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    Helium-Neon Lasers

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  • Back to Helium-Neon Lasers Sub-Table of Contents.

    HeNe Laser Characteristics, Applications, Safety

    Note: Due to the amount of material, information on specific commercial helium-neon lasers has moved to their own chapters: Commercial Unstabilized HeNe Lasers for the vanilla flavored (or actually mostly cherry flavored!) variety, and Commercial Stabilized HeNe Lasers for frequency and/or intensity stabilized scientific and metrology types.

    Introduction to Helium-Neon Lasers

    A helium-neon (henceforth abbreviated HeNe) laser is basically a fancy neon sign with mirrors at both ends. Well, not quite, but really not much more than this at first glance (though the design and manufacturing issues which must be dealt with to achieve the desired beam characteristics, power output, stability, and life span, are non-trivial). The gas fill is a mixture of helium and neon gas at low pressure. A pair of mirrors - one totally reflective (called the High Reflector or HR), the other partially reflective (called the Output Coupler or OC) at the wavelength of the laser's output - complete the resonator assembly. This is called a Fabry-Perot cavity (if you want to impress your friends). The mirrors may be internal (common on small and inexpensive tubes) or external (on precision high priced lab quality lasers). Electrodes sealed into the tube allow for the passage of high voltage DC current to excite the discharge.

    Note that a true laser jock will further abbreviate "HeNe laser" to simply "HeNe", pronounced: Hee-nee. Their laser jock colleagues and friends then know this really refers to a laser! :) While other types of lasers are sometimes abbreviated in an analogous manner, it is never to the same extent as the HeNe.

    I still consider the HeNe laser to be the quintessential laser: An electrically excited gas between a pair of mirrors. It is also the ideal first laser for the experimenter and hobbyist. OK, well, maybe after you get over the excitement of your first laser pointer! :) HeNe's are simple in principle though complex to manufacture, the beam quality is excellent - better than anything else available at a similar price. When properly powered and reasonable precautions are taken, they are relatively safe if the power output is under 5 mW. And such a laser can be easily used for many applications. With a bare HeNe laser tube, you can even look inside while it is in operation and see what is going on. Well, OK, with just a wee bit of imagination! :) This really isn't possible with diode or solid state lasers.

    I remember doing the glasswork for a 3 foot long HeNe laser (probably based on the design from: "The Amateur Scientist - Helium-Neon Laser", Scientific American, September 1964, and reprinted in the collection: "Light and Its Uses" [5]). This included joining side tubes for the electrodes and exhaust port, fusing the electrodes themselves to the glass, preparing the main bore (capillary), and cutting the angled Brewster windows (so that external mirrors could be used) on a diamond saw. I do not know if the person building the laser ever got it to work but suspect that he gave up or went on to other projects (which probably were also never finished). And, HeNe lasers are one of the simplest type of lasers to fabricate which produce a visible continuous beam.

    Some die-hards still construct their own HeNe lasers from scratch. Once all the glasswork is complete, the tube must be evacuated, baked to drive off surface impurities, backfilled with a specific mixture of helium to neon (typically around 7:1 to 10:1) at a pressure of between 2 and 5 Torr (normal atmospheric pressure is about 760 Torr - 760 mm of mercury), and sealed. The mirrors must then be painstakingly positioned and aligned. Finally, the great moment arrives and the power is applied. You also constructed your high voltage power supply from scratch, correct? With luck, the laser produces a beam and only final adjustments to the mirrors are then required to optimize beam power and stability. Or, more, likely, you are doing all of this while your vacuum pumps are chugging along and you can still play with the gas fill pressure and composition. What can go wrong? All sorts of things can go wrong! With external mirrors, the losses may be too great resulting in insufficient optical gain in the resonant cavity. The gas mixture may be incorrect or become contaminated. Seals might leak. Your power supply may not start the tube, or it may catch fire or blow up. It just may not be your day! And, the lifetime of the laser is likely to end up being only a few hours in any case unless you have access to an ultra-high vacuum pumping and bakeout facility. While getting such a contraption to work would be an extremely rewarding experience, its utility for any sort of real applications would likely be quite limited and require constant fiddling with the adjustments. Nonetheless, if you really want to be able to say you built a laser from the ground up, this is one approach to take! (However, the CO2 and N2 lasers are likely to be much easier for the first-time laser builder.) See the chapters starting with: Amateur Laser Construction for more of the juicy details.

    However, for most of us, 'building' a HeNe laser is like 'building' a PC: An inexpensive HeNe tube and power supply are obtained, mounted, and wired together. Optics are added as needed. Power supplies may be home-built as an interesting project but few have the desire, facilities, patience, and determination to construct the actual HeNe tube itself.

    The most common internal mirror HeNe laser tubes are between 4.5" and 14" (125 mm to 350 mm) in overall length and 3/4" to 1-1/2" (19 mm to 37.5 mm) in diameter generating optical power from 0.5 mW to 5 mW. They require no maintenance and no adjustments of any kind during their long lifetime (20,000 hours typical). Both new and surplus tubes of this type - either bare or as part of complete laser heads - are readily available. Slightly smaller tubes (less than 0.5 mW) and much larger tubes (up to approximately 35 mW) are structurally similar (except for size) to these but are not as common.

    Much larger HeNe tubes with internal or external mirrors or one of each (more than a *meter* in length!) and capable of generating up to 250 mW of optical power have been available and may turn up on the surplus market as well (but most of these are quite dead by now). The most famous of these (as lasers go) is probably the Spectra-Physics model 125A whose laser head is over 6 feet in length. It was only rated 50 mW (633 nm), but new samples under optimal conditions may have produced more than 200 mW. Even more powerful ones have been built as research projects. I've seen photos of a Hughes HeNe laser with a head around 8 feet in length that required a 6 foot rack-mount enclosure for the exciter. See Monster Vintage Hughes HeNe Laser System. (Photo courtesy of Bob Hess.) Its output power is unknown, but probably less than that of the SP-125A. The largest single transverse mode (SM, with a TEM00 beam profile) HeNe lasers in current production by a well known manufacturer like Melles Griot are rated at about 35 mW minimum over an expected lifetime of 20,000 hours or more, though new samples may exceed 50 mW. However, HeNe lasers rated up to at least 70 mW SM and 100 mW MM are available. Manufacturers include: CDHC-Optics (China), Spectral Laser (Italy), and PLASMA, JSC (Russia). However, the lifetime over which these specifications apply is not known and may be much shorter.

    Highly specialized configurations, such as a triple XYZ axis triangular cavity HeNe laser in a solid glass block for an optical ring laser gyro, also exist but are much much less common. Most HeNe lasers operate CW (Continuous Wave) producing a steady beam at a fixed output power unless the power is switched on and off or modulated (or someone sticks their finger in the beam and blocks it!). (At least they are supposed to when in good operating condition!) However, there are some mode-locked HeNe lasers that output a series of short pulses at a high repetition rate. And, in principle, it is possible to force a HeNe laser with at least one external mirror to "cavity dump" a high power pulse (perhaps 100 times the CW power) a couple of nanoseconds long by diverting the internal beam path with an ultra high speed acousto-optic deflector. But, for the most part, such systems aren't generally useful for very much outside some esoteric research areas and in any case, you probably won't find any of these at a local flea market or swap meet, though eBay can't be ruled out! :)

    Nearly all HeNe lasers output a single wavelength and it is most often red at 632.8 nm. (This color beam actually appears somewhat orange-red especially compared to many laser pointers using diode lasers at wavelengths between 650 and 670 nm). However, green (543.5 nm), yellow (594.1 nm), orange (604.6 and 611.9 nm), and even IR (1,152, 1.523, and 3,921 nm) HeNe lasers are available. There are a few high performance HeNe lasers that are tunable and very expensive. And, occasionally one comes across laser tubes that output two or more wavelengths simultaneously. Although some tubes are designed this way, it is more likely to be a 'defect' resulting from a combination of high gain and insufficiently narrow band optics. Such tubes tend to be unstable with the relative power varying among the multiple wavelengths more or less at random.

    Note that the single wavelength described above usually consists of more than one longitudinal mode or lasing line (more on this later). However, some HeNe lasers are designed to produce a highly stable single optical frequency or two closely spaced optical frequencies. These are used in scientific research and metrology (measurement) applications, described in more detail below.

    Current major HeNe laser manufacturers include Melles-Griot, JDS Uniphase, and LASOS. This is far fewer than there were only a few years ago. So, you may also find lasers from companies like Aerotech, Hughes, Siemens, and Spectra-Physics that have since gotten out of the HeNe laser business or have been bought out, merged, or changed names. For example, the HeNe laser divisions of Aerotech and Hughes were acquired by Melles Griot; Sieman's HeNe laser product line is now part of LASOS; and Spectra-Physics which was probably the largest producer of HeNe lasers from the very beginning gradually eliminated all HeNe lasers from its product line over the last few years. HeNe tubes, laser heads, and complete lasers from any of these manufacturers are generally of very high quality and reliability. A more complete list can be found in the Photonics Buyers' Guide and in the chapter: Laser and Parts Sources. Information on many specific HeNe lasers can be found in the chapters: Commercial Unstabilized HeNe Lasers and Commercial Stabilized HeNe Lasers.

    HeNe lasers have been found in all kinds of equipment including:

    Nowadays, many of these applications are likely to use the much more compact lower (drive) power solid state diode laser. (You can tell if you local ACME supermarket uses a HeNe laser in its checkout scanners by the color of the light - the 632.8 nm wavelength beam from a HeNe laser is noticeably more orange than the 660 or 670 nm deep red from a typical diode laser type.)

    Melles Griot (now part of IDEX Optics and Photonics Marketplace. Catalogs used to include several pages describing HeNe laser applications. I know this was present in the 1998 catalog but has since disappeared and I don't think it is on their Web site.

    Also see the section: Some Applications of a 1 mW Helium-Neon Laser for the sorts of things you can do with even a small HeNe laser.

    Since a 5 mW laser pointer complete with batteries can conveniently fit on a keychain and generate the same beam power as an AC line operated HeNe laser almost half a meter long, why bother with a HeNe laser at all? There are several reasons:

    However, the market for new HeNe lasers is still in the 100,000 or more units per year. What can you say? If you need a stable, round, astigmatism-free, long lived, visible 1 to 10 mW beam for under $500 (new, remember!), the HeNe laser is still the only choice.

    Some Applications of a 1 mW Helium-Neon Laser

    There are many uses for even a 1 mW helium-neon laser. Most of these same sorts of things can also be done with a collimated diode laser (though some laser diodes may not have the needed coherence properties for applications like interferometry and hologram generation).

    Below are just a few possibilities.

    (Portions from: Chris Chagaris (pyro@grolen.com).)

    For many more ideas, see the chapters: Laser Experiments and Projects and Laser Instruments and Applications and the many references and links in the chapter: Laser Information Resources.

    HeNe Laser Safety

    As with *any* laser, proper precautions must be taken to avoid any possibility of damage to vision. The types of HeNe lasers mostly dealt with in this document are rated Class II, IIIa, or the low end of IIIb (see the section: Laser Safety Classifications. For most of these, common sense (don't stare into the beam) and fairly basic precautions suffice since the reflected or scattered light will not cause instantaneous injury and is not a fire hazard.

    However, unlike those for laser diodes, HeNe power supplies utilize high voltage (several kV) and some designs may be potentially lethal. This is particularly true of AC line powered units since the power transformer may be capable of much more current than is actually required by the HeNe laser tube - especially if it is home built using the transformer from some other piece of equipment (like an old tube type console TV or that utility pole transformer you found along the curb) which may have a much higher current rating.

    The high quality capacitors in a typical power supply will hold enough charge to wake you up - for quite a while even after the supply has been switched off and unplugged. Depending on design, there may be up to 10 to 15 kV or more (but on very small capacitors) if the power supply was operated without a HeNe tube attached or it did not start for some reason. There will likely be a lower voltage - perhaps 1 to 3 kV - on somewhat larger capacitors. Unless significantly oversized, the amount of stored energy isn't likely to be enough to be lethal but it can still be quite a jolt. The HeNe tube itself also acts as a small HV capacitor so even touching it should it become disconnected from the power supply may give you a tingle. This probably won't really hurt you physically but your ego may be bruised if you then drop the tube and it then shatters on the floor!

    However, should you be dealing with a much larger HeNe laser, its power supply is going to be correspondingly more dangerous as well. For example, a 35 mW HeNe tube typically requires about 8 mA at 5 to 6 kV. That current may not sound like much but the power supply is likely capable of providing much more if you are the destination instead of the laser head (especially if it is a homemade unit using grossly oversized parts)! It doesn't take much more under the wrong conditions to kill.

    After powering off, use a well insulated 1M resistor made from a string of ten 100K, 2 W metal film resistors in a glass or plastic tube to drain the charge - and confirm with a voltmeter before touching anything. (Don't use carbon resistors as I have seen them behave funny around high voltages. And, don't use the old screwdriver trick - shorting the output of the power supply directly to ground - as this may damage it internally.)

    And only change electrical connections or plug/unplug connectors with power OFF, being aware of the potential for stored charge. In particular, the aluminum cylinder of some HeNe laser heads is the negative return for the tube current via a spring contact inside the rear end-cap. So, pulling off the rear end-cap while the laser is powered will likely make YOU the negative return instead! You will probably then bounce off the ceiling while the laser bounces off the floor, which can easily ruin your entire day in more ways than one. :( :) This connection scheme is known to be true for most JDS Uniphase and many Melles Griot laser heads, but may apply to others as well.

    See the document: Safety Guidelines for High Voltage and/or Line Powered Equipment for detailed information before contemplating the inside or HV terminals of a HeNe power supply!

    Now, for some first-hand experience:

    (From: Doug (dulmage@skypoint.com).)

    Well, here's where I embarrass myself, but hopefully save a life...

    I've worked on medium and large frame lasers since about 1980 (Spectra-Physics 168's, 171's, Innova 90's, 100's and 200's - high voltage, high current, no line isolation, multi-kV igniters, etc.). Never in all that time did I ever get hurt other than getting a few retinal burns (that's bad enough, but at least I never fell across a tube or igniter at startup). Anyway, the one laser that almost did kill me was also the smallest that I ever worked on.

    I was doing some testing of AO devices along with some small cylindrical HeNe tubes from Siemens. These little coax tubes had clips for attaching the anode and cathode connections. Well, I was going through a few boxes of these things a day doing various tests. Just slap them on the bench, fire them up, discharge the supplies and then disconnect and try another one. They ran off a 9 VDC power supply.

    At the end of one long day, I called it quits early and just shut the laser supply off and left the tube in place as I was just going to put on a new tube in the morning. That next morning, I came and incorrectly assumed that the power supply would have discharged on it own overnight. So, with each hand I stupidly grab one clip each on the laser to disconnect it. YeeHaaaaaaaaa!!!!. I felt like I had been hid across my temples with a two by four. It felt like I swallowed my tongue and then I kind of blacked out. One of the guys came and helped me up, but I was weak in the knees, and very disoriented.

    I stumbled around for about 15 minutes and then out of nowhere it was just like I got another shock! This cycle of stuff went on for about 3 hours, then stopped once I got to the hospital. I can't even remember what they did to me there. Anyway, how embarrassing to almost get killed by a HeNe laser after all that other high power stuff that I did. I think that's called 'irony'.

    Comments on HeNe Laser Safety Issues

    (Portions from: Robert Savas (jondrew@mail.ao.net).)

    A 10 mw HeNe laser certainly presents an eye hazard.

    According to American National Standard, ANSI Z136.1-1993, table 4 Simplified Method for Selecting Laser Eye Protection for Intrabeam Viewing, protective eyewear with an attenuation factor of 10 (Optical Density 1) is required for a HeNe with a 10 milliwatt output. This assumes an exposure duration of 0.25 to 10 seconds, the time in which they eye would blink or change viewing direction due the uncomfortable illumination level of the laser. Eyeware with an attenuation factor of 10 is roughly comparable to a good pair of sunglasses (this is NOT intended as a rigorous safety analysis, and I take no responsibility for anyone foolish enough to stare at a laser beam under any circumstances). This calculation also assumes the entire 10 milliwatts are contained in a beam small enough to enter a 7 millimeter aperture (the pupil of the eye). Beyond a few meters the beam has spread out enough so that only a small fraction of the total optical power could possible enter the eye.



  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Theory of Operation, Modes, Coherence Length, On-Line Course and Tutorials

    Instant HeNe Laser Theory

    For much more than I can provide here (should you care), see the section: On-line Introductions to Lasers. These sites are well worth checking out as they include substantial material on HeNe lasers.

    The term laser stands for "Light Amplification by Stimulated Emission of Radiation". However, lasers as most of us know them, are actually sources of light - oscillators rather than amplifiers. (Although laser amplifiers do exist in applications as diverse as fiber optic communications repeaters and multi-gigawatt laser arrays for inertial fusion research.) Of course, all oscillators - electronic, mechanical, or optical - are constructed by adding the proper kind of positive feedback to an amplifier.

    All materials exhibit what is known as a bright line spectra when excited in some way. In the case of gases, this can be an electric current or (RF) radio frequency field. In the case of solids like ruby, a bright pulse of light from a xenon flash lamp can be used. The spectral lines are the result of spontaneous transitions of electrons in the material's atoms from higher to lower energy levels. A similar set of dark lines result in broad band light that is passed through the material due to the absorption of energy at specific wavelengths. Only a discrete set of energy levels and thus a discrete set of transitions are permitted based on quantum mechanical principles (well beyond the scope of this document, thankfully!). The entire science of spectroscopy is based on fact that every material has a unique spectral signature.

    The HeNe laser depends on energy level transitions in the neon gas. In the case of neon, there are dozens if not hundreds of possible wavelength lines of light in this spectrum. Some of the stronger ones are near the 632.8 nm line of the common red HeNe laser - but this is not the strongest:

    The strongest red line is 640.2 nm. There is one almost as strong at 633.4 nm. That's right, 633.4 nm and not 632.8 nm. The 632.8 nm one is quite weak in an ordinary neon spectrum, due to the high energy levels in the neon atom used to produce this line. See: Bright Line Spectra of Helium and Neon. (The relative brightnesses of these don't appear to be accurate though at present.) More detailed spectra can be found at the: Laser Stars - Spectra of Gas Discharges Page. And there is a photo of an actual HeNe laser discharge spectra with very detailed annotation of most of the visible lines in: Skywise's Lasers and Optics Reference Section. The comment about the output wavelength not being one of the stronger lines is valid for most lasers as if it were, that energy level would be depleted by spontaneous emission, which isn't what is wanted!

    There are also many infra-red lines and some in the orange, yellow, and green regions of the spectrum as well.

    The helium does not participate in the lasing (light emitting) process but is used to couple energy from the discharge to the neon through collisions with the neon atoms. This pumps up the neon to a higher energy state resulting in a population inversion meaning that more atoms in the higher energy state than the ground or equilibrium state.

    Please refer to Helium-Neon Excitation and Lasing Process for the following description.

    It turns out that the upper level of the transition that produces he 632.8 nm line (as well as the other visible HeNe lasing lines) has an nergy level that almost exactly matches the energy level of helium's owest excited state. The vibrational coupling between these two states s highly efficient.

    1. A DC electrical discharge or RF field excites He atoms to the 2s energy state.

    2. Collisions efficiently transfer energy raising Ne atoms to the 3s2 energy state. Note the relatively high energy levels involved - over 20 eV for the upper energy states.

    3. Stimulated emission (lasing) causes a drop to one of several Ne 2p states.

    4. Radiative decay (spontaneous emission) drops Ne from the terminal lasing state to the 1s state.
    5. Collisions with the tube wall drops Ne from the 1s state to the Ground state.

    For 632.8 nm, one mirror will be highly reflective at 632.8 nm (typically 99.9 percent or better). This is the "High Reflector" or HR. The other mirror will have a typical reflectivity of 99 percent at 632.8 nm. This is the "Output Coupler" or OC from which the useful beam emerges. In order to suppress lasing at other wavelengths, the mirrors will generally be designed to have lower reflectivity there. (Though given the low gain of all the HeNe lasing lines, especially the "other color" lines, this isn't much of a problem at 632.8 nm.)

    The rate at which (4) and (5) can take place ultimately limits the power of a HeNe laser and explains why increasing the excitation (1) actually reduces power above some optimum level.

    The gas mixture must be mostly helium (typically 5:1 to 10:1, He:Ne), so that helium atoms can be excited. The excited helium atoms collide with neon atoms, exciting some of them to the state from which they can radiate at 632.8 nm. Without helium, the neon atoms would be excited mostly to lower excited states responsible for non-laser lines. And the gas mixture has to be super pure as any contamination results in excitation of rogue atoms (like H, O, and N) to lower energy states where all that will happen is that they will glow like a poorly made neon sign.

    A neon laser with no helium can be constructed but it is much more difficult and the output power will be much lower without this means of energy coupling. Therefore, a HeNe laser that has lost enough of its helium (e.g., due to diffusion through the seals or glass) will most likely not lase at all since the pumping efficiency will be too low.

    However, pure neon will lase superradiantly in a narrow tube (e.g., 40 cm long x 1 mm ID) in the orange (611.9 nm) and yellow (594.1 nm) with orange being the strongest. Superradiant means that no mirrors are used although the addition of a Fabry-Perot cavity (e.g., mirrors!) does improve the lateral coherence and output power. This from a paper entitled: "Super-Radiant Yellow and Orange Laser Transitions in Pure Neon" by H. G. Heard and J. Peterson, Proceedings of the IEEE, Oct. 1964, vol. #52, page #1258. The authors used a pulsed high voltage power supply for excitation (they didn't attempt to operate the system in CW mode but speculate that it should be possible).

    (From: Steve Roberts.)

    "Various IR lines will lase in pure neon, and even the 632.8 nm line will lase, but it takes a different pressure and a much longer tube. 632.8 nm also shows up with neon-argon, neon-oxygen, and other mixtures. Just about everything on the periodic table will lase, given the right excitation. See "The CRC Handbook of Lasers" or one of the many compendiums of lasing lines available in larger libraries. These are usually 4 volume sets of books the size of a big phone book just full of every published journal article on lasing action observed. It's a shame that out of these many thousands and thousands of lasing lines, only 7 different types of lasers are under mainstream use.

    There are many possible transitions in neon from the excited state to a lower energy state that can result in laser action. (Only the three found most commonly in commercial HeNe lasers are shown in the diagram, above.) The most important (from our perspective) are listed below:

          (1)         (2)           (3)           (4)          (5)         (6)
         Output       HeNe       Perceived       Lasing      Typical     Maximum
       Wavelength  Laser Name    Beam Color    Transition   Gain (%/m)  Power (mW)
     ------------------------------------------------------------------------------
         543.5 nm    Green         Green        3s2->2p10   0.52   0.59    2 (5)  
         594.1 nm    Yellow    Orange-Yellow    3s2->2p8    0.5    0.67    7 (10)
         604.6 nm                  Orange       3s2->2p7    0.6    1.0     3
         611.9 nm    Orange      Red-Orange     3s2->2p6    1.7    2.0     7
         629.4 nm                Orange-Red     3s2->2p5    1.9    2.0
         632.8 nm     Red          "    "       3s2->2p4   10.0   10.0    75 (200)
         635.2 nm                  "    "       3s2->2p3    1.0    1.25
         640.1 nm                   Red         3s2->2p2    4.3    2.0     2
         730.5 nm             Border Infra-Red  3s2->2p1    1.2    1.25    0.3
         886.5 nm                  "    "       2s2->2p10   1.2    1.25    0.3
       1,029.8 nm   Near-IR      Invisible      2s2->2p8    ???
       1,062.3 nm    "   "         "   "        2s2->2p7    ???
       1,079.8 nm    "   "         "   "        2s3->2p7    ???
       1,084.4 nm    "   "         "   "        2s2->2p6    ???
       1,140.9 nm    "   "         "   "        2s2->2p5    ???
       1,152.3 nm    "   "         "   "        2s2->2p4    ???            1.5
       1,161.4 nm    "   "         "   "        2s3->2p5    ???
       1,176.7 nm    "   "         "   "        2s2->2p2    ???
       1,198.5 nm    "   "         "   "        2s3->2p2    ???
       1,395.0 nm    "   "         "   "        2s2->2p?    ???            0.5
       1,523.1 nm    "   "         "   "        2s2->2p1    ???            1.0
       3,391.3 nm    Mid-IR        "   "        3s2->3p4    ???  440.0    24
    

    Notes:

    1. Output Wavelength is approximate. In addition to slight variations due to actual lasing conditions (single mode, multimode, doppler broadening, etc.), some references don't even agree on some of these values to the 4 or 5 significant digits shown.

    2. HeNe Laser Name is what would be likely to be found in a catalog or spec sheet. All those that have an entry in this column are readily available commercially.

    3. Perceived Beam Color is how it would appear when spread out and projected onto a white screen. Of course, depending on the revision level of your eyeballs, this may vary someone from individual to individual. :)

    4. Lasing Transition uses the so-called "Paschen Notation" and indicates the electron shells of the neon atom energy states between which the stimulated emission takes place.

    5. Typical Gain (%/m) shows the percent increase in light intensity due to stimulated emission at this wavelength inside the laser tube's bore. This is the single pass gain and will be affected by tube construction, gas fill ratio and pressure, discharge current, and other factors. The first column is from various sources. The second column is from Hecht, "The Laser Guide Book". However, a newer text: Mark Csele, "Fundamentals of light sources and lasers" (ISBN 0-471-47660-9, Wiley-Interscience, 2004) lists the typical gain as 1.2 to 1.5 at 633 nm. And measurements by myself and others seem to show that this slightly higher value may be more accurate, at least under some conditions. Also see the section: The Single Pass Gain Test.

      Gain at 1,523 nm may be similar to that of 543.5 nm - about 0.5%/m. Gain at 3,391 nm is by far the highest of any - possibly more than 100%/m. I know of one particular HeNe laser operating at this wavelength that used an OC with a reflectivity of only 60% with a bore less than 0.4 m long. Yet, the output power of the largest 3,391 nm commercial HeNe laser is still only a fraction of that at 632.8 nm.

    6. Maximum Power shows the highest output power lasers commercially available in a TEM00 beam for each wavelength. The first number is rated power while the number in () is achieved output power for a particularly lively tube. Lasers operating with multiple (spatial) modes (non-TEM00) may have somehwat higher output power.

    See the section: Instant Spectroscope for Viewing Lines in HeNe Discharge for an easy way to see many of the visible ones.

    The most common and least expensive HeNe laser by far is the one called 'red' at 632.8 nm. However, all the others with named 'colors' are readily available with green probably being second in popularity due to its increased visibility near the peak of the of the human eye's response curve (555 nm). And, with some HeNe lasers with insufficiently narrow-band mirrors, you may see 640 nm red as a weak output along with the normal 632.8 nm red because of its relatively high gain. There are even tunable HeNe lasers capable of outputting any one of up to 5 or more wavelengths by turning a knob. While we normally don't think of a HeNe laser as producing an infra-red (and invisible) beam, the IR spectral lines are quite strong - in some cases more so than the visible lines - and HeNe lasers at all of these wavelengths (and others) are commercially available.

    The first gas laser developed in the early 1960s was an HeNe laser operated at 1,152.3 nm. In fact, the IR line at 3,391.3 is so strong that a HeNe laser operating in 'superradiant' mode - without mirrors - can be built for this wavelength and commercial 3,391.3 nm HeNe lasers may use an output mirror with a reflectivity of less than 50 percent. Contrast this to the most common 632.8 nm (red) HeNe laser which requires very high reflectivity mirrors (often over 99 percent) and extreme care to mimize losses or it won't function at all.

    When the HeNe gas mixture is excited, all possible transitions occur at a steady rate due to spontaneous emission. However, most of the photons are emitted with a random direction and phase, and only light at one of these wavelengths is usually desired in the laser beam. At this point, we have basically the glow of a neon sign with some helium mixed in!

    To turn spontaneous emission into the stimulated emission of a laser, a way of selectively amplifying one of these wavelengths is needed and providing feedback so that a sustained oscillation can be maintained. This may be accomplished by locating the discharge between a pair of mirrors forming what is known as a Fabry-Perot resonator or cavity. One mirror is totally reflective and the other is partially reflective to allow the beam to escape.

    One mirror may be perfectly flat (planar) or both may be spherical with a typical Radius of Curvature (RoC = 2 * focal length) slightly longer that the length of the cavity (L) or even longer. Where both mirrors have an RoC equal to L, the configuration is called 'confocal' (the focii of the two mirrors are coincident), but it is marginall stable, so the RoCs will be at least slightly longer than L. A cavity with two planar mirrors is borderline stable and essentially impossible to align or maintain in alignment over time, so it is never used in HeNe lasers (but is in some pulsed solid state and other lasers). Curved mirrors result in an easier to align more stable configuration but are more expensive than planar mirrors to manufacture and are not as efficient since less of the lasing medium volume is used (think of the shape of the beam inside the bore). The confocal arrangement represents a good compromise between a true spherical cavity (r = 1/2 * L) which is easiest to align but least efficient and one with plane parallel mirrors (f = infinity) which is most difficult to align but uses the maximum volume of the lasing medium. (But as noted above, for a practical confocal cavity, RoCs slightly longer than L are used to assure stability.) For more on this topic, see the section: Common Laser Resonator Configurations.

    These mirrors are normally made so that the two mirrors together has peak reflectivity at the desired laser wavelength. (For technical reasons, it's sometimes easier to make mirrors like cliffs - high reflectivity that drops to low reflectivity at a given wavelength, in either direction - than to guarantee a particular peak reflectivity.) When a spontaneously emitted photon resulting from the transition corresponding to this peak happens to be emitted in a direction nearly parallel to the long axis of the tube, it stimulates additional transitions in excited atoms. These atoms then emit photons at the same wavelength and with the same direction and phase. The photons bounce back and forth in the resonant cavity stimulating additional photon emission. Each pass through the discharge results in amplification - gain - of the light. If the gain due to stimulated emission exceeds the losses due to imperfect mirrors and other factors, the intensity builds up and a coherent beam of laser light emerges via the partially reflecting mirror at one end. With the proper discharge power, the excitation and emission exactly balance and a maximum strength continuous stable output beam is produced.

    Spontaneously emitted photons that are not parallel to the axis of the tube will miss the mirrors entirely or will result in stimulated photons that are reflected only a couple of times before they are lost out the sides of the tube. Those that occur at the wrong wavelength will be reflected poorly if at all by the mirrors and any light at these wavelengths will die out as well.

    Summary of the HeNe Lasing Process

    The HeNe laser is a 4 level laser (see the table above for the specific energy level transitions for the common wavelengths):

    For most common IR wavelengths, level 4 is the 2s state and level 3 are various 2p states. However, the very strong 3.93 um line originates from the 3s state just like the visible wavelengths - and is the reason it competes with them in long HeNe tubes and must be suppressed to optimize visible output.

    The 's' states of neon have about 10 times the lifetime of the 'p' states and thus support the population inversion since a neon atom can hang around in the 2s state long enough for stimulated emission to take place. However, the limiting effect is the decay back to level 1, the ground state, since the 1s state also has a long lifetime. Thus, one wants a narrow bore to facilitate collisions with its walls. But this results in increased losses. Modern HeNe lasers operate at a compromise among several contradictory requirements which is one reason that their maximum output power is relatively low.

    Longitudinal Modes of Operation

    The physical dimensions of the Fabry-Perot resonator impose some additional constraints on the resulting beam characteristics.

    While it is commonly believed that the 632.8 nm (for example) transition is a sharp peak, it is actually a Gaussian - bell shaped - curve. (Strictly speaking, it is something called a "Voigt distribution" which is a conbination of Gaussian and Lorentzian - but that's for the advanced course. Gaussian is close enough for this discussion since the discrepency only shows up way out in the tails of the curve.) In order for a linear or (Fabry-Perot) cavity to resonate strongly, a standing wave pattern must exist. This will only occur when an integral number of half wavelengths fit between the two mirrors. This restricts possible axial or longitudinal modes of oscillation to:

                       L * 2                 c * n 
                 W = ---------    or   F = --------- 
                         n                   L * 2
    

    Where:

    The laser will not operate with just any wavelength - it must satisfy this equation. Therefore, the output will not usually be a single peak at 632.8 nm but a series of peaks around 632.8 nm spaced c/(2*L) Hz apart. Longer cavities result in closer mode spacing and a larger number of modes since the gain won't fall off as rapidly as the modes move away from the peak. For example, a cavity length of 150 mm results in a longitudinal mode spacing of about 1 GHz; L = 300 mm results in about 500 MHz. The strongest spectral lines in the output will be nearest the combined peak of the lasing medium and mirror reflectivity but many others will still be present. This is called multimode operation.

    Think of the vibrating string of a violin or piano. Being fixed at both ends, it can only sustain oscillations where an integer number of cycles fits on the string. In the case of a string, n can equal 1 (fundamental) and 2, 3, 4, 5 (harmonics or overtones). Due to the tension and stiffness of the string, only small integer values for n are present with a significant amplitude. For a HeNe laser, the distribution of the selected neon spectral line and shape of the reflectivity function of the mirrors with respect to wavelength determine which values of n are present and the effective gain of each one. And n will be much greater than 1!

    For a typical HeNe laser tube, possible values of n will form a series of very large numbers like 948,161, 948,162, 948,163, 948,164,.... rather than 1, 2, 3, 4. :-) A typical gain function showing the emission curve of the excited neon multiplied by the mode structure of the Fabry-Perot resonator and the reflectivity curve of the mirrors may look something like the following:

                    |                  632.8 nm
                   I|                     .
                    |                  |  |  |
                    |               |  |  |  |  | 
                    |            |  |  |  |  |  |  |  
             _______|______.__|__|__|__|__|__|__|__|__|__._______
               n=948,166  -5 -4 -3 -2 -1 +0 +1 +2 +3 +4 +5
    

    Or, see the following for some slightly more esthetically pleasing diagrams of the longitudinal modes of random polarized HeNe lasers. :)

    1. Longitudinal Modes of Typical Random Polarized 1 mW HeNe Laser.
    2. Longitudinal Modes of Typical Random Polarized 3 mW HeNe Laser.
    3. Longitudinal Modes of Typical Random Polarized 8 mW HeNe Laser.
    4. Longitudinal Modes of Typical Random Polarized 30 mW HeNe Laser

    Note 1 to the purists: Due to mode competition, particularly for short lasers like the 1 mW and 3 mW, the envelope of the lasing mode power curves will not be a nice Gaussian. However, factors besides the length and/or power output affect this shape, so these idealized ones are used here. More on this below.

    Note 2 to the purists: The actual location of the lasing lines does not quite coincide with the cavity modes except at the very center of the gain curve due to a phenomenon called "mode pulling". But this is for the advanced course :-) and the offset is usually much less than 1 percent of the mode spacing so it would be almost undetectable on the scale of these diagrams.

    There is much more on both of these effects below.

    The optical frequency of each line is than n * (c/2L) and is thus inversely proportional to the mirror spacing. Very short lasers (e.g., 1 and 2) lase on very few longitudinal modes. In the case of (1), at most 2 modes; for (2), 3 modes are possible if there was one near the center of the neon gain curve. This also means that the total output power varies significantly depending on mode position - as much as 20 percent in a laser like (1).

    For longer lasers like (4), over a half dozen longitudinal modes are present at all times and the variation in output power is less than 2 percent. For very short tubes, the output power scale is different because less of the total cavity length is actual gain. For example, a 150 mm tube typically has a bore with a length of around 75 mm (1/2 the cavity length) while a 800 mm tube may have a 700 mm bore. So, there are more modes in a long tube, but also more power per mode.

    Mode Sweep

    Since the mode locations are determined by the physical spacing of the mirrors, as the tube warms up and expands, these spectral line frequencies are going to drift downward (toward longer wavelengths). However, since the reflectivity of the mirrors as a function of wavelength is quite broad (for all practical purposes, a constant), new lines will fill in from above and the overall shape of the function doesn't change.

    In the diagrams above, a single arbitrary mode position is shown, but for well behaved lasers, the lasing lines will move smoothly through the gain curve as the laser warms up. This is called by various names including "mode sweep" and "mode cycling". While present with most lasers, the effects are quite striking with low to medium power HeNe lasers due to their relatively narrow neon gain bandwidth (which is only a small multiple of the longitudinal mode spacing in low to medium power HeNe lasers), the rather fortuitous phenomenon that for red (633 nm) HeNe lasers at least, adjacent longidutinal modes tend to be othogonally polarized, and nearly ideal behavior in other respects with the Physics mostly cooperating. (Murphy has seen the LASER DANGER signs and stays away!) Much more on all this below (except perhaps for Murphy).

    In the nice diagram above :) of the 8 mW laser, there are 5 longitudinal cavity modes that see gain above the lasing threshold (the right-most just barely). These become lasing modes (red and blue) producing a total output power of somewhat over 8 mW in this specific example. For the 30 mW laser, there are twice as many lasing modes one half the distance apart, and each mode has more power. Interestingly, adjacent modes in a so-called "random polarized" red (632.8 nm) HeNe laser are almost always orthogonally polarized, with the polarization axes fixed relative to the tube. (Here, one of them is arbitrarily referenced as 0 degrees, more on this later). As the distance between the mirrors is increased, the number of oscillating modes increases as well, though the actual power in each mode increases only slightly.

    The animated Power Point shows below demonstrate the mode sweep behavior for a variety of random polarized red HeNe lasers. For all of them, the default speed is one increment per second, so the shorter lasers in the animations will take longer to complete a full cycle since the number of "phases" in the slides is larger. (The phases are shown only to be able to identify the specific slides.) This is somewhat similar to real behavior as longer tubes expand faster. The left and right arrow keys can be used to go back and forth at a much faster rate, or to "simulate" the effect of a stabilized laser control circuit keeping a mode or modes in a particular place on the neon gain curve.

    These open in a separate window and are known to work in PowerPoint 2003, 2007, and 2010:

    For the 8 mW laser, there is the animation alone in a GIF file if you're using a real computer that doesn't have PowerPoint: :) It cycles at about 1 complete mode sweep cycle per second, which is several times faster than a real HeNe laser even at startup, when it would be fastest. However, some stabilized HeNe lasers will mode sweep this fast initially due to the heater used to control cavity length. See Mode Sweep of 8 mW Random Polarized HeNe Laser.

    For very short HeNe tubes, the width of the gain curve may be similar to or even narrower than the spacing between modes. With those, the output power will become very low or go to zero during portions of the mode sweep. Very few HeNe lasers were produced with cavity lengths where this would be an issue since maximum output power would be very low. The only one I know of was the Spectra-Physics 119 stabilized laser with a 100 mm cavity length (mode spacing of 1.5 GHz). The very short cavity was required to provide special characteristics for this system.

    The effects of mode competition where multiple lasing lines are drawing from the same upper state population also become more pronounced with shorter lasers, typically under 3 or 4 mW. For those, the actual appearance may differ substantially from these somewhat idealized plots. For example, the one in the show below can be identified because of its unique shape:

    In fact, it's often possible to go so far as to identify a specific manufacturer and even model of a HeNe laser tube based solely on the plots of its polarized mode sweep, providing a sort of "fingerprint" for lasers. :) For example, the type of tube installed in a Zygo or Teletrac/Axsys stabilized laser can be determined without opening the case! This is shown for several tubes in HeNe Laser Mode Sweep Fingerprints. These tubes are all physically similar yet have dramatically different mode sweep plots. And, it's often possible to determine key information about the health of a laser tube by comparing its mode sweep with that of a new one. Over most of its life, the general shape will remain the same, but as the power declines, in addition to the total height of the plot decreasing, the amplitude of the variation (i.e., the AC component) relative to the total will increase. However, near end-of-life when power is way down and fewer modes are oscillating, the distinctions will tend to disappear.

    The effects of mode sweep are more dramatic with short low pressure carbon dioxide (CO2) lasers because for a given resonator length, the ratio of wavelengths (10,600 nm for CO2 compared to 632.8 nm for HeNe means that the longitudinal mode spacing is 16.7 times larger). In these cases, the laser output will turn on and off as it heats up and the distance between the mirrors increases due to thermal expansion. For this to happen in a 632.8 nm HeNe would require the tube to be less than about 75 mm (3 inches) in length.

    A linearly polarized HeNe laser would have the same longitudinal mode spacing, but all the lasing modes would have the same polarization orientation (red or blue) as shown in the diagrams and animations, above. As an example, see Longitudinal Modes of Typical Linearly Polarized 8 mW HeNe Laser. So, someone with red/blue color-blindness (if there is such a thing) would see the diagrams for all them as being linearly polarized!

    A label on the polarized laser will indicate the plane or orientation of polarization of the output beam. For a random polarized HeNe laser, a polarizer oriented at 45 degrees with respect to the plane of polarization would produce an output with respect to mode sweep that is similar to that of a linearly polarized laser, except that even with an ideal polarizer, the output power would be cut in half.

    Now for some actual numbers: The Doppler-broadened gain curve for neon in a red (632.8 nm) HeNe laser has a Full Width Half Maximum (FWHM, where the gain is at least half the peak value) on the order of 1.5 or 1.6 GHz. So, for a 500 mm long (high gain) tube with its mode spacing of about 300 MHz (similar to what is depicted above), 5 or 6 lines may be active simultaneously and oscillation will always be sustained (though there would be some variation in output power as various modes sweep by and compete for attention). However, for a little 10 cm tube, the mode spacing is about 1,500 MHz. If this laser were to be really unlucky (i.e., the distance between mirrors was exactly wrong) the cavity resonance might not fall in a portion of the gain curve with enough gain to even lase at all! Or, as the tube heats up and expands, the laser would go on and off. There are very few commercial HeNe laser tubes that short. It is possible to widen the gain curve somewhat by using a mixture of neon isotopes (Ne20 and Ne22) rather than a single one since the location of their peak gain differ slightly. This would allow a smaller cavity to lase reliably and/or reduce amplitude variations from mode sweeping in all size HeNe lasers. The actual lasing threshold will also determine the effective width of the neon gain curve over which lasing occurs, so it may be wider than the FWHM.

    A high speed silicon photodiode and oscilloscope or RF spectrum analyzer can be used to view the frequencies associated with the longitudinal modes of a HeNe laser. The clearest demonstration would be using a short tube where at most two longitudinal modes are active. This will result in a single difference frequency when both modes are lasing. A polarized tube is best as it forces both modes to have the same polarization as a photodiode will not detect the difference frequencies for orthogonally polarized modes. Adjacent longitudinal modes of random polarized tubes are almost always orthogonally polarized (for 633 nm HeNes at least). But, adding a polarizer at 45 degrees to the polarization axes can compensate for this with a slight loss in signal strength. Without a polarizer, the beat frequencies of a random polarized laser will tend to be at multiples of twice the mode spacing since only those modes with the same polarization orientation beat with each-other in the photodiode. (If measured very accurately, it will be seen that these frequencies will not generally be exactly at multiples of the mode spacing based on c/2L and will vary slightly during mode sweep. The is due to mode pulling or pushing effects, reserved for the advanced course!)

    Passive stabilization (using a structure made of a combination of materials with a very low or net zero coefficient of thermal expansion or a temperature regulator) or active stabilization (using optical feedback and piezo or magnetic actuators to move the mirrors, or a heating element to control the length of the entire structure) can compensate for these effects. However, the added expense is only justified for high performance lab quality lasers or industrial applications like interferometric based precision measurement systems - you won't find these enhancements on the common cheap HeNe tubes found in barcode scanners. See the section: Stabilized Single Frequency HeNe Lasers.

