). The reason became obvious once I realized that the mirrors were planar - my ramp generator was driving only one of the three PZT stacks so the mirror was tilting rather than translating. Several view of the head alone are shown in Burleigh Plane-Plane Scanning Fabry-Perot Interferomter Head. The bottom views are looking into the head with the rear focusing lens and photodetector assembly removed.

    Testing with a Melles Griot 05-LIR-151 1,523 nm HeNe laser and ramp driver wired to all three PZTs in parallel, resulted in a good enough display to deduce the FSR. The distance between longitudinal modes of the 05-LIR-151 is 438 MHz appearing at 1/32nd of the distance between where the display repeats for an FSR of around 14 GHz. In the interest of round numbers, I'm guessing it is actually 15 GHz. :) It may that the mirror spacing is adjustable but I have no idea how.

    Three screws at the back of the head provide for coarse mirror parallelism adjustment with the PZT bias settings doing the fine alignment. Then the Pan and tilt of the mount are used to set the beam to be exactly normal to the mirrors. This, of course, results in a strong back-reflection directly into the laser, so ideally some type of optical isolator is required. However, simply attenuating the beam with a neutral density filter allowed for acceptable stability. The finesse is poor - optimistically 75 - but this may be due to imperfect mirror parallelism, the slightly expanding beam of the laser, and the coatings not being optimal for 1,523 nm.

    Some time later, I found the Burleigh/EXFO TL Series Laser Spectrum Analyzer Brochure which seems to fit. :) So, this is probably the TL-15-15-14-99 laser spectrum analyzer (15 GHz FSR interferometer body with TL-115-14-99 1,450 to 1,650 nm mirror set). Now I need to find a proper RC-93 controller, or better yet, FPS-250-NuView PC USB controller and EXFO NuView Laser Spectral Analysis Software. ;-)

    Burleigh RC-42 Ramp Generator

    The Burleigh RC-42 is a high voltage ramp generator includes the usual ramp amplitude and speed controls as well as three bias controls and three gain trim-pots. These provide for electrical fine tuning of mirror alignment and compensation for unequal PZT sensitivity on interferometers with triple PZT stacks like the Burleigh triple-pass SFPI described above. It can also be used with single-PZT instruments by ignoring 2 of the 3 outputs.

    Here are some relevant specifications, determined by testing. All values are approximate:

    Here are two photos:

    The RC-42 is both larger and heavier than it appears in the photos. At first I was wondering if there might be vacuum tubes inside! The power transformer may be from the tube era - a Stancor 8418 (230-0-230 V rated 50 mADC, 6.3 V at 2.5 A) but it is all solid state and even has several ICs, though a few of them are TO5 cans jammed into 8 pin DIP sockets for some reason! As it turns out, much of the weight is in the thick steel chassis and cover. That doesn't help it to look any smaller though. :-)

    Here is the pinout for the DB9M PZT connector:

      Pin   Function
     ----------------------------
       1    PZT Drive 1
       2    PZT Bias 1
       3    PZT Drive 2
       4    PZT Bias 2
       5    PZT Drive 3
       6    PZT Bias 3
       7    NC ??
       8    Chassis Ground
       9    125K ohms to Ground
    

    There is a wire attached to pin 7 so it may actually do something, but it was not obvious as there was no voltage on it and infinite resistance to Ground.

    TecOptics FPI-25 Scanning Fabry-Perot Interferometer

    The TecOptics FPI-25 is a general purpose instrument with a plane-plane cavity that can be set for an FSR of more than 300 GHz (mirrors almost touching) to a much lower FSR when the mirrors are at their maximum extent. They accommodate easily interchangeable mirror sets covering several wavelength ranges. There's no reason the FPI-25 couldn't also be configured as a confocal SFPI with suitable mirrors. Of course, like other voltage-controlled Fabry-Perot etalons, it may also be used in other ways in addition to as an SFPI for CW lasers. With a slow ramp, it can generate the time-averaged spectrum of a quasi-CW laser, and with a fixed (or feedback-controlled) drive voltage, it can act as a tunable optical filter.

    The FPI-25 is similar in basic capabilities to the "Burleigh Thing" described above (when used with only a single pass) but has a somewhat better set of adjustments designed to be used for optics experiments. Cavity spacing is varied by a knob turning a fine-threaded rod that moves the massive Invar resonator assembly. It may be locked in place once the desired position is reached. Three knobs which turn differential screws are used for extremely precise mirror adjustment, though this is considered "coarse" compared to what the PZTs can provide. The triple PZT with its mirror is fixed to the base. Changing cavity spacing still may require fine tuning of the mirror alignment, but not by much so that individual control of the three PZT offset voltages may be enough for this. The entire instrument sits on a pan-tilt mount so aligning it to the input source is quite easy. It may also be removed from the base and mounted on a (sturdy) post.

    Based on operation manuals I've seen, some versions of the FPI-25 were sold for awhile by Melles Griot, possibly after TecOptics gave up or sold the interferometer product line. The manual for systems that appear identical to those from TecOptics lists three models differing only in the maximum cavity spacing/minimum FSR: W1000 (60 mm/2.5 GHz), W2000 (100 mm/1.5 GHz), and W3000 (150 mm/1 GHz). There were at least two ramp generators. The basic version (FPZ-1-RG, photo below but not referenced by Melles Griot) doesn't have individual controls for bias and offset as does the fancier FPZ-3-RG. Melles Griot also offered an updated version of an optical head similar (but not identical) to the W1000 (50 mm/3 GHz) with their own redesigned ramp generator - the 13-FPC-001. (At least it has a more stylish front panel!) The main change to the optical head aside from reducing the FSR slightly seems to be that the resonator with the adjustable mirror and photodiode assembly is attached to the base and the fixed mirror on the PZTs is what moves to vary the FSR. However, as of 2010, there are no references to either system or their components on the Melles Griot Web site. But here are the two manuals (with permission from Melles Griot):

    Both manuals also include nice sections on basic interferometer theory.

    I have used an original TecOptics FPI-25 with the FPZ-3-RG, but don't actually own one. That system had a separate photodiode preamp box, the DA-1 (mentioned in the Melles Griot manual but with no description). Here are some photos of a TecOptics FPI-25 from an eBay auction. This would be equivalent to the Melles Griot W3000. I should have bought it but wasn't an SFPIs enthusiast back then. :) (I'll be happy to acknowledge the source of the following photos if the owner will come forward):

    Tropel 350 Scanning Fabry-Perot Interferometer

    Tropel was a manufacturer of a variety of laser-related equipment including interferometers and stabilized HeNe lasers, but apparently sold off these products to Coherent, possibly in the late 1970s. I don't know if the Tropel metrology division of Corning is a descendent of the same Tropel.

    The Tropel 350 is a real beauty, but a monster. OK, a beautiful monster! Or, would be if it was cleaned of years of neglect and given a nice polish. :) It has a heavy 4-bar (probably Invar) frame roughly 8x8x12 inches overall, and weighs in at over 30 pounds. Everything is visible which make it extremely useful in a teaching lab (which is where this one is located). It has a large planar mirror mounted on one end-plate and a large planar mirror that can be positioned along the length of the resonator by loosening some set-screws and moving it on the rods. The fixed mirror has three very smooth precise micrometer adjustments. The plate on which the movable mirror is mounted is attached to it by three cylindrical PZTs (about 1-1/4 inches long by 3/4 inch in diameter) that are fully exposed. The mirrors are 2 inches in diameter.

    The controller is fairly basic with 10 turn pots for scan amplitude and offset, a selector for speed, and three bias adjustments for fine mirror alignment. A Thorlabs photodetector with focusing lens (of course not original equipment!) was mounted externally. (Though there really is no "inside" to this SFPI!)

    Although there was a Burleigh manual with the setup, this appears to be Tropel through and through (unless the two companies had some connection in the past). There were no photos or diagrams of the equipment in the manual, so it was rather generic.

    The design wavelength of the installed mirrors is not known but it is definitely not for red and not for green. The reflectivity for 633 nm (red HeNe) is probably less than 75 percent and the reflectivity for 543.5 nm (green HeNe) is probably around 90 percent. (It might be slightly better at 532 nm.) Using a JDS Uniphase 1674 green HeNe laser results in really dreadful finesse - optimistically maybe 20. With the mirror spacing set for an FSR of 3 GHz (about 2 inches), it can barely resolve the longitudinal modes spaced 325 MHz apart. The display looks more like hands with upward pointing fingers than nice narrow peaks. :)

    So, the mirrors might be designed for argon ion blue wavelengths but there was no suitable laser to try. Or, as with my Burleigh Thing, they may be designed for 1.5 µm. However, this instrument probably predates the telecom age (and any need for 1.5 µm) by a few decades!

    I've offered to find a set of 98 to 99 percent 633 nm mirrors that could be installed (possibly with an adapter). These would result in a much more respectible finesse - 150 to 300 under optimal conditions, but at least half of this with ease. Anything over 1/2 inch in diameter is probably adequate, though 1 inch would be desirable simply to prevent the monster from looking totally silly with tiny mirrors. :-)

    Here are some photos (coming soon):

    I even found a reference to the use of the Tropel 350 as a triple-pass SFPI like the one described in the section: The Burleigh Triple-Pass Scanning Fabry-Perot Interferometer, with Tropel RC22 cube corners and RC70-B4 2" mirrors (whatever those are!). Unfortunately, the authors don't mention why this configuration was used. (And the rest of the paper is absolutely boring.) Google easily found several other papers referencing the Tropel 350 so it must have been fairly popular (as these things go) at some point in the past.

    The reason I was able to play with the Tropel 350 was that I received an urgent email from the lab instructor (who I had sold a one-Brewster laser to a few years ago) that the SFPI seemed to be misbehaving. It was occasionally making an electrical arcing sound and then ceasing to move the mirrors. You probably never realized that laser doctors make house calls! :-) In the end, all I could find was that perhaps there was some dirt or some other contamination causing an intermittent short circuit in one of voltages to the PZTs. It sounded like it was originating at the PZT itself, but it could have been in the HV connector or elsewhere since the resulting rapid change in voltage would make the PZT and mirror act like a loudspeaker of sorts. And unplugging and replugging the HV connector and jiggling wires seemed to make it go away, at least temporarily. So, I recommended carefully cleaning the exterior with alcohol including the HV connector and to email me in the morning. :) This would also greatly approve the appearance for the photos I requested! If external cleaning didn't help, then it would be necessary to disassemble the movable mirror mount to get inside the PZTs for cleaning, but that too should be very straightforward.

    Later I found a question about this same instrument with the same arcing problem 7 years ago on a teaching apparatus list server! Here is the only relevant reply:

    "The one that we used to use was modified for an MHV terminal for the high voltage piezo-drive to correct arcing from the previous connector. This also allowed us to use our own driver (that one died).

    There are special spring-like washers and insulators that keep the cell from shorting, perhaps the piezo is touching somewhere in the tube? Or the drive lead has detached from the ceramic?

    If it is like our old one, you must make a tool, like a hollow 3/16" tube with a 1/16" "tee" pin at the end, (shaped like a T) to tighten and adjust the cell after disassembly. This is for a lens ring that held it in, but also lets the beam through.

    Good luck, S. Anderson"

    The instrument described in this posting sounds like it has only a single PZT, but the advice about replacing the connector is even more valid for one with three PZTs! The original 6 pin connector is definitely not rated for 1,000 V and would be a prime suspect, especially given that unplugging and replugging it seemed to make a difference. The replacement doesn't really need to be an official high voltage connector. A Molex or AMP multi-pin nylon shell would have the needed dielectric strength, especially if only every other position were used. In addition, I'd suggest adding a resistor in series with each PZT to protect the driver should a short occur in the PZT assemblies. Something around 100K ohms should provide adequate current limiting without excessively distorting the drive waveform. (This doesn't really matter if it uses shunt regulators like the SP-476.)

    Burleigh CFT-500 Scanning Fabry-Perot Interferometer

    This one looks more like a fat 20 mW HeNe laser than an SFPI. :) There was no model number on the unit I have but based on photos and a spec sheet, it is probably a CFT-500 with an FSR of 150 MHz and minimum finesse of 125. See "CF Series" under Burleigh Brochures. However, it's also possible this was a custom unit and may have been intended as a tunable etalon (filter) rather than as an SFPI, though the difference is simply in application. (But there is no built-in photodetector as would be desirable for an SFPI.) More likely, the detector simply vanished. The unit consists of a HEAVY beautifully machined chrome-plated resonator cylinder about 22 inches in length and 1-3/8 inches in diameter installed in an insulated heater jacket with a 1.5K ohm thermistor temperature sensor for thermal regulation. The front mirror is mounted in an end-cap that has threads for fine tuning the cavity length and a ring to lock it in place. The back mirror and PZT is in an end-cap that screws on and seats against a fixed joint. It also has the 1K ohm thermistor and an SMA connector for the high voltage PZT drive. Both front and back have glass windows to prevent contamination. I don't know if it is intended to be hermetically sealed but it does come at least close with O-rings on all joints. And there is a small flat-head screw also with O-ring seal to perhaps allow for the pressure to be equalized - or something. ;-) The distance between the mirrors is just about 19.7 inches (0.5 m) for an FSR of 150 MHz. This entire assembly - which weighs in at around 8 pounds - is housed within the insulated heater in a light-weight aluminum cylinder about 23-1/2 inches in length and 2.5 inches in diameter. The total weight is about 11 pounds. There are separate cables for the high voltage PZT drive and the heater/thermistor.

    I acquired this unit on eBay. Well, parts of one plus a second outer cylinder with insulated heater and not much else. In fact, on eBay they were listed as "Burleigh Lasers". :) At first I was unable to identify the wavelength of the mirror set. It had the appearance of having high reflectance for the all too common (and useless to me) 1,5xx nm IR wavelength - blue/green in reflectance and pale pink/orange in transmission. However, these usually have at least some amount of reflection at 532 nm. This had very little - under 25 percent. But using an Ocean Optics USB2000+ with a tungsten lamp to get an idea of the transmission in the visible range, there was a very obvious stretch of low transmittance from around 475 to 510 nm. Indeed, at 473 nm, the transmission is around 1 percent and at 488 nm the transmission is only around 0.2 percent. Bingo! This should have a nice high finesse for the quite common 488 nm Coherent Sapphire OPSL and other modern replacements for the 488 nm argon ion laser. However, this does not appear to be one of the standard mirror sets, which have a wider wavelength range (450 to 550 nm would be the relevant one). The finesse may be more than 750 at 488 nm for a resolution of better than 200 kHz. It's still possible that the design wavelength is something other than 488 nm. It could even be an IR wavelength 3 times one somewhere in the range of the visible wavelengts (475 to 510 nm) or 1,425 to 1,530 nm, in which case its reflectance would be even closer to 1 and the finesse could be higher by as much as a factor of 10 or more. This in fact seems like a more realistic range except that none of the standard CFT-100 mirror sets have a wavelength range in the IR consistent with the measured VIS transmission.

    Unfortunately, one of the first things I found out was that the PZT with mirror attached had broken free of its glue and was dangling by a single wire. (Not surprizingly, there was a scribbled note on the outside of the outer cylinder: "Broken, please fix.". The seller didn't mention that or show it in the auction photos!) A chunk of the PZT material was also missing from the PZT cylinder, pieces bouncing around elsewhere. The good news is that neither mirror appears to have been damaged and the missing piece of the PZT probably won't make that much difference in performance. Re-attaching the PZT cylinder was a bit of a challenge though because it is recessed inside the end cap of the main SFPI cylinder and that doesn't come apart. I was afraid at first that it wouldn't be possible to assure decent mirror alignment. But at what is probably the original orientation, it appeared to seat square to the axis. Some careful application first of 5 minute Epoxy in 3 equidistant spots, then slow-cure stronger stuff in between seems to be satisfactory. The hole where the missing bits of PZT used to be came in handy though as the electrical connection to the outer surface of the PZT cylinder used to be via spring clips between it and the inner surface of the metal end-cap. (I have no explanation as to why a wire wasn't soldered to that like the inner surface.) There was no practical way of installing them ahead of time, but once the glue had cured, it was a simple matter to use a dental pick to carefully insert a clip on each side of the hole. (I never did find a 3rd clip, though I suspect there may have been 3 originally.) Part of the SMA socket had also broken off and was stuck in the cable plug. That was soldered back in place. It broke off again later, so a wire was simply soldered to the connector and attached via the screw that sealed the interior. The wires to the thermistor had been cut flush, so the remains of the cable had to be drilled out and the wires were then reattached directly. Aside from these minor problems, it was in perfect condition. :) But the final result should be very close to what it was when new. Burleigh High Resolution Scanning Fabry-Perot Interferometer (Tunable Etalon) shows the resonator cylinder, heater (foam insulating cylinder not shown), and outer aluminum cylinder, along with closeups of the front and back of the resonator and the interior of the rear end-cap housing the PZT and rear mirror. (My designations of front and rear are of course arbitrary.) The SMA connector is for the PZT HV, the twisted pair is for the temperature sensor, and the flat head screw provides access to the interior. The mirrors are pale orange/pink in transmission and blue in reflection, so lighting and angle determined how they appear in the photos.

    The rear mirror is glued into a holder which attaches to a flange glued to the end of the PZT cylinder via the 3 screws that are visible in the bottom middle photo. The other end of the PZT cylinder is simply glued to the end-cap, deep inside, adjacent to the window. Repair would have been somewhat easier if the window were removed, but that could not be done non-destructively. So, I had to use a long narrow stick to reach in and apply glue taking care to avoid getting it all over the window and inside of the PZT cylinder. This was mostly successful. ;-)

    My initial test with a JDSU 2201 488 nm air-cooled argon ion laser was inconclusive as the signal was extremely noisy with the PZT drive turned off, even after minimizing back-reflections, but maximum amplitude when the interferometer was perfectly aligned. The noise wasn't apparent with the photodiode directly in the beam. So, perhaps, it was actually frequency jitter due to high ripple in the ion laser power supply interacting with the highly selective interferometer. Yes, I know this is grasping at laser straws.... However, the expected spots did appear at the output indicating that alignment is probably acceptable. And the PZT makes nice ticking sounds when driven with my SP-476.

    Final testing had to await a suitable narrow line-width low noise 488 nm laser. A Coherent Sapphire 488-50 was the perfect candidate. It was first checked on a Spectra-Physics 470 SFPI head, which has an 8 GHz FSR, and was found to be pure single longitudinal mode from 12 to 50 mW (the range over which output power can be adjusted via RS232) both when stable and during the transitions. Then the Burleigh high resolution SFPI head was mounted on the same platform used to test the Ultra-High Resolution Scanning Fabry Perot\ Interferometer.

    The results so far are disappointing. The PZT works well, with ~200 V to span 1 FSR. There is no evidence of asymmetry from the missing chunk of PZT. But the maximum finesse appears to only be around 100, not the ~750 or more I was expecting. No amount of alignment was able to reduce it significantly. I'm quite sure the mirrors are clean and aligned well enough (parallel to each-other and centered). I did try several random focusing lenses at the input with no improvement. Proper mode matching at the input could still be the key though. But now I wonder about the line-width of the Sapphire laser. Could it be that the SFPI resolvance is actually much better than the laser and what's being displayed is really the laser line profile? There are no published specifications for its line-width. For that matter, Sapphire lasers are not even guaranteed to be SLM, just that many like this one are SLM. The Sapphire is a doubled OPSL - Optically Pumped Semiconductor Laser. But will its line-width be closer to that of a diode or a solid state laser? My inclination is that the wide peaks are still likely due to an SFPI issue but cannot be certain at this point.

    Tropel 2440 Scanning Fabry-Perot Interferometer

    This is a very unusual compact SFPI head best described as "cute". :) See Tropel Model 2440 Scanning Fabry-Perot Interferometer. It is about one half the diameter and length of most of the other common confocal SFPI heads such as the Coherent/Tropel 240 and SP-470. Note the AA cell for comparison. (No, unfortunately, it doesn't run on batteries!) If one were to use a miniaturizer on a more common SFPI head, the Tropel 2440 might be the result. :-) It is small enough to fit in some one inch adjustable mirror mounts, though minor modifications to the mount may be necessary. But an adapter (not shown) enabled it to be installed in a standard two inch optical mount - which of course partially eliminates the benefits of being so compact! But there are at least two other notable characteristics associated with the 2440 beyond its size:

    The 2440 head is easily disassembled (reversibly) to its major components: The detector assembly unscrews from the body of the SFPI revealing the recessed lock ring securing the rear mirror. A shroud enclosing the front of the head is simply pressed onto the main body cover over an O-ring. (The cover is normally sealed with adhesive and not supposed to be removable minor details like that never got in the way of a nice dissection. But that's also reversible.) The shroud protects the front mirror and has a threaded hole in front for an aperture or possibly other optic (like a polarizer). Pulling off the shroud reveals the threaded cap securing the front mirror to the end of the PZT cylinder.

    From left to right at the bottom of the photo are: photodiode assembly, main body with PZT cylinder and rear (curved) mirror (hidden), front (planar) mirror, threaded ring to secure front mirror, main body cover, front shroud, and front aperture (to assist with alignment). The rear mirror is recessed but can also be removed using the appropriate spanner or other suitable tool to unscrew its lock ring. However, I don't know if the mirror sets were supposed to be replaceable by the user. It's not obvious how the precise mirror spacing is adjusted, although it may simply be that the rear mirror seats against a spring or O-ring and the lock ring is then able to move it enough to cover the required adjustment range. I haven't attempted to remove the rear mirror - yet.

    While the entire detector assembly is easily replaced, it's locked together with glue and this one had a bad an intermittent connection inside! So I had to rip it apart, which required the use, among other things, of a heat gun, files, pliers, wrenches, clamps, and a liberal sprinkling of 4 letter words. :( :) But after reassembly, it still looks decent. And although the PD itself was still good, it is now socketed just in case. ;-)

    This particular 2440 probably has a mirror set with an FSR of 7.5 GHz. That's somewhat of a guess based on eye-balling the mirror spacing and assuming the curved mirror is the same as one of those for the 2440's bigger brother, the Model 240. It's possible there is sufficient space to install a mirror for an FSR of 1.5 GHz, requiring a hemispherical confocal mirror spacing of around 25 mm, one half that of the standard confocal cavity. With a suitable spacer, a 50 mm RoC rear mirror could be installed about 1 cm further back, and the front planar mirror (which would be unchanged assuming the wavelength range remained the same) could be positioned forward to extend the cavity (though it may not fit inside the shroud). Or, mirrors for a non-confocal Mode Degenerate Interferometer (MDI) spacing could be installed. This would enable the head to be set up for an intermediate FSR using a 1.5 GHz mirror set. Now that's a thought. ;-) For example, the 3rd order resonance would have an FSR of around 2.0 GHz with a mirror spacing of 1.25 cm, which should be easily accommodated by adding a spacer in front of the rear mirror. (See the section: Selectable FSR Mode Degenerate Fabry-Perot Interferometers.)

    The wavelength range of this unit appears to include 1,064 nm. There is, of course, nothing on the label, only "Tropel 2440". The mirrors are bluish in reflection, slightly pink in transmission, which normally suggests either VIS blue/green, or IR at 1,5xx nm. But wavelengths from 457 to 544 nm (multi-line argon ion and green HeNe) pass right through, as do wavelengths near 1,319 nm (YAG) and 1,523 nm (IR HeNe). And the photodiode is silicon since its voltage drop is around 0.7 V and it has good sensitivity to 532 nm. An Si PD would be blind above around 1,100 nm. Finally, I dug up a 1,064 nm laser and bingo! The mirrors block its beam, so 1,064 nm is likely within the wavelength range. The beam from a 870 nm diode laser passes through without much attenuation so the wavelength range can't extend below around 900 nm. My guess would be that it has the same coating, and is similar or identical to the mirrors for the 1,000 to 1,100 nm range or possibly the 900 to 1,070 nm ranges available for the Tropel model 240. But to achieve similar finesse in the hemispherical cavity (which cuts finesse in half), it would need to have a higher reflectance (assuming the coatings on both the planar front mirror and curved rear mirror are the same). (See the section: Coherent/Tropel 240 Scanning Fabry-Perot Interferometers.) Thus the complexion of the mirror coatings was a false trail. Actual testing will have to await a low power reasonably well behaved (longitudinal mode-wise) 1,064 nm laser I can conveniently power where my SFPI test setup is located. ;-)

    The only reference I have to the Tropel 2440 is a 1971 advertisement which states: "The Model 2440 is the only spectrum analyzer that converts to Pz drive. Unmode-matched finesse is >150. Scan 1 free-spectral range in 25 V. And it's inexpensive.". I'm not sure what "Pz drive" means :) but everything else is consistent with a hemispherical confocal cavity SFPI. The complete print Ad which also lists other Tropel SFPIs may be found at Tropel Laser Spectrum Analyzers (1971).

    I'm tempted to replace the mirrors ones for 633 nm. The rear one would be 43 mm RoC/1.7 GHz mirror as in my home-built SFPIs, and the front one would be a planar HR or near-HR. Even if the cavity length can't be extended to the required 21.5 mm for the hemispherical confocal cavity, a higher order (shorter) setting could be used, with the benefit of being able to be set for a larger FSR as noted above.

    The miniature RF connectors it uses are the same as those on some other Burleigh SFPI heads, but so far, I've been unsuccessful in identifying them. If anyone knows what they are called and/or has mating cables available, or more information on or other examples of this type of SFPI, please contact me via the Sci.Electronics.Repair FAQ Email Links Page.



  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Monochromators

    Basic Description

    While there are many ways of determining the wavelengths produced by a laser or other light source, the simplest one beyond the use of calibrated eyeballs is probably a monochromator. It's possible to construct one from inexpensive parts but they also show up surplus by themselves or as part of other optical devices like spectrophotometers, DNA analyzers, fluorescence spectrometers, and other lab equipment with even more obscure names.

    A monochromator is an instrument which accepts a light source as its input and can select not quite a single wavelength, but a narrow band of wavelengths. A monochromator is probably the simplest device for determining wavelength of a laser or other light source where standard calibrated eyeballs aren't sufficient. (The longer harder to pronounce term "monochronometer" is also commonly used but it refers to the same type of device.)

    There are many types of monochromators but here we only describe one of the simplest, consisting of the following:

    The arrangement above using a planar diffraction grating is acceptable if the input light source is well collimated and aligned with the monochromator's optical axis. But, making the diffraction grating slightly concave with its two focal points at the slits makes the system less sensitive to the orientation or divergence of the input beam and provides better selectivity since it is essentially imaging the light at the entrance slit into the exit slit. Alternatively, spherical lenses or mirrors can be used with a planar diffraction grating to achieve the same effect.

    Due to the way a diffraction grating selects wavelength, if the linear travel of the lead screw is converted into rotation of the diffraction grating by a lever of the proper length, the result is a linear relationship between the "nut" location on the lead screw and wavelength. As long as the pitch (lines/mm) of the diffraction grating is known accurately, the relationship will be exact. Thus, a simple multiturn precision dial can be used to read off wavelength. Or, for an automated instrument, a stepper motor will have a constant nm/step size.

    A fully optical monochromator - with no electronic detector - is perfectly adequate and may actually be preferred for measuring laser wavelengths in the visible range at least since almost any laser is powerful enough to result in a beam at the output of the monochromator to be easily seen. However, for measuring the spectrum of something like a glow discharge (as in the bore of a HeNe laser tube) or UV or IR lasers, a high sensitivity detector is essential. Spectra for varioue elements and compounds can be easily found by searching the Web. The NIST Atomic Spectra Database has an applet which will generate a table or plot of more spectral lines than you could ever want.

    CAUTION: When exploring the interior of a monochromator, DO NOT touch the surface of the diffraction grating. Cleaning a grating without damage is difficult at best, and may be impossible for some types of gratings

    The following sections describe some typical monochromators.

