Sam's Educational Michelson Heterodyne Interferometer Project Manual V2.1

Abstract

An experimental setup is presented which allows for several types of interferometers to be easily implemented without requiring any special tools or test equipment. The behavior of various interferometer configurations will be explored as well as the use of the interferometer for sensing and extensions to actual measurements like displacement (change in position) down to nm precision. The set of parts may be easily duplicated and/or modified for specific interests.

Introduction

IMPORTANT: If the interferometer is from Excel, it will be necessary to confirm that the Polarizing Beam Splitter (PBS) cube (labeled 1011A or 1012A) functions correctly. I have recently come across a small number of Excel PBSs - even new/NOS ones - where the coating on the diagonal is defective even though it looks pristine by eye. At the normal angle of incidence they are either marginal or totally ineffective at behaving like a PBS.

The simplest way to test the PBS uses the 5517 laser with a linear polarizer (LP, which can be one for a camera but NOT a circular polarizer). Power the laser until READY comes on solid. Remove everything from the interferometer - cube corner(s) and quarter wave plate(s) - and place it in the beam with the label at the top and an edge parallel to the beam. Position the LP in the beam between the laser and PBS with its axis of polarization vertical. If the PBS is good, there will be almost no light coming out the front with most being reflected to one side. (If the axis of polarization for the LP is not labeled, rotate it until almost no light gets through. For a defective PBS, there may be no such orientation.) Rotate the PBS a few degrees either way around the vertical axis and very little light should still get through. Rotate the PBS 90 degrees around the vertical axis and repeat. The behavior should be similar except that the reflected beam will exit from the opposite face. If either test behaves strangely, contact me for a replacement. I will pay shipping for the replacement and to return the original. Sorry about that. These have been included in the Hobby Special kits for several years but just recently, someone found a defective one. It had never occurred to me to even test these as high quality PBS cubes should not go bad, or be bad from the factory.

IMPORTANT: This manual applies to version 2.1 of Sam's Educational Michelson Heterodyne Interferometer kits. (There is no V1.x or V2.0 for heterodyne.) Here are links to all of them:

• Sam's Educational Michelson Homodyne Interferometer Project Manual V1.x: Original versions developed for a local college.

• Sam's Educational Michelson Homodyne Interferometer Project Manual V2.0: Version for sale prior to 2022.

• Sam's Educational Michelson Homodyne Interferometer Project Manual V2.1: Differences compared to V2.0 are minor. Versions available starting in 2022.

• Sam's Educational Michelson Heterodyne Interferometer Project Manual V2.1: This manual, there is no V1.x or V2.0 for heterodyne. Kits will be available soon.

For the combined kit (heterodyne + homodyne), both of the relevant manuals will need to be referred to, though there is a lot of overlap.

Detailed information and instructions on using and constructing most of the sub components of these kits like the various custom PCBs may be found at Sam's Electronics and Laser Kit Information and Manuals. There will also be links to them throughout this manual.

Homodyne interferometers employ a laser which nominally produces a single optical frequency ("laser line") while heterodyne interferometers employ a laser that generates two closely spaced optical frequencies. They each have their advantages and drawbacks. Much more on this below.

The photo below shows two typical configations of the Compact and Extended Heterodyne setups.

   

Typical Compact and Extended Heterodyne Setups with Ball Bearing Rails and OR3

Interferometers are the key technology is numerous applications in manufacturing and testing where the very minute wavelength of light is the "yardstick" providing non-contact measurements down to nanometer precision. In short, a light source is split into two parts that may travel different paths and then recombined at some type of detector. Where the path lengths differ by an integer number of wavelengths, the result will be constructive interference and the output of the detector will be high; where it differs by an integer number of wavelengths plus one half wavelength, the result will be destructive interference and the output of the detector will be low. In between, the output will vary sinusoidally. With suitable detectors and electronics, remarkably precise measurements can be performed. For example, nearly every microchip manufactured in the explored universe has been done with wafer steppers whose stages were positioned using interferometry based on HeNe lasers.

While interferometers are employed in a wide array of applications, the general emphasis for these experiments relate to the use of interferometery in metrology - precision measuremens of physical characteristics like displacement, velocity, angle, straightness, and more. Therefore unlike numerous interferometer experiments that may be found via a Web search, the emphasis here is on the signals that the setup provides, not so much on the interference patterns. With the homodyne interferometers using a non-two frequency laser (stabilized or otherwise), nothing precludes the observation of these. However, with a two frequency laser, the fringes aren't stationary and would have a bandwidth extending to MHz. So, unless you've opted for the high performance eyeball upgrade, that's out of the question. :( :-)

The experimental setups will enable various interferometer configuration to be easily implemented and then tested with one arm being on a micrometer linear stage and/or with some other device or material that can vary the path length precisely.

The light source is a Class II 633 nm Helium-Neon laser (HeNe for short) with an output power typically between 0.25 and 0.5 mW. Unlike the common HeNe laser, it produces two optical frequencies ("lasing lines") separated by a value called the "split frequency", and are implemented using a technique called "Zeeman Splitting". In essense, an axial magnetic field applied to the laser tube causes it to produce a pair of lasing lines (rather than a single one), typically 10s of kHz to several MHz apart. The "Z" used in various filenames and links throughout this manual is an artifact of the Zeeman laser that is used. ;-)

The basic detector is a photodiode with AC preamp called an "Optical Receiver". A dual channel digital oscilloscope will be used for initial testing with µMD2 for actual displacement measurment. Variations and enhancements to these will be offered as options.

Among the areas that can be explored with the Basic setups are:

• Techniques for aligning the interferometer.
• Inserting optics in one of the beam paths.
• Using the interferometer as a pressure and/or temperature sensor.

There is no need to construct all of the interferometer configurations described below. Doing the Linear Interferometer (LI) first makes sense since there are detailed instructions on its construction, alignment, testing. Building the High Stability Plane Mirror Interferometer (HSPMI) would be the logical next step moving from cube corners to plane mirrors. It also permits the loudspeaker and/or PZT actuators to be added. Then after that one of the others. Perhaps coordinate with the other project students using this same kit so that each of you do different ones.

This minimal set of experiments can all be done using parts in the Basic Kit:

• Construction of Linear Interferometer (LI).
• Interferometer as air pressure or index of refraction of air sensor.
• Interferometer as temperature sensor.
• Heterodyne measurement display using µMD2.

The following additional projects can be done using parts in the Deluxe kit:

• Construction and testing of other types of interferometers including the PMI, HSPMI, and SBI.
• Construction of Original Michelson Interferometer (BMI) and comparison with the LI.
• Resolution of the various interferometer configurations.
• Sensitivity to alignment based on configuration.
• PZT actuator for sub-wavelength-scale movement.
• Loudspeaker actuator for wavelength scale movement and "interferometer microphone".
• Interferometer as earthquake and/or vibration detector.

The following are more advanced projects, but they may require additional parts and/or different parts including the laser that are not included in the either kit:

• Interferometer as gas partial pressure sensor.
• Construction of the laser itself (rather than the complete HP/Agilent 5517 laser) using a bare laser tube, magnet, heater, power supply, mounts, and the µSLC1 Arduino-based controller for stabilization.

There is some information on these in this manual and links will be provided to learn more.

As of Winter 2022, there are 3 versions of the setups. Around 5 each of V1.0 and V1.5 (which differ in minor details) have been built and are being used for in-person and remote project labs at a local college; V2.1 is the one for sale going forward and comes in several flavors. The detailed asseembly instructions in this manual are for the Heterodyne V2.1 and is most similar to the original Homodyne V2.1 but modified for use with the two frequency laser.

Various configurations of V2.1 are or will be available on eBay under my user ID siliconsam or by searching for "Sam's Educational Michelson Interferometer Experimental Setups for Education-Signal Oriented". Or directly from me with more options and slightly lower cost. If interested, contact me via Sci.Electronics.Repair FAQ Email Links Page.

Currently, there are three minor variations that differ only in the location of the laser and the length of the ball bearing rail for Arm 2. The "Compact" version places the laser entirely on the breadboard and is more convenient for transport. Its main limitation is that the ball bearing rail is limited to 150 mm so it fits on the breadboard. The "Mid-Size" and "Extended" versions moves the laser to the left with an extension bar to support the back of the laser and uses a 200 or 300 mm rail. Versions include a Motion Control Platform (MCP) in place of the ball bearing rail for Arm 2. In principle, a closed-loop positioning system could be implemented using the interferometer and µMD2 for feedback, though the required additional hardware and software is expected to be developed by advanced project students. ;-) There is also a more major variation permitting a 400 mm rail or 300 mm MCP to be installed even on the Compact version, though without detailed assembly instructions. Stay tuned.

As noted above, there will also be a combined "Het + Hom" kit which frequency laser into a single frequency laser by blocking one of the polarized components and orienting the remaining one at 45 degrees. That modification will be detailed here, but for the experiments, one is referred to the Sam's Educational Michelson Interferometer Experimental Setup Project Manual V2.

Some of the photos are of the original prototype on a custom aluminum optical breadboard. They are only for reference (and because I'm too lazy to reshoot them!).

Note: Off-page links (including any clickable graphics) open in a single new window or tab depending on your browser's settings. A suitable fixed width or monospace font like "Courier New" must be specified in your browser to make sense of the simple ASCII diagrams. For Firefox, go to: "Settings", "General", "Fonts", "Advanced", "Monospace", and confirm that it is "Default (Courier New)".

Interferometers for Metrology Applications

Classic Michelson Interferometer

The textbook version is shown below:

In short, a light beam is split into two parts which are bounced off of a pair of reflectors and recombined at a detector. Any change in the relative path lengths of the two "arms" formed by the reflectors results in a phase shift between the waves in the two beams resulting in constructive or destructive interference, which can be measured and converted to displacement (change in position) down to nanometer precision. All other types of measurements made by these systems are based on opto-mechanical configurations designed such that changes in the measured variable are detectable by what is in essense a Michelson interferometer.

Where the Path Length Difference (PLD) between the two arms is small, the requirements for the laser are not very stringent. In fact, for very small PLDs, an LED or even a totally incoherent source like an incandescent lamp may be substituted for the laser. However, to be useful for the PLDs necessary for most applications (millimeters to 10s of meters or more), the light source must be a laser. And not just any laser, but one that has a narrow "linewidth". While the popular concept of a laser is of a light source that is monochromatic (single color or wavelength), in reality most lasers do not even come close. It takes careful design and implementation to achieve that. For these metrology applications using the so-called "homodyne" interferometer, the laser should ideally produce an output that is a single optical frequency with a linewidth approaching zero. In practice, it isn't that narrow but can still result in a linewidth of much less than 1 MHz, resulting in a usable PLD of 100s of meters. This is usually a low power stabilized HeNe laser. However, for PLDs of a few inches, a common (much less expensive) unstabilized HeNe laser will be adequate. Within this range, all the modes will be sufficiently correlated and the fringe contrast/signal amplitude will be acceptable.

For the heterodyne interferometer, instead of a single optical frequency, there are two spaced by up to 20 MHz. The usable PLD is determined by this "split" frequency and is thus still very large. The split frequency only limits the maximum slew rate of the remote reflector. More on this later.

Engrave PLD on your brain. It will be used throughout this manual. :)

Cube Corner Retroreflectors

Near the end of this manual is an experimental setup similar to the original Michelson Interferometer configuration adapted for the Polarizing Beam-Splitter. So you can experience these issues for yourself.

The diagrams below show some variations on the angles of the Beam-Splitter (BS) and Mirrors (M1, M2).

The first one is essentially the same as the diagram, above. The "Single Angled" one does nothing to prevent back-reflections. And while the "Double Angled" version does eliminate back-reflections, the two beams are angled at the detector which means their relative phase will vary across the detector. That is undesirable for our experiments in at least two ways: First, it will mean that the detector won't see a clean fringe signal. In fact there may be little or no signal depending on how the fringes average across its face. Second, alignment will change as either mirror is moved making testing with respect to PLD very tricky. But can you suggest an application where it may be useful?

The first enhancement of the Michelson interferometer is to add a means of separating the outgoing and return beams so that there is vitrually no optical power returned to the laser. The simplest way to do this is to replace the mirrors with Retro-Reflectors (RRs), typically cube-corner (trihedral) prisms, which have the property of returning the beam directly back parallel with the outgoing beam, but which may have an offset. In this way, virtually none of the reflected light ends up back at the laser. The use of the RRs also greatly reduces the sensitivity to alignment as any change in their angle is converted to a small change in the distance between the outgoing and return beams, but they remain parallel.

All of the practical interferometer configurations include at least one cube corner retro-reflector.

Polarizing Beam-Splitter

The beams reflected to the two arms of the interferometer then have orthogonal polarization which effectively makes them independent until they are combined at the detectors.

The result is then one of the most widely used configurations - the Linear Interferometer (LI), which was the first one used by HP in their original 5500A laser interferometer displacement measuring system. (As an aside, I do NOT know where that name "Linear Interferometer" comes from except perhaps that the inital configuration was in-line with the laser and thus "linear".) In practice, Arm 1 is used as the reference and is made as short as possible with the Cube Corner (CC) attached directly to the PBS cube. Both arms can move where differential measurements are required.

With the use of the PBS, the maximum amount of the laser optical power is available - virtually nothing exits out the unused side of the beam-splitter as it would in the original Michelson setup, above. However, at least half the power is lost in the detection scheme that is typically used so it would end up being similar. In principle, this wasted power can be diverted to a second detector. Their difference will then have twice the amplitude and the signal-to-noise ratio will nearly double (but with opposite phase). This is rarely, if ever, done though. But for the more complex multi-pass interferometer configurations described later as well as for use with two frequency lasers, the use of the PBS is essential to avoid incurring very large losses, or for the schemes to work at all.

As noted for the heterodyne interferometer - probably the most popular type used for high performance metrology applications and the topic of this kit - the laser actually produces a pair of optical frequencies that are othogonally polarized. They are typically separated by 100s of kHz to 10s of MHz called the "split" or "REF" or reference frequency. Most commonly the split frequency is in the small number of MHz range. For the HP/Agilent 5517 laser in this kit, it has been adjusted to be between 1.5 and 2.0 MHz. F1 is the lower optical frequency and is the horizontally polarized component; F2 is the higher optical frequency and is the vertically polarized component. As long as the difference frequency between F1 and F2 is reasonably stable, the specific value only impacts the maximum slew rate possible for the moving mirror or cube corner. It has no effect on the resolution. Think of the split frequency as a carrier that can be conveniently manipulated by AC electronic circuits. So just as with radio, the carrier does not carry information but makes it convenient to process that information. There are also several other advantages including reduced affects of signal amplitude and less sensitivity to interferometer alignment. Everything with respect to the interferometer optics and configurations is similar except that instead of the phase change between the two (essentially base-band) polarized components derived from the laser, it is the phase change between the REF signal (F2-F1) and a "MEAS" signal (F2-F1+ΔF1) with ΔF1 being a result of the Doppler shift from the moving reflector.

