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

Assembly and Experiments

Version 2.04 (9-Jan-2022)

Copyright © 1994-2022
Sam Goldwasser
--- All Rights Reserved ---

For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.

Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:
  1. This notice is included in its entirety at the beginning.
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Table of Contents


Author and Copyright

Author: Samuel M. Goldwasser

For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.

Copyright © 1994-2020
All Rights Reserved

Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:

1. This notice is included in its entirety at the beginning.
2. There is no charge except to cover the costs of copying.


The information in this document is intended for use in hobbyist, experimental, research, and other applications where a bug in the hardware, firmware, or software, will not have a significant impact on the future of the Universe or anything else. We will not be responsible for any consequences of such bugs including but not limited to damage to the $100,000,000 wafer FAB that was purchased on eBay for $1.98 + shipping, financial loss from the waste of 28 spools of ABS due to the office 3-D printer fabricating a part with random dimensions due to loss of lock, or bruising to your pet's ego from any number of causes directly or indirectly related to the implementation and use of this system. ;-)


The Michelson Interferometer experimental setups V1.0 and V1.5 were originally developed for Engineering student projects at Swarthmore College, Pennsylvania.


The only safety issues with the experiments to be performed using this kit are with respect to the low power Helium-Neon (HeNe) laser. Sure, you could drop the breadboard on your foot, but that's outside our control. :( :-)

Even though this laser is not likely to cause any harm, one should always take laser safety seriously. Someday you may be working with one that is truly dangerous.

  • Back to Sam's Educational Michelson Homodyne Interferometer Project Manual Table of Contents.


    The Michelson interferometer is one of the earliest and simplest to be developed, but also likely the most widely used configuration in a variety of applications including metrology (precision measurement). The type found in most introductory textbooks use single frequency or unstabilized lasers in what are known as "homodyne" interferometers and those are the subject of this manual. Another version uses a special two frequency laser to implement the "heterodyne" interferometer, which are more likely to be found in high performance commercial applications like semiconductor wafer foundaries and are the subject of the companion document: Sam's Educational Michelson Heterodyne Interferometer Project Manual V2.1.

    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 how the characteristics of the laser impact performance. Various enhancements are also described for both the laser and detector, as well as 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.

  • Back to Sam's Educational Michelson Homodyne Interferometer Project Manual Table of Contents.


    IMPORTANT: This manual applies to version 2.1 of Sam's Educational Michelson Homodyne Interferometer Kit. Here are links to all of them:

    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 the Basic version configured for the LI. There are minor variations in the actual kits depending on version and whether they are the "Basic", "Deluxe", or "Deluxe+" versions, though this layout will work for all of them. Differences will be noted below. Where the total path length and PLD is small, no beam expander is needed for the laser as in the Basic version. This may result in a larger signal since the entire beam hits the detector. But the smaller beam makes it more sensitive to alignment.

    Typical Linear Interferometer Setup with Quadrature Decoder and µMD0

    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. (Though nothing precludes the observation of these.

    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 of between 0.4 and just over 1 mW. The basic detector is a biased photodiode connected to a dual channel digital oscilloscope. Variations and enhancements to these will be offered as options.

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

    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:

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

    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:

    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 V2.1.

    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 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.

    Some of the photos here 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)".

  • Back to Sam's Educational Michelson Homodyne Interferometer Project Manual Table of Contents.

    Interferometers for Metrology Applications

  • Back to Sam's Educational Michelson Homodyne Interferometer Project Manual Table of Contents.

    Detectors - Optical to Electrical Conversion

    With the emphasis on the signal behavior of interferometers, the conversion from light to electrical is critical - but very simple to implement. To be able to view or do anything with the output of the interferometer requires some means of converting light to an electrical signal. The type of detector use here is called a "biased photodiode" and is essentially a current source with an output proportional to incident optical power.