    Thus, a typical HeNe laser is not monochromatic though the effective spectral line width is very narrow compared to common light sources. Additional effort is needed to produce a truly monochromatic source operating in a single longitudinal mode. One way to do this is to introduce another adjustable resonator called an etalon into the beam path inside the cavity. A typical etalon consists of a clear optical plate with parallel surfaces. Partial reflections from its two surfaces make it act as a weak Fabry-Perot resonator with a set of modes of its own. Then, only modes which have the same optical frequency in both resonators will produce enough gain to sustain laser output.

    The longitudinal mode structure of an optional intra-cavity etalon might look like the following (not to scale):

                    |                  632.8 nm
                   I|      .              .              .
                    |      |              |              |
                    |      |              |              |
                    |      |              |              |
             _______|______|______________|______________|_______
               m=13,542   -1             +0             +1
    

    Notice that since the distance between the two surfaces of the etalon is much less than the distance between the main mirrors, the peaks are much further apart (even more so than shown). (The etalon's index of refraction also gets involved here but that is just a detail.) By adjusting the angle of the etalon, its peaks will shift left or right (since the effective distance between its two surfaces changes) so that one spectral line can be selected to be coincident with a peak in the main gain function. This will result in single mode operation. The side peaks of the etalon (-1, +1 and beyond) will may coincide with weak peaks in the main gain function shown above but their combined amplitude (product) is insufficient to contribute to laser output.

    This is shown, again more esthetically in Intracavity Etalon for Line Selection in a Single Mode HeNe Laser. This example is based on the same 30 mW laser as in the diagram in the section: Longitudinal Modes of Operation. Adding an etalon inside the cavity introduces an additional loss function with peaks every GHz or so. (Note that such an etalon would be about 15 cm long, so the plasma tube for this laser needs to be short enough to allow for that much space between it and one of the mirrors, but that's just a detail!) Only where the product of the original net (round trip) gain and the etalon transmission is above one will the laser lase. For this example, there is only place where a cavity mode and etalon mode conincide - just to the left of center of the neon gain curve peak. And, now that there is only a single mode oscillating, it will have an output power of over 15 mW, rather than the ~3 mW or less in each of several multiple modes. There is always some loss in adding an etalon, so the full 30+ mW originally present isn't usually possible, though the ~50 percent reduction in output power shown here may be excessive.

    (From: Prof Harvey Rutt (h.rutt@ecs.soton.ac.uk).)

    The standard, small HeNe laser normally lases on only one transition, the well known red line at about 632.8 nm.

    The HeNe gain curve is inhomogeneously Doppler-broadened with a gain bandwidth of around 1.5 GHz (at 632.8 nm). (The width of the Doppler-broadened gain curve depends on the lasing wavelength. At 3,391 nm, it is only about 310 MHz.) For a typical laser, say 30 cm long, the axial modes are separated by about 500 MHz. Typically, two or three axial modes are above threshold, in fact as the laser length drifts you typically get two modes (placed symmetrically about line centre) or three modes (one near centre, one either side) cyclically, and a slow periodic power drift results. Shorter lasers, less modes, more power variation unless stabilized. But it needs a huge HeNe laser to get ten modes, and since they are closer of course they still only spread over the 1.5 GHz line width.

    Most HeNe lasers which do not contain a Brewster window or internal Brewster plate are randomly polarized; adjacent modes tend to be of alternating orthogonal polarizations. (Note that this is not always the case and can be overridden with a transverse magnetic field, see below. See the section: Polarization of Longitudinal Modes in HeNe Lasers. --- Sam.)

    Some frequency stabilized HeNe lasers are NOT single mode, but have two, and the stabilization acts to keep them symmetrical about line centre - i.e., both are half a mode spacing off line centre. A polariser will then split off one of them or a polarizing beamsplitter will separate the two.

    (From: Sam.)

    The party line is that adjacent modes in a HeNe laser will be of orthogonal polarization. However, I've seen samples of small (e.g., 5 or 6 inch) random polarized tubes only supporting 2 active modes where this is not the case - they output a polarized beam that remains stable with warmup and in any case, applying a strong transverse magnetic field will override the natural polarization. So, it's not a strong effect. Only if everything inside the tube is reasonably symmetric, will the modes alternate. Modes may also remain one polarization as they move through part of the gain curve and then abruptly - and repeatably - flip polarization. But the majority of tubes are well behaved in this regard.

    For a tutorial on both longitudinal (axial) and spatial (transverse) modes, see An Investigation of the Cavity Modes of the HeNe Laser.

    Resonator Length and Mode Hopping

    Here are some additional comments that address the common fear of the novice laser enthusiast that the resonator length has to be stabilized to the nm or else the laser will blink off.

    (Portions from: Steve Roberts.)

    Flames expected, as I'm ignoring some of the physics and am trying to explain some of this based on what I observe, aligning and adjusting cavities on HeNe and argon ion lasers as part of repairing them. Anyone who only goes by the textbooks has missed out on the fun, obviously having never had to work on an external mirror resonator. It can be quite a education!

    Due to the complex number of possible paths down the typical gain medium, you will see lasing as long as the mirrors are reasonably aligned. The cavity spacing is not always that critical and will change anyway as the mirror mounts are adjusted (there will always be some unavoidable translation even if only the angle is supposed to be changed). No, lasers don't really flash on and off in interferometric nulls as you translate the mirrors - they instead change lasing modes. They will find another workable path. You will in some cases see this as a change in intensity but it is more properly observed on a optical spectrum analyzer as a change in mode beating. Eventually you can translate them far apart enough that lasing ceases, but this is a function of your optics not the resonator expansion.

    I have seen what you fear in some cases by adding a third mirror to a two mirror cavity with a low gain medium such as HeNe where the third mirror can be positioned in such a way to kill many possible modes. This usually occurs when I use a HeNe laser to align an argon laser's mirrors and the HeNe laser will flicker from back reflections. See the section: External Mirror Laser Cleaning and Alignment Techniques. But unless you have a extremely unstable resonator design, translation will just cause mode hopping, this becomes important on a frequency stabilized or mode locked laser if you have a precision lab application. Otherwise, most commercial lasers are not length stabilized in the least. There are equations and techniques for determining if you have a stable optical design - stable in this case meaning it will support lasing over a broad range of transverse and longitudinal modes. For examples see any text by A. E. Siegman or Koechner. If your library doesn't have any similar texts, find a book on microwave waveguides. It might aid you in visualizing what is going on.

    Either an intracavity etalon or active stabilization systems are usually used on single frequency systems anyways, by either translating the mirror on piezos or by pulling on mirror supports with small electromagnets, or in the case of smaller units, heaters to change the cavity length on internal mirror tubes. An etalon is basically a precision flat glass plate in the lasing path between the mirrors, its length is changed by a oven and it acts as a mode filter.

    Length stabilization to the 50 or 100 nm you might have expected to be needed would be gross overkill anyhow, and would be impossible to achieve in practice by stabilizing the resonator alone. Depending on the end use of the product, most lasers are simply built with a low expansion resonator of graphite composite or Invar, although in many products a simple aluminum block or L shape is used, a few rare cases use rods made of two different materials designed to compensate by one short high expansion rod moving the mirror mount in opposition to the main expansion. A small fraction of a millimeter is a more reasonable specification.

    (From: Prof Harvey Rutt (h.rutt@ecs.soton.ac.uk).)

    The basic idea, that the laser can only work at the frequencies where an integral number of half waves fit in the cavity, is perfectly correct. The separation between adjacent modes is just 1/(2*L) where L is the cavity length in cm. From this we get the separation in 'wavenumbers'. One wavenumber is 30 GHz, so in more usual units it is just 30 GHz/(2*L). Or, to make it easy, in a 50 cm long laser the modes are 300 MHz apart. That is not very far optically.

    The laser operates by some molecule, gas, ion in a crystal, etc. making a transition between two levels. But those levels are not perfectly 'sharp'; we say they are 'broadened'. The reason can be many things:

    In any case no transition is *perfectly* sharp, the fact that it has a finite lifetime gives it a certain width, but this is not often the real limit, something else is usually more important.

    These broadening mechanisms 'blur out' the line - we see optical gain over that *range* of frequencies, the gain bandwidth.

    An example is carbon dioxide. The 'natural width' is very small, of order Hz. The Doppler width at 300 °K is about 70 MHz. The collision-broadened width increases about 7 MHz/Torr; so well below 10 Torr the width is Doppler-limited, ~70 MHz; above 10 Torr pressure broadened (e.g. ~700 MHz at 100 Torr).

    If I take a typical HeNe laser it might 'blur' out over a GHz or so - **more** than that 300 MHz mode spacing - so there are *always* two or thee modes within the 'gain bandwidth' and it will always lase. For a glass laser there might be *thousands* of modes, because the glass gain is very wide indeed.

    But there *are* cases that go the other way. For carbon dioxide, at low pressure, the line is Doppler-broadened and about 70 MHz wide, much **LESS** than that 300 MHz mode spacing. So short carbon dioxide lasers really do turn on and off as the cavity length changes, and you have to 'tune' the cavity length to get a mode inside the gain width. This mainly happens with short, gas lasers in the infrared.

    For a *high pressure* CO2 laser at 760 Torr (1 atm), the line width is several GHz, much more than the mode spacing, so the effect disappears.

    Observing Longitudinal Modes of a HeNe Laser

    Monitoring the output power of any HeNe laser while it's warming up will show a variation in output power due to longitudinal mode cycling. There is even a specification called the "Mode Sweep Percentage" which indicates how large the variation is in relation to the output power. For short tubes, the power fluctuations can approach 20 percent; for long tubes, they may be less than 2 percent.

    There are many ways to actually "see" the modes of a laser including the use of an instrument called a Scanning Fabry-Perot Interferometer (see the section: Scanning Fabry-Perot Interferometers). However, for a short tube with only 1 or 2 modes, it's quite straightforward to interpret what's going on from the output power and polarization alone. All that's need is a photodiode and multimeter (or continuous reading laser power meter), and polarizing filter. (A lens from a pair of polarized Sun glasses or a photographic polarizing filter will do.) The power monitor can be set up in the output beam and the polarizing filter in the waste beam from the HR mirror. Alternatively, a non-polarizing beamsplitter can be used to provide the two beams. Adding a polarizing beamsplitter oriented so that it separates the two polarization orientations in one of the beams can simplify the interpretation of the polarization changes.

    Changing the orientation of the polarizer will affect the amplitude of the intensity variations. For most HeNe lasers, the longitudinal modes will generally remain at two fixed orthogonal orientations, with adjacent modes usually being orthogonal to each other. As the tube heats and the cavity length increases, the modes march along under the gain curve with those at one end disappearing and new ones appearing at the other end as described above. But for well behaved tubes, they don't flip polarization. When the polarizer is oriented at 45 degrees to the polarization axes of the tube, the reading will remain constant. When aligned with the polarization axes of the tube, the reading will fluctuate the most.

    As a specific example, consider an HeNe laser tube with a mirror spacing of 120 mm (about 4.75 inches, one of the shortest commercially available laser tubes). This corresponds to a mode spacing of about 1.25 GHz - rather close to the FWHM of 1.5 to 1.6 GHz for the neon gain bandwidth. With this tube, at most 2 modes will be oscillating at any given time. When the output power and polarization is monitored while the tube is warming up, a very distinctive behavior will be observed. One might think that it should be a periodic variation in output power with a simple sinusoidal or similar characteristic. However, there will actually be two peaks for each cycle: A large one corresponding to when there is a single lasing mode at the center of the gain curve, and a smaller one when there are two modes symmetric around the center of the gain curve. For most tubes, the polarization of adjacent modes is orthogonal and will remain fixed with the mode. So, as the modes cycle under the gain curve successive large peaks will have opposite polarization. The small peaks will have equal components of both polarizations. Even though two modes are oscillating, the gain for each one is so much closer to the lasing threshold that their combined power is still lower than for the single mode at the peak of the gain curve. There may also be rather sudden changes in output power as modes on the tails of the gain curve come and go. However, for some tubes which are affectionately called "flippers", the polarization of the modes will tend to suddenly change orientation as they move through the gain curve. This should also be apparent when viewing the beam through a polarizing filter.

    For more on these types of experiments along with typical plots, see the section: HeNe Laser Output Power Fluctuation During Warmup.

    Longitudinal Mode Pulling

    It turns out that most lasers don't actually oscillate on exact multiples of the cavity resonance frequency, c/2L, as stated in introductory textbooks. (The exceptions would be where the gain curve is essentially flat but that's another story.) Longitudinal modes that aren't exactly centered on the gain curve will be at frequencies very slightly offset from these, pulled toward the center of the gain curve with those that are farthest away seeing the most shift. This is a well known effect called "mode pulling" with highly developed theory to back it up. (Mode pulling isn't unique to lasers. For example, a quartz crystal oscillator can be tuned over a small range using an external capacitor even though its mechanical resonance frequency differs from the output frequency.)

    Although the math can get to be rather hairy, one way of thinking of mode pulling is that the cavity bandwidth has a finite width which depends primarily on the reflectivity of the mirrors and cavity length. So, if the net gain is greater slightly off to one side due to the position of the gain curve relative to the cavity resonance, the lasing line will be shifted in that direction.

    When the laser beam hits a high speed photodetector like a photodiode, which is a non-linear (square law) device, in addition to the DC power term, there are the primary difference frequencies which are close to multiples of c/2L (but not exactly due to mode pulling), but also the differences of the difference frequencies - the second order intermodulation products - which will be at (relatively) low frequencies compared to c/2L. As the cavity length changes and the lasing modes drift across the gain curve, the mode pulling effect on each one varies slightly. But, small differences between large numbers can result in dramatic changes in these second order terms, rapidly rising and falling in frequency, and coming and going as modes drop off one end of the gain curve and appear at the other. The amplitude of the second order beat will be much lower than that of the primary beat but is still detectable with a spectrum analyzer, or in some cases with an audio amplifier.

    For a HeNe laser, the range of second order frequencies is typically in the 1 to 100s of kHz range while for a solid state laser it will be in the MHz to 10s or 100s of MHz range. Note that there will generally not be any beat in the range from 0 Hz to some minimum frequency (e.g., 1 kHz or so in the case of the HeNe laser) as would be expected when the modes are almost symmetric on either side of the gain curve where there would be very low second order frequencies. Apparently, a self mode-locking effect occurs to force these to be exactly zero frequency over a small range of mode positions. This behavior can easily be observed in the mode beat RF spectrum of a medium power (e.g., 5 mW) HeNe laser. See the next section.

    For these second order beat frequencies to be present, the laser has to be able to oscillate on at least 3 longitudinal modes simultaneously. (With only 2 modes, there will be only a single difference frequency.) The Doppler-broadened gain curve of neon for the HeNe laser is about 1.5 GHz Full Width Half Maximum (FWHM) at 632.8 nm. To get 3 modes requires the modes to be less than about 500 MHz apart implying a c/2L tube length of about 30 cm or more - typical of a 5 mW or more (rated) HeNe laser. It should be polarized to force all modes to be of the same polarization - orthogonal polarizations do not mix in a photodetector. For a randomly polarized laser which typically produces alternating polarizations for adjacent modes, a longer tube length would be required to guarantee enough same-polarized modes and/or a polarizer at 45 degrees to the beam polarizations could be added (but this would cut the power to the photodiode by 50 percent or more).

    This effect can be demonstrated using a medium length HeNe laser, high speed photodiode, and audio amplifier. Initially when the laser is turned on and is heating up and expanding the fastest, they may sound like clicks or pops or just non-random noise. As the expansion slows down, more distinct chirps and other interesting sounds will appear. The complexity of the symphony will also depend on the tube length and thus how many modes are oscillating.

    A more precise way to look at mode pulling would be to monitor the beat frequencies produced by a high speed photodiode using an RF spectrum analyzer. By expanding the region around c/2L, the changes during mode sweep will be clearly evident. There will be smooth movement as well as sudden shifts corresponding to mode hops. I even did this by beating not a single laser, but two identical stabilized HeNe lasers against each-other. With two modes from each laser, there are then as many as 6 beat frequencies if 45 degree polarizers are placed in front of each laser and they are then combined in a non-polarizing beam-splitter. I'll leave analysis of this behavior as an exercise for the student. It is at first a bit confusing, but with some thought, makes perfect sense. Simply concentrating on the mode pulling of each laser's longitudinal mode where one laser was locked and the other was allowed to mode sweep yielded a shift of about 500 kHz.

    (From: Roithner Lasertechnik (office@roithner-laser.com).)

    You can "listen" to a single mode HeNe tube: Take an X-rated photodiode and an AC power amplifier - guide a small part of the HeNe laser beam to the photodiode (don't let it saturate!) - and listen to the "chirping oscillations" during warming up with a speaker. Hint: There are no birds inside the tube. ;-) But it sounds similar! Looks like sin(x)/x.

    Waveforms and RF Spectrum of Longitudinal Modes

    While the beam from a healthy HeNe laser appears by eye to be constant (except possibly for the normal variation in output power during mode sweep), only a single frequency laser has an output which is truly DC. With a high speed photodiode and basic test equipment, a great deal of information can be determined as a result of the interaction among the multiple longitudinal modes (also called axial modes) that are present in all but the shortest HeNe lasers (or stabilized single frequency lasers). OK, well perhaps this requires some not quite so basic test equipment like a high speed oscilloscope and/or RF spectrum analyzer. :) While these instruments may not be something you have handy, if you're friendly with someone in a research lab at a local college or university, they may have may be able to help and then everyone could learn a lot from some simple experiments! :)

    The photodiode (PD) must have a frequency response that extends beyond at least the longitudinal mode spacing of the laser. A fancy costly one may not be essential, only that the PD is quite small. One with a 1 GHz response is typically around 1 mm square, with the frequency response being roughly inversely proportional to area. Candidate PDs may turn up in all sorts of equipment, even old optical mice. The PD should be back-biased with a few volts to improve frequency response and set up to drive into a 50 ohm load terminating at the scope input. Basing the circuit on something like the Thorlabs DET10A would be perfect. (Search for this on the Thorlabs Web site. The spec sheet will have the circuit diagram.)

    The first approach is to view the resulting mode beating on a fast oscilloscope. For a random polarized laser, a linear polarizer will be required in front of the PD oriented at 45 degrees to the principle polarization axes of the laser to force adjacent modes that are usually orthogonal to have the same polarization at the PD. The adjacent longitudinal modes will then produce a beat equal to their difference frequency. There will also be weaker beats from all other combinations of modes. Common HeNe lasers have a fundamental mode spacing of between 1.5 GHz (for a tiny 0.5 mW barcode scanner tube, around 10 cm between mirrors) and 161 MHz (for a 35 mW SP-127, around 95 cm between mirrors). Typical Longitudinal Mode Beat Waveforms of Melles Griot 05-LHP-151 HeNe Laser shows some snapshots during mode sweep. This laser is rated 5 mW with a mode spacing of 438 MHz (around 58 cm between mirrors). The waveforms were taken using a Thorlabs DET210 photodetector and my special edition laser-zapped Tektronix 2467 oscilloscope - formerly resident in the test department of a major laser manufacturer - evident from the 5 unsightly black blobs on the lower part of the screen where the CRT phosphor has been blown away by a high power pulsed laser! :) While the fundamental can usually be seen, information about any higher difference frequencies is hard to interpret. And even this relatively fast scope doesn't have much sensitivity beyond the 438 MHz fundamental. The screen shots are in no particular order in the montage other than to make the sequence somewhat pleasing. :) This is further complicated by higher order effects like mode pulling, which slightly shift the positions of the modes based on their location relative to the center of the neon gain curve. Thus, beyond confirming that the mode spacing is as expected, not much more can be easily determined and switching to the frequency domain will be more fruitful.

    The output from the PD may also be applied to an RF spectrum analyzer, there will be significant power detected at the longitudinal mode spacing and its harmonics (hundreds of MHz or more) due to beating between longitudinal modes, as well as under 1 MHz (due to second order beats and mode pulling).

    RF Spectra of Melles Griot 05-LHP-151 HeNe Laser During Mode Sweep shows the primary beat signal for the same laser head using a Thorlabs DET210 1 GHz silicon photodetector and an HP 8590L RF spectrum analyzer. (As with the scope, for a random polarized laser, a polarizer would need to be placed in front of the PD oriented at 45 degrees to the polarization axes to detect adjacent beats.) The center frequency is around 437 MHz and the span is 1 MHz. (Each box is 100 kHz.) (The spec'd value for the mode spacing of this laser is 438 MHz but it's possible the spectrum nalyzer is in need of calibration! Otherwise, complain to Melles Griot!) The sequence of screen shots show about half the full mode sweep cycle. Load the Movie Clip of RF Spectra of 5 mW HeNe Laser During Mode Sweep into Windows Media Player in repeat mode and it will look exactly as it does in real life. :) As can be seen, the spectrum goes from being nearly a pure single frequency at the longitudinal mode spacing, to a series of many peaks 10s of kHz apart to 4 peaks almost 200 kHz apart. The displayed width of each peak is much larger than it actually is, an artifact of the spectrum analyzer bandwidth setting.

    If there were no mode pulling, the display would always look like the one in the upper left corner (or even narrower) - a single frequency. However, the individual modes move slightly compared to the cavity resonances, so the spectrum spreads out as a function of the position of the modes on the neon gain curve.

    Interestingly, the display remains where there is a single narrow peak for longer than could be accounted for based on the normal speed with which the frequencies are changing. In fact, it's impossible to capture a situation where the peak is just slightly wider - it snaps from a FWHM of about 1/5 box (top left in composite photo) to approximately 1 box (top center) and vice-versa. Nothing in between ever apperas. This suggests that there is a self-locking process taking place, as mentioned in the previous section.

    When set for a frequency range covering 0 to 200 kHz, peaks are present similar to what appears on the right side of those shown above. But a linear HeNe laser power supply had to be used to avoid seeing the ripple frequency and harmonics of the switchmode brick overlayed on the beats! There are multiple strong beats at around 874 MHz as well, 2 times the mode spacing. They vary a way similar way as the others. This makes sense since there are are 3 longitudinal modes oscillating most of the time, with 4 modes for a brief period during mode sweep. The spectrum analyzer also claims there are weak peaks at around 1,311 MHz and 1,748 MHz during most of the mode sweep cycle, not simply that period where the self mode-locking takes place. However, it's not clear where these originate, or if they are even real. To be direct, the one at 1,748 MHz would require 5 modes to produce a beat at 4 times the mode spacing of 437 MHz. But there are never 5 modes present, let alone for most of the cycle. Perhaps they are the sum of second-order beats. Or, they could simply be an artifact of the analyzer, perhaps leakage from an internal mixer.

    Typical Longitudinal Mode Beat Waveforms of JDS Uniphase 1145P HeNe Laser shows scope display for a JDS Uniphase 1145P, with a mode spacing of 438 MHz (around 34 cm between mirrors). The additional complexity is due both to the lower beat frequencies (and thus better response of the scope) as well as the greater number of modes oscillating. An RF spectrum of this laser would have many more peaks closer together, but would look generally similar to that of the 5 mW laser.

    Typical Longitudinal Mode Beat Waveforms of Melles Griot 05-LHP-121 HeNe Laser, with a mode spacing of 687 MHz (around 22 cm between mirrors). With only 3 longitudinal modes oscillating (and stressing the bandwidth capabilities of the poor scope), the display is a fairly clean sinewave. The only obvious difference during mode sweep is that the amplitude changes slightly. However, most of the time, it is a relatively clean sinewave and for a higher power healthier tube, the reduction in amplitude is not that great as in this example.

    Transverse Modes of Operation

    Lasers can also operate in various transverse modes. Laser specifications will usually refer to the TEM00 mode. This means "Transverse Electromagnetic Mode 0,0" and results in a single beam. The long narrow bore of a typical HeNe laser forces this mode of oscillation. With a wide bore multiple sub-beams can emerge from the same cavity in two dimensions. The TEM mode numbers (TEMxy) denote the number (minus one) or configuration of the sub-beams.

    Here is a rough idea of what transverse modes might look like for a rectangular cavity:

    
                            O        OO        OOO      Each 'O' represents
         O        OO        O        OO        OOO       a single sub-beam.
    
       TEM00     TEM10    TEM01    TEM11      TEM21
    
    

    I have only shown the rectangular case because that's the only one I could draw in ASCII!

    Other (non-cartesian) patterns of modes will be produced depending on bore configuration, dimensions, and operating conditions. These may have TEMxy coordinates in cylindrical space (radial/angular), or a mixture of rectangular and cylindrical modes, or something else!

    To achieve high power from a HeNe laser, the tube may be designed with a wider but shorter bore which results in transverse multimode output. Since these tubes can be smaller for a given output power, they may also be somewhat less expensive than a similar power TEM00 type. As a source of bright light - for laser shows, for example - such a laser may be acceptable. However, the lower beam quality makes them unsuitable for holography or most serious optical experimentation or research. An example of a high power multimode HeNe laser head is the Melles Griot 05-LHR-831 which has a rated output power of 25 mW. Compared to their 05-LHR-827 which is a 25 mW TEM00 laser head, the multimode laser is about 2/3rds of the length and runs on about 3/5ths of the operating voltage at lower current.

    (Note that it is easy in principle to convert the output of a TEM00 laser into multimode by using a length of fiber-optic cable with lenses at each end to focus the beam into it and collimate the beam coming out. If the core diameter of the fiber is greater than that needed for the fiber itself to be single mode, then the result will be that multiple modes will propagate inside and the output will be multimode. To assure single mode propagation at 632.8 nm with the index of refraction of a typical glass fiber, a 4 um or smaller core is needed. The actual core diameter of the fiber will determine how many modes are actually generated. A core diameter of 10 um will result in a few modes while one of 125 um will produce dozens of modes. Why this would be desired is another matter.) However, all these modes will be exactly the same wavelength since they originate from a single TEM00 beam.

    Sometimes, laser companies don't quite get it right either and a laser tube that is supposed to be TEM00 may actually be multi-transverse mode all the time or whenever it feels like it (e.g., after warmup). I have a 13.5 mW Aerotech tube that is supposed to be TEM00 but produces a beam that has an outer torus (doughnut shape) with a bright spot in the middle. I've also seen an apparently factory-new Uniphase green HeNe laser that produces a similar doughnut beam. Both of these are probably the result of one or both mirrors having a radius of curvature that is too short for the bore diameter. They may have been manufacturing goofups. Everyone can have a bad day, even if it results in a bunch of dud lasers. :) Good for us though. Everyone (well everyone who cares!) has seen a nice TEM00 HeNe laser. How many have one that does three wavelengths with different mode structures! :) (See the section: The Weird Three-Color PMS HeNe Laser Head.)

    Note that the mode structure implies nothing about the polarization of the beam. Single mode (TEM00) and multimode lasers can be either linearly polarized or randomly polarized depending on the design and for the multimode case, each sub-mode can have its own polarization characteristics. HeNe (and other) lasers will be linearly polarized if there is a Brewster window or Brewster plate inside the cavity. The majority of HeNe laser tubes produce a TEM00 beam which has random polarization. For internal mirror tubes, linear polarization may be an extra cost option. External mirror HeNe lasers also generally produce a TEM00 beam but are linearly polarized since the ends of the tube are terminated with Brewster windows.

    A fast photodiode (PD) and oscilloscope or RF spectrum analyzer can be used to view the frequencies associated with transverse modes. The transverse difference frequencies are very low compared to the longitudinal mode spacing so a really high speed PD isn't needed. A response of a few MHz should be sufficient. Typically less than 2 mm square silicon PD will have an adequate frequency response if back biased. But the modes do have to overlap on the detector so it may be necessary to spread the beam of a multimode HeNe laser using a lens. A polarized tube is best as it forces the modes to have the same polarization (a PD will not detect the difference frequencies for orthogonally polarized modes). But, adding a polarizer can partially compensate for this, though the polarization may drift with a randomly polarized laser.

    For a tutorial on both longitudinal (axial) and spatial (transverse) modes, see An Investigation of the Cavity Modes of the HeNe Laser.

    Multi-Transverse Mode HeNe Lasers

    As noted, most HeNe lasers are designed to operate with a single transverse (spatial) mode or TEM00. However, to obtain the highest power for a given tube size or by a goof-up in design, a higher order mode structure may be produced. A non-TEM00 mode may be present if:

    All of these are really somewhat equivalent and simply mean that more than one mode fits inside the available active mode volume.

    Note that a speck of dirt or dust on the inside of a mirror or window (if present), or damage to an optical surface, can result in a multi-transverse mode beam even if the bore and mirror parameters are correct for TEM00 operation. Unfortunately, convincing a bit of dust to move out of the way isn't always easy on the inside of an internal mirror HeNe laser tube! Yes, though not common, it can happen. This is one reason not to store tubes vertically. I've heard of people successfully using a Tesla (Oudin) coil to charge up the errant dust particle, causing it to just out of the way via electrostatic repulsion. Your mileage may vary. :)

    Coherence Length of HeNe Lasers

    Common HeNe lasers have a coherence length of around 10 to 30 cm. By adding an etalon inside the cavity to suppress all but one longitudinal mode, coherences lengths of 100s of meters are possible. Naturally, such HeNe lasers are much more expensive and are more likely to be found in optics research labs - not mass produced applications.

    The following actually applies to all lasers using Fabry-Perot cavities operating with multiple longitudinal modes. It was in response to the question: "Why does the coherence length of a HeNe laser tend to be about the same as the tube length?"

    (From: Mattias Pierrou).

    In a HeNe laser you typically have only a few (but more than one) longitudinal modes. These cavity modes must fulfill the standing-wave criterion which states that must be an integer number of half wavelengths between the mirrors. In the frequency domain this means that the 'distance' between two modes is delta nu = c/(2L), where L is the length of the laser.

    The beat frequency between the modes gives rise to a periodic variation in the temporal coherence with period 2L/c, i.e. full coherence is obtained between two beams with a path-difference of an n*2L (n integer).

    If you have only one frequency, the coherence length is infinite (that is, if you neglect the spectral width of this mode which otherwise limit the coherence length). If you have two modes, the coherence varies harmonically (like a sinus curve).

    The more modes you have in the laser, the shorter is the regions (path-length differences) of good coherence, but the period is still the same.

    You can try this by setting up a Michelson interferometer and start with equal arm-lengths which of course gives good coherence. Then increase the length of one arm until the visibility of the fringes disappear. This should occur for a path-difference slightly less than 2L (remember that the path-difference is twice the arm-length difference!). If there are only two modes is the laser the zero visibility of fringes should occur at exactly 2L. Now continue to increase the path-difference until you reach 4L (arm-length difference of 2L). You should again see the fringes clearly due to the restored coherence between the beams.

    What is Mode Locking?

    The normal output of a HeNe or other CW laser is a more or less constant intensity beam. Although there may be long term variations in output power as well as short term optical noise and ripple from the power supply, these are small compated to the average intentsity. Mode locking is a technique which converts this CW beam to a periodic series of very short pulses with a length anywhere from picoseconds to a fraction of a nanosecond. The separation of the pulses is equal to the time required for light to make one round trip around the laser cavity and the pulse repetition rate (PRF) will then be: c/(2*l). For example, a laser resonator with a distance of 30 cm (1 foot) between mirrors, would have a mode locked PRF of about 500 MHz.

    Mode locking is implemented by mounting one of the mirrors of the laser cavity on a piezo-electric or magnetic driver controlled by a feedback loop which phase locks it with respect to the optically sensed output beam.

    Without mode locking, all the modes oscillate independently of one another with random phases. However, with the mode locked laser, all the cavity modes are forced to be in phase at one point within the cavity. The constructive interference at this point produces a short duration, high power pulse. Destructive interference produces a power of almost zero at all other points within the cavity. The mode locked pulse then bounces between the two laser mirrors, and a portion passes through the output coupler on each pass.

    As a practical matter, you probably won't run into a mode locked HeNe laser at a garage sale!

    Cavity Dumped Pulsed HeNe Laser

    Here's another one that won't turn up at a swap meet. Inside the cavity of a typical HeNe laser, the circulating power is 50 to 1,000 times the output power. If only all those photons could be accessed! Well, it turns out there is a way to do this sort of, at least in principle and for a short time. It's called "cavity dumping". The idea is use a high speed optical switch to briefly divert the Intra-Cavity (IC) beam outside the cavity. This sounds simple, right? :) There are just a couple of problems. With the low gain of the typical HeNe laser, any optics inside the cavity has to either be at the Brewster angle or have very good AR-coated surfaces so as not to significantly impact circulating power, or kill lasing entirely. And since any practical HeNe laser is limited in size perhaps 2 meters at most, the switching has to take place in nanoseconds to get any significant fraction of the IC power to exit the laser.

    The optical switch is the key to making this work. Either it has to actually deflect the beam or change its polarization so some other optical element will then reflect it out of the cavity. There are devices like Kerr cells and Pockels cells that could potentially be fast enough but they require high voltage to operate and may have excessive losses. Other approaches use an Acousto-Optic Modulator (AOM) as a deflector to divert the IC beam just enough to be reflected out of the cavity. However, AOMs don't operate instantaneously since they depend on a high frequency acoustic wave to propagate in their crystal. Even a high performance AOM would require 10s or even 100s of nanoseconds to switch states.

    I'm sure a literature search would turn up some moldy papers describing the cavity dumping technique with a HeNe laser, but a "proof of concept" experiment was performed recently by Kevin Zheng, a very talented high school student, while at the Stony Brook Laser Teaching Center. See Kevin's Research Journal and Poster. While the performance wasn't that fabulous (forget any ideas of a hole burning HeNe laser!), just being able to get this work at all with relatively limited resources is impressive.

    HeNe Laser Output Power Fluctuation During Warmup

    While not generally visible by eye alone except possibly for very short or tired (low gain) HeNe lasers, there is a quasi-periodic variation of output power with time. For the typical HeNe laser tube shortly after turn-on, the frequency is quite rapid (a cycle every few seconds) and gradually slows down as the tube temperature reaches a steady state value (after a half hour or more).

    Note that while the frequency of the power variations in output power of a HeNe laser goes to beyond the GHz range, the following deals with what can be seen by human eyeballs with the aid of only a photodiode and multimeter or chart recorder (or a PC with a data aquisition module).

    Thanks to Ryan Haanappel, here is a plot of the measured output power of a typical HeNe laser tube from power-on to 20 minutes: Typical HeNe Laser Output Power Versus Time During Warmup. More plots and photos can be found on Ryan's HeNe Lasers Experience Page, and later in this section.

    Examining the actual plot of output power versus time such as shown in HeNe Laser Output Power Fluctuation During Warmup (or careful observation of laser power meter readings) of a HeNe laser reveals that the curve is not simple but may include several types of behavior:

    Goofups in design and manufacturering can result in various combinations of these and other effects, though for the most part, HeNe laser companies generally know what they are doing! :) But see the plots below for both normal and abnormal behavior, and a link near the end of the section for a case study of one dramatic example of an "oops". :)

    Plots of HeNe Laser Power Output and Polarized Modes During Warmup

    Here are some plots of power output versus time for a variety of typical HeNe laser tubes and heads from nearly the shortest available through mid-size. (For longer tubes, the appearance will be very similar, but with even a smaller short term fluctuation in power.) The shape of the plots is mostly the result of what's called "mode sweep" or "mode cycling" as the longitudinal modes of the laser move with respect to the neon gain curve due to thermal expansion of the laser cavity. However, where the plot covers a long time (e.g., most of the warmup period), there will generally be an increasing trend in output power due to other factors as noted above.

    Most of these are from Melles Griot but the behavior of lasers from other manufacturers will be relatively similar, though the detailed shape of the individual polarized modes (more below) can differ significantly. The majority are healthy samples but a few show some rather dramatic peculiarities. There are also plots of a Coherent model 200 and Hewlett Packard model 5517A frequency stabilized HeNe lasers from power-on to locking.

    Plots such as these are almost like fingerprints for HeNe lasers. Many of the physical characteristics of the laser can be determined by their appearance, and some features are unique to a particular model or manufacturer.

    For most of the plots, my "instrumentation" consisted of a pair of $2 photodiode feeding two of the analog inputs of a DATAQ RS232 Chart Recorder Starter Kit attached to my ancient 486DX-75 Kiwi laptop running Win95. The photodiodes are reverse biased by 30 VDC from a +/-15 VDC power supply with a variable load resistor to set the calibration. The output is taken between the junction of the resistor and the photodiode, and power supply common (0 VDC). One channel is shown below:

                   R1     PD1
     +15 VDC o----/\/\----|<|----+
                  100            |
                                 /
                                 \<----------+----+---o A/D Input (+/-10 V range)
                                 / R2        |    |
                                 \ 25K       |    /
                    R3           |       C1 _|_   \ 200K ohms (Zin of A/D module)
     -15 VDC o-----/\/\----------+      1uF ---   / 
                   68K                       |    \
                                             |    |
       0 VDC o-------------------------------+----+----o A/D Ground
    

    The values shown were selected for lasers with a maximum power output of around 1 mW. For higher power lasers, R2+R3 can be decreased or an attenuation filter can be placed in the beam. The later is preferred to avoid shifting the 0 mW reference level, and is what I did for most of the plots.

    The capacitor across the input is intended to minimize noise pickup. The resulting filter rolls off at around 0.1 Hz. For reasonably well behaved HeNe lasers, even during the initial warmup period, this bandwidth is more than adequate. The sampling rate for all the plots is at least 10 Hz to allow for averaging since the A/D seems to have an uncertainty of about 2 LSBs.

    In most cases, the two photodiodes are positioned at the outputs of a Polarizing Beam-Splitter (PBS) cube and the laser tube is oriented so that they are aligned with its natural polarization axes.

    For monitoring power from the waste beam (which is much lower), a dedicated beam sampler assembly was constructed, which along with a photodiode preamp, enabled power levels as low as a few uW to fully utilize the 20 V p-p range of the A/D.

    Some of the plots have been acquired with the same photodiodes, but feeding a dual channel preamp and also a summer tp compute the total power without requiring a separate photodiode channel. (Of course, for this to be meaningful, the photodiodes and premaps have to be set up so the two channels have equal gain.) The premap results in lower noise in the plots expecially for low power lasers.

    And as of 2011, I've "upgraded" to USB versions of the DATAQ device (DI-158U and DI-145) and laptops that are only 10 years old. :) The original DATAQ RS232 module died and the Kiwi laptop doesn't have USB and is falling apart (though is still usable). The PDs are now each attached to a general purpose trans-impedance amp rather than the simple bias network. [See the section: Sam's Photodiode Preamp 1 (SG-PP1).] And in addition to channels 1 and 2, the outputs also feed a pair of resistors and a pot to adjust balance to channel 3 so their sum (which would usually be total power) is always present.

    Although some of these plots aren't as nicely annotated as the one above, zero power is near the bottom of the plot so relative power variations can still be easily seen (who cares about absolute power anyhow!) and the time/division is indicated. The plots are arranged by increasing laser tube length.

    For the following, "Total" means all the power in the beam; "Polarized" means a polarizing beam-splitter is used to separate the two orthogonally polarized modes with either one or both plotted. (This is Only done for random polarized lasers.) The scale factor for the "polarized" plot has been adjusted so that the peak amplitude is approximately the same as for the "total" plot for ease of viewing. However, it should be understood, that the sum of the power in the two orthogonal polarizations must add up to the total power. All are red (632.8 nm) HeNe lasers unless otherwise noted.

    Note: I have "edited" (doctored?) some of the plots to clean up unsightly randomness and other blemishes, mostly due excessive electrical noise at low optical power levels. However, the important features are unchanged.

    These have all been high quality HeNe lasers and except as noted, have relatively predictable mode performance. For information and plots for a really ill-mannered beast, see the section: Far East HeNe Laser Tubes 1.

    Mode Competition in Short HeNe Lasers

    If you haven't been wondering why some of the output power plots are so strange, you should be. :)

    The primary reason that the output power in any give longitudinal mode doesn't vary in a nice smooth (Gaussian) manner is due to mode competition. If not for mode competition, the gain would not saturate and be the same for all modes. Everyone would thus trace out the envelope of the neon gain curve. However, since the lasing modes are actually competing for a limited resource - the atoms in the upper lasing state - whenever there are more than one mode present, they have to be nice and share. This is most dramatic when only 2 or 3 modes are present since each one has a large fraction of the total output power. With those, the shapes of the envelopes of the polarized output power curves can be decidedly non-Gaussian. And for Zeeman-split lasers, downright weird. But once the various regions are understood - where there are 1, 2, 3, or more modes competing - then the resulting shapes make more sense:

    1 mode: The output power will change smoothly during mode sweep roughly following the profile of the Gaussian neon gain curve (minus the lasing threshold). The only way a real laser could be single mode throughout mode sweep would be either for the cavity to be around 10 cm or less (in which case lasing may cease entirely for a part of mode sweep) or for there to be an additional means of forcing SLM operation (such as an etalon inside the cavity). But slightly longer tubes will operate with a Single mode over a portion of mode sweep with 2 modes for the remainder.

    Plot of Mode Sweep of Typical 1 mW Random Polarized HeNe Laser Tube shows the appearance for a Melles Griot 05-LHR-007, the shortest modern laser tube I'm aware of. Over approximately 50 percent of the mode sweep cycle, it is pure single mode with power sharing during the remainder.

    2 modes: When a second mode appears, it will start eating into the power of the first mode. Where the modes are balanced on either side of the neon gain curve, their power will be equal. Between these 2 points, they will share power. The total output power may remain relatively constant or increase slightly when equal (usually up to around 20 percent). Tubes with a cavity length of 12 to 16 cm will operate with 1 or 2 modes.

    3 modes: When a third mode appears, it will start eating into the power of the other two. The relative and total power will depend on their location on the neon gain curve and is at the very least, not intuitively predictable. :) Tubes with a cavity length of 20 to 25 cm will operate with 2 or 3 modes during mode sweep.

    Plot of Mode Sweep of Typical 3 mW Random Polarized HeNe Laser Tube shows the appearance for a Spectra-Physics 088 (same as the Melles Griot 05-LHR-088) used in the SP-117/A/B/C and Melles Griot 05-STP-901 stabilized lasers. It is similar a common barcode scanner tube. At the peaks of the polarized modes (minimum for total power), there are 2 modes. Where the polarized modes cross, there are 3 modes. The overall shape of the mode sweep depends on many factors including the exact length of the cavity which determines where it switches from 2 to 3 modes.

    4 or more modes: The same general rules apply, but since the contribution of each mode is smaller, the effects of mode competition are also smaller and more difficult to see and interpret.

    Inhomogeneous Broadening in Neon and Mode Sweep

    The shape of the neon gain curve is by now familiar, but what does it really mean? The popular notion of it being the result of some magical process is fine as a first step, but doesn't help in attempting to understand how it is affected by wavelength, or for explaining phenomena like the Lamb Dip.

    What is really being depicted in the gain curve is a combination of a curve derived from what's called the "natural line width of neon" which is homogeneously broadened, and the distribution of atomic velocities of excited neon atoms as they translate into a distribution of Doppler shifts in optical frequency.