    Intruments SA Model H2O 1200 Monochromator

    An example of a simple monochromator is the Instruments SA H20 1200VIS, designed to span the visible spectrum with either manual or motorized control. A slightly battle weary sample of this unit is shown in Instruments SA H20 1200VIS Monochromator. A diagram of its organization is shown in Basic Monochromator Opto-Mechanical Layout and the actual underside mechanism in Instruments SA H20 1200VIS Monochromator Lead Screw Lever System.

    The two mirrors allow the input and output beams to be co-linear but have no other effect. A variety of input and exit slits permit the resolution and sensitivity to be easily changed. All inner surfaces of the monochromator as well as some additional "Absorbers" are coated with a super flat black material to absorb as much stray light as possible. This includes both unavoidable scatter as well as the zeroth and higher order diffracted beams (not shown) from the grating.

    In the diagram, a hypothetical light source consisting of red and green lines is shown, perhaps from a very strange Ar/Kr ion laser. The green beam is diffracted less than the red beam and is thus not passed through the exit slit.

    Assuming the mechanical design of the monochromator is correct, only two adjustments are needed for calibration: The angle of the diffraction grating with respect to the lever, and the dial setting with respect to the lead screw.

    When I found the 1200VIS, both of these were far off. Simply adjusting the dial to coincide with a 632.8 nm red HeNe laser resulted in a green 532 nm DPSS laser pointer reading 529 nm. This indicated that the lead screw wasn't moving the lever enough over that range. To remedy this, I slightly loosened the set screw locking the lever to the diffraction grating shaft, but still tight enough so that normal dial twiddling wouldn't affect the relationship. Then, using the 632.8 nm and 532 nm lasers as references, the diffraction grating was rotated incrementally with respect to the lever until the difference between the readings was exactly 100.8 nm. It would have been even better to use more extreme wavelengths like a 457 nm DPSS or argon ion laser and 647 krypton ion laser, but these will have to do for now. Checking some other known wavelengths including 543.5, 594.1 and 611.9 nm (green, yellow, and orange HeNe lasers), as well as a 640 nm (errant laser line being produced by the red HeNe laser) showed them to be quite accurate.

    For determining laser wavelengths, a simple white card is all that is needed on the output. However, in its original application of detecting spectral signatures of plasma flames and such, a sensitive photodiode or PhotoMultiplier Tube (PMT) detector would be mounted beyond the exit slit.

    The overall performance (including wavelength precision and repeatability) using the dial, is now better than 0.5 nm, limited by very noticeable backlash in the multiturn dial mechanism. Originally, the 1200VIS also had a stepper motor (which I removed), PMT detector, and controller with data acquisition system (all whereabouts unknown). That was probably much more accurate but for my intended uses, this will be fine.

    Verity Instruments Model EP200 Monochromator/Detector

    These devices have been turning up on eBay lately in a variety of flavors, both manual (EP200Mmd) and motorized (EP200Msd). The latter is really only more useful if one of the mating controllers is also acquired - or one is built. Contructing one would not be that difficult with a microcontroller and stepper motor driver. Both versions include a micrometer adjustable monochromator, PhotoMultiplier Tube (PMT) detector, its high voltage power supply, and preamp, all built into a case about 7.5" x 7.2" x 2.6" inches. It runs on +/-15 VDC.

    As its name implies, the optical output of the EP200's monochromator is sent directly to a detector inside the unit which produces a voltage between 0 and +10 V proportional to light intensity at the selected wavelength. There is no exit port for light. The case is very well sealed against stray light - the light goes in but it never comes out. :)

    An annotated photo can be found in Verity EP200 Monochromator/Detector Organization. In addition to the parts being labeled, the beam paths for the a sample input, zeroth order (reflected input), and first order (useful) spectral lines are shown. The input here may be from the bore discharge of a HeNe laser tube. The wavelength micrometer is set for 587.6 nm in the photo thus selecting the intense yellow (helium) line which passes through the exit slit to the PMT. Of all the other lines shown (there are many more in the spectra of He and Ne), only the green one even makes it to the vicinity of the exit slit, but it's off to one side. Note that only the central ray for the incoming beam and each of its spectral components has been drawn on the photo. However, for a diffuse source like a glow discharge (as in this example), light bulb, or even an LED, the internal beam will expand to fill the large concave holographic diffraction grating (which provides high light gathering power). The bounce mirrors must also be larger than what might be expected due to the size of the internal beams. Only with a collimated laser, would the actual beam paths closely resemble the narrow ones shown.

    This model uses an actual machine shop type micrometer assembly to select wavelength. While not as convenient as a direct reading multidigit dial, rotation selectivity is better since there are only 25 nm per revolution compared to 100 nm for the typical dial. And, there is no backlash in the readout so the precision is better. However, when using narrow slits, the wobble in the micrometer becomes significant. The motorized version with its long shaft may be even better but it's useless without the matching controller because there is no readout.

    The sensitivity using the PMT is truly amazing. With the HV nearly as low (close to 0 V) as it can be, -200 VDC, and the gain turned nearly all the way down (10 percent), simply placing a small neon lamp power indicator near the entrance slit overloaded the preamp. Numerous lines in the neon lamp spectrum could be easily found. Since the PMT provides most of the amplification, the preamp really isn't that sensitive in the grand scheme of things. I measured 10 V out for 3.75 uA in at the 100 percent gain setting. Thus, replacing the PMT with a photodiode would only be useful for high intensity sources or low power lasers aimed directly into the entrance slit. But that would be such a waste of the EP200 since one of its main benefits is having the super high sensitivity detector built-in. In fact, when using the EP200 with coherent sources like lasers, there may be a small amount of ripple in the peak response versus wavelength and orientation of the instrument, presumably due to interference effects similar to visible speckle. This is not present with gas discharge sources. The EP200 is also polarization sensitive with a difference in response of about 2:1 for s and p polarized light. Thus, using the EP200's output to monitor the mode sweep of a random polarized HeNe laser may result in excessive amplitude fluctuations. But why would anyone want to do that with a monochromator?!

    Detailed information on this series of instruments can be found at Verity Instruments Monochromators. A description and photo of the interior is there as well as connector pinouts. One thing I did determine that isn't on the Web site is that there is a small slide switch on the PMT HV PCB inside the unit to select internal (adjustment pot) or external PMT high voltage control on pin 1 of the DB9 (+2 to +10 VDC for -200 to -1,000 VDC). The Web site simply mentions that internal or external HV control is selected at the time the order is placed. Some units (don't know if they would be newer or older than the ones I've seen) may only have a jumper. The lid can be removed without much risk of contamination as the box is not sealed. Just don't touch the diffraction grating as it cannot be cleaned.

    A sticker under the micrometer cover as well as another one inside the unit details the function of the 4 position DIP switch, which is to control the PMT preamp bandwidth as follows (0=Off):

          Switch
         Position     Preamp
           4321      Bandwidth
      ----------------------------
           0000      530    Hz
           0001        5.3  Hz
           0010        0.53 Hz
           0100        0.24 Hz
           0110        0.17 Hz
           1000        0.11 Hz
           1010        0.09 Hz
           1100        0.08 Hz
           1110        0.07 Hz
    

    For manual control, only the first or second setting would probably be useful. Otherwise, the response speed is so slow that spectral features would be missed.

    The pinouts for the DB9 connector are available on the Verity Web site but here is a summary with the voltage polarities explicitly noted:

      Pin  Function
    -------------------------------------------------
       1   HV Programming (if enabled, +2 to +10 V)
       2   -15 VDC (225 mA, polarity protected)
       3   +15 VDC (50 mA, polarity protected)
       4   Remote HV Monitor (-2 to -10 VDC)
       5   Signal Output (0 to +10 V)
       6   DC Offset (Zero voltage)
       7   Circuit Ground (Power and signal return)
       8   NC
       9   Circuit Ground (Power and signal return)
    

    I think the HV will actually go down (up?) to 0 V but probably isn't very useful much closer to 0 V than about -200 VDC. Note: Some documentation I've seen shows the HV Programming input being -2 to -10 V but all the units I have work fine with +2 to +10 V. I do not know for sure what the DC offset pin is used for. It is connected to the wiper of the ZERO pot and produces a small DC voltage (less tha 1 VDC) that varies with the pot. But, it may also be an input to allow the controller to set the DC offset remotely.

    The next test was to look at the spectral lines of the discharge in the bore of a 1 mW HeNe laser tube. Placing the bare tube next to the EP200 entrance slit and approximately level with it resulted in a large response. (HV of -300 VDC and gain of 50 percent.) However, this particular EP200 came with 500 um slits, too wide for my taste. :) While the strong lines could be seen, weaker ones adjacent to them were buried. :) So I modified the slits by using 5 minute Epoxy to glue a piece of a single edge razor blade to each, positioned to reduce the slit width to between 100 and 150 um - about as narrow as could be done by eye. (See below for more details on modifying the slit width.) This worked great in improving the resolution and allowed weak lines adjacent to strong ones separated by well under 1 nm to be resolved. However, since each slit was narrowed from one side only (that was enough of a pain in itself!), the wavelength calibration shifted by about 1 nm. (I must have guessed wrong since if they had been narrowed from the proper side relative to each-other, the wavelength calibration shouldn't have changed.) To remedy this, the micrometer mounting plate screws were loosened just enough to allow the micrometer to be nudged by about 1/1000th of an inch using the 585.25 nm and 587.56 nm yellow lines of the HeNe laser tube bore discharge as references, and confirmed with the 632.8 nm red lasing line.

    Those 585.25 nm and 587.56 nm lines are significant in that they are from neon and helium, respectively, and if reasonably similar in amplitude, the ratio of helium to neon is correct inside the tube. On the tube I tested, the intensity of the He line was about double the Ne line, indicating that the He:Ne ratio was high. That's better than the other way around. :) This thing makes such determination so easy. :)

    I've built a DC power supply and detector meter box (from junk parts of course!) to drive the EP200 heads conveniently. By default, its analog meter shows the detector output. However, by pressing a button, it will show the HV, which is adjusted via a 10-turn lockable knob if the EP200 is set up for external HV programming.

    I also constructed my own fiberoptic cable adapter (these are also available from Verity) which positions the tip of an SMA fiber connector at the entrance slit. The other end of the fiber would then have a focusing lens that can be positioned conveniently near the spectral source. Even though the amount of light coupled through the fiber into the monochronometer is generally quite small for anything but a laser, with the high sensitivity of the EP200, there is easily enough to take readings.

    A second EP200 I acquired had a bad photomultiplier tube but I replaced the original (Hamamatsu R928HA Hamamatsu R928 Datasheet) with an RCA 931A I had laying around. That seems to work OK, certainly well enough for my needs - it's way too sensitive! There are probably many other compatible PMTs. As long as the PMT has the same side-input, pinout, and fits the socket and housing, that's probably good enough for non-critical uses of the EP200.

    On a third EP200, the grating had fallen out of its mount. How the set screw loosened up will probably remain a mystery. Although there is a long scratch on the grating (possibly from the trauma, possibly from bouncing around during shipping, or possibly it was there even when new). Since the scratch just happens to be perpendicular to the rulings, it really doesn't cause any degradation in performance of any consequence. This unit worked fine after reinstalling the grating and calibration.

    On a forth EP200, the PMT had actually cracked - the main cover had a major ding in exactly the wrong place. Although no internal damage was visible, this must have whacked the PMT. It's otherwise in good condition awaiting a transplant.

    Yet another one had a bad PMT (very high dark current or something) AND a bad PMT transconductance preamp op-amp. The main problem replacing it was that the space is very limited. It's also the only IC that's not socketed. An LM741H in a TO-99 can is pin compatible, but with some creative lead bending, a jelly bean LM741CN DIP (which is much less expensive) can be made to fit. The original very high quality OPA111AH op-amp from Burr Brown (now TI) is quite expensive, but the 741 should work well enough for measuring wavelengths since long term stability is not required. Almost any other common op-amp could probably be shoe-horned in its place. Pin 2 is the input and pin 3 comes from the Zero Offset trim-pot on the front panel. The op-amp offset trim pins are not used.

    CAUTION: Do NOT attempt to measure the output of even a very low power laser by aiming it into the entrance slit. That will completely overload the system and may damage the photomultiplier tube. For a typical 1 mW laser, just arranging the beam to hit a white card positioned at 45 degrees near the entrance slit will provide more than enough signal with the PMT HV near the lower end of its useful range (say -250 to -300 VDC).

    CAUTION: Do NOT attempt to adjust calibration unless you have a source of a known unambiguous wavelength to use as a reference! It's way too easy to shift it too far to get back easily without one. A low power HeNe laser or green DPSS module shining on a white card would be suitable, but NOT a red pointer or other diode laser unless its peak wavelength is known exactly.

    If anyone has proper narrow slits or anything else related to these monochromators that they don't need, please contact me via the Sci.Electronics.Repair FAQ Email Links Page.

    Initial Testing and Adjustment of an EP200

    Checking one of these units for basic functionality is quite easy, requiring only a spectral test source like a neon lamp, HeNe laser tube, or other gas discharge lamp. Given the extremely high sensitivity of the EP200, using the light shining through the window from an outdoor high intensity sodium or mercury vapor street lamp may even be possible.

    Additional equipment that will be needed are regulated +15 VDC and -15 VDC power supplies (50 and 225 mA maximum, respectively), a 10K ohm pot and 5K ohm resistor to build a circuit for setting the high voltage (HV) if using external HV Programming, and a DC voltmeter capable of reading 10 V full scale. A flashlight may be useful as a quick test for confirming that the photomultiplier tube (PMT) detector is responding to light.

    Referring to the pinout for the DB9 interface connector, above, wire up the 15 VDC power supplies, Signal output, and HV programming (just in case). Circuit Ground is the common for everything including all voltage measurements. The COM test point is connected to Circuit Ground. The 5K resistor goes to +15 VDC and then to the top (clockwise end) of the pot, the wiper goes to the HV Programming input, and the bottom (counterclockwise end) goes to Circuit Ground. The value of the resistor and pot aren't critical as long as their ratio is 1:2 so that the HV Programming input is 0 to +10 VDC. Anything from 1K to 50K should be fine for the pot. For a permanent setup, a 10 turn pot may be desirable as the gain is quite sensitive to HV.

    There is no need to remove the large cover which encloses the optics and electronics of the EP200 except to flip the PMT HV selector switch if needed, or a major problem is found. For all tests, this cover should be in place with the screws tightly secured. Ample details on what's in there can be found on the Verity Web site or from the photo Verity EP200 Monochromator/Detector Organization.

    However, we all know that curiosity will get the better of you, so as long as it's open, check that nothing has fallen out, that the diffraction grating is secure in its clamped mount, it rotates freely against spring tension, and that the metal shroud surrounding the grating glass itself is pressed in as far as it will go. DO NOT touch the surface of the grating as it can't be cleaned without degrading its performance!!! DO NOT attempt to remove or adjust the grating in its mount - that affects focus and precise wavelength calibration (beyond what is discussed below). It's not supposed to be all the way in. If for some obscure reason it must be removed, use a depth gauge or other instrument to determine exactly how far in it should go and note which side is up (so that the blaze angle is correct when reinstalled). If there are any serious dents in the cover, confirm that there is no corresponding internal damage.

    CAUTION: On all the EP200s I've tested, simply removing and replacing the cover may alter wavelength calibration by a fraction of a nm. I assume that any slight change in stress on the baseplate deforms it enough to shift the wavelength peak. So, expect to have to do the basic wavelength calibration described below if you do go inside.

    Now that that's out of the way, block the entrance slit with a piece of black tape and double check your wiring before proceeding. It doesn't matter if the micrometer compartment cover plate (two thumbscrews) is installed for any of these tests.

    1. With your HV Programming pot fully counterclockwise (0 V), apply power and confirm that +15 VDC and -15 VDC are present at the proper DB9 connector pins.

    2. Connect your DC voltmeter or a multimeter between the HV monitor test point or pin 4 on the DB9 and Circuit Ground or COM.

      • If there is between -2 VDC and -10 VDC present, the internal HV pot is in control. Set it for -3.0 VDC corresponding to -300 VDC, which is close to the minimum useful HV.

      • If there is no voltage present, adjust your HV Programming pot to confirm that it is in control. If so, set it for about -3.0 VDC output on the HV test point (-300 VDC to the PMT). Note: According to some documentation I've seen, the HV Programming input should be negative. However, all the units I have work fine with positive HV Programming input. But if for some reason yours doesn't, flip the polarity and try again.

      If it is not possible to obtain any high voltage or the full -10 VDC (-1000 VDC to the PMT) on the HV test point with either the EP200 pot or external HV control, the HV power supply (a potted brick, an EMCO model 6858) or associated circuitry may be defective, or there may be a short in the PMT or its wiring.

      To change from internal to external HV programming or vice-versa, remove the main cover on the EP200 by taking out all the screws around its periphery and locate the HV select slide switch near the edge of the electronics PCB between the PMT housing and slit mounting plate. Flip it to the opposite position (toward the control/connector panel to select the EP200 HV pot).

      However, I highly recommend using external HV control as it is more flexible and convenient allowing for very quick and easy control of PMT gain, and won't wear out the internal pot! This is very likely the default for most of these units.

    3. Once the presence of HV has been confirmed and it is set for about -300 VDC, connect the Signal test point or Signal Output (pin 5 on the DB9) to your voltmeter. The reading could be anywhere from negative to off scale at this point.

    4. Set the 4 DIP switches to the OFF position to select maximum signal bandwidth.

    5. Press and hold the "GAIN" pushbutton and adjust the GAIN pot for approximately +5 VDC (on the Signal test point or pin 5 of the DB9) which corresponds to 50 percent gain. (The useful range is from 0.1 to 10.)

    6. With no light input, it should be possible to set the ZERO pot to produce near 0 VDC on the meter, though it may be quite touchy and like to go way negative. Keep it just positive if it won't cooperate by going to exactly 0 VDC. If the Signal Output remains pegged beyond +10 V and won't settle down in positive territory, try turning down the HV to 0 VDC. If the meter can now be zeroed but pegs to over +10 V with almost any HV, the PMT is probably defective. Else, it may be a circuit problem. (Some dark current which varies with HV may be normal but with -200 to -300 VDC, it should be negligible.)

    7. Set the wavelength micrometer for approximately 600 nm. (If the micrometer doesn't turn easily, check the black locking ring near its base, counterclockwise a fraction of a turn to unlock.) Each turn of the micrometer corresponds to 25 nm with each division being 1 nm. Multiply the direct reading by 100 to obtain the actual wavelength.

    8. Shine a flashlight (if available) head-on into the entrance slit. With a broadband source, there should be a response on the meter regardless of wavelength. However, some LED flashlights may not he broadband. So, if using an LED flashlight, rotate the micrometer through the visible range and look for a response. If this test is successful, the EP200 is likely fully functional but may need to be calibrated for wavelength.

    Before proceeding with wavelength checks or calibration, the EP200 main cover should be securely screwed down including the locking screw of the input slit since a slight shift in wavelength may occur when this is done. The EP200 should be placed on a solid surface so it can't wobble as any position or orientation change may result in a variation in the signal output and confusion as to where a peak is located.

    1. Set up your spectral test source in front of the entrance slit, or aim the monochromator to the source. the f/3.5 acceptance of the system is low enough that precise alignment isn't needed but arranging it to be fairly close to in-line with the slit will be best.

      CAUTION: DO NOT shine a laser beam directly into the entrance slit. Almost any laser - even a half dead laser pointer - is way too powerful to be used directly. However, it is safe to shine a low power laser on a white card placed near the entrance slit. More on this below.

    2. Refer to the appropriate spectral charts to determine what wavelength emission lines should be present with your source. For example, three relevant lines for the bore discharge of a HeNe laser are: 585.25 nm (neon yellow), 587.56 nm (helium yellow), and 640.2 nm (neon red). The normal HeNe red 632.8 nm is also present, but quite weak. There are dozens of other lines present but the three cited are particularly strong. If these can be located exactly where they should be, you're done. If they are there but not exactly where they should be, you're almost done. :) When turning the micrometer, avoid applying side-pressure as this will result in a wavelength shift and hysteresis.

      Note that with the typical 500 um slits, it is difficult to resolve the 585.25 nm and 587.56 nm lines as their spacing is less than the spec'd resolution of the EP200. But it's no problem with 200 um or narrower slits.

    Other lines that can easily be checked in a HeNe laser tube bore discharge include: 447.1 nm, 471.3 nm, 492.2 nm, 501.6 nm, 587.6 nm, and 667.8 nm from helium, and 540.1 nm, 585.2 nm, 588.2 nm, 603.0 nm, 607.4 nm, 616.4 nm, 621.7 nm, 626.6 nm, 633.4 nm, 638.3 nm, 640.2 nm, 650.6 nm, 659.9 nm, 692.9 nm, and 703.2 nm from neon. Note that the lasing wavelength of 632.8 nm is not among these medium to strong lines.

    If you're not using a HeNe laser tube but a source like a gas discharge spectral lamp containing some other gas(es), spectra for varioue elements and compounds can be easily found by searching the Web. The NIST Atomic Spectra Database has an applet which will generate a table or plot of more spectral lines than you could ever want.

    If the spectral lines located above are at the proper locations on the micrometer or within the uncertainly due to what minimal backlash there is, you're done. Otherwise, adjustment of the micrometer assembly will be required. This is best done with a HeNe laser or other source with a single spectral line. Otherwise, it may be difficult to know which line you're seeing. The use of such a laser is assumed below. Note that a red diode laser pointer is NOT suitable as its wavelength is not known precisely, nor is the line very narrow. However, a green DPSS laser pointer at 532 nm is perfectly fine.

    1. Set up a low power HeNe laser with its beam shining on a white card placed at a 45 degree angle next to the entrance slit, approximately centered and level with it. CAUTION: Do NOT shine the laser into the entrance slit as it will be way too powerful. This would grossly overload the system and may damage the PMT.

    2. Attempt to locate the 632.8 nm wavelength (assuming a red HeNe laser) by adjusting the micrometer. Unless someone has totally disassembled the EP200 at some point, it should appear somewhere within the adjustment range of the micrometer, probably quite close to where it should be. A HV setting of -300 V and gain setting of 5 V (50 percent) should result in adequate response for a 1 or 2 mW laser. You can try increasing the HV slightly if the response is very weak so that the reading reaches at least half scale.

    3. If the wavelength location is too low, the micrometer must be moved toward the entrance slit side of the unit. If too high, it needs to move away. The return spring on the grating tends to push it away but with the cover in place and pressing on the seals, some persuasion may be needed.

    The following three steps can probably be skipped if the wavelength is within a few nm of the correct location.

    1. Use a right-angle hex wrench to loosen both of the micrometer plate mounting screws inside the micrometer compartment just so they are barely touching the plate. The return spring will move the micrometer assembly away from the entrance slit side of the EP200 to its stop.

    2. Set the micrometer to 632.8 nm exactly.

    3. Use a narrow screwdriver or other suitable tool as a lever vertically between the micrometer mounting plate and EP200 main cover. Slowly move the micrometer assembly toward the entrance slit side of the unit (pull the tool toward the control/connector panel) while monitoring the Signal Output test point or connector pin. When the position coincides with 632.8 nm, the reading will jump. With care, it should be possible to lever the plate back and forth very slowly around this point stopping exactly at peak output so the mounting screws can be tightened. 1/1000th of an inch movement corresponds to 1 nm. With care, a precision of 1/10,000th of an inch is possible. This sounds more daunting than it really is.

    1. Check the peak setting. If the wavelength is only slightly off, it may be possible to loosen the micrometer mounting screws so they are just snug enough to hold the plate in position. Then, gently nudge the plate in the appropriate direction to center the peak at the 632.8 nm setting.

    2. If possible, check some of the lines in the HeNe tube discharge spectrum to confirm calibration. If fine adjustment is needed, it's better to use one of those because there are negligible interference effects to confuse the peak location, as there are with the coherent laser output, which may be somewhat touchy and dependent on input position and orientation.

    3. If another laser is available with a much different wavelength (like 532 nm), it would be worth checking that it too is now lined up correctly with the correct micrometer setting. Should this for some reason be off by more than a fraction of a nm, the relation of the grating shaft to its lever arm has changed. Adjustment of that is a more involved process reserved for the advanced course. :) But, no one should have touched and unless the unit was terribly abused, it shouldn't have changed on its own.

    This description probably makes the procedure sound like it will take all day. Wavelength calibration should require only a few minutes unless you are an absolute perfectionist, in which case it will take forever. :)

    Narrowing the Slits in an EP200

    For end-point detection in whatever processes these instruments normally monitor, the most common 500 um slit is perfectly adequate. But for looking at closely spaced spectral lines, a narrower slit is almost essential. Although the slits width isn't adjustable on the EP200, modifying the slits to be 100 to 200 um is relatively easy. The first procedure is reversible:

    1. Remove both slit assemblies (slit plate in plastic holder). The entrance slit is locked into position by the cover screw next to it. The slit assembly can then be pulled out. The exit slit is under the circular cover plate held in place by 4 small screws on the bottom of the EP200. Once the cover plate is removed, the slit assembly can be pulled out. The entrance and exit slit assemblies are identical.

    2. Gently confirm that the thin metal slit plate itself is secure in each plastic holder. On some units, I've found that the glue has weakened and almost any pressure results in the slit plate popping out. If loose, fallen out, or in doubt, use a tiny dab of 5 minute Epoxy to secure it. Make sure it is parallel to the edge with the shiny side should be down

    3. Prepare a sacrificial single edge razor blade. It doesn't have to be new but the edge should be free of any nicks or dings. Use a pair of pliers to break off pieces that will fit in the hole in the plastic behind the slit.

    4. Narrowing the slits from both sides is not essential but is desirable to avoid a wavelength shift depending on where an small diameter input source is located - above, below, or centered on the slit. This is because the blades on each side will be at the different distances from the diffraction grating. This may result in an error of a fraction of a nm depending on source location, though resolution will not be affected significantly. But, to prevent a fixed wavelength calibration shift if doing only one-side surgery, the entrance slit should be narrowed from the opposite side as the exit slit.

    5. Put a tiny dab of 5 minute Epoxy in the recess near the edge on the appropriate side and use tweezers to place the bit of razor blade in position. I just eyeballed the slit width but using something more sophisticated to measure it is permissible. :)

    6. Check the status of each slit as the adhesive cures to make sure it hasn't shifted position.

    7. Reinstall the slits and check calibration. Since the slits are much narrower than before, increasing the HV and/or gain may be necessary. If the calibration did change significantly, use the procedure in the previous section to correct it. With the narrower slits, calibration error will be more noticeable.

    Where it is known that going back to the wider slit will never be desired, then the following may result in better performance:

    1. Carefully pop the original metal slit insert off of the plastic holder.

    2. Use a sharp pair of scissors to split the slit into two parts without harming the slit edges.

    3. File each side to the two halves can be mounted at the appropriate closer distance.

    4. Use 5 minute Epoxy to reattach them to the plastic holder confirming the correct spacing and adjusting as needed before the adhesive sets.

    The focus is more critical with narrower slits. If the diffraction grating's position in its mounting clamp isn't exactly right, resolution will suffer. If it's never been touched, then don't touch it now as it's unlikely to have moved on its own, and wavelength calibration may be affected by position. But, if you've been fiddling with the grating, now is the time to adjust focus using a diverging beam (laser or LED) as the input. Since this has to be done with the cover removed, the source will need to be bright with gain set low enough so the ambient light doesn't overwhelm the system.

    Monolight Model 6100 Scanning Monochromator

    These consist of a "head" unit which is a compact monochromator where the diffraction grating rotates continuously on a motor shaft. With a suitable controller, an optical spectrum over a wide range (e.g., from 300 to 1,100 nm) can be acquired in about 85 ms. Thus one application is in a fast, though not particularly high resolution, optical spectrum analyzer. The resolution is about 1.4 nm for my particular unit.

    Some general info used to be found at Macam Photonics: Monolight Monochromators. But now it simply says to contact the company. And it may be that Gamma Scientific and/or EG&G (now part of EXCELITAS), sold these systems under their name.