    Biased Photodiode (BPD1)

    A silicon photodiode (PD) when reverse biased by a positive DC voltage (battery or power supply) allows a current (designated Ipd) to flow with a sensitivity measured in amps / watt (A/W) or for our purposes, mA/mW of incident laser power. The sensitivity for silicon at 633 nm is typically between 0.3 and 0.4 mA/mW and linear up to several mW for the types of PDs used here. This relationship holds even when a load resistor R-Load is installed between the PD and circuit common (negative of the bias supply), resulting in an output voltage that is linear with respect to incident laser power based on Ipd * R-Load. For example, with a laser power of 0.5 mW, a PD sensitivity of 0.35 mA/mW, and R-Load of 10K ohms, Vo would be 1.75 V.

    The most basic circuit is shown below:

       +---------|<|-------+-------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). Actual usable detectors are almost identical to this simple circuit.

    Single Channel Detectors

    Here is a widely used commercial biased photodiode detector and a version used in the kits:

    Testing of either detector can be done using the laser or even a flashlight to confirm sensitivity to light. However, even a super-bright flashlight will likely result in only a small signal compared to the laser.

    Dual Channel Detectors

    These only use the custom detector. It would be silly, space consuming - and expensive - to require two DET110s! So, for two channels, all parts except for the power source and protection resistor are replicated:

           R-Protect    PD1      Yellow
       +-----/\/\----+--|<|---<<-----------------------+----------o Scope Channel 1
       |             |                                 |
       |             |  PD2      Blue                  |
       |             +--|<|---<<---------------------------+------o Scope Channel 2
       |                                               |   |
       |                                               /   /
       |                                       R-Load1 \   \ R-Load2
       |                                               /   /
       |                                DC Power       \   \
       |                          Red    +| |-   Black |   |
       +-----------------------<<---------||||---------+---+------o Scope Ground
                                          | |
       |<--- SBB or QDx PCB --->|<----- Scope / Power Wiring ------>

    PD Pins: Facing Front of PD with Legs Down: Anode on left, Cathode on right.

    R-Protect: 250-1K ohms typical. R-Load: 10K-1M ohms typical.

    The same PCBs may be used for this circuit.

    What is done with the light before it hits the photodiodes is the interesting part, which will be described where relevant. ;-)

    Dual Channel Quad A B Preamp (AB2)

    The problem with the simple circuits above is that there is no amplification. So to achieve both a usable signal level and reasonable bandwidth is not possible. By using a load resistor with a high enough value, it the signal level will be acceptable but then the badnwidth may as low as a few kHz. While this is satisfactory for initial testing and many of the experiments, greater bandwidth will eventually be desired for the displacement readout.

    The purpose of the Quad-A-B Preamp (henceforth referred to as QAB2 or simply AB2) is to provide a simple solution that accepts photodiode inputs and generates differential RS422 A and B signals that can be input to µMD0, µMD1, µMD2, or another compatible displacement measuring system.

    QAB2 is on a 1.6 inch by 2.25 inch PCB and runs on 12 to 15 VDC. (The PCB itself is called SG-AB2.) The optical input is a beam up to ~3 mm in diameter (using the default photodiode) with an optical power from <25 µW to >1 mW. QAB2 has >3 MHz bandwidth (full cycle) which is more than adequate for systems using the kit lasers as well as for many real applications. With a Linear Interferometer which has a full cycle of ~316 nm, the slew rate can be greater than 1 meter per second, which is a fairly nutty velocity. ;-). 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. ;-)

    Populated AB2 V1.00 PCB

    Although shown with photodiodes plugged into the PCB, in actual use, they would probably be on the detector PCB with short twisted wire cables to connect them to the SG-AB2 PCB.

    Parts to construct AB2 will eventually be included as part of the homodyne setups. More information on AB2 including complete "Heathkit™-style" assembly instructions may be found at Quad-A-B Preamp 2 (QAB2) Assembly and Operation Manual.

  • Back to Sam's Educational Michelson Homodyne Interferometer Project Manual Table of Contents.

    Linear Interferometer

    Although HP called the combination of the PBSC and reference CC attached to it, the "Linear Interferometer", the term "LI" here will refer to the entire setup. And to enable the PLD to be set to zero, the CC normally attached directly to the PBSC will be mounted a few inches away.