    Ignoring Special Relativity (which is acceptable for the velocities involved), the Doppler shift in optical frequency is equal to the relative velocity of the excited atom divided by the speed of light multiplied by the optical frequency or:

                  va
      Δf = -f0 * ----
                  c
    

    Where:

    At any temperature above absolute zero, all atoms are in motion and have a probabilistic distribution of velocities (speed and direction), which all contribute to the Doppler broadening. For a Fabry-Perot (linear) cavity, the photons traveling in either direction "experience" the relative speed of the excited atoms. Stimulated emission will only occur when the Doppler-shifted energy of the photon matches a possible lasing transition of an excited atom. The width of the Doppler broadening is directly proportional to optical frequency, but it is also affected by other factors including temperature and pressure, since these impact the distribution of atomic velocities. The shape of the Doppler broadening curve is then the result of the aggregate of the motion of all the atoms available for stimulated emission. And the width of the inhomogeneously broadened neon gain curve is the width due to homogeneous line-width of neon plus the inhomogeneous Doppler broadening. Since they are added like independent noise souces using the square-root of the sum of the squares, the increase in neon gain bandwidth due to the homogeneous line-width is quite small (just over 5 percent even at 3,391 nm). Thus, the change is close to 1/5th even if the homogeneous part is ignored.

    Assuming the FWHM value of 1.6 GHz for the entire inhomogeneously Doppler-broadened gain bandwidth of the common red wavelength of 632.8 nm, at the mid-IR wavelength of 3,391 nm it is only 315 MHz. And at the green wavelength of 543.5 nm it is about 1.86 GHz. The optical frequency difference between cavity modes (c/2L) is only dependent on cavity length and the speed of light. Thus, the number of lasing modes possible for a given cavity length decreases as the gain bandwidth becomes narrower at longer wavelengths.

    Note that the lasing modes themselves will have a very narrow bandwidth - possibly as small as 5 kHz or even lower for a laser operating with a single mode. At that point, physical vibrations, laser power supply noise, and other external effects are the limiting factors, not the theoretical minimum bandwidth for the HeNe laser which is under 1 Hz! (Schawlow-Townes linewidth). I originally thought that finding values for the bandwidth of commercial HeNe lasers would be straightforward, but it seems to be near impossible. The only specifications I am aware of from a laser manufacturer are in Laboratory for Science brochures. The best is for their model 220 Ultra Stable HeNe laser, which lists 5 kHz over a period of 1 second. But the value for the type of HeNe laser tube that used to be found in barcode scanners may not be all that much greater if it is mounted to minimize vibrations and driven with a well filtered HeNe laser power supply.

    One would expect that with the much smaller gain bandwidth at 3,391 nm of 315 MHz, there would be fewer longitudinal modes oscillating compared to 632.8 nm. Or equivalently, a laser tube would need to be much longer for the same number of modes to fit within the FWHM of 315 MHz. But because of the very high gain at 3,391 nm, the lasing threshold will be lower and thus the effective gain bandwidth of the neon gain curve is going to be wider. I do not know by how much, but with a potential gain over 40 times that of the 632.8 nm transition, it could be very significant. There might even be more modes than at 632.8 nm.

    Due to the longer wavelength, mode sweep for a laser tube at 3,391 nm will have a complete cycle that is over 5 times as long as one at 632.8 nm. These same numbers would apply to mode competition at 3,391 nm interfering and stealing power from a 632.8 nm laser.

    Number of Longitudinal Modes at Other HeNe Wavelengths

    As described above, the gain bandwidth of neon is roughly inversely proportional to the wavelength (or proportion to the frequency) of the lasing transition. However, this assumes that the lasing threshold is at the same location relative to the peak of the neon gain curve, often specified as the Full Width Half Maximum or FWHM. At 632.8 nm, this turns out (not coincidentally!) to be reasonable and results in the expected number of lasing modes and mode sweep plots to go along with them.

    For very low gain wavelengths like green (543.5 nm) and yellow (594.1 nm) - which may have 1/10th the gain or less compared to the common red (632.8 nm) wavelength, the lasing threshold will be far higher on the roughly Gaussian shaped gain curve, where it is narrower. So, while the FWHM of the neon gain curve may be slightly wider at these wavelengths, fewer modes will be oscillating because of the narrowing due to the higher lasing threshold. However, until the lasing threshold approaches the peak of the gain curve, the reduction in number of modes won't be that dramatic. And every effort is made to eliminate losses inside the cavity for these low gain lasers, so in fact, the lasing threshold may not even get that high relative to the peak during the expected life of the laser.

    For very high gain wavelengths, the reverse will happen. There's really only one - the mid-IR transition at 3,391 nm which behaves more like a solid state laser with a gain over 40 times that of 632.8 nm. The lasing threshold will be much lower on the gain curve extending the useful region well out into the tails of the distribution. In this situation, many more modes could end up oscillating than would be accounted for by the much narrower FWHM of the neon gain curve of 315 MHz - roughly 1/5th the width compared to 632.8 nm. If calculations based solely on this small gain bandwidth were valid, a 75 cm 3,391 nm laser would have a similar number of longitudinal modes to a 14 cm 632.8 nm HeNe where there are only 1 or 2 active modes at any given time. Since 3,391 nm lasers much shorter than 75 cm are commercially available and don't have dramatic variations in output power with mode sweep, this must not be the case. For example, REO has one with a cavity length of less than 50 cm and maximum power variation of 5 percent, which implies that there are several longitudinal modes always present.

    Here are results so far:

    Intensity Stability of HeNe Lasers

    There are at least three kinds of intensity variations present with HeNe (or other gas) lasers: long term as various longitudinal modes compete for attention, short term due power supply ripple or discharge instability, and beat frequencies between modes that are active.

    Common internal mirror HeNe laser tubes include a specification called "Mode Cycling Percent" or something similar. This relates to the amount of intensity variation resulting from changes in longitudinal modes due to thermal expansion. Typical values range from 20 percent for a small (e.g., 6 inch, 1 mW) tube to 2 percent or less for a long (e.g., 15 inch, 10 mW) tube. These take place over the course of a few seconds or minutes and are very obvious using any sort of laser power meter or optical sensor. Even the unaided eyeball may detect a 20 percent change. The more modes that can be active simulataneously, the closer those that are active can be to the same output power on the gain curve. Very short tubes or those with low gain (other wavelengths than 632.8 nm or due to age/use or poor design) may vary widely in output intensity or even cycle on and off due to mode cycling. (Note that since the polarization for each mode may be different, reflecting the beam of one of these HeNe lasers from a non-metallic reflective surface (which acts somewhat as a polarizaer) can result in a large variation in brightness as the dominant polarization changes orientation over time.) Trading off between tube size and mode cycling intensity variations is one reason that HeNe tubes with otherwise similar power output and beam characteristics come in various lengths.

    There are also stabilized HeNe lasers which use optical feedback to maintain the output intensity with a less than 1 percent variation. (They usually also have a frequency stabilized mode but can't do both at the same time.) An alternative to doing it in the laser is to have an external AO modulator or other type of variable attenuator in a feedback loop monitoring optical output power. See the next section for more info.

    Short term changes in intensity may result from power supply ripple and would thus be at the frequency related to the power line or inverter. These can be minimized with careful power supply design.

    Intensity variations at 100s of MHz or GHz rates result from beats between the various longitudinal modes that may be simultaneously active in the cavity. For most common applications, these can be ignored since they will be removed by typical sensor systems unless designed specifically to respond to these high beat frequencies.

    Also see the section: Amplitude Noise.

    Stabilized Single Frequency HeNe Lasers

    The common red (633 nm) HeNe laser, while highly monochromatic, generally does not produce just a single frequency (or equivalently, wavelength) of light. As noted in the section: Longitudinal Modes of Operation, several closely spaced frequencies will generally be active at the same time and their precise values and intensities will change over time. For many applications, this doesn't matter. However, for others, it makes such a laser useless.

    If you have, say, $5,000 to spend, you can buy a red (633 nm) HeNe laser that actually produces a single frequency with specifications guaranteed stable for days and that don't change over a wide temperature range. While the operation of such a HeNe laser is basically the same as the one in a barcode scanner (and in fact may use the identical model HeNe laser tube!), several additional enhancements are needed to eliminate mode sweep and select a single output frequency. Simply constructing the laser cavity of low thermal expansion materials isn't enough when dealing with distances on the order of a fraction of a wavelength of light! Active feedback is needed. The most common implementation of these lasers starts with a short red (632.8 nm) tube that can only oscillate on at most 3 longitudinal modes. (For technical reasons, stabilized lasers at the other common visible and IR HeNe wavelengths are more difficult to implement and are much less common. More on this below.) It then adds optical feedback to keep them in a fixed location on the HeNe gain curve by precisely adjusting the distance between the mirrors over a range of about 1/2 the lasing wavelength. This is most often done with a heating coil (inside or outside the tube), but a PieZo Transducer (PZT, an expensive version of the beeper element in a digital watch) may also be used. The PZT reduces the time for the system to stabilize to a few seconds, compared to up to 30 minutes for the heater. But, for a laser that will be left on continuously, this probably doesn't matter. Some lasers use a means of cooling in addition to the heater like a piezo fan, probably to allow the laser to run stably over a wider temperature range. And a few including the Melles Griot 05-STP-909/910/911/912 (originally based on the Aerotech Syncrolase 100) use a miniature RF induction heater surrounding the HR mirror mount to control only its length, not that of the entire tube. With direct heating of such a small mass, the response is quite fast. This also makes for a more compact package than a full tube heater.

    Many schemes work well and it's amazing how dirt simple these really are considering their hefty price tags! It's easy to build perfectly usable systems from a common surplus HeNe laser tube and a few common junk parts.

    The common ones are listed below:

      Type of Stabilization Technique                  Variation  Precision
     -----------------------------------------------------------------------
      Normal (multimode) HeNe laser                       ---        ---
      Single mode without stabilization                 1.5 GHz     3x10-6
      Single mode amplitude stabilization                10 MHz     2x10-8
      Lamb dip stabilization                              5 MHz     1x10-8
      Gain peak stabilization                             5 MHz     1x10-8
      Dual mode polarization stabilization                1 MHz     2x10-9
      Second order beat stabilization                   200 kHz     4x10-10
      Zeeman beat frequency stabilization               100 kHz     2x10-10
      External reference (iodine) cell stabilization     <5 kHz     1x10-11
      External reference (F-P resonator) stabilization   <1 Hz      1x10-14
    

    Note that an etalon inside the laser cavity could also be used to select out a single longitudinal mode. For high power lasers which would require long tubes supporting many modes, this would be needed with both the overall mirror spacing and etalon being feedback controlled. But for low power lasers (e.g. 1 to 3 mW), the use of a short tube to limit the number of modes in conjunction with basic feedback control is a much less complex lower cost approach.

    Stabilized lasers (or anything that needs to be regulated to some precision) can be classified as two types. The technique is "intrinsic" - basically derived from an internal reference - if what is used to regulate the device is a fundamental property of its construction - the laser physics in this case. It is "extrinsic" if some external reference is used. Most commercial stabilized HeNe lasers are of the first type since they exploit the known and essentially fixed frequency/wavelength and shape of the neon gain curve in the E/M spectrum. Additional techniques may be used to further reduce the uncertainty.

    Most common commercial stabilized HeNe lasers are red at 633 nm, partially because of all the available HeNe wavelengths with a single frequency output power of less than 2 mW. Systems like this are both relatively easy to implement and generally useful for a wide range of applications. The approaches usually fall into one of two subclasses:

    1. One or Two Mode stabilized systems: These use random polarized HeNe laser tubes that are short enough that only a few modes will oscillate at the same time. Adjacent modes of a random polarized HeNe laser tube are almost always orthogonally polarized. So, where two modes are oscillating, separate signals corresponding to the amplitude of each mode can be easily obtained by feeding a pair of photodiodes from a polarizing beamsplitter. (If a tube has modes that aren't orthogonally polarized or that behave strangely, it gets recycled into another application or the dumpster.) The signals may be obtained from the waste beam out of the HR mirror of the laser or by sampling a portion of the output beam. Either one or both of the photodiode signals can then be used for the feedback loop depending on whether intensity or frequency stability is most important. Note that under some conditions, up to 3 or even 4 modes may be permissible in a tube that is to be used for these purposes. More below.

      • Where the best frequency stability is desired, the ratio of the mode signals (usually made 1:1) is used in the feedback loop. This results in better absolute frequency stability since this ratio is independent of the actual output power, which may change as the tube warms up and ages due to use. With a ratio of 1:1, the two modes are parked equally spaced on either side of the gain curve. Even if the tube oscillates on 3 modes if one is near the center of the gain curve (1 strong one and 2 weak ones), there will only be 2 modes when stabilized. The overall approach is shown in Dual-Mode Single-Frequency Stabilized HeNe Laser. Commercial examples include the Coherent 200, Spectra-Physics 117/A/B/C (and identical Melles Griot 05-STP-901), REO SHL. Axsys/Teletrac 150, and many others.

        Some inexpensive (this is relative!) stabilized HeNe lasers only use a single mode for frequency locking. When on the slope, this will be reasonably stable after warmup once the output power has reached equilibrium.

      • When the best intensity stability with a polarized output is desired, the signal from a single mode (one photodiode channel) is compared to a reference voltage and this becomes the error signal in a feedback loop to put its mode near the center of the gain curve. Even if the tube oscillates on up to 4 modes if there are two on either side of the gain curve, with one near the center of the gain curve when stabilized, there will be at most 2 weaker modes on the tails of the gain curve. Since these will be orthogonally polarized to the dominant center mode, they can be blocked by the output polarizer. The overall approach is shown in Single-Mode Single-Frequency Stabilized HeNe Laser. Commercial examples include the Spectra-Physics 117A (and identical Melles Griot 05-STP-901), and REO SHL.

      • When the best intensity stability of the total output (without regard to polarization) is desired, a non-polarizing beam sampler is used or the signals from the two photodiode channels are summed and compared to the reference. I am not aware of any commercial lasers using this approach.

    2. Zeeman split systems: A magnetic field is used to create a pair of lasing modes that differ from each other by a relatively small frequency. The stable optical frequency along with the Zeeman difference frequency are used for a variety of metrology applications. These may be classified as either axial or transverse based on the orientation of the magnetic field:

      • Axial: Like the single mode systems described above, the tube length is such that only a single longitudinal mode will oscillate. However, a powerful axial magnetic field splits this single mode into two sub-modes with counterrotating circular polarization states. When passed through a polarizer at the output, this results in a beat frequency in the 100s of kHz to several MHz range (depending on the magnetic field strength and other factors) which may be used to derive the stabilization feedback signal and is also key to the measurement technique for which these are designed. The overall approach is shown in Dual-Mode Stabilized Axial Zeeman-Split Dual-Frequency HeNe Laser. Commercial examples include the HP/Agilent 5501B, 5517, 5518A, and 5519A/B (though the heater is actually *inside* the tube for these); Excel 1001; Zygo 7705; and others.

      • Transverse: Like the two mode systems described above, the tube length is such that a pair of modes can oscillate when straddling the gain curve but only a single mode when at the peak. A moderate transverse magnetic field in conjunction with the natural birefringence of the mirror system results in a beam frequency in the 10s to 100s of kHz range. Since the beat frequency varies slightly with the mode position, it may be used in a PLL feedback loop for frequency stabilization. One example is the Laboratory for Science model 220.

    Most commercial stabilized HeNe lasers for general laboratory applications are of type (1) and operate with 2 orthogonal modes for frequency stabilization, though some use 1 mode for intensity stabilization (or can select between them with a switch). (Regardless, only a single longitudinal mode - thus a single optical frequency - may be allowed to exit the laser, the other being blocked with a polarizer.) These include the Coherent 200, Spectra-Physics 117 and 117A (and the identical Melles Griot 05-STP-901), many from Zygo, and various models from REO, Thorlabs, and others. For example, in the Melles Griot 05-STP-901 frequency and intensity stabilized HeNe lasers (no longer in production), the laser cavity permits a pair of orthogonal polarized longitudinal modes to be active and can provide very precise control by straddling these on either side of the gain curve (frequency stabilized mode) or a single longitudinal mode that is also used for the output on one side of the gain curve (intensity stabilized mode). Those from other companies are generally similar.

    All the interferometry lasers manufactured by Agilent (formerly Hewlett Packard), Excel, and one model from Zygo (the 7705) are of type (2). While lasers from Teletrac/Axsys, Optodyne, Renishaw, and others are type (1).

    And there are hybrid approaches. For example, the Zygo 7701/2/12/14 lasers generate and lock a single frequency via dual mode stabilization, But it is split into two using an Acoutso-Optic Modulator (AOM) rather than the Zeeman effect.

    For some photos of the (quite simple) Zeeman split stabilized HeNe tube used in the Hewlett-Packard 5517 laser head, see the Laser Equipment Gallery (Version 1.86 or higher) under "Assorted Helium-Neon Lasers". And for more information on these lasers, see the sections starting with: Hewlett-Packard/Agilent Stabilized HeNe Lasers.

    It isn't really possible to convert an inexpensive HeNe tube that operates on many longitudinal modes into a single frequency laser. Adding temperature control could reduce the tendency for mode hopping or polarization changes, and the addition of powerful magnets can force a polarized beam. But, selecting out a single longitudinal mode would be difficult without access to the inside of the tube. However, if the HeNe tube is short enough that the mode spacing exceeds about 1/2 the Doppler-broadened gain bandwidth for neon (about 1.5 GHz), it will oscillate on at most 2 longitudinal modes at any given time and these will each be linearly polarized and usually orthogonal to each-other. Then, stabilization is possible using very simple hardware. In fact, even if the mode spacing is a bit smaller - down to 500 or 600 MHz - then only 2 modes will be present most of the time but 3 may pop up if one is close to the center of the gain curve. This, too, is an acceptable situation since the tube can be stabilized with the modes straddling the gain curve and then only 2 modes will oscillate. For intensity stabilization, 4 modes may even be permitted. Note that while the modes of a random polarized and linearly polarized tube are similar (except for polarization), a random polarized tube is desirable to be able to use a tube that supports 2 modes with the benefits they provide, while being able to eliminate the second mode from the output. Also see the section: Inexpensive Home-Built Frequency or Intensity Stabilized HeNe Laser for details.

    It may be possible with a combination of what can be done externally, as well as control of discharge current, to force a situation where gain is adequate for only 1 or 2 modes even for a longer tube. Whether this could ever be a reliable long term approach for a HeNe tube that normally oscillates in many longitudinal modes is questionable. What I don't think will have much success are optical approaches such as feeding light back in through the output mirror. Doing this would likely have the exact opposite of the desired effect but may work in special cases (it's called injection locking and is used with great success for other applications).

    Coherent, Melles Griot, Spectra-Physics, and others will sell you a small stand-alone stabilized HeNe laser for $5,000 or so and Agilant (HP) and others have interferometers and other similar equipment which includes this type of laser (and are even more expensive!). Other manufacturers includ Zygo, Teletrac, Nikon, Micro-g Solutions, SIOS, NEOARK, and REO. The lab lasers that I've seen all use short HeNe tubes with thermal feedback control of the resonator length and all operate at the red HeNe wavelength (632.8xxxxxx nm to 8 or more significant figures). One typical system is described in the section: Coherent Model 200 Single Frequency HeNe Laser. The Spectra-Physics model 117A/118A laser actually uses a lowly SP088-2 tube similar to those in older grocery store barcode checkout scanners as its heart. A tube like this is visible in the Spectra-Physics Model 117 OEM Stabilized HeNe Laser Assembly. However, some do employ a custom tube with the heater inside to greatly speed up response and reduce heat dissipation to the outside. A stabilized HeNe laser for green or other color visible HeNe wavelength or one of the IR wavelengths is also possible using the same techniques.

    As noted above, the actual stabilization mechanism for the general purpose stabilized lasers may be the ratio of amplitudes of two longitudinal modes (which is better for frequency stabilization) or the amplitude of one mode (which is better for intensity stabilization). These are usually stable to within a few parts in 109. However, the frequency drift when intensity stabilized is still not much - probably less than 1 part in 108. Output power variation may be 0.2 percent if intensity stabilized and 1 percent if frequency stabilized. Some allow either method to be selected via a switch, as well as providing for an external tuning input to vary the frequency over several hundred MHz. (However, due to the thermal control most often used, the response time is not exactly fast.)

    The Zeeman split interferometer lasers may lock the difference frequency to a crystal clock, though most seem to use the basic polarized modes for stabilization, with the Zeeman beat used only as the reference for the interferometer. See the sections starting with: Hewlett-Packard/Agilent Stabilized HeNe Lasers. A few do lock the Zeeman frequency to a PLL. One of these was the Laboratory for Science Model 220. (Laboratory for Science is now out of business.) See the section Laboratory for Science Stabilized HeNe Lasers. Another example is the NEORK Model 262 Transverse Zeeman Laser.

    More sophisticated schemes with even better precision and lower long term drift may lock to the "Lamb Dip" at the center of the neon gain curve or to one of the hyperfine absorption lines of an iodine vapor other type of gas cell, achieving stabilities on the order of 1 part in 1010 or even better. See, the section: Iodine Stabilized HeNe Lasers.

    Due to the performance, simplicity, reliability, and relatively low cost of stabilized HeNe lasers, they are still often the preferred frequency reference for many applications. As noted, a typical system might go for $5,000. While this may seem high, it is small compared to many other technologies. The cost is not the result of expensive components or complex manufacturing, but more to the relatively limited number of units produced. If stabilized HeNe lasers were as popular as laser pointers, they would probably cost under $100.

    Much additional information and specific details of actual systems may be found in the chapter: Commercial Stabilized HeNe Lasers.

    Digital Control of Stabilized HeNe Lasers?

    These types of lasers have been designed using simple analog techniques for over 35 years. So why change? A few op-amps, a monostable or two, and a handful of discrete parts is sufficient for any conceivable level of performance in a mode-stabilized HeNe laser. There are at most two signals that need to be monitored (the polarization modes) with the objective of maintaining them equal or in a fixed ratio. Yet, I've seen at least 3 examples of dual polarization mode stabilized HeNe lasers that have gone from a simple analog approach to a much more complex digital approach, apparently with no obvious technical justification:

    All are basic mode stabilized HeNe lasers. The 5517 is a Zeeman-split laser but the stabilization is mode-based.

    The redesign in each case must have cost a fortune. Since none of these lasers had many adjustments in their analog designs, ease of manufacturing is probably not the justification. And there is no need for preventive maintenance as components age - lasers like this will run for years on-end without any adjustments. Cost of components is also not a viable excuse as jelly bean op-amps and other common parts are adequate for any of these lasers. Nor do any require an external computer interface like more complex lasers.

    However, one obvious benefit from the company's point of view is serviceability, or lack thereof for anyone not supported by the manufacturer. The new designs are virtually impossible to troubleshoot and repair without detailed service information, and possibly support software. Unless the problem is obvious like a broken wire or blown fuse, attempting to find an electronic fault in these high density surface mount PCBs controlled by firmware programs is just about impossible. And Marketing can promote the "benefits" of digital technology, as bogus as that may be here. If anything, the additional electrical noise from digital signals is a detriment. Digital has to be better, right? :)

    Iodine Stabilized HeNe Lasers

    Unlike the more common HeNe stabilized lasers like those that lock to some intrinsic feature of the lasing process like the neon gain curve, an Iodine Stabilized HeNe Laser (ISHL) uses a external gas cell containing iodine vapor, so that a line in the iodine absorption spectrum is used as the reference wavelength. In principle, this provides an improvement in long term wavelength accuracy of 1 to 2 orders of magnitude - down to 0.1 parts per billion, corresponding to a few 10s of kHz - or better.

    An ISHL operating on the common red (633 nm) wavelength consists of a HeNe laser tube with one or two Brewster windows, a gas cell containing iodine at low pressure, and at least one external mirror on a PieZo Transducer (PZT) for fine cavity length control. The iodine cell needs to be installed inside the laser cavity to benefit from the high intra-cavity circulating power as the sensitivity in the vicinity of 633 nm is very low. However, when operating on the green (543.5 nm) wavelength, the cell can be external despite the lower power generally achievable with green, because the sensitivity is higher.

    The basic principles of operation for an ISHL are rather straightforward: The iodine (or actually I2) has a very complex absorption spectra with hundreds of absorption lines. A very small portion of it is shown in: Iodine Absorption Spectrum Near 532 nm. By dithering the laser cavity length via a PZT, a lock-in amplifier (also known as a phase sensitive detector or synchronous demodulator) can maintain the wavelength at the very center of any selected absorption peak (or dip, depending on your point of view!). The challenging part is to be able to reliably select a specific absorption line to lock to. So, although locking to a given line is fairly simple, the overall electronics can get to be quite complex if automatic line selection is desired, though nowadays, an embedded microcomputer does the line selection.

    Here are some photos of an iodine stabilized laser based on the classic NIST (National Institute of Standards and Technology, formerly the National Bureau of Standards) design originally described in the paper: Howard P. Layer, "A Portable Iodine Stabilized Helium-Neon Laser, "IEEE Trans. on Inst. and Meas, IM-29, pp358-361, 1980. The photos are actually of two different samples of the NIST design. The first one is of a complete laser head while the others are of a physically similar resonator only where it's easier to see the individual components.

    Although the laser head does not presently lase, I am hopeful that it will someday. The discharge color of the HeNe laser tube is normal and there is no visible brown crud in the bore indicating that it should be healthy. The iodine cell still has iodine in it based on its response to a green (532 nm) DPSS laser pointer beam. This thing has probably been sitting on a warehouse for years, if not decades (next to the lost Ark), so the non-lasing condition isn't exactly a surprise. However, there seemed to be some type of contamination inside one of the B-windows. So, it may require a replacement 05-LHB-290. I do have one that lases, though it's a bit weak. However, the NIST paper states that the reflectivity of the OC mirror is only 93 percent, presumably to force single longitudinal mode operation by reducing gain, but this also dramatically reduces output power. And the tube would need to be quite healthy to lase at all. Replacing that mirror with a 99 percent OC might be an option. Then mirror alignment or some other means could be used to force SLM. It would seem like a more logical solution to force SLM would be to add a PZT-controlled etalon that tracks cavity length tuning. Then, the output power would be close to the maximum available from the tube - 5 to 10 times higher than this design produces. But I've not seen that anywhere. The paper also states that the laser tube and cavity are 20 and 30 cm long, respectively. On my samples, they are at least 25 and 35 cm. And, their laser tube appears to not be a Melles Griot 05-LHB-290. So perhaps the original prototype was not identical to the versions later reproduced by Frazier (and others), though it's quite clear that Frazier copied nearly every aspect of the laser design down to the controller-in-a-scope and its front panel layout and labeling. ;-)

    More on ISHLs:

    And multi-wavelength iodine stabilized HeNe laser have also been built. See: "A Tunable Iodine Stabilized He-Ne Laser at Wavelengths 543 nm, 605 nm, and 612 nm", J. Hu, T. Ahola, K. Riski, and E. Ikonen, Digest of the 1998 Conference on Precision Electromagnetic Measurements, July 6-10, 1998, IEEE Cat. No. 98CH36254. This one used the tube from a PMS/REO LSTP-1010 5 color tunable HeNe laser with a pair of PieZo Transducers (PZTs) behind the rear mirror (tuning prism) and a lock-in amplifier for feedback control. For these wavelengths, the iodine cell can be outside the cavity, but notice that the red wavelength, 633 nm, is not included. Multi-Wavelength Iodine Stabilized HeNe Laser

    The only modification to the laser itself was to add a pair of PZT cylinders between the back of the tuning prism and its mount so that the cavity length could be tuned electronically. The iodine cell and laser power detector are external to the cavity.

    What I found curious with this (as well as the NIST laser) is that the laser cavity is way too long to restrict the laser to single longitudinal mode operation as would be required for the system to be useful. The authors of the paper don't appear to address this, nor have I found it mentioned elsewhere.

    So I performed a quick experiment using a REO tunable HeNe laser. As expected, with the power in each wavelength maximized, there are multiple longitudinal modes oscillating. And also as expected, there would be a range of the mode sweep cycle where the output would be pure SLM if either the Wavelength Selector or Transverse adjustment were set so as to reduce output power below a specific value, differing for each wavelength as follows:

       Wavelength      Maximum SLM Power
     -------------------------------------
        632.8 nm             56 µW
        611.9 nm             97 µW
        604.6 nm            169 µW
        594.1 nm            320 µW
        543.5 nm            240 µW
    

    These values are very approximate and don't necessarily mean that the laser can be tuned over any significant range and remain SLM as is required to be useful to lock to an I2 line - that would require even lower power. The 543.5 nm SLM power may be somewhat higher than 240 µW but that's as much as my laser wanted to put out at the time. It would appear that 594.1 nm would be a very usable wavelength at higher power, but apparently the authors did not find a suitable I2 absorption transition at that wavelength, or at 632.8 nm either. The latter is rather strange as we know that there are more than a half dozen suitable I2 lines within the normal 632.8 nm gain bandwidth to which the Frazier and NIST lasers can be locked.

    The NIST (and presumably Frazier) ISHLs use an OC reflectance of only 93 percent to raise the lasing threshold and force SLM operation. (Common red HeNe lasers of this size typically have an OC reflectance of 99 percent.) This option is not available for the multi-wavelength ISHL since the authors used a stock PMS/REO tunable laser tube which has a relatively high reflectance (much greater than 99 percent) internal OC.

    Assuming this analysis with respect to usable SLM power to be correct, it does explain why direct locked ISHLs typically have very low power. To achieve higher power, some companies offer what is known as an "offset-locked iodine stabilized HeNe laser". With these, a normal SLM HeNe laser with a typical output power of 1 to 2 mW (at 632.8 nm) has its optical frequency phase locked to the lower power ISHL. Implementation is actually easier than it sounds but nonetheless is left as an exercise for the motivated student. ;-)

    Stabilized HeNe Lasers at Other Wavelengths

    All types of schemes for stabilizing red (633 nm) HeNe lasers have been developed, but most of those that are commonly used in commercial stabilized HeNes are based on monitoring of one or both polarized modes in the output or waste beams and locking their position to the neon gain curve. For well behaved so-called "random polarized" 633 nm HeNe laser tubes, adjacent modes are generally orthogonally polarized. So, to assure a single mode (single frequency) output, the tube simply has to be short enough that at the lock position, either one mode or two polarized modes are present. In the latter case, a polarizer at the output can block the unwanted mode.

    While it might be assumed that exactly the same approach could be taken for "other color" lasers, this turns out not usually be the case. The principle reason is that the nice behavior that has been counted on to keep the lasers well mannered may not be present. So while the tube will still have a pair of orthogonal axes of polarization, adjacent longitudinal modes will not necessarily be orthogonal and/or even have a consistent relative polarization - they may flip like a banshee.

    So, where it is desired to implement a stabilized HeNe laser at other wavelengths (visible or IR), the polarization may be the primary issue, but there are a number of other complications including differences in the neon gain bandwidth and generally much lower power:

    1. Orthogonal polarization: For the 633 nm HeNe laser, the Physics has cooperated (or Murphy took a millisecond off) with adjacent modes being orthogonally polarized. Since this is not necessarily true at other wavelengths, the use of a short tube may be required so that only a single mode is permitted at the lock point. For example, to assure that only a single mode can oscillate at 543.5 nm would require a tube less than about 12.5 cm in length, which would have an extremely low output power if it could be made to work at all - probably well under 0.1 mW.

    2. Neon gain bandwidth: The width of the inhomogeneously-broadened neon gain curve depends on optical frequency. It is roughly equal to the (lasing wavelength/633 nm*1.6GHz)+100 MHz (added as the square root of the sum of the squares). (For most purposes, the add 100 MHz term can be ignored since its contribution will be small.) Thus, the length of the tube must be selected based on wavelength to assure that only the desired number of longitudinal modes can oscillate. However, FWHM or other definition of the gain bandwidth has to be adjusted depending on the actual gain and losses of the tube. For example, the mid-IR 3,391 nm line has a gain over 40 times that of the 633 nm red line, so the lasing threshold will be much lower effectively widening the gain curve.

    3. Power output: The gain and/or efficiency for most of the non-red wavelengths is much lower than for 633 nm. Normally, this can be handled using a longer tube. But that directly conflicts with (1) for the green (543,5 nm), yellow (594.1 nm), and orange (604.6 or 611.9 nm) wavelengths since these tubes need to be shorter than even for red.

    Various tricks may be used to stabilize HeNe lasers at other wavelengths but in general, it's often not as easy! Also see the section: A Stabilized HeNe Laser at 1,523 nm.

    Reverse Incremental Efficiency of HeNe laser?

    You say: "Huh, what?". ;-) Until recently, it never occurred to me to even think about how the HeNe lasing process and electrical input might be related other than that the HeNe laser is extremely inefficient. Then someone asked the obvious question: "Does the power input to the laser depend on the output power in the beam?". With a bit of thought, it should be obvious for there to be some relationship. But even for other types of lasers, this is not something that is often considered. The slope efficiency is an important measurement for any laser, being how the laser output changes as a function of the electrical (or other) input. For example, with a laser diode, all that is needed is to measure the input electrical power and output optical power at two points where lasing is occurring and calculate the ratio of the differences. But this is from input to output. For a HeNe laser, such measurements can be done over a portion of the range where the power supply is stable resulting in a typical value of 0.3 mW/W or 0.3 percent, similar to the pathetic absolute efficiency for the HeNe laser!

    But what we want here is the opposite - how the input power is affected by the laser output, which I'll call the "Reverse Incremental Efficiency" or RIE. In other words, compare the input power with the laser operating normally and with the output suppressed, for example, by misaligning a mirror. For a HeNe laser, would there be a detectable change in input power if this were done? With a normal constant current HeNe laser power supply, the result should be a change in tube voltage. If for want of a better term, the "reverse slope efficiency" were 100 percent, then "spoiling" the beam of a 1 mW laser should result in a reduction of 1 mW in power consumed by the tube.

    So I did an experiment using a high-mileage JDS Uniphase 1145P laser head with a Melles Griot 05-LPL-915 power supply set at 6.5 mA. The lasing was spoiled using a tube-type Nylon mirror adjuster pushing on the OC mirror mount to kill lasing in a totally reversible manner. Measurements were made while the laser was warming up and outputting 12 mW and then once fully warmed up and outputting 19 mW. The results were rather intriguing:

        ΔPo       ΔVt      ΔPt       RIE
      ------------------------------------
       12 mW     4.1 V   26.65 mW   45.0%
       19 mW     5.2 V   33.80 mW   56.2%
    

    ΔPo is the output power, ΔVt is the change in tube voltage from 0 mW to ΔPo, and ΔPt is the corresponding change in the tube's power consumption.

    At first, my measurements were made with a DMM with only 4 digits of resolution and it appeared as though the the RIE might be exactly 50 percent, which could have had some cosmic significance. :) But it wasn't to be. With the full 5 digits of a Fluke 87, while the RIE isn't far from 50 percent, it isn't 50.00000000%. Too bad. But what this does say is that the incremental efficiency of getting coherent photons out the front of a HeNe laser once it's running at the normal voltage and current and outputting near rated power is order of 50 percent, not a miniscule value like that 0.3 percent! Note that the results depend on whether the laser is running at reduced and full power. If this had been some obscure effect of mechanical stress on the discharge voltage, then the change in tube voltage would be about the same at both output powers. And pushing on the mirror mount beyond where lasing ceases has no effect on tube voltage. At least until it breaks off. :)

    To further confirm that this is a true lasing effect, I repeated the experiment with a Melles Griot 05-LHB-570 one-Brewster laser where lasing could be suppressed simply by poking something in the cavity between the tube and OC mirror:

         ΔPo       ΔVt       ΔPt      RIE
      -------------------------------------
       2.55 mW    0.9 V    5.85 mW   43.6%
    

    Even at this much lower output power, the RIE is still fairly high, though uncertainly is greater due to the much lower power and corresponding change in tube voltage.

    Then, I did multiple sample points while a like-new 1145P head was warming up:

        ΔPo       ΔVt      ΔPt       RIE
      ------------------------------------
        6 mW     4.8 V   32.1 mW    21.0%
       10 mW     5.7 V   37.1 mW    27.0%
       14 mW     5.8 V   37.7 mW    37.1%
       17 mW     6.0 V   39.0 mW    43.6%
       19 mW     6.3 V   41.0 mW    46.3%
       21 mW     6.3 V   41.0 mW    51.3%
       24 mW     6.4 V   41.6 mw    58.0%
    

    Just when I thought this was making some sense, these data appear to show an unexpected very non-linear relationship. Most of the voltage change occurs between 0 mW a few mW, and it is then nearly constant, perhaps due to the gain saturating. There is still significant uncertainty as the measured values for both absolute tube voltage and the voltage difference fluctuate over time.

    And finally on the same head when fully warmed up with a stable 24 mW of output power undisturbed, with controlled misalignment of the OC mirror to generate a few intermediate values:

        ΔPo       ΔVt      ΔPt       RIE
      ------------------------------------
        1 mW     1.3 V    8.5 mW    12.0%
        6 mW     4.4 V   26.8 mW    21.0%
        9 mW     5.2 V   34.5 mW    26.1%
    
       24 mW     6.3 V   41.0 mW    58.6%
       "" mW     6.5 V   42.3 mW    56.8%
       "" mW     6.8 V   44.2 mW    54.3%
    

    These are generally similar to the measurements during warmup. The last two full power entries reflect the variation that may be present even when the laser is in thermal equilibrium. Even so, there can be small changes in the longitudinal mode positions and thus relative efficiencies of the lasing lines or something. :)

    A reference to this phenomenon can be found on page 38 of an old NASA report: An Experimental and Theoretical Investigation of Striations in a HeNe Laser. (If this link should decay, simply search for the title.) I'm sure there are many more in depth studies but locating them is left as an exercise for the student. :)

    On-line Introductions to HeNe Lasers

    There are a number of Web sites with laser information and tutorials.



  • Back to Helium-Neon Lasers Sub-Table of Contents.

    HeNe Laser Tubes, Heads, Structure, Power Requirements, Lifetime

    Early Versus Modern HeNe Lasers

    In the first HeNe lasers (see the diagram below), exciting the gas atoms to the higher energy level was accomplished by coupling a radio frequency (RF) source (i.e., a radio transmitter) to the tube via external electrodes. Modern HeNe lasers almost always operate on a DC discharge via internal electrodes.

    
          Bellows                                                Bellows
          /\/\/\      Discharge tube with external electrodes     /\/\/\
         ||     \________________________________________________/     ||
         ||             | |                | |              | |        ||===> Laser
         ||      ___  __|_|________________|_|______________|_|__      ||===> Beam
         ||     /   ||   |                  |                |   \     ||
          \/\/\/    ||   |                  o                |    \/\/\/
     Adjustable     ||   +-----------o RF exciter o----------+     Adjustable
      totally       ||                                              partially
     reflecting     ||<-- to vacuum system                         reflecting
       mirror                                                        mirror
    
    

    Early HeNe lasers were also quite large and unwieldy in comparison to modern devices. A laser such as the one depicted above was over 1 meter in length but could only produce about 1 mW of optical beam power! The associated RF exciter was as large as a microwave oven. With adjustable mirrors and a tendency to lose helium via diffusion under the electrodes, they were a finicky piece of laboratory apparatus with a lifetime measured in hundreds of operating hours.

    In comparison, a modern 1 mW internal mirror HeNe laser tube can be less than 150 mm (6 inches) in total length, may be powered by a solid state inverter the size of half a stick of butter, and will last more than 20,000 hours without any maintenance or a noticeable change in its performance characteristics.

    Older brochures from several manufacturers of HeNe lasers can be found at Vintage Lasers and Accessories Brochures and Manuals

    Structure of Internal Mirror HeNe Lasers

    The following applies to most of the inexpensive internal mirror low to medium power (0.5 to 5 mW) HeNe tubes available on the surplus market. Depending on the original application, the actual laser tube may be enclosed inside a laser head or arrive naked. :-)

    This fabulous ASCII rendition of a typical small HeNe laser tube should make everything perfectly clear. :-)

    
                    ____________________________________________
                   /                         _________________  \
            Anode |\  Helium+neon, 2-5 Torr   Cathode can ^   \  |
            .-.---' \.--------------------------------------.  '-'---.-.     Main
        <---| |::::  :======================================:   :::::| |===> beam
            '-'-+-. /'--------------------------------------'  .-.-+-'-'
     Totally    | |/  Glass capillary ^      _________________/  | |  Partially
     reflecting |  \____________________________________________/  |  reflecting
     mirror     |                                                  |  mirror
                |          Rb          +               -           |
                +---------/\/\---------o 1.2 to 3 kVDC o-----------+
    
    

    The main beam may emerge from either end of the tube depending on its design, not necessarily the cathode-end as shown. (For most applications it doesn't matter. However, when mounted in a laser head, it makes sense to put the anode and high voltage at the opposite end from the output aperture both for safety and to minimize the wiring length.) A much lower power beam will likely emerge from the opposite end if it isn't covered - the 'totally reflecting' mirror or 'High Reflector' (HR) doesn't quite have 100 percent reflectivity (though it is close - usually better than 99.9%). Where both mirrors are uncovered, you can tell which end the beam will come from without powering the tube by observing the surfaces of the mirrors - the output-end or 'Output Coupler' (OC) mirror will be Anti-Reflection (AR) coated like a camera or binocular lens. The central portion (at least) of its surface will have a dark coloration (probably blue or violet) and may even appear to vanish unless viewed at an oblique angle.

    For a diagram with a little more artistic merit, see: Typical HeNe Laser Tube Structure and Connections. And, for a diagram of a complete laser head: Typical HeNe Laser Head (Courtesy of Melles Griot) and actual structure in X-ray View of Melles Griot 05-LHR-911 HeNe Laser Head. For some photos, see: Typical Small to Medium Size Melles Griot HeNe Laser Tubes. The ratings are guaranteed output power. These tubes may produce much more when new. Another type of construction that is relatively common is shown in the Hughes Style HeNe Laser Tube and a photo in Hughes 3227-HPC HeNe Laser Tube. These are probably disappearing though as Melles Griot bought the Hughes HeNe laser operation and is converting most to their own design but many still show up on the surplus market, including newer ones with the Melles Griot label. Another design that is similar is the NEC Style HeNe Laser Tube. Some specifications for various NEC HeNe lasers can be found at SOC under "Gas Lasers". Most common higher quality HeNe tubes will be basically similar to one of these two designs though details may vary considerably. Most have an outer glass envelope but a few, notably some of those from PMS/REO, may be nearly all metal (probably Kovar but with an aluminum liner which is the actual cathode) with glasswork similar to that of Huches or NEC at the anode-end.

    Tubes up to at least 35 mW are similar in design but proportionally larger, require higher voltage and possibly slightly higher current. and of course, will be more expensive.

    HeNe tubes used in barcode scanners tend to use a simpler (possibly cheaper) design. Some typical examples are the Uniphase 098-1 HeNe Laser Tube and Siemens LGR-7641S HeNe Laser Tube. A typical small barcode scanner tube is shown in Uniphase HeNe Laser Tube with External Lens. That negative lens is used in the barcode application to expand the beam at a faster rate than with the bare tube. A second positive lens about 4 inches away is then used to recollimate the beam. (In many cases, the required curvature is built into the output mirror but not here. The lens was removed by soaking the end of the tube in acetone overnight.)

    CAUTION: While most modern HeNe tubes use the mirror mounts for the high voltage connections, there are exceptions and older tubes may have unusual arrangements where the anode is just a wire fused into the glass and/or the cathode has a terminal separate from the mirror mount at that end of the tube. Miswiring can result in tube damage even if the laser appears to work normally. See the section: Identifying Connections to Unmarked HeNe Tube or Laser Head if in doubt.