The maximum PLD is virtually unlimited for the two frequency laser with usable PLDs of 10s or even 100s of meters or more - impacted more by issues of beam divergence and collimation than the stability of the laser itself. However, since it is the PLD that is being measured, you can't purge it from your brain, sorry. ;-)

Common Interferometer Configurations for Displacement Measurements

• Linear Interferometer (LI): Simplest, cube corner for remote reflector, single pass: resolution X1.

• Plane Mirror Interferometer (PMI): Plane mirror for remote reflector, double pass: resolution X2.

• Single Beam Interferometer (SBI): Compact, small cube corner for remote reflector, single pass: resolution X1.

• High Stability Plane Mirror Interferometer (HSPMI): Plane mirror for remote reflector, sensitivity to temperature changes <1/10th that of PMI, double pass: resolution X2.

• Modified Linear Interferometer (MLI): Same architecture as SBI but beam paths separated, cube corner for remote reflector, single pass: resolution X1. It really only has "linear" in its name because it uses cube corners like the basic LI.

• Double Pass Linear Interferometer (DPLI): Similar to the HSPMI but with cube corners for the arm reflectors, double pass, resolution X2. Like the MLI, it really only has "linear" in its name because it uses cube corners like the basic LI.

• Single Pass Plane Mirror Interferometer (SPPMI): Uses NO cube corners, single pass: resolution X1. The SPPMI may also be called the "No Cube Corner" interferometer. It is most similar to the original Michelson interferometer but uses QWPs to avoid back-reflections. However, it is similarly very sensitive to misalignment and useful only over short changes in PLD.

• High Resolution Plane Mirror Interferometer (HRPMI): Doubles the resolution compared to the PMI or HSPMI but is much more complex and tricky to align. Quadruple pass, resolution x4. A few additional parts and much determination may be required to construct this.

When used in a displacement measuring system with a 633 nm HeNe laser, 1X represents a full cycle resolution of 1/2 wavelength or ~316 nm; 2X of ~158 nm, and 4X of ~79 nm. For a homodyne system with a Quad-A-B deterctor, there are 4 counts per cycle so that gets multiplied by up to 4. For a heterodyne system using µMD2, there are 2 counts per cycle so that gets multiplied by up to 2. The very commonly used PMI or HSPMI will then have a heterodyne resolution of ~79 nm and interpolation techniques can extend it down to under 1 nm. However, if the signals are noisy or asymmetric, the benefits may not be achievable.

Using the Heterodyne Setup as a Homodyne Interfermeter

So while it could be made to work, everything would be more finicky. But even so, a combined kit will probably be offered in the future. ;-)

Detectors and Waveform Display

All signals are AC. For this kit, they will be either fixed (for REF - 1.5 to 2.0 MHz) or varying for MEAS (theoretically from <100 kHZ to 4 MHz or more), though for reasonable velocity of the reflector, they will tend to be close to REF.

So for the heterodyne interferometer, the detector is slightly more complicated than a photodiode since it must operate at frequencies up to several MHz and the available beam power using two frequency lasers tends to lower.

There need to be two signals:

• REF: There is a dedicated optical receiver inside the 5517 laser which monitors the difference between F1 and F2 and generates the RS422 reference signal output via a pair of pins on the laser head connector. This electrical signal is called REF, which shouldn't be confused with the reference optical path in the interferometer which is also called REF. ;-)

• MEAS: For the output of the interferometer, an external optical receiver is required. The one used here is called OR3 (for historical reasons!) and is a small PCB with a photodiode at one end and a 3 pin connector or screw terminal block for the MEAS RS422 signal at the other. This electrical signal is called MEAS which also shouldn't be confused with the MEAS optical path in the interferometer.

Note that by convention, ARM 1 of the interferemeter (at the back of the breadboard) is for REF; ARM 2 of the interferometer (with the stage) is for MEAS. Even though they appear symmetric at first glance, for some interferometer configurations, there is a difference in behavior, usually a scale change. For example with the Linear Interferometer (LI), moving either reflector results in the same change in PLD. However, for the Plane Mirror Interferometer (PMI), Arm 1 has 1/2 the sensitivity as Arm 2.

Biased Photodiode (BPD1)

The most basic circuit is shown below:

           Silicon 
          Photodiode
   +---------|<|-------+-------o Output to scope or DMM
   |  Cathode   Anode  |
   |                   /
   |                   \ R-Load
   |                   /
   |     Bias Supply   \
   |       +| | -      |
   +--------||||-------+-------o GND / Common / Return
            | |

Note the polarity of the PD with its cathode connected to the positive of the power supply and thus reverse biased. With no light incident on the PD, only the so-called "dark current" will flow, which is generally small enough to be ignored (nanoamps or less).

A very simple custom PCB is included to implement a similar circuit for initial testing with an oscilloscope. It is called BPD1 but the PCB may be unmarked or labeled something else. The bias supply can be +15 VDC from the laser power supply, 12 V from the power pack used for various experiments, or a 9 V battery. The load should be 2-3K ohms.

Optical Receiver (OR3)

OR3 is on a 1" by 2.75" PCB and remedies most of these deficiencies. The optical input is a beam up to ~3 mm in diameter (using the default photodiode) with an optical power from <10 µW to >1 mW. While OR3 as tested doesn't have the frequency response of the commercial ORs, its >3 MHz bandwidth is more than adequate for systems using the kit lasers. And it is expected that the bandwidth limit can be extended with trivial changes to only a few part values. This is left as an exercise for the student. ;-)

Where optical power low and electrical noise pickup becomes an issue (or for simply for convenience and aesthetics), a case with a copper foil liner or wrap for shielding could easily be fabricated for this using that lonely 3-D printer eagerly awaiting something to 3-D print. ;-)

The actual SG-OR3 PCB is four layers with internal power and ground planes. This does appear to have better sensitivity without stray noise pickup than a two layer PCB with the same component placement but no planes. The gain can be set at the half way point and still be quiet with no signal without shielding. And the sensitivity there is better than 2 µW at 3 MHz. The populated SG-OR3 PCB is shown below. It may look HUGE in your Web browser. Don't be fooled. ;-)

   

Typical Populated OR3 V1.02 PCBs - with Headers and Screw Terminal Blocks

REF (F2, vertical) and MEAS (F1+ΔF1, horizontal) are passed through a linear polarizer at 45 degrees to a back-biased photodiode. The optical receivers provided with this kit add circuitry to boost the signal and then convert to the RS422 signal format required by µMD2. See Optical Receiver 3 (OR3) - Assembly and Operation Manual. These are provided in kit form with Heathkit™-style assembly instructions so construction is not overly painful. ;-)

More information on OR3 including complete "Heathkit™-style" assembly instructions may be found at Optical Receiver 3 (OR3) Assembly and Operation Manual.

Using the Oscilloscope

(Even though REF and MEAS are differential pairs - each pair being similar but with polarity - we will usually refer to them simply as REF and MEAS except as noted.)

• For REF, either pin E or F on the laser head connector can be used. They will appear as un ugly square wave. With no load, it will probably have a peak-to-peak (p-p) amplitude of 1 to 5 V depending on the version of Control PCB inside the laser. Pin R can be used as the GND (though any of the Grounds would probably be just as good here).

    Laser Head Connector Pin   Funtion
   ------------------------------------
                E              ~REF
                F               REF
  

       Laser Head Connector Pin F o---------o Scope Channel 1
       Power Head Connector Pin R o---------o Scope Channel 1 Ground
  

  An AC signal on REF (and ~REF) from the laser is only present after READY turns on solid. Since the rest of the interferometer does not affect REF, it can be viewed any time the laser is powered and READY is on solid.

• For MEAS, either BPD1 or OR3 can be used. Since at this point, the SG-OR3 PCB may not have been built and tested, use BPD1. The pinout is as follows (from left too right on the screw terminal block:

  BPD1 Pin     Function
 -------------------------
     1      GND
     2      +Bias Voltage
     3      PD 1 Anode
     4      PD 2 Anode

PD1 is probably the one to use. The other one is intended to be used where a beam-splitter separates the polarized components. As with the Thorlabs DET110, a load resistor is required so the photodiode current can develop a voltage to be viewed with the scope. At the frequencies that MEAS will have, a value of a few K ohms is suitable.

                 2   R Protect      PD1              3
 + Bias Voltage o-------/\/\---+-----|<|------+-------o Scope Channel 2
                      1K ohms  |              |                    
                      typical  |              /
                              _|_ C Bypass    \ R-Load
                              --- 0.1 µF      / 2-3 K ohms typical
                               |              \
                 1             |              |      1   
 - Power Ground o--------------+--------------o-------o Scope Channel 2 Ground

The signal will be a fairly decent sinewave at the REF frequency with an amplitude proportional to optical power and R-Load.

However, the MEAS electrical signal will only be present when the F1 and F2 polarized components are both present and combined using a linear polarizer at 45 degrees. This will be true directly from the laser but the signal from the interferometer, correct alignment will be required.

With REF and MEAS displayed on the scope and synced to REF, any change in PLD will show up as a change in the horizontal position (phase) of MEAS. With the interferometer aligned, gently touching the breadboard or even just walking across the floor will probably be easily detectable.

More on all this will be explained in more detail in the sections on the Linear Interferometer.

Displacement Measurement

REF and MEAS Phase

A variety of techniques can be used to extract this information but virtually all are ultimately based on digital counters for REF and MEAS whose difference is used to calculate displacement, or a single up/down counter with suitable logic to deal with race conditions. These result in a basic resolution of 1/2, 1/4, or 1/8 wavelength depending on the type of interfermeter. Additional hardware, software, or firmware are then (optionally) be used to extend the resolution down to or below the nm range. A variety of techniques can be used including Phase Locked Loops (PLLs) to multiply the REF and MEAS frequencies by 16 or 32 or more before applying them to the counters, or digital estimation of the phase difference between the REF and MEAS signals. PLLs are used in commercial systems like the HP 5508A while phase estimation is used in µMD1, the predecessor to µMD2 - since it can be done entirely in firmware. This is works-in-progress for µMD and may be available before the Sun goes Nova. ;-) However, by using averaging, similar resolution can be achieved but it just takes longer.

Early measurement displays like the HP 5505A were based on SSI-MSI TTL and occupied 5-1/4" high rack mounted units. Modern ones are typically implemented with a combination of microprocessors, FPLDs, and custom LSI parts and would easily fit inside a box of playing cards.

Micro Measurement Display 2 (µMD2)

While in principle, the scope discplay could be used for many of the experiments later in this manual, a proper displacement readout makes everything much easier. The Zeeman interferometer kit is intended to use Micro Measurement Display 2 (µMD2), though any measurement readout compatible with HP/Agilent lasers would also be satisfactory (including µMD1). µMD2 runs on an inexpensive Teensy 4.0 microcomputer board which includes all the required hardware except for the line receivers for REF and up to three axes of MEAS signals. Counters inside the CPU implement the basic measurement computations along with high performance firmware in C and ARM assembly code which estimates the phase difference between REF and MEAS(s) to extend the resolution. (The latter is a "works in progress" as noted above). The source code is available (though modification is highly discouraged).

Precision Measurement System using Two-Frequency Laser and µMD1 Measurement Display

The graphic above show the specific example of a Linear Interferometer (LI) for displacement (change in position) measurements, with velocity calculated as the rate of change of position. By substituting different interferometer configurations, higher resolution displacement as well as angle, straightness, and other physical variables can be measured.

The µMD Graphical User Interface (GUI) runs on PC or laptop under Windows™ (XP/Vista/7/10/11 or later) via a USB interface. Raw measurement data from the Teensy board can also be input directly to something like Excel™ or Matlab™, or a user-developed analysis application. The screenshots below of µMD1 show the displacement of a mirror on a PZT driven by a triangle waveform from a function generator. The p-p amplitude is around 60 nm and 10 nm for the left and right plots, respectively. As noted, µMD2 doesn't currently support sub-wavelength interpolation, but if averaging is enabled and motion is relatively slow, the display will be similar.

   

µMD1 Main Window Typical Display (Left: 60 nm p-p, Right:10 nm p-p)

Complete information on µMD2 can be found at Micro Measuremnt Display 2 (µMD2) Installation and Operation Manual. It includes complete step-by-step "Heathkit™"-sylte assembly instructions for the SG-µMD2 PCB as well as information on the Windows µMD GUI.

However, if you would prefer µMD1, it is still possible to purchase the parts via a Digikey "Cart" with the SG-µMD1 PCB and programmed PIC from me.

Linear Interferometer

The designations m-n show the paths taken by the Arm 1 and Arm 2 beams where "m" is the Arm and "n" is the sequence number.

Linear Interferometer Setup

Linear Interferometer Diagram and Suggested Compact Version Breadboard Layout

The other configurations will have a few additional or substitute parts and small variations in the horizonatal position of the laser and placement of the optical receiver but are otherwise similar. Therefore only the LI setup will be described in more detail.

• Baseplate: This is a 8x24x1/2" aluminum optical "breadboard" with an array of 192 1/4-20 tapped holes on one inch centers to which all the components are attached with cap-head screws or set-screws. (We may use the terms "baseplate" and "breadboard" interchangeably.)

• HeNe laser: This is a Hewlett Packard (HP) or Agilent 5517 two frequency Zeeman laser mounted on 3 studs or set-screws that enable its position and alignment to be easily adjusted. Power for the laser is ±15 VDC from a pair of wall adapters. (The high voltage for the HeNe laser tube is safetly tucked away inside the case. And even if it is desirable to remove the cover to show off the interior, there are no exposed high voltages.) The 5517 laser is of very high quality with a sticker price if new above $8,000. It is the same type laser used in numerous commercial applications including countless wafer Fabs. The laser is the only somewhat fragile item so please don't drop it or use it to hammer nails. Your instructor (and I), and perhaps your grades won't like that. :( :)

  Beam diameter from most of these lasers is 6 mm so that experiments could actually be performed with optics up to several meters away. Where it is desirable to do experiments over even longer distances, a 9 mm beam may be requested. And a 1-2 mm beam can be substituted where everything is on the breadboard and the path length difference isn't too large.

  A narrow beam is a bit trickier to align and to maintain alignment as the stage in Arm 2 is moved, but it is quite adequate for a small range of PLD. But where the PLD is large (e.g., for use as an earthquake sensor), the larger beam eases alignment and is required so that the divergence of the beam doesn't affect the detector response.

• The heart of the setup is an assembly using optical components from Hewlett Packard (HP), Agilent, or Keysight, or equivalents from Excel or Zygo, which are all physically and funcationally interchangeable. These may be configured like LEGO™ blocks in a variety of ways to create several types of interferometers. Most descriptions will use HP/Agilent/Keysight part numbers:
  • HP/Agilent 10702A or 10706A Polarizing Beam Splitter Cube (PBSC). (The two part numbers are physically and functionally identical except for the label.) This is a high quality 1" PBSC mounted in a precision stainless steel frame.

  • HP/Agilent 10703A Cube Corner (CC) retro-reflector. This is a glass trihedral prism installed in a stainless steel cylidrical case. It can be attached directly to the PBSC.

  • A pair of HP/Agilent 10722A or equivalent Quarter WavePlates (QWPs) can also be attached to the PBSC.

  As a point of interest (or trivia), these parts cost something like $4,000 if purchased new. Fortunately for us, there is eBay. :-)

  This assembly attaches to an aluminum "PBS Mount Adapter Plate" which itself is secured to the breadboard with optical posts and spacers.