    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

    The diagram below shows the general arrangement of the laser, beam splitter, cube corners, and detector.

    Linear Interferometer Diagram and Suggested 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 detector but are otherwise similar. Therefore the LI setup will be described in more detail.

    A variety of mounting schemes are used:

    Setting the Heights

    The following diagram shows the relationships of the various mounts for setting the heights during assembly. This diagram applies to all V2.1 setups. 4-1/4" places the laser in the center of the PBS vertically:

    Required Optical Components and their Suggested Heights.

    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 the Deluxe version configured for the HSPMI:

    Deluxe Version Configured for the High Stability Plane Mirror Interferometer with µMD0

    The Deluxe+ version would be similar except for the larger breadboard and longer rail; The Basic version does not include the beam expander, mounted CC, QWPs, or planar mirrors.

    Assembly and Alignment of the Linear Interferometer

    The following procedure may be used to install each of the parts and then get to the point of "first signal" using the oscilloscope. The procedure for the other interferometers will be virtually identical in most respects, but may be trickier depending on the type.

    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.

    It is also assumed that the laser is linearly polarized. Slight changes are required if it is random polarized.

    1. Laser mount posts: Attach two 3 inch posts to the breadboard using 1/4-20 set screws.

      If the laser head has a beam expander, or it is anticipated that one will be added later, it is recommended that the posts be mounts 1 hole to the left of where the are in the photo.

      Note: To assure that there are ample threads engaged in both parts here and in subsequent steps with set-screws, the set-screw should be installed approximately half-way into the baseplate or mounting plate and then a thin tool or even the edge of a metal plate or stiff cardboard can be used to keep the set-screw from turning as the post or post holder is threaded onto it before tightening.

    2. Laser mount rings: Secure through to the top of the posts with 8-32 1/2 inch cap-head screws. If the mounting hole in the rings is threaded, there should be plastic washers to go between the rings and the spacers to provide compliance so the rings can be tightened with the correct alignment.

    3. Laser head: These steps secure and align the laser head cylinder.

      • Install the Nylon thumbscrews in the ring mounts if not already present. If there are two different lengths of thumbscrews (e.g., 1 and 1-1/4 inch), the longer ones should be in the front side of the rings. If the thumbscrews are much longer than needed, they can be trimmed. However, since not all interferometer configurations have the laser head in the same position, make sure not to cut them too short!

      • Slip the laser head into the rings. Initially set it to approximately centered horizontally and vertically. Then shift it 1/4 inch toward the back of the baseplate.

      • Plug the big white male "Alden" connector of the laser head into the female Alden connector of the power supply. Make sure it is seated fully, usually against the shoulder with no part of the prongs visible.

        • If the power supply is "lab style" - a chessy plastic box with keylock switch and power indicator - then it plugs into the AC line. An IEC type line cord will be included if in the USA; you will have to provide the approriate like cord if elsewhere. If there is a voltage switch on the back of the supply, confirm that it is set corretly for your location. If set wrong, the power supply may be damaged.

        • If the power supply is a bare "brick", it requires 12, 15, or 24 VDC from a matching wall adapter that should be included. Confirm that the voltage ratings match. There may be more than one wall adapter in the kit that appear similar. Check the labels! Using the wrong one may damage the brick and/or wall adapter. The wiring is probably already done via a barrel to screw terminal adapter, but confirm that the wires are secure and attached to the correct terminals. Positive is red and negative is black. If there is a yellow wire, it should be attached to the black wire and thus negative as well.

      • Power up the laser by turning the keylock switch or plugging in the wall adapter. It may take a few seconds or even up to a minute to light. If it doesn't light in a reaosnable length of time, check the wiring. Shining a flashlight in the front of the laser will sometimes help it along. DO NOT stare into the beam with your remaining good eye. :-)

      • Confirm that the laser is polarized: The model number should be 1107P, 1108P, or 05-LHP-211. There should be an alignment mark near the front of the laser. If not or just to confirm, rotate an LP or LP/CP in front of the laser. If the laser is polarized, there will be orientations where virtually no light gets through at all times. The polarization axis is then at 90 degrees to this. Add an alignment mark if there is none. (If it is random polarized, this orientation will swap by 90 degrees over a few seconds as the laser warms up. More on this below in the section on linear versus randon polarized lasers.) DO NOT assume the yellow arrow of the safety sticker (if present) identifies the polarization orientation (though it probably does if there is no other mark). But check it anyhow!