    Gas Fill and Getter

    In order for an HeNe laser to operate efficiently (as such things go) or at all, there must be a very precise and pure mixture of helium and neon gas in the tube. The total amount of gas in a typical 1 mW HeNe tube is much less than 1 cubic cm if it were measured at normal atmospheric pressure. It fills the tube only because the pressure is very low. However, with this small amount of gas, it doesn't take much contamination or leakage to ruin the tube.

    Mirrors in Sealed HeNe Tubes

    (See: Typical Small to Medium Size Melles Griot HeNe Laser Tubes for views of the types of mirrors and mirror mounts discussed below.)

    The mirrors used in lasers are a bit more sophisticated than your bathroom variety:

    Mirror Reflectances for Some Typical HeNe Lasers

    Here are some (approximate) typical OC reflectances for red (632.8 nm) HeNe lasers determined by measuring the actual transmission (R = 100 - T) of a red HeNe laser beam through the optic with a simple photodiode based laser power meter:

    The HRs in all cases showed greater than 99.9 percent reflectivity (T less than 0.001 - virtually undetectable on my fabulous meter).

    Due to the behavior of the photodiode at low light levels, the absolute precision of the readings is somewhat questionable. However, the relative reflectivities of these mirrors is probably reasonably accurate. Note, in particular, the high R of 99.4% for the very long external mirror laser compared to the low R of 97.7% (T of 2.3%) for a shorter internal mirror tube. I expect that in addition to the length of the bore, part of this difference is due to the absence of Brewster window losses in the internal mirror tube resulting in a higher gain so that more energy can be extracted via the OC on each pass.

    Mirrors for non-red HeNe lasers must be of even higher quality due to the lower gain on the other spectral lines. The OC will also have higher reflectivity for this reason. For green HeNe tubes (which have the lowest gain of all the visible HeNe wavelengths), the transmission is about 1/10th that of a similar length red tube. For example, the reflectivity of a typical green HeNe tube OC is 99.92 to 99.95 percent (.08 to .05 percent transmission) at 543.5 nm.

    Notes on making these measurements:

    More About HeNe Dielectric Mirrors

    In the mid 1980s, before Ion Beam Sputtered (IBS) coatings really made their commercial debut, some mirrors were still Epoxied (soft-sealed), particularly those with a lot of coating layers (like 20 or 30), mostly green, yellow, and IR HeNe lasers. These tubes need sharp cutoffs (to kill lasing on unwanted wavelengths) and/or ultra high reflectivity (due to their very low gain) in the coatings - which means a lot of layers. The packing density on Electron-Beam (E-Beam) coatings is not great, so water molecules get into all the layers. When you hard-seal the mirror by heating the frit, the water comes out and cracks the coating (called a 'crazed' mirror). Another problem with mega-stack E-Beam coatings is that the transmittance curve can shift as much as 10 nm (to longer wavelengths - the layers get thicker) during the oven cycle (again a water-thing). If you have to, say, highly reflect at 594.1 nm (for a yellow output tube) and highly transmit beyond 604.6 nm (to kill the orange and red), and your coating shifts 10 nm in the oven cycle, another batch of tubes ends up in the dumpster. :( No! Send the my way. :)

    Ion Beam Sputtered (IBS) coatings have a much higher packing density, so they withstand the (i.e., 450 °C) frit sealing temperatures and don't even shift 1 nm. Nowadays, everything is hard sealed, with the exception of the high-end (long precision) Brewster tubes. Hard-sealing a BK-7 window puts a lot of stress on it, and that just isn't acceptable on the high-Q tubes. So, those get fused silica windows optically contacted (lapped and polished surfaces that are vacuum tight.) (In fact, with this type of seal, if there is no adhesive present, the windows can be easily removed from your dead, leaky, or up-to-air tubes by heating the Brewster stem and window with a heat gun. The window can then be popped off with your thum bnail!)

    Random and Linear Polarized HeNe Tubes

    Most common HeNe laser tubes are randomly polarized since for many applications the polarization of the beam doesn't matter. As noted elsewhere, the term "random" here really doesn't mean that the polarization is necessarily jumping around to totally arbitrary orientations. In fact, such behavior would be rather unusual. It just means that nothing special is done to control the polarization. The typical HeNe laser will lase on several longitudinal modes (how many will depend on tube length of the resonator) with adjacent modes having polarizations orthogonal to each-other. Each of the modes will change their relative intensities periodically over time.

    "Random polarized" is actually a poor choice of terminology since most random polarized HeNe lasers do NOT exhibit random and/or high speed fluctuations in polarization. Rather there are generally two polarization axes that are orthogonal to each-other and the output power slowly varies between the two axes as the tube cavity length changes due to termperature and the lasing modes drift under the neon gain curve. (In fact, the tube used in a stabilized HeNe laser must be a random polarized tube!). The orientation of these polarization axes is usually fixed for the life of the tube and determined by slight asymmetries in the tube geometry and/or mirror coatings (either deliberate or simply as a result of manufacturing tolerances).

    For the special case of a short tube where only two modes fit under the gain curve (typically 5 or 6 inches in length) at the instants when they are equal, the output will appear to be non-polarized (constant intensity as an external polarizer is rotated in the beam) but as the modes shift under the gain curve, one or the other polarization will dominate and for a portion of the entire cycle, the tube will be pure linearly polarized in each of these axes. For longer tubes, there will be much less of an effect because there will be multiple modes with both polarizations at all times.

    The main physical effect resulting in a particular polarization direction being favored in a random polarized HeNe tube is a slight preferred axis in the dielectric mirror coatings or in subtle aspects of the geometry of the tube due to manufacturing tolerances. Where these effects are very small or cancel, the resulting polarization axes may indeed not be restricted to a fixed orientation, but this would be extremely unusual. Most often, the polarization axes are fixed for the life of the tube. It's possible to design a tube with a known orientation for the polarization axes but this turns out to be more complex and expensive, so usually it's left up to natural selection. :-)

    Most linearly polarized HeNe laser tubes are similar to their randomly polarized cousins but include a Brewster plate or window inside the cavity which results in slightly higher gain for the desired polarization orientation. Such tubes produce a highly polarized beam with a typical ratio of 500:1 or more between the selected and orthogonal polarization. External mirror HeNe lasers almost always use Brewster windows and so are inherently linearly polarized. A strong transverse magnetic field can also be used to force linear polarization and indeed, long before I observed this phenomenon, some commercial HeNe lasers offered a "polarization option" which was a set of magnets to be placed next to the bore. See the section: Unrandomizing the Polarization of a Randomly Polarized HeNe Tube.

    Another way to force linear polarization in a HeNe laser (or any other low gain laser) is to add a mirror at 45 degrees reflecting to the actual HR mirror, which is then at 90 degrees to the optic axis (facing sideways). The 45 degree mirror will have a slight polarization preference so its reflectance will be extremely high at the desired polarization and slightly lossy at the unwanted one. Like the Brewster plate, this is enough to force linear polarization in low gain lasers. The undesirable losses from the extra mirror bounce may be less than the losses through a less than perfect Brewster plate or one with a slight orientation error, which is particularly important for "other color" HeNe lasers, especially green, which has the lowest gain. However, this approach is much less common than using a Brewster plate (even for green). I've only seen it in PMS green HeNe laser heads. Based on a test of the mirrors from a broken tube, the reflectance of the 45 degree mirror was about 99.997% for the preferred polarization orientation and 99.9% at the unwanted one. The 90 degree mirror had a reflectance of about 99.997% regardless of polarization. This difference in loss is far less than for a Brewster window but is still more than adequate for the green laser, though probably not for a higher gain red one. And the one PMS polarized yellow HeNe laser head I've had used a Brewster plate. For more info, see: U.S. Patent #6,567,456: Method and Apparatus for Achieving Polarization in a Laser using a Dual-Mirror Mirror Mount.

    Linearly polarized HeNe lasers tended to be used in older laser printers (since the external modulator often required a polarized beam) and older LaserDisc players (because the servo and data recovery optics required a polarized beam). Randomly polarized lasers were used in older barcode scanners since polarization doesn't matter there. Note the use of "older". Nowadays, this equipment all use diode lasers which are inherently polarized. I've heard of people retrofitting such equipment to use diode lasers without much difficulty, but your mileage may vary. :)

    More on Random Polarized HeNe Lasers

    As noted above, the term "random polarized" doesn't mean that the polarization is jumping around at random, but rather that nothing special is done to control polarization. Only natural sources of light such as incandescent lamps produce anything approaching true random polarization since each of the emitters (e.g., atoms, etc.) is oscillating more or less independently of its neighbors in both polarization and wavelength (or frequency). Thus the resulting net polarization will be varying on a time scale of femtoseconds (10-15 seconds) and testing with a polarizer will simply show a uniformly non-polarized source - the intensity of the light that passes through the polarizer will be independent of its orientation.

    However, the output of a laser consists of one or more "lasing lines" which correspond to those optical frequencies which match a cavity resonance ("cavity mode") AND where the round trip net gain within the laser cavity is greater than one. These are the longitudinal (or axial) modes of the laser and each one will have a specific polarization and optical frequency. The cavity modes are spaced at a distance of f=c/2L (called the "Free Spectral Range" or FSR, where f is optical frequency, c is the speed of light, and L is the distance between the mirrors). For the typical HeNe laser, there are between 1 or 2 (for a 15 cm 1 mW tube) and 10 or 12 (for a 1 meter 35 mW tube) present at any given time. Longitudinal Modes of Typical Random Polarized 8 mW HeNe Laser illustrates this for a medium size laser. Much more can be found in the section: Longitudinal Modes of Operation.

    For the red (632.8 nm) HeNe laser, unless something specific is done to control the polarization inside the laser tube, adjacent longitudinal modes will usually be orthogonally polarized (the red and blue lines in the diagram, above). The orientation of their two axes will be determined by some very slight asymmetries in the tube's construction or mirror coatings, and will usually remain fixed for the life of the tube. For reasons that are not clear, in Melles Griot tubes at least, one of the two axes often tends to line up approximately with the exhaust tip-off even though nothing special is done to make this happen and there is no obvious structural characteristic of the tube to cause it. The polarization axes can also be forced to be at a particular orientation, though some tubes using this technique may have other quirks. (See the section: The Strange Mischievous REO Tube from a Stabilized HeNe Laser Head.)

    Should the temperature of the laser cavity change, the distance between the mirrors increases or decreases resulting in a shift in the position of the cavity modes. For most HeNe lasers, this happens inadvertently as a result of the heating caused by the bore discharge during warmup. But it can also be caused by changes in ambient temperature as well as heating or cooling intentionally applied, usually for the purposes of laser stabilization. For the most common situation, as the tube warms up and the cavity expands, longitudinal modes will drift through the neon gain curve, disappearing at one end (longer wavelength, lower optical frequency) as the gain falls below the lasing threshold, and being replaced at the other end (shorter wavelength, higher optical frequency) as the gain there rises above the lasing threshold. The total output power in each of the two polarization axes will correspond to the sum of the power in its lasing modes. The total output power of the laser is the sum of the output power in both polarizations. In most real HeNe lasers, the variation versus time as the tube warms up - called "mode sweep" or "mode cycling" - is smooth and occurs on a time scale of seconds to hours depending on how close the tube is to thermal equilibrium, being fastest just after the laser is turned on. The modes are not jumping around on a time scale of nanoseconds as has been suggested by at least one major supplier of HeNe lasers! :) However, depending on the size of the laser, there can be high frequency variations in power in each polarization, or in a combination of the two observed with a high speed photodetector and oscilloscope. More on this below.

    There are several specific cases depending on the length of the laser cavity. To simplify the explanation, it is assumed that the laser tube has been rotated in its mounts so that the natural polarization axes are at 0 and 90 degrees. In addition, the second order ripple and noise in the output from imperfect power supplies or other external factors are assumed to be small (which is typically the case). Also, fine points like mode pulling (which shift the modes very slightly in position, a small fraction of 1 percent) are ignored. So, the FSR (Free Spectral Range or cavity mode spacing) is equal to the longitudinal (or axial) mode spacing of the lasing lines. And the lasers are assumed to be well behaved and not be "flippers" or "stutterers" or have other pathologic disorders:

    For both of these cases, exactly two modes can be maintained by a feedback circuit with one on either side of the neon gain curve to implement a stabilized HeNe laser. Under these conditions, both of the polarizations are pure single modes with a constant CW output. They are a very pure single optical frequency with ultra-long coherence length when one of them is selected with a polarizer.

    For extremely low power tubes with a cavity length less than 8 or 9 cm, there will never be more than 1 lasing mode present at any time and during a portion of the mode sweep, there may be exactly 0 modes and no beam at all. There will never be any beat frequency detectable in the output. Since two adjacent modes are needed to force orthogonal polarizations and that never occurs, these tubes may lase with the same polarization each time the single mode appears, or the polarization may come up randomly one way or the other (but will remain the same while it's present). So perhaps, such tubes can be truly called random polarized. :) However, they are now almost non-existent.

    Finally, for most linearly polarized HeNe lasers, a Brewster plate or Brewster window(s) within the laser cavity provide enough gain asymmetry to force the polarization to be in one plane only. The polarization purity is usually very high - 500:1 or more. Everything above about mode sweep still applies except that all the longitudinal modes have the same polarization. So the diagrams, Power Power shows, and plots will look identical except that all the modes would be the same color. :) A polarizer will not affect the relative amplitude of the modes, only the intensity and angle of the linearly polarized beam. And whenever more than 1 longitudinal mode is present, there will be a beat signal detectable using a fast photodiode which will contain one or more frequencies depending on the possible distances between all the lasing modes.

    So what this all shows is that random is all in the eyes of the polarized beholder. :)

    More on Mode Cycling in Short HeNe Lasers

    As noted, a randomly polarized HeNe laser doesn't really produce arbitrary polarization but the individual longitudinal modes may switch polarizations as the tube warms up and expands. Where the distance between the mirrors is small - 5 or 6 inches as is the case with small HeNe laser tubes, only two adjacent modes will fit under the inhomogeneously Doppler-broadened gain curve of neon. With only two active modes, effects of mode changes may be obvious even without anything more than Mark-I eyeballs and a polarizing filter but fancy equipment may be needed to fully characterize what's going on.

    (Portions from: Lynn Strickland (stricks760@earthlink.net).)

    Our testing suggested that adjacent modes always have orthogonal polarization - (lets go with S and P designations). BUT, in some two-mode tubes, a given mode doesn't always REMAIN S or P as it changes in frequency (it flips polarization). In "flippers", certain frequencies only support one polarization. If this frequency range is around the center of the gain curve, most power will be of one polarization regardless of temperature (so it appears to be linearly polarized). (However, the extinction ratio varies over time, and is generally poor).

    Here's a test setup that shows what's going on if you have access to some nice instrumentation: Send the beam from a two mode, randomly polarized HeNe tube (Example: 05-LHR-006) into a Scanning Fabry-Perot Interferometer. (SFPIs are generally exorbitantly priced, but you can build one if so inclined. See the sections starting with: Scanning Fabry-Perot Interferometers. --- Sam.) Put a polarizer in the beam path, aligned to maximize P polarization (or S polarization, doesn't matter). Normally, the P mode will remain P polarization at all frequencies under the gain curve. So as the frequency changes (due to cavity length changes with temperature), the P mode will trace out a nice pretty sort of bell-shaped curve with a width of about 1.6 GHz FWHM. Bottom line, you can get P-polarized light at every frequency under the gain curve.

    In a 'flipper', your curve has missing sections. In other words, there are some frequencies where you cannot get P polarization. When the observed, P mode reaches one of these frequency ranges, it will flip and become S-polarized. When the flip occurs, the other, formerly S mode, turns into a P. If you're just looking at one polarization (as the experiment describes), the observed P mode disappears and pops up again at a frequency delta equal to the longitudinal mode spacing (where the S mode used to be). Some call it mode hop, but it really isn't, because both modes are still there. Both modes still have, and always had, orthogonal polarization - they just swapped. Some tubes flip at one point under the gain curve, some flip many times under the gain curve.

    This has to do with gain asymmetry. What brought it to our attention, is that when the polarizations flip, you get high frequency 'noise' if you have polarization sensitive components in your beam path. Solutions are to specify a laser that doesn't flip, go to a three mode (longer) laser, go to non-polarization sensitive optics all the way through the beam delivery/detection train, or put a bandwidth filter on your detector.

    A magnetic field will sometimes make a flipper stop, and sometimes make a non-flipper start - but not always. Sans magnetic field, over time (several thousand operating hours) our test population suggested that flippers always flip, non-flippers always behave.

    There is more on flippers below.

    HeNe Mode Flipper Observations

    The longitudinal modes of a HeNe laser tube sweep through the gain curve as the resonator heats and expands. On a random polarized red (632.8 nm) tube, adjacent modes tend to be orthogonally polarized due to non-linear mode competition (or something). With well behaved tubes, once a mode starts lasing with a given polarization as it exceeds threshold on one side of the gain curve, that polarization is fixed until the mode ceases lasing on the other side of the gain curve. The Power Point show HeNe Laser Mode Sweep: 200 mm (~8 inch) Cavity Length demonstrates the effect of changing cavity length on the lasing modes in a well behaved 2 to 3 mW random polarized tube.

    A "flipper" tube is one where the polarization orientation of adjacent longitudinal modes swap places at a fixed location on the neon gain curve as the modes sweep through it. Some will flip at multiple locations on the gain curve but this is less common. The Power Point show HeNe Flipper Mode Sweep: 200 mm (~8 inch) Cavity Length demonstrates the effect of changing cavity length on the lasing modes in a classic 2 to 3 mW flipper tube.

    The issue of why some tubes are flippers is apparently one of those grand mysteries of the Universe that even the Ph.D. types at major laser companies have been pondering for eons without resolution, as it's still not always possible to manufacture a tube that is guaranteed to be well behaved. :) Flipper behavior may not be detected where the laser is simply used as a source of photons for the same reason that polarization effects of normal mode sweep tend to be minimal since the total power doesn't vary that much. However, polarization flips will introduce short noise spikes. And if there are any polarization sensitive optical elements (intentional or not), significant sudden power fluctuations will also be evident in the polarized beam(s).

    As with random polarized HeNe lasers not being random at all, flipper behavior is also mostly deterministic in that for a given tube, flipping will usually always occur at the same place(s) in the mode sweep, but there are exceptions.

    While I haven't seen any discussion of flipper theory, here are some thoughts.

    In the absence of external influences like magnetic fields, the mode orientation in a laser will be determined by at least two factors:

    Since a transverse magnetic field can also introduce a polarization preference, it is possible to cause a well behaved HeNe laser tube to exhibit flipper behavior by the careful placement of s strong magnet near the tube. I've demonstrated this with a normal Uniphase 098 laser. With no magnet, the mode sweep is perfectly ordinary with no tendency to flipping. By placing a single rare earth magnet next to the tube near the middle, it can be made to turn into a flipper with a mode plot very similar to that of a natural flipper. With too weak a magnetic field, there is no effect or a sort of shortened aborted flipping. With too strong a magnetic field, the polarization becomes locked to the magnetic field and the output ends up being linearly polarized.

    For that peculiar tube above which reverts to normal behavior at the very end of the warmup period, a very weak magnetic field will cause it to continue to flip after the point of transition where flipping ceases under normal conditions.

    Plot of "Flipper" Aerotech OEM1R HeNe Laser Head with Various Magnetic Fields Applied (Combined) shows the effect of a rare earth magnet at 4 orientations about 4 inches from the center of the laser head compared with no magnetic field. The magnetic field axis was horizontally aligned with one of the polarization axes of the laser. The magnet was rotated 90 degrees approximately every 30 seconds. The first and last orientation shows a mode sweep pattern that is relatively normal. They probably differ slightly because the magnet wasn't in exactly the same position. The tube was allowed to completely warm up with the magnets in the last orientation with no significant change in the plot, even after the transition point where the tube reverts from flipper to normal behavior with no magnetic field A closeup is shown in Plot of "Flipper" Aerotech OEM1R HeNe Laser Head with Magnetic Field Induced Somewhat Normal Behavior (Combined). While very different than the mode plot of the tube after warmup with no magnetic field, the flips are gone (no vertical jumps) and it's relatively well behaved.

    Conversely, it should be theoratically possible to suppress flipper behavior with a suitably placed magnet. Getting this to work is more problematic since the magnetic field has to exactly counteract the natural polarization birefringence. But I was able to somewhat do this with my flipper head so that the mode sweep became well behaved. This was more finicky than going the other way. Almost any magnetic field did disrupt the normal flipper behavior. But getting it to be really well behaved was more difficult.

    Of course, a magnetic field will also introduce other effects due to Zeeman splitting which may be detrimental depending on the application.

    Note that mirror alignment which may affect the resonator orientation preference had no effect on flipper behavior, at least for the one sample I tested. Pressing on the mirror mount of my flipper tube in any direction would reduce the output power significantly due to changing mirror alignment. But the mode flips still occurred, and appeared to be at approximately the same location on the gain curve.

    Some observations and questions:

    Speculation:

    1. No asymmetry: Polarization can be at any orientation at random. Very rarely, if ever seen with HeNe lasers.

    2. Small assymetry: Normal case. Polarization will always be at fixed orientation and 90 degrees to it. Alternating modes will have orthogonal polarization.

    3. Moderate asymmetry: Flipper. Mirror or tube will slightly favor one polarization orientation. When a mode starts with orthogonal polarization, it will progress until the lower energy state is one where the polarization flips.

      This state can be forced from (2) by a small transverse magnetic field.

    4. Large asymmetry: Polarized tube, Brewster plate.

      This state can be forced from (2) or (3) by a large transverse magnetic field.

    I now have been able to borrow a dual perpendicular window HeNe (gain) tube and was hoping to shed light (no pun...) on some of these issues by constructing a setup similar to the one described in the section: Transverse Zeeman Laser Testbed 1. This enabled the tube or one of the mirrors to be rotated without affecting alignment. The tube is longer than I'd like - about 14 inches resulting in a mirror spacing of about 16 inches - so it was necessary to really kill the gain with low reflectance mirrors and/or an aperture to get only 2 or 3 modes oscillating. But it should have been adequate to answer some of these questions. However, the somewhat unexpected result turned out to be that the polarization always remained with the tube regardless of mirror orientation even if the intracavity beam was much smaller than the bore so that any imperfections in its shape should not have had any effect. I attribute this to a very small amount of asymmetry in the transmission through the perpendicular windows. It might be AR coating or stress birefringence, distortion, or even the windows not being mounted quite perfectly perpendicular. With the intracavity photons traversing the windows an average of perhaps 100 times, even a miniscule asymmetry would be amplified into something significant. So on to Plan B, putting everything inside the gas envelope and doing away with the perpendicular windows entirely. Unfortunately, implementation of Plan B is currently not a funded project. :( :)

    Polarization of Longitudinal Modes in HeNe Lasers

    It is well known that adjacent longitudinal modes in red (632.8 nm) HeNe lasers (at least) tend to be orthogonally polarized as discussed above. This is a weak coupling as a magnetic field, Brewster plate, or even some asymmetry in the cavity can affect it or kill it entirely. And some lasers will cause the polarization to suddenly flip as modes cycle through the gain curve. However, the majority of modern well designed red HeNe lasers will exhibit this phenomenon.

    This is not necessarily true of "other color" HeNes. My informal tests suggest that in general it is *not*. Long green (543.5 nm), short and long yellow (594.1 nm), and medium length orange (611.9 nm) random polarized HeNe laser heads all exhibited varying degrees of erratic behavior with respect to polarization. Usually, modes when part of the way through the gain curve and then either flipped abruptly or oscillated between polarizations for a short time and then flipped. The long yellow head liked to have pairs of adjacent modes with the same polarization but exhibited the flipper behavior as well. However, adding a modest strength magnet near the long green seemed force it to behave with adjacent modes having orthogonal polarization. I have no idea if this is significant or the long green HeNe was simply a cooperating sample.

    But what is the underlying cause?

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

    The reason that HeNe lasers can run - more accurately, like to run - in multiple axial modes is associated with inhomogeneous line broadening (See section 3.7, pp. 157-175 of my book) and "hole burning" effects (Section 12.2, pp. 462-465 and in more detail in Chapter 30) in the Doppler-broadened laser transitions commonly found in gas lasers (though not so strongly in CO2) and not in solid-state lasers.

    The tendency for alternate modes to run in crossed polarizations is a bit more complex and has to do with the fact that most simple gas laser transitions actually have multiple upper and lower levels which are slightly split by small Zeeman splitting effects. Each transition is thus a superposition of several slightly shifted transitions between upper and lower Zeeman levels, with these individual transitions having different polarization selection rules (Section 3.3, pp. 135-142, including a very simple example in Fig. 3.7). All the modes basically share or compete for gain from all the transitions.

    The analytical description of laser action then becomes a bit complex - each axial mode is trying to extract the most gain from all the subtransitions, while doing its best to suppress all the other modes - but the bottom line is that each mode usually comes out best, or suffers the least competition with adjacent modes, if adjacent modes are orthogonally polarized.

    There were a lot of complex papers on these phenomena in the early days of gas lasers; the laser systems studied were commonly referred to as "Zeeman lasers". I have a note that says a paper by D. Lenstra in Phys. Reports, 1980, pp. 289-373 provides a lengthy and detailed report on Zeeman lasers. I didn't attempt to cover this in my book because it gets too complex and lengthy and a bit too esoteric for available space and reader interest. The early (and good) book by Sargent, Scully and Lamb has a chapter on the subject. You're probably aware that Hewlett Packard developed an in-house HeNe laser short enough that it oscillated in just two such orthogonally polarized modes, and used (probably still uses) the two frequencies as the base frequencies for their precision metrology interferometer system for machine tools, aligning airliner and ship frames, and stuff like that.

    (From: Sam.)

    Indeed, HP has several models of two-frequency HeNe lasers but the ones I'm familiar with actually use an external magnet to create Zeeman splitting. Rather than two longitudinal modes, a PZT or heater is used to adjust cavity length so that only a single mode is oscillating, which is split by the Zeeman effect. Then, the difference frequency (in the low MHz range) is used in the measurement system as a reference and possibly for stabilizing the (optical) frequency. See the section: Hewlett-Packard/Agilent Stabilized HeNe Lasers.

    The Spectra-Physics model 117A frequency stabilized HeNe laser is designed more like what you are describing - two modes, no magnets. A heater is used to adjust cavity length in a feedback loop using a pair of photodiodes to monitor the two orthogonal polarized modes. However, I would assume that based on its description, the desired operating conditions would be for it to run with a single mode (which it can with carefully controlled cavity length). See the section: Description of the SP-117A Laser. The Coherent and Melles Griot stabilized HeNe lasers are similar.

    Power Requirements for HeNe Lasers

    Power for a HeNe laser is provided by a special high voltage power supply (see the chapter: HeNe Laser Power Supplies and consists of two parts (these maximum values depend on tube size - a typical 1 to 10 mW tube is assumed):

    A few HeNe lasers - usually larger or research types - have used a radio frequency (RF) generator - essentially a radio transmitter to excite the discharge. This was the case with the original HeNe laser but is quite rare today given the design of internal mirror HeNe tubes and the relative simplicity of the required DC power supply.

    Operating Regions of a HeNe Laser Tube

    There are several distinct operating regions for a HeNe plasma discharge as a function of tube current each of which has its own properties. The following summary is partially extracted from the HeNe Laser Manual by Elden Peterson and is mostly just for curiosity sake as there is little reason to run a HeNe laser tube at anything other than close to the nominal current (which results in maximum power output and rated life) listed in the tube specifications except possibly to implement low level modulation for laser communications. However, some manufacturers do run their tubes at lower current when maximum power isn't needed, possibly to extend life.

    Note that the visual effect of increasing current from dropout to cessation of output will just be a smooth increase and then decrease in coherent optical output power. To detect the single frequency or broadband noise will require a sensor and oscilloscope with a bandwidth of at least a few MHz.

    I've also seen lasers where single frequency noise occurred close to the dropout current and below the point of maximum output power. However, this was only present with some high mileage tubes in HP-5517 lasers so it's not clear whether this should be listed as a separate regime, or just a special case of a particular tube and power supply combination.

    Also of note is that the HeNe laser power supply itself will contribute to optical ripple and noise. A DC input switchmode (inverter) power supply will have ripple at the switching frequency. This is typically in the range of 1 to 5 percent of the operating current and will result in an optical power variation of a few tenths of a percent. An AC input linear power supply will have some ripple at 1X or 2X of the line frequency (with some harmonics) even with a regulator. An AC input switcher (most bricks) will have both types of ripple. Special low noise power supplies are available for critical applications. However, for most common uses, the additional cost is not justified. There are some more comments on this topic in the section: Intensity Stabilized HeNe Laser.

    HeNe Tube Dimensions, Drive, and Power Output

    A large number of factors interact to determine the design of a modern HeNe laser. Beam/bore diameter, bore length, gas fill pressure, voltage, current, and mirror design, are all critical in determining how much output power will be produced - or whether a given tube will lase at all. Hundreds (at least) of technical papers and entire phone book size volumes filled with equations have no doubt been written on these topics and we can't hope to do anything serious in a few paragraphs, but at least, may be able to give you a feel for some of the relationships among power output, bore dimensions, gas pressure, and drive requirements in particular.

    You have probably wondered why the beam from a typical HeNe laser (without additional optics) is so narrow. Is it that making a tube with larger mirrors would be more costly?

    No, it's not cost. Even high quality and very expensive lab lasers still have narrow bores. The very first HeNe lasers did use something like a 1 cm bore but their efficiency was even more mediocre than modern ones. A wide bore tube would actually be cheaper to manufacture than one requiring a super straight narrow capillary. However, it wouldn't work too well.

    A combination of the current density needed in the bore, optimal gas pressure, gain/unit length in the bore, the bore wall itself aiding in the depopulation of lower energy states, and the desire for a TEM00 (single transverse mode) beam (there are multimode tubes that have slightly wider bores), all interact in the selection of bore diameter.

    In fact, there is a mathematical relationship between bore size, gas pressure, and tube current resulting in maximum power output and long life.

    The optimal pressure at which stimulated emission occurs in a HeNe laser is inversely proportional to bore diameter. According the one source (Scientific American, in their Amateur Scientist article on the home-built HeNe laser - see the chapter: Home-Built Helium-Neon (HeNe) Laser), the pressure in Torr is equal to 3.6 divided by the ID of the bore. I don't know whether this exact number applies to modern internal mirror tubes but it will likely be similar. Power output decreases on either side of the optimal pressure but a laser with a low loss resonator may still produce some output above twice and below half this value.

    Thus, as the bore diameter is increased, the optimal pressure drops. Aside from having fewer atoms to contribute to lasing resulting in a decrease in gain, below a pressure of about .5 to 1 Torr, the electrons can acquire sufficient energy (large mean-free-path?) to cause excessive sputtering at the electrodes. This will bury gas atoms under the sputtered metal (which may also coat the mirrors) leading to a runaway condition of further decreasing pressure, more sputtering, etc. Even with the large gas reservoir of your typical HeNe tube (which IS the main purpose of all that extra volume), there may still be some loss over time. A drop in gas pressure after many hours of operation is one mechanism that results in a reduction in output power and eventual failure of HeNe tubes.

    As a result, the maximum bore diameter you will see in a commercial HeNe laser will likely be about 2 mm ID (for those multimode tubes mentioned above where the objective is higher power in a short tube). Most are in the 0.5 to 1.2 mm range. This results in high enough pressure to minimize sputtering, maximize life, provide maximum power output, and optimal efficiency (to the extent that this can be discussed with respect to HeNe lasers! Well, ion lasers are even worse in the efficiency department so one shouldn't complain too much. Since total resonator gain is proportional to bore length and approximately inversely proportional to bore diameter (since the optimal pressure increases resulting in a higher density of lasing atoms), this favors tubes with long narrow bores. But these are difficult to construct and maintain in alignment. Wide bore tubes have lower gain but a higher total number of atoms participating with potentially higher power output at the optimal pressure and current density. Everything is a tradeoff!

    However, all this does provide a way of estimating the power output and drive requirements of a HeNe tube or at least comparing tubes based on dimensions. Assuming a tube with a particular bore length (L) is filled to the optimum pressure for its bore diameter (D), power output will be roughly proportional to D * L, discharge voltage will be roughly proportional to L (probably minus a constant to account for the cathode work function), and discharge current will be roughly proportional to D. (Note that D instead of the cross-sectional area is involved because the optimal pressure and thus density of available lasing atoms is inversely proportional to D.)

    So, do the numbers work? Well, sort of. Here are specifications for some selected Melles Griot red HeNe tubes rearranged for this comparison:

       Total    Bore      Bore    --- Ratio of ---  Discharge  Discharge   Output
       Lgth   Lgth (L)  Dia. (D)  L   D   (D * L)    Voltage    Current    Power
     ------------------------------------------------------------------------------
       135 mm   80 mm    .46 mm   1   1     1         900 V      3.3 mA     .5 mW
       177 mm  115 mm    .53 mm   1.4 1.15  1.6     1,130 V      4.5 mA    1.0 mW
       255 mm  190 mm    .72 mm   2.4 1.57  3.7     1,360 V      6.5 mA    2.0 mW
       370 mm  300 mm    .80 mm   3.8 1.7   6.4     1,800 V      6.5 mA    5.0 mW
       440 mm  365 mm    .65 mm   4.6 1.4   6.4     2,150 V      6.5 mA     10 mW
       930 mm  855 mm   1.23 mm  11.1 2.7  29.9     4,500 V      8.0 mA   25-35 mW
    

    (Bore length was estimated since the cathode-end of the capillary is not visible without X-raying the tube or by optically determining its position through the mirror!)

    The general relationships seem to hold though large tubes seem to produce higher output power than predicted possibly constant losses represent a smaller overhead. As noted elsewhere there is also a wide variation even for tubes with similar physical dimensions. Oh well...

    There are more examples in the section:Typical HeNe Tube Specifications. You can do the calculations. And, some large IR HeNe lasers may use a somewhat wider bore. See the section: Spectra-Physics 120, 124, and 125 HeNe Laser Specifications for a comparison of visible and IR HeNe tubes for the same model laser.

    Note that there are some multi-mode (non-TEM00) HeNe tubes with wider bores and a different mirror curvature that produce up to perhaps twice the power output for a given tube length. However, with multiple transverse modes, these are not suitable for many applications like interferometry and holography. They are also not very common compared to single-mode TEM00 HeNe tubes.

    Higher Power HeNe Laser?

    (From: Chris Leubner (cdleubner@ameritech.net).)

    The most powerful HeNe laser I have ever seen was 160 mW of real power and was the only time I've ever seen a HeNe laser burn anything before with raw beamage. It would slowly burn electrical tape placed in the beam and felt warm on your skin. It was made of two almost 6 foot long Spectra-Physics model 125 tubes hooked electrically to separate power supplies and optically in series in a custom made double-wide sized 125 head. Sadly, it doesn't work anymore and is currently resting piecefully in the NTC laser department's laser graveyard. :-(

    (From: Steve Roberts.)

    I've seen a normal SP-125 break 160 mW on its own. Two tubes at only 160 mW sounds like it was misaligned, not that I'd like to try to align that one! :)

    The current record is for a Chinese researcher using 2 tubes with a flattened elliptical profile in a V fold resonator to get 330+ mW into a fiber. The beam shape and divergence from this are not what you would expect from a typical HeNe laser, even one that runs multi (transverse) mode. Remember that a HeNe laser's power is limited by collisions with the tube wall returning Ne atoms to the ground state, so using a flattened tube means more wall area, hence more power. Optimal gas pressure is a function of bore diameter as well. So you're limited to about a 1 meter tube in most cases by other optics reasons and sputtering. With collisions with the wall increased by a larger wall surface area, what the folks in China did is try tubes with different cross sections. To get enough length they folded the resonator using a 3 optic V-fold. You don't want to see the beam profile. It's nasty! It looks kind of like this: <{[=]}>. And the divergence is high as the optics need to fill that whole lasing volume.

    Please note, however, that going to a large rectangular or star shaped tube is not possible due to some quirks in the plasma at the pressure required for HeNe laser operation. Details are in a 1996 issue of Review of Scientific Instruments. A few years ago, Cornell University attempted to sell the rights to the unit in the United States, on behalf of the Chinese Inventor. U.S. patent and marketing were assigned to a group that sadly dropped the ball. At the time, the picture of the unit looked like one of those old foldaway sewing machines like my mom used to have, an ornamental blue box about the size of a PC Tower turned on its side with 4 wooden legs.

    In the early days, very long HeNe lasers were constructed in an attempt to obtain higher power. But optimal gas-fill and bore diameter weren't known, and mirrors weren't as good as they are now. Aligning multiple segments with a long narrow bore needed for best gain would have been virtually impossible in any case. Thus, such experimental lasers probably had mediocre performance.

    (From: Sam.)

    Using a folded resonator, high power HeNe lasers could be constructed in compact packages but the initial machining and/or alignment would be a real treat. I've seen a spec sheet for some with up to 55 mW of output power using a mono-block folded resonator with a volume of 326x280x95 mm (about 13"x10"x4"). I can't imagine this being cost effective though except maybe for space applications where money is no object!

    Boosting the Power Output of a HeNe Laser?

    Unfortunately, given the existing laws of physics, there usually isn't much you can do to increase the output power of a HeNe laser above its specified ratings. Unlike an ion laser where higher tube current usually increases power output (at the expense of tube life), boosting current to a HeNe tube beyond the optimal amount actually *decreases* power output. Options like Q-switching don't exist for HeNe lasers.

    Bare HeNe Tubes and Laser Heads

    What you have may be a 'bare' tube or it may be encased in a cylindrical or rectangular laser head - or something in between:

    If you have a laser head that is missing the Alden connector, replacements should be available from the major laser surplus suppliers or salvage one from another (dead) head. I also have many available. Where the end-cap on a cylindrical laser head is also missing, there are no readily available commercial sources - fabricate one from a block of wood and paint it black or find some other creative solution. A suitable ballast resistance must also be installed between the positive power supply output and the HeNe tube anode.

    The cylindrical head serves another purpose besides structural support and protection. This is the distribution of heat and equalization of thermal gradients. Thus, removing a long HeNe tube in particular from its laser head may result in somewhat random or periodic cycling of power output due to convection and other non-uniform cooling effects.

    Often, particularly inside equipment like barcode scanners, you will see something in between: A HeNe tube wrapped in several layers of thick aluminum foil probably to help distribute and equalize the heating of the tube for the reason cited above. However, I haven't really noticed any obvious difference in stability when this wrap was removed. Spectra-Physics is very fond of this but others may have copied it to sell compatible tubes.

    HeNe Tube Seals and Lifetime

    Neon signs last a long times - years - how about HeNe laser tubes?

    The operating lifetime of a typical HeNe laser tube is greater than 15,000 hours when used within its specified ratings (operating current, proper polarity, and not continuously restarting). Under these conditions, end-of-life occurs when the oxide "pickling" layer of the cathode can gets depleted. Larger diameter (1.5 or 2 inch) tubes last the longest - up to 50,000 hours or more. Small diameter (0.75 or 1 inch) tubes have the shortest lifetime - 10,000 hours or so. Since even 10,000 hours is still very long - over 1 year of continuous operation - HeNe laser lifetime is not a major consideration for most hobbyist applications. Chances are that even a surplus laser will still have thousands of hours of life remaining.

    However, the shelf life of the tube depends on types of sealing method used in the attachment of the optics. There are two types of internal mirror HeNe tubes:

    A very few tubes apparently have frit at one end and a soft-seal at the other so check both ends. This probably applies only to some low gain "other color" HeNe lasers with a mirror that would be affected by even the relatively low temperature at which the frit melts.

    Note that other parts of most tubes (except for Brewster windows, if present) use glass-to-metal seals but since these must be manufactured at high temperature, they are not an option for delicate optics. The very best tubes with one or two Brewster windows do not use frit because even at the low temperature at which it is fired, there may still be some unavoidable stresses introduced - these tubes continued to be soft-sealed even after frit was common but now use optical contacted seals. With optical contacted seals, the two pieces are ground and polished optically flat and brought together under clean room conditions. The resulting seal is gas-tight. Just a bit of Epoxy is used for mechanical stability but it doesn't do the sealing.

    The HeNe gas doesn't 'wear out'. A HeNe tube, when properly connected has a substantial portion of its power dissipated by the bombardment of positive ions at the cathode (the big can electrode) which is made large to spread the effect and keep the temperature down and is "pickled" (coated) to reduce its work function. Hook a tube up backwards and you may damage it in short order and excessive current (operating current as well as initial starting current from some high compliance power supplies) can degrade performance after a while. Electrode material may sputter onto the adjacent mirrors (reducing optical output or preventing lasing entirely) or excessive heat dissipation may damage the electrodes or mirrors directly.

    As the tube is used (many thousands of hours or from abuse), operating and starting voltages may be affected as well - generally increasing with the ultimate result being that a stable discharge cannot be initiated or maintained with the original power supply. See the section: How Can I Tell if My Tube is Good?.

    (From: Lynn Strickland (stricks760@earthlink.net).)

    Typical failure mechanism in a HeNe is cathode sputtering -- seldom gas leakage in the newer (like since 1983) tubes. Shelf life is stated to be about 10 years, but it's not uncommon at all to see HeNe lasers built in the early 1980's that still meet full spec.

    Interesting lifetime note - it used to be that you left a HeNe 'on' at all times to prolong life. Since hard-sealing, you should turn it off while not in use. If it's a 20,000 hour tube, and you only turn it on for a few hundred hours a year, it will last a heck of a long time. Not uncommon at all for the HeNe to outlive several power supplies. The larger diameter tubes tend to last longer, but it also depends on fill pressure and operating current (higher fill-pressure tubes last longer). The typical 5 mW red HeNe will commonly live to 40k to 50k operating hours.

    As for cathode sputtering, the tube has an aluminum cathode that is 'pickled' during the production process to add a layer of oxidation about 200 microns thick. The oxidation layer prevents aluminum from being bombarded away from the cathode during plasma discharge. As the tube ages, the oxide layer is depleted until aluminum is exposed. Sputtered aluminum can stick to the mirror, causing power decline, or to the inside of the glass envelope, causing the discharge to arc internally. This arcing, if allowed to continue for a period of time, will also cook the power supply. A tube with no oxidation layer on the cathode will die in about 200 hours of use. OR, once the oxidation layer is depleted, the tube will die in about 200 hours. This is why a HeNe life curve is usually pretty flat, then quickly degrading to nothing over about a 200 hour period.

    An Older HeNe Laser Tube

    The Spectra-Physics 084 (SP-084) was popular for applications like barcode scanners. It was rated at 2 to 3 mW when new. Several shots of one are shown in Photos of Spectra-Physics Model 084-1 HeNe Laser Tube. (These photos courtesy of Meredith Instruments.) A diagram is shown in Construction of Spectra-Physics Model 084-1 HeNe Laser Tube. While the main glass tube and end-plates use glass-to-metal (hard) seals, the mirrors appear to be Epoxied in place (soft sealed). Thus, one would expect these tubes to leak over time. However, out of 31 that I have tested, 20 appear to be nearly as good as new showing only slight leakage which their getters have taken care of nicely and no detectable reduction in power output. (Of the others, 7 had weak or no output but most could be at least partially revived - see the section: Attempting to Revive Some Soft-Seal HeNe Tubes. The remainder were totally dead.)