• Two unmounted glass cube corner retro reflector trihedral prisms: These can be secured in the one inch Thorlabs KM100 kinematic mirror mounts with the KM100s rotated 180 degrees so the knobs face the PBSC.

  For some of the other interferometer configurations, these will be replaced either with 1" planar mirrors installed in the KM100s, or with a thinner mirror glued to a loudspeaker or PZT. (More on these later.)

• The turning mirror: is simply a small rectangular high quality planar first surface aluminized mirror that redirects the output beam to the detector if needed. The exit direction of the beam depends on the specific interferometer configuration but most of them direct the output to the left so to get it to the detector requires a right angle reflection.

• The optical receiver shown is the SG-OR3 PCB which provides most of the capabilites of the HP/Agilent/10780 but may not look as cool. Or way cool depending on our perspective. ;-) Feel free to install it in a custom 3-D-printed case. :)

• There are either one or two types of Polarizers in the kit:
  • Circular Polarizers (CP) are a Linear Polarizer (LP) combined with a Quarter WavePlate (QWP). The LP axis is at 45 degrees to the sides.

  • Linear Polarizer (LP) sheet with its polarization axis lined up with one of the sides.

  The CP type can be used for most purposes in place of an LP. Specifically here for placing in front of the detector and/or photodiodes.

  Do NOT remove the protective film until ready to use. Also note that the CP sheet has a weak adhesive on the QWP surface and it will attract dust and debris (including grubby fingerprints!) Cleaning can be done with isopropyl alcohol but the adhesive will still be there when it evaporates.

  Only small pieces of these are required for any given purpose so they can be cut as desired. Four or 9 equal pieces of each is probably a decent choice. Use masking tape to stick them whever they need to be stuck to. ;-)

  In fact, the primary use is to be mounted in front of the optical receiver photodiode with the linear polarization axis at 45 degrees to combine the H and V F1 and F2 polarized components from the interferometer. For that, a ~4x6 mm piece of CP sheet is ideal and can be stuck directly to the photodiode.

A variety of mounting schemes are used:

• Laser: As noted above, the laser head comes with a set of "feet" - aluminum blocks with U-shaped cutouts. These attach to the bottom of the laser and sit on nuts threaded onto studs (setscrews) threaded into holes on the breadboard. By adjusting the position of feet and their height, very precise alignment can be fine tuned and then locked in place. For the interferometers, the beam will need to be positioned at a height of 4-1/4 inches above the baseplate and either centered or offset approximately 1/4" toward the back of the setup (as shown above) depending on the specific interferometer being implemented.

• PBSC assembly and turning mirror: The mount for this is the "PBS Mount Adapter Plate" attached to a pair of 3" Thorlabs posts with 1/4" spacers. This replaces the funky wood block in the really early versions of these kits. ;-)

• Interferometer Arm 1: This would normally be the "Reference Arm" in the basic Linear Interferometer. A Cube corner (CC) or one inch first surface mirror is installed in a one inch Thorlabs KM100 adjustable mount on a post in a post holder which allows for height and rotation about the vertical axis, and also fine adjustment pan and tilt of the CC itself via the knobs on the KM100.

• Interferometer Arm 2: This is what the "Tool" of a displacement measureing system (for example) would mount on. It has a similar KM100 for a mirror or CC, but mounted on a micrometer controlled linear stage so that its position can be adjusted very precisely, which in itself in on a lockable ball bearing slide for coarse positioning. A loudspeaker with a mirror glued to its cone to be used as an electrically driven actuator or as a laser microphone can also be installed in the KM100.

• Detector: The SG-OR3 optical receiver PCB attaches to a plate that mounts on a Thorlabs post.

Setting the Heights

Required Optical Components and their Suggested Heights.

The only difference between the Compact, Mid-Size, and Extended versions is mounting of the back of the laser. For the Compact version, the stud is attached the breadboard; for the others, the stud is attached to the xtension Plate.

The heights of any Retro-Reflectors (RRs) in the setup will be what most affect the beam height. This is true of the Linear Interferometer (LI). Where there is an RR attached to the PBS cube like the Plane Mirror Interferometer (PMI), the alignment of the laser will need to be used. However, there is a wide tolerance and enough degrees of freedom so in the end, it should not really be much of a problem to set it up.

Here is an annotated photo of a typical setup configured for the DPLI:

Compact Version of Heterodyne Interferometer with Major Parts Labeled

Assembly and Alignment of the Linear Interferometer

Please refer to the appropriate layout diagrams below.

Linear Interferometer Breadboard Layouts for the Compact Version (left), Mid-Size Version (center-Left), Extended Version (center-right), and with Long Rail or MCP (right)

An noted above, the only real difference between first these setups is in the mounting of the laser and the length of the ball bearing rail (or motion control platform). The Compact Version is most easily transported while the Extended Version provides the maximum range of movement of the Arm 2 reflector. The Mid-Size Version represents a nice compromise.

The one on the right supports an extra length 400 mm ball bearing rail or 300 mm motion control platform. Assembly is basically similar but detailed instructions are not provided, sorry. ;-) And it may be necessary to trim one of the BA1Ss used to mount the laser to provide clearance for the mount on the rail.

For the following it is assumed that nothing has been mounted, but depending on the previous use, some of these steps have already been completed. Refer to the layout diagrams, above, parts locations that are known to work.

Parts attached with fasteners should be snug but don't overtighten unless noted.

1. Breadboard: The default assembly is to put 5 rubber bumpers on the bottom of the breadboard - 4 at the corners and 1 near the center to avoid damaging the breadboard or surface on which is it placed. This provides clearance underneath for at least one part (the MGN15 rail) that is best fastened using a screw and nut (Compact version) or 2 screws and nuts (Extended version). If it is desired to mount the breadboard flush on an optical table, then the MGN15 rail can be mounted using a pair of screws into holes tapped into the breadboard (not included).

2. Laser mount: Thread 1/4-20 nuts onto three 2" 1/4-20 setscrews or threaded rods so around 1/4" of the thread is exposed. Thread the two studs for the sides of the laser into the breadboard at the locations with the "Xs" in the diagrams, above, to a depth such that the nuts just touch the breadboard. Tighten the nuts against the breadboard with an open-end or adjustable (e.g.,"Crescent™") wrench.

   Then thread another 1/4-20 nut onto each stud so that the spacing between them is exactly 3/8". Add a 1/4 washer to each

   • Compact version: Thread the stud for the back foot into the breadboard at the far left location "X" to a depth such that the nuts just touch the breadboard. Tighten the nut against the breadboard with an open-end or adjustable (e.g.,"Crescent™") wrench. Thread a 1/4-20 nut onto the stud so that the spacing between them is exactly 3/8". Add a 1/4 washer.

   • Extended version: Secure the Extension Plate to the breadboard in the location shown with a pair of 1/4-20 5/8" cap-head screws in the coutnersunk holes. (The laser can also be mounted 1 inch further to the left if it is desired to have more space on the breadboard but only one hole may mate with a breadboard hole depending on the Extension Plate length.) Thread the stud for the back foot into the Extension Plate at the far left location "X" to a depth such that the nuts just touch the Extension Plate. Tighten the nut against the Extension Plate with an open-end or adjustable (e.g.,"Crescent™") wrench. Then thread another 1/4-20 nut onto the stud so that the spacing between them is exactly 1/8". Add a 1/4 washer.

3. Laser feet: A set of 3 mounting brackets called "feet" are included with the laser. Two of these have been modified so that the "U" slot is longer. They are for the sides. The other one is for the back.

   With the laser upside-down on a padded surface, install the modified feet on both sides of the laser extended out using only a single M4 flat head screw for each in holes "S". Makes sure the feet extend at right angles to the laser. With only a single screw securing the feet, these should be a bit tighter than other fasteners. ;-) Install the unmodified foot at the rear of the laser using 2 M4 flat head screws in holes "R".

4. Rear bracket: Secure a BA1S to the rear foot at a right angle to the laser with a 1/2" 1/4-20 cap head screw and 1/4" nut.
   • Compact version: The BA1s should be placed below the rear foot with the recessed hole facing down to accept the screw head.

   • Extended version: The BA1s should be placed above the rear foot with the recessed hole facing up to accept the screw head.

5. Laser: Flip the laser over and set it on the 3 studs in the approximate location shown in the diagram. Add another 1/4" washer to each stud and thread a 1/4" nut onto each but only make them finger-tight for now as they will be used to fine tune alignment..

6. Laser power supplies: (These may already be wired.) The laser requires 15 VDC at up to 2.3 A and -15 VDC at under 200 mA. There are a pair of wall adapters / power packs for the laser and optical receiver. One has a current rating greater than 3 A while the other may be lower. If they have barrel connectors, the screw terminal adapters may be used to make the wiring convenient, though soldering is better.

   The power packs have US plugs or a socket for a power cord. For use overseas, appropriate plug adapters will need to be provided. These are not included since there are approximately 653,248 different power standards Worldwise and shipping would be kind of high if all were included. ;-) In that case, the cord may also not be included.

   The higher current power supply may be rated at 16 VDC. If that is the case, a high current diode should be installed in series with the positive (center) lead to drop the voltage to an acceptable range. The bar on the diode should face AWAY from the supply.

   The common point is the negative (-) of the higher current supply to the positive (+) of the lower current supply.

7. Laser head connector wiring: Only ±15 VDC, GND, and the REF signals need to be wired. For a short cable as is likely used here, the shields and other wires can be ignored. "Short" means a meter or less.

   The pinout for the HP-5517 power/reference connector (J2) is as follows:

   
      Pin      Function
    ----------------------------------------------------------
       A       No Connection on 5517
       B          "          "
       C          "          "
       D          "          "
       E       ~REF (Zeeman beat signal from internal optical
       F        REF  receiver's differential line driver)
      G,H      Ground
       J       +15 VDC Sense
       K       +15 VDC
       L       -15 VDC
       M       +15 VDC
      N,P      Cable Shield
       R       Signal Return (REF)
       S       Ground
       T       +15 VDC
       U       Cable Shield
   

   The diagram is of the connector on the laser. and the contacts on the rear of the cable connector.

   Use AWG 22-20 stranded insulated hookup wire, preferably color coded for +15 VDC (yellow or orange), -15 VDC (blue or violet) and GND (black or green). And any other convenient colors for the REF signals.

   Strip around 1/8" of insulation and insert into the connector socket. With tinned hookup wire and the gold-plated sockets, soldering is very easy. Add a short length of heatshrink tubing to protect and insulate each connection after each solder joint is made as this also prevents solder bridges between pins.

   For the short cable, it is also not necessary to connect all the duplicate pins, though that won't hurt. And for those, there is no need to run all the wires the full length of the cable; they can be joined near the connector to a fatter wire.

   The REF/~REF cable MUST include a GND to tie the µMD2 PCB to the system GND.

   Double check that the connections are correct and secure.

8. Powering the laser: It is best to plug both power packs into a switched surge-protected filtered power strip but this is not essential.

   When power is applied, both of the Power LEDs as well as the Laser ON LED should come on immediately. The laser then will go through a warmup sequence that takes 4 to 5 minutes. Around halfway through it, the READY LED will start flashing. When it stays on solid, the laser is stable and ready to use. ;-)

   If the Power and Laser LEDs do NOT come on, power down and troubleshoot. It means there is a bad connection, bad power supply, or incorrect wiring. The laser is fairly well protected against the latter, but an internal fuse may blow if, for example, +15 were connected to -15. (Don't try it - I an not sure of what will happen, but it won't be good.)

9. Laser alignment: For the Linear Interferometer (and most others), the beam needs to be offset 1/4" toward the far side of the breadboard at a height of 4-1/4 inches.

   Doing this accurately is critical to the ease with which the subsequent alignment can be performed since not all mounts have sufficient degrees of freedom to accomodate an arbitrary beam location and direction. Fabricating an "alignment aid" out of carboard may be useful. This would have a hole at the optimal height offest 1/4" toward one side from a mark at the bottom.

10. Interferometer assembly: This consists of the Polarizing Beam-splitter cube, mounting plate, 2x 3" posts. 2x 1/2" 1/4-20 setscrews, 2x 1/4" spacers, 2x 8-32 x 1/2" cap-head screws.
    1. Attach a pair of 3" posts to the breadboard with 1/4-20 x 1/2" setscrews in the locations shown above. Thread the setscrews about half-way into the breadboard. Then with something thin pressing on their sides to prevent them from turning, thread the posts onto the setscrews until they contact the breadboard. Use a thin rod (e.g., hex wrench) through the hole in the post to tighten them.

    2. Attach the "PBS Mount Adapter Plate" to the posts with a pair of 8-32 x 1" or 1-1/8" cap-head screws with 3/4" spacers between the plate and post. The screw on the right should be made fairly tight as it won't be accessible once the PBSC is installed.

    3. Remove any optics that may be attached to the PBSC. They will not be required for the LI. Store them wrapped in soft paper towels, bubble wrap, etc., to protect the optical surfaces.

    4. Place the PBSC on the PBS Mount Adapter Plate so that the diagnonal marking is front-left to back-right with the "In" arrow pointing left-to-right.

    5. Use four 4-40 x 1-3/4" cap-head screws to secure the PBSC to the plate from the top if the adapter plate holes are tapped or four 6-32 x 1/2" cap-head screws from the bottom if they not tapped.

    The laser beam should pass through the PBSC centered vertically and 1/4 inch toward the back. It should be at the same location relative to the breadboard at the far end. If not, fine tune alignment. :) Getting this dialed in via the nuts on the studs will greatly simplifiy alignment later.

11. Turning mirror: Use tape or a *tiny* bit of adhesive to attach the turning mirror to the right angle "Turning Mirror Bracket" centered vertically with respect to the PBSC and flush with the right edge. Attach the bracket to the PBS Mount Adapter Plate with a 3/8" 4-40 cap-head screw and #4 washer. Center the screw in the elongated hole for now.

12. Arm 2: These steps assemble the components of Arm 2. Arm 2 is assembled first because the lateral position of the CC cannot be adjusted relative to the laser.
    1. Thorlabs rail, ball bearing rail, or motion control platform: This assumes an initial installation, not swap.
       • Thorlabs rail (Homodyne only):
         1. Attach the Thorlabs RLA0600 6 inch rail (Basic and Deluxe) or Thorlabs RLA1200 12 inch rail (Deluxe+ version) to the breadboard. The recommended location is shown in the diagram above.

         2. Add 8-32 screws and #10 washers to the ends of the rail to act as stops.

         3. Attach the linear stage to the RC1 carrier using a 4-40 cap-head screw and washer.

         4. Install the stage/carrier assembly onto the RLA rail.

       • Ball bearing rail (Heterodyne or combined):
         1. Attach the MGN15-150mm rail with MGN15C carriage to the breadboard. Try to avoid removing the carriage from the rail as it is then easy for some ball bearings to attempt to escape. :( ;-) Since the holes in the MGN15-150mm rail do not line up with breadboard holes, it is attached to the breadboard with either a pair of 6-32 cap-head screws installed in special tapped holes, or one of these along with a 6-32 screw with nut and lockwasher through a 1/4-20 hole. The recommended location is shown in the diagram above.