      • Orient the laser head so the polarization axis is at -45 degrees (counterclockwise) from vertical when viewed from the front. What differences will there be if it is at +45 degrees?

      • Using the thumbscrews, adjust the position so that the beam is at a height of exactly 3-3/4 inches (which should be centered vertically in the rings) and perfectly horizontal, and 1/4 inch toward the back of the setup. DO NOT overtighten the thumbscrews - just enough so the laser head won't move on its own.

        Doing this accurately is critical to the ease with which the subsequent alignnment 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.

    4. As a quick test, place a linear polarizer (LP or LP/CP with the non-adhesive side facing the beam) with the polarization axis at 45 degrees in the combined return beam. If the alignment is close, very slight rotation of the linear stage micrometer should result in the intensity varying dramatically. Further fine tuning of the alignment may be required to maximize the variation and uniformaty of the fring pattern. The optimal alignment will be where it goes almost totally dark to light. Of course, much more more can be done using these simple observations including effects of alignment, expansion using a lens, display using a Webcam, etc.

  • Set up the oscilloscope: (If using a USB scope, this assumes that the required software and device drivers have already been installed on your PC or MAC.) For input, use one of the scope probes on the 1X setting (there is a slide switch on its body).

  • Single Channel Detector: These steps assemble the components of the fringe detector using the Thorlabs DET110 or home-built equivalent.

  • Adjust the position of the detector so that the combined return beams are centered on the active area of the sensor.

  • If alignment is close, the amplitude of the signal on the scope should vary dramatically as the micrometer is rotated by the smallest amount - (or from just touching the breadboard or anything on it)! The wavelengths of light are TINY! Each full cycle is 1/2 wavelength or around 316.5 nm. The micrometer moves the stage by 0.5 mm per full rotation, or around 1,389 nm/degree.

  • The amplitude can be maximized using the knobs on the Arm 1 and Arm 2 adjustable mounts. The signal amplitude may vary slightly (up to ~20 percent) in a periodic cycle over a time scale of seconds to minutes in addition to it probably increasing slightly as the laser warms up. The time scale will depend on how long the HeNe has been on. Why? There can be several causes.

    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 1/3,160th 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.

  • Now you're set to explore all sorts of exciting aspects of interferometry. ;-)

  • Back to Sam's Educational Michelson Homodyne Interferometer Project Manual Table of Contents.

    Observing the Effects of PLD on Fringe Contrast

    The tests above were done with the PLD near 0. What happens otherwise? If the laser operated with a Single Longidudinal Mode (SLM), the PLD would not matter up to a very large number in the 100s of meters or more. (Such lasers are also called "single frequency".) However, the laser used here is NOT SLM but has 1 or 2 modes depending on its cavity length, which changes due to thermal expansion during warmup as the modes sweep through the neon gain curve. (There is much more on this in the section: Linear polarized versus random polarized laser.)

    For the first of the following tests, the lasing modes must all have the same polarization. And the (single pass) Linear Interferometer should be used.

    For all these tests, it will be better to shut off the laser for a few minutes before starting. Then when it is turned on, the mode sweep due to cavity expansion will be fastest.

    1. Check behavior with the PLD set as close to 0 as possible by measurements of the distance from the PBSC to the CCs in each arm. ±1 mm will be acceptable. Monitor the behavior of the detected signal over time by twiddling the micrometer periodically over a few minutes. Note any significant change in signal amplitude. A change of 10 or 15 percent can be attributable to the normal variation in total power during mode sweep and warmup, but anything more will be due to the interferometer.

    The cavity length of the tube in the 1107 and 1108 lasers is around 137.6 mm or 5.417 inches. One half of this is 68.8 mm or 2.7085 inches.