    As is typical of Spectra-Physics internal mirror HeNe tubes, these have thick glass walls (at least compared to tubes from most other manufacturers). For the barcode scanner application (at least) there was an outer wrap (removable) of several layers of thick aluminum foil, apparently for thermal stabilization but it would also reduce electrical noise emissions and light spill from the discharge. (The foil wrap also seems to be common with more modern Spectra-Physics HeNe barcode scanner tubes when not installed in cylindrical laser heads.) A 100K ohm ballast resistor stack in heat shrink tubing was attached with a clip and RTV Silicone to the anode end-plate stud, and both ends were capped with rubber covers for protection (of the tube and user).

    The SP-084-1 is about 9-1/2" (241 mm) by 1" (25.4 mm) in diameter with a bore length of 5.5" (140 mm). Its output is a TEM00 beam about 0.8 mm in diameter exiting through a hole in the cover on the cathode-end of the tube. Power supply connections are made to a stud on the anode end-plate and the exhaust tube on the cathode end-plate. Their optimal operating point is around a tube current of 5 mA resulting in a total operating voltage (across tube + Rb) of about 1.9 to 2.0 kV using the 100K ballast.

    Note from the diagram that unlike modern tubes where the mirrors are on mounts that can be adjusted (by bending) after manufacturer, alignment of the SP-084-1 would appear to be totally fixed. Some possible ways of setting alignment might be:

    1. The mirrors were just glued in place expecting alignment to be adequate (but the end-plates do not appear to be specially machined).

    2. The mirrors were aligned at installation using external optics but before the tube had been pumped down and filled with helium and neon.

    3. The manufacturing process provided a means of adjusting the mirrors after filling but before the glue had fully set or by softening it with heat.

    4. There was some means of distorting the end-plates (but this doesn't seem likely given their thickness).

    From appearances, I would guess (2). Since the mirrors are slightly curved (non-planar), their position could be used to adjust alignment slightly - and some were attached very visibly off-center to compensate for end-plates fused to the glass tube at a slight angle.

    HeNe Laser Pointers

    While modern laser pointers fit comfortably on a keychain and can be had for $1 or less if you know where to look, the first laser pointers were, well, HUGE and at least several hundred dollars. :) One of the earliest laser pointers using a HeNe laser tube I've seen (dating from the late 1970s) was about 12 inches long by 1-3/4" in diameter (just like a common HeNe laser head). The name on it is Bergen Expo Systems, Inc. and it is a model LP6-227 should you want to order one. :) The date of manufacture was 1978. This pointer was tethered via a six foot cord to a separate high voltage power unit. See Bergen HeNe Laser-Based Laser Pointer. The beam on/off button on the side not surprisingly didn't control the power supply but rather moved a sliding shutter. The actual manufacturer was probably Spectra-Physics as the tube inside was an SP-084 (a common barcode scanner type). It also has the funny 3 pin power supply connector mainly used by Spectra-Physics, though some other Bergen pointers have used the standard 2-pin Alden connector. I don't have the power supply so can't say what it looked like.

    More recent HeNe laser-based laser pointers became more compact and some ran off a bunch of AA or 9 V batteries. But they never achieved keychain status, unless they were keys for elephants. :) I have HeNe laser pointers badged Kodak and Hitachi which output almost 1 mW and run off a pair of 9 V batteries or a DC wall adapter. Battery life is, well, short. :)

    It is still possible (in 2010) to buy a HeNe laser in a compact package. The Metrologic model 811 (red, $399) or 815 (green, $750) is not much over 1" x 2" x 7" and houses a 5 or 6 inch HeNe laser tube with HV power supply built-in. However, these are still tethered to a DC wall adapter, though a bettery box option is available.

    There's not much demand for these as pointers anymore, though they still are useful as compact lasers for alignment and other optics lab applications. But they are still very cute. :)

    HeNe Lasers using External Mirrors

    While most of what you will likely come across are the common internal mirror HeNe tube, having the optics external to the tube is essential for some applications.

    A One-Brewster HeNe Laser Tube

    I was given a CLIMET 9048 HeNe laser head which contains a Melles Griot HeNe tube with a normal HR mirror at one end but with a frit-sealed Brewster window instead of an OC mirror at the other end. In this case, it is the cathode-end which is nice since there is no high voltage to deal with near the Brewster window. But identical tubes also come with the Brewster window at the anode-end but why anyone would want this excapes me. :) (And, several other models of one-Brewster tubes are common - see the section: Melles Griot Brewster and Zero Degree Window HeNe Tubes.)

    The tube is a Melles Griot model 05-LHB-570. It has an internal HR mirror and Brewster window at the other end of the tube. The HR is similar to those on other Melles Griot tubes (including the use of a locking collar) though the somewhat more silvery appearance of its surface may indicate that it is coated for broadband reflectivity and/or perhaps for higher reflectivity than ordinary HRs. (The mirror reflectivity of the HR on at least some versions of the 05-LHB-570 is greater than 99.9% from 590 to 680 nm but I don't think this one, which is quite old, has these characteristics.) The total length is about 265 mm (10.5 inches) from the HR mirror to the Brewster window. There is also a power sensor inside the head for (I assume) monitoring what gets through the HR mirror (untested).

    CLIMET 9048 One-Brewster HeNe Laser Head shows the aluminum cylinder with its mounting flange at the Brewster window end, ballast resistor, and Alden connector. The other black wire attaches to the solar cell power sensor.

    These one-Brewster HeNe tubes are generally used in applications like particle counting which requires high photon flux to detect specks of dust or whatever. Access to the inside of the resonator is ideal since with appropriate highly reflective mirrors at both ends, several WATTs of "virtual" circulating power can be produced inside the cavity of this HeNe laser. Thus, for these applications, they have the benefits of a high power laser without the cost or safety issues. There are even HeNe tubes similar to this that will do up to 45 W using super high quality mirrors and Brewster window. And, of course, they are also super expensive. Of course, you can't siphon off all that power - only be extremely envious and frustrated that it is trapped in there - but also safe from any sneak attacks on an unsuspecting eyeball. :)

    A rig similar to the one from which the Climet 9048 was removed is a model 8654, whatever that means. It is shown in Climet Particle Counter Assembly - Front and Climet Particle Counter Assembly - Rear. There really isn't much inside - just some passages for the particle-containing gas which is directed to through the intracavity beam at one focus of a large aspheric lens which directs any scattered light onto a PhotoMultiplier Tube (PMT). The PMT is inside the black box at the lower left with its high voltage power supply above in the front view. The three-screw (sort of) adjustable mount for the external HR mirror is visible in the rear view. What's interesting is that there is really nothing physical to protect either the B-window or mirror from contamination by the flowing gas, except presumably by the flow pattern and pressure. There are separate compartments for the B-window and mirror, but they aren't sealed. However, it appears that during operation, those compartments are provided with a flow of higher pressure gas, filtered by the large canister visible in the photos. But, how they are expected to remain clean when the thing is shut down is a mystery. It is a particle counter after all. Aren't particles basically dust? :) OK, well, part of the secret is that apparently these things are intended to be looking at really clean air without many particles. A typical use would be in a semiconductor Fab Class 10 cleanroom - 10 or fewer particles (2 microns or larger) per cubic foot. This isn't your normal room air, which would be Class 10000 to Class 100000! :) Even so, the recommended service interval printed on the label is only 6 months.

    With its wide bore, this tube has an optimal operating point (maximum power) of about 7.5 to 8 mA at about 1 kV (though the recommended current is actually 6.5 mA). This may just be a peculiarity of the sample I tested.

    I have constructed a simple mirror mount so that various mirrors could be easily installed and there is easy access to the inside of the cavity. See HeNe Laser Tube with Internal HR and Brewster Window with External OC for a diagram showing this laser assembly. Using various mirrors, both from deceased HeNe lasers as well as from laser printers and barcode scanners, output power reached more than 3 mW and the circulating power inside the resonator peaked at over 1 W (but not with the same mirrors). With optimum high quality mirrors, it should be capable of more power in both areas. Photos of this laser are shown in Sam's External Mirror Laser Using One-Brewster HeNe Laser Head.

    See the section: Sam's Instant External Mirror Laser Using a One-Brewster HeNe Tube for details on these experiments and the design of the mirror mount.

    I have attempted to get wavelengths other than boring 632.8 nm red out of this and similar 1-B tubes. However, all attempts have failed but one - installing a somewhat larger 05-LHB-670 in place of the dead tube of a PMS/REO tunable HeNe laser. (This 1-B tube did 7.5 mW with the same OC mirror as used above. The 1-B tube in the Climet head probably woudn't have enough gain.) The HR mirror on the tuning prism is broadband coated for 543.5 to 632.8 nm. In this case, I was able to convince just a few 611.9 nm orange photons to cooperate and lase. However, the only way to collect them was from the reflections off the Brewster surfaces of the tube or prism, or from the HR mirror of the 1-B tube. The total orange power was around 225 microwatts - 50 uW from the HR mirror, 65 uW reflected from the Brewster prism, and 110 uW reflected from both surfaces of the tube's Brewster window. When 633 nm was selected, the output from the HR mirrors was about 350 uW (I didn't measure the red power from the Brewster reflections).

    Designing a Helium-Neon Laser Tube

    (From: Lynn Strickland (stricks760@earthlink.net).)

    H. Weichel and L.S. Pedrotti put out a good summary paper which includes the equations used in the design process of a gas laser. In particular, section V tells you how to calculate mode radius at any point, given mirror curvature, spacing and wavelength. If you know that, the aperture size (the capillary bore usually) and the magic number for the ratio between the two, you can design a TEM00 gas laser. Using a HeNe tube with a Brewster window, you could do some fun stuff with predicting aperture sizes and locations to force TEM00 operation.

    The paper was published by the Department of Physics, Air Force Institute of Technology, Wright-Patterson Airforce Base, OH. The title is "A Summary of Useful Laser Equations -- an LIA Report". Don't know where you'd find it, but the Laser Institute of America (LIA) might be a good start.

    Parallel Plate HeNe Laser Tube

    When HeNe lasers were becoming really popular in the late 1970s, efforts were under way to reduce costs. Not surprising, huh? :) IBM reported on a novel approach using molded parallel plates which had some similarity to flat planel display fabrication. See:

    Needless to say, the parallel plate HeNe laser never took off but it was an interesting approach.



  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Wavelengths, Beam Characteristics

    HeNe Laser Wavelengths

    While what comes to mind when there is mention of a HeNe laser is a red beam, those with other wavelengths are manufactured.

    Typical maximum output available from (relatively) small HeNe tubes (400 to 500 mm length) for various colors: red - 10 mW, orange - 3 mW, yellow - 2 mW, green - 1.5 mW, IR - 1 mW. Higher power red HeNe tubes (up to 35 mW or more and over 1 meter long) and 'other-color' HeNe tubes (much lower - under 10 mW) are also available. However, these will be very large and very expensive.

    Tunable HeNe Lasers

    If it were possible to select any available wavelength desired, then some people would be content beyond description. :)

    A few tunable HeNe lasers have been produced commercially. These provide wavelength (color) selection with the turn of a knob. However, due to the low gain of most HeNe lasing lines, producing a useful tunable HeNe laser is not an easy task. Everything must be just about perfect to get the "other color" lines to lase at all, and even more so when a laser is to be designed to work at more than one wavelength with a TEM00 beam. The most widely known such laser (as these things go) is manufactured by Research Electro-Optics, Inc. (REO). It produces at least 5 of the visible wavelengths: normal red, two oranges, yellow, and green. A Littrow (or Brewster) prism with micrometer screw adjusters takes the place of the HR mirror in a normal HeNe laser. See the section: Research Electro-Optics Tunable HeNe Lasers.

    There used to be a model ML-500 tunable HeNe laser from Spindler and Hoyer that did *14* lines between 611 nm and 1,523 nm. So no 604 nm orange, 594.1 nm yellow, 543.5 nm green, or 3.39 µm IR. The mirror set had to be changed to go between the visible and IR wavelengths. It used a Birefringent Filter (BRF) for wavelength selection instead of the Littrow prism in the REO tunable laser. A BRF has the advantage that there is no loss from a slightly incorrect Brewster angle for all but one wavelength, unavoidable with a Littrow prism. This is because the BRF is always set at exactly the Brewster angle. The birefringent crystal in the BRF filter produces a different optical delay for polarization components oriented in the direction of its slow and fast axes. Only when this difference is a multiple of a full cycle for any given wavelength, will the polarization be unchanged and thus result in minimal loss through the BRF. By rotating the BRF around its optical axis (still maintaining it at the Brewster angle to the laser's optical axis), the wavelength where minimum loss occurs can be selected. In 1987, it was only $5,800 for laser with either wavelength range, an additional $750 for the other mirror set

    I don't know why Spindler and Hoyer would have admitted defeat in not including those other wavelengths as they were certainly known at the time. Perhaps, the losses through the two Brewster windows of their laser tube and the Brewster angled plate of the BRF compared to those of the Brewster window and Brewster prism of the PMS/REO tunable laser were just too high. Perhaps, their mirror coating technology was not as good as what PMS/REO had available.

    Unfortunately, Spindler and Hoyer no longer makes this laser, only boring normal HeNe lasers and other optical equipment. However, a scan of the original ML-500 product brochure can be found at Vintage Lasers and Accessories Brochures and Manuals. With modern technology, a 17 line tunable HeNe laser should be possible. :) A tube with internal mirrors and a BRF *inside* would reduce the number of Brewster angle reflective surfaces to only 2, compared to the 3 of the PMS/REO design. A magnetic coupling can be used to move the BRF from outside the tube. In addition, the mirrors can be recessed away from the ends of the tube so they don't experience any high temperatures during the sealing process. The tube itself would be hard-sealed with frit or regular glass. Then optical contacting or leaky Epoxy seals can be avoided. Then use a Brewster angle window to pass the laser beam out of the tube. One of the mirror mounts would be attached via a metal bellows to allow for alignment.

    Exact Frequency/Wavelength of HeNe Lasers

    There is, of course, no single precise HeNe wavelength since any given cavity will only oscillate at the permitted longitudinal modes and the gain curve is something like 1.5 GHz wide. Thus, for a common HeNe laser, there is no single wavelength and those that are present drift over time (mostly due to thermal expansion of the cavity). A single mode frequency stabilized HeNe laser will have very nearly a constant single wavelength precise to 9 or more significant figures but it too will depend on the physical size of the laser's cavity - there is no one correct answer!

    For example, one typical stabilized HeNe laser from Hewlett-Packard, has a precise vacuum wavelength of 632.991372 nm. Another one from Melles Griot (as noted below) is 632.991058 nm in vacuum or 632.81644 nm in air (divide by the index of refraction of air, n=1.00027593).

    (Portions from: Jens Decker (Jens.Decker@chemie.uni-regensburg.de).)

    The Melles Griot catalog claims a nominal frequency of 473.61254 THz for their 05-STP series of frequency stabilized lasers. (Elsewhere in the same catalog they are more precise and lists 473.612535 THz for the 632.8 nm line.) Anyhow, with c = 2.997925E8 m/s this gives 632.991058 nm in vacuum or 632.81644 nm in air for n = 1.00027593 (formula from J Phys.E, vol. 18, 1985, pp. 845ff). To find reliable values for all the other HeNe lines is quite difficult. One has to compare a number of books to be sure whether the values are for air or vacuum.

    (From: D. A. Van Baak (dvanbaak@calvin.edu).)

    Well, here it is exact:

    The metrologists' answer for a 632.8 nm HeNe laser stabilized to the a-13 component of the R(127) line of the 11-5 transition of the 127-Iodine dimer molecule is:

    under certain specified conditions, with uncertainty 2.5x10-11. See: "Metrologia", vol. 30., pp. 523-541, 1993-1994.

    HeNe Laser Beam Characteristics

    Compared to a diode laser, the beam from even an inexpensive mass produced HeNe tube is of very high optical quality:

    Ghost Beams From HeNe Laser Tubes

    If you project the output from some HeNe laser tubes (as well as other lasers) onto a white screen a meter or so away, you may see a main beam and a weak beam off to the side a few cm away from it. Maybe even another still weaker one after that.

    Most internal mirror HeNe tubes should not have any higher order transverse (non-TEM00) modes. And, for multimode tubes, such modes should show up as part of, or adjacent to the main beam anyhow.

    One possible cause for this artifact is that the output-end mirror (Output Coupler or OC) has some 'wedge' (the two surfaces are not quite parallel) built in to move any reflections - unavoidable even from Anti-Reflection (AR) coated optics - off to the side and out of harm's way. Where wedge is present, the small portion of the light that returns from the outer AR coated surface of the OC will bounce back to the mirror itself and out again at a slight angle away from the main beam. In a dark room there may even be additional spots visible but each one will be progressively much much dimmer than its neighbor. Note that if the laser had a proper output aperture (hole), it would probably block the ghost beams and thus you wouldn't even know of their existence!

    Without wedge, these ghost beams would be co-linear with the main beam (exit in the same direction) and thus could not easily be removed or blocked. This could result in unpredictable interference effects since the ghost beams have an undetermined (and possibly varying) phase relationship with respect to the main beam. Sort of an unwanted built-in interferometer! The wedge also prevents unwanted reflections from that same AR coated front surface back into the resonator - perfectly aligned with the tube axis - which could result in lasing instability including cyclic variations in output power.

    Thus, the ghost beam off to one side is likely a feature, not a problem! The effects of wedge on both the output beam and a beam reflected from a mirror with wedge is illustrated in Effects of Wedge on Ghost Beams and Normal Reflections. Note that his diagrams shows the effect of a beam coming in from the right and reflecting off the mirror. Where the beam is from the tube itself, the main beam corresponds to the one marked "1st Back Surface".

    If it isn't obvious from close examination of the output mirror itself that the surfaces are not parallel, shine a reasonably well collimated laser beam (e.g., another HeNe laser or laser pointer) off of it at a slight angle onto a white screen. There will be a pair of reflected beams - a bright one from the inner mirror and a dim one from the outer surface. As above, if the separation of the resulting spots increases as the screen is moved away, wedge is confirmed (there may be higher order reflections as well but they will be VERY weak - see below). Where the mirror is curved, the patterns will be different but the wedge will still result in a line of spots at an angle dependent on the orientation of the tube.

    Wedge is often present on the other mirror (High Reflector or HR) as well (in fact, this appears to be more likely than the OC). Wedge at the HR-end won't affect the output beam at all but performing the reflectance test using a collimated laser (as above) at a near-normal angle of incidence may result in the following:

    With the exaggerated amount (angle) of wedge in Effects of Wedge on Ghost Beams and Normal Reflections, another effect becomes evident: The weaker spots are spaced further apart. It is left as an exercise for the student to determine what happens when a laser beam is reflected at an angle from such a mirror! Note that his diagrams shows the effect of a beam coming in from the right and reflecting off the mirror. Where the beam is from the tube itself, the main beam corresponds to the one marked "1st Back Surface".

    The appearance resembles that of a diffraction grating on such a beam (but for entirely different reasons). The behavior will be similar for an OC with wedge but because the HR mirror isn't AR coated, the higher order spots (from the HR) are much more intense.

    It is conceivable that slight misalignment of the mirrors may result in similar ghost beams but this is a less likely cause than the built-in wedge 'feature'. However, if you won't sleep at night until you are sure, try applying the very slightest force (a few ounces) to the mirror mounts (the metal, not the mirrors as they are very fragile) in each while the tube is powered (WARNING: High Voltage - Use a well insulated stick!!!!).

    Depending on the type of laser you have, see the sections: Checking and Correcting Mirror Alignment of Internal Mirror Laser Tubes, Quick Course in Large Frame HeNe Laser Mirror Alignment, and External Mirror Laser Cleaning and Alignment Techniques, for more information.

    Another much simpler cause of an ugly beam from a HeNe (or other) laser is dirt on the outside of the output mirror since this will decrease the effectiveness of the AR coating. The dirt may also be on other external optics. Some HeNe laser heads have either a debris blocking glass plate glued at an angle to the end-cap or a neutral density filter to adjust output power. Even if AR coated, either of these may also introduce one or more ghost beams and if not perfectly clean, other scatter as well. I'm gotten supposedly bad HeNe lasers where the only problem was dirt on either the output mirror or external plate or filter.

    (From: Steve Roberts.)

    The mirror is wedged to cut down on the number of ghost beams, however even with a wedged mirror there is almost always one ghost. Nothing is wrong with your coatings on the mirror, it is simply a alignment matter. The mirrors need to be "walked" into the right position relative to the bore. There are many many paths down the bore that will lase, but only a few have the TEM00 beam and the most brightness, this generally corresponds to the one with minimum ghosts.

    See the section: Quick Course in Large Frame HeNe Laser Mirror Alignment for more information.

    Other Spectral Lines in HeNe Laser Output

    While there is no such thing as a truly monochromatic source - laser or otherwise, the actual output beam of even an inexpensive HeNe laser is really quite good in this regard with a spectral line width of less than 1/500th of a nm. For a frequency stabilized HeNe laser, it can be 1,000 times narrower!

    But if you look at the output of a HeNe laser with a spectrometer, there will be dozens of wavelengths present other than one around 632.8 nm (or whatever is appropriate for your laser if not a red one). Close to the output aperture, there will be a very obvious diffuse glow (blue-ish for the red laser) visible surrounding the actual beam. So why isn't the HeNe laser monochromatic as expected?

    With one exception, this is just due to the bore light - the spill from the discharge which makes it through the Output Coupler (OC) mirror. As your detector is moved farther from the output aperture, the glow spreads much faster than the actual laser beam and its intensity contribution relative to the actual beam goes down quickly. It is not coherent light but what would be present in any low pressure gas discharge tube filled with helium and neon. However, the presence of these lines can be confusing when they show up on a spectral printout.

    The exception is that with a 'hot' (unusually high gain) tube or one with an OC that is not sufficiently narrow-band, one (though probably not more though not impossible) of the neighboring HeNe laser lines (e.g., for other color HeNe lasers) may be lasing though probably much more weakly than the primary line. For example, a red (632.8 nm) laser might also produce a small amount of output at 629.4 or 640.1 nm though this isn't that common. For many applications, a bit of a "rogue" wavelength output is of little consequence and specifications for general purpose HeNe lasers usually don't explicitly include any mention of them. However, rogue output will cause reduced accuracy in metrology applications and since they may not be TEM00, even where the beam is simply used for alignment.

    I have a couple of 05-LHP-171 lasers that produce up to 10 percent of their output at 640.1 nm. The first is of unknown pedigree obtained in a lot laser junk from a well known laser surplus dealer. It may have been rejected for other reasons since the output at 632.8 nm is only about 4 mW when it should be well over 7 mW. The 632.8 nm is the normal TEM00 but the 640.1 nm beam may be TEM01 or TEM10 (2 modes) or even TEM11 (4 modes) depending on mirror alignment. With optimal mirror alignment for 632.8 nm, there may be no 640.1 nm at all. The other is a 25-LHP-171-249 system sold to a university lab. It has a manufacturing date of 2000, so this isn't only a problem with old lasers as some people have claimed.

    I have one 'defective' yellow (594.1 nm) HeNe tube that also produces a fair amount of orange (604.6 nm), and another that produces in addition some of the other orange line (611.9 nm).

    While the probability of a commercial HeNe laser outputting at a rogue wavelength is low, where such a laser is used for measurements assuming pure 632.8 nm, errors could result. For more on this topic, see the paper:

    In the course of research for this paper, the first author, Jack Stone, borrowed one of my interesting Melles Griot 633 nm lasers that produced 5 to 10 percent of its output at 640.1 nm! :)

    (From: Prof Harvey Rutt (h.rutt@ecs.soton.ac.uk).)

    For gas lasers the plasma lines are typically 80 dB or more below the output (measured, of course, within the very small laser mode divergence). This is unlike most semiconductor lasers, which typically have broad 'shoulders' close in to the line, as well as 'lines' due to other modes and instabilities because the initial divergence of the diode is high, and spontaneous emission from the junction high, the broad background tends to be large.

    For gas lasers it is usually in the form of narrow lines at remote wavelengths, very easily removed with an interference filter and/or spatial filtering in the *rare* cases where it matters. There is presumably a weak broad background from processes involving free electrons (bound/free and free/free), but I've never seen it even mentioned, let alone observed it. More likely to be significant in the high current density argon laser than the very low current density HeNe.

    The only cases I have seen where the plasma lines caused problems were Raman measurements on scattering samples with photon counting detection, and weak fluorescence measurements which are similar.

    In most cases scattered light in the monochromator is much more of an issue (hence double monochromators for Raman) and will obscure plasma lines in many cases.

    Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes

    As a practical matter, the only wavelength that is useful from an internal mirror HeNe laser is the one for which it was designed. (Or the pair in the case of a couple of Research Electro-Optics (REO) lasers.) However, it is often possible to at least obtain unstable lasing at other wavelengths by extending the cavity using an external mirror. The output power of the other lines can be anywhere from almost non-existent to greater than the power at the original wavelength. This probably works best obtaining a some red from a long "hot" yellow (594.1 nm) or orange (611 nm) tube since at least one mirror is likely coated broadband to include yellow through red. Due to the low gain of the non-red lines, going the other way - getting yellow from a red tube, for example - is not likely to succeed unless the tube is very long. But obtaining lasing at other red wavelengths - and even orange - may be possible with a moderate size red HeNe laser tube. Even a 1 mW tube may give you 1 or 2 other red lines. I doubt it will work at all with a green HeNe tube having mirrors that appear orange in transmission since both mirrors are probably too transparent at even the yellow wavelength (except possibly if two external mirrors are used). However, if a mirror is more red in transmission, there might be a chance. See the section: Instant HeNe Laser Theory for a table of HeNe lasing wavelengths and relative gains.

    I've gotten most of the well known HeNe lasing lines in this manner including up to 4 mW of red from a 2 mW yellow HeNe laser, both orange lines, various other red lines, and one of the wavelengths that isn't even mentioned in most texts dealing with HeNe lasers. More below. I've only heard of one instance of any yellow being produced from non-yellow tubes, that being a REO 612 nm laser. And I haven't even attempted to obtain green from non-green tubes.

    Here's how to get other wavelengths from your HeNe laser. Either a bare tube or complete laser head can be used for these experiments.

    Using my Melles Griot 05-LYR-170 yellow HeNe tube which for my "broken" sample, actually lases a combination of yellow (594.1 nm) and orange (604.6 nm) from both ends (see the section: The Dual Color Yellow/Orange HeNe Laser Tube), it was quite easy to achieve red output, and all three colors were occasionally present at the same time - an impressive achievement for a HeNe laser. My setup is shown in 05-LYR-170 HeNe Laser Tube Mounted in Test Fixture for Multiline Experiments. The output from the tube's OC was directed at an AOL CD used as a reflective diffraction grating with the first-order beam projected on a white card several feet away. An MSN CD would work just as well :) but a CD-R or CD-RW may not. The lens from a pair of eyeglasses (mildly positive, about 4 diopters or 1/4 meter focal length) narrowed the spots to improve spectral resolution. This rig could easily resolve lines separated by less than 1 nm. The first external "red" mirrors I tried were from an SP-084 HeNe laser tube but due probably to their relatively short RoC, the 05-LYR-170 had to be pushed quite close to the mount to get any red output. Mirrors designed for a longer laser worked better but there wasn't much difference between the behavior using an HR or OC (99 percent).

    Then to add to the excitement, with a bit of twiddling, I was able to obtain the other orange line (611.9 nm) as well, and at times, all 4 lines were lasing simultaneously! As expected, this additional line was only present when using an external HR. Depending on the original makeup of the yellow and orange beam (for this tube, their absolute and relative intensities varied with time and were also a very sensitive function of mirror alignment), it was possible to get mostly red or to vary the intensities of the other colors, most easily suppressing yellow in favor of orange and red. The intensity of the red output was never more than 1 mW or so. Its transverse mode structure varied from TEM00 to a star pattern with nothing in the center. Strange. Due to both surfaces of the HeNe tube's HR mirror reflecting some of the intracavity beam resulting in a multiple cavity interference effect, there was a distinct lack of stability. To help compensate for this, a micrometer screw to precisely adjust cavity length without affecting mirror alignment would have been nice.

    I also tried this with the external mirror mounted beyond the tube's OC mirror but although there was a definite effect on yellow and orange lasing, it wasn't possible to obtain any red output. (For the 05-LYR-170, the OC already reflects red quite well and the HR doesn't.) Finally, I replaced the red external mirror with a green HR (from a tube of about the same length) mounted beyond the 05-LYR-170's OC (since its HR by appearance looked like it might be a good mirror for green). But, not surprisingly, while this could affect the lasing of the yellow and orange lines, I could detect no coherent green photons. However, I would expect that with a appropriately coated mirrors (or possibly two such mirrors, one beyond each end of the tube), obtaining lasing at the relatively high gain 640.1 nm red line would be easy - the usual "red" mirrors may deliberately kill this line to prevent it from lasing. Although I couldn't detect any evidence of lasing at the other red lines of 629.4 nm and 635.2 nm, these should also be possible with appropriate mirrors as they have higher gain than the yellow and oranges. Another interesting one would be the "Border Infra-Red" line at 730.5 nm. Lasing at the IR lines might also be possible but they are so boring. :)

    Next, determined to do something with a more normal HeNe laser tube, I tried a Siemens tube but that refused to do anything interesting. Then, I tried a Melles Griot 05-LHR-150 which typically outputs a 5+ mW red (632.8 nm) beam. Since the OC for this laser is probably around 99% reflective at most, peaking at 632.8 nm, I figured that it would be best to place the external mirror beyond the OC rather than the HR. And, with the same external HR as used above, it was possible to obtain 6 lasing lines, count'm 6: 629.4 nm, 632.8 nm, 635.2 nm, 640.1 nm, a line popping up around 650 nm (all variations on red), ****AND**** 611.9 nm orange! However, since the output is being taken from the HR, none of the colors was more than a fraction of a mW.

    Lasing of the 650 nm line was hard to obtain - it only showed up for a few seconds off-and-on every few minutes and increasingly rarely after the tube warmed up. The exact wavelength is very close to 650 nm (649.98 nm) as determined later with an Agilant 86140B Optical Spectrum Analyzer (OSA) which is a lot more expensive than my AOL CD. :) (The wavelength was referenced to the 632.8 line from the same laser resulting in a measurement error bound of +/- 0.02 nm assuming the 632.8 nm line is actually 632.8 nm. But since this could also be slightly shifted, the error may be higher.) Getting anything at 650 nm is really puzzling as there are no HeNe lasing lines between 640.1 nm and 730.5 nm. But I have no doubt it is a true lasing line since it was fluctuating independantly of the others (later confirmed, see below). And all those other lines were quite accurately located corresponding to their handbook wavelengths in the diffracted pattern (and later confirmed with the OSA). So there is little reason to suspect that the funny one isn't as well. When present, it appeared as strong (or weak) as all the expected ones, (except of course, the original 632.8 nm line which was usually, but not always, the strongest). If 650 nm is not a HeNe lasing line - it's certainly not in the sequence of energy level transitions that produce all the other visible HeNe lines - one possible explanation is that there is some trace element present inside the tube and that is what's lasing, not neon. I figured this to be a distinct possibility since the particular tube I am using originally had gas contamination and I revived it by heating the getter. (See the section: Repairing the Northern Lights Tube.) Therefore, the 650 nm wavelength may not be present with another more normal tube. But as it turned out, contamination has nothing to do with it.

    I don't think the 730.1 nm line was present but given its low relative perceived brightness, it may not have been visible at all using my AOL Special CD diffraction grating but I couldn't find it with the OSA either. It took awhile to detect the evidence of the 635.2 nm line which only appeared sporatically (but it is the lowest gain of all the known ones above).

    A few days later, I tried the same experiment with a couple of my old Spectra-Physics 084-1 HeNe laser tubes which are of soft-seal design so have almost certainly leaked over time (but still work fine). With my "hottest" SP084-1 (about 2.9 mW), I could almost duplicate the results of the 05-LHR-150 including the funny line around 650 nm but minus anything at 635.2 nm. Using a more normal 2.4 mW SP084-1, it was possible to obtain (non 632.8 nm) lines at 629.4 nm and 640.1 nm. For these, an SP084-1 HR worked almost as well for the external mirror as the longer RoC HR I had been using with the 05-LHR-150. I then installed a SP098-1, a common hard-seal barcode scanner tube (this sample puts out about 1.4 mW). With that, the only additional line was at 640.1 nm. Which particular lines appear in each case seem consistent with the length of the tubes (and thus the single pass gain) and the relative gain of the lasing lines.

    Some quick calculations predict that the real effect of the external HR mirrors is the obvious one - to increase the circulating power. A 1 percent OC (typical) followed by even a 90 percent external mirror would result in greater than a 99.9 percent effective mirror for a range of wavelengths/modes. An external 99.9 percent HR would result in an even better effective mirror. It looks like the reflectance peak is relatively broad with respect to wavelength (the transmission peak is rather narrow). Specific modes for each of the wavelengths will be enhanced or suppressed. This would also appear to be consistent with the apparent lack of need for the external mirror to result in a stable resonator. All it has to do is form a Fabry-Perot cavity.

    These have to be classified right up there in the really fascinating experiments department. Seeing any HeNe laser operating with multiple spectral lines is really neat.

    For more examples of these stunts using an already interesting "defective" HeNe laser, see the sections starting with: Melles Griot Yellow Laser Head With Variable Output and in particular, the section: External Mirror Therapy for Variable Power 05-LYR-171 Yellow Laser Head.

    As always, depending on mirror reflectivity and other factors, your mileage may vary. But feel free to try variations on these themes. The results from using an HeNe HR beyond the OC of almost any red HeNe laser tube should be easily replicated (except perhaps for the funny 650 nm line). Almost any mirror will do something since even an aluminized mirror will be returning over 90 percent of the otherwise wasted photons to the cavity - enough to boost the gain of all but the weakest lines enough for lasing if everything lines up just right. Aside from getting zapped by the high voltage or dropping the tube on the floor, they are low risk, high reward experiments.

    And, can you believe that people get stuff like this published in scholarly journals? I was recently sent an article entitled: "Yellow HeNe going red: A one-minute optics demonstration" by Christopher Hopper and Andrzej Sieradzan, American Journal of Physics, vol. 76, pp. 596-598, June 2008. Geez, they could have saved a lot of time and effort and come here instead. Or, perhaps they did. :)

    (From: Bob.)

    For neutral neon at low pressure, the lines 640.3 nm, 659.9 nm are listed. For neutral helium, there is one at 667.8 nm. None of the other noble gases have wavelengths listed this short. As far as ionized species go, singly ionized argon has a line at 648.30 nm. Singly ionized krypton has a hand full of lines from 647 nm to 657 nm. Finally, xenon has one at 652 nm.

    For atmospheric gases, there is a singly ionized nitrogen line at 648.3 nm. There are no neutral lines of interest for atmospheric gases. The footnotes for the above line were listed as CW lasing in 0.02 torr of krypton. Whats the standard operating pressure of a HeNe laser? Not THAT far out of the ball park I would guess.

    (From: Sam.)

    The last one sounds promising and would make sense given the history of the particular 05-LHR-150 and the soft-seal design of the SP084-1. Though HeNe lasers operate in the 2 to 3 TORR range - about 100 times higher pressure, the partial pressure of any N2 contamination could very well be down around 0.02 Torr.

    However, I now know exactly where the 650 nm line is coming from and it has nothing whatsoever to do with contamination. The exciting writeup from someone who beat me to this by about 15 years follows in the next section preceeded by a condensed version, below.

    I've also found a commercial laser that appears to produce a very stable 650 nm line. See the section: The PMS/REO External Resonator Particle Counter HeNe Laser.

    (From: Stephen Swartz (sds@world.std.com).)

    Lasing of certain HeNe tubes at 650 nm is a known phenomenon and not just a hallucination. The 650 nm line which is never discussed in most standard texts is not due to a "normal" transition of neon. It comes instead from a Raman transition. The 650 nm line is not often observed but when it is it will always be seen simultaneously with operation on a multitude of other lines. A large number of other "unusual" colors have been seen over the years. Higher power tubes with mirrors that are excessively broadband are your best bet for observing them. Often these lines flicker on and off over a few seconds to minutes time scale. A diffraction grating is a good way to look for them.

    (From: Someone at a major laser company.)

    The 650.0 nm Raman line is a known problem in that it competes for power with the 632.8 nm line intermittently, particularly in long tubes with high circulating power. Polarized tubes are much less susceptible to this effect and using a lower reflectance for the OC mirror helps since it reduces circulating power without affecting output very much (over a reasonable range).

    Bruce's Notes on Getting Other Lines from Red (633 nm) HeNe Laser Tubes

    This, to make a gross understatement, would appear to be the definitive word on coaxing other colors from surplus HeNe laser tubes. And I thought six lines (including the mysterious 650 nm line) was an achievement. :)

    (From: Bruce Tiemann (BruceT@ctilidar.com).)

    I have gotten many lines from many different HeNe lasers. In my experience almost every tube is capable of giving at least one other line than 633 nm. (Most wavelengths have been rounded to save bits. So, 632.8 nm becomes 633 nm.) I have never tried doing this with lasers that give other lines than 633 nm, but since that line has the highest gain, it should be no mean feat to at least get that line from lasers that are supposed to not give it. It is also not my experience that calculations to ensure resonator stability, etc., are necessary. Just try it! My best results, in terms of output power, were with a flat grating as the external feedback mirror, and my best results in terms of new lines was obtained with a flat dielectric mirror, formerly used as a facet in a polygonal scanning assembly. Flat mirrors are not stable at any separation for a diverging beam, and HeNe lasers are very rare that give converging beams for their output.

    The home stuff had the mirrors on blocks, with the steering accomplished by adjusting the HeNe tube by lifting one or the other end of the tube with sheets of paper, and the azimuth by moving the laser tube back and forth. The lab experiments were done with "real" mirror mounts, supplemented by a single PZT that tilted the feedback mirror a few microns.

    (I like PZTs a great deal, and would like to observe that you can get PZT elements from little piezo alarms, from which the useful element can be extracted with some hand-tools and the mind-set of a 9-year-old kid dissecting a bug. :) These are only about $1 each, as opposed to tens to hundreds of bucks for "real" PZTs that you buy from Thor, etc. One of them and a 0 to 50 VDC power supply can precision-wiggle a mirror on the micron scale, which is all that is needed for these experiments.)

    (From: Sam.)

    I have indeed done something similar using the piezo beeper from a dead digital watch to move a mirror in a HeNe laser based Michelson interferometer. With 0 to 25 V, it went through 4+ fringes which means over 2 full wavelengths at 633 nm. The configuration in these is called a "drum head" piezo element because the movement resembles that of a musical (depending on your point of view!) drum head with the most shift in the center. The piezo material itself doesn't change by very much in thickness but is constructed so it distorts to produce the shape change. With care, the piezo material can be cut to size or drilled to pass light through its center. Much more voltage could have been safely applied if needed.

    (From: Bruce.)

    Something I also did is cast the spots from a smaller (approximately 3/4 m) spectrometer directly onto the CCD element of a small camera with no lens. I also fabricated a beam block by taping little wires to the side of a block, that would protrude up just in the locations of the very bright lines, like 633, 650, and 612 nm, to block them, but letting light of other colors pass in the ample space between the wires. You could still see when the bright lines were on from light leaking around the wires, but it wouldn't wash out the image when they were.

    In this case, when the feedback mirror was tilted, speckle, which was cast everywhere, would kind of shift around all over the place, but the new lines looked like ghostly bullseyes, which would breathe in and out as the mirror was tilted, but remain in the same location unlike the speckle. This was an easy way to see the weakest lines like 624 nm, and it was also how I discovered 668 nm, the CCD being more sensitive than the eye in the deep red. (I searched for but did not find the normal laser line 730 nm even with this very sensitive method.)

    All in all this laser produced 17 different lines, many at one time, from a "single line" 633 swap-meet laser. :)

    References:

    The 650 nm discovery paper is:

    Miscellaneous Comments on Getting Other Lines from HeNe Laser Tubes

    (From: Flavio Spedalieri.)

    I have a small Yellow Tube - 05-AYR-006 (as a combo with power supply 05-LPM-496-037). This tube is physically the same size at the 1mW reds, but has a larger bore resulting in multimode output.

    I have managed to get the red (632.8nm) line to lase and perhaps orange lines by placing a HR from a 632.8nm HeNe at the HR end of the yellow tube.

    Further, I obtained a broadband mirror from an Argon Laser tube, the OC worked best at the OC end of the Yellow tube, have got the laser to output green...

    The mirrors hand-held - next to build a small external resonator assembly.

    About the Waste Beam from a HeNe Laser

    The so-called High Reflector (HR) or totally reflecting mirror in a HeNe laser isn't really perfect, though the actual reflectivity is generally 99.95 percent or better. For a 1 mW laser tube with a 99 percent Output Coupler (OC) mirror, there is about 100 mW of intracavity power. Of this, about 50 uW will exit the rear through a 99.95 percent HR mirror. Unless the back of the HR mirror is painted or covered, there is always some small beam exiting the rear of the laser.

    Normally, what comes out in that direction is, well, waste, and is of no consequence. But, there are times where it's convenient to use this low power beam as a reference, expecting its power to track that of the main output beam. Unfortunately, it is sometimes not well behaved in this regard.

    In constructing some amplitude stabilized HeNe lasers which depend on the waste beam feeding a photodiode for their feedback loop, an annoying characteristic of the waste beam has become evident with some otherwise perfectly normal and healthy HeNe laser tubes. Namely, that the relative power in the waste beam and the main beam does not remain constant as the tube warms up. In fact, one tube I was using had a variation of almost 2:1 in relative waste beam and output beam power depending on the tube's temperature. This is probably due to one or both of the following:

    1. Variation in mirror reflectivity. Designing and manufacturing high reflectivity mirror coatings is somewhat of an art and they don't always come out right. There may be ripples, a slope, or other variations in the reflectivity-versus-wavelength function. For an HR mirror on a 1 mW tube of, say, 99.97 percent resulting in 30 uW, a change of only 0.01 percent would add 10 uW to the waste beam.

    2. Lack of wedge or insufficient wedge between the inner and outer surfaces of the HR mirror. This will result in an etalon effect, effectively modulating the reflectance as a periodic function of temperature by perhaps 10 or 20 percent, which would appear as a similar change in the waste beam power. From room temperature to the operating temperature of a typical enclosed HeNe laser head, the power variation would go through several cycles.

    The coating problem is more likely to result in a strictly increasing, or at least slow change in waste beam power with higher temperature while the etalon would be periodic with temperature going through several cycles, it might be possible to determine which of the two effects is present.

    Normally, the waste beam is not used for anything and no one cares. Though there will also be a change in the power of the output beam (inversely relative to the waste beam) from these issues, it will be too small to be detectable without careful measurements, being swamped by the normal mode sweep power variations. But when the waste beam is used as the amplitude reference in a stabilized laser, the supposedly stabilized output will vary based on the relative waste beam power. That 10 uW change would result in the output power changing by 33 percent.

    For some plots the mode sweep of normal and naughty tubes, see the section: Plots of HeNe Laser Power Output and Polarized Modes During Warmup. In particular, compare the plots of the Spectra-Physics 088 with those of the tubes that immediately follow it.