    Simply applying 15 VAC to the power jack will make the motor spin, though the stability may not be that great. Although motor speed is regulated based on a voltage-to-frequency function from the 36 pulse/rotation optical encoder disc, it's likely intended to be more precisely phase locked to a crystal reference by the controller. The 6100 has a built-in trans-impedance (current to voltage) preamp for a photodiode. If displaying the spectrum of a laser, the photodiode can be almost anything as long as it's relatively small area (low capacitance). I used one from a barcode scanner for testing, just positioning it near the output slit. The 6100 provides a trigger signal that can sync an oscilloscope which can then be used to display the spectrum in leu of the controller and data acquisition system. Although a digital storage scope is desirable, my Tek 465B worked just fine in showing the 3 lines of my funny yellow-orange-orange PMS/REO LHYR-0100M HeNe laser head. However, the resolution is orders of magnitude poorer than would be required to view the individual longitudinal modes of any HeNe laser, as wtih a Scanning Fabry-Perot Interferometer (SFPI). The main problem was excessive sensitivity. My photodiode detector had no gain control, being, well, just a photodiode. Perhaps the intended detectors have an adjustment. I had to use neutral density filters to reduce the intensity to a level that didn't saturate the preamp.

    If anyone has more specific information including schematics for the 6105 head unit, or has Monolight hardware they'd be willing to contribute to the cause, please contact me via the Sci.Electronics.Repair FAQ Email Links Page. I had to add a pot to adjust the pullup resistance on the optical encoder on my sample as the signal was about half the amplitude it should have been leading to some peculiar behavior. Although it appears as though this pullup is a "select on final test" resistor, being in a two terminal header rather than being soldered into the PCB, I assume that somehow the output has gone down due to a weak LED or other problem in the opto-detector reading the disc pulses by reflection, similar to the reel rotation sensors in some VCRs. It is probably a standard 3 terminal device (LED and photodiode) which could be replaced but I can't read the part number without disassembling the unit.



  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Optical Wavelength Meters

    Principles of Operation

    While instruments like monochromators and optical spectrum analyzers are capable of determining the wavelength of light sources from light bulbs to lasers, their accuracy depends on the precision of multiple mechanical parts and the quality of the initial calibration. This is because they use what might be termed an indirect method of analysis - typically a diffraction grating moved by a precision mechanism. If there is any real-time reference, it is likely only at a single wavelength so there could be significant error at wavelengths not close to it.

    Where the source to be measured is broadband or has multiple spectral lines, such techniques are generally the easiest and fastest (but see below). However, where a single wavelength CW laser's output needs to be determined very precisely, alternative methods are generally used. (Wavelength meters capable of reading pulsed lasers also exist. See the section: Pulsed Wavemeters.) Instruments of this type may have an accuracy and resolution that is orders of magnitude higher than a monochromator or optical spectrum analyzer.

    The following description is for one common approach and the one used in the Burleigh WA-20 a typical older model dating to the early 1980s.

    The basic wavelength meter or "wavemeter" compares the unknown input with a reference laser by counting fringes for both sources simultaneously in a Michelson interferometer where the path length difference is varied periodically by a motor-driven mechanism. The unknown wavelength (or frequency) is then related to the reference by the ratio of the number of fringes for each during a fixed period chosen to be near where the path length difference is small (to minimize the effects of the coherence length of the unknown laser). Where the desired display is in wavelength (e.g., nm), the reference wavelength is divided by N/No (where N is the number of fringes for the unknown and No is the number of fringes for the reference). Where the desired display is in frequency (cm-1), the reference frequency is multiplied by N/No. (Frequency here is the meaning used by spectroscopy-types: 1 cm/wavelength, or wave number.) The division and multiplication can be easily accomplished with digital counters and simple control logic, similar to that in any vanilla-flavored electronic counter/timer. Phase-Locked Loops (PLLs) will generally be used for both the reference and unknown detector signals to multiply the fringe counts and thus the resolution.

    A red HeNe laser is generally used for the reference laser. For the Burleigh WA-20, an Aerotech OEM1P laser head with an Aerotech brick power supply was found in one unit I checked. (I don't know if all WA-20s used the same laser.) The brick ran from 12 VDC (it's labeled 10-14 VDC) and had a fixed output of 4 mA with a compliance range of 1,200-2,000 V. The specs of the OEM1P laser head (from an old Aerotech brochure) are:

    I doubt that any of these specifications are really critical. A rated output power between 0.5 and 2 mW should be acceptable. Where a replacement laser head is not electrically compatible, another HeNe laser power supply can be easily substituted. Even 0.5 mW is much more than actually required. The only "adjustment" is a linear polarizer that can be rotated to reduce REF power. Even the fact that the OEM1P is linearly polarized probably doesn't make a lot of difference, though a random polarized laser might result in a larger variation of REF signal amplitude during mode sweep and the polarizer/attenuator won't work so an variable ND filter would need to be substituted. To avoid this, a linear polarizer can be added at 45 degrees to the polarization axes of the laser resulting in a beam indistinguishable from that of a random polarized laser at the expense of 50 percent of the output power. (More will be lost with a sheet polarizer, but there will still be 30 or 40 percent of the original power.) With so little power actually needed, this should not be a problem.

    However, one thing that would make a very slight difference is the isotope ratio of neon in the gas-fill, which can shift the peak of the neon gain curve by almost 1 GHz or 1.4 pm, more than 1 count in the LSD of the WA-20 display. This in addition to the contribution of REF frequency change due to mode sweep. The hardest part may be in mounting a non-exact replacement laser - there are a pair of narrowed sections on the Aerotech laser head which mate with brackets in the WA-20! (Melles Griot may have a suitable laser head that would be a drop in replacement.)

    Even if not stabilized (but with a known gas-fill), its wavelength will be accurate to better than 1 part in 106. A stabilized HeNe laser locked to the gain curve can be a couple orders of magnitude better and an iodine line stabilized HeNe laser, even better. (Some later Burleigh/EXFO wavemeters do incorporate stabilized HeNe lasers for the reference.) Another source of error is the change in the refractive index of air over the typical wavelength range of at least 400 to 1,000 nm. For this reason, for better accuracy, some wavelength meters put the interferometer optics in a vacuum chamber (less than 10 Torr). However, simply providing a lookup table for wavelength correction would be nearly as effective and much less of an implementation issue, though the actual pressure and temperature have to be taken into consideration.

    The performance of this fundamentally simple device is quite amazing. The resolution and accuracy of the Burleigh WA-20, which is one of the earliest commercial wavemeters, is better than 1 part in 106 (less than 0.001 nm or 1 picometer over the measurement range of 400 to 1,000 nm!). No routine calibration is required. While degradation in alignment is possible, the effect will be to increase the power level needed to take a reading but will not noticeably effect the resolution and accuracy. As long as the instrument is happy with the signal levels, the resulting display should be accurate. The most common problem may be a bad belt between the motor and interferometer drive! And an elastic band or tape player belt will work just fine, thank you. :-) (But really old belts may decay into a gooey black mess.)

    The successors to the WA-20 are the Burleigh/EXFO WA-1000 and WA-1500. They are virtually identical mechanically to the WA-20 but keep the interferometer at atmospheric pressure instead of in a vacuum. Thus their weight is much lower and similar that of a WA-10. Software correction for the non-linearity of the index of refraction of air is then used with inputs from pressure and temperature sensors. The WA-1000 uses the same reference HeNe laser as the WA-10 and WA-20 and has similar accuracy. The WA-1500 uses a stabilized HeNe laser providing for better accuracy - +/-0.2 picometer compared to +/-1 pm for the WA-20. Both are microcprocessor-controlled resulting in a There is more information on the WA-1000 and WA-1500 in subsequent sections.

    A note to those out there who believe in running wavemeters continuously because they believe this results in better performance: There is no need. The performance of wavemeters like the WA-20 or WA-1000 is essentially the same as soon as the reference laser turns on as if run for a year. There is nothing to warm up that matters. So, save the reference laser and mechanics (where appropriate) and turn your wavemeters off when not being used! This also applies to those like the WA-1500 that incorporate stabilized HeNe lasers for the reference except that a warmup period of a few minutes is required for the laser to lock. But after that, the wavelength drifts at most by much less than 1 count in the last digit (0.1 picometer).

    It should be noted that this implementation of a wavemeter is a subset of a more general technique called Fourier Transformer Spectroscopy which is capable of dealing with arbitrary spectra. (See, for example: World of Physics: Fourier Transform Spectrometer.) Rather than simply counting fringes, the Fourier transform is taken of the fringe waveform during one or more scans of the path length difference. For a single spectral peak as with a CW single frequency laser, the FT is a single peak. For a source with multiple peaks, the fringe pattern becomes visually complex, but the Fourier Transform will be the desired spectrum. This approach is also used in some wavemeters that can deal with multi-line laser input. For example, the WA-650 is an add-on that converts the WA-1500 or WA-1000 into an optical spectrum analyzer by Fourier Transform processing of the fringe pattern. And later models called "multimavelength meters incorporate FFTs as standard equipment. :)

    In fact, it should be possible to process signals from the back of almost any wavemeter which has a built-in reference laser to use it as an optical spectrum analyzer. The interference signal for the unknown source, the interference signal for the reference laser, and a scan position sync pulse are required. This would be very simple if the scan was linear. But with wavemeters using a motor-driven scan like the WA-20 or WA-1500, the speed and thus fringe frequency isn't perfectly constant and this would totally mess up the FFT. It should be possible to correct it as long as a reference laser signal is available. The details are left as an exercise for the student. In fact, this would make a nice term project in DSP course. :)

    But, the beauty of the basic single wavelength wavemeter is at least in part due to the simplicity in terms of its principles of operation, mechanical construction, and electronics.

    A Scanning Fabry-Perot Interferometer (SFPI) may have better resolution, it typically doesn't have very good accuracy or stability with respect to absolute wavelength or frequency unless additional techniques are used, adding to complexity.

    While minor enhancements like the use of a voice coil magnetic drive instead of a motor can improve the speed and reduce the size of the Michelson interferometer-based wavemeter, higher performance instruments may use something called a Fitzeau interferometer with no moving parts. Multiple wedged etalons generate fringe patterns which depend on the source wavelength. These are captured via CCD arrays and analyzed in software. These instruments can deal with pulsed lasers and have much faster dipslay rates (100s of Hz or more compared to a few Hz for motor driven interferometers) and even more immune to alignment problems.

    Tuneup of a Burleigh WA-20 Wavemeter

    The Burleigh WA-20 is a typical older wavelength meter that uses a motor-driven moving interferometer mirror and fringe counting to determine wavelength (in um) or inverse frequency (in cm-1) of CW lasers between 0.4 and 1.0 um for the visible, which may be extended to 4.0 um with the IR option (which substitutes a different beamsplitter and detector). This model dates from the early 1980s, though the specific unit I'm working on has a manufacturing date of 1995. There was also a WA-10, with the only difference being that while the WA-20 maintains the entire interferometer inside a chamber that can be evacuated to below 10 Torr to for better accuracy, the WA-10 simply has a dust cover. The reason that a vacuum is beneficial is that there is a small non-linear depedence of the index of refraction of air on wavelength so a measurement of a laser with a wavelength far away from the 633 nm reference might see an error of as much as 3 parts in 106 in air. If the temperature, pressure, and humidity are known, a lookup table can be used to eliminate the error, but that's probably more trouble than it is worth. However, the nice thing about the WA-10 is that it's a lot easier to work on it when doing alignment not having to deal with the vacuum chamber and vacuum-tight covers over the mirrors! And it weighs less. :)

    A red HeNe laser (polarized but not stabilized) is used as both the wavelength reference, and to provide a "tracer" beam to facilitate alignment of the unknown laser to the input of the WA-20. It passes through the same interferometer optics, but more-or-less in reverse. Thus, alignment of the reference laser beam is sufficient to guarantee alignment of the entire system.

    Neither the WA-10 or WA-20 are manufactured or supported now, but the modern replacements, the WA-1500 (with stabilized HeNe laser reference) and WA-1000 (without) are substantially similar in design, though they both operate without a vacuum, but have pressure and temperature sensors using software correction for the non-linear index of refraction of air. With its stabilized reference laser, the WA-1500 has somewhat better accuracy than the WA-20 while the WA-1000 is similar. They both have better sensitivity (20 uW instead of 100 uW). For more information, see the section: Burleigh WA-1000 and WA-1500 Wavemeters

    The WA-20 I had on loan for testing and adjustment was so misligned when I received it that the tracer beam was partially cut off and less than 1/10th the intensity it should have been, and the "Ref Error" light was flashing due to low signal level. Yet, despite these problems, it was still able to measure the wavelength of an external red HeNe laser to the expected accuracy, though higher than spec'd power was required. That is, after the drive belt which had fallen off was put back in place. :) Consider yourself lucky if the belt simply falls off; on many of these, the rubber has decomposed and turned into a gooey black mess. A common elastic band will suffice until a proper drive belt can be obtained.

    There is an alignment procedure in the user manual that is strightforeward, if somewhat tedious. It uses the reference laser entirely to align the mirrors and beamsplitter in relation to the input aperture. Once this is done, the unknown laser input is also automagically aligned since it uses the same optics. Once this procedure was complete, the system was able to read the red HeNe laser as well as a highly attenuated C315M green (532 nm) laser at power levels below the spec'd minimum of 100 uW.

    Additional items that still require attention are obtaining a replacement drive belt and replacing the O-ring in the motor vacuum seal feed-through since it's leaking at too high a rate. However, except for being run at 1 atm instead of a vacuum and accepting the slight reduction in accuracy, it's now in good shape.

    And, the Power indicator uses a strange 60 V, 20 mA incandescent lamp, and of course is likely to be burnt out on a well-used WA-20. I replaced it with a high brightness LED, soldered to the slide contacts of the original lamp along with a 1N4148 across it for reverse polarity protection. An additional 3.9K, 2 W resistor and 1N4007 diode were added in series with the original 3.9K resistor in series with the one already there. An alternative that wastes less power would be to tap off one of the 5 VDC or 12 VDC supplies but this makes it easy to restore the original arrangement if desired.

    Burleigh WA-1000 and WA-1500 Wavemeters

    The WA-1000 and WA-1500 are more modern Michelson interferometer-based instruments. They are both smaller and lighter than the WA-20, and are microprocessor-controlled. The use of environmental sensors for wavelength compensation eliminates the need for the massive vacuum chamber of the WA-20. The primary functional difference between the two models is that the WA-1000 uses a normal 1 mW polarized HeNe laser head for the reference laser while the WA-1500 uses a frequency stabilized HeNe laser. The use of the stabilized reference laser allows for approximately a 5X increase accuracy and a 10X increase in resolution. But one cannot simply install a stabilized HeNe in the WA-1100 and obtain all the features of a WA-1600 as there are other differences. However, the result would be less variation in readout value even if the number of digits of the display doesn't change.

    The WA-1000/1500 come in three wavelength ranges that are documented: VIS (400 to 1100 nm), NIR (600 to 1800 nm, and IR (1500 to 4000 nm). There is also a UV version but its specifications are unknown. It's possible to convert from one version to another in the field by at most changing the beam-splitter (the only transmission optic in the interferometer beam paths) and signal photodiode. Except for the IR version, all accept either a free-space beam fed in from the side (similar to the WA-10/20) or a fiber-coupled input fed in via an FC/PC connector on the front panel. (There's really no reason why an appropriate optical fiber could not be used with the IR version, so I don't know why that is not spec'd. The WA-1000-IR I tested had the fiber port, but that may have been a conversion from VIS or NIR.) The physical layout of the interferometer is similar to that of the WA-10 but thankfully, a higher quality motor and belt are used to drive the moving retroreflectors, so belt replacement and belt goo cleanup are probably no longer a part of regular maintenance. :) The motor-driven attenuator is internal and can be set for automatic or manual (using front panel buttons). Complete specifications for both instruments are in the operation manual, easily found on-line by searching for "Burleigh WA-1000 manual" or the later "EXFO WA-1000 manual" (which for the most part is identical except for some formatting enhancements).

    Burleigh WA-1100 and WA-1600 Wavemeters

    The WA-1100 and WA-1600 are more modern Michelson interferometer-based instruments. They are both smaller (shorter) and lighter, and like the WA-1000/1500, are microprocessor-controlled along with environmental sensors providing for wavelength compensation and eliminating the need for the massive vacuum chamber of the WA-20. The primary functional difference between the two models is that the WA-1100 uses a normal 1 mW polarized HeNe laser head for the reference laser while the WA-1600 uses a frequency stabilized HeNe laser. The use of the stabilized reference laser allows for approximately a 5X increase accuracy and a 10X increase in resolution. However, one cannot simply install a stabilized HeNe in the WA-1100 to convert it to a WA-1600 as there are other differences including the firmware and possibly the actual interferometer as the update rate on the WA-1100 is 10/second compared to only 1/second for the WA-1600. The WA-1100 and WA-1600 appear in some ways to be cost/size/weight-reduced versions of the WA-1000 and WA-1500. The input to both of these instruments is fiber-coupled, with an FC/APC as the default connector. (There is no free-space option.) Complete specifications for both instruments are in the operation manual, easily found on-line by searching for "Burleigh WA-1100 manual".

    There are several subsystems inside the unit as shown in Burleigh WA1100 Wavemeter Interior View: DC power supply (upper right), reference laser with its HeNe laser power supply brick (right), interferometer optics with processing PCB on top (covered, bottom), microcontroller PCB (upper left), display PCB (attached to front panel, hidden), and a little motor driver PCB (left).

    The interferometer in the WA-1100 has been greatly reduced in size and complexity. (I have not seen the internal organs of a WA-1600.) It uses a single retro-reflector (cube-corner) instead of a dual-sided one as in the WA-10 and WA-20. It's on a much much smaller motor driven linear slide, and the remaining optics are installed in a compact single-piece precision milled aluminum structure. There is still a rubber belt but hopefully, it won't decay and require replacement like those in the WA-10 and WA-20 as gaining access to the belt appears to be more involved, buried beneath the optics. A single PCB (the Sensor Board) mounts above the interferometer as shown in Burleigh WA1100 Wavemeter Sensor Board. There are 3 photodiodes (PDs) facing downward into it, one each for input laser power, reference fringe detector, and signal fringe detector. Repair would be more difficult as (1) most parts on the PCB are SMT and (2) it's not possible to get to the optics with the PCB (and PDs) in place. However, like the WavemeterJr, there really is only a single mirror that requires alignment and those adjustments are accessible without disassembly. That is accessible from the right side of the interferometer without any further disassembly and consists of 2 kinematic adjustments screws and locking set-screws near them. Peaking the reference fringe amplitude should also result in optimal signal alignment. Everything else is glued in place.

    The reference lasers are typically OEM versions of the Melles Griot 05-LHP-491 for the WA-1100 and 05-STP-910 for the WA-1600, fiber-coupled directly to the interferometer. The fibers are not connectorized, only terminated and glued. The input fiber needs to be able to pass wavelengths up to 1,700 nm with minimal losses, so it is assumed to have a rather large core. However, the reference fiber may be the same and thus multi-mode at 633 nm, and it is sensitive to power changes with even modest bending radii or changes in routing. There is a beam sampler and silicon photodiode that monitors laser power before fiber coupling, which may be read from the front panel, so it won't detect problems with the fiber. There is nothing special about these particular model reference lasers, but if the beam diameter and divergence of a replacement are not close to the original, the coupling efficiency may be reduced unless the focus position of the fiber (which is glued) at the laser end is changed. The WA1100 in the photo actually has a JDSU 1107P installed as the replacement reference laser. And it indeed appears to have significant power coupled into the fiber cladding for this reason. Attempting to break the glue bond was deemed to risky.

    The specifications list a wavelength range of 700 to 1,700 nm. And indeed, the lower bound seems to be quite strict. There is even a VIS-blocking filter in the optical path, with the reflection from it going to the laser power PD. It's not clear why the wavelength range doesn't extend further into the visible. Certainly the IR PD still has some sensitivity at 633 nm so it should be possible to measure the wavelength of a common HeNe like its own reference! And for that matter, why not include a second signal PD so that the measurement range could be extended down to 400 nm or beyond as is done with the WavemeterJr? With minor modifications to the firmware, it should not require much more than a dichroic (or even broad-band) beam-splitter and $2 silicon PD. Darn Marketing! Cost reduced, strip out useful features to promote sales of the higher priced spread. ;-)

    Tuneup of a Burleigh WA-2500 Wavemeter

    The WA-2500 is appropriate named "WavemeterJr" as it is a much smaller, lighter, and simpler instrument than the WA-20 which may be used to measure wavelengths from 400 to 1,800 nm using separate detectors for VIS (400 to 1,100 nm) and IR (up to 1,800 nm). The precision is lower (5 digits instead of 6 digits) and it lacks the built-in reference HeNe laser, so an external HeNe laser must be used for calibration. However, this can be done regardless of whether the instrument is set for VIS or IR as the IR photodiode still has decent sensitivity at 633 nm. Given the lower resolution, this isn't nearly as important as with the WA-20. But the WA-2500 also lacks the tracer beam, and in fact, only allows for fiber coupled input via an FC connector on the rear panel. It is microprocessor-controlled and includes an RS232 port for data collection. A Web search for "Burleigh WA-2500" will locate an operation manual with description, specifications, and alignment information.

    Like the WA-20, the WA-2500 is based on a Michelson interferometer, but the optical setup is much simpler. The main component is a single Cube-Corner (CC) on frictionless dual flexure mounts with a electromagnet ("voice coil") to "excite" it at its mechanical resonance of about 10 Hz, providing 10 readings/second (or 1/s if averaging is turned on). An automatic locking mechanism keeps it from bouncing around when power is off, but makes an annoying loud clunk in doing so. :) The other interferometer optics include a 45 degree beam-splitter mirror, and two 0 degree mirrors. (A detailed layout is in the manual.) The alignment of one of these mirrors is critical, with adjustments accessible from the back panel. I just wish they had used a higher quality mount with finer pitch screws! There is also a fixed (45 degree) fold mirror which simply directs the internal beam to the adjustable one on the backpanel to make the optical layout work out in the available space.

    The only time there's a need to go inside is to flip the detector board to select VIS or IR. (One screw and one jumper.) A photo is shown in Photodiode Preamp PCB from Burleigh WA-2500. On this WA-2500, the final op-amp (LF347) was found to be blown along with a toasted (but apparently still functional) resistor. Someone may have plugged the cable in incorrectly, though this would seem to be difficult. :) Or it may be that the "Monitor" BNC on the rear panel is in parallel with this output, so perhaps someone plugged something in there that shouldn't have been plugged in there. :) So, the signal level was very low almost never showing up on the bar-graph display and only a few hundred mV at most from the Monitor BNC, with the machine either producing Lo Signal or Alignment Error. There was also no Window signal (middle BNC) at all which initially led me to suspect there might have been logic problems. But apparently, that isn't generated until some minimum signal level is detected. Once the op-amp was replaced, it instantly sprang to life. However, for optimum performance, using the identical op-amp (or at least one with adequate bandwidth) for that stage at least seems to be critical. Substituting an OP27 in place of the LF347 (which was the only single op-amp I had available at the time) resulted in a non-uniform signal envelope and incorrect calibration with respect to Hi Sig and the bargraph as well as limiting the range of the bargraph. With correct op-amp, the trim-pot can be adjusted for a reasonably flat signal envelope.

    The tuneup then consisted simply of peaking the signal level using the mirror alignment screws.

    The WA-2500 works very well with single longitudinal mode (single frequency) and multimode lasers where the modes stable and closely spaced, as they are in all HeNe lasers. Ifthe bandwidth of the lasing modes is larger than the coherence period over which the Mechelson interferometer samples, the WA-2500 will reduce the resolution so that a meaningful measurement can still be made. This is done by looking at the envelope of the fringe signal and only sampling during a segment between where it goes to zero. However, (not surprisingly) there can be real problems with multimode lasers where the modes may be jumping around. I could not get reliable measurements using a green laser pointer or crappy green DPSS laser module (which is probably similar). But it works flawlessly with any HeNe and high quality DPSS lasers like the Coherent C315M. There can also be some loss of resolution if using a multi (spatial) mode fiber for the input though this is usually minor, may be worth the greatly reduced hassle in coupling to the large fiber core.

    Performance is somewhat better in terms of sensitivity and consistency with a single mode fiber having a core size appropriate for the laser wavelength, but a multimode fiber can be used without too much difficulty. Even the one Burleigh provided can only be truly single mode for the longer wavelength range. A 9/125 telecom fiber will not be single mode for a 532 nm green laser, which requires a 3 or 4 µm core to be single mode inside the fiber.

    Most of the above also applies to the WA-2200, of which there appears to be no record on-line. The sample I tested did not allow for swapping between VIS and IR, and the detector PCB was slightly larger. But otherwise, the optical components and layout are identical to that of the WA-2500. My unit had a broken glass Moire plate, probably from being dropped hard. There are two glass plates with patterns of fine lines in close proximity to generate the reference signal in lieu of the HeNe reference laser. But the mass of the moving part of the optics could indeed cause them to contact if the shock is severe enough, even when locked. I remounted the remaining good piece as best I could - the other piece was no where to be found, presumably lost by the previous owner. I believe there is enough remaining to provide the necessary signal. I finally found the PLL test-point. Here are all the test-points with their function:

       ID    Function
     ------------------
      TP1    +5 VDC
      TP2   +12 VDC
      TP3   -12 VDC
      TP4    +5 VDC
      TP5   PLL Signal
      TP6   Common/Gnd
    

    I have no idea why there are two test-points that appear to have +5 VDC on them, but perhaps one is a reference voltage or digital and analog, or something. :) TP1 through TP5 are near the power supply; TP6 is on the other side of the PCB near the front panel. There are 4 trimpots on the main PCB:

       ID   Function
     ------------------------------------------------------------
       R3   PLL Gain (set fully CW)
      R36   Bargraph Sensitivity?
      R49   ??? but critical setting, else PLL and other errors
      R53   ??? no obvious effect (set fully CCW)
    

    Adjusting alignment of the two Moire plate varies the PLL signal amplitude as expected. But the separation appears to have to be at the limit the mounting screws will permit to get a clean signal. With some fiddling, it was possible to get it up to almost 1.5 V p-p, a bit below the 2.0 V p-p the manual wants. But behavior is essentiaally identical down to less than 0.5 V p-p so that's probably OK. "PLL Err" is displayed on power-up a couple times but it then disappears, and that may be totally unrelated to signal level.

    I also found that the pot on the photodiode preamp board changes its bandwidth and setting it lower produces a flatter signal envelope at the expense of some gain. I'm not sure that this WA-2200 is quite as stable as the WA-2500 but it isn't too bad now. However, there is an annoying intermittent problem: Occasionally, usually a few minutes after being powered on, it will get into some state where it's not happy with any signal level producing "Lo Sig" or "Hi Sig" or possibly other errors continuously. This may continue for a few minutes and then totally disappear. There's a slight possibility it has to do with back-reflections destabilizing the test laser (Melles Griot 05-LHR-911) which is directly fiber-coupled to the wavemeter without an optical isolator, but that's not too likely.

    If anyone has service information on these (or other) wavemeters, please contact me via the Sci.Electronics.Repair FAQ Email Links Page.

    Burleigh WA-7100 and WA-7600 Multi-Line Wavemeters

    The WA-7100 and WA7600 appear are geared to the telecom industry providing spectral analysis capabilities using the same Michelson interferometer technology as the WA-1100 and WA-1600. Rather than simply displaying the precise wavelength of a single laser line, these instruments provide a spectrum analyzer type display using the Fourier transform of the fringe signal rather than simply counting fringes and comparing the count to a reference laser or grating. In other fields, this would be called a Fourier Transform Infra-Red Spectrometer or FTIR. ;-)

    These are larger and heavier than the WA-1100 and WA-1600, probably primarily to provide space for the larger LCD screen on the front panel. But the interferometer "engine" appears to be similar or identical. The microprocessor-based controller is physically similar but provides much more sophisticated measurement and display capabilities. Similar environmental sensors provide for wavelength compensation. As above, the primary difference between the two models is that the WA-7100 uses a normal 1 mW polarized HeNe laser head for the reference laser while the WA-7600 uses a frequency stabilized HeNe laser. The use of the stabilized reference laser allows for approximately a 5X increase accuracy and a 10X increase in resolution. And, a stabilized laser cannot simply be installed to increase resolution. The input to both of these instruments is fiber-coupled, with an FC/APC as the default connector. (There is no free-space option.) Complete specifications for both instruments are in the operation manual, easily found on-line by searching for "Burleigh WA-7600 manual".