    1. Change the location of the mirror in Arm 2 so the PLD is within ±1 mm of one half the cavity length by relocating the stage and/or adjusting its position using the micrometer. Now observe the fringe signal again and describe what you see over the course of a few minutes. (Turn off the laser again and allow it to cool for a few minutes as above.) Check alignment to confirm that a change in alignment is not the cause of the effects being seen.

    2. Try intermediate locations for the Arm 2 mirror.

    3. If the Arm 1 adjustable mount is positioned as close as possible to the PBS cube using the BA1S holddown, it will be possible to achieve a PLD of at least the tube cavity length. See how the signal amplitude there compares with the one with a PLD of 0. If it's noticeably lower, why might that be? There are several factors involved.

    Now explain the behevior in each case. And what are special about a PLD of zero and one half the cavity length?

    How might these results differ if the HSPMI were used instead of the LI?

    If your laser is random polarized, it is possible to perform the following additional tests with the CP removed from the front of the laser:

    1. Repeat the above tests with the laser oriented so the outer lines are aligned with the horizontal and vertical axes.

    2. Repeat with the lines on the laser oriented at ±45 degrees.

    Explain your results with respect to the longitudinal mode behavior.

    What would happen if the PLD could be extended to more than the cavity length of the laser as would be possible with the 12 inch rail in Deluxe+ kit?

    All of these tests can also be done with the other interferometer configurations. Predict how the results would change, if at all. What about the HRPMI?

  • Back to Sam's Educational Michelson Homodyne Interferometer Project Manual Table of Contents.

    High Stability Plane Mirror Interferometer (HSPMI)

    The basic Plane Mirror Interferometer (PMI) as its name implies uses a plane mirror instead of a cube corner for the remote reflector. It has a double pass architecture which halves the distance for a full fringe cycle at the detector for a movement of the reflector in Arm 2 (called the "measurement arm"). However, it is not desirable to use a PMI here because it is double-pass only for Arm 2 but single pass for Arm 1. Thus while the PLD can be set to zero, the spacings or lengths of the two arms (as well as the change in displacement) are not the same, which at the very least is confusing. (More on this in the section on the PMI.)

    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.

    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 used in metrology applications. But as with the LI, we need the PLD to be close to zero or ohter specific value for experiments using a multi-longitudinal mode laser. (The "other specific value" would normally be a small integer multiple of the laser's internal cavity length. Why?)

    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.

    High Stability Plane Mirror Interferometer Setup

    Assuming the LI was already built, not many changes/additions are required:

    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:

  • Back to Sam's Educational Michelson Homodyne Interferometer Project Manual Table of Contents.

  • Back to Sam's Educational Michelson Homodyne Interferometer Project Manual Table of Contents.

    Other Interferometer Configurations

    There are some other variations on the Michelson interferometer that may be put together using parts in this kit, starting with the original form of the Michelson Interferometer.

    Original Michelson Interferometer (BMI)

    This is identical to the original except that it uses the polarized laser and PBS cube as the beam-splitter. But note that the PBS passes and reflects the X (parallel to the baseplate) and Y (orthogonal to the baseplate) polarized components and they retrace their steps on the return journey. Thus a Non-Polarizing Beam-Splitter (NPBS) must be inserted to pick off both of them before passing through the linear polarizer at 45 degrees to combine them for the detector. The NPBS can be the variable Attenuator Plate (AP) that is part of the quadrature decoder, but mounting it and determination of the optimal attenuation is left as an exercise for the student.

    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)

    The PMI is probably the most common of the interferometers that use a planar mirror.

    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)

    The MLI adds a pair of QWPs to direct the beam to the detector out the side of the PBSC and is achitecturally similar to the SBI but with offset beam paths that have two additional advantages: (1) retro-reflections back to the laser are reduced further and (2) the beams don't hit the apex or edges of the cube corner trihedral prisms. However, only under very specific cercumstances would the additional cost of the QWPs likely be justified.

    Double Pass Linear Interferometer (DPLI)

    The remote reflector is a cube corner which is better for long distances yet it has double the resolution of the normal LI, the same as the PMI.