         2. Attach the linear stage adapter plate to the MGN15C carrier using four M3-6mm flat-head screws. Tighten these securely as they won't be accessible after the other parts are installed.

         3. Attach the linear stage to the MGN15 adapter plate using four 4-40 1/4 inch cap-head screws at the corners.

         4. Add stops at both ends of the rail using the rubber bumpers (150 mm rail) or screws, spacers, and nuts (300 mm rail) provided. ;-)

       • Motion Control Platforms: These instructions will differ slightly depending on whether the MCP is Type 1 or TYpe 2, and on the length.
         1. If purchased at the same time as the interferometer, holes will already have been drilled in the breadboard for the stepper motor rig, with the motor on the right. These locations will enable the maximum usable travel. Else holes wlil need to be added.

            Secure the platform on the breadboard using four 4-40 1/2" cap-head screws if the holes are tapped, or 4-40 3/4" screws and nuts if not. Take care to assure that it is aligned.

         2. Attach the linear stage adapter plate to the platform using four M3-6 flat-head screws. To maximize travel, the plate should extend to the left (toward the PBS).

         3. Attach the linear stage to the linear stage adapter plate using four 4-40 1/4 inch cap-head screws at the corners.

         For more on setting up and using the MCP, see the section: Motion Control Platform

    2. Refer to the "Suggested Heights" diagram and attach a 2" post holder (rail or slide) or 1" post holder (MCPs) to the linear stage with an M3-12mm cap-head screws and #4 washer through the hole in the bottom of the post holder. Orient it so the knob on the post holder is easily accessible. Center the screw as best as possible and tighten it securely. Note: The hole may be tapped 4-40, in which case the screw should be 4-40 1/2".

       CAUTION: Make sure the tip of the screw does NOT contact the fixed part of the linear stage. If it does, an addition washer may need to be added. Or the screw can be shortened slightly with a metal file or grinding wheel.

    3. Adjust the micrometer so the stage is approximately in the center of its travel range.

    4. KM100 or similar mirror mount: Secure to a 1" post (rail or slide or MCP Type 1) or 3/4" post (MCP Type 2) with an 8-32 3/8" cap-head screw. Slip the post into the Arm 2 post holder and hand tighten its thumbscrew. Adjust the two alignment knobs so that the mounting plate is parallel to the base in both directions. If the adjustments are too tight, check that any locking screws are not tightened. Else, total removal, cleaning, and lubricating with a tiny amount of light grease or machine oil will be required.

    5. Cube Corner (CC) trihedral prism: Install the CC in the KM100 with its apex facing out and oriented to that a flat is at the top or bottom. (This reduces the chance of the beams hitting an edge of the prism.) It should be secured with either a soft-tipped set-screw or Nylon wide-head screw. DO NOT overtighten - it should be snug enough not to fall out (these are fragile!) but not so tight as to smash the CC! Note that the CCs mount backwards from what might be expected so that their edge can be secured properly. It's flat surface is facing through the mount.

    6. Set the knobs of the adjustable mount so its fixed and moving plates are approximately parallel and orthogonal to the beam axis as in the diagrams.

    7. The horizontal and vertical position (not orientation) of the CC relative to the incident beam adjusts the corresponding coarse position of the return beam.
       • Since the adjustable mount on Arm 2 and thus its CC cannot be moved laterally relative to the laser, the laser alignment should be double checked and fine tuned via its mounts if necessary so the incident beam is offset 1/4" toward the back of the setup from the center of the CC and aligned with the holes, and thus the return beam will be offset 1/4" toward the front of the setup and parallel to the incident beam.

       • The vertical position of the CC is adjusted by loosening the post holder's thumb-screw and should be set so the incident and return beams are at the same height.

    8. Place a piece of paper where the detector would be for the beam from the turning mirror. Fine tune the orientation and height of the mirror mount so there is a bright return beam there from Arm 2.

1. Attach a 2" post holder to a BA1S holddown with a 1/4-20 3/8" cap-head screw. Orient it so the thumbscrew (knob) is accessible.

2. Attach this assembly to the baseplate using a 1/4-20 1/2" cap-head-screw.

3. KM100 or similar mirror mount: Secure to a 2" post with an 8-32 3/8" cap-head screw. Slip the post into the Arm 1 post holder and hand tighten its thumbscrew. Adjust the two alignment knobs so that the mounting plate is parallel to the backplate in both directions. If the adjustments are hard to turn, check that any locking screws are not tightened. Else, total removal, cleaning, and lubricating with a tiny amount of light grease or machine oil will be required.

4. Cube Corner (CC) trihedral prism: Install a CC in the mount with its apex facing out and oriented so that a flat is at the top or bottom. This is to orient the prism so that the beams do not hit an edge. It should be secured with either a soft-tipped set-screw or Nylon wide-head screw. DO NOT overtighten - it should be snug enough not to fall out (these are fragile!) but not so tight as to smash the CC! Note that the CCs mount backwards from what might be expected so that their edge can be secured securely. :) They thus face through the mount.

5. Start with the knobs of the adjustable mount set so its fixed and moving plates are approximately parallel and orthogonal to the beam axis as in the diagrams. The BA1S can be moved to adjust the coarse horizontal position of the return beam. The post holder's thumb-screw may be loosened to move the adjustable mount up or down to adjust coarse vertical position of the return beam. The return beam should hit the PBSC at the same height as the incident beam, and offset 1/4" to the left of center. The spacing between the centers of the incident and returns beams should then be 1/2".

6. Using your piece of paper with the Arm 2 return beam unblocked, there should now be two spots reflected from the turning mirror corresponding to the returns from Arms 1 and 2. (Unless, that is, you're very lucky and they are already superimposed perfectly!)

7. Adjust the position and orientation of the CCs to superimpose the return beams from Arm 1 and Arm 2. This can be fine tuned with the adjustable mount knobs initially by eye. Then later once the oscilloscope is set up, it can be further optimized by maximizing signal amplitude from the detector.

Connect the REF signal to channel 1 using one of the REF wires from the laser head and GND. Set the scope to trigger on channel 1. With the laser powered and READY, there should be a stable ugly squarewave displayed at the REF frequency (between 1.5 and 2.0 MHz).

1. Attach a BA1S Holddown to a 2" post holder with a 1/4-20 3/8" cap-head screw.

2. Install a 2" post into the post holder with the small (8-32) tapped hole at the top with the lock screw just snug.

3. Clamp the assembly down loosely with a 1/4-20 1/2" cap-head screw.

1. Install a photodiode, 1K ohm resistor, and male-male jumper wires to the solderless breadboard based on the circuit in the section: Biased Phoatodiode (BPD1), or use the BPD1 PCB.

2. install a 2.2K ohm resistor for R-Load. If you don't have one or something close in your private stock ;-) one can be borrowed from the SG-OR3 parts for now.

3. Tape or place a piece of LP or LP/CP to the front of the sensor with its polarizaiton axis at 45 degrees (edges at 0/90 degrees). If using the CP, remove the protective film from both sides and the sticky side can be stuck directly to the face of the PD.

4. Apply bias power to the home-built detector from the +15 VDC laser power supply, the 12 VDC adapter, or a 9 V battery.

(Even if one beam is blocked, there could still be a weak signal due to imperfect optical coatings and such resulting in cross-talk between REF and MEAS. But it will be stable and locked to REF. That's the tip-off that the interferometer is not working.)

If the scope is triggered on channel 2 and the Arm 2 cube corner is moved rapidly back and forth on the rail, the MEAS frequency should change - increasing if the PLD is decreasing and decreasing if the PLD is increasing. If it doesn't change, alignment is not correct or an optic is installed incorrectly.

And for those new to interferometers, to reiterate, the optimal alignment will also be where the signal instability is maximized. ;-) Almost ANYTHING will affect it from touching the apparatus or table on which it is on, to just walking across the floor. The wavelength of light is really really small. ;-) To put this in some perspective, a full cycle of the signal with the Linear Interfemeter is a change in PLD of 316.4 µm (1/2 wavelength of 632.8 nm or 1/3160th of a mm). That's about 1/158th the diameter of an average human hair (~50 µm) or 1/22th the diameter of a human red blood cell (7 µm). Street traffic will be detectable, as will drafts from the A/C, changes in temperature, and siesmic events. Some of these effects can be further explored using parts in these kits.

High Stability Plane Mirror Interferometer (HSPMI)

The HSPMI on the other hand is perfectly symmetric: The beam paths for both Arms 1 and 2 are double pass and go through the CC. However, the change in PLD is double the change in position of the mirror in either arm. Thus it could also be used as a differential HSPMI where the relative displacement of Arms 1 and 2 is to be measured.

For use with a single frequency laser for homodyne or two frequency laser for heterodyne, the unequal path lengths is of little consequence. However, if building only a single type of plane mirror interferometer, it might as well be the HSPMI.

Normally, the Arm 1 mirror would be mounted along with the QWP on the PBSC as the reference since absolute PLD doesn't matter with the single frequency or two frequency lasers. For this setup, it's more convenient to mount the Arm 1 QWP on the laser and the associated mirror separate.

As with the LI, above, the designations m-n show the paths taken by the Arm 1 and Arm 2 beams where "m" is the Arm and "n" is the sequence number.

The only real difference in the assembly procedure is the addition of the\ QWPs and substitution of the planar mirrors for the cube corners. Alignment will be similar though the effects of beam height and offset will change subtly. But what fun would it be if nothing was different? ;-)

High Stability Plane Mirror Interferometer Setup

1. Install the HP retro-reflector (10703A) on the PBSC face closest to the detector. The orientation should be such that the beam doesn't hit an apex. This usually means the serial number (if present) runs up and down.

2. Install QWPs (HP 10722A or unmarked) on the faces toward the Arms. Their orientation does not matter as long as the screw slots are used.

3. Replace the unmounted cube corner retro-reflectors with circular 1" planar mirrors. The mirror mounts can be rotated 180 degrees to make them easier to adjust.

Adjust the location of the Arm 2 planar mirror so that the PLD is close to zero. Since the Arm 1 and Arm 2 beam paths are identical, distances from the mirrors to the faces of the PBSC block can be used.

Alignment will be similar to that for the LI, differ in some respects due to the planar mirrors and double pass architecture:

• Adjustment will be more finicky. While grabbing and manipulating the entire mirror mounts may be acceptable for the LI, the knobs should probably be used for the HSPMI. Why?

• The signal level will be lower by as much as a factor of 2 or more. Trace the beam paths and compare them to those of the LI. Why?

• How will the rate of waveform shift change?

Monitoring the PLD

Direct Observation

1. Using the DSO to directly view REF and MEAS: This is simplest. Display the waveforms for REF from the laser head cable and MEAS from BPD1 or OR3 above the one-another on the scope. Trigger on REF. Then the MEAS waveform will move back and forth relative to REF as the PLD changes. Almost any type of oscilloscope with a bandwidth of at least a few MHz would be adequate.

   With care, both the gross change of the PLD in integer wavelengths as well as the subtle variation in rate of change at sub-wavelength resolution can be observed either directly or by recording the display and playing it back in slow motion. Since the change is analog - not quantized into bits - more subtle behavior can be easily seen at a resolution down to a few nm. And the extreme sensitivity of the interferometer to any disturbance will be obvious.

2. REF-MEAS difference: Another method that would provide a display that might be useful is to combine the REF and MEAS signals using a simple resistor network. However, this does lose the direction information. A 10K ohm pot wired with terminals 1 and 3 connected to REF and MEAS, and the wiper connected to a DSO with a 10K ohm resistor to GND would provide a voltage at the split frequency with an amplitude that would vary based on their phase relationship - maximum when in phase and minimum when 180 degrees out of phase. The pot would be used to maximize the variation to account for any difference in amplitude of the REF and MEAS signals. The envelope of the combined signal can then be displayed on the DSO.

Using a Measurement Display

• Displacement readout: Although nothing is moving for an interferometer sensor like the gas cell, the displacement readout and graph will correlate with a change in the index of refraction of what's inside, which can thus be calculated. In effect, the value will represent by how much the number of wavelengths that fit in Arm 1 is changing. This is subject to a scale factor, which will be affected by the "Interferometer Type" and "Wavelength" settings. It may be possible to finagle the scaling to display an index of refraction offset, but probably not an actual index of refraction. If the graph "Displacement Range" for the GUI is locked, then the display will appear similar to that of a scope, scrolling continuously right-to-left rather than sweeping repetitively from left-to-right.

• Firmware-only enhancements to generate analog output: The addition of some simple C code to generate a voltage proportional to displacement and a means to reset it would result in a display on a DSO similar to the Homodyne version, above. Any additional circuitry can be wired to the pad array on the PCBs or installed on a separate solderless breadboard or prototyping board Two simple additions are required:
  • Analog voltage: This is easily generated in C code based on the value of DISP(lacement) in the firmware. It is sent one of the microprocessor's WPM-capable output pins. A simple RC filter can be used to remove the PWM switching noise.

  • Reset to zero voltage: This is necessary because the µMD GUI "Reset Display" button only zeros the value used by the GUI and there is currently no means of sending information from the µMD GUI to the firmware - and that's not likely to change. An SPST momentary switch can be added between an IO pin and GND. Firmware would simply look for a LOW and clear the REF and MEAS counters. If this was done at the same time the "Reset Display" button was "pressed" on the GUI, they would track.

Source code for the GUI can be made available, but changing anything is highly discouraged! Hacking the firmware for µMD0 (Homodyne only) is straightforward. For µMD2 (Homodyne and Heterodyne versions), it is more dicey but shouldn't explode the Universe if care is taken to limit additions to the code just before the values are sent to the USB port - and not messing with any values that impact what is sent! However, there is no - zero - tech support for unauthorized changes, even to a single bit in a comment fields. I've lost too many brain cells in the creation of the GUI and firmware to want to revisit them. And after overloading my brain, many of the relevant memories have been off-loaded to long term storage. ;( :-)

Controlling the PLD

Before proceeding with any of these, please read the sections on Monitoring the PLD since changing the PLD without measuring it is like a tree falling in a forest with no one around. ;-)

Thorlabs or Ball Bearing Rail

Micrometer Linear Stage

The micrometer stages permit µm-scale displacement to be manually set, though as a practical matter doing *anything* at that resolution manually is a challenge. And even just touching the knob introduces a displacement shift and vibrations which are easily detectable and jitter in the fringe display or measured displacement.

Motion Control Platform

It would replace the Thorlabs or ball bearing rail and carrier with a stepper motor-driven platform and an Arduino-compatible board to drive it. Among other things, this will enable the characterization of non-linearity, backlash and displacement scaling error. These are critical parameters in CNC machine calibration. In addition, controlling Arm 2 motion electronically should introduce far lower vibration than actually touching the micrometer knob.

However, these are not super precision rigs. Their selection was based on the small size to fit the interferometer setup - and cost. So they may have significant free-play, backlash, and other errors. But that's exactly the sort of thing that experiments can aim to characterize. The main issue is that they use a lead screw (not a ball screw) into a threaded brass block. And aside from a setup like that being inherently imprecise, the machining is sometimes, well, not so great as well. :(

(Doing something similar using a servo motor rather than stepper motor is possible and theoretically superior, but likely much more expensive and complex.)