    There are several subsystems inside the unit as shown in Burleigh WA7600 Multi-Line Wavemeter Interior View: DC power supply (upper right), fiber-coupled Melles Griot 05-STP-910 stabilized reference laser with its HeNe laser power supply brick (right), interferometer optics with processing PCB on top (covered, bottom), microcontroller PCB (upper left), display PCB (attached to front panel, hidden), and a little auxiliary PCB (assumed to be for the motor driver as with the WA-1100, left).

    As noted, the interferometers in the WA-7100 and WA-7600 are probably similar or identical to the ones in the WA-1100 and WA-1600, respectively, though I have not removed the cover to inspect it. The reference lasers are the same as well. See the previous section for more info.

    The VERE WM4100 Wavelength Meter

    While one of the systems described above is named "Wavemeter Junior", this one should perhaps be called "Wavementer Lite". And it's not really a wavemeter since it doesn't count fringes in any way, shape, or form using either an actual laser or mechanical reference. And there's no interferometer, so it really doesn't compute wavelength directly. Rather, a diffraction grating directs the incoming light from a 600 µm fiber onto a Position Sensitive Detector (PSD). Analog circuitry generates a voltage proportional to the wavelength based on the centroid of the spot position. An A/D converts that to a display of wavelength. The spec'd default wavelength range is 500 to 1,000 nm. The manual including specifications may be found at WM4100 Wavelength Meter Operation Manual. (A Web search will also find this manual.) Hamamatsu, one supplier of PSDs, has an on-line "optics handbook" extensive technical info on PSDs in Chapter 02: Silicon Photodiodes. Or, for a quick intro, see Wikipedia Position Sensitive Device. The 1-D version would only require the equation for X:

                      Ib-Ia
            x = Kx * -------
                      Ib+Ia
    

    where Ia and Ib are photocurrents from the ends of the PSD and Kx is a scaling factor. The analog circuitry of the WM4100 implements this equation directly using AD706 dual op-amps, PMI AMP03 differential amplifiers, and an Analog Devices AD632 divider IC. However, since for the diffraction grating, angle is a function of the arcsin of the wavelength, and the spot is projected onto a flat surface, additional calculations are required to correct for these non-linearities. Thus there is an AD538 "Real-Time Analog Computational Unit (ACU)" involved somehow. More on this when I figure it out. :)

    Where incredible accuracy is not of key importance, this approach works well. Any source with a single peak wavelength that can be coupled into the input fiber will produce a reading and the response is a fraction of a second - essentially the update rate of the A/D. So, multi-longitudinal mode laser diodes as well as high brightness LEDs will be acceptable. If you can eyeball a peak wavelength using a spectrometer, this thing will find it. More or less. :)

    And it is relatively small and light weight!!!

    I now have reverse engineered circuit diagrams for most of the system.

    The unit I acquired appeared to work fine in general and displayed around 633 nm for a red HeNe. But a green (532 nm) laser pointer known to have an effective IR-blocking filter produced 1,070 nm - around twice what it should be. It turned out that the 2nd order green spot appeared at the high end of the sensor rather than being blocked (or so I assumed) and the 1st order spot was blocked, and that obviously some calibration would be required. Someone *had* been inside as the internal ST cable at the diffraction grating-end was only half pushed into the connector and some mounting screws were loose. By slightly adjusting the angle of the diffraction grating (which secured tightly), the green spot was easily positioned in the Hopefully the 3 user adjustments will now suffice to tune it up and they didn't also twiddle the 3 unmarked trim-pots!

    However, upon further testing - after attempting to adjust the grating and all of the pots (including those that shouldn't be twiddled), I came to the conclusion that this instrument must have been set up for something like 600 to 1,100 nm. There was no electrical or optical adjustment that would ever result in a reading much below 600 on the display. Indeed. the full model number is WM4100C. So, perhaps the "C" means something. While an electronic failure could have resulted in a bogus 100 nm offset messing up the readings, that wouldn't explain how the 2nd order 532 nm spot hit the PSD. The grating position didn't appear to have been changed - at least its screw was very tight. No other optical adjustment could have had such a large effect. The screws securing the input lens were just snug, but that optic has very limited adjustment range.

    At present it is awaiting final calibration, but does read down to the HeNe 594.1 nm (yellow) line - barely.

    Here are a couple of photos and more information on calibration adjustments.

    Pulsed Wavemeters

    Most of the previous descriptions are for older wavemeters that only can be used with CW lasers since they are based on Michelson interferometer with motor driven mirrors and require significant time to take a reading. While these type of systems are still manufactured, modern high performance wavemeters that can be used with both CW and pulsed lasers are now fully "solid state" with no moving parts. They typically employ one or more Fizeau interferometers and linear diode (e.g., CCD) detectors. The interferograms (fancy name for fringe pattern images) are then transferred via to a PC for analysis and display. For more info, check out companies like Bristol Instruments and High Finesse GmbH.



  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Ring Laser Gyros

    Basic Description and Operation

    (Includes contributions from Richard Graham of the Ring Laser Gyro Group at Canterbury University.)

    Mechanical gyroscopes measure rotation by measuring forces on a rotating mass which has been machined and spun at high precision. Having moving parts they are often clunky, bulky and distinctly 'low-tech'. They often take a long time to 'spin up' and stabilize.

    In principle the ring laser gyroscope can replace these with a fully optical system using counter-rotating laser beams, photodetectors, and digital electronics with no moving parts larger than photons and electrons.

    In practice, it isn't so easy.

    In its simplest form, the ring laser gyro (RLG) consists of a solid triangular block of glass with a hold drilled out parallel to each edge. Mirrors are added at each corner, a laser gain media such as Helium-Neon is added and a stable laser cavity is established. The gain media is chosen to allow two counter-rotating laser beams to be established - one clockwise (CW) and the other counter-clockwise (CCW).

    At some point two of the output beams are mixed together and (due to the slightly different optical frequencies) a beat frequency in the audio band will result and this can be measured to very high accuracy with a photosensor and appropriate electronics.

    The frequency measured is known as the Sagnac frequency. It is proportional to the rotational velocity, operating wavelength and ratio of cavity area to perimeter.

    A complete 3-axis inertial platform would require 3 RLGs mounted at 90 degrees to each-other. The entire affair can be fabricated inside a solid glass block!

    However, there are problems with this simplistic implementation.

    Fabricating a large monolithic slab of low thermal expansion glass is extremely difficult. As a result, very large cavities have been constructed in a discrete fashion, i.e., from individual pipes mounted on a concrete base. But it is difficult to maintain mechanical stability of the device.

    For the most part, these difficulties have been overcome to a degree sufficient to allow for navigation of aircraft, spacecraft and submarines. In such applications RLGs are increasingly being used in place of mechanical gyroscopes.

    Photos of an RLG laser assembly from a commercial inertial platform can be found at Flavio's Ring Laser Gyro Page. This is a triangular ring cavity dual discharge HeNe laser with part of the beam path being external to the block. One or more of the mirror mounts include coils, presumably to do the dithering. (Hopefully, there will eventually be descriptions there as well once we figure out what's going on.)

    There is some interesting information on RLGs at the Canterbury Ring Laser Projects Page.

    (From: A. E. Siegman (siegman@stanford.edu).)

    If you go to the Laser Gyros Directory, you'll find photographs of an early square He-Ne ring laser gyro built by Sperry and some early designs for Honeywell's monolithic ring laser gyros.

    The Sperry gyro couldn't actually be rotated in the lab - kind of hard to spin a one ton or thereabouts optical table. So they relied on the Earth's rotation, or at least the vector component of it perpendicular to the table at Sperry's latitude, to test their system.

    I recall a conference talk on their work in which the speaker noted that, given the backscattering and lock-up problems associated with a ring laser at this low rotation rate, their primary conclusion was that as best they could tell the Earth was still rotating, but at a highly uncertain rate.

    Home-Built Ring Laser Gyro?

    So you want to build one? Good luck! :-)

    (From: Douglas P. McNutt (dmcnutt@macnauchtan.com).)

    The mechanical precision is the hard part and that's what makes it virtually impossible for an amateur to construct a ring laser gyro. The two opposite traveling waves have to have extremely high spectral purity which translates to high quality, high reflectance flats at the corners. Not a home job.

    It might be easier to build a fiber gyro in which the light passes many times around an effective ring through a wound fiber.

    (From: Christopher R. Carlen (crobc@epix.net).)

    The mechanical part is horrendous. We have an open cavity HeNe at my school's lab, and it is a challenge to keep lasing on a heavy damped breadboard with the mirrors mounted on a thick dovetail rail, bolted to the breadboard.

    Then you complicate that by going from a straight, two-mirror cavity to a three or four mirror cavity ring configuration, and then spin it real fast. Can you say "centrifugal force?"

    A fiber loop isn't quite the same as a ring laser, because the ring laser actually has the laser gain medium in the ring. As opposed to having the beam directed into a ring. The gain medium in the ring cavity ensures a standing wave is set up in the cavity, which would not be so for the fiber loop.

    Of interest for the future of laser gyros are the new photorefractive polymer devices that exhibit the property of two-beam coupling. This device allows coherent transfer of energy from one beam to another, when the beams are intersected in the material. This can be used to assemble a ring resonant cavity, pumped from the outside by a laser. This can be done with a small diode laser resulting in an assembly much smaller and easier to keep still while spinning than a gas laser ring cavity.

    Photorefractive oscillators using inorganic PR crystals have been studied for some time. The first announcement of a resonant cavity using a PR polymer has just occurred in the past few weeks (March, 1998).

    (From: Douglas Dwyer (ddwyer@ddwyer.demon.co.uk).)

    If you are trying to make a laser gyro as a home project you've got a lifetime project.

    I think the ring laser is often carved out of a solid block for stability , a major problem with both ring lasers and fibre gyros is locking of the two phases - when rotated the phase relationship between the two paths sticks until a certain rotation rate is reached at which point the two paths unlock and it starts to work properly The solution to this could be to deliberately modulate the phase of the light with pseudo random noise and demodulate at the phase detector. Also as stated the fibre gyro is less attractive because of the inherent greater spectral width of the laser.

    I wonder if one could bake a Mossbau gyro. I once saw turntable rotation detected by the relativistic effects on the gamma radiation and absorption. That could be easier.



  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Fourier Optics

    Introduction to Fourier Optics

    The Fourier Transform (FT) of a signal - be it one dimensional such as audio or RF, or multidimensional such as an image (picture) - is a powerful tool for the analysis and processing of information. In a nutshell, the FT provides information on the frequency content of the signal. The signal and its FT form what are known as a 'transform pair'. The FT is a completely reversible operation so if the FT of the signal is completely known, the signal is also completely determined.

    Some applications for the Fourier transforms include:

    Refer to any book on signal processing for more details Fourier analysis and applications including all of the exciting equations!

    The usual modern way of performing the Fourier transformer operation is to digitize the data and use a special optimized computer algorithm called the 'Fast Fourier Transform' or FFT. However, even the most efficient variation of this approach is highly computationally intensive - especially when large multidimensional arrays like high resolution images are involved. To achieve adequate performance, digital signal processing accelerator cards, multiprocessors, or even supercomputers may be needed!

    Enter Fourier optics.

    It turns out that under certain conditions, a simple convex lens will perform the Fourier transform operation on a two dimensional (2D) image totally in *real time*. The theoretical implications of this statement are profound since real-time here means literally at the speed of light. In practice, it takes great effort and expense to make it work well. Many factors can degrade the contrast, resolution, and signal-to-noise ratio. Extremely high quality and expensive optics, precision positioning, and immaculate cleanliness are generally essential to produce a useful system. However, to demonstrate the basic principles of Fourier optics, all that is required is a common HeNe laser and some relatively simple low cost optics.

    Basic Setup for Simple Fourier Optics Experiments

    You don't even want to think about what a high quality Fourier optics setup for serious research would cost. However, for demonstrating the fundamental principles, it is possible to get away with much less. The necessary components are shown below:
       
       +-------+     Spatial Filter  Input          Fourier Transform        Output
       | Laser |===>()===---:---===()::():::><:::():::><:::():::><:::():::><:::()
       +-------+    FL     PH      CL  TR        TL        TP       ITL        OP
                     |<-f1->|<-f2->|   |<-- f -->|<-- f -->|<-- f -->|<-- f -->|
    
    
    A laser with a long coherence length is required. A diode laser will probably not work well. Therefore, this is likely to be a HeNe type. A medium power laser (i.e., 10 mW) will make for a brighter display but a 1 mW should work just fine. CAUTION: Take appropriate precautions especially with a higher power laser. However, once the beam has been collimated to a large diameter, the hazards are reduced.

    Ideally, you have a nice optical bench to mount all these components. Otherwise, you will have to improvise. The first three items (the spatial filter components) really do need to be accurately and stably positioned. See the section: Laser Beam Cleanup - the Spatial Filter.

    Laboratory quality lenses for Fourier optics research cost thousands of dollars each. However, you can demonstrate the basic principles and do some very interesting experiments with inexpensive optics.

    Comments on Fourier Optics

    (From: EandorY (ehusman@zianet.com).)

    I just finished a class in this, using "Linear Systems, Fourier Transforms, and Optics", by Gaskill (Wiley).

    A coherent source yields a Fourier transform of the electric field, including the phase factors. An incoherent source will perform essentially the same effects on the radiance, rather than the field. A coherent source is used to develop the concepts, and so most of the books show the experimental verifications of spatial imaging with coherent sources.

    A negative lens will give a virtual image. If you want to perform spatial filtering, I think you're forced to use a positive lens. You also perform the inverse transform with another positive lens. You should therefore be able to confirm basic spatial filtering concepts with a hobbyists' telescope.

    Gaskill talks about a few special configurations, but the easiest to get to is to locate a laser to one side of the lens, place the transparency at the front focal plane, and find the Fourier transform plane at the point where the point source (a laser) comes to focus. To make things really simple, put the laser twice the focal distance away from the lens, the image at the focal distance, and find the FT at twice the focal distance on the far side of the lens. An alternative is to take a laser, collimate the light to obtain plane wave illumination, place the image anywhere between the source and the lens, and find the FT plane at the focal distance on the other side of the lens. It is the focal point of the light source that determines the position of the FT plane.

    Like I say, I just took the class, am still shell-shocked, and haven't had a chance to absorb or experiment with these techniques, so I could be misunderstanding the text. (From: Norman Axelrod (naxelrod@ix.netcom.com).) Yes, you need a laser. HeNe works, but not a diode (the laser needs to have good coherence). Focus the laser through a pinhole (focusing lens and pinhole combination is called a spatial filter). then re-collimate the light with a lens. Place the image or aperture 1 focal length from the collimating lens, then you can either use a bare screen placed at distance away, or a second collimating lens. This is necessary to get the far-field pattern.

    (From: Brian Rich (science@west.net).)

    A really cool book about this that I have a copy of but may be out of print is "Laser Art and Optical Transforms" by T. Kallard. Look for it at a good university library.

    (From: Norman Axelrod (naxelrod@ix.netcom.com).)

    There is another way to phrase what is happening that might make it more intuitive for folks with more of an optics background.

    First, the light used should be parallel and coherent.

    The light transmitted through the transparency (or light reflected from a 2-dimensional image) is diffracted by the transmission and phase changes provided by the image. As is done in elementary physics, a lens (here, a high quality lens) is used to take the light that is diffracted at different angles and focus them at a distance of one focal length from the lens (just like a burning lens, except you use parallel coherent light coming into the initial transparency and you have more than one beam at the burning distance).

    The key physical point is that the Fraunhofer diffraction pattern of an object is the Fourier transform of that object. This is true in the sense that the amplitude and the phase of the radiation at any point in the diffraction pattern are the amplitude and phase at the corresponding point in the Fourier transform.

    For simple examples:

    Arrays of identical apertures provide diffraction patterns that are the product of the intensity patterns from the individual apertures and the intensity patterns from the geometry of the array. If the array is random, you get the diffraction pattern of the individual apertures. Young (of Young's Double Slit) used this for one of the earlier measurements on the diameter of blood cells. One of the more amazing things (at least to me) that you can do with this is to take remove the horizontal OR vertical lines from an image of a wire screen with crossing vertical AND horizontal lines. By a simple modification of the light in the transform or diffraction pattern plane, you can produce an image that ONLY has either vertical or horizontal lines! The Fourier transform or diffraction pattern from a wire screen (like a screen from nylon stockings or from on a screen door - - but with tighter geometry) with periodic holes on a square grid consists of bright regions on a similar square grid. If you take an opaque screen and put a long narrow opening to allow ONLY the light from near the x-axis to get through, the resulting image has only vertical lines! This is called the Abbe-Porter experiment (and is discussed in Goodman's book). We have patents on this in which we used a simple opaque cross (in the transform plane) to eliminate perpendicular lines in an image and re-image only non-rectangular features. The perpendicular lines (lined up with the two axes of the cross) are effectively eliminated, but circular features and irregular features are imaged just fine! My favorite book on this remains Optical Physics by Lipson & Lipson (Cambridge Univ Press).

    (From: Tom Sutherland (tom.sutherland@msfc.nasa.gov).)

    Please allow me to recommend Professor Goodman's excellent and recently updated text "Fourier Optics". If I had my (last edition) copy in front of me I'd give you a better answer, however I do recall that the exact fourier transform of a pattern illuminated by a coherent plane wave is produced at the back focal plane of a lens if the pattern is located at the front focal plane of the lens. The intensity (but not the phase) of the fourier transform is produced if the pattern is located anywhere else in front of the lens (but of course there are some questions of scaling). (From: Robert Alcock (robert@fs4.ph.man.ac.uk).) Have a look at the book "Introduction to Fourier Optics" by J.W. Goodman. McGraw-Hill Book Company 1968. The first few chapters set the theoretical framework for the book by explaining 1D and 2D fourier transforms and scalar diffraction theory. I think that the chapters that you may find particularly interesting are:

    It's a fantastic book that should answer all your questions.

    (From: Herman de Jong (h.m.m.dejong@phys.tue.nl).)

    Let me explain the optical Fourier Transform by lenses with an example: Suppose for simplification we essentially look at a two dimensional system: we use cylinder lenses and slit object.

    When you use a broad laser beam and eliminate a slit (a pulse function), it will have a near field and a far-field pattern that is not exactly the same. The far-field pattern is a utopia but you get very close to the utopia the further away you put your screen. The intensity pattern is a squared sinc function (the sinc function is the FT of the pulse function) that scales with distance. We conclude the infinity pattern to be the squared of the FT of the slit and the associated E -field is actually the FT. If you use a cylindrical lens to image the slit on a screen you also get an FT provided you collect all relevant light from the slit onto your lens and the lens is perfect. It scales with the ratio of object an image distances It so happens that the FT of the FT the original but for a scaling factor and a minus sign in the inverse FT. I'm not sure how but in otical intensity FT's it makes no difference probably because of the squared of E -field that eliminates the minus sign.

    It gets much more difficult to grasp with 3D and rotationally symmetrical optics, objects and images. You wouldn't want to know and I wouldn't be able to answer many questions.

    (From: James A. Carter III (carter@photon-sys.com).)

    It is possible to form the Fourier transform by placing the transparency in a convergent-cone optical field formed by a single laser. This technique is used when one wishes to scale the transform to be optimally sampled by a detector with fixed spatial sampling. Changing the location of the transparency with respect to the focus of the cone (i.e., changing the quadratic phase of the optical filed) will change the scale of the transforms as it maps spatial frequency (sometimes called the "plane wave spectrum") to spatial coordinates. Actually, no lens is required at all if you have a large enough lab and can invoke the "far field" condition. The "Fraunhofer" condition uses the quadratic phase of the lens to negate the second order term in the scalar diffraction integral using denoted as "Fresnel" diffraction. The far field condition puts the observation plane far enough away from the transparency plane to make it essentially a constant term in the integral and again you have a 2-D Fourier transform.

    The lens can be thought of as a way to image the far field (ideally at infinity) to the back focal plane. If the transparency is not at the front focal plane, then the transform field (amplitude and phase) at the focal plane will have a quadratic phase term. The quadratic phase is irrelevant if the field is detected (with detector or film) because then all phase information is lost. If the field is recorded with a reference phase (i.e., a hologram), or is filtered for subsequently inversing the transform, then the quadratic phase should be corrected. The simplistic way to do this is to use a plane wave illumination (collimated source) and place the transparency at the front focal plane. Using your imagination and knowing the symmetry of the Fourier transform should justify this rational.

    The field at the transform plane contains only the information that is collected and sampled by the lens. Thus, the ability to sample higher spatial frequencies depends on the collection angle (numerical aperture) of the lens. Some feel that the illumination beam must be spatially filtered to produce a uniform distribution. This is no more the case than saying that every Fast Fourier Transform should just be zero padded. Hamming, Hanning and other windowing algorithms are used to suppress the side-lobes produced by the finite sample extent. The Gaussian distribution of the laser can actually improve the fidelity of the transform and eliminate "ringing." The quality of the lens in terms of wavefront aberrations is important, but no more important than the wavefront quality of the beam. These phase aberrations may effect the point spread function of the system (seen when no transparency is present) and it is the point spread function that convolves with the transform and limits fidelity.

    The text by Jack Gaskill and Joe Goodman are excellent for details. Another excellent source is the "(The New) Physical Optics Notebook: Tutorials in Fourier Optics" by Reynolds, DeVelis, Parrent, Thompson. This is available from Optical Engineering Press (SPIE). The "old" version of this was used in my training at the U. of Rochester when I took physical optics from one its early authors (Brian J. Thompson).

    Many interesting things can be done with this simple engine.

    (From: Jeff Hunt (jhunt@ix.netcom.com).)

    I'm a grad student at the Optical Sciences Center at the University of Arizona, and I think that Jack Gaskill's book on the subject is quite good. Just like Gaskill says, it covers what Goodman's text does, but it explains things in a way that is easier to understand (Goodman is the authority on the subject, from what I understand.)

    (From: DeVon Griffin (devon@baggins.lerc.nasa.gov).)

    Having done Gaskill ten years ago, I would say that the main drawback of the book is his notation. The m double-hat triple prime sort of thing makes trying to pick it back up after not having looked at it for awhile a daunting task.



  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Barcode (UPC) Scanners

    Introduction to Barcode Scanners

    The use of the Universal Product Code (UPC) has revolutionized grocery/supermarket and other retail store checkout and inventory control as well as being applied to other numerous and varied applications including package routing and tracking, and even tagging of wild animals and an aborted attempt to use similar codes printed in your weekly TV section to program your VCR with a hand-held barcode wand!

    Some would argue that the use of such technology in supermarkets at least, has dehumanized the buying experience and stacked the deck in favor of the merchant since prices tend to no longer be printed on each item and the checkout process is now so fast that it is virtually impossible to catch mistakes should they occur. Since the price-to-item relationship is stored in a computer somewhere, it is indeed possible for there to be errors - but in reality, these are generally rare.

    Space and other factors prevent me from going into the details of the Universal Product Code itself but here are some Web sites that have info and many links to barcode manufacturers, barcode specifications, barcode generating software, and other information that may be useful:

    The quick summary is that the pattern of black lines familiar on virtually all products nowadays - the UPC code - has been carefully designed to be easily decoded when scanned in either direction, at an arbitrary angle, and with variable speed. There are actually many other barcodes besides the UPC, used for inventory control, tracking, and other diverse applications. (If you should need to stay in a hospital, you will be given a barcode!)

    The UPC consists of 12 total digits. The first digit is the type of product (0 is for groceries, 3 is for drugs, etc.), the next 5 digits on the left half are the manufacturer code, the first 5 digits of the right half are the product code, and the last one is a modulo check digit. Each digit as its name implies can have a value from 0 to 9, encoded as a set of 4 alternating bars and spaces, each of which may have a width of 1, 2, 3, or 4 units called "modules". The total width of each digit is defined to be 7 which allows for 20 unique codes - 10 used for the left 6 digits the other 10 for the right 6 digits. The left six digits are coded with odd parity; the right six digits with even parity. Additional details can be found at the first Web site, above.

    Anatomy of a Barcode Scanner

    For the purposes of the discussion below, we restrict our attention to the type of equipment found at your local supermarket - the barcode scanner that is mounted under or beside the conveyer counter (and may include an electronic scale but that is another story). While details vary, the basic architecture of these devices tend to be very similar. Once you are familiar with one model, parts identification and the optical path of any other one will be almost immediately obvious. Hand-held scanners may not even use a laser but a linear array of LEDs. Large industrial barcode scanners may contain a much more powerful laser and somewhat different optical path. Some of the newest barcode technology does away with the laser scanner altogether and uses a 2-D video camera (CMOS or CCD) based imaging system and high speed DSP (Digital Signal Processor) instead. This eliminates most of the complex and costly optical and mechanical components making for a compact robust system. But currently, the traditional electro-mechanical laser scanner is still most common.

    The basic principle is to use a collimated laser beam, rotating multifaceted mirror, several stationary mirrors, and other optics, to generate a scan pattern above or beside the scanner which will intercept the UPC code printed on the item to be scanned in almost any orientation. While the scan may appear to consist of multiple lines or a continuous pattern, it is in reality a single rapidly moving spot.

    Looking through the glass of the scanner, it may appear that all sorts of stuff is arranged at random. However, this is not the case. :) Refer to Optical Path of Typical Checkout Barcode Scanner as you read the description below (which also includes some comments on potentially useful parts that may be obtained from these units):

    And for those who fear lasers of all types, there is absolutely NO risk to vision or anything else in looking into the business end of a barcode scanner. The laser is low power and the beam is moving, so it's unlikely you'll even experience any afterimages. Staring into the scanner all day would cause no harm except possibly cramps from being stuck in one position for too long. ;-)

    The outgoing beam is set up to be a small spot in the active area above or beside the scanner - the scanned item volume. However, the return from the UPC printed on the item is in general not well focused but is a diffuse reflection. Thus, as noted, all the mirrors have to be large to capture as much of this as possible to feed to the photodetector. The return path is the same as the outgoing path until the objective combo lens. This focuses the return beam onto the photodetector:

    See the document: Sam's Gadget FAQ for more on salvaging parts from barcode scanners.

    Apparent Brightness and Safety of Barcode Scanners

    There really aren't too many safety issues with respect to these devices even though they contain a Class IIIa (1 to 3 mW) laser and the beam may appear to be quite bright. (Note that barcode scanners systems are listed as Class II laser devices since access to the laser and optics requires some disassembly.)

    Metrologic Model MH290 Hand-Held Barcode Scanner

    This hand-held HeNe laser based barcode scanner apparently was the source of the power supply described in the section: HeNe Inverter Power Supply Using PWM Controller IC (IC-HI1). The entire HeNe laser (tube and power supply) is about 1"x1.5"x5" and weighs only about 3-1/2 ounces!

    (From: Art Allen, KY1K (aballen@colby.edu).)

    The unit I have which uses a power supply 100 percent identical to the schematic and PCB layout of IC-HI1 is a Metrologic Model MH290. It is labeled with a 1990 date of manufacture and says 12 VDC at 550 mA on the scanner unit itself. The wall wart that runs the system is rated at 12 VDC at 1 A.

    The MH290 is a hand-held unit with a trigger, you pull the trigger when you are ready to scan and the laser starts scanning for 4 or 5 seconds and then shuts down. To attempt a second scan, you have to pull the trigger again. Inside the hand unit there is the receiver, a second PCB to support the receive electronics and the spinning mirrors (driven by a small 15 degree per step stepper motor). The MH290 is smart enough to know when the laser is on, and the error is produced if it doesn't come on OR if it stays on longer than it should.