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    Magnets in High Power or Precision HeNe Laser Heads

    Effects of Magnetic Fields on HeNe Laser Operation

    If you open the case on a higher power (and longer) HeNe laser head or one that is designed with an emphasis on precision and stability, you may find a series of magnets or electromagnetic coils in various locations in close proximity to the HeNe tube. They may be distributed along its length or bunched at one end; with alternating or opposing N and S poles, or a coaxial arrangement; and of various sizes, styles, and strengths.

    Magnets may be incorporated in HeNe lasers for several reasons including the suppression of IR spectral lines to improve efficiency (such as it is!) and to boost power at visible wavelengths, to control its polarization, and to split the optical frequency into two closely spaced components. There are no doubt other uses as well.

    The basic mechanism for the interaction of emitted light and magnetic fields is something called the 'Zeeman Effect' or 'Zeeman Splitting'. The following brief description is from the "CRC Handbook of Chemistry and Physics":

    "The splitting of a spectrum line into several symmetrically disposed components, which occurs when the source of light is placed in a strong magnetic field. The components are polarized, the directions of polarization and the appearance of the effect depending on the direction from which the source is viewed relative to the lines of force."

    Magnetic fields may affect the behavior of HeNe tubes in several ways:

    In principle, varying fields from electromagnets could be used for intensity, polarization, and frequency modulation. I do not know whether any commercial HeNe lasers have been implemented in this manner.

    But if magnets were not originally present, the only situation where adding some may make sense is for older longer or "other color" HeNes where a series of weak magnets may actually boost output power by 10 to 25 percent or more. On the other hand, most non-Zeeman stabilized HeNes do NOT like magnets at all. Even a relatively weak stray magnetic field from nearby equipment may result in a significant change in behavior. However, unless ferrous metals are used in the laser's construction, any change will likely not be permanent.

    Typical Magnet Configurations

    Here are examples of some of the common arrangements of magnets that you may come across. In addition to those shown, magnets may be present along only one side of the tube (probably underneath and partially hidden) or in some other peculiar locations. I suspect that for many commercial HeNe lasers, the exact shape, strength, number, position, orientation, and distribution of the magnets was largely determined experimentally. In other words, some poor engineer was given a bare HeNe tube, a pile of assorted magnets, a roll of duct tape, and a lump of modeling clay, and asked to optimize some aspect(s) of the laser's performance. :-)

    For the magnet configuration used in a commercial laser, see the section: Description of the SP-124 Laser Head.



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    Internal Mirror HeNe Tubes up to 35 mW - Red and Other Colors

    Typical HeNe Tube Specifications

    Prior to the introduction of the CD player, the red HeNe laser was by far the most common source of inexpensive coherent light on the planet. The following are some typical physical specifications for a variety of red (632.8 nm) HeNe tubes (all are single transverse mode - TEM00):

       Output       Tube Voltage       Tube         Tube Size
       Power        Operate/Start     Current      Diam/Length
     ------------  ---------------  ------------  -------------
      0.3-0.5 mW    0.8-1.0/6  kV    3.0-4.0 mA     19/135 mm
      0.5-1 mW       .9-1.0/7  kV    3.2-4.5 mA     25/150 mm
       1-2 mW       1.0-1.4/8  kV    4.0-5.0 mA     30/200 mm
       2-3 mW       1.1-1.7/8  kV    4.0-6.5 mA     30/260 mm
       3-5 mW       1.7-2.4/10 kV    4.5-6.5 mA     37/350 mm
       5-10 mW      2.4-3.1/10 kV    6.5-7.0 mA     37/440 mm     
      10-15 mW      3.0-3.5/10 kV    6.5-7.0 mA     37/460 mm
      15-25 mW      3.3-4.0/10 kV    6.5-7.0 mA     37/600 mm
      25-35 mW      4.0-5.2/12 kV    7.0-8.0 mA     42/900 mm
    

    Where:

    Tubes like this are generally available in both random and linearly polarized versions which are otherwise similar with respect to the above characteristics (for red tubes at least, more below).

    At least one other basic specification may be critical to your application: Which end of the tube the beam exits! There is no real preference from a manufacturing point of view for red HeNe lasers. (For low gain "other-color" HeNe laser tubes, it turns out that anode output results is slightly higher gain and thus slightly higher output for the typical hemispherical cavity because it better utilizes the mode volume.) However, this little detail may matter a great deal if you are attempting to retrofit an existing barcode scanner or other piece of equipment where the tube clips into a holder or where wiring is short, tight, or must be in a fixed location. For example, virtually all cylindrical laser heads require that the beam exits from the cathode-end of the tube. It is possible that you will be able to find two versions of many models of HeNe tubes if you go directly to the manufacturer and dig deep enough. However, this sort of information may not be stated where you are buying surplus or from a private individual, so you may need to ask.

    The examples above (as well as all of the other specifications in this and the following sections) are catalog ratings, NOT what might appear on the CDRH safety sticker (which is typically much higher). See the section: About Laser Power Ratings for info on listed, measured, and CDRH power ratings.

    Note how some of the power levels vary widely with respect to tube dimensions, voltage, and current. Generally, higher power implies a longer tube, higher operating/start voltages, and higher operating current - but there are some exceptions. In addition, you will find that physically similar tubes may actually have quite varied power output. This is particularly evident in the manufacturers' listings. (See the chapter: A HREF="laserhcl.htm#hcltoc">Commercial Unstabilized HeNe Lasers.)

    These specifications are generally for minimum power over the guaranteed life of the tube. New tubes and individual sample tubes after thousands of hours may be much higher - 1.5X is common and a "hot" sample may hit 2X or more. My guess is that for tubes with identical specifications in terms of physical size, voltage, and current, the differences in power output are due to sample-to-sample variations. Thus, like computer chips, they are selected after manufacture based on actual performance and the higher power tubes are priced accordingly! This isn't surprising when considering the low efficiency at which these operate - extremely slight variations in mirror reflectivity and trace contaminants in the gas fill can have a dramatic impact on power output.

    I have a batch of apparently identical 2 mW Aerotech tubes that vary in power output by a factor of over 1.5 to 1 (2.6 to 1.7 mW printed by hand on the tubes indicating measured power levels at the time of manufacture).

    And, power output also changes with use (and mostly in the days of soft-sealed tubes, just with age sitting on the shelf):

    (From: Steve Roberts.)

    "I have a neat curve from an old Aerotech catalog of HeNe laser power versus life. The tubes are overfilled at first, so power is low. They then peak at a power much higher than rated power, followed by a long period of constant power, and then they SLOWLY die. It's not uncommon for a new HeNe tube to be in excess of 15% greater than rated power."

    And the answer to your burning question is: No, you cannot get a 3 mW tube to output 30 mW - even instantaneously - by driving it 10 times as hard!

    I have measured the operating voltage and determined the optimum current (by maximizing beam intensity) for the following specific samples - all red (632.8 nm) tubes from various manufacturers. (The starting voltages were estimated.):

       Output     Tube Voltage       Tube         Supply Voltage     Tube Size
       Power      Operate/Start     Current        (75K ballast)    Diam/Length
     ----------  ---------------  ------------   ----------------  -------------
        .8 mW        .9/5  kV        3.2 mA           1.1 kV         19/135 mm
       1.0 mW       1.1/7  kV        3.5 mA           1.4 kV         25/150 mm
    9   1.0 mW       1.1/7  kV        3.2 mA           1.4 kV         25/240 mm
       2.0 mW       1.2/8  kV        4.0 mA           1.5 kV         30/185 mm
       3.0 mW       1.6/8  kV        4.5 mA           1.9 kV         30/235 mm
       5.0 mW       1.7/10 kV        6.0 mA           2.2 kV         37/350 mm
      12.0 mW       2.5/10 kV        6.0 mA           2.9 kV         37/475 mm
    

    Melles Griot, Uniphase, Siemens, PMS, Aerotech, and other HeNe tubes all show similar values.

    The wide variation in physical dimensions also means that when looking at descriptions of HeNe lasers from surplus outfits or the like, the dimensions can only be used to determine an upper (and possibly lower) bound for the possible output power but not to determine the exact output power (even assuming the tube is in like-new condition). Advertisements often include the rating on the CDRH safety sticker (or say 'max' in fine print). This is an upper bound for the laser class (e.g., Class IIIa), not what the particular laser produces or is even capable of producing. It may be much lower. For example, that Class IIIa laser showing 5 mW on the sticker, may actually only be good for 1 mW under any conditions! The power output of a HeNe laser tube is essentially constant and cannot be changed significantly by using a different power supply or by any other means. See the section: Buyer Beware for Laser Purchases.

    Also see the section: Locating Laser Specifications.

    In addition to power output, power requirements, and physical dimensions, key performance specifications for HeNe lasers also include:

    With manufacturers like Aerotech, Melles Griot, and Siemens, a certain amount of information can be determined from the model number. For example, here is how to decipher most of those from Melles Griot (e.g., 05-LHP-121-278):

    The vast majority of Melles Griot lasers you are likely to come across will follow this numbering scheme though there are some exceptions, especially for custom assemblies. (Some surplus places drop the leading '05-' when reselling Melles Griot laser tubes or heads so an 05-LHP-120 would become simply an LHP-120.)

    For other manufacturers like Spectra-Physics, the model numbers are totally arbitrary! (See the section: Spectra-Physics HeNe Lasers.)

    HeNe Tubes of a Different Color

    Although a red beam is what everyone thinks of when a HeNe laser is discussed, HeNe tubes producing green, yellow, and orange beams, as well as several infra-red (IR) wavelengths, are also manufactured. However, they are not found as often on the surplus market because they are not nearly as common as the red variety. In terms of the number of HeNe lasers manufactured, red is far and away the most popular, with all the others combined accounting for only 1 to 2 percent of the total production. In order of decreasing popularity, it's probably: red, green, yellow, infra-red (all IR wavelengths), orange. Non-red tubes are also more expensive when new since for a given power level, they must be larger (and thus have higher voltage and current ratings) due to their lower efficiency (the spectral lines being amplified are much weaker than the one at 632.8 nm). Operating current for non-red HeNe tubes is also more critical than for the common red variety so setting these up with an adjustable power supply or adjusting the ballast resistance for maximum output is recommended.

    Maximum available power output is also lower - rarely over 2 mW (and even those tubes are quite large (see the tables below). However, since the eye is more sensitive to the green wavelength (543.5 nm) compared to the red (632.8 nm) by more than a factor of 4 (see the section: Relative Visibility of Light at Various Wavelengths), a lower power tube may be more than adequate for many applications. Yellow (594.1 nm) and orange (611.9 nm) HeNe lasers appear more visible by factors of about 3 and 2 respectively compared to red beams of similar power. To get an idea of the actual perceived color at each wavelength, see the section: Color Versus Wavelength.

    Infrared-emitting HeNe lasers exist as well. In addition to scientific uses, these were sued for testing in the Telecom industry before sufficiently high quality diode lasers became available.Yes, you can have a HeNe tube and it will light up inside (typical neon glow), but if there is no output beam (at least you cannot see one), you could have been sold an infrared HeNe tube. However, by far the most likely explanation for no visible output beam is that the mirrors are misaligned or the tube is defective in some other way. Unfortunately, silicon photodiodes or the silicon sensors in CCD or CMOS cameras do not respond to any of the HeNe IR wavelengths, so the only means of determining if there is an IR beam are to use a GaAs photodiode, IR detector card, or thermal laser power meter. IR HeNe tubes are unusual enough that it is very unlikely you will ever run into one. However, they may turn up on the surplus market especially if the seller doesn't test the tubes and thus realize that these behave differently - they are physically similar to red (or other color) HeNe tubes except for the reflectivity of the mirrors as a function of wavelength. (There may be some other differences needed to optimize each color like the He:Ne ratio, isotope purity, and gas fill pressure, but the design of the mirrors will be the most significant factor and the one that will be most obvious with a bare eyeball, though the color of the discharge may be more pink for green HeNe tubes and more orange and brighter for IR HeNe tubes compared to red ones, more below.) Even if the model number does not identify the tube as green, yellow, orange, red, or infra-red, this difference should be detectable by comparing the appearance of its mirrors (when viewed down the bore of an UNPOWERED tube) with those of a normal (known to be red) HeNe tube. See the section: Determining HeNe Laser Color from the Appearance of the Mirrors. (Of course, your tube could also fail to lase due to misaligned or damaged mirrors or some other reason. See the section: How Can I Tell if My Tube is Good?.)

    As noted above, the desired wavelength is selected and the unwanted wavelengths are suppressed mostly by controlling the reflectivity functions of the mirrors. For example, the gains of the green and yellow lines (yellow may be stronger) are both much much lower than red and separated from each other by about 50 nm (543.5 nm versus 594.1 nm). To kill the yellow line in a green laser, the mirrors are designed to reflect green but pass yellow. I have tested the mirrors salvaged from a Melles Griot 05-LGP-170 green HeNe tube (not mine, from "Dr. Destroyer of Lasers"). The HR (High Reflector) mirror has very nearly 100% reflectivity for green but less than 25% for yellow. The OC (Output Coupler) also has a low enough reflectivity for yellow (about 98%) such that it alone would prevent yellow from lasing. The reflectivities for orange, red, and IR, are even lower so they are also suppressed despite their much higher gain, especially for the normal red (632.8 nm) and even stronger mid-IR (3,391 nm) line.

    However, to manufacture a tube with optimum and stable output power, it isn't sufficient to just kill lasing for unwanted lines. The resonator must be designed to minimize their contribution to stimulated emission - thus the very low reflectivity of the HR for anything but the desired green wavelength. Otherwise, even though sustained oscillation wouldn't be possible, unwanted color photons would still be bouncing back and forth multiple times stealing power from the desired color. The output would also be erratic as the length of the tube changed during warmup (due to thermal expansion) and this affected the longitudinal mode structure of the competing lines relative to each other. Some larger HeNe lasers have magnets along the length of the tube to further suppress (mostly) the particularly strong mid-IR line at 3,391 nm. (See the section: Magnets in High Power or Precision HeNe Laser Heads.)

    In addition, you can't just take a tube designed for a red laser, replace the mirrors, and expect to get something that will work well - if at all - for other wavelenghts. For one thing, the bore size and mirror curvature for maximum power while maintaining TEM00 operation are affected by wavelength.

    Furthermore, for these other color HeNe lasers which depend on energy level transitions which have much lower gain than red - especially the yellow and green ones - the gas fill pressure, He:Ne ratio, and isotopic composition and purity of the helium and neon, will be carefully optimized and will be different than for normal red tubes.

    Needless to say, the recipes for each type and size laser will be closely guarded trade secrets and only a very few companies have mastered the art of other color HeNe lasers, especially for high power (in a relative sort of way) in yellow and green. I am only aware of four companies that currently manufacture their own tubes: Melles Griot, Research Electro-Optics, Uniphase, and LASOS, with the last two having very few models to choose from. Others (like Coherent) simply resell lasers under their own name.

    And, the answer to that other burning question should now be obvious: No, you can't convert an ordinary red internal mirror HeNe tube to generate some other color light as it's (almost) all done with mirrors and they are an integral part of the tube. :) Therefore, your options are severely limited. As in: There are none. (However, going the other way, at least as a fun experiment, may be possible. See the section: Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes.) For a laser with external mirrors, a mirror swap may be possible (though the cavity length may be insufficient to resonate with the reduced gain of other-color spectral lines once all loses taken into consideration). But realistically, this option doesn't even exist where the mirrors are sealed into the tube.

    There are also a few HeNe lasers that can output more than one of the possible colors simultaneously (e.g., red+orange, orange+yellow) or selectively by turning knob (which adjusts the angle of a Littrow or other similar dispersion prism) inside the laser cavity using a Brewster window HeNe tube). But such lasers are not common and are definitely very expensive. So, you won't likely see one for sale at your local hamfest - if ever! One manufacturer of such lasers is Research Electro-Optics (REO). See the section: Research Electro-Optics's Tunable HeNe Lasers.

    However, occasionally a HeNe tube turns up that is 'defective' due to incorrect mirror reflectivities or excessive gain or magic :) and actually outputs an adjacent color in addition to what it was designed to produce. I have such a tube that generates about 3 mW of yellow (594.1 nm) and a fraction of a mW of orange (611.9 nm) but isn't very stable - power fluctuates greatly as it warms up. Another one even produces the other orange line at 611.9 nm, and it's fairly stable. But, finding magic 'defective' tubes such as these by accident is extremely unlikely though I've heard of the 640.1 nm (deep red) line showing up on some supposedly good normal red (632.8 nm) HeNe tubes.

    As a side note: It is strange to see the more or less normal red-orange glow in a green HeNe laser tube but have a green beam emerging. A diffraction grating or prism really shows all the lines that are in the glow discharge. Red through orange, yellow and green, even several blue lines (though they are from the helium and can't lase under any circumstances)!! The IR lines are present as well - you just cannot see them.

    See the section: Instant Spectroscope for Viewing Lines in HeNe Discharge for an easy way to see many of the visible ones.

    Actually, the color of the discharge may be subtly different for non-red HeNe tubes due to modified gas fill and pressure. For example, the discharge of green HeNe tubes may appear more pink compared to red tubes) which are more orange), mostly due to lower fill pressure. The fill mix and pressure on green HeNes is a tricky compromise among several objectives that conflict to some extent including lifetime, stability (3.39 µm competition), and optical noise. This balancing act and the lower fill pressure are why green HeNes don't last as long as reds. Have I totally confused you, color-wise? :)

    The expected life of 'other color' HeNe tubes is generally much shorter than for normal red tubes. This is something that isn't widely advertised for obvious reasons. Whereas red HeNe tubes are overfilled initially (which reduces power output) and they actually improve with use to some extent as gas pressure goes down, this luxury isn't available with the low gain wavelengths - especially green - everything needs to be optimal for decent performance.

    The discharge in IR HeNe tubes may be more orange and brighter due to a higher fill pressure. Again, this is due to the need to optimize parameters for the specific wavelength.

    Determining HeNe Laser Color from the Appearance of the Mirrors

    Although most HeNe lasers are the common red (632.8 nm) variety (whose beam actually appears orange-red), you may come across unmarked HeNe tubes and just have to know what color output the produce without being near a HeNe laser power supply.

    Since the mirrors used in all HeNe lasers are dielectric - functioning as a result of interference - they have high reflectivity only around the laser wavelength and actually transmit light quite well as the wavelength moves away from this peak. By transmitted light, the appearance will tend to be a color which is the complement of the laser's output - e.g., cyan or blue-green for a red tube, pink or magenta for a green tube, blue or violet for a yellow tube. Of course, except for the IR variety, if the tube is functional, the difference will be immediately visible when it is powered up!

    The actual appearance may also depend on the particular manufacturer and model as well as the length/power output of the laser (which affects the required reflectivity of the OC), as well as the revision number of your eyeballs. :) So, there could be considerable variation in actual perceived color. Except for the blue-green/magenta combination which pretty much guarantees a green output HeNe tube, more subtle differences in color may not indicate anything beyond manufacturing tolerances.

    The chart below in conjunction with Appearance of HeNe Laser Mirrors will help to ideentify your unmarked HeNe tube. (For accurate rendition of the graphic, your display should be set up for 24 bit color and your monitor should be adjusted for proper color balance.)

          HeNe Laser          High Reflector (HR)          Output Coupler (OC)
       Color  Wavelength   Reflection   Transmission    Reflection   Transmission
     ------------------------------------------------------------------------------
        Red    632.8 nm    Gold/Copper      Blue        Gold/Yellow   Blue/Green
       Orange  611.9 nm   Whitish-Gold      Blue       Metallic Green   Magenta
       Yellow  594.1 nm   Whitish-Gold      Blue       Metallic Green   Magenta
       Green   543.5 nm   Metallic Blue  Red/Orange    Metallic Green   Magenta
    
       Broadband (ROY)    Whitish-Gold      Blue
    
        IR     1,523 nm    Light Green  Light Magenta   Light Green  Light Magenta
        IR     3,391 nm       Gold (Metal) Coated         Neutral        Clear
    

    The entry labeled 'Broadband' relates to the HR mirror in some unusual multiple color (combinations of red and/or orange and/or yellow) internal mirror tubes as well as those with an internal HR and Brewster window for external OC optics. And, the yellow and orange tubes may actually use broad band HRs. The OCs would then be selected for the desired wavelength(s) and may also have a broad band coating.

    For low gain tubes, they play games with the coatings. I guess it isn't possible to just make a highly selective coating for one wavelength that's narrow enough to have low reflectivity at the nearby lines so they won't lase. So, one mirror will be designed to fall off rapidly on one side of the design wavelength, the other mirror on the other side. That's one reason front and back mirrors on yellow and green tubes in particular have very different appearances.

    As noted, depending on laser tube length/output power, manufacturer, and model, the appearance of the mirrors can actually vary quite a bit but this should be a starting point at least. For example, I have a Melles Griot 05-LHR-170 HeNe laser tube that should be 594.1 nm (yellow) but actually outputs some 604.6 nm (orange) as well. It's mirror colors for the HR and OC are almost exactly opposite of those I have shown for the yellow and orange tubes! I don't know whether this was intentional or part of the problem And, while from this limited sample, it looks like the OCs for orange, yellow, and green HeNe lasers appear similar, I doubt that they really are in the area that counts - reflectivity/transmission at the relevant wavelengths.

    I do not have any data for the 1,152 nm (IR) HeNe laser wavelength. If you have access to a 1,152 nm or any other non-red HeNe tube and would like to contribute or comment on their mirror colors (or anything else), please contact me via the Sci.Electronics.Repair FAQ Email Links Page.

    More on Other Color HeNe Lasers

    Here are some comments on the difficulty of obtaining useful visible output from HeNe lasers at wavelengths other than our friendly red (632.8 nm):

    (From: Steve Roberts.)

    You do need a isotope change in the gases for green, and a He:Ne ratio change for the other orange and yellow lines. In addition, the mirrors to go to another line will have a much lower output transmission. The only possible lines you'll get on a large frame HeNe laser will be the 611.9 nm orange and 594.1 nm yellow. The green requires external mirror tubes in excess of a meter and a half long and a Littrow prism to overcome the Brewster losses and suppress the IR.

    The original work on green was done by Rigden and Wright. The short tubes have lower losses because they have no Brewsters and thus can concentrate on tuning the coatings to 99.9999% reflectivity and maximum IR transmission. There is one tunable low power unit on the market that does 6 lines or so, but only 1 line at a time, and the $6,000 cost is kind of prohibitive for a few milliwatts of red and fractional milliwatt powers on the other lines. But, it will do green and has the coatings on the back side of the prism to kill the losses.

    Also look for papers by Erkins and Lee. They are the fellows who did the green and yellow for Melles Griot and they published one with the energy states as part of a poster session at some conference. Melles Griot used to hand it out, that's how I had a copy, recently thrown away.

    Even large HeNe lasers such as the SP-125 (rated at 50 mW of red) will only do about 20 mW of yellow, with a 35 mW SP-127 you're probably only looking at 3 to 5 mW of yellow. And, for much less then the cost of the custom optics to do a conversion, you can get two or three 4 to 5 mW yellow heads from Melles Griot. I know for a fact that a SP-127 only does about 3 mW of 611.9 with a external prism and a remoted cavity mirror, when it does 32 mW of 632.8 nm.

    So in the end, unless you have a research use for a special line, it's cheaper to dig up a head already made for the line you seek, unless you have your own optics coating lab that can fabricate state-of-the-art mirrors.

    I have some experience in this, as I spent months looking for a source of the optics below $3,000.

    (From: Sam.)

    I do have a short (265 mm) one-Brewster HeNe tube (Melles Griot 05-LGB-580) with its internal HR optimized for green that operates happily with a matching external green HR mirror (resulting in a nice amount of circulating power) but probably not with anything having much lower reflectivity to get a useful output beam. In fact, I could not get reliable operation even with the HR from a dead green HeNe laser tube as the Brewster window would not remain clean enough for the time required to align the mirror. See the section: A Green One-Brewster HeNe Laser for more info.

    I would expect an SP-127 to do more than 3 to 5 mW of yellow, my guess would be 10 to 15 mW with optimized mirrors but no tuning prism. If I can dig up appropriate mirrors, I intend to try modifying an SP-127 to make it tunable and/or do yellow or green. :)

    (From: Lynn Strickland (stricks760@earthlink.net).)

    You can find 640.1 nm in a lot of red HeNe lasers. I have a paper on it somewhere, and cavity design can influence it to a large extent. If you have a decent quality grating, it's pretty easy to pick up. 629 nm is the one you don't see too much.

    I'm no physicist, but the lower gain lines can lase simultaneously with the higher gain lines, no problem, as long as there is sufficient gain available in the plasma. It's really pretty easy to get a HeNe laser to output on all lines at the same time (if you have the right mirrors). The trick is optimizing the bore-to-mode ratio, gas pressure, and isotope mixture to get good TEM00 power. Usually the all-lines HeNe lasers are multi (transverse) mode. I don't know of anyone who makes them commercially though - at least not intentionally.

    Steve's Comments on Superradiance and the 3.39 µm HeNe Laser

    Generally, when a gas laser is superradiant, there is a limit to its maximum power output (with exceptions for nitrogen and copper vapor laser, although nitrogen's upper limit is defined by the maximum cavity length into which you can generate a 300 ns or less excitation pulse.

    The 3.39 µm HeNe laser's gain is still, like all other HeNe lines limited by a wall collision to return the excited atoms to the ground state. 3.39 µm HeNe lasers have larger bores then normal HeNe lasers, and the bores are acid etched to fog them and create more surface area, but still the most power I've ever seen published was 40 mW - nothing to write home about. The massive SP-125, the largest commercial HeNe laser, could be ordered with a special tube and special optics for 3.39 µm, and it still only did about 1/3rd the visible power. Superradiance and ultimate power are not tied together.

    The reason 3.39 µm got all the writeups it did was that it started on the same upper state as all the other HeNe lines, was easily noticed when it sapped power from the visible line, and was, at the time, a exotic wavelength for which there were few other sources.



  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Viewing Spectral Lines in Discharge, Other Colors in Output

    For accurate measurements, you'll need an optical instrument such as a monochromator or spectrophotometer or optical spectrum analyzer. See the section: Monochromators. But to simply see the complexity of the discharge spectrum inside the bore of a HeNe laser tube, it's much easier and cheaper.

    (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.)

    Instant Spectroscope for Viewing Lines in HeNe Discharge

    It is easy to look at the major visible lines. All it takes is a diffraction grating or prism. I made my instant spectroscope from the diffraction grating out of some sort of special effects glasses - found in a box of cereal, no less! - and a monocular (actually 1/2 of a pair of binoculars).

    The shear number of individual spectral lines present in the discharge is quite amazing. You will see the major red, orange, yellow, and green lines as well as some far into the blue and violet portions of the spectrum and toward the IR as well. All of those shown in Bright Line Spectra of Helium and Neon will be present as well as many others not produced by the individual gas discharges. There are numerous IR lines as well but, of course, these will not be visible.

    Place a white card in the exit beam and note where the single red output line of the HeNe tube falls relative to the position and intensity of the numerous red lines present in the gas discharge.

    As an aside, you may also note a weak blue/green haze surrounding the intense main red beam (not even with the spectroscope). This is due to the blue/green (incoherent) spectral lines in the discharge being able to pass through the output mirror which has been optimized to reflect well (>99 percent) at 632.8 nm and is relatively transparent at wavelengths some distance away from these (shorter and longer but you would need an IR sensor to see the longer ones). Since it is not part of the lasing process, this light diverges rapidly and is therefore only visible close to the tube's output mirror.

    Dynamic Measurement of Discharge Spectra

    The following is trivial to do if you have a recording spectrometer and external mirror HeNe laser. For an internal mirror HeNe laser tube, it should be possible to rock one of the mirrors far enough to kill lasing without permanently changing alignment. If you don't have proper measuring instruments, don't worry, this is probably in the "Gee wiz, that's neat but of marginal practical use" department. :)

    (From: George Werner (glwerner@sprynet.com).)

    Here is an effect I found many years ago and I don't know if anyone has pursued it further.

    We had a recording spectrometer in our lab which we used to examine the incoherent light coming from the laser discharge. This spectrum when lasing was slightly different from the spectrum when not lasing, which one can expect since energy levels are redistributed. As with most detectors, ours used a chopper in the spectrometer light beam and a lock-in amplifier.

    Instead of putting the chopper in the path of light going to the spectrometer, I put it in the path of the internal laser beam, so that instead of an open/closed signal going to the amplifier it was a lasing/not-lasing signal. What was recorded then was three kinds of spectrum lines: some deflected positive in the normal way, others deflected negative, and the third group were those that were unaffected by chopping, in which case when we passed over the line we only saw an increase in the noise level. Setting up such a test is easy. The hard part is interpreting the data in a meaningful way.

    Other Color Lines in Red HeNe Laser Output

    When viewing spectral lines in the actual beam of a red HeNe laser, you may notice some very faint ones far removed from the dominant 632.8 nm line we all know and love. (This, of course, also applies to other color HeNe lasers.)

    For HeNe lasers, the primary line (usually 632.8 nm) is extremely narrow and effectively a singularity given any instrumentation you are likely to have at your disposal. Any other lines you detect in the output are almost certainly from two possible sources but neither is actual laser emission:

    Since the brightness of the discharge and superradiance output should be about the same from either mirror, using the non-output end (high reflector) should prove easier (assuming it isn't painted over or otherwise covered) since the red beam exiting from this mirror will be much less intense and won't obscure the weak green beam.

    Note that argon and krypton ion lasers are often designed for multiline output where all colors are coherent and within an order of magnitude of being equal to each other in intensity or with a knob to select an individual wavelength. Anything like this is only rarely done with HeNe lasers because it is very difficult (and expensive) due to the low gain of the non-red lines. For more information, see the sections: HeNe Tubes of a Different Color and Research Electro-Optics Tunable HeNe Lasers.



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    Demonstration HeNe Lasers, Weatherproofing

    Putting Together a Demonstration HeNe Laser

    For a classroom introduction to lasers, it would be nice to have a safe setup that makes as much as possible visible to the students. Or, you may just want to have a working HeNe laser on display in your living room! Ideally, this is an external mirror laser where all parts of the resonator as well as the power supply can be readily seen. However, realistically, finding one of these is not always that easy or inexpensive, and maintenance and adjustment of such a laser can be a pain (though that in itself IS instructive).

    The next best thing is a small HeNe laser laid bare where its sealed (internal mirror) HeNe tube, ballast resistors, wiring, and power supply (with exposed circuit board), are mounted inside a clear Plexiglas case with all parts labeled. This would allow the discharge in the HeNe tube to be clearly visible (and permit the use of the Instant Spectroscope for Viewing Lines in HeNe Discharge). The clear insulating case prevents the curious from coming in contact with the high voltage (and line voltage, if the power supply connects directly to the AC line), which could otherwise result in damage to both the person and fragile glass HeNe tube when a reflex action results in smashing the entire laser to smithereens!

    A HeNe laser is far superior to a cheap laser pointer for several reasons:

    Important: If this see-through laser is intended for use in a classroom, check with your regulatory authority to confirm that a setup which is not explicitly CDRH approved (but with proper laser class safety stickers) will be acceptable for insurance purposes.

    For safety with respect to eyeballs and vision, a low power laser - 1 mW or less - is desirable - and quite adequate for demonstration purposes.

    The HeNe laser assembly from a barcode scanner is ideal for this purpose. It is compact, low power, usually runs on low voltage DC (12 V typical), and is easily disassembled to remount in a demonstration case. The only problem is that many of these have fully potted "brick" type power supplies which are pretty boring to look at. However, some have the power supply board coated with a rubbery material which can be removed with a bit of effort (well, OK, a lot of effort!). For example, this HeNe Tube and Power Supply is from a hand-held barcode scanner. A similar unit was separated into its Melles Griot HeNe Tube and HeNe Laser Power Supply IC-I1 (which includes the ballast resistors). These could easily be mounted in a very compact case (as little as 3" x 6" x 1", though spreading things out may improve visibility and reduce make cooling easier) and run from a 12 VDC, 1 A wall adapter. Used barcode scanner lasers can often be found for $20 or less.

    An alternative is to purchase a 0.5 to 1 mW HeNe tube and power supply kit. This will be more expensive (figure $5 to $15 for the HeNe tube, $25 to $50 for the power supply) but will guarantee a circuit board with all parts visible.

    The HeNe tube, power supply, ballast resistors (if separate from the power supply), and any additional components can be mounted with standoffs and/or cable ties to the plastic base. The tube can be separated from the power supply if desired to allow room for labels and such. However, keep the ballast resistors as near to the tube as practical (say, within a couple of inches, moving them if originally part of the power supply board). The resistors may get quite warm during operation so mount them on standoffs away from the plastic. Use wire with insulation rated for a minimum of 10 kV. Holes or slots should be incorporated in the side panels for ventilation - the entire affair will dissipate 5 to 10 Watts or more depending on the size of the HeNe tube and power supply. (However, if you want to take this thing outdoors, see the section: Weatherproofing a HeNe Laser.

    When attaching the HeNe tube, avoid anything that might stress the mirror mounts. While these are quite sturdy and it is unlikely that any reasonable arrangement could result in permanent damage, even a relatively modest force may result in enough mirror misalignment to noticeably reduce output power. And, don't forget that the mirror mounts are also the high voltage connections and need to be well insulated from each other and any human contact! The best option is probably to fasten the tube in place using Nylon cable ties, cable clamps, or something similar around the glass portion without touching the mirror mounts at all (except for the power connections).

    Provide clearly marked red and black wires (or binding posts) for the low voltage DC or a line cord for AC (as appropriate for the power supply used), power switch, fuse, and power-on indicator. Label the major components and don't forget the essential CDRH safety sticker (Class II for less than 1 mW or Class IIIa for less than 5 mW).

    See: Sam's Demo HeNe Laser as an example (minus the Plexiglas safety cover), contructed from the guts of a surplus Gammex laser (probably part of a patient positioning system for a CT or MRI scanner). The discrete line operated power supply is simple with the HV transformer, rectifier/doubler, filter, multiplier, and ballast resistors easily identified. This would make an ideal teaching aid.

    See the suppliers listed in the chapter: Laser and Parts Sources.

    The Ultimate Demonstration HeNe Laser

    Rather than having a see-through laser that just outputs a laser beam (how boring!), consider something that would allow access to the internal cavity, swapping of optics, and modulation of beam power. OK, perhaps the truly ultimate demo laser would use a two-Brewster tube allowing for interchangeable optics at both ends, be tunable to all the HeNe spectral lines, and play DVD movies. :) We'll have to settle for something slightly less ambitious (at least until pigs fly). Such a unit could consist of the following components:

    Everything needed for such a setup is readily available or easily constructed at low cost but you'll have to read more to find out where or how as each of the components are dealt with in detail elsewhere in Sam's Laser FAQ (but I won't tell you exactly where - these are all the hints you get for this one!).

    A system like this could conceivably be turned into an interactive exhibit for your local science museum - assuming they care about anything beyond insects and the Internet these days. :) There are some more details in the next section.

    Guidelines for a Demonstraton One-Brewster HeNe Laser

    The following suggestions would be for developing a semi-interactive setup whereby visitors can safely (both for the visitor and the laser) adjust mirror alignment and possibly some other parameters of laser operation. The type of one-Brewster (1-B) HeNe laser tube like the Melles Griot 05-LHB-570. See the sections starting with: A One-Brewster HeNe Laser Tube Note that the 05-LHB-570 is a wide bore tube that runs massively multi (transverse) mode with most mirrors configurations unless an intracavity aperture is added. This is actually an advantage for several reasons:

    1. The multi-transverse mode structure is interesting in itself and provides additional options for showing how it can be controlled.
    2. Mirror alignment is easier and the tube will lase over a much wider range of mirror orientation.
    3. Output power is higher for its size and power requirements.

    Here are some guidelines for designing an interactive exhibit:

    Weatherproofing a HeNe Laser

    If you want to use a HeNe laser outside or where it is damp or very humid, it will likely be necessary to mount the tube and power supply inside a sealed box. Otherwise, stability problems may arise from electrical leakage or the tube may not start at all. There will then be several additional issues to consider:



  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Interesting, Strange, and Unidentified HeNe Lasers

    When Your Laser Doesn't Fit the Mold

    The vast majority of HeNe tubes and laser heads you will likely come across will be basically similar to those described in the section: Structure of Internal Mirror HeNe Lasers. However, when rummaging through old storerooms or offerings at hamfests or high-tech flea markets, you may come across some that are, to put it bluntly, somewhat strange or weird. I would expect that in most cases, these will be either really old, developed for a specific application, or higher performance lab quality models which are just not familiar to someone used to surplus specials. Consider these to be real finds if only for the novelty value! Refurbishing of the lab-grade lasers may be worth the effort and/or expense resulting in a truly exceptional (and possibly valuable) instrument. And, simply from an investment point of view, it is amazing what some old (and even totally useless dead) but strange lasers have fetched on places like Ebay Auction recently. See the section: Auctions. Here are some descriptions of what I and others have come across:

    Segmented HeNe Tubes

    I have several medium power HeNe tubes that do not have a single long bore (capillary) but rather it is split into about a half dozen sections with a 1 or 2 mm gap between them. Each of the short capillaries is fused into a glass separator without any holes. Two of these tubes look like the more common internal mirror HeNe tubes except for the multiple segments as shown below:

    
                    ____________________________________________
                   /       |            |            | _______  \
            Anode |\       |            |            |        \  | Cathode
            .-.---' \.-----'-----..-----'-----..-----'------.  '-'---.-.
        <---| |::::  :===========::===========::============:   :::::| |===> 
            '-'---. /'-----.-----''-----.-----''-----.------'  .-.---'-'
                  |/       |            |            | _______/  |
                   \_______|____________|____________|__________/ 
    
    

    Or, for a more esthetic rendition, see: Helium-Neon Laser Tube with Segmented Bore.

    The third has Brewster angle windows at both ends with an external (fixed) HR mirror and an external screw-adjustable OC mirror. The cathode is also in a side-tube rather than the more typical coaxial can type but is otherwise similar.

    Only one of the 3 HeNe tubes of this type that I have works at all and it has a messed up gas fill probably due to age despite its being hard sealed. Its output is perhaps 1 or 2 mW (where it should be around 20 mW). However, to the extent that it works, there doesn't appear to be anything particularly interesting or different about its behavior. Of the other two tubes, one has a broken off mirror (don't ask) but before the mishap, did generate some decent power (perhaps 5 to 10 mW but still nowhere near its 20 mW rating) but erratically. I suspect this was due to a contaminated gas fill resulting in low gain rather than the segmented design since a couple of other similar length tubes of conventional construction behaved in a similar manner. The funky tube with the external mirrors was not hard-sealed at the Brewster windows and leaked over time.

    The only obvious effect this sort of structure should have on operation would be to provide gas reservoirs at multiple locations rather than only at the cathode-end of the bore as is the case with most 'normal' HeNe tube designs. I do not know whether this matters at all for a low current HeNe discharge. Therefore, the reason for the unusual design remains a total mystery. It may have been to stabilize the discharge, to suppress unwanted spectral lines, easier to maintain in alignment than a single long capillary, or something else entirely. Then again, perhaps, the person who made the tubes just had a spurt of excessive creativity. :)

    I have also acquired a complete laser head with a similar tube, rated 25 mW max with a sticker that says it did 22 mW at one time. It is unremarkable in most respects but does have a large number of IR suppression magnets arranged on 3 sides over most of the length of the tube. Currently, it does not lase because the gas is slightly contaminated but it is also misaligned. The discharge color is along the lines of "Minor - Low Outupt" in Color of HeNe Laser Tube Discharge and Gas Fill so there may be some hope.

    Strange High Power HeNe Laser

    This is a on-going project on finding information and restoring a strange HeNe laser acquired by: Chris Chagaris (pyro@grolen.com). Research to determine the specifications and requirements involved postings to sci.optics, email correspondence, and a bit of luck - seeing a photograph of the mysterious laser in a book on holography.

    Here is the original description (slightly reformatted):

    (From: Chris Chagaris (pyro@grolen.com).)

    I have recently acquired what I have been told is a 35 mW Helium Neon laser head. However, it is unlike anything I have ever seen before. (See the diagram, below.)

    
                    Capillary tube/external starting electrodes
    
       Starting pulse  o-------+----------------------+
                              _|_                    _|_
        ||     //==================================================\\     ||
        ||   //=====. .==================. .=================. .=====\\   ||
                    |||                  | |                 |||     
      Mirror        '|'  25K             | |           25K   '|'        Mirror
             Anode 1 +---/\/\---o +HV    | |   +HV o---/\/\---+ Anode 2
                         .---------------' '--------------.
                      ---|-+                            +-|---
                         |  ) Main               Spare (  |
                      ---|-+                            +-|---
                         '--------------------------------'
     
                  Gas reservoir with heated cathodes and getters
    
    
    Jodon Laser Head shows the construction in more detail.

    Here is one reply Chris received by email from someone else named Marco. As you will see, this turns out to be a dead end.

    (From: Marco.)

    "Hi Chris,

    This seems to be a really old one, or from other location than west Europe, Japan, and the USA. The 'SM' could be an abbreviation for Siemens, they had manufactured lasers from 1966 to 1993; until last year Zeiss/Jena has taken over the production; and since 1997 Lasos has overtaken the production by a kind of management buy-out. You can send them the number, it will be possible that they know it. Contact Dr. Ledig. I will also look around if I can help you further.

    HeNe lasers with a heated filament are no longer built. To see if it still runs you can attach a 3.3 V supply to the filament and see if it glows red, not more, to much heat will destroy it. You could use transformers from tube amplifiers for the filament and an old HeNe laser power supply for the anode.

    This laser will need around 5,000 V and 10 mA I think. If you could only get a smaller power supply, you may not see any laser beam, but you can see if it will trigger."

    (From: Sam.)

    Here are my 'guesses' about this device. (I have also had email discussions with Chris.)

    I agree with much of what Marco had said.

    Unfortunately, Chris has determined that regassing will be required and he is equipped to do this but there will be some delay in the results.....

    (From Chris (a few months later).)

    Well, tonight while looking through the "Holography Handbook" I spied what looked suspiciously like that elusive laser I have. It said it was made by Jodon Engineering Associates of Ann Arbor, MI. I immediately called them and was fortunate to have the engineer (Bruce) who has built their tubes for the last 18 years answer the phone. I told him of my plight and read off the numbers that were on the plasma tube. Sure enough, it was one of their early lasers. They have been manufacturing HeNe's since 1963. He provided me with many of the details that I had been searching for.

    I explained that I planned on trying to re-gas this antique and he offered to help with what ever information I needed. It is truly refreshing to find someone in the industry that is willing to help the amateur without an eye on just making a profit.

    I finally located a small supply of HeNe gas, just yesterday. While visiting North Country Scientific to purchase a pair of neon sign electrodes (in Pyrex), I mentioned my need for a small amount of laser gas for my laser refurbishing project. (This was formally Henry Prescott's small company that supplied all the hard to find components for the Scientific American laser projects.) Lo and behold, there on a shelf, covered with dust, were a few of the original (1964?) 1.5 liter glass flasks filled with the 7:1 He/Ne gas mix. He let them go at a very decent price!