The basic components are:

• Mini stepper motor driven platform: The Arm 2 reflector and micrometer stage will be mounted on this in place of the ball bearing rail.

  There are currently 2 variations designated Type 1 and Type 2. Type 1 is smaller (height and width), but is not as precise as Type 2, which is also slightly more expensive. But both are quite compact. Other types could be used but they quickly become massive and just a wee bit overkill to move a mirror mount. ;( :-)

  The default models that have been selected have a travel length of either 100 or 200 mm. The screw pitch for Type 1 is 1 mm resulting in 200 full steps per 1 mm of linear movement. For Type 2 the pitch may be 2, 4, 6, or 8 mm resulting in 100, 50, 37.5, or 25 full steps per 1 mm of linear movement. The default will either be 2 mm or 8 mm (my choice).

• Stepper motor controller: The default will be a a STEPPERONLINE DM320T NEMA micro-step controller. It is capable of generating up to 32 micro-steps per for each basic motor step, though how linear they will be is not known. For the coarsest setting, 1 input pulse moves the platform by 1/2 the basic step or 1 mm / 400 = 2.5 µm. When set for 32 micro-steps, the resolution becomes 0.078125 µm. Funny how that works out to be close to 1/8th of 633 nm. ;-) Of course, backlash and free play in the system means achieving anything close to this is probably a total fantasy, but characterizing the various errors is part of the goal here using the interferometer as the reference.

  For more of a DIY approach, an Arduino A3967 EasyDriver or something similar may be used, but their maximum current and micro-step resolution are usually much more limited. An even more rudimentary approach using an H-bridge driver shield and suitable Arduino sketch could also be used. But doing something like either of these is not really worth it unless what you really want to do is explore the finer points of motor control rather than interferometers. ;-)

• Arduino or PC software to issue commands to the motor controller. These would mostly be in the form of direction and pulses to move the stepper motor. For stand-alone control, an Arduino NANO or UNO would probably have enough capabilities. Or the motion control functions could be integrated into µMD2 since the Teensy 4.0 should have more than enough horsepower.

• A PC or MAC (GASP) Graphical User Interface (GUI) to implement the top-level motion control and comparison via data from µMD2 of the error between the issued commands and actual displacement.

The setup below was used for the initial implementation and is similar to what will be included in the kit, (currently an option). This rail with a travel of 200 mm would fit easily fully on the "Extended" version of the interferometer. In fact, rails with a 250 mm travel would also fit and they may be available in the future. For the shorter setups, a rail with a travel of 100 mm would be used. Putting the motor on the left enables a micrometer stage with the knob on the end to be used without hitting the motor housing, which is at a greater height than the platform. However, to minimize the PLD for the homodyne setups, the motor should be on the right using a micrometer stage with side-mounted knob. For Heterodyne, the absolute PLD doesn't make any difference so there's no real need for the reflector to be able to move as close as possible to the interferometer.

Holes will need to be drilled or drilled and tapped on the breadboard to mate with the Metric hole spacing of the rail. BA-type hold-down clamps or even industrial strength double sticky tape would also work, if a bit clunky.

To mount the micrometer stage on the stepper platform, an adapter plate is needed unless one is willing to simply use glue. It's the same one used with the ball bearing rail, so that can be transferred if available. 5-Minute Epoxy would be satisfactory, and could be removed if necessary. (If installed on a V2.0 setup, the optical centerline increases by 1/2 inch so some parts change, mostly Thorlabs posts and spacers.)

The Atmega 328P Nano runs the firmware in the link, below. This is for testing only or if all that's desired is a manually-controlled motorized stage. ;-) It has no error checking so twiddling the knob faster than the motor can move so it just whines is very easy, and there are no limit switches so the flaform can smack into the end plates. But as can be seen, there are many extraneous useless LEDs. ;-) The DM320T controller is set for 400 pulses/mm and the lowest motor current of 0.3 A. Higher current will increase the maximum speed to some extent, but don't go above a sustained current of more than 0.6 A. CAUTION: I you decide to play with a rig like this, make sure the DC to the DM320T is clean and comes on quickly. Else the DM320T may get into a funky state which among other things can send lots of current through the motor, as well as not respond to drive pulses. At least, that's what appeared to have happened on more than one occasion when ramping up the input voltage manually. A 12 VDC wall adapter solved that. I don't know if the red Fault LED on the DM320T being off is sufficient to guarantee correct operation.

              

Test of 200 mm Travel MCP (left), on Extended Heterodyne Setup (Left-Center), Test of 100 mm Travel MCP (Right-Center), and on Basic Homodyne Setup (Right)

The photos show samples of the 100 and 200 mm travel MCPs along with typical installation on the optical breadboards. The electronic Solderless BreadBoard (SBB) with the Nano driving the 200 mm versions is my original rig for testing the quad decoder sketch; the one with the 100 mm versions is all that's needed and is what will be included in the kit. Feel free to add decorative LEDs. ;-) And yes, if you look closely, the tip of the micrometer is not touching the bracket in one of the pics. That's because the stage can be locked in place and wasn't unlocked for the photo. ;-)

The hardware is the easy part and a mock-up has been constructed as shown above. For V2.1, the motion control platform can be substituted for either the Thorlabs or ball bearing rail with virtually no changes to anything else. But for V2.0, it will not quite be a drop-in as the heights have increased by 1/2 inch. So some opto-mechanical components will need to change - mostly Thorlabs posts and spacers, but there would be negligible additional cost involved.

The software to make this educational and useful is the challenge.

An Arduino sketch for an initial version that simply enables the platform to move based on Quad-A-B encoder signals - NOT from the interferometer! - can be found at:

NEMA Motor Driver Demo Firmware: NEMA_Test1_fw_v01, implemented in an Atmega 328P Nano 3.0 (although almost Arduino-compatible could be used). The input is an encoder that provides TTL-compatible Quad-A-B signals; The outputs are Motor Direction (both polarities) and Motor Drive Pulse with a resolution of 1 step or microstep for each transition of either A or B (4 per cycle). This firmware is based on a general quad-pulse demo sketch so there's a bunch of other stuff in there, mainly to drive useless but pretty LEDs. ;-) While currently implemented on a solderless breadboard as shown, there will be an SG-µMC1 PCB in the near future as part of the motion control option providing this function - along with all the useless LEDs. ;-)

The SG-µMD0, SG-µMD1, or SG-µMD2 PCBs can have firmware added for motor control in addition to their other duties. Or another microcomputer could be used.

Suggestions for actual challenging applications (and coding volunteers) are welcome. :( :-)

Mechanical installation

1. Motion control platform: If purchased at the same time as the interferometer, holes will already have been drilled in the breadboard for the stepper motor rig, with the motor on the right. These locations will enable the maximum usable travel.

   Secure the platform on the breadboard using four 6-32 1/2" cap-head screws if the holes are tapped, or 6-32 3/4" screws and nuts if not. Take care to assure that it is aligned.

2. Adapter plate and stage: These are the same as used for the ball bearing rail. If transferring them, first remove the stage and then the flat head M3 screws securing it to the ball bearing rail carriage Reinstall in the opposite order on the MCP using the appropriate set of holes. To maximize range, have the plate extend on the side away from where the knob is located.

Electrical installation

Please refer to the connection diagram below.

Typical Connection Diagram for Motion Control Platform Testing

(To reverse direction, do only ONE of the following: Swap A and B, Mov DIR from D16 to D14, swap A+ and A-, swap B+ and B-.)

1. Motor wiring: These may vary. Two schemes are known:
   1. Red/blue, green/black.
   2. Yellow/blue, red/green.

   There could be others, so confirm with a multimeter. The coil resistance should be no more than a few ohms. Connect them to the A+/A- and B+/B- terminals of the DM320T. As a practical matter, it doesn't much matter which motor winding goes to A or B, or even the polarity as long as they aren't cross-connected (which should not damage anything but the motor won't move). Direction of motion is reversed by swapping any single pair of wires going to a specific winding. The DM320T connector blocks may be removed by grasping them and pulling straight ou if that is more convenient to secure the connections.

2. DM320T power: 12 to 15 VDC is recommended. For the homodyne systems, a 12 VDC power pack is included for this purpose. For the heterodyne system, DM320T power can come from the +15 VDC power supply for the laser.

3. Control signals: These are opto-isolated logic level signals. OPTO should be connected to +5 VDC from the Nano. DIR(ection) and PUL(se) can be driven from a Nano digital output as in the sample sketch. ENA(ble) defaults to active if left open. PUL triggers the motor to move on the negative edge of the electrical signal (logical rising edge).

4. DM320T DIP switches: For the NEMA11 stepper motor, set "PK Current" to 0.3 A. If there are problems with vibration, go to 0.5 A but not more. A higher current will simply cause the motor to get hotter even at idle. Initially set the "Pulse/rev" to 400 (which is the coarsest resolution). Rotating the encoder will result in fairly fast movement of the platform. For these settings, all the switches should be ON (down). You can play with the Pulse/rev settings later. ;-)

5. Arduino wiring: Carefully plug the Nano into the solderless breadboard with its USB connector at one end. Power can come from a USB port or USB charger cube, 5 VDC to 5V, or a 7-12 VDC power supply to VIN and GND. (A USB backup battery pack is not recommended as these may shut down after a few seconds due to insuffient current.) If using VIN, adding a zener diode, three-terminal regulator (e.g., 7808), or some other means to drop the voltage to between 7 and 8 V is recommended to reduce the power dissipation in the Nano's on-board regulator. For example, if using 12 VDC for the DM320T, a 1N4732 (4.7 V) will drop the input voltage to around 7.3 V. 5V from the Nano then powers the encoder and OPTO for the DM320T.

   Here are the required connections. Referring to the sample sketch, there are other pins that may be used to monitor up/down pulses and PWM signals to drive various flavored LEDs for your own amusement. ;-)

     Arduino Pin   Nano Pin   Name   Function
    ----------------------------------------------------------------------------
          D2           5       A     Encoder input
          D3           6       B     Encoder input
         D14          19      DIR-R  Direction reverse to DM320T*
         D15          20      Pul    Motor drive pulse (low) to DM320T
         D16          21      DIR-F  Direction forward to DM320T*
         GND          29      GND    Ground/common for optical encoder+
          5V          27      +5V    +5 VDC for DM320T OPTO and optical encoder
         VIN          30      VIN    +7 to +12 VDC input power (optional)
   

   * Only one of the Direction signals should go to the DM320T. They are complements of one-another. So if the platform moves the wrong way, use the other one.

   + Since the control signals to the DM320T are opto-isolated, the DM320T GND does not need to connect to the Arduino or encoder GND.

6. Encoder wiring: The typical wire colors/functions are:

      Color   Function
    -------------------------------------
       Red    Vcc (+5 to +24 VDC)
      Black   GND/Common
      Green   Output A (open collector)
      White   Output B (open collector)
   

   Double check with the labeling on the encoder. Tin the wire ends or solder them to #24 AWG solid wire to insert into the SBB. The required pullup resistors are provided by the Nano so no other components are required.

   The encoder can also be used to drive µMD0 or µMD2 with Homodyne firmware. For that, pullups will need to be added to the A and B signals and a reference voltage will be required on the A- and B- inputs at about 1/2 the signal swing.

When power is applied to the DM320T, the green LED should come on but NOT the red one. If it does, check connections and that power is clean and being applied quickly (not ramping up). The motor should not get more than slightly warm even after being on for a while at the 0.3 or 0.5 A current setting. The motor may make a hissing noise when stationary due to the DM320T's drive. I assume it is some sort of dither to overcome stiction (or something). It is not known yet how much the resulting vibrations will be seen by the interferometer. That may be an interesting thing to quantify. Ears and interferometers are very sensitive. ;-)

The typical rotary encoder included in the kits has a resolution of 400-600 cycles per 360 degree rotation. So turning the shaft should get the platform moving at a reasonable clip and it is easily possible to exceed the pulse rate where the motor responds. In that case, the motor will stall and whine with the platform remaining stationary. This is a mechanical limitation, not the Nano firmware. The lead screw has a small pitch (1 mm) which is desirable to maximize resolution for interferometer testing. But it also means the motor has to exert itself to produce fast movement. At the DP320T's coarsest Pulse/rev setting, the maximum platform speed is about 1 cm/second corresponding to 6,000 steps/second, or an encoder rotation rate of 2.5 revs/second. (6,000 steps/second / (600 cycles/rev * 4 pulses/cycle). If the DM320T DIP switch Pulse/rev setting is increased, the rate of movement will decrease proportionally and the maximum (input) pulse rate before the motor stalls will increase proportionally. Mounting a massive knob or wheel (not included) on the encoder would enable the friction and imbalance of the shaft to be measured by the interferometer. ;-)

Voice Coil Linear Actuator

The mini loudspeaker 4 ohm woofer can move its cone a couple of mm with 1.5 V at 0.375 A. But for these tests, it only needs to move a few µm.

One of the interferometer configurations using planar mirrors is simpler to use for this since it's more difficult to attach a cube corner to the loudspeaker. The HSPMI is recommended. To use it:

1. The 1" diameter 1/4" thick "Speaker Mounting Disk" should be glued to the back of the loudspeaker with Epoxy. This will permit it to be installed in a KM100 mount just like a 1" diameter mirror.

2. One of the small mirrors should be attached to the front of the loudspeaker with the tiniest of tiny dots of Epoxy at the corners. as shown below. Position it to be as parallel as possible to the loudspeaker frame.

However, a cube corner can be atteched to the loudspeaker with the aid of a 1 inch ID x ~3/4 inch tall ring. This provides the needed support and can be glued to the loudspeaker cone. Then the Linear Interferometer and others requiring CCs in the moving arm can be used.

For seeing how its movement will affect the interferometer, the loudspeaker can be driven with a 1.5 V battery (not included) and series resistor. Even with a fairly high resistor value like 10K ohms, the mirror will move decent amount, probably much more than one fringe cycle. Or it can be driven through a resistor via the 10K ohm potentiometer and 10K ohm series resistor from a 9 V battery or 12 VDC power supply. One of these should be included in the kits.

       + o--------+
                  |
     Battery      /     10K
    or Power  10K \<---/\/\---> +
      Supply      /
                  \       Loudspeaker
                  |
       - o--------+-----------> -

Calculate the sensitivity of movement in nm with respect to loudspeaker voice coil current.

DO NOT connect the loudspeaker directly to the 12 VDC power supply or 9 V battery as they both may be damaged or destroyed. No more than 1.5 V should ever be applied to the speaker. This can be done using a series resistor between it and the power supply or battery or with the potentiometer and series resistor as shown above. Only a very small current will be needed to move the mirror enough to be readily detected and it won't be visible by eye.

An electronic function generator with simple buffer amplifier can also be used to drive the loudspeaker over a larger range as long as the voltage to the loudspeaker doesn't exceed around 1.5 V. For a small range of travel, a current limiting resistor is that would be needed. With a triangle wave and µMD0, it should be possible to demonstrate the linearity (or non-linearity) of the loudspeaker cone with respect to voltage.