    The MH290 connects to another unit via a 9 pin RS232 type connector, the other unit has the EEPROM and related components for decoding and interfacing to the computer itself. The MH290 hand held scanner does not connect directly to the computer and all power sent to the MH290 comes from this other box.



  • Back to Laser Instruments and Applications Sub-Table of Contents.

    CD, DVD, and Other Optical Disc/k Systems

    There are two consumer oriented applications for lasers that are by far dominant, at least in terms of the number of units produced. These would be laser printers and related equipment, and CD, DVD, and Blu-ray and HD DVD players and drives. Laser printers aren't really very interesting from a technology perspective - an IR laser diode with fancy focusing and scanning optics. (I suppose laser pointers are also something that should be included as being a common consumer laser but it's not clear how many of these are actually used for their intended purpose!)

    Optical storage, of which CD, DVD, MiniDisc, and LaserDisc are a subset, are all based on very similar technology requiring extreme precision to be able to read (and perhaps write) micro-size features on a spinning disc/k. The first optical drives were developed in the 1970s using HeNe lasers. LaserDisc players were the first consumer electronics to benefit. Early ones used HeNe lasers but even more modern LaserDisc players seemed to simply substitute an IR laser diode for the HeNe laser while retaining much of the other optics without significant miniaturization. However, LaserDisc players were never the same sort of mass produced product as the CD player and were more directed to high-end and specialized markets like interactive education and training since the disc format allowed rapid access to video snippets or up to 54,000 individual video frames on a LaserDisc. With the introduction of the personal computer around the same time, the LaserDisc was an ideal video storage peripheral, unsurpassed until the advent of the DVD. (And some people would claim still superior.)

    For more information on optical storage technology, see the Notes on the Troubleshooting of Compact Disc Players and CDROM Drives. In addition to descriptions of how the technology works, there are photos and diagrams of optical pickups ranging from one in an HeNe laser-based LaserDisc player prototype through modern DVD drives. The size difference is dramatic with the typical DVD pickup being roughly 1/1000th the volume of that LaserDisc pickup. Yet, it must perform all the same functions.



  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Laser Printers and Similar Equipment

    Introduction to Laser Printers

    All modern laser printers use IR diode lasers of 5 to 30 mW maximum output. Their wavelength is generally around 780 nm (like those of CD and many other optical disc/k systems).

    Very old laser printers used helium-neon lasers but these are even rarer than HeNe laser based LaserDisc players. However, if you do find one, there will likely also be an Acousto-Optic Modulator (AOM) and driver since directly controlling HeNe lasers at high speed isn't feasible - don't neglect these very desirable components!

    And, of course, those large graphic arts machines may have large HeNe lasers and even air-cooled argon ion lasers though newer ones will use Diode Pumped Solid State Frequency Doubled (DPSSFD) green lasers.

    Anatomy of the Optical System of a Laser Printer

    The optical path from laser to photosensitive drum is in the order listed below: The laser and optics components in laser Fax machines are similar but in addition, there will be the cold cathode fluorescent lamp, imaging lens, and CCD array of the input section. In principle, this could also be a laser scanner with virtually identical construction to that of the printer but I don't know if this is ever done in practice.

    See the document: Notes on the Troubleshooting and Repair of Printers and Photocopiers for information on how the image exposure and fixing portions of this equipment works as well as warnings and precautions with respect to the hazards of toner dust. See the document: Sam's Gadget FAQ for more on salvaging parts from deceased equipment.



  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Laser Light Shows Lasers

    Some Basic Info on Light Show Lasers

    For more information on lasers suitable for light show and related multimedia entertainment applications, see the Chapter: Argon/Krypton Ion Lasers. For more information on all aspects of laser light shows, check out the LaserFX.com Web site.

    (Portions from: Erik Huber (erik.p.huber@uibk.ac.at).)

    I worked in a big disco as LJ - Did a lot of raves and such stuff. I also DJ a little just for fun. The laser power you need depends on the room you have. If you want to scan pictures you need more power. If you just use rays, you won't need so much.

    The prices for such lasers look like these:

    WARNING: Be aware that the maximum laser power level for the human eye is about (2.5 mW)/(cm2). Never look into the beam!

    (From: Steve Roberts.)

    If you wish to scan graphics on clouds, it takes from 10 to 20 watts of well collimated argon light to do so, and the problem is only people within about a 10 degree cone around the laser site from where it hits the cloud will see the graphics. Everybody else at best sees a faint flash from within the cloud, and in most places in the US the conditions for doing it will only be right a few days a year. It's also not a good surface for images, any thing more then a simple logo or spirograph pattern is unlikely to be recognizable. Scrolling text didn't work. How do I know, I was the one running the spirograph generator as a guest at the laser site.

    Safety of Laser Shows

    (From: L. Michael Roberts (newsmail@LaserFX.com).)

    In the USA, laser shows in clubs/bars/parks are regulated by the CDRH (Center for Devices and Radiological Health, a division of the FDA). Audience scanning is NOT permitted in the USA while it is common in the rest of the world. A large scanned effect spreads the laser power over a wide area and usually has some motion to it (such as the sine waves used to make rippling sheets of light). This means that the energy density and the exposure times are low.

    If the laser beams are not scanning directly on the audience (dancers) then the effects are probably safe. If the system uses scanned beam effects, then it is probably following the rules of it's jurisdiction and is probably safe.

    Having done a few of those shows overseas, it's not just moving fast, but that's part of it. In fact, moving too fast can in some cases brighten the beam to exceed the MPE (Maximum Permissable Exposure) because of the dynamic characteristics of the scanners. It's the dwell time on each point of the image as the scanners are tracing it out, it has to be carefully measured for each animation or effect with a scope, fast photodiode, and a laser power meter. Each image has to be carefully designed using the show software to avoid sharp corners and other hotspots. Just scanning it fast is not enough - you will note that only very large scans flow over the audience. There is what is referred to as the zero line, well above the audiences head. As the images dip below the zero line, they are reduced in brightness by the hardware and by the show programmer. A scan fail system is also usually in use that will cut off the system should a scanner fail, and this has to happen fast if the MPE is not to be exceeded, in a fraction of a millisecond, so very careful engineering has to go in this.

    Please folks, just because you saw a beam scanned over the audience in a club and you have a laser, don't try it at home without getting the equipment to make the measurements and calculate the MPEs. It is not possible to determine if a effect is eye-safe by eyeball alone. The European clubs pay between $50,000 and $100,000 for these systems so a lot of time and money is spent on doing the safety analysis when programming a show. There are permits and licenses involved as well. Each frame of the show - and there is usually 6 to 15 frames per second - must be checked and carefully designed when doing this. The show must be checked for each facility it is ran in as well. You really need to take a class in how to do it safely. Such classes are offered at ILDA and ELA meetings and by safety inspectors/laser providers in Europe.

    Please note that this is not normally legal in the US as we have lower MPEs that make it ineffective when done anyways. It was suspended in much of Europe recently for a review of the power levels in use, new standards were implemented with tighter controls and it is again legal in parts of Europe. It is also legal in Canada, but again, measurements have to be made.

    If you're gonna do a show and you don't know what your doing, the basic guidelines for where the beam may go are a minimum of 3 meters up from the highest point in the audience and a two meter horizontal separation from the audience to any beam. In the USA, a CDRH Variance is required for any public show above 4.95 mW, and the penalties are draconian for failure to obtain them. The MPE in the US is about 2.3 mW per square centimeter per second for visible lasers.

    Single or Multiple Lasers for Color Presentation?

    For multiple color presentations, it's possible to use either a single laser that produces more than one wavelength, or two or more lasers combined into a single beam using dichroic mirrors. The only commercially available multiple wavelength solution is to use a 'white light' mixed-gas argon/krypton ion laser. The advantage is that it's possible to get a reasonable mix of colors and relative intensities, and the beams are already aligned to each-other. The disadvantages include large power requirements, high cost, and possibly relatively short life. The wavelength balance also changes with age, use, and output power. However, there is really is no viable alternative at present to obtain suitable red, green, and blue wavelengths from a single laser. Solid state RGB lasers are never built as single lasers, but either multiple lasers, or one or more lasers with some other wavelength conversion scheme added on. These are very complex and expensive but will no doubt improve and come down in price as demand increases.

    Where a non-gas laser solution is desired, what is often done is to combine green (532 nm) and blue (473 nm) DPSS lasers with a red (635 or 650 nm) diode laser, though red DPSS lasers (656.5, 660, or 671 nm) are now becoming available. However, this is 3 separate lasers whose beams need to be matched in size and divergence, and possibly polarization, and then combined into one.

    (From: Wes.)

    For our 1.4 to 1.5 Watt RGB system, we use the following:

    This combination makes a nice white balance. You could use 50% less red if you have 635 nm red. If you have 671 nm from a red DPSS laser, might as well forget it (would require way too much power since very very low visibility of that wavelength). FWIW, I think 650 nm makes better colors than 635 nm.

    (From: John R. (scifind@indy.net).)

    In my opinion, I would rather have a single mixed-gas 'white light' laser to avoid the hassles of beam collimation of two independent lasers. This is especially true if you do shows on the road where everything is jostled around. You may get better life with a red-only krypton tube, but you are almost always fiddling with near- and far-field collimation to keep your PCAOM output efficient across the entire spectrum.

    The color balance in a single mixed-gas laser will slowly change over time, but it is easy to make software color palette corrections on the white-light balance in a few minutes. (At least until the red output drops too much.)

    As for tube lifetime, I think it is function of art, science, tube current, luck, and the phase of the moon when the tube was installed. I know one laserist who only got 600 hours on a tube. I know another that has lasted for many years.

    About the Schneider High Power DPSS RGB Laser/Projector

    This laser has been reported on in various laser trade publications and discussed on the USENET newsgroup alt.lasers. Such systems represent the future direction of technology for RGB laser show and laser TV equipment due to their higher efficiency and more robust construction. Cost is still a problem though. :)

    (From: Patrick Murphy (pmurph5@attglobal.net).)

    The Schneider solid-state RGB laser does exist and is in use for laser shows, including the Hershey Park outdoor show in the U.S. There are two main versions of the laser. One is just the laser for light-show type applications. The other is the laser plus a video projection head (scanning mirror type) to create infinite-focus, wide color-gamut video. I saw both versions doing a combined show (video + laser graphics + laser beams), a few weeks ago at the Schneider factory in Germany.

    The following information relates just to the light-show model, imaginatively called "Showlaser" .

    The original $160,000 price mentioned elsewhere was an estimate; the actual U.S. price will be somewhat lower than this ($120K? $140K?). This is still a lot, but not quite as much as the estimate. Schneider realizes the price is high for the laser light show market and will be seeing if it is possible to lower it.

    The useful output power is 13 watts of modulated white-light from the end of a fiber (e.g., into your scanners). The colors are nicely spread -- red at 628 nm, green at 532 nm, blue at 446 nm -- so you get very dramatic violet and purple. (In video applications, there is no speckle, skin colors are normal, and saturated colors are quite striking.)

    The input power is 220 VAC at 3,000 W (e.g., about the same as two hair dryers). It has its own internal chiller, which you fill every few months with a gallon of distilled water. So in this sense it is "air-cooled", as you don't have to hook up an external chiller.

    Because everything -- laser head, modulators, chiller, power supply -- is built into one unit, the Showlaser weighs 660 pounds. This is roughly the same as all the parts of a medium- or large-frame ion laser together. The unit is compact and is on casters so the weight is not quite as bad as it could be.

    The working part of the laser is manufactured by Jenoptik (it says so it in a big decal on the Showlaser's side). The working principle is described in this paper: RGB Lasers for Laser Projection Displays. Here is the abstract:

    "JENOPTIK Laser, Optik, Systeme GmbH has developed the first industrial all-solid-state Red-Green-Blue laser system for large image projection systems. Compact in design (0.75 m 3 , 180 kg, 3 kW power consumption), the system consists of a modelocked oscillator amplifier subsystem with 7 ps pulse duration and 85 MHz pulse repetition frequency, an optical parametric oscillator (OPO), and several non-linear stages to generate radiation at 628 nm, 532 nm and 446 nm with an average output power above 18 W. Each of the three colors is modulated with the video signal in a contrast ratio of 1000:1 and coupled into a common low order multi mode fiber. The system architecture relies on efficiently manufacturable components. With the help of FEM analysis, new engineering design principles and subsequent climatic and mechanical tests, a length stability below 50 um and an angle stability below 10 uR have been achieved. The design includes efficient laser diodes with integrated thermo-electric cooler and a life time above 10,000 hours. The stability of the output power is better than +/-2% in a temperature range from 5°C to 40°C. The system operates reliably for more than 10,000 hours under field conditions. The design is based (among others) on work by Laser-Display-Technologie KG and the University of Kaiserslautern."

    The working part contains numerous optical components on a breadboard. Although it looks like a nightmare to align, everything is actually controlled by a computer. Once it is factory-set, in theory you never need concern yourself with what is inside. Schneider says the laser will last 10,000 hours before the diodes need replacing.

    "AVI-Imagineering With Lasers" is the U.S. distributor. They've received the one for Hershey Park, with more on order. So far, the Hershey Park laser has traveled well for AVI. It was trucked five times and four times there were no problems at all when the laser was turned on. The fifth time there was a power loss which may or may not have been due to traveling. (The cause is still being studied.) Since the solid-state laser is much newer than decades-old ion technology, I think people should expect a few "teething pains" to be worked out.

    Schneider also makes high-end TVs sold in Europe. I have been through the factory (same place as the laser division) and it is an amazing place, with raw materials such as plastics and electronic components coming in one end, and consumer boxed TVs coming out the other. Schneider also recently bought a majority interest in "tarm", the well-known German laser show company. So Schneider does things on a big scale, they know what they are doing in laser, and they want to do it at a consumer level.

    Obviously, it's pretty amazing for an RGB laser to get 13 watts of modulated light from a standard 220 VAC dryer-type outlet, with only occasional water top-offs, and a 10,000 hour claimed life. On the downside is the weight and the natural bugs that come with development of any new technology. The price is the biggest obstacle at this moment. With luck that may be coming down to a more affordable level, as volume, development, technology etc. improve.

    Inexpensive Combining of Argon Ion and HeNe Laser Beams

    Also see the section: Combining Light from Multiple Lasers.

    (From: John R (scifind@indy.net).)

    White-light color control with a red HeNe and multiline argon ion laser and be done without a PCAOM, but you may not like the answer. It is much cheaper than the PCAOM method, but still involves lots of work and moderate costs. Of course, if you are a laser hobbyist, nothing is cheap, especially if you want laser beams other than 632.8 nm red!

    For a minimum white light color control system:

    1. You need a multiline argon ion laser with at least the 488 (cyan-blue) and 514.5 (green) lines.

    2. You will need three separate dichroic filters. (Edmund Scientific and others sell these).

    3. One dichro is used to split the multiline Argon beam into a transmitted blue line and a reflected green line at 90 degrees. This gives you the isolated blue and green beams.

    4. Once you have the separate blue/greens, you need some method of intensity color. Three possibilities are single-channel AOMs, blanking scanners, or simple beam shutters.

    5. Once you get the blue/green beams through intensity controllers, they must be recombined using another dichro.

    6. Using a third dichro, the Argon beams and then super-imposed onto the red HeNe laser beam. (Of course, you should have some type of intensity controller for the red HeNe beam as well.)
    Thus, the final "white light" beam is made up resultant actions of three dichros and three intensity controllers. If you have some type of analog controller for each R/G/B color, you can blend them produce an incredible amount of colors.

    I once built one of these "RGB color boxes" using an argon and HeNe laser. It worked quite well, but there was the major hassle of alignment of multiple dichros, other mirrors, and three AOMS. A significant portion of the Argon power may be lost because it has to pass through three dichros.

    As for costs, if you can get surplus AOMs, dichros, and make your own mirror mounts, maybe $200 to $400 - if you're lucky!

    Unfortunately, there is no simple or cheap way of doing it.

    And, if you are thinking about mixing yellow and orange HeNe's with argons and red HeNe's, I seriously doubt you will achieve the performance (and ultimate cost) of even a used PCAOM.

    Why?

    1. Both yellow and orange HeNe's only give a few milliwatts. They will easily be over-powered by the argon laser not only in terms of actual milliwatts, but in apparent visual brightness to your eyes.

    2. Unless you are just shining the independent laser beams onto the same spot on the wall, accurate near- and far-field collimation of a multiline argon with three yellow, orange, and red HeNe's is almost impossible.
    You will need some lots of custom dichros to combine the beams and numerous beam leveling mirrors to achieve it. Lots of dichros and lots of mirrors translates into "lots of losses" and a bitch to establish and maintain collimation. Three dichro color systems are still lots of work. In this case, you would have a FIVE-color dichro system.

    You may also run into problems as each independent laser has its one beam diameter, divergence, and spatial TEM characteristics. So if you could collimate them, the resultant "white light" beam will have lots of color fringes.

    Of course, it is your time, money, and effort, therefore, I wish you good success. But using a higher power red HeNe and then blending it with the multiline argon is still the better approach.

    For more information, try Laser FX. Their Website author also has an excellent handbook on lasers and laser shows. There are a couple of chapters devoted to RGB color control in lasers, including HeNe/Argon methods. If you are serious about making white light beams (and learning about lasers and shows), this is the book to have!

    Also, other ideas. Neos Technology has a 4-channel PCAOM crystal for $680 and driver for $600. If you are a hobbyist, this is not cheap. However, if you can get a PCAOM system, it is vastly superior to the RGB/dichro color method.

    (From: L. Michael Roberts (NewsMail@LaserFX.com).)

    To combine the two lasers your best and lowest cost solution would be a dichotic. Firstly you need to have a set of two FS mirrors on optics mounts (E.g., Newport MMI or RMSM OM3/4) to level and steer the beam. Purchase a cyan or red dichro (from Edmund or PPS); mount it on another optics mount. With a cyan dichro, you shine the argon through the dichro (which transmits green/blue wavelengths). Set the dichro in the beam at 45 degrees at the point where the ar and HeNe beams are made to cross at a right angle.

    Careful adjustment of the steering mirror pair on each laser will allow you to produce two beams that are level relative to each other (and the baseplate of your projector) and cross at right angles. Set the dichro in the position where the beams cross at a 45 degree angle relative to the Ar beam (with the 45 degree angle such that the HeNe beam is reflected away from the Ar source).

    Adjust the beams until the HeNe and argon beams overlay each other on the dichro (near field adjustment). Now look at the resultant beam at some distance or on the projection surface. Adjust the dichro so that the two spots overlap (far field adjustment).

    Adjusting the dichro will cause some change in the position of the Ar and HeNe beams so you then re-adjust the near field (laser steering mirrors to overlap the beams on the dichro); then the far field (dichro to overlap beams on the screen). 2-4 adjustments going back and forth form near to far field may be required, but in the end you will have the two beams exactly overlaid on each other. To the eye, the beam will appear a pinkish white - colour balance can be adjusted by varying the brightness of the Ar laser.

    A cyan dichro is recommended as it reflects red and you want to conserve red photons. You will note that some of the argon beam is deflected in the direction the HeNe would have been going if not reflected. This is due to beam splitting at the surface of the dichro. If you use a red dichro, those would be red photons you would be throwing away.

    You can now place a PCAOM (from NEOS or MVM) in the combined beam. Make sure the polarization of the HeNe is vertical (check the ar while you are at it - they are usually polarized vertically but poor alignment could have you a bit off) and that the PCAOM cell is correctly oriented. Varying the control voltages to the PCAOM will allow you to have additive (RGB) colour control. You can get 16.7 million colours or more depending on the PCAOM and the system used to control it.

    Dichroic Mirrors for Separating Multiline Beams

    Dichroic (dielectric) mirrors can be used to split a multiline laser beam into two or more sets of separate lines. They enable the construction of simpler, smaller, and more efficient systems compared to dispersive techniques like prisms or gratings. But good quality dichros are not cheap.

    (From: Steve Roberts".)

    There are 3 quality sources of laser show dichros that I have used:

    For pricing, you're looking at $20 to $50 a square inch, depending on quality, and whether a precut size is available. Some may charge a cutting fee or a little more for the AR coated units. Keep in mind you need to know if you want CMY or RGB and 0 or 45 degree incidence, as most folks will stock the whole set of combinations. Be clear - specify that you want "transmit blue reflect green at normal incidence" Or "pass blue/green combine red at 45 degrees". Most people don't think about it, but "pass deep blue and violet" for a argon laser turns out to be a nice dichro to have.

    Prisms are generally only useful for separating one line, and for laser display purposes, you need all the power you can get, so you want all the blue or all the green lines, etc. They are also a pain in the neck as dispersion versus angle is constant, and a dichro can be tilted off axis quite a bit and still have throughput. Many traditional laser projectors for planetariums did just that, have a prism and a color selection galvo, but this takes up several feet of space to do and is difficult to support from a control systems point of view and to align. With a prism, you're wasting from 60 to 85% of your light at any one time, as you're only using one line.

    Also beware that Edmund Scientific's dichros are more or less coated for TV/spotlight applications and thus leak some blue or green that a laser show dichro wouldn't. This spoils the effect of clean contrasting colors, so you need a dichro designed for laser display. Edmund's dichros are great with a tungsten source however.

    When you order, ask for backside AR coats on your dichros if available. Otherwise you'd have 8 to 10% Losses from the Fresnel losses.

    Visibility of High Power Laser Beams

    The following applies to the visibility of the beam itself (i.e., Star Wars Light Saber style), not to its appearance then it strikes a surface.

    (From: L. Michael Roberts (newsmail@LaserFX.com).)

    To create visible beams in *total* darkness you can get away with as little as 100 mW. For beam effects in a club or other venue with some ambient lighting, 1 watt is about the minimum you need to make visible beam effects. Outdoors you will need 5-6 watts to make visible beams (again depending n ambient lighting conditions).

    In all cases, a scattering medium (smoke or dust) is required to deflect the light towards the observer's eyes. In clean, clear air in winter, I have seen the beams from a 20 watt argon look lamer than the beams from a 1 watt indoors with a good haze.

    (From: Steve Roberts.)

    In a dark room with average dust levels and high humidity you can start to see the forward scattering of an HeNe beam at about 1 mW! 30 to 40 mW of argon makes an OK side view beam in a dim room, but its not exactly a Star Trek photon torpedo kind of glow. It helps if the argon is configured multiline and is doing more green then blue, as the eye peaks in the green. To see the beam in a well lit room requires smoke of some form.

    Most laser light show types don't like the common aquafog, it irritates your lungs after constant exposure, so we use hazers indoors. A hazer works by making very tiny particles of medical grade oil. These are small enough to be flushed out of your lungs by normal breathing and if properly set up, are odorless and OSHA approved. Fog machines for the most part are crackers, they work by incomplete combustion of glycols (aquafog) or burning of oil in air. Hazers fragment the oil in CO2 and thus are almost odorless. Plans for a homemade hazer of sorts that uses air are at LaserFX on the "Backstage" pages. It has a slight odor but is not that bad to be around, and mind you I have asthma! I have done indoor shows for 1,200 people using 60 mW and a cracker. I have also done shows indoors for 100 people with a 5 mW hene, it depends on ambient lighting and air circulation/humidity.

    It is a minimum of about 5 watts of argon light for a decent outdoor smokeless beam show, with 20 watts being more typical.

    (From: Steve Quest (Squest@cris.com).)

    Visible wavelength lasers are more visible in 'plain air' if the angle of incidence is low (you're close to the same angle of the beam) and if the power is greater than about 5 watts. I perform an outdoor laser show using a 30 to 57 (max) watt YAG (frequency doubled to 532 nm) which is plainly visible in mostly clear air (no need to smoke, or fog the air). When I want to do beam effects with a 5 watt argon/krypton white-light laser, I have to fog the air up.

    Plain outdoor air has enough particulate matter to scatter a laser beam so long as it is above 25 or so watts, thus making the beam visible. Of course, the more power, the brighter the beam looks, but CDRH has limits, and that limit is .9725 mw/cm2 at 750 feet, so the days of power beam shows going all the way to outer space and beyond is over :-(.

    I use a Laserscope laser, which is FDA (Food and Drug Administration) approved, and am following CDRH (Center for Devices and Radiological Health) guidelines, receive FAA (Federal Aviation Administration) approval and air clearance before every show, and make sure that NOTAM (NOtice To AirMen) are issued to pilots flying in the area of my shows, giving exact details as to what is going on. Pilots love the shows, and air traffic routes planes WAY out of their flightpaths to fly near the beam shows to get the best seats in the house. :) However, I have to beam-off when they get too close, then they return to their flightpath, and I can resume the show.

    I used to be able to sparkle off the new moon with my YAG at full power and full convergence. It takes some doing but you can see the sparkle from the Sea of Tranquillity with the naked eye off the corner cube reflector, aka: retroreflector left there in 1969 by the astronauts.

    (From: Sam.)

    WARNING: Shooting a laser into the sky is irresponsible and highly illegal without prior approval from the proper agencies. Airline pilots do not appreciate being blinded!

    Here are some additional comments on the effects of viewing direction on apparent brightness:

    (From: Johannes Swartling (Johannes.Swartling@fysik.lth.se).)

    What you see is light that has been scattered by the small particles in the fog or smoke. This kind of scattering is called Mie scattering, and occurs when the size of the particles is comparable to or a little smaller than the wavelength of the light. In Mie theory, there is something called a scattering profile - i.e., the probability that the light will scatter in a certain direction.

    Now, in the case of very small particles, such as molecules, this scattering profile is isotropic. That means that the light will scatter in all directions with equal probability. This special case is called Rayleigh scattering, and can be seen from pure air if you have a strong enough laser, such as an Ar-ion laser. When the particles get larger, however, the light will tend to scatter more and more in the forward direction. That is what you see from the smoke. When you look along the beam in the direction where it comes from, you see a lot of light that has been scattered just a little bit off the direction of the beam. When you look along the beam away from the laser, there's a lot less light that has been scattered backwards.

    (From: Pissavin (pissavin@aol.com).)

    One interesting phenomenon; Depending on whether dust or smoke is used, there is an asymmetry: With smoke, if you put your head near the laser and look down the beam, you see almost nothing. Now, look toward the laser (BUT NOT DIRECTLY INTO THE BEAM!) and you see a clear beam. Then replace the smoke with dust and the effect will be reversed.

    (From: NeoLASE (neolase@lasers.org).)

    Large particles like dust have more back scattering centers while small particles like smoke and haze have more forward scattering centers. Mie scattering effects, and all that stuff, I've heard/read of but I haven't studied in detail. Used a lot in laser particle size analysis.

    Limitations of Lasers for Large Scale Shows

    (From: Dean Glassburn (Dean@niteliteproducts.com).)

    For the most power available, usually a krypton ion laser running red only and an argon ion laer for the blue and green is combined. The krypton red wavelength (647.1 nm) is not the best for color combination for true RGB mixing but it is about all that is available with adequate power. Remember, even if the argon were to produce 20 watts evenly split between green and blue, and 10 watts of red from the krypton, a total wattage of only 30 watts is available for the entire picture area. This really isn't that much for a large scale presentation and is why Vegas uses RGB light bulb boards as well and stadiums use Jumbotrons or Diamond visions, not lasers. The total light available is 1,000's of times brighter, and even with coarse resolution, the distance from the screen blends the image. Raster scanning with a laser is very inefficient, but with vector scanning and raster some unique effects can be created. Better yet use 10,000 watt lamps, one for each color via the proper filtering and use light valves to control the each device for each color. Like a projection TV except on a huge scale. And cost is always a factor.

    Use of Pulsed Laser for Laser Shows?

    (From: Steve Roberts.)

    How well this works depends on the pulse rate and pulse width of your laser and how fast you are scanning, and how much you like dots and dashes in your image. It also depends on how you are shaping your image - i.e ,, some non-galvo imaging systems use pulsed YAGs for projection video.