    (Hopefully, those tiny weeny slippery He atoms have not leaked out! --- sam)

    Now, about the magnets:

    The magnets are of rectangular shape, one inch long, 3/4 inch in width and 3/8 inch thick. There are a total of 26 magnets placed flat against the top (14) and flat against the bottom (12) of the plasma tube as viewed from the side. All but the ones on the very ends of the plasma tube are attached exactly opposite from one another, top and bottom. (See Jodon Laser Head for placement and field orientation).

    They are placed with the long side (1") parallel to the plasma tube with the north and south poles along this axis.

    They appear to be of ceramic construction and not very powerful. Sorry, I don't have any means of measuring the actual field strength.

    The current status of this project is that the laser needs to be regassed. Chris is equipped to do this and has acquired the needed HeNe gas mixture.

    To be continued....

    Photos of a similar but much larger Jodon HeNe laser (3.39 µm IR in this case) can be found in the Laser Equipment Gallery (Version 1.41 or higher) under "Jodon Helium-Neon Lasers".

    The Aerotech LS4P HeNe Laser Tube

    This is a 1970s HeNe laser tube contributed by Phil Bergeron who also refired the getter (see below) before sending it to me. It was probably manufactured just before companies realized that putting the mirrors inside the gas envelope would work just fine and is best and cheapest. The construction of the LS4P is generally similar to that of modern tubes with a hollow cold cathode and narrow bore. However, it is basically a two-Brewster laser with mirrors sealed to short glass extensions that are the same diameter as the main tube. See Aerotech LS4P HeNe Laser Tube.

    The Brewster windows appear to be glued in place. The OC is a normal 7 or 8 mm diameter curved mirror glued to the inside of the output aperture plate - basically a metal washer. The HR is a square, almost certainly planar mirror, glued to the outside of a 4 screw adjustable mount of sorts. Why is the HR square? Probably because it was cut from a large coated plate, rather than being coated individually. Why 4 screws instead of 3, making mirror adjustment much more of a pain? Another unsolved mystery of the Universe. :) Though it's not obvious from the photo, the Brewster windows aren't quite oriented the same - the angle differs by perhaps 5 degrees - so the gain is already slightly reduced from what's possible. However, I have been assured that this laser did meet specifications when new. The output is still polarized - probably half way in between - but the polarization extinction ratio is certainly lower than it could be. If the laser is still under warranty, it might be worth complaining. ;) As can be seen, this sample still lases after refiring the getter and then letting it run for several hours to allow the cathode to adsorb remaining impurities. The refiring was actually done using a can crusher demonstration apparatus and the remains of the getter coating can be seen as the ugly brown ring encircling the tube just to the left of the anode connection. I don't know whether the getter coating was any the worse for wear after that exciting event as I was not present.

    What's a "can crusher"? :) Basically an electromagnetic pulse (EMP) generator: Discharge a really large high voltage capacitor bank into a couple of turns of wire wrapped around the tube (in this case). Since the getter electrode in this tube is conveniently oriented as a ring around the bore and thus acts as the secondary of a transformer, the high current discharge induced enough current to heat the ring to heat it instantly. I wish I could have witnessed that!

    The output is only about 2 mW though, when the spec is 4 mW. Spectral line measurements of the discharge in the bore suggest that it's low on helium and low pressure in general. A helium soak may be in its future.

    I have a most likely even earlier Aerotech tube which is constructed along the same lines as the LS4P except that:

    1. It is nearly 3 times as long and twice the diameter.
    2. It has a side-arm cathode.
    3. The HR mirror is round instead of square.
    4. The bore is segmented as described in the section: Segmented HeNe Tubes.

    It doesn't lase and has a very pink discharge - running it now to see if that helps but not much hope by the time it gets that far. The tube originally put out 22 mW according to a hand-written sticker. I had picked it up on eBay in a big blue case and substituted another only slightly newer hard-sealed Aerotech tube which at least lased - 6 mW, wow. :) Its problem appears to be a bad recipe for the gas fill, mirrors, or both.

    A Really Old HeNe Laser

    This one isn't really that strange but it must be quite old. The American Optical Corporation model 3100 was a red (632.8 nm, the usual wavelength) HeNe laser that used an external mirror (Brewster window) tube with a heated filament for the cathode.

    The cover on one unit bears a sticker from El Don Engineering, 2876 Butternut, Ann Arbor, Michigan 48104, Phone: 1-313-973-0330. The laser was serviced and repaired on 9/28/80 and its output was 2.3 mW, TEM00. Another one had "Tube No. 1170, 2.1 mW TEM00, Jan. 13, 1970". I wonder if they still exist. :)

    The AO-3100 appears to be made by Gaertner (whoever they are/were, their model number is not known). Two samples are shown in the Laser Equipment Gallery (Version 2.08 or higher) under "Assorted Helium-Neon Lasers".

    The bore is about 2.5 mm in diameter which is extremely wide for a red HeNe laser. I would have expected it to be multi-mode (not TEM00). However, both samples say TEM00 and they must know. The Brewster windows are Epoxy-sealed so needless to say, most of these lasers no longer work (aside from the slight problem that when I received the first tube from one, it was in pieces. While I never expected it to work, being intact would have been nicer.)

    AO-3100 HeNe Laser Plasma Tube shows a (dead) tube removed from an AO-3100 laser. Note the wide but thin-walled bore. Cathode in AO-3100 HeNe Laser Plasma Tube is a closeup of the filament and expired getter below it.

    Not surprisingly, most of these lasers no longer lase or even light up since the tubes are soft-seal and long past their expiration dates. But if you happen to own a working time machine, it seems that Metrologic was supplying replacement tubes and power supplies for the AO-3100 as late as 1980. And, a bargain at only $225 and $100, respectively. You'll have to pay with old bills though. :)

    However, I now have obtained an AO-3100 that does still lase. More below.

    Lasing specifications:

    HeNe laser tube:

    Resonator:

    Power Supply:

    I have acquired a sample of the AO-3100 that was quite battle weary but the tube did survive cross-county shipping. The case, on the other hand, looks like it lost a fight with one of those Sherman Tanks. :) It was bent and dented in multiple places. How the tube didn't turn to a million bits of glass is amazing.

    The better thing about this laser is that the discharge color of the old soft-seal tube looks pretty good and there is still a very distinct getter spot. A measurement of the ratio of the He 587.56 nm and Ne 585.25 nm spectral lines in the discharge show that they are about equal in intensity. This means that the He:Ne fill pressure is still decent, though compared to a barcode scanner HeNe laser tube I tested, about 1/2 the helium intensity. A helium soak might be in its future.

    After realigning the mirrors and cleaning the Brewster windows, I now have 0.35 mW of red photons squirting out the front of the laser. Probably only the front mirror was misaligned originally, but since I had to remove them both to get the rubber Brewster covers off, realignment of both were required. Fortunately, getting an alignment laser beam through the wide bore was straightforward. The HR mirror mount was then installed and adjusted to return the alignment beam cleanly through the bore. The OC mirror mount was then installed and that's when it became clear that its alignment was way off. Now I wonder who did that. :) Once the alignment screws were tweaked to center its reflected spot, a bit of fiddling resulted in a weak beam. Some mirror walking and Brewster cleaning helped, but it's not finished.

    The discharge color appears to be improving as it is run as well but output power has been decreasing as it is run. I hadn't realized that the spec'd lifetime is only around 100 hours - and I've put on 5 or 10 percent of that just testing it! It might be a power supply problem though since it produces a nice bright beam for an instant when started, but then settles down to perhaps 100 uW on a good day. I do turn it on for a few seconds almost everyday just to keep it happy.

    The photos for "Gaertner/American Optical 3100 Helium-Neon Laser 2" in the Laser Equipment Gallery are of this laser in action. The color rendition of my digital camera isn't very good. The color in the main bore and larger sections of tubing actual should look close to that in normal HeNe lasers. But the cathode glow (the bright blob) is actually more yellow, (though not quite the yellow in these photos. :) The double coiled glowing hot filament is clearly visible in Views 03 to 05. A careful examination of Views 03 and 06 reveals the scatter from the Brewster windows at each end of the tube. Note the large difference in scatter size due to the hemispherical resonator. View 07 shows that there is indeed a beam from this laser (if that wasn't obvious from the Brewster windows), though due to its relatively low power, bore light is competing for attention.

    I now run this laser for a short time on roughly a weekly basis just to keep it happy. I've never reinstalled the boots, so Brewster cleaning is required every few weeks. The maximum power is now only about 0.2 mW and seemed to be declining with extended run time. Once one realizes that the rated life is only 100 hours or so, it's likely that the few hours I ran it sucked up a substantial percentage of its life. However, the short runs don't seem to be hurting it much. This laser was acquired in July, 2005 and it had been over 2 years now without obvious degradation.

    However, as of 2009, it lights up with an seemingly normal discharge color but will not lase despite repeated B-window cleaning. It's possible that the mirrors have become contaminated due to not being sealed, or even degraded since they are soft coated. Eventually, I'll deal with that.

    The Dual Color Yellow/Orange HeNe Laser Tube

    Multiline operation is common in ion lasers where up to a dozen or more wavelengths may be produced simultaneously depending on the optics and tube current. However, most HeNe lasers operate at a single wavelength. The only commercial HeNe lasers I know of that are designed to produce more than one wavelength simultaneously are manufactured by Research Electro-Optics (REO). They have 1,152/3,390 nm and 1,523/632.8 nm models.

    Through screwups in manufacturing (incorrect mirror formula, extra "hot" emission, etc.), an occasional HeNe laser may produce weak lasing at one or more ("rogue") wavelengths other than those for which it was designed. For red tubes, the most likely spurious wavelength is a deeper red at 640 nm since it is also a fairly high gain line. For a low gain yellow laser, orange is most likely since it is a relatively close wavelength and any goofup with the mirror reflectivities may allow it to lase.

    I have a tube made by Melles Griot, model number 05-LYR-170, which is about 420 mm long and 37 mm in diameter and can be seen as the middle tube in Three HeNe Tubes of a Different Color Side-by-Side. Its only unusual physical characteristics are that the bore has a frosted exterior appearance (what you see in the photo is not the reflection of a fluorescent lamp but the actual bore). Apparently, larger Melles Griot HeNe tubes are now made with this type of bore - it is centerless ground for precise fit in the bore support. I don't know if the inside is also frosted; that is supposed to reduce ring artifacts. And, of course, the mirrors have a different coating for the non-red wavelengths.

    According to the Melles Griot catalog, this is a HeNe laser tube operating at 594.1 nm with a rated output of 2 mW. However, my sample definitely operates at both the yellow (594.1 nm) and orange (604.6 nm) wavelengths (confirmed with a diffraction grating) - to some extent when it feels like it. The output at the OC-end of the tube is weighted more towards yellow and has a power output of up to 4 mW or more (you'll see why I say 'up to' in a minute). The output at the HR-end of the tube has mostly orange and does a maximum of about 1 mW. Gently pressing on the mirrors affects the power output as expected but also varies the relative intensities of yellow and orange in non-obvious ways. They also vary on their own. The mirror alignment is very critical and the point of optimum alignment isn't constant. In short, very little about this tube is well behaved. :)

    Why there should be this much leakage through the HR is puzzling. The mirror is definitely not designed for outputting a secondary beam or something like that as there is no AR coating on its outer surface. Thus, that 1 mW is totally wasted. Perhaps, this was an unsuccessful attempt to kill any orange output from the OC. The OC's appearance is similar to that of a broadband coated HeNe HR - light gold in reflection, blue/green in transmission. The HR appears similar to one for a green HeNe laser - light metallic green in reflection, deep magenta in transmission. (However, it's hard to see the transmission color in the intact tube. The OC may be more toward deep blue and the HR may be more toward purple.)

    As would be expected where two lines are competing for attention in a low gain laser like this, the output is not very stable. As the tube warms up and expands - or just for no apparent reason - the power output and ratio of yellow to orange will gradually change by a factor of up to 10:1. Very gently pressing on either mirror (using an insulated stick for the anode one!) will generally restore maximum power but the amount and direction of required pressure is for all intents and purposes, a random quantity. If the mirror adjuster/locking collar is tweaked for maximum output at any given time, 5 minutes later, the output may be at a minimum or anywhere in between.

    I surmise - as yet unconfirmed - that at any given moment, the yellow and orange output beams will tend to have orthogonal polarizations. But, as the distance between the mirrors changes, mode cycling will result in the somewhat random and unpredictable shifting of relative and total output power as the next higher mode for one color competes with the opposite polarized mode of the other. Is that hand waving or what? :)

    A few strong magnets placed along-side the tube reduce this variation somewhat. I'm hoping that adding some thermal control (e.g., installing the tube in an aluminum cylinder or enclosed case) may help as well. I was even contemplating the construction of a servo system that would dither the cathode-end mirror mount to determine the offset direction that increases output and adjusts the average offset to maximize the output. This might have to be tuned for yellow or orange - an exclusive OR, I don't know if maximizing total optical power will also maximize each color individually.

    Using an external red HR or OC (99 percent) mirror placed behind the tube's HR mirror, I was able to obtain red at 632.8 nm as well as a weak output at the other orange line (611.9 nm), and at times, all four colors were lasing simultaneously. :) See the section: Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes.

    (From: Steve Roberts.)

    Ah, the Melles Griot defects... These show up from time to time and are highly prized in the light show community for digitizing stations and personal home lumia displays.

    The yellow/orange combo is not a goof. I've seen a 7 mW version of that that was absolutely beautiful, but rejected because it was too hot. It's probably slight differences in the length of the tube or bore size. They cut them for a given mode spacing, but fill them all at once with the same gas mixture. A few companies do make dual line tubes, but you can imagine the initial cost is murder.

    I used to have a short tube that switched from red (632.8 nm) to orange (611.9 nm) that appeared brighter then the red when it felt like it.

    I sometimes wonder if there are a few more HeNe transitions we don't know about. I know they exist in ion lasers. I have seen a 575 nm yellow line in krypton that's not on the manufacture's data and a red in Kr that is between 633 and 647 nm. I had that red in my own laser. 575 nm is preferred for show lasers because it doesn't share transitions with 647 nm like 568 nm does.

    When I was interviewing at AVI in Florida they used 4 color 4 scan pair projectors for digitizing - 6 mW of yellow, 5 mW of green, and 8 mW of red, all from HeNe lasers. The blue came up from an ILT ion laser in the basement to each of the four stations via optical fiber. The guy who owned AVI said if you call Melles Griot and ask nicely they will grade some tubes for you for a slight extra cost. Methinks they make all the special colors up and tune them in power somehow, so they can make a price differential, those lines should be consistent by now.

    Every two years of so it seems Melles Griot cleans out their scrap pile, and somebody always seems to get there hands on them, grades them and sells em.

    (From: Daniel Ames (Dlames2@aol.com).)

    The yellow and orange HeNe energy transitions are very similar and possibly competing with each other, especially if the optics are questionable. I have learned that Melles Griot and other HeNe laser manufacturers sometimes suffer from costly mistake on a batch of tubes due to the optics being incorrectly matched to the tube and/or the optics themselves not being correct for the desired output wavelength. One such batch was supposed to be the common red (632.8 nm) but the optics actually caused the gain of the orange to be high enough that the output contained both red and orange (611.9 nm). Then I believe they are rejected and tossed out, only to be saved by professional dumpster divers to show up on eBay or elsewhere. Actually, these misfits such as the yellow/orange tube can be quite fascinating. It would be interesting to shine a 632.8 nm red HeNe laser right through the bore of that tube while powered and see what color the output is. I have been told that if you shine a red HeNe through a green HeNe that it will cause the green wavelength to cease. I have not had this opportunity to try this, so I do not know for sure what really happens, maybe the red just overpowered the green beam. This could be verified with 60 degree prism or diffraction grating on the beam exiting the opposite end of the green tube. Happy beaming. :)

    (From: Sam.)

    I have tried the experiment of shining a red HeNe laser straight down the bore of a green HeNe laser (my green One-Brewster tube setup). I could detect no significant effect using a low power (1 or 2 mW) laser. This isn't surprising given that the intracavity power of the green laser was probably in the hundreds of mW range so the loss from the red beam would be small in a relative sense. However, wavelength competition effects are quite real as evidenced from experiments with the two color 05-LYR-170 tube.

    The Weird Three-Color PMS HeNe Laser Head

    I picked up a surplus PMS (now Research Electro-Optics) LHYR-0100M HeNe laser head (with power supply) on eBay for a whopping $30 including shipping. This model supposedly produces a pure yellow (594.1 nm) multimode beam with a minimum power output of 1 mW. See REO LHYR-0100M. But mine is happily outputting the yellow (594.1 nm) and two orange (604.6 and 611.9 nm) lines (determined by splitting the beam with a diffraction grating, something I routinely do with all newly acquired HeNe lasers!).

    Its actual total power output after warmup is over 2.50 mW. The 594.1 nm (most intense, LG01/TEM01* doughnut) and 604.6 nm (LG01/TEM01* or TEM10 depending on its mood) are relatively stable but the 611.9 nm (least intense, TEM01) visibly fluctuates. Nonetheless, overall power stability and mode cycling behavior are similar to that of a typical medium power red (632.8 nm) HeNe laser, which contrasts dramatically with the very unstable yellow/orange Melles Griot laser described above. REO does have a couple of dual wavelength HeNe laser heads listed but nothing like this. They are 1,152/3,391 nm and 1,523/632.8 nm.

    There is also an additional 2 pin connector on this laser head. The resistance between pins is about 20 ohms and I assume it to be a heater on the OC mirror, though driving it with about 10 V had no detectable effect whatsoever. (This is supposedly used to prevent the formation of "color centers" in the mirror coating. Many older PMS lasers have the heaters and I've never seen any noticeable effect on any of those I've tested either!)

    However, I wonder if there is also some screwup in the REO model descriptions as the size of this laser head actually matches that of the REO LHYR-0200M, being almost 17" in length rather than the 13" listed for the LHYR-0100M. I kind of doubt that shorter length can be accounted for by dramatic improvements in HeNe laser technology since my sample was manufactured (1988), though I suppose that's a possibility. But the electrical specifications of the two lasers are supposed to be identical, which doesn't make sense and I don't believe in coincidences. :) And the output power of my sample peaks at 6.5 mA which isn't consistent with the specs for either the LHYR-0100M or LHYR-0200M which are both 5.25 mA.

    The Weird Four-Color REO HeNe Laser Tube

    (With extensive contributions from: Sean Reeber.)

    And this one is only supposed to be 611.9 nm orange. However, it's doing stable 604.6 nm (orange toward yellow), 594.1 nm (yellow), AND a wavelength that few if any people have ever seen in a HeNe laser, which appears to be 609.05 nm (orange). The tube is labeled LTOR-0150ODE, which would normally mean 1.5 mW (rated) 611.9 nm (orange). But we know and love PMS/REO - many of their "other color" HeNes are not what they are spec'd to be. For example, see the section: The Weird Three-Color PMS HeNe Laser Head, above. This is a bare tube which by design (I assume) has about equal output from both ends. (Confirmed because both ends have the strange extra optic glued to the mirror glass, presumably to correct divergence.) Originally, it was misaligned, so the total output power was only about 2 mW consisting of the three common lines - 611.9, 604.6, and 594.1 nm. (Already out of spec but not unusual for REO.) After aligning the OC mirror with a car key (!!), it now produces almost 4 mW total output from both ends. AND a lasing line popped up between 611.9 and 604.6 nm. At first I thought it was simply an artifact of the diffraction grating since it was too unstable to really analyze in detail. But then it came on and stayed on for almost an hour during which photos could be taken of the lasing line spectrum and the wavelength could be measured precisely. And it seems to be very close to 609.05 nm.

    A search of the NIST database and other sources has shown that there is a transition at 609.5 nm (but not at 609.05 nm) between the 2P4 to 1S4. But this is still not out of the question, though I do believe my measurements to have an uncertainly of less than 0.2 nm. However, if it is indeed 609.6 nm then there is another mystery: 2S4 is the lower lasing level for 632.8 nm (common red). But there is normally no lasing at 632.8 nm for this laser! (Though 632.8 nm can be produced using external mirrors.) So, if that wavelength is accurate and originates there, it may be another Raman transition. The source of the peculiar lasing line is unknown. It has not turned up in a literature search for lasing wavelengths so far. However, in "Gas Lasers", edited by Masamori Endo, Robert F. Walter, pg. 501, there is a diagram with a radiative decay transition at 609.6 nm. This should not be a lasing line though. See link for the graph from "Gas Lasers" Gas Lasers: The Helium-Neon Energy Level Diagram.

    Another possibility is that the 609.05 nm lasing line is from some gas contamination. Here are some possible emission lines close to the unknown lasing line:

    The most likely is argon, with its emission line at 609.0785 nm, within 0.03 nm of the mystery line. It could be that REO used neon intended for NE2 indicator lamps, which apparently may have 0.5% Ar to reduce the starting voltage. Or, perhaps they added Ar for that purpose figuring it would make no difference in lasing - which would be true in most cases, and any additional lines would go unnoticed by 99.9% of users.

    Stay tuned. The jury is still out on this one. :) The next step will be to accurately measure the wavelengths of nearby lines, as well as major lines for other gases, in the bore discharge.

    The Ancient Hughes HeNe Laser Head

    These old laser heads have been showing up in various places including eBay with one particular model number being: 3184H. See Hughes Model 3184H HeNe Laser Head. They date from the 1970s, some possibly quite early in the decade. Their external appearance is unremarkable - a heavy gold-colored cylinder about 12.25 inches long and 1.75 inches in diameter, with end-plates each attached with 4 cap screws. Power connections to most are via a pair of rather thin red and green wires (with red being the positive input), though later ones may use an Alden cable. There is a 30K ohm, 5 W metal film internal ballast resistor which by itself is insufficient for stable operation with most power supplies - an external ballast of 50K to 75K is required. The power supply that appears to be intended to drive this laser head has a 60K ballast on board. (See the section: Hughes HeNe Laser Power Supply for the Model 3184H Laser.) So far, ho hum. :)

    But the remarkable thing about these laser heads revolves around what is inside: A two-Brewster HeNe laser tube! Except for some very early units, the tips of the 2-B tube extend to very nearly touch the mirror plates. On some early ones, the tube is about an inch shorter. (I don't know if this is just a physical difference or whether the newer tubes are actually slightly higher power.) So, these are really external mirror lasers in a nice compact stable package. See View Inside Hughes Model 3184H HeNe Laser Head and Hughes Model 3184H HeNe Laser Head Construction. The end-plates press against aluminum gaskets which allow for mirror adjustment as well as providing a mostly decent environmental seal. The mirror glass is held in place in the end-plate with an aluminum ring press-fit against a rubber cushion. Note the threaded inserts to provide steel-on-steel contact for the adjustment screws. The Brewster window and potting material can be seen within the massive aluminum cylinder - the wall thickness of the sections near each end is at least 5/16ths inch! It's actually made up of 3 pieces (in addition to the end-plates) press-fit together along with a rubber O-ring and an additional rubber ring (maybe just squirted in before completing the press-fit) for sealing. The center section has thinner walls and I found out that clamping it in a vice will crunch the tube. :( But at least the broken heads still make decent hammers. :) The actual tube is the typical Hughes-style but with B-windows at both ends. Although the potting material is soft rubber and not RTV, it appears to mostly fill the inner space, just allowing the Brewster stem at the anode/wiring-end of the tube to poke out and nearly covering the cathode-end, so removing the tube intact would be a challenge. More below.

    Several other models may also contain 2-B tubes like this including the 3072H, 3176H, 3178H, 3193H, and 3194H.

    Unfortunately, dating from the 1970s, most samples are deader than the standard door nail. They might light up but don't lase. I acquired two of these awhile ago. One, from 1976, appeared to have approximately the correct discharge color (as best as I can determine viewing it from the end) and the tube voltage seemed reasonable. But, no red photons no matter what I've tried. Another, from 1979, did start a couple years ago, though the discharge color and tube voltage characteristics were obviously wrong. But now it only flashes, indicating that it's nearly up to air. However, several of the oldest lasers, dating from the early 1970s, have survived and lase and even produce an output power not much different than what was measured in 1973, the last time they were tested! The beam is TEM00 with low divergence and less scatter than many modern HeNe lasers. I suspect that for those fortunate individuals, the Brewster windows were optically contacted instead of being sealed with Epoxy.

    One of the working heads I tested outputs about 3.5 mW at 6.5 mA with an operating voltage to the head of about 1,610 V. The test power in 1973 was 3.4 mW. Based on the 4 in the model number and a CDRH sticker rating of 6.5 mW, I suspect that the rated output power is actually 4 mW. Power continues to increase slightly above 6.5 mA. This may mean that either the optimal current is higher, or more likely, that the tube is low on helium or has some other slight gas fill problem, or it's just high mileage. (Although the power supply that apparently went with these heads is not very well regulated, its behavior suggests that 6.5 mA is correct.) Due to the way the tube is potted inside the metal cylinder, there is no way to easily assess the discharge spectrum to evaluate the gas fill without test instruments.

    The mirrors appear to be hard-coated with the HR being flat and the OC having an RoC of about 30 cm. This results in a nearly hemispherical resonator with a mirror spacing just under 30 cm, confirmed by the very small spot visible on the HR mirror when the laser is operating. The OC is AR coated on its outer surface (though it is not as robust as modern AR coatings), and on most of the laser heads, the HR is fine-ground on its outer surface.

    Interestingly, the bore of the 3184H appears to be tapered and is wider at the OC-end than at the HR-end. This makes sense to more closely match the mode volume of the hemispherical resonator and thus increase the gain slightly. A tapered bore was apparently an optimization that was popular in the early days of HeNe lasers but went out of fashion due to its higher cost compared to using a uniform size capillary tube for the bore. I've only come across a tapered bore (or at least noticed it) in one modern-style HeNe laser tube, a Melles Griot 05-LHP-170, manufacturing date unknown but it has a serial number of 675P - sounds kind of old! With this asymmetry, the HR and OC cannot simply be swapped without likely seeing a severe penalty in output power. It also would likely not be advantageous to use a confocal or any other symmetric configuration. However, going to a long-radius hemispherical resonator might work even better than the existing arrangement.

    With 4 screws holding the end-plates in place against the aluminum gasket, mirror adjustment is somewhat awkward but with persistence, optimal alignment including mirror walking can be performed relatively quickly. However, the aluminum gasket isn't ideal, so for testing, I've replaced it with a rubber O-ring to provide some real compliance. That is, until I decide what to do with the 2-B tube inside! :)

    There apparently were some of these for other wavelengths. I've found a (dead) sample of a 3176 that was probably for 1,152 nm as the mirrors are highly transparent at all visible wavelengths but without the greenish tint typical of 1,523 nm mirrors. I suppose it's possible someone replaced the mirrors but they appear to be original.

    For a description of several more of these lasers, and a test jig and tests using external mirrors, see the section: Some Semi-Antique Hughes Laser Heads.

    Where one is really determined to get the tube out, here is more info on what's involved. But why bother? Aside from esthetics, it's perfectly happy in there and very well protected. The risk of destroying the tube may not be worth the rewards. The press-fit end-sections must be pulled straight out (not twisted) with something along the lines of a gear puller as they are a very tight metal-to-metal press fit with ridges all around. Or, they can be carefully cut off with a metal cutting lathe or band saw. But serious vibrations will likely destroy the tube. Then, the rubber potting material would have to be chipped/gouged/cut/sliced away to actually extract the tube. Then all the remnents of the rubber stuff must be removed from the tube.

    Having said that, I was able to get the end-sections off of a dead laser head without serious tools. (I'm not about to risk a good one!) Since the center section has a slightly larger outside diameter than the end-sections, an aluminum HeNe laser head clamp tightened just snug around the end-section provided a way of pressing on the center section to pull the end-sections free. Four clearance holes were drilled in a 1/2" thick piece of aluminum plate and 4-40 screws were then passed through these holes and threaded into the laser head. By carefully tightening these screws in a cyclic manner (e.g., 1,2,3,4,1,2...), the end-section could be pulled out about 1/8". Once this was done, the HeNe head clamp was removed and shorter screws were used to attach the 1/2" plate directly to the head. With the plate clamped in a vice, the entire head could be worked back and forth until it came free. (Alternatively spacer plates and/or shorter screws could be added/substituted to continue the original process until the end-section comes free.) This was not fun, a set of screws survived for only about one end-section, and as noted, this is really only the beginning of the tube extraction process. I have not yet attempted to go any further. But someone else has succeeded in removing one of these tubes intact. Apparently it wasn't much fun.

    I've recently come across a 3170H, which is similar in construction to the heads described above - but on steroids. It is 22-3/4" long by 2-1/4" diameter with a thick-walled cylinder for its entire length. The mirror adjustments are equally mediocre with the same aluminum foil seals. The 2-B tube inside is about 22" from Brewster tip to Brewster tip. It had a manufacturing date of 1978. Unfortunately, the HV cable was cut flush with the body of the cylinder, so there was no chance of beaing able to safely apply power, but using an Oudin coil, it does ionize with possibly decent color. It must have been good for 10 or 15 mW.

    And I was given a 3178H that is under 9 inches in length with an Alden cable coming out the side instead of the red and green wires, but is otherwise identical to the 3184H. See Hughes 3178H HeNe Laser Head on Original Mount with 3598H Power Supply. It produces over 1 mW at 6.5 mA (a bit under 0.9 mW on 5 mA), which is probably close to the power when new.

    The PMS/REO External Resonator Particle Counter HeNe Laser

    This is a particle counter assembly labeled: ULPC-3001-CPC, 18861-1-16 with the actual HeNe laser tube labeled: LB/5T/1M/E(HS), PMS-4877P-3594. The unit is shown in PMS/REO ULPC-3001 Particle Counter HeNe Laser Assembly. When I found it on eBay, the listing was for a One-Brewster tube. However, this one is really strange. For one thing, it is not a Brewster tube but rather a somewhat normal internal mirror HeNe laser tube. Well, at least normal by PMS/REO standards - mostly metal with Hughes-style glasswork at the anode-end. Except it is a very multimode tube having an output that is rather high (greater than 7.5 mW) for its length (11 inches between mirrors) and power requirements 1,900 V/5.25 mA. That would be only modestly interesting. But there is an additional mirror beyond the OC (inside in the area between the two red dots next to the red sticker at the left) which forms an external resonant cavity with the (internal) OC mirror. The external HR mirror is actually coated on the end of a transparent crystal about 1 cm in length, mounted by a pair of electrodes attached to opposite sides which most likely is piezoelectrically active and probably changes length when a voltage is applied to it. A photodiode is mounted beyond the crystal (far left in photo). The signal from the photodiode shows resonance effects at several relatively low frequencies (two dominant ones are around 175 and 350 kHz). The waste beam from the HeNe laser HR mirror can actually be seen to flicker and become much lower in power at the resonance points. The crystal and photodiode may be used to dither the output so that the effects of the inherent laser noise are eliminated. I doubt its supposed to be a very high frequency because the wires to the electrodes are not shielded. It might also be used in a feedback loop at low frequencies.

    PMS has a patent for this setup - U.S. Patent #4,594,715: Laser With Stabilized External Passive Cavity. By linearly oscillating the external mirror at a modest frequency (enough to produce a few cm/sec of movement), the resulting Doppler broadening of the wavelength spectrum will be sufficient to effectively decouple the external cavity from the active cavity. This gets around the stability issues present with open cavity (e.g., Brewster window) particle counter designs. There is a great deal of information in the patent on this and other principles of operation.

    Any hapless particles that may pass through the beam in the cavity between the OC of the HeNe laser tube and the external mirror will result in scatter detectable from the side. A large reflector and aspheric lens collects the side-scatter and focuses it on another photodiode (under yellow CAUTION sticker). There is a preamplifier in the box.

    It gets better. Viewing the waste beam out the unused HR-end of the tube (far right) with a diffraction grating reveals that the tube is lasing on the normal red line, but also on both of the HeNe orange lines (604.6 nm and 611.9 nm), three other red lines (629.4 nm, 635.2 nm, and 640 nm), *and* on the very rare Raman shifted red line at 650 nm. And there may be others but it's difficult to resolve them since the beam is multimode and the spectra cannot be focused to small spots. Here's a photo of spectrum: Lasing Lines of PMS/REO External Particle Counter HeNe Laser 1. From left to right, the wavelengths are: 604.6 nm, 611.9 nm, 629.4 nm, 632.8 nm, 635.2 nm, 640 nm (very weak), and 650 nm. The 650 nm is actually probably the second strongest after 632.8 nm. How many 7 line HeNe lasers have you seen? :) This is similar but even better than what I've observed in my experiments using external mirrors with normal internal mirror HeNe laser tubes although this one seems particularly stable with little obvious variation in the intensities of the lines, at least over a period of a few minutes. Obtaining the 650 mm line is particularly unusual, especially since it is so stable. See the section: Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes. These non-632.8 nm lines are probably not an objective of the design but are just an interesting artifact.

    I have estimated the reflectivities for the three mirrors which are in this laser. These values are based on measurements of the output power of the HeNe laser tube without the external mirror (about 8 mW after warmup) and the assumption that the internal OC is about 99 percent:

                                                Power with external HR?
        Mirror Description     Reflectivity         No         Yes
      ----------------------------------------------------------------------
        HeNe laser tube HR     99.99%             0.9 mW     1.00 mW
        HeNe laser tube OC     99% (assumed)      8.00 mW   80.00 mW
        External HR            99.9%                --       0.09 mW
    

    The "Power" refers to the optical power passed by the specified mirror depending on whether the external HR mirror is present and aligned. In the case of the HeNe laser tube OC with the external HR, this is the circulating power in the external cavity which is what's available for the particle scatter. Note that the circulating power inside the HeNe laser tube is around 10 WATTS but isn't accessible.

    I obtained another particle counter assembly with an internal mirror HeNe laser tube and external resonator. However, there were some differences, most notably an electronics PCB attached to the back of the resonator, and possibly a PZT instead of EO device for cavity length dithering. The tube in this one must be soft-sealed as it arrived with a putrid blue/pink discharge requiring more than a week of run time to clean up until the output power leveled off at about 1.2 mW (50 percent higher than the other laser). It then produced 6 of the same 7 lines through the HR (all but 604 nm). The 650 nm Raman line had been growing steadily during cleanup and is as intense or perhaps even more intense than the 633 nm line. It is also rock stable which is supposed to be impossible. The 640 nm line is very weak, possibly as a result of mode competition with the Raman line but that's just a wild guess. :) There is also a very weak output at around 652 nm - probably another Raman line or something more exotic. But it is only there sporatically. See Lasing Lines of PMS/REO External Particle Counter HeNe Laser 2. Too bad the color rendition of the digital camera is so poor.

    And here are some comments on particle counter technology:

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

    There exist particle counters using external resonant cavities, and also intracavity Nd:YAG setups. Intracavity measurements *look* as though they give you amazing sensitivity, but they usually don't. Not only is the circulating power amazingly sensitive to temperature gradients and tiny amounts of schlieren from air currents, but the signal you get is wildly nonlinear and highly position-dependent. Intracavity measurements are a great way to lose sleep and hair. Passive cavities are usually much better, and nonresonant multipass cells are better still.

    The Ohmeda Raman Gas Analyzer REO One-Brewster Laser

    This unit is somewhat similar to a particle counter in that there is a very high-Q 1-B HeNe laser tube with a second HR mirror some distance away. In between is a space for an absolute filtered unknown gas to pass through with 8 "viewing ports" - 4 on each side. Sensitive photon counting detectors would normally go behind individual narrow band filters, each with a different center wavelength.

    Raman spectroscopy is used to identify gases by passing a laser beam through the unknown sample. Raman scattering results in a shift toward longer wavelengths depending on the atomic/molecular composition of the gas. By measuring the intensity of the Raman scatter at several longer wavelengths, the gas composition can be determined. For these units, the relevant gases were apparently N2, O2, and N2O based on "linearization constants" printed on a label on the lasers.

    To get any sort of sensitivity, the beam must be high power since a very small percentage of photons actually undergo the Raman shift. For the Ohmeda unit, this is achieved by utilizing the intracavity power between 2 super polished HR mirrors and super-polished Brewster window. While I don't know for sure what the intracavity power should be, based on tests of the mirror reflectivities and output power with an external OC mirror with known reflectivity, it is at least several watts and could be over 100 W when using the original exteranl HR mirror!

    The relevant patents include:

    The first one describes the principles of Raman spectroscopy though the actual drawings do not correspond to the Ohmeda laser assembly. But the other two have diagrams which closely match the specimens I have, though I'm not sure which they are.

    A photo of a mostly complete unit is shown in Ohmeda Raman Gas Analyzer Assembly. The metal HeNe laser tube can be seen poking out the left side with the red cap covering its internal HR mirror. The brick power supply is behind it. The tuning prism assembly is at the right, partially hidden by an absolute filter and one of the detector PCBs. That elaborate set of filters and dessicant containers is designed to eliminate *all* particles and condensible vapors from the laser cavity, which must remain perfectly clean. I'm not really sure why the heatsink is clamped to the lsaer tube. It doesn't get *that* hot. :)

    The laser tube, Brewster prism, and external mirror are probably made by REO, Inc.. (Other parts of the assembly may be made by REO as well.) The tube looks like a slightly shorter version of the REO/PMS tunable 1-B tubes, but its internal HR mirror is coated so that in conjunction with the HR mirror at the other end of the cavity, the reflectance for 632.8 nm is maximized. Using a 60 cm RoC OC mirror with a reflectance of approximately 98 percent at 632.8 nm, the laser produces about 5.4 mW, multimode. I assume that with an optimal OC mirror, the power would be somewhat higher. (This test was done without the Brewster prism assembly. There would be some loss with the prism present in the cavity.)

    At 5 mW - implying 250 mW of intracavity power with the 98 percent OC - the waste beam is about 5 uW and the reflectivity of the internal HR mirror is thus about 99.998 percent. There is very little scatter visible on the B-window under these conditions. (I did have to clean it, but there is a handy access port that can be used for this purpose.) If there were no other losses, putting a similar HR at the other end would result in 125 W of intracavity power! Of course, this is impossible as there ARE other losses, but it is likely to be several watts and perhaps much more. With an SP-084 HR, the output from this mirror was about 0.5 mW and the output from the internal HR was 32 uW corresponding to about 1.5 W of intracavity power. Not too shabby. But with the REO HR (and Brewster prism), the waste beam power for 633 nm was a whopping 122 uW implying about 6 WATTs inside. Not too shabby at all. :) I have cleaned the Brewster prism with no significant change in performance. However, a careful cleaning of all three surfaces would almost certainly improve things some more, especially for this case. Interestingly, with the REO mirrors, the beams exiting the laser appears to close to, if not pure, TEM00.

    When used in the normal way, there is a 632.8 nm narrow band filter between the external mirror and a silicon photodiode. So, that is almost certainly used to monitor the power transmitted by that mirror, and by inference, intracavity power.

    The 632.8 nm intracavity power would no doubt be greater without the prism but that's where it gets interesting. With the prism in place, the wavelength is tunable with both orange wavelengths being easily selectable for 2 of the lasers. (The 604.6 nm orange line is not present in Laser 3 for unknown reasons, but probably due to mirror reflectivities.)

    Here are the stats for three similar laser assemblies with different dates of manufacture:

    Laser 1 (Ohmeda PN 6090-2000-513, 15-Jul-04, Tube #IB826-5, S=0.35, T=0.57, Laser Power=3.91. REO tube MN SB/1M/BW, S/N 2856-2204-1063):

                   Power from    <------- External Mirror ------->   Intracavity
       Wavelength  Internal HR     Type     Reflectivity    Power       Power
     -----------------------------------------------------------------------------
        632.8 nm       5 uW      60 cm OC     98.0%       5,400 uW      0.25 W
         "    "       32 uW      SP-084 HR    99.966%       500 uW      1.5  W 
         "    "      122 uW       REO HR      99.9984%      186 uW      6.0  W !!
        611.9 nm     166 uW        "   "        ---       1,140 uW       ---
        604.6 nm      14 uW        "   "        ---       0.280 uW       ---
    

    Laser 2 (Ohmeda PN 6090-2000-513, 20-Feb-03, Tube #I2348-8, S=1.37, T=0.53, Laser Power=3.45. REO tube MN SB/1M/BW, S/N 1151-0603-911):

                   Power from    <------- External Mirror ------->   Intracavity
       Wavelength  Internal HR     Type     Reflectivity    Power       Power
     -----------------------------------------------------------------------------
        632.8 nm      381 uW      REO HR        ???        141 uW        ???
        611.9 nm    1,120 uW       "   "        ---         93 uW        ---
        604.6 nm      710 uW       "   "        ---         32 uW        ---
    

    Laser 3 (Ohmeda PN 6090-0803-507, 9-Aug-02, Tube #2890-3, S=2.57, T=0.37, Laser Power=2.0. REO tube MN SB/1M/BW(HS), S/N 6093-0501-607):

                   Power from    <------- External Mirror ------->   Intracavity
       Wavelength  Internal HR     Type     Reflectivity    Power       Power
     -----------------------------------------------------------------------------
        632.8 nm      864 uW      REO HR        ???        147 uW        ???
        611.9 nm    2,080 uW       "   "        ---         29 uW        ---
        604.6 nm        0 uW       "   "        ---          0 uW        ---
    

    There were three measured parameters hand-printed on the tube casings of these lasers, but without units: "S", "T", and "Laser Power". Note that S and T have approximately the same ratio as my measured 632.8 nm output power for the internal and external HR mirrors, respectively. While it's not known what these stand for, if the units of these parameters are mW, then this suggests that when new with perfectly clean optics surfaces, the performance at 632.8 nm may be 3 to 4 times what I've measured so far! (There would also be an increase in 611.9 nm output but since significant power is being coupled out of the cavity, the difference won't be nearly as dramatic.) It's also not known what the parameter Laser Power means since nowhere would there be an output where this could be measured.

    But these 1-B tubes are considerably shorter than PMS/REO tunable 1-B lasers tubes - 10.25 inches versus 13 inches from internal mirror to B-window. The relative length of the bore discharge differs by a larger relative amount: approximately 8.75 versus 11.5 inches or about 1.3:1. So, their gain will be much lower. And, there is an additional optical surface in the intracavity beam path compared to the tunable laser system since a (2-surface) Brewster prism is used rather than (1-surface) Littrow prism. Thus, even the performance I've measured is rather impressive, especially for Laser 3's orange output (which is really just an accident of the mirror coatings, and not something that was designed in).

    Also note that Laser 3 has a different part number than Lasers 1 and 2. I have no idea what differences there may be in the laser part of the system, if any. There is no obvious physical difference.

    The orange 611.9 nm beam on Laser 3 when peaked is doughnut mode with a distinct hole in the middle (LG01/TEM01*). There is also an annoying amount of mode-hopping, so adjusting for maximum power is sometimes a challenge as the power jumps around. On Laser 2, the orange beam is TEM00.

    I did not test Lasers 2 or 3 with non-REO mirrors, thus the exact reflectivity and intracavity power is not known. Note how the relative mirror reflectivities for these lasers are all different. This may be the reason of a total lack of 604.6 nm orange for Laser 3. Now, Laser 3 was originally sick with a pink discharge and no lasing and had to be run for 100 to 200 hours to recover anything. But since it's total power out of both ends is greater than the others at both 632.8 and 611.9 nm, I doubt low gain to be the cause, though that's still a possibility. Also note that 2+ mW of 611.9 nm orange from a tube of this length with mirrors not optimized for that wavelength is already somewhat impressive. And, the power is actually slightly higher than listed above since that is only the last time all 4 measurements were made. For the complete exciting saga of Laser 3, and up to date measurements, see the section: REO One-Brewster Tube - No Lasing.)