There are all sorts of electronic function generators. For this application, something very basic like an antique Wavetek would be acceptable. Function generator kits are available on eBay for under $10. With minor modifications, those based on the XR2206 chip would be more than adequate. The modifications would be to extend the range down below 0.05 Hz by replacing the capacitor for the highest range with 100 µF and to provide a constant (0 Hz) output by replacing the capacitor for the next highest range with a jumper wire. Unfortunately, there is no offset adjustment but the amplitude knob will then change the output over a few tenths of a volt. Alternatively or in addition, and external amplitude control may be desirable since the kit knob does not go to 0. An offset adjust could be added at the same time. ;-)

The loudspeaker will also be sensitive as a microphone so monitoring the detector output on the scope should result in a fairly sensitive response to voice and music, though the frequency response will be poor due to the large mass of the mirror. Send that to the line input of a stereo system and listen to it on headphones. Why? The quality will be terrible but it will demonstrate possibly the most complex way of going from acoustic sound to electronic sound. ;-)

Why might any of these NOT behave as expected? Think of not only issues with the interferometer but other causes.

Piezo Transducer

The PZT beeper element in the kit is 27 mm in diameter with an active area of around 20 mm in diameter. It is what's called a "drum head" PZT because the surface moves in and out at its center when a voltage is applied. It can move a few µm with 15 V at essentially no current - it has some capacitance but an infinite resistance, which for slow movement is all that matters.

Since the PZT must be free to change shape so the center can move in and out, it must ONLY be secured around its perimeter. One approach is to attach it to the 1 inch mirror mount with 3 or 4 dabs of 5 Minute Epoxy. But almost any technique that doesn't interfere with the area of the disk beneath the white Piezo material should be satisfactory. Further as shown in the diagram above, the mirror must be attached only at its center with something like a small washer.

As with the loudspeaker, one of the interferometer configurations using planar mirrors must be used for this since it's not practical to attach a cube corner to the PZT. The HSPMI is recommended.

1. A small platform should be fashioned from a washer or something similar so that one of the small mirrors can be glued to the center ONLY of the PZT. This is necessary because a large mounting area may impede its movement.

2. The PZT can be mounted directly on one of the KM100s with three tiny dots of Epoxy around the edges, or taped in place. Take care attaching the wires as the solder connections are fragile. Putting a small amount of Epoxy over them (but only in their vicinity so as not to impede movement) would be useful.

3. A 9 V battery or the 12 VDC power supply can be used along with the 10K ohm potentiometer to vary its voltage:

          + o--------+
                     |
        Battery      /     10K
       or Power  10K \<---/\/\/\---> +
         Supply      /
                     \              PZT
                     |
          - o--------+-------------> -
   

   (The 10K ohm series resistor is not required for the PZT but using it makes the circuit identical and safe for the speaker.)

Calculate the sensitivity of mirror movement in nm with respect to PZT voltage.

As with the speaker, an electronic function generator can be used to drive the PZT. With a typical output voltage swing of 20 V peak-peak, the change in position will be several wavelengths. By using a triangle wave and µMD0, it should be possible to observe how good the linearity of movement of the PZT is with respect to voltage, though there will be fewer full cycles compared to the speaker.

Or it can driven from the loudspeaker output of an audio amplifier through a step-up transformer.

The PZT may be sensitive enough to act as a microphone as well.

Gas Cell Compensator

It may be done using any of the interferometer configurations, though the sensitivity will depend on which one is used.

The concept is that an increase in air pressure will change its index of refraction, and while this is totally invisible to the human eye, the interferometer should be able to easily detect it as a shift in the fringe signal. In fact many fringe cycles even for a space of a couple inches. With some simple calculations, it is possible to corelate the pressure reading on the gauge with the phase change of the fringe signal. If it's sealed well enough, even warming the gas cell by holding it tightly should result in a detectable fringe shift. However, doing that without introducing vibrations that totally swamp any change due to the expansion would be a challenge.

The Gas Cell Compensator (GCC) consists of a ~2" length of 1" OD Acrylic tube, a pair of planar windows sealed to the ends, the pressure bulb and gauge for a blood pressure cuff (sphygmomanometer), and some simple plumbing. It can mount on a Thorlabs post and post holder using a BA1S hold-down.

GCC Assembly:

The Acrylic tube will already be cut to length and drilled and tapped for the 10/32 hose barb and 8-32 set-screw to attach it to a Thorlabs post. The ends will have been ground to be close enough for government work. :) They don't need to be perfectly perpendicular to the tube or parallel to each-other. Nor do they need be polished - the rough cut surface is better for gluing. Only that they can seal to the windows.

The windows are 1-1/8" in diameter and made of either glass or Acrylic. The Acrylic will have paper protective brown paper on both sides.

1. If there is more than one hose barb with threads, one of them may have the threads shortened so as not to protrude inside the Acrylic cylinder. Partially thread it into its 10-32 hole. Mix the tiniest amount of 5 minute Epoxy and apply it to the exposed threads. Then rotate it clockwise until fully seated. Wipe off any excess Epoxy and allow time to cure.

2. Apply a small amount of the remaining Epoxy to the threads of a 8-32 1/2" set-screw. Intstall it in the 8-32 tapped hole so that no more 3/8" is exposed. Apply some more Epoxy at the threads where they meet the tube. (Since this one is vertical and can't interfere with the beams, some portion of it protruding inside the Acrylic tube is OK. Add an 8-32 nut as shown in the diagram for added strength with the Epoxy joining the nut to the cylinder.

3. Clean one window end one end of the 2" tube with alcohol if available. Soap and water is also acceptable. DO NOT use anything stronger. Make sure it is completely dry and dust-free by shining a bright light through it before proceeding. Repeat as needed.

4. Carefully place the tube on top of the cleaned window. Put a weight on top of it or use some other means to prevent it from moving accidentally.

5. Prepare a small amount of 5 minute Epoxy and apply the tiniest bead all around the outside of the joint between the 2" tube and window.

   Avoid getting any Epoxy inside the tube, especially on the windows, as much of their area may need to be unobstructed depending on the type of interferometer and/or whether the laser has a beam expander.

   If it gets messed up before curing, the Epoxy can be careully wiped off and then the glass and/or Acrylic can be cleaned with alcohol (but nothing stronger!). After curing, a single edge razor blade can be used to remove Epoxy, then cleaned with alcohol. Take care to avoid scratching the window(s).

6. Repeat the previous three steps for the other end. A cotton swab can still be used to clean the previously attached window if necessary. It's especially important that the inner surfaces of both windows be clean as they will be inaccessible once the second window is installed.

7. Wait an hour or so before proceeding to allow the Epoxy to fully cure.

8. Connect the hose barb on the tube with the pressure gauge and bulb using the rubber tubing. Test to confirm the connections are reasonably gas-tight and correct if necessary. For this experiment, a small leak may be preferred as it will allow for a controlled pressure decline which can be used to correlate with the fringe signal.

The photos show the Basic homodyne version with the GCC installed in Arm 1 of the interferometer. The mirror mount post holder is moved further out to make room for it. The arrengement is identical for the heterodyne setups except for the substitution of the laser and optical receiver.

For more details on the setup to view the signal, refer to the section: Monitoring the PLD.

     

This photo shows the complete setup with the prototype of the GCC and a scope trace of the photodetector out showing the GCC loosing presure some of its pressure over 20 seconds or so. Hands-off bleed-down is better for visualizing what's happening as any vibrations - which are inevitable during pump-up - will show in the display.

Closeup of GCC Assembly using a Piece of PVC Pipe (left), Overall Setup Showing the Fringe Signal as the Pressure Declines

Note: If doing this using one of the plane mirror interferometers, the Arm 1 mirror mount may need to face away from the PBSC (with the mirror installed backwards) as with the CCs in the LI to provide enough clearance for the GCC. Then adjust the Arm 2 mirror position so the PLD is 0.

The blood pressure gauge reads up to 300 mm/Hg (almost 6 psi), but there should be no need to go anywhere near the extreme hypertension region for these tests! :) 100 mm/Hg will be more than enough.

The gas cell can be mounted in either arm of the interferometer, though using Arm 1 is probably better as it has nothing else. It can be positioned so that either one or both beams (where present) pass through it. (How will this change the calculations?) Avoid aligning the gas cell so that the windows are perfectly perpendicular to the beam paths - angle it slightly so the reflections from the surfaces of the windows do not coincide with the main beams.

The Arm 1 and Arm 2 path lengths do not need to be the same so that at least is simpler than for the homodyne setup with unstabilized laser.

Fine tune the alignment of the interferometer to maximize signal amplitude. Close the bleeder valve and slowly pump up the bulb while watching the scope display and pressure gauge. The index of refraction, n, will be approximately equal to 1 + P * k. By measuring the number of cycles and partial cycles as the pressure is changed, it is possible to calculate k. Check it against a value found in a search. Why might it not be the same? Knowing k, an arbitrary pressure can be measured with the interferometer.

Based on the NIST Refractive Index of Air Calculator using Ciddor Equation, the index of refraction of air at 1 atm (760 mm/Hg), 20°C, and 50%RH, is 1.000271372. As an example, at a pressure above 1 atm of 100 mm/Hg, it is 1.00030715. What is the value of "k"? Over the 3 inches (76.2 mm) inside the GCC, the change in path length is approximately 2.73 µm or 4.31 full wavelengths at 633 nm. You can complete the calculations. ;-) Perform the test with 100 mm/Hg and your favorite interferometer configuration. Explain your results. What are the possible sources of error? Hint: What effect will reflections from the parallel surfaces of the windows have on transmission?

You might be wondering if it would be possible for the interferometer to act as a microphone using only the change in air pressure from sound waves in one arm. This could be done in principle, but the sensitivity would be extremely poor. In fact to get a detectable response due only to the air pressure variations would require sound levels similar to what might be found a few feet from a jet engine or directly in front of the loudspeaker array at a rock concert. Of course the entire interferometer would be vibrating (assuming it didn't totally disintegrate) and that would dominate any response. Original equipment human ears are extremely sensitive. ;-) See, for example: Engineering Toolbox: Sound Pressure.

Thermal Expansion

This shows how a change in temperature of an object undetectable by eye can produce a noticeable effect if in one arm of the interferometer. A glass block with two polished surfaces actually called a "compensator plate" is included in the kit, along with 1 or 2 power resistors to heat it.

It may be done using any of the interferometer configurations, though the sensitivity will depend on which one is used.

Thermal Assembly Assembly:

1. Thread the ~1x1-1/4x1/4" block with 8-32 stud into a Thorlabs post and install it in a post holder attached to the breadboard for convenience in mounting the other parts.

2. Use a drop of Epoxy to secure the 25 ohm 5 watt resistor to the block using one of its flat faces. Wait at least 15 minutes for the Epoxy to cure.

3. Use a drop of Epoxy to secure a long frosted surface of the compensator plate to the top (labeled side) of the power resistor. Wait at least 15 minutes for the Epoxy to cure.

4. This assembly can then be installed in Arm 1 of the interferometer using a Thorlabs 1.5" post, post holder, and BA1S Holddown. Angle the broad polished faces of the compensator plate very slightly to avoid back-reflections into the beam paths.

The 12 VDC power pack is used to do the heating. DO NOT use a 9 V battery, it won't last very long. The power into the resistor(s) is 12*12/R - 5.76 watts for the 25 ohm resistor.

 

The Coefficient of Thermal Expansion (CTE) for optical glass is around 8x10^-6/°C. (It varies slightly depending on the specific type, which is not known for the compensator block.) That means a 1 °C change in temperature will result in its length changing by 8 ppm (parts per million or 0.000008 x its length). Assume that the index of refraction of optical glass, n_g, is approximately 1.5. (Again not precisely known .) Calculate the expected number of full cycles from the detector for a 10 °C change in block temperature. Don't forget that it's the net change in PLD that matters.

Monitor the signal using your choice of method as the block heats or cools and use the results above to estimate the temperature of the glass block based on its length. Without actually knowing the temperature of the block throughout its volume, and knowing it actual CTE and n_g. it is not possible to be precise. That's OK.

The effect will not be as dramatic as with the GCC, above, but with care, should be easily detectable.

CAUTION: Do NOT leave the resistors plugged into the power pack continuously for too long as bad things may happen.

What else may be impacting the PLD change besides the block itself? For example, is there any detectable response to the heating if the block is rotated and/or offset so it just misses both beams?

Index of Refraction of Air

The index of refraction of air, n, varies by just under -1 ppm/°C. Or more precisely, according to the same NIST Web site, -9.517x10^-7. So heated air in one of the interferometer arms should change the path length due to its change in n.

This can be tested with the same Gas Cell Compensator assembly and interferometer configuration used for the air pressure measurements. Heat it with a blow dryer with the inlet port unplugged from the hose so that the pressure won't be affected. Do this well away from in the interferometer to avoid heating other components. Then quickly install it in Arm 1 and wait a few seconds for the vibrations to die away. Watch the signal as its temperature (and that of the air inside) declines. The heating could also be put into an oven on LOW. Just don't get it so hot that the Epoxy decomposes (which could be as low as 160 °C). :( :) While the sensitivity of n with respect to temperature compared to the effect on the glass block is around 1/10th as great, the GCC is ~5 times as long, so it should still produce an easily detectable signal.

Note that the expansion of the Acrylic cylinder itself is not a significant factor for these measurements. Why?

Gas Partial Pressure Measurement (Advanced)

Engineering Toolbox - Refractive Index for some common Liquids, Solids and Gases lists the values for many common substances.

Parts to do these tests are not included in the kits, but with a bit of resourcefulness, it should be possible to provide a suitable vessel either for a gas or liquid (with its vapors actually being what's measured). The Gas Cell Compensator, above, can also be used for this purpose if a way is devised to fill it with the test gas, for example by adding a second hose barb so there are entry and exit ports.

Inexpensive glass cuvettes with polished parallel sides would make suitable containers to introduce liquids, or with an improvised cover, gases, without the need for additional plumbing. Cuvettes are typically 1 cm wide but may be up to 5 cm or more in length. 1 cm is not enough width for both beams in an arm to fit and the length is desirable to maximize the sensitivity. So, two cuvettes side-by-side may be needed. Or a custom cuvette could be constructed from pieces of microscope slides sealed with RTV Silicone.

As a simple test, start with the cuvette(s) being empty and allow the interferometer to come to thermal equilibrium. Then carefully add some alcohol (at the same temperature and filled to below the level of the beam) and watch the fringe signal as vapors come off the liquid.

Mounting the cuvette(s) on the power resistor heater could allow the effects of temperature to be explored either with a liquid or gas. But interpreting the results may be more complex than it appears at first.

More on this is left as an exercise for the student's imagination. ;-)

Earthquake or Vibration Detector

For these experiments where one of the arms is of considerable length, the 6 mm beam (up to around 5 meters) or 9 mm beam (up to at least 10 meters) will need to be needed. A narrower beam would expand too much to be useful. But don't get carried away putting the reflector 25 meters away. Remote alignment will be a large challenge at the very least.

Start with perhaps 0.5 meter. Then extend it gradually, tweeking alignment at each step.