    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)

    This doubles the resolution over the PMI or HSPMI. The HRPMI is also high stability because the two beam paths have the same length through the optics, and a PLD of 0 if the distance in Arm 1 and Arm 2 are the matched. This will be much more complex to align and will require a few additional parts and a lot of determination. ;-)

    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. :-)

  • Back to Sam's Educational Michelson Homodyne Interferometer Project Manual Table of Contents.

    Displacement Measurement

    One of the most important and wide spread application of interferometry is for precision measurement and positioning. By keeping track of the fringes rather than just observing them, a change in position (called displacement) can be computed down to sub-nanometer resolution. With clever modifications to the Michelson interferometer, angle, straightness, squareness, and others can also be computed. Semiconductor Fabs for every microchip in the Universe utilize "steppers" (a fancy term for multi-axis stages) which position the wafers for exposure are controlled using laser interferometers.

    The output from an interferometer using a single frequency or two frequency laser may be processed to yield displacement information in digital form.

    In their simplest form, the measurement electronics for a homodyne system is just a quadrature decoder circuit driving an up-down counter; for the heterodyne system it is a pair of accumulators and a subtractor. The interferometer optics are identical. In practice, the electronics is considerably more complex, in part to provide sub-wavelength interpolation and extend the range down to nm resolution. And yes, kits are available for these as well.


    However, a basic implementation of a homodyne interferometer displacement measurement system for demonstration purposes can be done with a $3 microprocessor board and a few inexpensive parts as shown below. Where the path length difference is limited to be less than a few cm, a multi-longitudinal mode (not strictly single frequency) laser like the one in these kits may be used.

    Quadrature Decodor

    The basic detector using a single photodiode can generate a signal corresponding to light and dark fringes, but cannot provide direction information, essential for using an interferometer in metrology applications. The Quad-Sin-Cos decoder provides a pair of outputs that are 90 degrees offset from each-other in position, similar to the outputs of a rotary or linear encoder:

    This show a rotary optical encoder which uses a pair of LEDs and photodiodes physically offset by 90 degrees to generate Quad-Sin-Cos analog signals which are then thresholded to yield Quad-A-B digital signals. The specific type of sequence is called a "Gray" code (not based on color but attibutable to someone named Frank Gray) and has the property that any possible allowable change in value is a change in only a single bit. This eliminates the ambiguity with sensors using the normal binary order where two bits can change at not quite the same time.

    (The animated encoder graphic seems to be all over the Web. If anyone knows who the original copyright holder is, I will acknowledge them.)

    Many other types of encoders produce similar signals. They may use optical, mechanical, or magnetic sensing, among others.

    An interferometer with angled paths for the two interfering beams produces fringes similar to the pattern of an optical encoder so a quad detector could be built with offset photodiodes. However, more commonly, the 90 degree phase shift is done optically using a single combined beam as shown below. If thresholded and converted to digital form, the result would be a Quad-A-B format.

    This shows variations on one of several common implementations for a Quad-Sin-Cos decoder that provides Sine and Cosine outputs for use in a displacement measuring system. This is among the simplest. In most instances, the photodiodes would be reverse biased to provide a linear response. It may be possible to get away without that for initial testing but it will probably be needed if doing anything useful with the outputs. In addition, a third "Intensity" channel is almost always included to accommodate variations in detected power due to the laser aging, changes in alignment, and contamination over time. The Intensity channel can be implemented electronically or optically with a non-polarizing beam-splitter at the input and additional photodiode.

    The output signals from these will be close to sinusoidal with a relative phase close to plus or minus 90 degrees depending on the. direction of motion of the remote reflector or ring laser gyro.

    The purpose of the angled arrangement is to minimize the difference between the amplitudes of the two polarizations. Otherwise, with 45 degrees being close to the Brewster angle (around 57 degrees), one will be much larger than the other. Even so-called 50:50 beam-splitters may be subject to this, so using the angled arrangement for either one may be beneficial. The parts for the version using the Attenuator Plate (AP) are what are in the kit, which simplifies construction. Using the AP also permits the relative amplitudes of the Channel A and B signals to be changed somewhat without electronic adjustments.