    However if you are talking about an AO Q-switched YAG at a high rep rate, you can do, say, 10 to 12K galvo graphics. It just shimmers a lot and has faint spots that wander through the image. The real killer is that the divergence of pulsed YAG lasers of any significant power is extremely high and when the divergence magnitude starts to catch up with the resolution of the points in the image, you get a blob. When it catches up with the scan angle, you get a bigger blob. This happens at say a couple of hundred feet from the laser.

    I have witnessed this as a member of the crew on a show using a Q-switched YAG for beam effects. The company owner wanted to try scanning images on a building some distance away to see how his collimator worked. Up close it wasn't bad. But, more then a hundred feet or so from the laser, it was "The Green Blob".

    Holographic Laser Show Images?

    Being able to project a 3-D image hundreds of feet into open space is pure science fiction - there is no current technology and even basic theory that would make this possible without some medium to act as a screen. However, some pretty vivid illusions that may give the impression of such a display do exist and you may experience one at your next large scale laser show:

    (From: L. Michael Roberts (NewsMail@laserfx.com).)

    The most common way of creating this illusion is to use a scrim or a water screen. The scrim is a thin fabric screen, like mosquito netting, that is often dyed black or dark grey. It is rolled/lowered/flown into place while the audience is looking at something else, then used for laser graphics projections. Using typical modern 30K PCAOM projectors, flicker free images can be projected onto the scrim. While most of the laser beam goes through the scrim, enough of the laser is intercepted and reflected by the threads in the scrim to form an image.

    The water screen is a similar concept except that it uses a thin film of water droplets sprayed into the air as a projection surface. Both there techniques allow one to create the look of an image suspended in mid-air - especially if the audience is fixed in relation to the projection surface.

    There is a beam interference technique in the early stages of development but it isn't likely to ever result in a large scale display out in open air. It was pioneered by Dr. Elizabeth Downing. The image is generated inside a specially doped glass cube using scanned IR lasers. At present. the display s barely 2" on a side. For details see 3D Laser Based Volumetric Display.

    Laser Show on a Shoe String

    A low cost way of getting into laser shows is described at LaserFX.com's Low Budget Laser Graphics System which includes information on suggested lasers, galvos, modifications to a sound card to pass DC, and the computer system and software. Much more info is of course available on the LaserFX.com Web Site.

    (From: Gronk (gronk@concentric.net).)

    I am fairly new to lasers (been studying and researching on internet for about 1.5 years now, especially Sam's Laser FAQ) and decided a few months ago to do my own laser show for our New Millennium eve party. We had about 30 or 35 people in attendance, and a musical show that lasted about 40 minutes. The equipment consisted of a home built Lissajous pattern generator (not the spinning motor kind) with laser modulation driving a GAL-2, a 1 watt stereo audio amp with raw audio from the show music driving a GAL-2, and 2 lumia wheels with 3 lasers shining through them.

    All this was projected on a silver screen (plastic tarp) suspended about 15 feet above the audience (no audience scanning done of course) . The lasers were all laser pointer types with the batteries removed and wires attached, and all connected to a home built laser power control station which controlled power to each individual laser. Fog beam effects were accomplished by spraying 'fog-in-a-can' at the beams. It turned out great, with all who attended enjoying it very much (granted, most of them had never seen a 'real' commercial laser show).

    It was a really fun project and will be done again at years end this year! I would encourage anyone who might be thinking of doing this to go for it! It was not really expensive, and was worth every penny for the all around experience. I also included my son (who was way better than me at operating the pattern generator) in the show, so he got a real kick out of it too. Highly recommended!

    (From: John Craker (watts@dccnet.com).)

    I built a basic laser show from a dead (semi dead?) LaserDisc player. When hooked up to my home stereo, it displays lovely (and useless) Lissajous patterns on my ceiling.

    I basically robbed a section of the chassis that housed the HeNe laser and another section that had two deflection mirrors. Pointed the output of the laser into the mirrors. I hooked up the coil of each mirror to each channel of my stereo. With the difference in the stereo signal, you have each mirror oscillating at a slightly different rate, and since one mirror deflects in the 'Y' axis, and the other in the 'X', you get this great ever changing display. Size is pretty much adjusted via the volume. :)

    (From: Sam.)

    Based on a photo that John sent me as well as the sample in Optical Pickup from HeNe Laser-Based LaserDisc Player, the deflector from this LaserDisc player would appear to be virtually identical to what Meredith Instruments used to sell as GAL-2 (I don't think they have them anymore). I wonder if that's where they got them. In the LD player, the galvos were used for fine tracking and tangential (timebase) correction. I also have seen similar deflectors in other somewhat newer Laserdisc player optical pickups. However, if an IR rather than a HeNe laser was used (as would be the case with anything after about 1983), the mirrors will likely not be highly reflecting at visible wavelengths (though very thin mirrors could perhaps be glued on top of the IR mirrors, with a slight sacrifice in performance). Along with a HeNe laser or laser pointer, and low power audio amp, you're in the instant light show business. Well, at least for those boring Lissajous patterns! :) The GAL-2 is sensitive enough to be driven by a personal stereo but the 4 ohm input impedance may overload and kill its output amp if it is designed for 32 ohm headphones.

    Note that while the GAL-2 (or LaserDisc deflector) appear superficially similar to a pair of loudspeaker voice coil/magnet assemblies, the pole pieces of their magnets are on either side of each coil rather than within and surrounding them as in a true loudspeaker. Thus, the coil, and thus mirror, pivots from side-to-side as expected and desired rather than moving in and out.

    Building a Beam Table

    If you are conducting high-precision scientific experiments, or doing holography, you will need one of the BIG (4 x 8 foot (1.2 x 2.4 m approximately), vibration isolated optical tables like the ones available from Melles Griot, Newport, and others. You will also need a large wallet, not to mention a solid foundation and space to locate it!

    If you are a starving laserist who wants to make something for mounting lasers, a few optical components and scanners, you can semi-DIY for a reasonable cost.

    Go to your local machine ship and have them order a sheet of 3/8" (9.5 mm) T6016 aluminum large enough to mount your laser(s) with space left over. I would suggest 5' (1.52 m) by 18" (0.46 m) for medium frame lasers, longer if you want to build a beam table. Now have the machine shop drill and tap 1/4-20 holes on a 1" grid (M6-1.0 holes on 25 mm grid for metric).

    To save money, do not have the entire plate drilled and tapped. Leave the area where you intend to mount the laser(s) blank - you could have the shop put in the mounting holes for the laser(s) or you could do it yourself. The area where you intend to mount the electronics can be drilled and tapped on a 2" (50 mm) grid. You will need some mounting holes in that area, but unusable holes under transformers and PSUs just cost money. The area at the output of the laser(s) should be drilled and tapped with the full 1" (25 mm) grid as this is where you need the most flexibility for mounting optics.

    When the plate is done, have them chamfer the edges and send it out for black matt anodizing. 4 or 5 years ago, I could have one of these made up for around $250 CAD - YMMV

    One last tip: When choosing the mounting position of the laser, make sure the output beam will fall between two lines of holes, and parallel to the holes, in the grid to allow the most flexibility on mounting items on either side of the beam.

    Galvo Type Deflectors for Laser Light Shows

    If you just want to get your feet wet in laser shows, you can mount small first surface mirrors on a pair of loudspeakers. But to get the full effect, they have to be modified to alter the angle of the mirror, not push it back and forth. An alternative is to mount mirrors on small hobby motors at a slight angle. Up to 3 motors can be used with the beam bouncing off the mirrors in succession. Varying the speed of the motors individually will then produce a large variety of Lissajous and/or spirograph type displays.

    One step of from these bare bones approaches is to obtain a set of inexpensive galvos. The most common source for these are from ancient HeNe laser-based LaserDisc players. See the section:

  • Laser Show on a Shoe String. However, if you want to play with the "big guys", then what's needed is a pair of high speed galvos and the driving electronics. These are probably even more important than the laser(s) in determining the ultimate performance of any laser show:

    (From: Steve Roberts.)

    May I suggest what I suggest to all beginners in Laser Shows?

    1. Buy a copy of L. Michael Roberts' (no relation) book on Laser F/X, its well worth the money and will save you much reinventing of the wheel.

    2. Save up and buy decent scanners, much fun can be had with the slower stuff, such as G330s, but in the long run you will find yourself needing to acquire faster scanners anyway and will be setting yourself back financially and time-wise with the slower units.

    Acceptable galvos for beginners:

    Don't bother with galvos like CECs - they are designed for exposing beams in small chart recorders using a ultraviolet arc source, they are referred to as "pen" galvos, and thats what they are, about the size and shape of a ink pin, with a small mirror about .5 mm across. They are thus too small to make a XY mirror pair, especially since the external magnet needed is huge.

    Controlling the Divergence of a Laser Projector

    (From: TOCKET.)

    I've been looking into installing a lens in my projector to increase the divergence enough to make audience scanning safe. I've seen recommendations to use -3 diopter lenses in small rooms, but I wanted to be able to calculate appropriate strengths for different conditions. So here goes:

    Lens strength is often specified in diopters, which is simply the inverse of the focal length, that is:

     D = 1/f m-1
    

    Where D is the strength in diopters and f is the focal length in meters. (A nice property of diopters is that lens strength is additive - the result of using lenses D1, D2,...,Dn is simply their sum D1+D2+,...,+Dn.)

    A collimated beam that passes through a lens will get a half-angle divergence of:

     θ = r0/|f| = r0*|D|
    

    Where r0 is the radius of the collimated beam. Note that it's the absolute value of the strength that is used. A concave lens will give the same divergence as a convex of the same power. However, the convex lens will have its focal point in front of the lens, which is not desirable. It is also interesting to note that the divergence is directly proportional to the beam diameter, which means that the initial size of the beams is very important and that they must be matched.

    The size (radius) of the beam at a distance z is given by:

      r = r0*z/|f| + r0 = r0*z*|D| + r0
    

    For example, using a -3 diopter lens to diverge a beam with a 3 mm diameter and measuring at 4 meters:

      r = 1.5 mm * 4 m * |-3| m-1 + 1.5 mm = 19.5 mm
    

    Which means that the beam will be 39 mm wide at 4 m. Assuming a circular beam with a flat profile (not very realistic though) the following equation can be used to calculate the irradiance, which is really what matters from a safety perspective:

     E = φ/A = φ/(2*π*r2) = φ/(2*π*r02*z2*D2)
    

    Where E is the irradiance (W/mm2) and φ is the radiant flux (W). To convert to W/mm2, multiply by 100. While the calculated values here should be taken with a grain of salt because beam profiles are never flat, it is useful to see that doubling the strength of the lens (or distance) gives 4 times lower irradiance. The actual irradiance must be measured to assure safety.

    Dye Laser for Red through Yellow Wavelengths?

    Green and Blue are generally produced by either a multiline argon ion laser (though a DPSS laser is often replacing the power hungry ion laser for green at least). However, getting high power red requires either a krypton ion (or mixed gas) laser or very expensive DPSS laser. Even the largest HeNe laser (SP-125, multimode if one exists) won't break the 200 mW barrier and it's very difficult and costly to get decent beam quality from a red diode laser. Orange and yellow are at least as much of a problem. So, what about pumping a dye laser with an argon ion laser?:

    (From: Steve Roberts.)

    As an example, a Coherent 930 medical system uses a modified I90 tube with a CR599 three mirror dye head. Threshold for the dye from the factory docs with fresh R6G, fresh optics, and a good tweak, is 1.5 watts all lines from the argon, lasing at a few milliwatts tuned at 640.2 nm. Note that the power is only about 40 mW at 2.5 watts pump, reaching a max of 3.2 watts with 9 watts pump. The specially selected MRA tube with extreme multimode optics reached 12 watts when new at 40 amps, but was designed to only sustain these powers for 30 seconds or so at a time. The opthalmologist or dermatologist who would use one of these needed about 0.7 watts of treatment power.

    2.5 watts pump is about 24 amps down the tube.

    If you moved the unit around without draining, the dye reservoir vents are set up in such a way that you would leak dye solution into the PSU. There is no drain on the unit, you'd have to suck it out. The dye solution is not just methanol, it has some nasty additives to quench triplet states that prevent the dye from lasing, the dye pressure is about 40 to 150 psi adjustable and it squirts across a air gap.

    I still have the R6G stains on the garage floor from scrapping a aurora dye a few years ago.

    Now if you have room for a second three-phase laser (in addition to your green/blue argon ion) laser at your rave and a large box truck with lift-gate, don't mind a 400 pound 6 foot long 18" wide console on wheels (build the beam table on top of it!) and like cleaning liquid cancer off your optics while ruining a change of clothes every time you open it up, then this is the laser for you. Splitting it into boxes would cost a lot as the linear PSU is spread out all over the thing. If you run it at 2 watts of tunable red through yellow it would be a hell of a show, especially if the stepper controller on the tuner was rewired. If the tuner is removed, it would lase broad-band by a few nanometers at the peak of the dye.

    By the way, the blue pump beam is nearly totally adsorbed if the thing is tuned to rock, and fluctuates and sputters like a lumia on the wall of the dye head.



  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Laser Based Systems for 2-D and 3-D Display

    Whatever Happened to Laser TV?

    I am sure everyone has heard of the predictions that there would be mural (or stadium) sized TV screens using lasers instead of the other silly technologies like LCDs and light valves. This was 10, 20 years ago. Where are they? The idea is simple: Replace the three electron guns in the color CRT with red, green, and blue lasers and raster scan a TV picture onto your favorite screen, barn, or mountain-side. :-)

    There are now companies marketing (or at least seriously demonstrating) laser based TV displays. The most recent versions use a single multi-color diode pumped solid state laser. One such unit has an optical output of about 13 W. To put this in perspective: The visible output of a 250 W incandescent bulb is about 13 W. So, that's a lot of light for a small screen but isn't going to compete in a theater setting. And, you don't want to ask about the cost! :) But, see the section: About the Schneider High Power DPSS RGB Laser/Projector for info on one such unit.

    Using laser diodes directly rather than solid state lasers has some fundamental problems. The first has to do with color. Untill recently, you could have any color of laser diode you want as long as it is red. :) While moderate power (perhaps up to 500 mW or 1 W) red laser diodes have been around for awhile, laser diodes with an actual blue wavelength (430 to 445 nm as opposed to deep violet - around 400 nm) are just becoming available as costly engineering samples with all sorts of strings attached and they have power outputs of only a few 10s of mW at most (see: Availability of Green, Blue, and Violet Laser Diodes). Of course, even 445 nm is more violet than blue, 460 would be better, but it's a start. Green laser diodes aren't even on the horizon in an commercial form and those tested in the lab have had very limited life and may have operated only at cryogenic temperatures. Unfortunately, even if high power RGB laser diodes could be purchased for $10, due to the fact that they would operate with multiple spatial modes along one axis, generating a tightly collimated beam suitable for direct scanning would be very complex and expensive, if not outright impossible. Better go to plan B. :)

    However, there is what might be described as a hybrid technology that still use lasers for the light sources but with a MEMS (Micro ElectroMechanical System) for the modulation. The Grating Light Valve (GLV) is a 1 dimensional array of MEMS-controlled diffraction gratings. See Silicon Light Machines Products and Technology. A typical system for TV or computer display would utilize 3 GLVs (one for each primary color). Each GLV would have enough channels for a vertical or horizontal line of the display and conventional (low speed) mechanical deflection such as a galvo would be used for the other axis. Such systems have been demonstrated and the GLV already has a track record in the printing industry where it is used to expose master printing plates a swath at a time using a high power IR laser diode line source. While there are no fundamental technical problems with this approach and it is certainly much simpler in some ways than direct scanned laser TV, there is still the not so minor issue of low cost high power lasers. But at least, multimode diodes can be used so when high power blue and green laser diodes are available, we'll be all set. :)

    (From: James A. Carter III (jacarter3@earthlink.net).)

    Just to let folks know where this Laser TV thing has been.

    In the 1920's, a company in England, Scophony Labs (I think that's right) patented a method for using Bragg diffraction on tanks of water (that's right H20) to display TV signals using white light (thermal) sources. They had to use BIG beams because they didn't have lasers. BIG beams mean low modulation rates due to acoustic transit time. Their idea was to scan the spot so that the acoustic pulse was stationary on the screen. I believe that they didn't use galvonometric scanners for the horizontal scan, instead they put mirrors on motor shafts (similar to what some cinemagraphic projectors used at the time). The scan rate and magnification were selected so that the scan velocity vector was equal and opposite to the image of the acoustic velocity vector. This may have been an idea way ahead of its time.

    Just ten years ago, I helped design the optics of a system that does display not only NTSC images but scan to HDTV as well. This is not a cheap system and is certainly is not suitable for avionics; although the Air Force (through TRW) did buy many systems. It used an air bearing motor to drive a many faceted polygonal mirror scanner for the horizontal scan and used a "galvo" scanner for the vertical. The AO modulators had enough band-width (at least 500 times what you get from PCAOMs) to project NTSC images in a flying spot mode. That is the scanner was going much to slow to give the Scophony condition. When we ramped the system (it was a closed loop continuous multiscan projector) to 1280 by 1024 sources, the scan was fast enough that we achieved the Scophony condition and realized over 35 MHz of video bandwidth per channel. This is somewhat inadequate for computer CAD graphics but was quite acceptable at the time. The display was dazzling, to say the least. Per laser color for each red, green and blue channel with red at a deep and rich 635 nm (dye laser pumped by the otherwise useless cyan lines), and the argon lines for green and blue. We used a 10 watt argon from Spectra-Physics to be the photon engine (SP was an investor here). One of these went to the NAB show and displayed our beloved President Ron.

    Unfortunately, the lasers were not reliable enough, to expensive to repair and replace, and more light is always better. Further, the big guys (TRW and SP) started to bicker and the company went under. The last time I saw one of these systems was at SP Corporate in San Jose. I was there to install a 25 watt laser, but that's another story.

    Current commercial work centers on dumping the high speed scanner and using an AO cell to modulate the whole line at one time. Bragg cell technology can give the Time-Bandwidth Product (TBP) required which is certainly over 1000 and closer to 2000. Unfortunately, acoustic attenuation (Beer's law in time and space) and the non-uniformity of the laser source (typically Gaussian) require losses to make a nice uniform display. Even with HIGH power pulsed lasers (repping at the horizontal line rate or at a multiple), the display can lack luster.

    As always, more photons... more photons...

    (From: Tony Clynick (tony.clynick@btinternet.com).)

    I am pleased to tell you that laser video projection is still very active in the UK. Based on the original laser video projector (LVP) made by Dwight-Cavendish in the early 1980's, the projector now made by the team at LCI (Laser Creations International in London) has been installed at several permanent sites in theme parks since 1994, mostly in East Asia, and has been used for dozens of temporary shows world-wide since 1987. Most applications are in exhibitions, outdoor shows and theme parks.

    The LCI-LVP uses SP white-light lasers with special optics to provide good flesh-tones so the need for dye lasers is eliminated. A polygon scanner (GEC Marconi - thanks Alan) provides the line scanning, at rates of up to 36kHz. AO modulation and Scophony balance provides video bandwidth up to 30MHz, so HDTV (1250/50 and 1125/60), as well as PAL/NTSC/SECAM are available in the LCI-LVP. Output on screen of a peak-white modulated raster of over 15 watts has been achieved. The largest image projected so far was 50 metres wide. The collimated scanned beam provides an infinite depth-of-field, which was put to good use last year at the Singapore National Day on a giant 35m x 28m high-gain screen laid over the slanted stadium seating. The difference in projection distance between the top and bottom of the screen was nearly 100 metres, so the LVP was the only machine capable of a focussed image over the whole screen. All LVP's supplied so far by LCI are also capable of vector scanning using the waste AO beam.

    (From: Chris Cebelenski).

    I know of one experimental project that uses an array of galvo's to project a raster image at 1/2 normal NTSC refresh rate (15 fps). The cost of this endevour so far has been, well, let's just say it's been expensive. :-)

    Currently it's configured like this:

    There are several problems with this:

    1. Size. Stadium sized projections are fine, but it doesn't work too well in a dome. U2 would love it. (Search for: "Popmart tour" on altavista).

    2. Cost. Enough said!

    3. Power loss. Even the 5 W laser can get dim. With some mods it could work with multiple large-frame lasers, but then there's #2 again.

    4. Unlike most laser systems, it works best when projected against a BLACK background. White backgrounds have much better reflectivity, but the image really doesn't look right due to bleed and scatter.

    5. Max and min sizes - make it too large and it breaks up and the scanners can't keep up. Too small and the resolution of the scanners isn't good enough to provide a clear image and cross-talk is rampant.

    (From: Steve Roberts (osteven@en.com).)

    Two years ago I was at a Laser-FX conference in Canada, we had the chance to watch (I have it on tape) a Russian made scan system with no moving parts, all acousto-optic and almost totally analog driven, that produced sharp clean monochrome images without flicker the size of a billboard using a 6 watt 532 nm YAG . The marketing person explained that RGB existed in the lab and was not far away. I believe the company name was Lasys Technologies. Scan head and laser was about the size of a PC/AT case and sat on a tripod, and was easily handled with low weight. Ran off 220 VAC three-phase, but I was told 220 single-phase would not be a problem. Further details can be obtained from: L. Michael Roberts (lmichael@laser-fx.com) who was the organizer of the conference.

    (From: L. Michael Roberts" (NewsMail@laserfx.com).)

    Some of the newer laser based video projectors (e.g., the Samsung unit) use a white light laser (Ar/Kr) as the source - 3.5 to 10 watts depending on the image size and brightness desired. The beam is split into it's prime component colours, modulated, recombined and then scanned.

    Many of the older units used a tandem laser pair - an Argon and a red-only krypton. Some units even use three lasers - an argon with blue optics, and argon with green optics and a red-only krypton. This takes a LOT of water and power to operate.

    There is presently a lot of work being done on producing compact diode pumped YAG based red and blue lasers. Laser Power showed prototypes of these lasers at the ILDA meeting in Amsterdam last November. This would allow people to build a fairly powerful (2 watts input approximately) laser based video projector that is air-cooled and can run on 115 VAC.

    (From: Sam.)

    Here is a link to an article about a system that may be commercially viable in the near future. It uses second and third harmonic generation to produce green (532 nm, 13 W) and blue (447 nm, 7 W) output, respectively, from a pair of Nd:YVO4 diode pumped solid state lasers along with a diode pumped optical parametric oscillator to generate the red (628 nm, 10 W) beam.

    The company claims their market advantage to include higher resolution (1,600 x 1,200) and better contrast ratio (1,700:1) than competing non-laser based technologies. They also cite lower maintenance than arc lamp based systems. However, the cost is also much higher at present and I question the brightness of 3,000 lumens at the screen (this is about equivalent to the total light output of a pair of 100 W incandescent bulbs) so it may still be inadequate for theater-size applications.

    And, here's a description with photos of a laser TV system built back in 1985 (along with some other related laser display gadgetry):

    And some comments from Doug:

    (From: Doug Dulmage.)

    One thing that is nice about TV using lasers is the use of a true red "gun". I've built 3 or 4 different versions of laser video projectors using argon and krypton lasers and the first thing you notice when you put a standard color bar signal up is that it looks "different". The reason is that in normal television there really is no such thing as a red phosphor. They are actually closer to orange than red, but by color mixing and a little fooling of the brain, you see red from the orange phosphor. So when you finally do see a video display that comes from a fairly dark red line (like the 650 of the krypton), things that normally look really bland like browns, violets, and other colors that depend on red, look stunning. It makes normal television look much more like film that video. Oddly enough, a couple of commercial laser video companies went to great lengths to produce the orange line instead of the red from a krypton by using argon pumped dye lasers to produce the orange. I could never, ever figure out why go to such trouble except that they were so anal about trying to follow NTSC standards for color that they ignored the benefit of having a true red. I had a little secret method for curing those situations where the client would complain about the color and I could give them orange back without the use of the dye laser, but normally once they saw real red, they wouldn't let you touch it. It makes sense, most color CCD camera (at least with three CCD's) use color dividing prisms that cutoff into the red more than orange.

    Laser Based 3-D Displays

    Displays capable of providing information about the three-dimensional aspects of a scene can be divided into two classes:

    There have been a number of volumetric (not true holographic) displays developed over the years using rotating mirrors, disks, LED arrays, disks inside cathode ray tubes, etc. These are all scanned in such a way as to cover a true volume of space at a rapid enough rate (at least that is the objective) to produce the illusion of a solid 3-D volume floating in space. The scanning source can be a laser, electron beam, or the projected output of another 2-D display like a CRT or LCD panel.

    Currently, there are technical issues to be resolved with respect to the bandwidth of the channel to get the information into the display (Gigabytes/second are required for adequate refresh rates). But more fundamentally, these techniques are incapable by their design of rendering solid shaded surface views. The volumetric display is one of 'look through' or 'structured fog'. However, such a technique in a practical application could be extremely useful.

    With technologies as yet unavailable, one could conceive of a 'selective activation' display where points in 3-space are rendered opaque or emissive by intersecting Laser beams or something like that. There has been progress in this area with emissive displays - intersecting laser beams resulting in the production of colored points of light. However, all these technologies suffer at present from serious resolution and bandwidth limitations - not likely to be solved for decades at least. (See below.)

    A true holographic display would be capable of an ***arbitrary*** viewing mode including the display of solid surfaces with shading which would be viewable with correct perspective and shading from a range of angles. I do not know of any actual examples of such technology at present. An emissive volumetric display like the one described below cannot implement hidden surface removal - essential for life-like rendition. While wire-frames and look-through displays have many uses, they aren't likely to be of much value for a boob-tube replacement! :)

    A brief description of some of the alternatives can be found at: Pangolin's Laser Show Guide - Making 3D, floating images. Additional details on one of these, the spinning helix approach, can be found at: Technical Description of a 3D Volumetric Display System.

    Also see the sections starting with: Introduction to Holography

    (From: L. Michael Roberts (NewsMail@laserfx.com).)

    Already in the works! A "Three-Colour, Solid-State, Three-Dimensional Display based on two-step, two-frequency upconversion in rare earth doped heavy metal fluoride glass is described. The device employs infrared laser beams that intersect inside a transparent volume of active optical material to address red, green, and blue voxels via sequential two-step resonant absorption. Three-dimensional wire-frame images, surface areas, and solids are drawn by scanning the point of intersection of the lasers around inside the material. The prototype device is driven with laser diodes, uses conventional focusing optics and mechanical scanners, and is bright enough to be seen in ambient room lighting conditions.

    The full article is available on-line at 3D Laser Based Volumetric Display.

    (From: Michiel Roos (roosmcd@dds.nl).)

    That's a block of (expensive) glass with some lights in it? Last thing I heard, they'd only got a low resolution. But a couple of years ago I was at a Philips trade show. There was a true (?!?) 3D laserTV system. In a room, a music video was shown. There were a number of layers displayed in air (fog?) so you'd get a 3D view. Nice thing was that you could walk right through the image and still see it. But I've never heard of it again. Anybody knows if they're working on this now?

    3-D Laser Engraving Inside a Glass Block

    Examples of art pieces made under computer control of a pulsed laser focused inside a glass block can be found at 3D Laser Art Co.. They have a basic explanation of the process but no specifics and no mention of the type of laser that is used.

    (From: Steve Roberts.)

    Engraving inside a block of glass is a pretty easy thing to do if you have a high power pulsed YAG laser. I've seen problems in labs with cheap glass lenses developing spectacular defects in the middle of the glass, so a variable focus lens, some galvanometer scanners for positioning, and a monster pulsed YAG - plus some decent software and you should be able to carve in flint or lead glass.

    It's all too easy to create microcracks on the insides of the cheap lenses.

    (From: David Toebaert (olx08152@online.be).)

    The December 1999 issue of 'Laser und Optoelektronik' has a beautiful picture on the cover of a piece of lead crystal with the Dresdner Frauenkirche inside, 3-D engraved using Nd:YLF (Q-switched AND mode locked) lasers. It was developed by the Fraunhofer Institut fur Werkstoff- und Strahltechnik.

    (From: A. E. Siegman (siegman@stanford.edu).)