    (PMS/REO tubes are soft-sealed since that results in minimal stress on the B-window and higher Q. However, this does mean they should be run periodically. I later found that Laser 2 had a mild case of low poweritis but it's not clear if extended run time will clear it up.)

    I do not know what the reflectivity of the internal HR is at 604 nm and 611.9 nm so the intracavity power is not known for these wavelengths either. The purpose of the Brewster prism is no doubt to select only one of the possible wavelengths, which based on the specifications of the filter between the external mirror and photodiode, is no doubt 632.8 nm. The very nice behavior on the orange lines is thus simply an artifact of the mirrors being so highly reflective at 632.8 nm. But note how the power balance between the two mirrors seems to be more or less reversed for Lasers 1 and 2. So, although the internal mirror for both lasers is not AR coated and the external mirror is, the coating formulas appear to have been interchanged.

    It would be quite risky to try to run the laser with only the external REO HR but no prism as the mirror glass is glued in place. While the plate that it's glued to could be mounted directly on the adjustable mount, the mirror would be very exposed and susceptible to damage. So, I'm probably not going to attempt that.

    Here are how the 8 filters intercepting Raman light from the side of the lasers were labeled and the 633 nm line selection filter in front of the photodiode:

       Location             Part Number             Wavelength
     -----------------------------------------------------------
          1A       BARR #4 4 375-003 7819 1 1993     781.9 nm
          1B       BARR #1 2 373-030 7777 2 3100     777.7 nm
          1C       BARR #9 2 373-024 6938   3991     693.8 nm
          1D       BARR #8   373-027 7421 2 2093     742.1 nm
          2A       BARR #8   373-026 7364 1 2293     736.4 nm
          2B       BARR      373-022 6753   4391     675.3 nm
          2C       BARR #1   373-021 6629 1 2193     662.9 nm
          2D       BAR  #473 CAVITY  7017 1 2293     701.7 nm
    
        Ext HR     BARR #1 2 374-002 6328   4102     632.8 nm
    

    I'm deducing the center wavelength based on the part number and observations of visible light transmittance for those in the 600 to 700 nm range. I don't think the exact location of the side mirrors matters except to the extent that it matches up with the appropriate sensor channel.

    While these center wavelengths would suggest a rather large wavelength shift, this apparently is the case for gases. But wouldn't there also have to be a 632.8 nm rejection filter in front of the detectors or else that would overwhelm the small Raman signal?

    While I had expected the photosensors to be PhotoMultiplier Tubes (PMTs) as in the similar Raman system using an argon ion laser, these are most likely Avalanche PhotoDiodes (APDs). They are in TO18 cans clamped to a ThermoElectric Cooler (TEC, Peltier device) on a large heatsink. Inside the can, there is a little gold colored block perhaps 1.5 mm square, with a 0.5 mm blue dot in the middle, which I presume is the active area. The APD is probably a S9251-05 (or very similar), one of the Hamamatsu S9251 Series Avalanche Photodiodes. There's a fair amount electronics to go with them, though nothing obviously recognizable.

    The REO One-Brewster Particle Counter HeNe Laser

    This unit is physically similar to the external resonator assembly described in the section: The PMS/REO External Resonator Particle Counter HeNe Laser, above. A photo is shown in REO LS27 Particle Counter HeNe Laser Assembly but this one has a one-Brewster HeNe laser tube with internal and external HRs. (Actually, "LS27" was on the tube itself; the entire assemly has no number.) I've since discovered that on PMS particle counter that uses this or a very similar assembly is the PMS Micro Laser Particle Counter Turbo 110, whatever that it. At least, a photo of the insides of one shows something that looks like this laser!

    The particle stream passes through the intracavity beam. An elaborate gas flow system maintains positive pressure of clean filtered gas to prevent contamination of the Brewster surface and external HR mirror by the separate gas stream containing the particles being counted. Having been manufactured in 1996, the 1-B design may predate the external resonator design.

    The tube is labeled Model: SB/1M, Serial Number: PMS-4638P-2296, and is physically similar to the one described in the section: The Ohmeda Raman Gas Analyzer REO One-Brewster Laser. The glass end of the tube can be seen near the middle of the photo with the Brewster window hidden by a cylindrical dust cover sealed with O-rings that can be pulled back for cleaning. Unlike any of the other PMS/REO lasers (except for the LSTP tunables), this laser also has 3 ceramic magnets glued to the side of the tube, and they do increase the output power by about 5 percent. There are 2 magnets opposite each other near the cathode-end and 1 near the anode-end. The second magnet near the cathode seems superfluous since its effect is minimal but might help a tiny bit. (They may not have put a second magnet near the anode because it would have been dangerously close to the anode connection!)

    The power supply is a Voltex brick (which someone had cut all the wires off of, literally 1/4" from the brick. But with wire extensions carefully spliced and insulated, it still works!). The power supply is labeled and set for 5 mA for some reason (perhaps for maximum life), compared to the usual 5.25 mA or 5.5 mA of the other PMS/REO tubes.

    With the external HR in place, lasing is mostly on the normal 632.8 nm (red) with a small percentage of several other lines:

       Wavelength   "Color"   Percent
     ----------------------------------
        611.9 nm    orange       3%
        629.4 nm      red        5%
        632.8 nm      red       80%
        635.2 nm      red        8%
        640.1 nm   deep red      4%
    

    For particle counting, only the total intracavity power matters, not the wavelength. Thus, there is no tuning prism in this unit.

    The photodetector appears to be identical to the one in the external resonator system (including the safety label), probably using an avalanche photodiode since there is a 200 VDC power supply attached to it. A reflector and big fat focusing lens directs flashes from any particles unlucky enough to pass through the intracavity beam into the photodetector. The only other sensor is a photodiode mounted on the tube's HR mirror, presumably to monitor waste beam power.

    As with one of the Ohmeda tubes, this one was also weak at first with an excessively pink slightly dim discharge. But it eventually recovered (though there were a few bumps in the way) with extended run time as the discharge now looks normal (salmon color and bright, possibly near-new and slightly overfilled) and the waste beam power has increased to something very respectable. (See the section: REO One-Brewster Tube - Very Low Output.) So far, the only sick soft-seal tubes that seem to consistently recover to near-new performance with extended run time (as long as there is no contamination from really annoying things like H2 and water vapor) are those from REO. Some other manufacturers' tubes may improve somewhat, but not to this extent, and others simply get worse.

    To determine the actual reflectivity of the mirrors and thus the intracavity power, I subsituted a 60 cm RoC, 99%@633nm mirror for the external HR. Rather than attempt to remove the REO mirror itsefl, I simply unscrewed the mounting plate and substituted an instant adjustable mount of my own. :) By measuring the output power from the OC, and knowing its reflectivity, the intracavity power could be calculated. The ratio of the waste beam power from the internal HR to intracavity power represents the transmission (ignoring losses) of the internal HR or Ti. Then, the transmission of the external HR or Te is just the ratio of external to internal waste beam power times Ti. This all went smoothly with the results shown below:

                   Power from    <------- External Mirror ------->   Intracavity
       Wavelength  Internal HR     Type     Reflectivity    Power       Power
     -----------------------------------------------------------------------------
        632.8 nm       2 uW      60 cm OC    99.0%        1,300 uW      0.13 W
         "    "       86 uW       REO HR     99.9959%       246 uW      6.0  W
         "    "      165 uW        "  "       "   "         472 uW     10.7  W
    

    (The last entry is after the full recovery.)

    Based on the 60 cm OC's measured reflectivity of 99% and the waste beam power from the internal HR of 2 uW with an intracavity power of 0.13 W, it is allowing only 1 part in 65,000 of the intracavity beam to excape for a reflectivity of around 99.99846%, Wow! If the external HR were that good, the intracavity power would be even higher.

    The Keuffel and Esser 71-2615 Autocollimating Alignment Laser

    (Perhaps this section would be more at home in the chapter: Laser Instruments and Applications. But since it has a vintage HeNe laser and didn't seem to fit any category there, here it is!)

    So someone sent me this "thing":

    The common autocollimator is an optical instrument for measuring extremely small angular deviations using a point light source, collimating telescope, and beamsplitter to enable the reflection of the light source to be viewed from the side on a graticule. A Web search for "autocollimator" should provide hours of bedtime reading on this subject. :)

    The autocollimating alignment laser uses, well, guess what, a laser for the light source and a pair of split photodiodes in place of a human observer. Such instruments can supposedly measure down to arc-seconds.

    The Keuffel and Esser 71-2615 is LARGE (over 20 inches long) and MASSIVE (over 10 pounds). And I thought that Metrologic military HeNe laser made a good hammer! :) It is all precision machined and must have cost a fortune new. The thing is also beautiful, with an exterior that is very nicely chrome plated..

    The beam out the front is about 1/2" in diameter, only a few hundred uW, rated 1 mW max. The connector on the back has 4 pins that test as diodes.

    > I did a brief patent search but didn't find anything relevant. Here is a discussion on the USENET newsgroup sci.optics precipitated by my request for info (loosely based on the description above).

    (From: Wade Kelman.)

    It's absolutely worthless, and you should send it to me. I'll throw it out for you. :)

    Actually, I think you have an alignment telescope that is accurate to a fraction of an arc-second, much better than the visual kind that use reticles for alignment.

    I'm surprised that the K&E - Brunson - Cubic Precision Web site doesn't have information on this. Or, you could just call them and ask about it.

    (From: Adam Norton.)

    What you have is an electronic autocollimator used to measure angle deviation of the reflected beam in the arc-second range. Along with tooling mirrors, penta prisms and such, it is used to do optical alignment, check machine tool way flatness & perpendicularity, surface plate flatness, shaft straightness, etc. In crappy used condition these are worth about $1K (check out ebay). If you had (or could make) the readout, you might get much more. Please do not disassemble as that will ruin the alignment.

    (From: Sam.)

    I wonder if this was an one of those ideas that never really caught on. There are others out there on eBay and elsewhere, but little (easily located) information.

    I did find 5 photodiode outputs on the back that respond to reflected light. I couldn't tell if they were sensitive to slight misalignment though. That would be my next experiment. I wonder what's needed for the readout? Just some op-amps and meters for X and Y?

    (From: Adam Norton.)

    I replied to the original post before seeing this branch of the thread. This is definitely an idea that has caught on. Check out the Brunson Instruments Web site (which acquired the Cubic Precision/K&E line). Also look at Davidson Optronics and Moeller-Wedel.

    To get a signal from the quadrant detector that is proportional to angle and insensitive to reflectance or beam power you need to use the following formulas: Q1, Q2, Q3, Q4 are the signals from the four quadrants:

       X = [(Q1+Q2) - (Q3+Q4)]/(Q1+Q2+Q3+Q4)
       Y = [(Q1+Q4) - (Q3+Q2)]/(Q1+Q2+Q3+Q4)
    

    Older systems used to do this all with analog amplifiers. On-Track technologies among others sell such amplifiers.

    (From: Sam.)

    This one uses a pair of split detectors so the denominators of the above equations should only have two terms, but would be otherwise similar.

    (From: Phil Hobbs.)

    In analog, you can do it right down to the shot noise, which typically means something in the hundreds of picoradians rms. Just needs a mildly modified laser noise canceler. See, for example: Ultra-sensitive laser measurements without tears.

    Of course, in real life the accuracy will be limited by stray fringes and QE drift in the diodes, but you really can see very very small angular movements this way.

    (From: Sam.)

    OK, I know you told me not to disassemble the thing. But I may want to do that since the laser tube is very weak - about 30 microwatts out the front and getting weaker with run-time. So, it's end-of-life and is unlikely to get better under any conditions. I assume it should be close to 1 mW when new.

    If it were just weak but stable, then the sensitivity would be lower but it will still work so it could be left alone. But it's getting worse. It's clearly an old laser which really needed to be run periodically to maintain its health and was not. (I even found a pic in an auction for one with a notice to this effect.) That would date it to no later than 1980 or so. Soft seal tubes like that went away by 1980. Well, this has probably sat unused for years, if not decades! (However, it seems the tube must have been replaced around 1986, see below.)

    It looks like there are 4 setscrews around the perimeter at several locations that do the alignment and lock the laser in place, though not having seen a diagram, there could be others further forward. Originally, I thought the setscrews were covered with hard Epoxy but it turns out that is just a crust over the top, and poking through it with an awl allows these caps to be popped out. Then, there is only some goopy tar-like stuff, for reasons unknown other than to discourage such tampering! :)

    If only 2 of the setscrews were removed, the alignment would be maintained, though of course a modern replacement tube - assuming one could be made to fit at all - will also not have the exact alignment of the original. But a jig could be made to adjust it.

    Any suggestions other than simply use it or sell it as-is?

    It's not a big deal either way. The only reason I have this at all is curiosity! :)

    (From: Adam Norton.)

    I do not know what this looks like on the inside, but given how stable this thing has to be, I would imagine practically everything would be potted in place inside. If you can replace the laser, trying to align it parallel to the outside housing within a fraction of an arc second might be very tricky. If you can not do that accurately, the gadget still might be useful to measure changes in angle.

    (From: Sam.)

    That's my feeling. It was useless they way it was with the power declining toward zero the more it was run.

    Fortunately, there is no potting anywhere, though some assemblies were locked in place with some globs of Epoxy.

    The setscrews seem to adjust the rear of the laser, the front of the laser, and the beam expander position, which makes sense.

    I've got the front and rear sections out now.

    The front section has the output collimating lens and beamsplitter and photodiode assembly.

    The rear section has the laser tube and rear laser mirror. The front laser mirror is still stuck inside. Go figure.

    This uses a two-Brewster laser tube with external mirrors. What I haven't figured out yet is hot to get the remaining section with the front laser mirror and expanding lens out. It's about as inaccessible as possible, more than 10 inches in from either end, and doesn't seem to want to move, though I may just need a bigger pry bar. :)

    I've also removed the diverging lens and spatial filter assembly.

    Unfortunately, so far I have been unable to remove the final remaining piece which holds this as aligns it with the HeNe laser. This also retained the output mirror and mount from the HeNe laser.

    Nothing has been damaged so far so it should go back together.

    I'll have to replace the laser tube with a modern internal mirror linearly polarized laser tube and arrange to mount it in a similar way. The polarization is needed to optimally separate the outgoing and return beams via a polarizing beamsplitter and Quarter-Wave Plate (QWP).

    BTW, the date on the laser tube is 1986. My guess is that it was replaced in 1986 and they used an original design tube, since by then, internal mirror polarized HeNe lasers were widely available and a lot cheaper and less finicky than this contraption. (It was almost recent enough that a red diode laser could have been used but probably not quite.)

    It's a custom Hughes two-Brewster HeNe laser tube, a model 3183M. This is short, about 8 inches from tip to tip. Perhaps "M" stands for modified? The mirrors are in massive stainless steel mounts and 1/2" or more in diameter mirrors - unusually large for such a laser. Why? The Radius of Curvatures (RoCs) are 30 cm for the OC and planar for the HR.

    I was able to remove the mirror mount deep inside the big cylinder with a hex driver extended with 3/8" copper pipe. :) What was left inside - the mounting plate for the spatial filter/beam expander, electrical connector for the photodiodes, and the OC-end of the HeNe laser - finally yielded to a scrap HeNe cylinder pounded by a 5 pound hammer. :) There appears to be some glue residue that was holding it in place, perhaps the last defense against revealing its secrets. Being able to lay out the parts on the bench will make it a lot easier to realign.

    Using a Melles Griot 05-LHP-605 laser head with just the front end-cap removed, it was quite straightforward to install and align the expanding lens and spatial filter to the axis of the main cylinder. The inside diameter of the 05-LHP-605 cylinder is about the same as that of the original laser, so it is a snug fit to the mounting plate at the front. The expanding lens was screwed to the mounting plate snug enough that it would not move on its own, but could be pushed around with the 4 setscrews around the perimeter of the mounting plate. The laser and mounting plate were slid into the main cylinder and then the beam was aligned with its optical axis using the setscrews. After pulling it back out, the spatial filter could be screwed in place and adjusted to cleanly pass the beam. With the 05-LHP-605, the output beam is only about 7 mm in diameter - around half of that with the original laser.

    So, I need to find a short polarized HeNe laser tube with a wide beam. A standard cylinder diameter will fit. The trick will be matching the beam diameter so that the expander works correctly and results in a large diameter final beam. I suspect the Hughes has a rather wide beam diameter and possibly a wide divergence as well with its 30 cm RoC OC and planar HR. That is similar to what the gold-cylinder Hughes lasers use. But it may be tough to test since it's so near dead that getting it lasing would be a major issue. Since the axial position of the collimating lens is slightly adjustable, the divergence won't be a big issue. But the laser beam diameter will be proportional to the final beam diameter, and finding a modern tube with sufficiently wide beam may prove challenging.

    The Melles Griot 05-LHP-605 I used for testing, about 1 mW, could work. But the divergence and beam diameter result in a final beam that is too narrow for the collimating lens of the autocollimator (about half the original). This would probably be acceptable but not optimal. Matching this may be the hardest part of this retrofit.

    A suitable normal tube might be the 05-LHP-410 which has a relatively wide beam (0.85 mm). But I've never seen one of those.

    Linearly polarized barcode scanner HeNe laser tubes may also be suitable Possibilities include the 05-LHP-004 and 05-LHP-690 but their beams are closer to 0.5 mm so the final beam diameter wouldn't be much better. But polarized barcode scanner tubes aren't common.

    An alternative could be a diode laser. But matching the beam quality of any HeNe would be a challenge.

    Far East HeNe Laser Tubes 1

    These are from a Chinese company called Artworldcn Enterprise Limited. Navigating this Web site is shall we say, challenging, so here's a direct link to Artworldcn's HeNe Laser Product Page, which has some basic specifications. (There used to be a jumbled mess at the bottom of that page supposed to be an ASCII diagram of an early RF-excited HeNe laser and was copied directly out of this chapter of Sam's Laser FAQ! But they neglected to also copy the HTML formatting specifying a fixed-width font, so it was totally unrecognizable (except to me! If you're at all curious, check out the diagram in the section: Early Versus Modern HeNe Lasers.)

    The chart on that Web page includes the following information:

            Cavity  Total     Tube    Working   Trigger  Working  Output   Diver-
     Model  Length  Length  Diameter  Current   Voltage  Voltage  Power    gence
    -------------------------------------------------------------------------------
      150   140 mm* 150 mm*  28 mm     3 mA      4 kV    1.3 kV   0.8 mW
      180   180 mm  190 mm   28 mm     4 mA      4 kV    1.4 kV   1.2 mW
      200   200 mm  210 mm   30 mm     4 mA      5 kV    1.5 kV   1.4 mW
      230   230 mm  240 mm   35 mm     4 mA      5 kV    1.5 kV   1.5 mW  1.25 mR*
      250   250 mm  260 mm   36 mm     4 mA      5 kV    1.5 kV    2 mW
      280   280 mm  290 mm   36 mm     5 mA      5 kV    1.5 kV    3 mW
      300   295 mm* 305 mm*  36 mm     5 mA      5 kV    1.5 kV    5 mW    3 mR*
      320   320 mm  320 mm   36 mm     5 mA      5 kV    1.5 kV    5 mW
      350   350 mm  360 mm   36 mm     5 mA      5 kV    1.5 kV    7 mW
      400   400 mm  410 mm   36 mm     5 mA      5 kV    1.5 kV    8 mW
      450   450 mm  460 mm   39 mm     5 mA      5 kV    1.5 kV   10 mW
      480   480 mm  490 mm   39 mm     5 mA      5 kV    1.5 kV   12 mW
    

    (The voltage specs for all these tubes are rather suspect since the values don't change much with output power and I haven't measured them even for the tubes I've tested. Values denoted with "*" were measured; all others from their Web site. This doesn't mean they are accurate, just that I haven't measured them.)

    Model 150: The first tube I tested was the model 150 (150 mm, ~6 inch tube), similar in performance to a common barcode scanner tube. The construction of this (as well as the others) is, well, strange as shown in Artworldcn 150 mm HeNe Laser Tube. As can be seen, the actual tubes they are shipping bear little resemblence to what's on their Web site. The entire tube is made of glass except for the mini-adjustable mirror mounts, which are similar to those used in Hughes-style tubes except that they have 4 slotted-head screws instead of 3 hex-head screws. Who uses slotted head screws in precision devices anymore? And at least one screw head was already broken! The mirror substrates appear to be attached via a thin layer of glass frit, not the bead that's present on virtually all "normal" tubes. Only the anode uses the mirror mount for the electrical connection; the cathode (which is a normal aluminum cylinder, partially hidden behind the label) has its own terminal via a glass-to-metal feed-through. And to make sure people don't do something stupid, the cathode mirror mount normally has heatshrink over it to prevent its use as the negative electrical connection (removed for the photos). Electrically, the tube behaves normally and should run on a typical HeNe laser power supply for 0.5 to 1 mW lasers. The specs call for 3 mA at 1.3 kV with a 4 kV start.

    The output by eye at least is close to TEM00 with a very low M-Squared. So, as a pointer, alignment laser, or barcode scanner, it would be fine. Using a diffraction grating, the only wavelength present appears to be 632.8 nm (at least in the visible). That's the good news, and without actually making measurements, it appears like any other HeNe laser tube of similar size and output power. However, with respect to modes and polarization, this is about the most cantankerous small HeNe laser I've ever seen.

    The first thing I noticed after admiring the most artistic (some would say primitive) glass work :-) was that there is no AR coating on the OC mirror, none, not even a puny attempt at an AR coating. I've never heard of any production HeNe laser lacking an AR coating on its OC. Even the 45 year old Spectra-Physics 115 laser had one! Second, as evidenced by the absense of any ghost beams, neither mirror substrate is wedged. So, there will be back-reflections from the outer surfaces of both mirror substrates directly into the laser cavity. I fully expected these back-reflections to make a mess of the tube behavior, and indeed they do. But I was not expecting it to be nearly as strange as reality, or reality to be so strange. ;-)

    Using a polarizer with a PC data acquisition system from a cold start to almost 30 minutes, the behavior is extremely bizarre. See: Mode Sweep of Artworldcn 150 mm HeNe Laser Tube During Warmup. This was taken after optimizing alignment (see below) so the total range of the vertical axis is approximately 1 mW. Virtually all common HeNe laser tubes of this length (about 150 mm) go through a predictable mode sweep with two orthogonal polarizations (here called S and P) alternating as the tube expands and the longitudinal modes drift through the neon gain curve. Compare this to plots for a typical tube of similar size and output power in Mode Sweep of Melles Griot 05-LHR-006 HeNe Laser Tube During Warmup. Adjacent modes are orthogonally polarized and the power in each mode goes to exactly zero for a portion of the mode sweep cycle in such a short tube. Even those tubes that are "flippers" generally produce a repeatable pattern, although it might change from flip to non-flip behavior at some point during warmup. But this tube tends to favor one polarization for a few minutes mode sweeping within it alone except for some random burps of the other polarization, and then slowly shifting over to where the other polarization dominats. Within each of these extended temporal regions, one polarization has the most power with occasional dips, while the orthogonal polarization only shows low level twitching and bumps. So it behaves like a poorly polarized tube for awhile (many mode sweep cycles) and then the polarization changes. A few flips can be seen (vertical green lines) but for the most part, the modes change smoothly, so it is not strictly speaking, a flipper. However, even though the total power output doesn't vary that much, it does so in such a way that there is a noticeable discrepancy in the shape of the plots of the P and S polarized modes, especially over a short time period. With normal tubes, they are virtually mirror images of each-other. Yet another very unusual characteristic of this tube.

    On a Scanning Fabry-Perot Interferometer (SFPI) the behavior is even more striking. It was necessary to add an ND1 filter to minimize back-reflections from the SFPI before the display settled down, but that's not unusual, and this could also be largely avoided by positioning the SFPI far away from the laser and aligning it slightly off-center. At first, with no polarizer, the display appeared as though it could pass for a normal tube, with the modes happily drifting through the neon gain curve. I wouldn't have given the display on the SFPI a second glance if this were a common tube. But knowing that there was already something very peculiar about this tube, using the polarizer oriented to pass the most power, the display was essentially unchanged from what it looked like with no polarizer - for awhile. Only linearly polarized HeNe lasers behave like that. But then the modes gradually disappeared and it was necessary to reorient the polarizer to get back a similar display. I've have never seen this type of behavior in literally hundreds of HeNe lasers tubes I've tested.

    As a test, I put a drop of alcohol first on the HR and then on the OC mirrors. There was little effect with the HR, but the output power in one polarized mode instantly increased dramatically when done on the OC. This was too fast to be a thermal effect, so perhaps an AR coating alone would be enough to make the tube behave. However, while my quick alcohol drop test showed that something changed, it was not clear if behavior was actually significantly improved. And, putting a glass plate at a very slight angle against the OC with some water to index match didn't seem to help, so it's quite possible that other more fundamental modifications would be required.

    This sample was also originally annoyingly weak (about 0.4 mW, well below spec) and that of course presented an irresistible challenge. However, it turned out to be rather easy to realign the OC mirror (cathode-end) by adjusting those antique slotted-head screws to boost the power output to over 1 mW. But just when I thought the situation couldn't get any worse, as a result of the optimization, a rogue mode appeared on the SFPI which wasn't there before! (And to confirm that I hadn't simply missed something, misaligning the mirrors makes them disappear, perhaps that's why it was adjusted to be so weak!) The rogue mode can be seen during part of what passes for a mode sweep cycle on this tube as shown in Longitudinal Modes of Artworldcn 150 mm HeNe Laser Tube. The Free Spectral Range (FSR) of this SFPI is 2 GHz. The longitudinal mode spacing of this tube is about 1.034 GHz based on a measured mirror spacing of 14.5 cm. I believe the two tallest peaks on the left photo correspond to the normal (expected) TEM00 modes. Based on the 2 GHz FSR of the SFPI, the tallest and next tallest to the right of it would be just about 1.034 GHz apart. The rogue mode is to the right of one of the main modes, usually but not always the largest one, about 125 MHz higher in frequency than the mode it's hugging (based on the direction of drift of the peaks on the display during mode sweep). At first I thought it was a longitudinal mode. A measurement of the beat frequencies, if any, would prove conclusively that the mode is indeed adjacent, and not aliased as a result of to the 2 GHz FSR of the SFPI (due, for example, to some other lasing line that's not supposed to be there, perhaps IR). And with a Thorlabs DET210 detector (1 GHz bandwidth), there could be no doubt: A beat of around 125 MHz was indeed present for a portion of the time, coming and going as expected. A rogue longitudinal mode would seem to be essentially impossible as there is no way for there to be any reflections inside the cavity at a shorter distance than the mirrors. It would have to be approximately 15.6 mm closer based on the 125 MHz difference. So, could it be a higher order spatial mode? This would seem to be the most likely explanation. And it gets even weirder. Looking closely at the SFPI plots, the rogue mode turns out to actually be a pair of modes, confirmed by their beat frequency to be about 15 MHz apart! At first everything appeared totally perplexing, but assuming these are higher order spatial modes not visible even by careful inspection of the beam profile, it begins to make sense, as they would totally scramble the SFPI display, which generally assumes a TEM00 beam.

    As an aside, the cavity geometry of this tube is backwards from nearly all others: The OC is planar and the HR is curved with a RoC measured to be unbelievably long at around 1 meter based on reflecting a parallel beam from outside. With that RoC, Matlab produces a frequency offsets for the first higher order spatial mode that is close to 125 MHz. The fact that it's split could perhaps be due to some asymmetry in alignment.

    The main reason that multi-spatial mode operation was the first thing to suspect was the nearly perfect the beam profile. In fact, it may even look better than a more normal short barcode scanner tube. Now in all fairness to Artworldcd, their Web site does say: "Wavelength 632.8nm multi made,long operating time warranty time 1year". OK. so perhaps they need an English translator in addition to some tube redesign. :)

    Here's a summary of observations and peculiarities:

    Thus, aside from the multitude of unknowns, everything is obvious. ;-)

    I've since tested 2 other samples of this same model tube. The serial numbers of two of them are 746 and 1,375, acquired from the company within the last month (April, 2011), so these were likely current production. The third one had no SN label. Even assuming they started at SN1, at most 1,375 had been built to date. Each of the three have unique personalities but generally similar overall behavior. The second tends to remain much more polarized before swapping polarizations while the third produces spikes of the opposite polarization that are fairly regular until it switches polarization and then does the opposite. SNs 746 and 1,375 both had similar higher order spatial modes displayed on the SFPI, while the unmarked tube appeared to be pure TEM00. However that tube was running at slightly lower output power (0.8 versus 1 mW). The alignment screws were too well sealed to attempt to boost it, where higher order spatial modes would be more likely. There are no obvious physical differences but a slightly narrower bore can't be ruled out.

    Some aspects of the glass-work are rather crude. For example, the orientation of the tip-off with respect to the cathode terminal differs on all three. And the bore end inside near the cathode-end of the tube has probably been cut by scoring and snapping, not with a diamond saw as there are obvious chunks missing on some places.

    So, as noted, using a tube like this for pointing or alignment or anything else that depends solely on the appearance of the beam should be fine. And, I've been told that it isn't too bad for demonstrating the basic principles of a Michelson interferometer. But anyone hoping to build a stabilized HeNe laser or do serious interferometry or holography - or even to explain what longitudinal modes are all about in a classroom - could end up totally frustrated.

    But this is a cute little tube! It's possible that only minor modifications would be required to eliminate all these deficiencies, starting with the use of wedged substrates for both mirrors and AR coating of the OC mirror. Using a slightly narrower bore possibly in conjunction with a different RoC for the HR mirror would suppress the higher order spatial modes. (A mirror with that large an RoC is probably the same one they use for their other longer HeNe laser tubes. They then control the reflectivity with the planar OC.) But why not simply copy the relevant parameters from a common 6 inch barcode scanner tube? All of these changes should have only a modest impact on manufacturing cost. Then the tube would not only be cute, but might actually work well and be rather boring like all the others. ;-)

    From what I've determined, these tubes are less than half the price of those of similar output power and size from companies like Melles Griot or JDS Uniphase. So there should be some room for well justified added cost while still being much less expensive than the others.

    I've since done tests of two other higher power tubes as shown in Several Artworldcn HeNe Laser Tubes The results were totally unremarkable. :) The longer tubes in the photo are rated 1.5 mW and 5 mW.

    Model 230: As with the model 150, there is no AR coating on the OC. The output is a nice low divergence beam which appears to be pure TEM00 with a measured output power after a brief warmup of 2.4 mW. Other than some instability when two modes approach equal amplitude, the mode sweep behavior was textbook in nature with no evidence of higher order spatial modes. In the instability region, the modes would bounce up and down, with a possible mode flip. This sort of behavior is not unusual even in some high quality HeNe laser tubes, though it is generally not present with most.

    Model 300:

    Again, no AR coatings. This one is highly multimode (not TEM00) which explains its relatively short length compared to common 5 mW (rated) tubes from other manufacturers. The output power after warmup is over 6.25 mW in an interesting beam. :)

    Bendix JL-1 RF-Excited HeNe Laser

    This one is truly ancient, certainly before 1965, perhaps much earlier. It was probably one of the first educational lasers ever sold. The laser head is covered in amber Plexiglas with the plasma tube clearly visible. The wavelength was probably common 633 nm red with an output power of 1 or 2 mW at most. It has huge bulbs holding the Brewster windows, possibly "repurposed" chemistry lab-wear based on the printing visible on them. There is an impedance matching coil inside the case with an RF connector on the back side. Regrettably, I have not seen the RF exciter. While one would assume that the tube is up to air after almost 50 years, this may not be the case. It is hard-sealed - no Epoxy anywhere. The glass is thin, no getter, no metal inside tube at all, nothing passes through its wall. So, while it may not lase due to He depletion either from use (RF tends to suck He out of thin-walled tubes) or from age, it may still be gas intact and retained its Ne. In that case, a He soak for 6 or 7 weeks (1 day for every year of age) should restore it to like-new condition. :) Stay tuned.

    Here are some photos (coming soon):

    Melles Griot Dual Output Green HeNe Laser Tube

    This is probably another "oops". :) It's supposed to be a Melles Griot 05-LGR-024, a short green (543.5 nm) tube with a spec'd output of 0.2 mW and TEM00 beam profile. However, someone at the Melles Griot factory must have been smoke'n sump'n that day and stuck OC mirrors on both ends. So, it actually produces 0.3 to 0.4 mW from each end and the beam profiles are multi-spatial mode, something along the lines of TEM11. (The total output power is much higher than the spec because it is a new tube and probably since it is multimode.) Optically, the mirrors are the same, both nearly planar by eye, but actually behaving concave using an external red laser reflected from them, which results in a focus at 3 or 4 meters. My assumption is that they actually have a normal positive Radius of Curvature (RoC) internally but it is masked by a curved outer surface, which is AR-coated and difficult to see let alone actually measure. With two curved mirrors, the intra-cavity mode volume would be incorrect thus resulting in multiple spatial modes. The mirror coatings have virtually the same reflectivity at 543.5 nm, but at 633 nm, the OC is around 6 percent while the HR is around 21 percent. This of course makes no difference for a green laser, but does support the hypothesis that they are from different batches. Thus, it probably wasn't entirely the fault of the assembler. More likely, there were a few OCs accidentally mixed in the the box labeled "Green HRs". (I know that at least 3 of these tubes were manufactured.)

    Russian (USSR) OKG-13 HeNe Laser Head

    This one is either really ancient, or Russian technology was backward for far longer than could be imagined. The OKG-13 is a HeNe laser head containing a two-Brewster plasma tube with heated filament/cathode and huge (~30 mm diameter) external mirrors. The casing bears some similarity to that of the much larger and very ancient Perkin Elmer HeNe lasers.

    Here is a rough translation of the general specifications for the OKG-13:

    And a similarly rough translation of the description:

    "The device OKG-13 is a generator of continuous coherent radiation in the visible part of the spectrum and is designed for use in automatic control systems along the line of for precision optical measurements."

    The tube itself appears to be of coaxial construction, with the single filament off to one side, but it is not a side-arm tube. The Brewster windows are attached to rather large bulbs at each end but the bore itself is narrow like that of a modern HeNe laser. The use of a heated filament/cathode went out of fashion :) for most USA HeNes in the late 1960s, though some legacy designs may have persisted into the early 1970s. I have yet to find anything on this laser head assembly that would provide an indication of the manufacturing date. There may be one on the actual glass tube but for reasons that will become clear, I have no current plans to remove it even for inspection or photos. The only label attached to the exterior has the OKG-13 model and a stamped serial number, but no date or date code.

    I acquired this laser head on eBay of course, shipped all the way from the Ukraine. :) Here are some photos from the auction (courtesy of Electronics Parts Choice maintained by eBay seller ID: zorolan). These show the laser head in the condition it was received:

    When 9 V was applied to the filament leads, the filament immediately lit up nice bright orange indicating that the tube was at least not up to air. Using a variable HeNe laser power supply, it was easy to initiate a discharge but the color was (not unexpectedly) sickly pink/blue. And at first, I was not optimistic about the chances for a miraculous recovery. However, within a few minutes, there was a very obvious improvement. And within less than an hour, the discharge looked relatively normal with a bright salmon complexion. And even though the OC mirror is in rather poor condition with numerous scratches, careful cleaning of the Brewster window at that end of the head resulted in weak lasing. Initially between 25 and 50 µW. (Since these may be soft-coated mirrors, there may be no practical way to clean them.) I did not remove the mirror at the HR-end because it did not appear to have ever been disturbed. The OC mirror had been removed for the auction photos - I would not prop up a mirror like that up intentionally!

    Of course, a dirty laser would never be happy, so it got a nice scrub and massage! Paint touchup and detailing will come later. ;-)

    It was obvious that much more power was possible as the power nearly doubled when run at 6 mA rather than the spec'd 5 mA current.

    After several hours, the output power has increased to a peak of over 250 µW at 5 mA, but with a 15 percent mode sweep variation. A maximum power of 340 µW could be reached at around 8 mA.

    I'm thinking of building a power supply with a fail-safe circuit preventing the HV from being applied unless there is filament current. The tube would light and lase on a modern power supply without the filament being hot, but that would probably destroy the tube rather quickly from sputtering. In the meantime, it's using my HeNe laser power supply protection widget simply to monitor the filament voltage directly from the tube's filament leads. (See HeNe Laser Power Supply Current Limit Protection.) This will instantly shut off the HeNe laser power supply if the filament voltage either increases significntly or goes away entirely.

    Since a proper cleaning of the OC-end Brewster window was never performed, it seemed like a perfect excuse to remove the OC mirror and test the laser with an external mirror.

    Using a randomly selected external mirror that was laying around (45 cm, 98.5%), it was possible to get over 0.53 mW easily. Thus, the plasma tube is still in at least decent, if not very healthy condition. Cleaning of the window could now be done while lasing using a generic grocery paper towel :), acetone, and cotton swabs. In fact, the window was already quite clean and the power only increased perhaps 5 percent.

    But when the original mirror was replaced, the output power had increased to almost 0.4 mW! I don't think this was due to the Brewster cleaning but simply the luck of the draw on the orientation and position of the mirror. The Russian mirror may simply be too far gone to either achieve optimal output power or consistent results. It is extremely sensitive as to position in the holder. (More so than can simply be accounted for by the associated change in alignment.) The coating quality may also be inferior. If I knew it was hard-coated, proper cleaning might help. But attempting to clean a soft-coated mirror with almost anything will damage or destroy it. Unfortunately, no modern mirror would fit without looking like it had been replaced.

    By fiddling with the mirror and tube alignment via the centering screws, it's now possible to get 0.35-0.4 mW sustained, with it actually peaking at almost 0.45 mW when warming up.

    However, running it any more may be counterproductive. It's not known what the life expectancy is of a tube like this. It may be as low as a hundred hours or more likely, several thousand. But not the 10K or 20K hours of a modern tube. The power had declined by a couple percent without doing anything. It's not clear if that's a tube life problem, or simply alignment changing slightly due to thermal cycling. Although the pieces seem to be locked tightly together, torquing the mirror locking ring does affect the power slightly.

    The person who sold me the OKG-13 laser (eBay seller ID: zorolan) was kind enough to send me a scan of the operation manual. But unfortunately, it is - no surprise - in Russian. :( :) From what I can deduce looking at the specifications, a photo of the laser head and power supply, and a diagram of the internal construction of the laser head, it's for a slightly newer model as that diagram lacks any reference to connections for the heated filament. See Internal Construction of OKG-13 Laser Head from Operation Manual. (The curvature of the mirrors is greatly exaggerated and some other details do not match my laser head either, but it's better than nothing.) And there may be a date (for the manual at least) in there - 1979 - but that's quite questionable as some of the other date listings don't make sense. That peculiar value of "0,6328 mkilometre" for the wavelength appears to have originated as "0,6328 mkm" in the manual. Perhaps mkm could be interpreted as a "thousand-thousandth" of a meter (1 micron) rather than a thousandth of a kilometer (1 meter). :) Google does find a few papers that reference the OKG-13 (and other OKG) HeNe lasers. One has a publication date of 1975 suggesting that the OKG-13 is likely from much earlier. But the others are much more recent suggesting that the OKG lasers may be or may have been very common. Unfortunately, I am unable to access the full text of these papers (probably also in Russian anyhow!).

    If anyone is even moderately fluent in technical Russian and willing to do at last a partial translation of the manual, please contact me via the Sci.Electronics.Repair FAQ Email Links Page.

    Russian Two-Brewster HeNe Laser Plasma Tube

    This tube is also from Russia (I assume the USSR, though I don't know that for sure). It too has a hot cathode/filament, but it is somewhat longer than the tube in the OKG13, above - about 10.5 inches tip-tip. See Russian Two-Brewster HeNe Laser Plasma Tube 1. The anode is on the left, though for some reason, it's not a simple wire electrode as in most other tubes of this type. The cathode/filament is on the right along with a pair of getters. It is not known if the getters have not ever been fired, or are simply exausted but only leaving a clear residue. This sample appears to be brand new, but it has leaked so a refill would be required to make it work. Just add gas. ;-)

    The for which this tube is intended uses a three-bar resonator that mounts inside an oversize cylinder as shown in Russian HeNe Laser Using Two-Brewster Plasma Tube 1. The resonator assembly appears to simply slip inside secured by screw caps at each end. Nothing else is known about the laser at this time.

    Bausch and Lomb HeNe Laser

    This one is old. It's built on a wooden base with wooden end-plates, which gives new meaning to the term "optical breadboard". :) See Bausch and Lomb HeNe Laser. And yes, that skinny thing is the laser tube with not much of a gas reservoir. :( It has internal Epoxy-sealed mirrors and is sort of RF-excited as there are no electrodes in the tube and is held in place with a spring behind the its back-end. There is a rock, well actually a small white pellet of something inside the tube, put there on purpose since it's in the extension partially pinched off from the main part of the tube to prevent it from migrating and blocking the bore.

    (From: Bob Arkin.)

    The white rock was a dessicant to remove water vapor from the crappy Epoxy sealed windows. So, it is a sort of very limited getter. (The Optics Technology brand HeNes had carbon chunks instead.)

    The power supply uses an bridge rectifier, SCR, and automotive-style induction coil to ionize the gas 120 times per second using two pieces of copper foil wrapped around the tube near the ends. A fluorescent lamp ballast inductor limits current. So it's a pulsed HeNe. :) The connections to the tube are simply pieces of foil. Originally they were copper but my replacements are aluminum. So be it. It is way beyond any hope of lasing but the power supply does work resulting in a blue-white discharge.

    The instruction manual (courtesy of Meredith Instruments) may be found at Vintage Lasers and Accessories Brochures and Manuals under "Bausch and Lomb". The only "instructions" are pretty much to plug it in. (There is no power switch.) If it doesn't lase, replace the tube. :-) However, there is a schematic.

    More to come.

    ENL-911 Two-Brewster HeNe Laser Plasma Tube

    This one also must be really old as the two-Brewster plasma tube has a hot filament. The markings on the glass are "ENL-911". See ENL-911 Two Brewster Hot Filament HeNe Laser Plasma Tube. As can be seen, it has a narrow bore like a modern tube and a small gas reservoir at one end. The single ballast resistor is only 15K ohms, so, there must be additional ballast in the mating power supply. There is also a lone magnet near the center glued to the bore, purpose unknown. For IR suppression, there are normally multiple magnets with opposite polarities all along the bore. Its purpose must not be to attract debris being in exactly the worst place for that. ;-) The mirror plates were missing but one of the rings to which they would attached can be seen at the upper left. The laser head has a connector identical to that used on some other more conventional Oriel HeNe lasers but that may just be a coincidence as there is no other evidence to suggest this is an Oriel laser. There are no markings on the head cylinder.

    (Portions from: Bob Arkin.)

    These are from a long defunct company called "Eletro-Nuclear Laboratories, Inc.". There were two models, this uses a single ended tube. There was also a double ended tube with the hot cathode in the middle. The single magnet was indeed for IR suppression.

    The power supply had the cathode lead out as two wires from a center tapped filament transformer with a wirewound adjustable Ohmite to set the filament temperature. So the plug had two pins for the cathode, one for the anode, and two used as a jumper to kill the PS circuit if no head was plugged in. And, yes, there was a limiting resistor in the power supply and just a single resistor thrown onto the tube as a ballast.



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