The remote reflector should preferably be mounted on a separate structure, not the same table as the rest of the setup. Another table or wall, for example. Or even suspended by springs or wires. Either arm can be extended but using Arm 2 with the linear stage may be simpler for mounting to a slab of wood or aluminum.

The use of µMD2 would be best as it would be possible to record data and (with some simple formatting in Excel) then display it like a seismograph. But just watching the REF/MEAS signal on the scope should provide some valuable insight into what's going on. Not only vibrations, but temperature changes and even air convection should be detectable.

Variations on the basic interferometers can be used to sense tilt (angle), lateral movement (straightness), deviation from right angle (squareness), and others. See Optics for Interferometers Using Two-Frequency Lasers. The same optics can be used with single frequency lasers.

Other Interferometer Configurations

Original Michelson Interferometer (BMI)

Without the cube corners and/or QWPs to separate the outgoing and return beams, everything will be jumbled together and there will be back-reflections directly to the laser. This won't cause damage but there could be serious instability in the resulting behavior. However, with a polarized HeNe, the effects may not be detectable either visually or even in the fringe signal. But for a random polarized laser, the result would be mode polarization switching, which could reak havoc with the signal.

No Retro-Reflector Plane Mirror Interferometer (NRRPMI)

The NRRPMI minimizes the required size of the optical components but with no retro-reflector, will require very precise in alignment during setup to maintain a usable signal with any significant movement. It is most similar to the original Michelson interferometer but the addition of the QWPs avoids (most) back-reflections to the laser.

Plane Mirror Interferometer (PMI)

However, it is asymmetric in terms of the beam paths. The reference (Arm 1) is single pass while the measurement (Arm 2) is double pass. To achieve a PLD close to 0 - required for the non-single mode laser - the positions of the two reflectors must differ significantly. Note how close the mirror on the stage is to the PBSC in the diagram - and that may not even be close enough for the paths to be equal!

For this reason, while the PMI is widely used, the HSPMI is recommended as the one to be built after the LI.

Single Beam Interferometer (SBI)

This is commonly used where space is tight since it doesn't require two offset beams. Normally, much smaller PBSC and CCs could and would be used. As with the NRRPMI, above, the use of the QWPs avoids most back-reflections to the laser.

Modified Linear Interferometer (MLI)

Double Pass Linear Interferometer (DPLI)

The signal level may be even lower than with the PMI since the CCs are slightly lossier than planar mirrors. A single pass through the silver coated CCs is ~86% resulting in a net transmission of ~74% since there are two passes. For the planar mirrors these values are closer to 90% and 81%, respectively. The CCs also mess slightly with the polarization. With linearly polarized light, the plane of polarization is rotated by ~±10 degrees depending on which set of internal surfaces are involved. What the effect is on the circularly polarized light is not known.

What would be the effect if only one of the QWPs were present in Arm 1 or Arm 2? Try it!

High Resolution Plane Mirror Interferometer (HRPMI)

The HRPMI is essentially an HSPMI in which instead of the return beam going to the detector, it is reflected back into the interferometer, but offset in position by an additional cube corner and traverses all of the optics a second time. So instead of 2 passes, it becomes 4 passes, and the losses will more than double reducing the signal level significantly. In principle, this could be extended to 6 or more passes using a similar approach, but as you will undoubtedly see if you're crazy enough to attempt to implement the HRPMI, it's already tough enough to align.

Drawing the detailed beam paths for the HRPMI showing how the photons are routed would be more work than it's worth. But since it is equivalent to the HP/Agilent/Keysight 10716A, a Web search will find information, but no need to bother Google, get it at HP/Agilent/Keysight 10716A High Resolution Plane Mirror Interferometer. However, the 10716A is normally used with a two frequency laser for heterodyne interferometry. So, wherever it refers to "ΔF", replace that with "ΔΦ" since we are changing the phase rather than the frequency.

The HRPMI setup requires some additional optics (another turning mirror and adjustable mount for an unmounted cube corner). The laser may also need to be positioned further to the left to make space. The only way to really test it without a measurement display would be with one of the methods of fine tuning path length - loudspeaker, PZT, air pressure, tmperature, etc. The micrometer stage will simply not have fine enough control to reliably detect a difference between X1 or X4. Thus the setup is shown with the loudspeaker.

Although drawn with all the beam paths in a plane, it is possible to implement it in 3-D as a 2x2 array within the PBSC by carefully offsetting the cube corners (as is done in the actual 10716A). Consider everything about the HRPMI to be a challenge. :-)

More Advanced Options

Other (non-Displacement) Interferometers

But many other parameters can be measured using interferometry including angle, straightness, and flatness. The layouts for these are not particularly complex but would require creativity in mounting and some optics and mechanical parts not included in the standard kits. See the information at Optics for Interferometers Using Two-Frequency Lasers.

DIY Single Frequency (SF) HeNe Laser

With an SF laser and the Quadrature Detector, the signal output from any of these interferometer configurations will provide complete displacement information that can be used with a measurement display or for closed-loop control. Systems using SF lasers are called homodyne interferometers.

SF HeNe lasers are available for order of $5,000 from a few laser companies (though this number has been dwindling). But fortunately, it is possible to construct one from readily available parts for less than 1/20th as much. The laser tube can be identical to the one used in the JDSU 1107 or 1108 random polarized HeNe laser head that comes with some of these interferometer kits. Adding a heater to control cavity length along with a simple controller using discrete analog components or an Arduino turns it into an SLM laser with performance similar to that of the high priced ones. If interested, a kit of parts along with detailed assembly instructions is available. For the manuals, go to Sam's Electronics and Laser Kit Information and Manuals.

Modifying the Output of a Two Frequency Laser for use in a Homodyne Interferometer

There are a couple of ways of doing this without actually rebuilding the laser itself. ;-) But unfortunately, both lose at least 1/2 the optical power. There's no easy way around that for a single axis as is the case here. (For two axes, a PBS could be used to separate the H and V polarized components with each one rotated to the desired polarization orientation or converted to circular polarization with waveplates. Each axis could then use 1/2 the total power and nothing would be inherently wasted.)

1. Linear polarizer at 45 degrees: This is simplest but strictly doesn't create a single frequency output. In fact, the output is similar to that of a multi-longitudinal mode laser like those used in the other kits, but the mode spacing is at the split frequency rather than close to the ~1 GHz of a short linearly polarized HeNe. If the detector is "blind" at or above the 1.5 to 2.0 MHz split frequency of the two frequency laser, this scheme will work fine. But that cannot be guaranteed even for the basic detection schemes used here. And the soon-to-be-released AB2 Quad-A-B preamp has a response exceeding 3 MHz. In fact, as should be recalled, an LP at 45 degrees is exactly the scheme used to combine the F1 and F2 components for the optical receiver. The only reason this isn't an issue with the other kits is that none of the electronics responds at ~1 GHz.

2. Linear polarizer at 0 or 90 degrees with Half WavePlate (HWP) to orient the beam at 45 degrees: This one is "pure". It blocks F1 or F2 (which one doesn't much matter) and then rotates the single frequency beam to 45 degrees.

In the interest of efficiency (since as noted, the theoretical best that can be done is to only cut the power in half), a small PBS cube is used rather than LP sheet. The HWP has only a 5-10 percent loss and will be attached to its output-side. (Or vice-versa as the operations are symmetric). The detailed mechanics are yet to be worked out but any special parts will be provided with the combined kit and the "Single Frequency Converter" will mount on the faceplate of the laser.

DIY Two Frequency Zeeman HeNe Frequency Laser

Where a random polarized HeNe laser tube meets certain requirements, applying an axial magnetic field will result in the normal single longitudinal mode splitting into two components that are left and right circular polarized, which are converted to orthogonal linear polarization with a QWP. It turns out that many of the HeNe laser tubes that used to be used in 100s of thousands of supermarket checkout barcode scanners satisfy these requirements, and the laser tube found in a $10,000 metrology laser is essentially the same. ;-) Stabilization is then similar to that of SF laser, above, and a kit is also available. Since the SF and TF lasers are very similar, it may be possible to use the same tube and controller. But while a homodyne interferometer can work using an unstabilized laser with restrictions on PLD, a two frequency Zeeman HeNe must be stabilized because the beat frequency actually appears for only a portion of mode sweep. So it must be locked to the center of that range.

Kits are available to construct stabilized Zeeman HeNe lasers that would perform well for use with the interferometer. They include a bare HeNe laser tube, power supply, heater, magnets, optional parts to construct an Arduino-based controller, and more. So if you really want to be able to claim that this was built from parts closer to stone tools and bear skins than HP/Agilent lasers, see the manual at: Stabilized Zeeman HeNe Laser Kit 1 with or without Arduino.

Heterodyne V2.1 Summary of Features and Options

Heterodyne V2.1 includes parts so that it can be configured as either the Compact or Extended configuration.

The only option for Heterodyne is the Motion Control Platform (MCP) in place of the MR15 ball bearing rail with MGN15-C carriage block.

Information

Required Tools, Equipment, and Supplies

• Hand tools:
  1. Hex driver set: 0.05 inch to 1/4 inch. The "ball-driver" variety are generally preferred if available.

  2. adjustable ("Crescent" wrench): One with a maximum opening of 3/4 to 1 inch should be acceptable.

  3. Screwdrivers: Small flat blade and Philips, jewelers screwdriver set.

  4. Needlenose pliers, flush-cutters, wire strippers:

  5. Assorted small metal files:

• Soldering equipment:
  1. Soldering iron: While it is possible to get away without soldering for the opto-mechanical construction and basic experiments, it will be required to assemble tne various PCBs - AB2, OR3, µMD0, µMD2, etc. A small temperature-controlled soldering iron is preferred. It doesn't need to be fancy or expensive. A usable setup can be purchased for under $25. A propane torch or Weller soldering gun is NOT suitable. ;-)

  2. Desoldering tool: This also doesn't need to be fancy or expensive. A hand desoldering pump like a full size "SoldaPullt™" is quite satisfactory. The smaller ones or other brands never have seemed to work as well. And yes, there will always be a need to remove components either because the wrong part was installed or was damaged.

  3. Solder: Rosin core solder, #20 typical. 60:40 tin:lead solder is close to the easiest to work with. There are lead-free solders now but they generally do not seem to work as well. DO NOT even think about using that old spool you found with your plumbing supplies!

  Just as important are the skills to do fine soldering. If you are not proficient, find someone who is or practice on scrap PCBs. Correcting sloppy work on a PCB is much more difficult than doing it right in the first place and may not even be possible.

• Supplies:
  1. Hookup wire: This is insulated wire for connecting various discrete components like the potentiometer used for the PZT or loudspeaker. #22-24 AWG stranded hookup wire is optimal.

  2. Male-male Arduino jumpers: These are typically used for attaching the solderless breadboard(s) to screw terminal blocks.

  3. 5-Minute Epoxy: Epoxy is for securing various parts like the gas cell end-plates and PZT mirrors. the 5-Minute variety is soft enough that it can be removed if necessary. DO NOT use Cyanoacrylate-based adhesives like Crazy Glue™! Aside from accidentally sticking your fingers together, it may ruin optics.

     TWo-part 5-Minute Epoxy is included in all the kits.

• Test equipment:
  1. Digital Multimeter (DMM): A DMM is always handy to have and one that costs under $20 will be more than adequate.

  2. Digital Storage Oscilloscope (DSO): While one can spend 10s of thousands of dollars on a DSO, the type needed here are under $150. Two suggestions are the Owon VDS1022I USB scope (around $110) or FNIRSI ADS1013D DSO (around $175). The Owon works in conjuction with a PC or MAC while the FNIRSI has a touch screen and is stand-alone. These are both color two channel scopes with a real bandwidth of 20-25 MHz. But if you're serious about electronics or photonics, consider something more capable. Spending under $500 will get you a scope with a bandwidth of 100-200 MHz and many features similar to those of the professional scopes. ;-)

     While it is possible to use an analog scope for some of the experiments, being able to capture a trace at 1 Hz or 10 MHz is nearly essential. Analog storage scopes were never very good and are almost as rare now as raw dinosaur eggs.

  3. Laser power meter: This can be constructed with one of the kit PDs using the biased photodiode circuit and a DMM.

Parts Identification

Polarization Control Optics

• Linear Polarizer (LP): A linear polarizer only passes light that is aligned with its polarization axis. More precisely, it obeys Malus's law whereby the optical power of the light passed is equal to:

  I_o = I * cos²(Θ_i)

  where:

  • I_o is the output power.
  • I is the input power.
  • Θ_i is the angle between the input and polarizer's axis.

  Where the input light has multiple polarized components with differing polarization orientations, the result is the sum of Malus's law applied to each one.

  One use of an LP in this interferometer is to combine the two orthogonally polarized components from the PBSC at 45 degrees so they line up and can interfere at the detector.

• Quarter WavePlate (QWP): A QWP is a part of the Circular Polarizer (CP) and are used in the quadrature-sin-cos detector. The effect of a QWP is summarized below. For the following, "θ" is the angle with respect to the fast axis of the waveplate.

            Input                      Output
    ------------------------------------------------
      Linear, θ = +45°            Right circular
      Linear, θ = -45°            Left circular
      Right circular              Linear, θ = -45°
      Left circular               Linear, θ = +45°
      Linear, θ not +/-45°        Elliptical
      Elliptical, θ = 0 or 90°    Linear at angle
  

  "Elliptical, θ = 0 or 90°" means its major and minor axes are aligned with the waveplate optic axes. This case can be easily decomposed into the vector sum of a linear component (which is unchanged) and a circular component which gets transformed into a linear component at 45°.

  QWPs are used in several of the interferometers. Their optical axes are always at ±45 degrees. (The handedness of the circular polarization will be swapped but that doesn't matter here - they are unmarked and can be mounted either way.) Thus when light polarized at 0 or 90 degrees passes through the QWP, it is converted to circular polarization. When that is reflected from a CC or plane mirror, the handedness flips and when it passes back through the QWP, the polarization axis is rotated 90 degrees. This allows the PBSC to redirect it resulting in the various beam paths required by the interferometers.

• Circular Polarizer (CP): The CP is a combination of an LP and QWP:
  

Some specific applications:

• For the detectors, the CP would be used like an LP since the output polarization doesn't matter to a photodiode.

• A CP may be used like an LP in front of a linear polarized laser (with its LP-side facing the laser) to convert it to circular polarization and to adjust the usable output power. However, the behavior of the interferometer will be subtly different than with linear polarization at 45 degrees. That will be something to analyze.

• A CP may be used like an LP in front of a random polarized laser (with its LP-side facing the laser) to convert it to circular polarization. (1) A single polarized component may be selected if lined up with it or (2) both of its linearly polarized components may be combined so it behaves like a linearly polarized laser with the same cavity length (though with reduced output power). As above, the behavior of the interferometer will be subtly different than with linear polarization at 45 degrees.

• A pair of CPs may also be used to select a single polarized component of a two frequency laser and orient it as desired. The first CP blocks one of the polarized components and converts the other to circular polarization. The second CP then takes that and produces a linearly polarized output aligned with its optical axis. The sticky sides of the two CPs will even be what gets stuck together after determining the correct orientations. ;-) This is the simplest and least expensive way to convert a two frequency laser to a single frequency laser. But there is a downside compared to using a true PBS and HWP or even some of the schemes: The output power will likely be cut by 50 percent or more (in addition to blocking the unwanted polarized component) due to losses from the two CPs.