    The basic process is a.k.a. "bulk (or internal) optical damage" produced by a focused laser beam. The basic effects were observed with the very earliest ruby and other pulsed lasers in the early 1960s, very often unintentionally and to the detriment of expensive optical components including sometimes the laser rods themselves. This led to a whole field of "laser damage" studies, including a series of NIST-sponsored symposia and other publications over the next several decades, and quite a lot of early work in the Soviet Union also..

    The physical process involves a complex mixture of photoionization, multiphoton ionization, melting, vaporization, and various stimulated scattering processes, leading to bubble formation, track formation, and "micro-explosions" occurring at either the focal spot or at various intrinsic defects inside the material. The exact details of what happens depend on the wavelength, intensity, and pulse duration of the laser pulse and the physical characteristics of the material.

    There are a number of small firms in the U.S. and elsewhere who will write the kind of decorative cubes you saw in the gift show, in glass or plastic cubes, using computer-controlled pulsed YAG or other lasers. They will also fabricate inexpensive customized versions as mementos for going-away parties, bowling trophies, and so forth.

    There is also a recent (late 1990s) patent by a British guy on a subsurface marking apparatus of this sort which has been used by a major distillery for writing subsurface serial numbers into the bottoms of zillions of Scotch whiskey bottles. I'll not provide a citation because IMHO given the prior art and state of knowledge of these effects the patent should never have been issued.

    (From: "Beric" (beric@ntlworld.com).)

    Its actually British Technology, but as usual developed overseas. The patent is owned by United Distillers. They are micro cracks, that are laser written into the glass.



  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Introduction to Holography

    What is Holography?

    Holography represents a class of techniques which capture 3-D information about a scene as an interference pattern on or in an extremely high resolution 2-D film. When the film is developed and viewed under the right conditions (some require a laser for viewing while others can use a suitable white light source), the result is a recreation in every detail of the original including the ability to move your viewpoint and look around objects, proper hidden surface removal (solid objects appear solid), shadows and highlights, and so forth. In principle, the hologram is optically indistinguishable from the original. A normal photo of a hologram would look the same as a photo of the scene itself.

    However, in so far as the technology exists today, holography is NOT what is often depicted in Sci-Fi and other movies and TV shows. Some of this deficiency is due to fundamental principles of what holography is and how it works while much of it is due to the inadequacy of present technology:

    (Portions from: Rick Poulin (rpoulin@rohcg.on.ca).)

    While holography is really still in it's infancy it already has many other fascinating applications. Just a few of these include:

    Description of Holography Technique

    While there are significant differences in the details of the process needed to produce those little logos compared to large white light holograms used for marketing or 3-D volume images for medical diagnosis, the basic techniques are similar and can be summarized very briefly. The following is the sort of holography setup that is within the capabilities of a determined amateur:

    (From: Joshua Halpern (vze23qvd@verizon.net).)

    As far as a stable optical table goes:

    1. You could buy one on eBay. You need to find one near you otherwise the moving costs will kill you. Otherwise you need to hire riggers. That costs between $500 to $1,000

    2. There are two simple ways to build one. They start with the same base, blocks of stiff styrofoam. The simplest thing then is to put a block of granite or marble on top. Of course, you now have the problem of attaching anything to the marble, but the table will be vibration isolated

    3. Otherwise get a thick piece of magnetic steel and put that on top. you can then use magnetic bases. You buy the bases from a machine shop supply place - not an optical supply house which would charge 2 to 3 times the as much.

    4. For even better isolation you put a sandbox on top of the styrofoam, fill it with sand (there are grades, you want a grade with fine round kernels). Put a sheet of plastic down before you float the steel/stone on the sand and seal the plastic to the sides of the sandbox to avoid having sand everywhere.

    Note that all of these options involve moving something very heavy. It will cost you money, but unless you have experience in doing such things pay it. The money is a lot cheaper than the medical costs.

    Basic Amateur Holography Setup

    See the section: Holographic Information Resources for alternatives - this is just one option.

    (From: Brian Hogan (bhogan@bjgate.com)).

    I haven't made holograms for a long time, but I started from the ground up. If you've got $3K to play with, you can really start off very well. But if you want to save money, you can build a complete setup for less than $1,000. It may be far more advanced than what you may have intended, but you'll be able to create pretty professional holograms.

    The best bet is to get a 5-10 mW HeNe surplus laser for about $200 to $300 dollars. This type of laser should have a coherence length of at least 6" or so. You'll also need some holographic film (I used to use Kodak stuff many years ago -- don't know if they still make it but it was relatively sensitive and easy to use). Next, you'll need to build a stable table. In a pinch, a heavy wooden plank, slab of marble, etc., laid on a few partially inflated inner tubes will probably be enough. I strongly recommend against a sandbox as it's more of a pain in the ass to keep things clean and to prevent optics from constantly shifting as you move things in the sand. Set the table up on the lowest floor, preferably on a concrete foundation, to minimize vibrations. Then you'll need to get some redirection mirrors and expanding lenses. Finally, you'll need the chemicals to develop the exposed film.

    From complete scratch, you are looking at an investment of about $350 to make a simple hologram.

    Here are more detailed suggestions:

    1. Ditch the sandbox idea. While it does work, it's a pain to keep sand from getting on all of the optics. Also, the light color of the sand means that you'll often have to mask out stray reflections. Lenses and mirrors have a tendency to shift when you move things around. I strongly recommend that you build a solid, rigid table and place it on inner tubes. For my setup, I made 6 columns out of cinder blocks about 3 ft high. I then put down on top of the columns a 2 inch thick pine plank measuring 4'x8'. I drilled six 4" holes in the plank spaced evenly out and then placed 6 forklift inner tubes centered around these holes. (The holes in the plank allowed for inflating the inner tubes later on from the bottom to adjust the air cushioning.) On top of the inner tubes I built a box out of wood measuring 4'x8'x4" inside dimensions. Into the box I poured about 11 cubic feet of Redimix concrete, using chicken wire and rebar for strengthening. The top was smoothed. After five days of curing, I glued a 1/16" thick sheet of steel (4'x8') to the top of the concrete. I painted the steel and the sides flat black. This was definitely a very heavy, solid table that was not really intended to be moved (except with dynamite!) Anyway, this might be more than what you'd like, but the table performed exceptionally well. The height was such that it made for comfortable working. The size meant I could do many intricate setups with multiple beams. The steel top meant I could use magnetic mounts for the optics. Total cost was less than $200 bucks in 1986.

    2. Get good optics. I got most of mine through Edmund Scientific. They're a bit expensive, though. All mirrors should be first surface (aluminum on the front surface, not on the back). I recommend getting several mirrors of about 2"x2". You'll also need to get two or three in the 4"x6" range and higher. You can never have too many mirrors. The lenses you'll need should mostly be concave. Look for the largest diameter, shortest NEGATIVE focal lengths you can find. These lenses will expand beams, which is generally what you'll be doing in holography. I would try to get an assortment of -6 to -20 mm double concave lenses at least 10 mm in diameter. If you don't use plate film, get some clear glass plates about 4"x6" to sandwich the film. I built a special jig that would clamp the film between the glass plates. Be creative, but try to make the clamp jig as small as possible -- you don't want it to interfere with any laser beams coming from behind the film to illuminate the object to be holographed. Also in the optics category, you'll need to get at least 1 variable beamsplitter mirror.

    3. Make or buy good optics mounts. You can go out and purchase optics mounts, but talk about EXPENSIVE. My table had a steel top, so I built magnetic mounts. The base of the mount was nothing more than a doghnut magnet (3 and 5" in diameter). I solidly epoxied 3/8" steel rods to these magnets vertically. Most were about 18" tall but some measured as much as 36" tall for overhead illumination shots. The optics themselves were glued to masonite pieces (with holes for the lenses). I used laboratory stand clamps to hold the optics in place. They clamp to the optic mount rods and can swivel the optics 360 degrees. Everything was painted flat black to reduce reflections. I built about 16 mounts in all. Like mirrors, you can never have too many.

    4. Get the most powerful laser you can afford. I did most of my holography with an 8 mW He-Ne laser that I purchased as surplus from Meredith Instruments. The more power means shorter exposure times and better results. You must get a TEM00 mode, single wavelength laser. I never tried a diode laser, but I don't recommend them because the beam is not round like a TEM00 laser. A good surplus HeNe laser will cost at least $300, but it's the most important part.

    5. Get the right film. Holography requires high resolution, special film for the purpose. I'm not sure Kodak is still in the holographic film business, but I had very good success with their film. I also used Agfa holographic film with pretty good results. Check around on the internet for sources. There are other types of media (e.g., dichromatic emulsions), but try films first. For processing, I used Kodak D-19 developer and Kodak fixers. I used a bleach mixture I made myself out of sulfuric acid (look in plumbing section of home center for drain cleaner -- very dangerous stuff!) and potassium dichromate. There are many other formulas out there so check around on websites. Processing must be done under clean conditions in a dark room. You can use a dim green safelight so that it won't exposure the red light sensitive film. (Also see below. --- Sam.)

    6. Though this is a long description, it should give you some ideas. There are many books out there that should give you much more information. The setup I described will cost somewhere around $1000. Once you've had some success with making basic holograms, you'll probably invest in specialty optics and other stuff to make more advanced holograms. With my setup, I was able to do practically anything anybody else could do with equipment costing many times as much as what my stuff cost. The key is to be creative not only with the actual holograms themselves but also with the equipment you use.
    Good luck and have fun.

    (From: Rick Poulin (rpoulin@rohcg.on.ca).)

    I used to be a holographic experimenter and got my supplies from Agfa but sadly they got out of the business and left many people scrambling for a new cheap source. If you want to pay through the nose, Edmund Scientific or MWK Laser Prodcuts are the high water marks for pricing.

    If you want cheap film or glass plates there is a source in Russia called Red Star. Go to the Royal Holographic Art Gallery Film Page for the North American dealer in British Columbia, Canada.

    (From: Jens Kilian (Jens_Kilian@agilent.com).)

    The difficulty of making holograms is *much* overrated. If you're not going for commercial quality or for fancy stuff (image plane, rainbow etc.), a simple Denisyuk (reflection) hologram can be made with *very* little equipment (laser, lens, plate, chemicals).

    With the right plate exposure time is in the seconds, not hours range; and the vibration problem can be reduced with a robust setup like this:

                                  \
          Laser =====> -------------\ Front-surface mirror
                                    | \
                                    |
                                 ======= Plate
                                 | XXX | Object + support
                                 +-----+
    

    I've been to a workshop (see below) which was held in a public building next to one of the main thoroughfares in Stuttgart, where *everybody* produced near perfect holograms, even the guy, not me :-), who carried out a developed plate from the darkroom into near full sunlight.

    The workshop was run by: Junker Holografie. We used HRT plates. Clickety click... *darn*, HRT has shut down (HRT Holographic Recording Technologies GmbH).

    (From: Fleetie (fleetie@fleetie.demon.co.uk).)

    Well, I ended up paying a lot of money for front-surface mirrors and an AR-coated beamsplitter, and such like when I briefly (!!!) took it up, but the plain fact is if you just want your first hologram, and you have the film and developing chemicals, you just need:

    Just put the lens right by the laser, get the beam nice and wide. Place the (ideally glassy or transparent or translucent) objects somewhere in the diverged beam. Put the film down-beam somewhere, so that the objects are between the laser and the film. You may find that the objects cast a shadow on the film; as long as a significant part of the film is not in shadow, it should be ok.

    Unless something moves really grossly, or you severely under- or over- expose, you'll get at least some kind of a transmission hologram out of it. (It won't be optimally efficient, but you really should see SOMETHING.)

    Even if something moves (but not TOO much), you'll often end up with a hologram that looks kind of stripy; the more movement, the more stripes.

    To view the hologram, just leave everything set up the way it was, remove the object(s), put the hologram back in the film holder in the SAME orientation in which it was exposed, let the laser illuminate it, look THROUGH the hologram towards the laser at the place where the objects were. You should see a holographic image of the objects. (I used to cut a little corner off the rectangle of film at the top right, to help get the film orientation the same when I wanted to view it. Then there are only 2 ways to orient the film, rather than 8. Just remember whether you had the emulsion side of the film facing towards or away from the laser. (Put a corner of the film between your lips; the emulsion side will feel sticky.))

    (This is all in my limited experience; standard disclaimers apply.)

    After that, you may want to try a 2-beam setup, with a reference beam shining directly onto the film, and another beam illuminating the object(s) but not the film. Then you can play with the relative brightnesses of the beams, and get better interference, and therefore a brighter hologram.

    It gets harder when you want to produce reflection holograms, which can be viewed in white light. You need more power, really, to get your exposure times down.

    Have fun anyway if you decide to go for it.

    Complete Holography Kits for Education

    Several companies provide all the equipment and materials needed to get started in holography. One example can be found at the Arbor Scientific Holography Page. Their prices may not be the best on individual pieces but the convenience of one-stop shopping may outweigh the additional cost (except probably for the laser especially if you opt to use a cheap laser pointer for this!). Also check the various companies listed in the section: New, Surplus, Walk-In, Mail Order, Kits/Plans (Commercial).

    The following is from a posting to the USENET newsgroup alt.lasers in early 1999. I have no direct knowledge of the contents or quality of these kits or whether they are still available.

    (From: Steve McGrew (stevem@iea.com).)

    I've just received and tested the first shipment of a new holography kit for education. It includes a HeNe laser, an optical breadboard, adjustable mounts, dielectric mirrors, and a detailed, understandable manual in good English (I helped with the translation). The manual details a series of experiments and explanations that will lead a student through all the basics of optics up through 3D holography. The kit and experiments are designed for a college-level optics course, but would be suitable as well for science enrichment at the high school level. The kits are made in China under the supervision of a university optics professor. Each kit fits neatly into an aluminum suitcase. If you were to buy all the parts for the kit in the U.S., they would cost somewhere in the range of $1,500.

    My cost is $525 plus shipping; I'll provide these kits to any bona fide school for my cost plus 10%, and will provide advice as needed to teachers and students. (Price subject to change, so please ask for confirmation of current price.)

    Also see the next section.

    The Litiholo 3-D Holography Kit

    Introduction

    Liti Holo, offers holography related components and supplies for the DIY'er. Liti's main attraction appears to be their instant no developing required holography plates. These literally develop as they are exposed, so once the lights are turned back on, the hologram (or lack thereof) is ready to be viewed. (Liti Holo is a division of Liti Holographics, a company specializing in the production of holograms (including full color and motion) for marketing and advertising. At least, that's what I gather from their Web site. There is no obvious mention or links to Liti Holo.)

    A single color holography kit using a red diode laser has been available for awhile for around $100. See Litiholo Hologram Kit.

    Now, a new version has been developed for making what they call full color holograms. (Link from the page above. Whether it is generally available - or ever will be - isn't entirely clear.) The kit consists of a set of 3 lasers (all under 5 mW) with battery packs, a holographic beam combiner, laser-cut plastic parts to mount everything, and two boxes of Liti's special instant no development 2x3" holography plates (20 total). Now before you get excited, there are just a few limitations. :( :) The most significant is that for the $250 to $300 price tag, it does NOT include $10,000 laboratory single frequency lasers. What a surprise? ;-) Due to the limited coherence length of the lasers, it appears as though only essentially 2-D objects (like the tops of the bottle caps that are provided) can be assured of being captured. This despite the spectacularly 3-D images shown on the Liti Holo Web site. So, if you thought this was going to make nice 3-D holograms, probably not. For that, you'll need lasers with decent coherence length. You might get lucky but more than likely, you won't, or at least not very often. That's the bad news. The good news is that this kit will make real holograms and the way things are arranged, vibrations are less of an issue than with conventional holography. So there's an excellent chance that your first exposure will be successful. Taken together, these may be enough to get you hooked. :-)

    However, since the 3 color kit is a superset of the basic holography kit, it is possible to make 3-D (single color) holograms if the limited dimensionality becomes excessively boring. The red and blue lasers may have adequate coherence length, at least some of the time.

    The Lasers

    What the instructions call "laser diodes" or simply "diodes" are either actually diode lasers which have built in drivers (red and blue) or a Diode Pumped Solid State (DPSS) laser (green).

    These are all typical laser pointer type modules. To provide a divergent beam, the collimating lenses must be removed. (This had already been done on all but the red laser.)

    Red diode laser

    This is the physically smallest of the three. If the output looks like a pointer beam, the front barrel must be unscrewed to remove the collimating lens and spring (save for something else). Install the laser in the mount and screw the barrel back on to help secure it. Add the "Special Clip" (heat-sink) to the barrel once the laser is installed. Once set up, carefully rotate the laser to maximize coverage of the spread out red beam on the Holographic Plate Holder.

    Green DPSS laser

    WARNING: There is NO IR-blocking filter. Thus high levels of both 808 nm and 1,064 nm are present in the output. To display anything on the SFPI or to accurately measure the output power, an external filter must be added.

    On my sample, the laser starts out very weak and requires 2 or 3 minutes before it suddenly transitions to decent power. This appears to be related to the temperature of the crystal as a great deal pump light leakage is present initially, visible by eye.

    The Special Clip (heat-sink) may be most important for the green laser.

    The beam from the green laser is fairly symmetric so orientation doesn't matter. But it diverges slowly, so it will generally need to be positioned farther from the setup than the red laser.

    Blue diode laser

    A switchmode boost driver provides the 4 to 6 V required by the blue laser diode. Once set up, carefully rotate the laser to maximize coverage of the spread out red beam on the Holographic Plate Holder.

    Unfortunately, without actually monitoring the longitudinal mode structure of these lasers continuously, it's a sort of crap shoot as to whether any given exposure will be capable of significant depth without artifacts.

    These are not $25,000 lab-quality lasers. Didn't I say that already? ;-)

    Guidelines for Using the Litiholo 3-D Kit

    The following were not mentioned in the instructions for this kit:

    My first hologram was of the top of a cell phone main PCB (a Casio G'zOne Rock if you must know, found along the side of the road). While I would classify this as a success - sort of - there was almost no blue and the contrast was rather poor (or at least poorer than I had expected). However, it was sharp and showed enough depth to be able to clearly see the limited 3-D nature of the circuit board components.

    The lack of blue was either due to the need for more blue and less red/green in the color balance mix, or bad luck on the blue laser not being very coherent (or changing coherence) during the exposure.

    Unfortunately, while the instant plates make it easy to make holograms, that also makes it easy to expose plates and end up with nothing on them. And the plates are not inexpensive, $3 or more for each 2x3" plate.

    Holography Using Cheap Diode Lasers

    If you ask most laser 'experts' about the possibility of using a laser pointer or inexpensive diode laser module for making holograms, the typical response will be to forget it - the coherence length is only a few mm and therefore inadequate. This apparently isn't the case. The coherence length for a typical laser pointer or diode laser module may actually be more like 200 mm (10 inches) - comparable to that of an HeNe laser and, with care, will remain stable for long enough to make an exposure. While it may be unreasonable to expect any old $8.95 laser pointer to produce the same quality results as a $500 HeNe laser, surprisingly good holograms can be obtained on a budget. And, it would appear, that in some cases, they can actually be superior.

    While I don't know how to select a laser diode to guarantee an adequate coherence length, it certainly must be a single spatial (transverse) mode type which is usually the case for lower power diodes but those above 50 to 100 mW are generally multimode. So, forget about trying to using a 1 W laser diode of any wavelength for interferometry or holography. However, single spatial mode doesn't guarantee that the diode operates with a single longitudinal mode or has the needed stability for these applications. And, any particular diode may operate with the desired mode structure only over a range of current/output power and/or when maintained within a particular temperature range.

    For for information on laser pointer holography, see:

    Also see the section: Holographic Information Resources.

    (From: Frank DeFreitas (director@holoworld.com).)

    I had my fingers crossed tighter than ever for this one -- moving up to 35 mW of power for holography using a diode source. It worked!

    The module used contained the Hitachi 35 mW, 658 nm diode, along with AR-coated anamorphic prisms (optional) and high-grade collimating optics. The measured optical output after collimating optics is 27 mW and total cost for putting the whole thing together was about $50 to $60.

    This little baby exceeds the performance of any HeNe in its power range, including the $5,000 Spectra-Physics at 25 mW.

    Those diodes are real little buggers once they're set up with an interferometer. Very strange behavior (at least strange after working with gas lasers for so many years) - and in a good way.

    In any case, this baby is ROCK solid. The final test which put us over the top was so incredible that I thought there was something wrong with the set-up. I would tap on the table just to make sure. It's almost as if a fringe-locker was in place. Even with the best HeNe that I've had here (Spectra-Physics 124B Stabilite) there would ALWAYS be some "drift" or what I call "float". (Float is the feeling that fringes are not entirely still -- it's not something that shows up very clearly to the eye. It's more of a "feeling" when testing). The fringes with the new diode are locked so tight it's almost like watching a still photograph.

    As far as the coherence length is concerned, I measured (using a Science and Mechanics PhotoMeter placed in the fringes) out to 14 feet without any change. As you may know, this amount of coherence would require a rather expensive etalon on any lab laser. Up until this point, we were only capable of recording a few inches using diode lasers.

    This diode created two very bright test holograms that exhibited depth all the way back with the object(s) (1. ocean coral, 2. angel statue with wings). For a special twist, I used an initial set-up for a 30 x 40 cm hologram and then just shot two 4 x 5s with the set-up as-is. Even though the size of the holograms are 4 x 5, they will give you an indication of what a 30 x 40 cm hologram would turn out like -- since your beam spread, exposure, etc. are calibrated for that size.

    For a complete report, along with photos of the module, the holograms, the visible beam in my lab and a interesting size comparison to a Spectra-Physics 124B HeNe laser go to the Our Own 25 mW Laser Page. (There are also other reports preceeding this one which may be accessed at the Holoworld site.) D and S Lasers is a spinoff of Holoworld offering plans, a kit, as well as an assembled 25+ mW diode laser system with long coherence length suitable for holography.

    As for using green laser pointers, realize that these are based on an entirely different technology than laser diodes in red pointers. Green pointers are Diode Pumped Solid State (DPSS) frequency doubled lasers. To be useful for holography, a laser has to have a decent coherence length. For a short cavity laser like a that in a laser diode (a fraction of a mm) or green laser pointer (2 to 10 mm typical), this implies single longitudinal (and of course single transverse) mode operation. Some red diodes do this under some conditions (by controlling diode current and diode temperature). Depending on the specific configuration of the laser cavity in a green laser pointer, some may also operate single mode. Maybe. But, stabilizing them without major modifications may be difficult. The CASIX DPM crystals generally do not operate single mode but may do so at times depending on pump power and pump beam alignment. A discrete cavity pointer laser will likely operate single mode up to a modest power level and then switch to multimode. Many or most green pointers are now quasi-CW and/or Q-switched which further complicates matters.

    (From: Colin K. (colinholo@yahoo.com).)

    Laser diodes do work. I would not say they work well. At least the APC style most amateur holographers use. There needs to be a method of locking the frequency to single mode. If you only need 5 mW then Integraf has a very reliable diode for $35 with a coherence length of more than 6 ft. I run one from two D-cell batteries and have made more than 30 holograms with it with no failures. As the red diodes increase in power it becomes increasingly hard to get the line to stabilize. I have a TEC based laser with the Panasonic 50 mW diode and I have had much difficulty keeping it in a single mode. When I can the coherence length is quite long. More than 12 feet.

    The 35 mW laser Frank sells from the Holoworld site (APC with Mitsubishi Diode) makes a good hologram most of the time but it will run in multiline mode at random times.

    The best laser I have found in red is the Analog Technologies TLM-S1 Tunable Laser Module but it's not cheap (don't ask!). There is also a less expensive non-tunable laser that will be available for about $800 very soon. I am hoping to test a sample with a 50 mW diode in a few days. The 25 mW has extremely long coherence lengths.

    (From: Tony (kilm02nspm@clara.co.uk).)

    I thought that laser diodes would be unsuitable for holography due to their supposedly very short coherence length until 1999, when I read of holograms being made using laser pointers. I didn't believe it, but thought it wouldn't hurt to try. I bought a laser pointer (the bullet style with light feedback regulation), broke it open and fixed the diode and board to an adjustable mount, powering it from 3 AA cells. It worked first time, producing brighter holograms that were ever possible with my old 1 mW He-Ne. Having only a small table I've never been able to confirm the long coherence lengths quoted by some but I have found reflections from objects at the back of the table, giving a coherence length (taking into account the path difference there must have been) of at least 50 cm. I tried a few pointers and found only the cheap no-regulator types with only a resistor and diode don't work. One thing to remember is they do need to warm up just like a gas laser so don't expect to click the power on and off for an exposure - it's still best to use a shutter. Set up an interferometer to check the warmup time as well as you table's stability. The simplest way (assuming you've already built a vibration damping table) to make a transmission hologram with a diode laser is: Remove the collimating lens from the pointer, this produces a 'stripe' of light which can be used instead of a beam expander. Screen off the edges of the stripe next to the laser until only your objects and reflector are illuminated. With the laser at the left centre for example, you would place your object below centre of the right side and your reflector for the reference beam above centre on the right. Arrange your plate at the bottom of the table, the fun part being to keep it out of the direct beam while facing the reflected light from your object and being fully illuminated by the reference beam at the correct angle. You'll have to use some white card in the plate holder to try and balance the light from the object and reference beams. All this is much easier with more mirrors of course but for a zero-budget experiment it does work. You can make a partial reflector for the reference beam by painting a piece of 6 mm glass black on one side and roughly control the intensity by moving it nearer or further from the plate or film.

    Monitoring the Wavelength Stability of a Laser Diode

    While some laser diodes are particularly good for use in holography and interferometry due to their natural tendency to operate in single spatial and longitudinal mode, many others can be convinced to behave by a combination of current and temperature tuning. However, some means is needed to check for mode hopping and multimode operation. This can be done with fancy and expensive instrumentation this is normally out of reach for even the well equipped holographer. There are low cost alternatives which provide some of the same information.

    (From: Jonathan Head (holosjmh@primus.ca).)

    Here's my problem - laser diode frequency stability. It used to be the holographer's biggest issue was vibration stability. Now it's frequency stability, at least if you use a laser diode. And given the huge advantage of coherence length, robustness, and lower cost over HeNe lasers, who wouldn't?

    I'm building a heat sink for it. A TEC maybe later, right now I'm going low-tech. I believe I can keep the temperature quite low (close to 0 C) and stable enough to shoot between mode hopping episodes, with the design (by Colin Kaminski) I have. I have run numerous monitoring tests with my interferometer and audio detector (solar cell/amp/headphones) which, believe it or not, (at first I didn't) can actually (and cheaply) detect mode hopping via the amplitude shifts in the beam. They are audible clicks, which turn into static when the diode starts into multi-mode operation between mode hop free temperature zones. The beam quiets down when solidly in multi-mode, then the static returns followed by dwindling clicks, as it transits to the next temperature zone.

    You can therefore easily detect mode hopping *without* an interferometer at a cost of only about 8% of the total beam diverted to the solar cell using a glass plate beamsplitter. A single beam will do. I've placed the BS before the shutter and can use it to time my hologram exposures. But I digress. Although since I haven't found this in your FAQ I thought I'd mention it.

    For some time I've been running tests with an interferometer in conjunction with the audio set-up mentioned above. The correlation between the two (audio and visual) is interesting and useful. Primarily I'm testing methods to control the temperature of the LD, and monitor its mode hopping and linewidth behavior, without the benefit of expensive instruments. (I think holographers have enough expenses just from the film, plates, optics, and time away from family.)

    The first report I saw of the possibilities came from Tom Burgess, who posted on Frank DeFreitas' holography forum that he noted clicks, and rasps, in the beam that he thought might be mode hopping since they were accompanied by jerks in the pattern, and fringe washouts, respectively. This turns out to be the case.

    It helps to have an interferometer set up simultaneously to observe beam activity, but it isn't strictly necessary (and quite impossible if you are shooting a hologram).

    It's been previously shown that there is a correlation between noise in the total intensity of the beam, and mode hopping. The solar cell will pick this up as output intensity fluctuations directly caused by the laser switching between wavelengths. A "click" is heard for each discrete mode hop, and sometimes the mode hopping is quite rapid, which results in a "static" like sound of various tempos and sound levels.