• A CP and QWP may be used in a way similar to a pair of CPs. The CP blocks one of the polarized components and converts the other to circular polarization. The QWP then takes that and produces a linearly polarized output aligned with its optical axis. The benefit is that the losses will be lower - only from the CP and reflections from the QWP's surfaces if not AR-coated. So the output power could be nearly double.

And as a point of interest CPs are used in front of LCD displays to increase their contrast under ambient illumination: With the LP side toward the viewer, ambient light will pass through it and the QWP, and be reflected by the display itself. But when CP light is reflected, the "handedness" flips and the result is linear polarization at 90 degrees to what it was originally, and thus blocked by the LP. That is part of the reason your smart phone screen looks dark when nothing is displayed on it.

However, while circular polarizers intended for use with cameras are constructed as shown above, the "polarizing films" sold for smart phones and the like may or may not be true CPs. Some are just linear polarizers or linear polarizers coated with a weak neutral density filter. And it's not clear how one can select the proper type since the sellers are typically clueless. The only actual CPs consistent with the above diagram I've found so far were listed as something like: "New Backlit Screen Modify Part Polarizing film For GBA GBC GBA SP N_ES". Those listed as "LCD Polarizer Film Polarization film Polarized Light Film For ip.ccHHH" may also be CPs but with (1) the adhesive and non-adhesive sides swapped and (2) the axes of the LP and QWP rotated 45 degrees. They may also have a weak ND filter. Sometime a faint colored line can be seen which is at the orientation of the LP or 90 degrees from it. But that doesn't help with whether there is a QWP, though sometimes there are also a series of small circles, presumably to indicate CP, though they tend to be on the opposite side from the QWP. Those listed specifically for iPhones were of the simple LP type (usually with a faint line at 90 degrees to the LP axis parallel to the short side). Interestingly though, the protective film that normally gets discarded is birefringent and may even be a QWP. Which of course makes little sense. :( :) A further issue is that some of these may be diffuse, not trasparent. That doesn't make much difference for use on the PDs, but means they won't work elsewhere in the beam path. Confirm if possible. Confused yet? I sure am. :( :)

This same scheme is also used in what is sometimes called a "poor man's optical isolator", whose purpose is to minimize back-reflections from an optical setup into the laser, which may destabilize it or worse. The combination of a linear polarizer (or PBSC) and QWP acts as a "diode" for polarized light. It's called "poor man's" because it is much less expensive than a Faraday isolator, and adequate for many purposes. But for it to work well, any reflective surfaces in the optical setup must not mess with the polarizaton.

The piece of CP provided in these kits is intended for such an application. It can be cut into smaller pieces since they only need to be slightly larger than the beam, which even if expanded is only ~4 mm. As noted shown in the diagram above, the optical axes of the QWP are at 0 degrees with respect to the edges of the CP and the axis of the LP is at 45 degrees.

The CP comes with protective film on both sides which MUST be removed because it acts like some type of waveplate and messes with the polarization. The side of the CP with the QWP is sticky since that was intended to go against the screen. To prevent it from collecting dirt and fingerprints, the smaller pieces to be actually used in the detector(s) should be stuck to microscope cover slips (included) or glass windows. CAUTION: Microscope cover slips are thin and fragile - don't press too hard.

Adjusting the Micrometer Stage

• Generic stage: There are 4 setscrews located on the side of the stage. Depending on the version, these may be on either side. It may also be necessary to loosen screws on the top which secure the inner track(s). If there is a locking knob, some may be concealed under the lock-plate.

• Parker stage: There are 3 setscrews located on the side opposite the lock knob (if present) which may be concealed by a sticker.

A 1/16" hex driver may be used to tighten or loosen them the smallest amount which should not be more than 1/10th of a turn.

The micrometer can be lubricated with light grease if it seems rough or tight. The ball bearings for the stage itself should not need lubrication unless serious dust or other contamination has gotten into the tracks.

Adjusting the Motion Control Platform

1. Move the platform to the motor-end of the rail by rotating the lead screw manually or with power applied temporarily to the DM320T controller using the Arduino MCP test sketch and encoder. If the lead screw appears to bind there and is more difficult to rotate or the platform gets stuck, loosen the two screws securing the motor and adjust the lateral position of the motor so that the rotation requires the least torque.

2. Move the platform to the far end of the rail in a similar manner and repeat by adjusting the lateral position of the bearing end-plate.

3. Move the platform to approximately the center of travel.

4. Loosen the 4 screws securing the platform to the carriage and then set them so they are just snug.

5. Gently twist the platform while testing the axial free play and set it so the free play is just barely detectable. Tighten the 4 screws and confirm that it hasn't changed. If the free play is set to exactly zero, then the the lead screw may be too tight. This is basically compensating for imprecise machine work in cutting the threads.

6. Run the platform fron end-to-end using a low current setting of the DM320T (e.g., 0.3 or 0.5 A) to confirm it doesn't bind anywhere.

7. Repeat all these steps as necessary to optimize.

It still won't be a $20,000 rig but should be quite acceptable.

Removing 5 Minute Epoxy

Where the adhesive is accessible, a single-edge razor blade can often be used to slice it or get underneath and peel it off. But that may not be possible if the turning mirror were attached in the wrong location with bits of Epoxy underneath it. Or reusing the mirror stuck to the PZT. Attempting to pry the thin mirrors off would likely result in bits of glass.

The easiest method where the parts can withstand it is to use a common heat gun. The exact temperature at which the typical Devcon 5 Minute Epoxy decomposes is not known. The datasheet only states that the "Operating Temperature" range is -40 °F to +200 °F but not what happens at +201 °F. ;-) It may exceed +500 °F. However, from experience, a sufficient temperature can be reached in a couple minutes to soften the Epoxy without damaging parts made of glass or metal. However, some plastics might melt. ;( As the critical temperature is reached, the Epoxy will soften, so work over a non-flammable surface that won't damage the part to be removed and gently prod it with a popsicle stick or something similar until it falls off. After cooling, the residue can be removed by scraping or with isopropyl alcohol. Since the Epoxy at least partially decomposes, this method should not be used for repositioning, only removal. Clean the two surfaces and then start fresh. ;)

CAUTION: Don't try this with the gas cell or loudspeaker, they would likely get damaged.

Sam's Educational Michelson Heterodyne Interferometer Kit Parts List

Differences between the Compact, Mid-Size, and Extended Versions (which are all V2.1) will be noted. They are identical except for the back mounting of the laser and the length of the ball bearing rail.

Parts in Heterodyne Kits ONLY

 Quantity Description               
------------------------------------------------------------------------------
 Baseplate/Optical breadboard:

    1x Aluminum optical breadboard 8x24" (BASE Lab Tools SAB0824).

 Laser Assembly: 

    1x Laser consisting of HP/Agilent/Keysight 5517 laser head (transducer).
       These are custom modified to be suitable for this
       kit with a split (REF) frequency betwee 1.5 and 2.0 MHz (most similar
       to a 5517A except in a small case.  The output power is greater than
       400 µW.

    1x Laser head connector.

    1x Set of 3 HP/Agilent "feet" for the laser head.  Two of these have been
       modified with extended slots for the sides of the laser; the other is
       unmodified for the back of the laser.

    3x Laser Mounts each consisting of:

      1x 1/4-20 2" setscrews.
      3x 1/4-20 nut.
      3x 1/4" washer.

    1x Extension Plate (1-1/2" x 1/2" x 9-12") with 2x 1/4-20 5/8" cap-head
       screw.  (Used for Mid-Size and Extended Versions only.)

    1x Thorlabs BA1S for back of laser with 1/4-20 1/2" cap-head screw and
       1/4" nut.

    1x Set of ±15 VDC power packs / wall adapters.  Their input voltage
       range is 100-240 VAC to function Worldwide, though the plugs will
       either be US or sockets for line cords.  At least one will have a
       current rating >3 A.
 
  Optical Receiver 3 (OR3) kit.

  Micro Measurement Display 2 (µMD2) kit.

Parts Common to both Heterodyne and Homodyne Kits

  Biased Photodiode Detector (BPD1):

    1x 1/2x1/2" piece of CP polarizer sheet (may be used as CP or LP with
       LP axis at 45 degrees), to be cut to size.

    1x 1/2x1/2" piece of LP polarizer sheet (LP axis XY), cut to size.
 
    1x BPD1 PCB with 8-32 1/2" cap-head screw and 1/4" #8 spacer.
    1x Silicon photodiodes + spare.
    1x 2 pin male to female socket strip for photodiode.
    1x 1K ohm resistor (PD protection).
    1x 0.1 µF capacitor.
    1X Load resistors, 2-3K ohms typical, or 10K trim-pot
    1x 4 pole screw terminal block.
    1x Solderless breadboard with adhesive back, 170 tie points.
    1x Set of jumper wires, etc.

  Common to BPD1, OR3, and AB2:

    1x Thorlabs PH2 post holder.
    1x Thorlabs TR3 post.

    1x BA1 or BA1S with 1/4-20 3/8" cap-head screw to attach to PH2 and
       1/4-20 1/2" cap-head setscrew to attach to base. 

  Interferometer Assembly. (HP part numbers are used here; The actual parts
   may be the physically and functionally equivalent from Excel or Zygo.)

    1x HP/Agilent 10702A or 10706A PBS cube in frame with 4x 4-40 x 1-3/4"
       cap-head screws.

    1x HP/Agilent 10703A Cube Corner or mounted equivalent with 2 4-40 cap
       head screws.

    2x HP/Agilent 10722A Quarter WavePlate or custom equivalent each with two
       4-40 cap-head screws.

    1x Turning mirror (Approximately 1/2" x 1"). 

    1x Turning Mirror Bracket V2.0 with 4-40 x 5/16" cap-head screw and washer.

    1x PBS Mount Adapter Plate V2.1 with 2x Thorlabs TR3 post, 2x 3/4" spacer,
       and 2x 8-32 x 1" or 1-1/8" cap-head screw.

  Arm 1:

    1x Thorlabs KM100 or Newport U100 mirror mount with 3/8" 8-32 cap-head
        screw to secure it to post.

    1x 1" bare Cube Corner.
    1x 1" diameter planar mirror.
    1x Thorlabs PH2 post holder.
    1x Thorlabs TR2 post.

    1x BA1 or BA1S with 1/4-20 3/8" cap-head screw to attach to PH2 and
       1/4-20 1/2" cap-head sscrew to attach to base.

  Arm 2:

    1x Thorlabs KM100 or Newport U100 mirror mount with 3/8" 8-32 cap
       screw to secure it to post.

    1x 1" bare Cube Corner.
    1x 1" diameter planar mirror.
    1x Thorlabs PH2 post holder (rail or slide) and/or PH1 (MCPs).
    1x Thorlabs TR1 post (rail or slide or MCP Type 1) or TR075 (MCP Type 2).

    1x Generic 40x40 mm linear stage, end or side micrometer adjuster,
       maximum of 20 mm thick.  A 4-40 or M3 screw into a tapped hole is
       used to secure the post holder the top of the stage.

  Thorlabs rail assembly (Homodyne only):

    1x Thorlabs RC1 carrier with screw and washer to secure it to the
       bottom of the stage.

    1x Thorlabs RLA0600 6 inch rail (Basic or Deluxe version) or RLA1200
       12 inch rail (Deluxe+ version).

  Ball bearing rail assembly (Heterodyne):

    1x MGN15C carriage block.

    1x MGN15 stage adapter plate with 4x 4-40-3/16" cap-head screws to
       secure generic stage to plate and 4x M3-8 flathead screws to
       secure plate to MGN15C carriage block.
 
    1x Locking bracket with 2x M2.5x16 mm screws to secure bracket to
       MGN15C carriage block and 4-40 1/2" Nylon thumbscrew.

    1x Rail:

       1x MR15-150 150 mm rail with 2x 6-32 3/8" cap-head screws to secure
          rail to breadboard and rubber bumpers to be used as end-stops.
          Breadboard requires addition of two 6-32 tapped holes. (Compact
          version.)

       1x MR15-200 200 mm rail with 2x 6-32 7/8" cap-head screws and nuts to
          secure rail to breadboard.  The screws with spacers also can act
          as the end-stops (Mid-Size version.)

       1x MR15-300 300 mm rail with 2x 6-32 7/8" cap-head screws and nuts to
          secure rail to breadboard.  The screws with spacers also can act
          as the end-stops (Extended version.)

  Motion Control Platform (Option):

    1x Mini-stepper driven platform, 100 mm, 150 mm or 200 mm travel, with
       4x 4-40 1/2" cap head screws or 4x 4-40 3/4" cap-head screws
       and nuts to secure it to the breadboard.  Each of these will nearly
       entirely fit on the breadboard for the Compact, Mid-Size,
       and Extended Versions, respectively.  The hole pattern may differ
       for the Type 1 and Type 2 MCPs.

    1x MCP adapter plate (if not already present with the generic stage, they
       are the same.

    1x STEPPERONLINE DM320T micro-step stepper motor controller.

    1x Atmega 328P Nano 3.0 microcomputer board with solderless breadboard.

    1x Optical high resolution rotary encoder to move the platform via
       the Nano at least for testing.

  Voice Coil Actuator:

    1x 1-1/2" to 2" loudspeaker. 
    1x 1" D x 1/4" T aluminum or Acrylic Speaker mounting Disk.
    1x Speaker mirror (Approximately 1/2"x 1", may be same as turning mirror).
    1x Optional: Cube corner mounting ring.

  Piezo Transducer (Deluxe):

    1x 27 mm PZT beeper element.
    1x #2 washer to use as spacer.
    1x PZT mirror (Approximately 1/2"x 1", similar to turning mirror).

  Gas Cell Compensator (Air Pressure and Temperature):
    
    1x 1" OD, 7/8" ID, 2" L Acrylic tube.
    2x 1-1/8" D glass or Acrylic window.
    1x 10-32 to hose barb adapter.
    1x 8-32 3/8" or 1/2" set-screw.
    1x Blood pressure bulb with valve.
    1x Blood pressure gauge.
    1x 3-3.5 mm (or 1/8") ID Rubbor tubing to connect.
    1x Hose barb "T" for 3 to 4 mm tubing.

  Thermal Expansion:

    1x  ~1x1x2 cm compensator plate or other glass block with polished sides.
    1x  25 ohm, 10 W power resistor.
    1x  12 VDC 1 A power pack (PD bias, voice coil, PZT, thermal expansion).
    1x  Screw terminal to 5.5/2.1 female barrel connector adapter.
    1x  ~1" x 1-1/4" x 1/4" Delrin "chip" with 8-32 set-screw.

  Parts common to Gas Cell Compensator and Thermal Expansion:

    1x Thorlabs PH2 post holder.
    1x Thorlabs TR3 post.

    1x BA1 or BA1S with 1/4-20 3/8" cap-head screw to attach to PH2 and 1/4-20
       1/2" cap-head setscrew to attach to base. 

    1x Two part 5 minute Epoxy.

-- end V2.09 --