    In addition to a small silicon solar cell, all you need is an amplifier equipped with phono jacks, a short RCA cable and headphones. A photodiode would work also. Using a plain piece of glass and perhaps a transfer mirror or two, divert part of the beam directly to the solar cell, which is connected directly to the amp inputs. Listen via headphones or you may get feedback interference from external speakers. I'd also recommend diverting the beam before the shutter, so that you can monitor the beam before an exposure. Once you've established that a background hum can be interrupted by blocking the solar cell with your hand, you can then attempt to "listen" to the beam for various manifestations of mode hopping activity.

    This is a practical means, especially for holographers on a budget, to determine suitable windows of opportunity in which to make their exposures. The wavelength stability of laser diodes depends on temperature and injection current, among other things, and unless these two factors are strictly controlled there will always be a chance for mode hopping to ruin an otherwise good hologram.

    The absence of audible indications (clicks and/or static sounds) will not, however, guarantee that the LD is operating in single mode, or at least with a narrow enough linewidth, to make a good hologram. This is because there are also times during multimode operation when no mode hopping occurs, and/or the intensity fluctuations are out of range to pick up. I've found this occurs as the diode moves through a zone of instability, of which there are many, determined by the particular combinations of case temperature and current. The audible indications occur as the LD enters and exits an unstable zone. In the middle of the unstable zone, it is often quiet (even while fringes are completely washed out). Therefore, one can determine where the laser is operating fairly easily, by simply monitoring the situation.

    This should save a significant amount of wasted film for those holographers using a bare-bones laser diode. For anyone using a TEC this is a way to find the zones of stability, and establish favorable set points.

    Holographic Video Displays

    To create a useful holographic display of a moving scene requires an almost unbelievably large amount of data processing and throughput. Suppose you just wanted to produce a holomovie of a 50 x 50 x 50 cm volume using a 50 x 50 cm display device. Given that your typical holographic film must have a resolution on the order of a wavelength of the light used to create/reconstruct the hologram - 1,000 line pairs/mm or better - this would mean that some sort of spatial light modulator (e.g., LCD) would be needed with a similar resolution to reproduce moving images. That implies over 1.25x1012 or 1.25 Terapixels! And you thought high resolution laptop screens were expensive! To make things easier, we'll assume 1 bit per pixel for the interference pattern, resulting in 100 Gbytes per frame! To provide smooth motion, one needs a minimum of 24 to 30 fps so you are looking at 2.4 Terabytes/second. Now, granted, various compression techniques (e.g., MPEG-26 by then) can be used to reduce this by perhaps a factor of 10 to 100 or more (and no doubt such processing will be much more advanced once this sort of folly becomes at all practical) but that is still 24 Gbytes/second through the communications channel. Hmmm, that doesn't look quite as impossible! This doesn't take into account the need for color but at least the laser(s) will probably be the least of your problems in bringing such technology to market!

    Such a display is simple in principle:

    I was actually discussing stuff like this (in a former life) in the early 1980s realizing that either a dedicated special purpose computer or something as yet non-existent would be needed to achieve any sort of througput.

    That is still the case.

    However, for stationary images (e.g., medical visualization where one wants to view anatomy from various angles with proper perspective, etc.), the speed may not matter as much as long as writing doesn't take more than a few seconds.

    So let's see.... For a 10 cm x 10 cm SLM, resolution order of a wavelength of visible light, that's only about 50 billion pixels. Not your ordinary CRT electron gun - more like a scanning electron microscope. A few 10s of Giga bytes per second (for a 1 second refresh rate) is the same order of magnitude as the internal memory busses on some of the latest microprocessors, so no big deal. :) Of course, then multiply that annoying frame rate thing. ;-)

    A search of a patent database at using keywords like "Three Dimensional Display" and "Holographic" should turn up a variety of interesting, though probably for the most part unrealistic (as yet) approaches to this problem.

    (From: Steve Roberts.)

    The problem is twofold, resolution and bandwidth. Resolution, because a hologram needs far more sensors per mm then available CCDs can provide, and bandwidth because only a dedicated direct array of fiber optic lines could handle the bandwidth. Your not going to see the scene shot with actual lasers. A computer and two or more cameras will be used, to synthesize the data. Experimental small scale displays have been made at low resolution, but the Cray computer they used to do the calculations is not something I'd have room for in my living room. Laser beams loose coherence after a short distance, so the guys at Monday Night Football aren't going to go blind, as lasers will not be used to gather the images. Maybe at the end of my lifetime in 30 years, but not any time soon.

    (From: James Hunter Heinlen (dracus@primenet.com).)

    There have been a few made. Right now, the only applications that can afford such tech is very high end medical, and government, mostly military, but I believe the DoE has one in their nuclear power simulation program. At any rate, they are fiendishly expensive, and the one I saw (when I was still doing consulting in the explosives industry) used a couple of Cray YM/P-2E's (when they were new) as signal processors, plus other computers to do the modeling, run the simulation, and produce a real time data stream to use as a signal to be processed. It was considered the low end of the tech, and produced a dim (but beautifully detailed) 3D moving image of whatever you wanted in real time. They were using it to display the progression of a shock-wave through multiple layers of (non-ideal, realistic) rock in fine detail. We had to turn off the lights to see the display. The 'monitor' looked like a plexiglass fish tank. If you want more info, there was a couple of good articles about the displays in Government Computer News when they first started making this type of system.

    (From: Ted (email address N/A).)

    There have been a few attempts to display true interference pattern holograms created by lasers on very high resolution LCD displays. I was at a digital imaging conference and they had one there. The screen itself, I think, had about 50,000 x 50,000 pixels. The actual holograms were scanned by a drum scanner at 90K x 90K pixels each and displayed at 1:10 (or something like that) on the screen, which was about 17"! The hologram was very bright, more brilliant than most I've seen on film. The spokesman said each hologram file took well over 100 MB.

    Note our eye process signals at about 27 fps, so about 30 fps is needed. At 30 fps, a one-second holographic animation of such would be 3 GB! An hour would be 180 GB+. Clearly, even true hologram motion, is still a long way. Artificial interference holograms created by computers would require even more storage and processing power. But, at the rate things are going in the computer industry, it is highly feasible in 10-20 years this could become a reality.

    (From: Andre de Guerin (mandoline@gtonline.net).)

    There is a new type of liquid crystal display that generates a hologram directly by producing the interference patterns on the surface of the LCD then illuminating it with visible light.

    The display this produces is a moving 3-D hologram in real time.

    One slight problem... The LCD density is something ridiculous like 3,800 x 3,800 pixels with a pixel size of 10 um x 10 um. There would be major problems with mass producing this sort of display, given that standard 1280 x 1024 laptop screens 1/10th the size have problems with dead pixels.

    (From: Sam.)

    Actually, there are bit more than one slight problem, not the least of which is that the resolution cited is at best marginal and feeding it with data must be a real treat, bandwidth and processing-wise! :) However, dead pixels, at least, would not be a major problem, just adding a bit to the background noise since localized defects in a hologram do not appear localized in the 3-D reconstruction.

    Suitable Lasers for Holography

    Here is a summary of the types of lasers with a long enough coherence length to be used to make holograms of macro-objects (more than a fraction of a millimeter in depth). See the relevant chapter on each type for more information.

    Helium-Neon (HeNe) lasers:

    Most polarized HeNe lasers can be used to make decent holograms if allowed to warm up for a half hour or more. A spatial filter will probably be needed (or at least highly desirable) to clean up the beam. The best will be large frame lasers like the SP-124 and SP-127, but internal mirror lasers in enclosed laser heads should be nearly as good with adequate warmup. HeNe lasers produce at most 40 or 50 mW at 632.8 nm (red-orange).

    Argon or krypton ion lasers:

    These have relatively short coherence length unless fitted with an etalon for single frequency operation. However, even a single wavelength without etalon argon or krypton ion laser may be acceptable for some holography work. The most useful (highest power) lines would be 488 nn (blue) and 514.5 nm (green).

    Diode lasers:

    I don't have specific recommendations but some should be excellent if temperature and current controller. Check on the holography Web sites or ask on the holography forums, below.

    Diode Pumped Solid State (DPSS) lasers:

    These must be single frequency to be useful for general holography. This is partially due to the typically short cavities of (non-ring) DPSS lasers, and the wide gain bandwidth of most SS lasing materials. DPSS lasers are mostly 532 nm (green). But there are some blue ones at 457 nm or 473 nm. Specific DPSS lasers I know to be excellent for holography are:

    I don't know of any new inexpensive DPSS lasers that are consistently acceptable for holography.

    Pulsed Solid State (PSS) lasers:

    Holograms can be made with some PSS lasers. While cavity lengths for PSS lasers are usually relatively long (at least compared to most DPSS lasers), the wide gain bandwidth of the SS lasing medium will mean many longitudinal modes are able to coexist unless specific means like an etalon are included inside the cavity. I don't know of any specific commercial PSS lasers to recommend, but with minor modifications, the widely available small rangefinder laser, SSY1, has been used successfully for holograms. See the section: Using SSY1 to Make Holograms.

    Coherence Length and Holography

    The Coherence Length (CL) of the laser is a critical parameter determining the usable depth of an object. This is the Path Length Difference (PLD) between the reference beam and the reflection of the object beam from any feature on the object. An ideal single frequency (Single Longitudinal Mode, SLM) laser would have an infinite CL; a true white noise "laser" would have a CL of 0. There have been extensive discussions on this topic on the various Internet forums like Photonlexicon without any definitive conclusions. Here is my take. Comments welcome.

    The following was written specifically for the HeNe laser but should apply to most other single spatial mode (TEM00) lasers.

    The useful coherence length is something else.

    The major conclusion from this would be that the advantage of a long cavity is offset by the increased number of longitudinal modes with significant power so that CL remains similar. The effects of the weak modes on the tails of the gain curve may be small for a holographic exposure so there may be some advantage to a long laser other than power.

    Holographic Information Resources

    See the chapter: Laser Information Resources, specifically:

    There is a weekly holography show on-line at Holotalk which has feature stories and special guests by hosted by The Internet Webseum of Holography. You may need special speech/video plugins for you browser to take advantage of this Web page.



  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Laser Communications

    Basic Description

    The term 'laser communication' can mean many things but generally refers to the transmission of information via a laser beam in free-space or a fiber-optic cable. A laser communications system must then consist of:

    Amateur Laser Communications

    For more information and discussions on amateur laser communications, join the Laser Reflector. It is run by ham radio operators who do long distance free-space communications. One is working on laser EME (Earth-Moon-Earth), and another is into non-line-of-site weak signal operation using low baud rate long term integration and advanced DSP techniques with coherent signals!

    The Laser Reflector Web site provides archives of past discussions indexed by date (year and month) and a large set of links to other laser and laser communications sites.

    Offers of inexpensive lasers, laser components, and other related items also appear from time-to-time via this email discussion group.

    Anyone with an interest in laser communications is welcome to join. You don't need to be a ham radio operator. Just send email to majordomo@qth.net with 'subscribe laser' (without quotes) in the message body.

    See the section: Laser (Email) Listservers for more information about these private email discussion groups.

    See the section: Amateur Laser Communications Sites for additional Web sites related to this endeavor.

    Early Laser Communications Experiment

    Not surprising, the potential of optical communications was recognized by researchers even long before the laser was invented. The following is just an example of how easy it is to turn a laser that can be modulated and solar cell into a line-of-site comm link. This was just an ad-hoc experiment but

    Bell Labs may have actually developed and produced some number of portable demonstrators to promote the idea of optical communications. The typical unit appears to have consisted of a HeNe laser tube, power supply, and modulator, along with a separate receiver based on a solar cell, all packed in a handy traveling salesman's type sample case. :) I say "may have" and "appears" because I can't quite tell from the limited information and photos I have if it actually had a working laser or just a cool-looking neon sign-type tube for show - and actually did the communications with a separate conventional modulated lamp (an arc lamp is mentioned in the description I have and its presence doesn't make much sense otherwise). In any case, laser or not, this unit was used in community relations and school programs to show how telephone signals could travel over an optical beam. Some photos of one of these units rescued from the dumpster can be found in the Laser Equipment Gallery (Version 1.76 or higher) under "Assorted Helium-Neon Lasers" (giving it the benefit of the doubt in actually containing a laser!).

    (From: George Werner (glwerner@sprynet.com).)

    Back in the middle 60's our group at Oak Ridge National Laboratory had built a HeNe laser for the purpose of demonstrating to interested groups. One time when I had brought it home in preparation to taking it "on the road" I decided to test its long distance transmission. For distant transmission we used a beam expander which was half of an 8x binocular with a 30 mm objective. We also had built into our power supply a jack into which we could plug in an audio modulation. I set up the laser on the kitchen table near a window with a little pocket radio supplying a signal to the modulator from the local radio station. With a mirror I directed the beam out the window and across the valley to the parking lot I could see where the city maintenance department has a number of vehicles parked.

    It was about a mile away. Looking with another telescope I could see that my beam was getting there when it retro-reflected from a car's tail light.

    Then, taking with me a Fresnel lens and an audio amplifier attached to a solar cell, I drove over there to see what it looked like up close. This was at about 5:30 in the afternoon, still bright daylight, so the red spot was not obvious, but I soon found it. About that time the night watchman, as he should, came to see what it was about. I explained that I was checking on this light that I was beaming down from halfway up the hill across the Turnpike. He looked in that direction but didn't see anything. Where he was standing, the beam was landing between his belt and his shoulders. "You'll have to scootch down a little bit to see it," I said. He found this hard to believe but he tried it and there was no mistaking there was a light. I would compare it to the brightness of a locomotive headlight about a half mile down the track at night (except that it was red).

    Then I put my 18 inch f/1 Fresnel lens in the beam and put the solar cell at the focus (now bright enough to see the reflected light) and the radio station came through loud and clear. With a Polaroid camera I photographed the light coming from my house. Shot from that distance, all the houses are very tiny, but magnification shows a white blob where my house should be.

    P.S. I did not get arrested for trespassing. :)

    (From: Sam.)

    Although George was definitely not an amateur in the laser field of the day, this could very well have been the earliest (or at least one of the earliest) examples of amateur laser communications since it I bet it wasn't part of his job description!



  • Back to Laser Instruments and Applications Sub-Table of Contents.

    Laser Induced Breakdown Spectroscopy

    (From: John Green (ulao10@gmail.com).)

    I have an interest in geology but I'm neither much of a mineralogist nor much of a petrologist. So it is often helpful to know the elemental composition of a particular sample. There have long been tests that one can perform to determine if a particular mineral contains a particular element but they are too often rather limited in the range of elements they can be used for or lack sensitivity or involve equipment that is far too expensive for the amateur. After surveying numerous possibilities I identified one technology that I thought might be within the price range an amateur could afford and yet be useful for a wide range of elements and be fairly sensitive. That technology is LIBS or Laser Induced Breakdown Spectroscopy.

    LIBS involves directing a very high power very short duration laser pulse at a target specimen which causes a tiny amount of the target to be ablated forming a plume of material which absorbs energy from the laser pulse ionizing some of the vapor. The ionized (plasma) vapor then produces light with very distinct wavelengths which can be analyzed to determine the composition of the target as shown in Principles of Laser Induced Breakdown Spectroscopy.

    LIBS is sometimes called Laser Induced Plasma Spectroscopy (LIPS), which I personally prefer, but LIBS is widely accepted. LIBS is a type of atomic emission spectroscopy and is fundamentally similar to, for example, flame emission spectroscopy which you may have seen demonstrated in high school or college chemistry or physics class. In that case an unknown element is introduced into a high temperature flame then a spectroscope is used to produce a spectrum which can be analyzed to determine the unknown element. This method can be extremely sensitive especially for certain elements e.g. alkali metals. Drawbacks are to analyze some elements very high flame temperatures are needed and to get repeatable results the unknown has to be introduced in a controlled manner. Usually this is as an atomized solution. Another variation is inductively coupled plasma spectroscopy but here rigid vacuum and extensive sample preparation are required. LIBS is generally stated to have the advantages of little or no sample preparation, little damage to the specimen, high sensitivity to nearly all elements, and stand off operation. The system described here hits the target with a pulse of infrared radiation (1064 nm) lasting a few nanoseconds but delivering nearly a gigawatt per square centimeter on a very small area for that brief interval. This power (maybe 10 millijoules) vaporizes a tiny volume of the material. The initial spark that is created may reach 100,000 K and radiates a lot of energy peaking around 28 nanometers which is extreme ultraviolet bordering on soft X-rays. Much of this energy is absorbed in the plume of vaporized material breaking chemical bonds and ionizing atoms. As time goes on this plasma expands at supersonic velocity and begins to cool. As it cools electrons begin to re-combine with atoms. This is the process that produces the line spectra that we are looking for and begins around 1 to 5 microseconds after the laser pulse and continues for as long as hundreds of microseconds.

    Upon selecting LIBS as a means to accomplish my goal I first did some research to determine the requirements in terms of laser power, spectral sensitivity, and resolution. The military surplus SSY-1 laser would meet the requirements and cost about $300 which I considered reasonable. My research also indicated that a section of a DVD disc would suffice as a diffraction grating at least for the purpose of evaluating the usefulness of such a system.

    This system was designed to be simple and economical to produce mainly with simple hand tools and from commonly and cheaply available materials and parts. The semi-finished version is shown in John Green's LIBS System. There are some exceptions. Most people won't have a variac handy but a small 3 amp variac can be bought for less than $40. I happened to have a spherical concave mirror on hand so I used it. This mirror does not have to be a high quality optical component it just has to be capable of projecting an image. I found a small bench lathe to be practically indispensable in manufacturing the spectrometer components from 1-1/2" PVC pipe. I am sure however that a resourceful individual who is good with hand tools can find ways of making these parts as extreme precision is not required. I also designed each subsystem to be adjustable because I knew that during the development process many changes would be required and that they might depend on some parameter in another subsystem so making it adjustable meant that I didn't have to rebuild it. I also bought the pulse forming network and trigger transformer from Meredith Instruments as I wanted a reliable system and military stuff is generally pretty solid. Both are pretty cheap (under $40 ea. from Meredith Instruments, Feb. 2014).

    The base of the entire system is mounted on a piece of 1/2"plywood which is approximately 2 ft by 3 ft. The Laser assembly support (1" aluminum square tube, Lowes item 216099) is mounted normal to this with 4, 4" zinc braces (Lowes item 19165). This provides stable support for the laser while allowing it to be adjustable.

    The laser itself is mounted on a 1/2" piece of plywood approximately 13" by 4-1/2" cut to fit the cover (12" plastic mud pan, (Lowes item# 58140). The two black cable clamps originally held a cat-toy laser (very Cheap) that functioned as a targeting beam, bore sighted to the main beam, however several of them each slowly lost power after a couple hundred shots by the main laser (I think the light from the laser somehow destroys them). The main laser should be parallel to the aluminum tube so the beam doesn't wander as the laser height is adjusted. Below the laser is the lens and focusing assembly consisting of the grey tube which is a short length of 3/4" PVC electrical conduit which was turned on the lathe to provide a snug fit into the next PVC pipe to enable focusing. The next part is a short length of 1inch PVC plumbing pipe which in turn fits tightly into a 1 inch coupling. A grove was cut inside the 1 inch coupling then a section just less than half the circumference was cut out to allow lenses to be changed. The lenses themselves which snap in and out of the coupling easily are not critical optical components I used a set of cheap eye loupes from Harbor Freight (item # 98722) said to be 2x, 3x, 5x, 7x, & 10x and priced at $1.99 (Feb. 2014). I have never actually measured the focal lengths of these lenses and don't really care. I am currently using the 3x lens.

    The next assembly is the sample table. The table itself is made from a plastic conduit box (less than $1.00 at any hardware store). The main complication was that a cutout had to be made to clear the mirror. Otherwise it is simply a box constructed of Masonite with a length of 10-24 threaded rod held captive at the top and bottom with a coupling nut epoxied to the table such that the table can be raised and lowered by turning the screw.

    The next assembly is the spectrometer including the concave mirror and camera. The mirror is used because it significantly increases the amount of light entering the spectroscope slit. While evaluating the efficacy of the mirror I found that it provided a power gain of approximately 6 compared to placing the slit 3/8" from the spark. In this case the source of the light rays is placed between the focus of the mirror so that it forms a real image of the source at some greater distance. For an explanation of this phenomenon see: Wikipedia Curved Mirror. Again the placement of these components is not critical and can be varied to suit conditions. The spectrometer is constructed of 1-1/2" Plumbing PVC pipe. The slit was produced by facing a PVC cap (Lowes item # 260594) on the lathe. Since the interior surface of the cap is domed the facing operation leaves a round hole and a convenient flat surface to which razor blades may be glued. A length of pipe connects the slit to the holder forthe diffraction grating. This length is not critical but a shorter length provides for a brighter spectrum while a longer pipe produces a higher resolution spectrum. I chose a full optical path length of approximately 13 inches which is just a little longer than the minimum focal distance of my camera lens. The other end of the spectrograph was constructed from a inch PVC "Y" fitting (Lowes part # 23377). The 43 degree angle (2-3 degrees to the camera lens axis) is fairly critical (+/-1 degrees) if you want the spectrum in the center of the camera field of view. The section of DVD is to be placed here. Tape it in place to begin with so you can adjust it as needed to obtain a straight level spectrum with respect to the horizontal image plane. The inside (spindle center) of the DVD should toward the slit side of the "Y". The other cut on the "Y" is not terribly critical because it simply interfaces with the camera lens. The base for the spectrograph assembly is cut from a piece of 1/2" inch plywood and pivoted on one end. The camera I am using is a Nikon D3000 with Nicor 18-55 mm zoom lens and I have been shooting max zoom (55 mm) f/5.6 ASA 1600 0.5-30 seconds. Focus is very critical. The inside of all spectrometer components should be painted flat black to reduce reflections.

    Now for the power supply. WARNING: LETHAL VOLTAGES ARE PRESENT INSIDE AND OUTSIDE THE POWER SUPPLY. You have been warned, if you are not sure that you know what you are doing don't try this. I built this thing with the knowledge that kids and cats would be around. That's why you have to use a screwdriver to open the power supply. Also note the warning labels on the device. Also notice that my power supply unit is constructed inside a wooden box as shown in John Green's SSY1 Power Supply. Not exactly fire safe. Find a better way.

    The fundamental power is provided from 120 VAC mains by a variable autotransformer (Variac) which is my only means of adjusting flash tube voltage and thus laser radiation fluence. I scrounged a transformer from an old microwave oven. These things are manufactured cheaply (but not sold cheaply) and have two secondary windings. A filament winding (which is not used but could be) and a high voltage (about 2 kV) which I used to provide high voltage to the pulse forming network. Warning, one side of the high voltage winding is usually grounded to the transformer housing. I also added a new secondary of about 40 turns (as I remember) because I anticipated the need for low voltages for other functions. The only components I had to buy for this were the pulse forming network and the trigger circuit (both from Meredith Instruments, PFN-1 $35.00 , SSY-1-XFM $25.00, Feb 2014) and the High voltage full wave rectifier (Mouser part # 844-GBPC3512W). I do not at this time have a circuit diagram of the power supply circuit however the only complication in this circuit is the low voltage circuit and regulators. Otherwise the circuit is straightforward and well documented elsewhere in Sam's Laser FAQ.

    At this point I must point out what I consider to be the most important shortcoming of this system. As noted above, the initial spark produces a lot of radiation. Most of this radiation is outside the narrow window to which the camera is sensitive (visible light) however a lot of such radiation nevertheless gets through. This radiation is essentially black body radiation (like that from the Sun or an incandescent light bulb) and is worthless for our purposes and is in fact quite detrimental to goal of obtaining a line spectrum. Expensive professional LIBS systems have ways of eliminating this radiation mostly by waiting a microsecond or two before turning on the light sensing device. There are (as far as I know) no cheap options for doing this. We are working on possible solutions but so far nothing looks very promising and that is one of the main reasons for this document. By interesting others we might get more minds working on the problem. This shortcoming however does not totally invalidate the method as I shall try to demonstrate.

    I have taken hundreds of spectra involving thousands of shots during the process of developing this instrument which speaks well for the longevity of the laser components. I find that with my present setup I typically have to take between 10 and 30 shots to get a proper spectrum. Individual lines of different elements vary greatly in power, for example while trying to get a spectrum of the sodium D line to assess the resolving of the present setup I had to keep reducing the number of shots of sodium chloride till I finally got a usable spectrum at 3 shots. Compare this to the sodium chloride spectrum which required 15 shots. In other cases I have had to take as many as 60 shots. The spectra produced by this instrument became useful only after it was calibrated and the component parts were rigidly fixed.

    Once the system became usable I began by building a spectrum library of elements I had on hand. Where possible I used pure elements. It is not always easy to find whether a particular sample is pure (or at least nearly pure) unless one buys them from reputable suppliers, and that gets expensive. The composition of coins for example is well documented. Otherwise I used what was available and as time goes on I will refine these spectra. One might be inclined to ask "Why not use spectral libraries available, for example, on the Internet?" Well I do but these were usually produced with very sensitive specialized equipment and are fine for locating specific lines but many elements produce a "forrest of lines" and it is simpler to make comparisons to spectra produced on the same system. With a small library in hand it was time to try some unknowns. The first sample I tried was a mineral called sodalite. The chemical formula for sodalite is given in Wikipedia as Na8(Al6Si6O24)Cl2 . The details of this elude me at present and I must brush up a bit (a lot) on chemistry but I think the strength of lines depends on mole fraction. Regardless one can clearly see that sodium, aluminum, and silicon are all significant components of this mineral. So what does the spectrum of sodalite look like and how does it compare to the components? See Several LIBS Spectra.

    Let's do one more. I have in my collection a specimen which I believe contains grains of kyanite (Al2SiO5 ). I also happen to have a specimen I bought which is labeled kyanite and which I believe to be kyanite. Compare them! Remember this tells us nothing about the chemical formulae only the elements present.

    This one isn't as dramatic but I think the two show that LIBS can be a valuable tool even in the hands of a rank amateur like me. Much work remains to be done. For example the spectra must be calibrated in terms of wavelength or wavenumbers to be really useful and there is software available (mostly for amateur astronomy) that facilitates this operation. One program in particular (Visual Spec) looks really useful and I have worked with it quite a bit with some success but it was written by a Frenchman and whoever translated the documentation into English does not appear to be a profoundly talented English speaker. I have several times given up exasperated but I will go back and try again.



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    Miscellaneous

    Use of Laser to Identify Stars in the Sky to a Group

    Of course you can't reach the stars but there may be enough scatter in the air to show the direction. :)

    (From: Louis Boyd (boyd@apt0.sao.arizona.edu).)

    In my experience a 5 mW red laser does not do the job unless there's a lot of dust or water droplets in the air. The problem is the dark adapted human eye is very insensitive to red. Also backscatter from small particles is reduced as wavelength increases. I can't give a specific power level because it's so dependent on the particles suspended in the air. Under the right conditions a 3mw green pointer would be easily visible for a few people standing together but probably won't be adequate in very clean air. Blinking the laser can make it easier to detect and reduce power consumption. You also didn't state the size of the group. The distance of the observer from the emitter makes a difference.

    The "vanishing point" for off axis viewer isn't at infinity and is dependent on the power level and the hight of suspended particles. The effect is that what you are pointing at may not be exactly where other's perceive the end of the "beam" to be. You may actually be better off with a larger beam diameter using a modified flashlight with a halogen bulb.

    One of the more powerful "MagLight" or "Surefire" flashlights with a an extension of a couple of feet of ABS plastic with internal baffle rings to prevent side scatter does a good job. This can put out around a watt of light and it's a lot cheaper than an adequately powerful laser. If this is for a large group get one of the "million candlepower" lamps and make the baffle out of a "honeycomb" of tubes with black flocking blown into them. Those have over 10 watts of light output. If you need to do this for a large crowd like a stadium use a xenon short arc lamp spotlight with hundreds of watts of output.



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