    Some resourcefulness will be required to mount the parts in this kit to put together a Quad-Sin-Cos decoder. A variable attenuator plate is included that may be used as the NPBS. Pieces of CP will be satisfactory for both the combination of the LP+QWP (since that's exactly what the CP is), as well as the LP (flipped) since the output polarization doesn't affect PD behavior. See the information on polarization, below.

    This would be a great excuse to finally make good use of that 3-D printer sitting idle. ;-) A simple frame could be designed to mount the AP via its spring and screw so its position, and thus reflection and transmission, would be adjustable in the beam. Slots and/or faces would be used to attach the pieces of CP and the PDs. Be creative! This is probably overkill though.

    Quadrature Detector using Variable Attenuator Plate and Circular Polarizer Sheets. Optical Layout, Parts), Constructed on Solderless Breadboard, Protoboard, and PCB Assembly

    These photos show a diagram for the preferred implementation of the Quad decoder itself, the typical parts, and 3 perfectly workable construction options, the first of which uses a small solderless breadboard and doesn't require any soldering. For that one, the AP and pieces of CP sheet could be glued to wires that would be stuck in holes. Or U-shaped pieces of wire could simply be stuck in holes to keep the CPs and AP in place. ;-)

    Before constructing the prototype or PCB versions, it was dedirable to to conclusively prove that the simple Type 3 scheme with CPs for the polarization optics actually worked as advertised, so a prototype version was installed on the Michelson Interferometer test-bed:

    In the interest of expediency, it cheats and used an NPBS rather than a plate beam-splitter or variable attenuator, two Thorlabs DET110s rather than bare photodiodes, but the QWP+LP for Channel B is a piece of CP (as in the diagram on the right, above) stuck to a microscope cover slip that is glued to a platter clamping ring from an ancient defunct harddrive. Got that? :)

    The ugly scope screen shots below were taken using this setup:


    Displacement Positive (Left) and Negative (Right)

    Capturing a decent photo while twiddling the micrometer screw is quite challenging. ;-) But the conclusions are clear: This simple Quad decoder does its job well with a phase shift of ±90 degrees. If the Arm 2 mirror or retro-reflector were on an electronically controlled positioner like a loudspeaker voice coil or linear motor driven with a ramp, the waveforms would be textbook quality. ;-) But with only a small stretch of the imagination, it can be seen that the screenshots agree with the expected behavior based on the diagrams, above.

    The prototype on a prototyping board was then constructed and tested to confirm similar behavior, and then the simple PCB was made so that mounting of the photodiodes and other electrical components would be simplified. There is nothing to really secure the AP/BS but that could be done with double-sided tape.

    Constructing the Quad-Sin-Cos Detector

    The mounting scheme doesn't need to be fancy or pretty but should hold the pieces securely while maintaining alignment. This can use bits of tape and Epoxy or other adhesive. The CPs, QWPs, and NPBS plate are expendible so feel free to chop them up if necessary for them to fit. :)

    The photos above show various possibilities not involving a 3-D printer :), including a simple PCB (which is available), but some soldering is required for that. The simplest approach is to use the same Solderless Breadboard (SB) as the Single Channel Detector, attached to the post using the Detector Adapter Plate as in this closeup:

    1. Set up the HeNe laser so that there is a linearly polarized beam. Orient the polarization to be at 45 degrees or preferably, the output of the interferometer as well.

    2. Mount the AP at an angle as close to normal to the incident beam as is convenient to still be able to locate the Channel B CP and PD in the reflected beam. The rational for this is to minimize the effect of the angled plate on the polarization of the reflected and transmitted beams. Why? If you're into fancy 3-D-printed mounts, it can be arranged to use the screw/spring combination to adjust it's position, and thus attenuation (or transmission and reflection). But that certainly is not required and the screw and spring can be relegated to your junk drawer. ;-)

    3. Mount one PD to intercept the transmitted beam. (This will be designated Channel A or 1.) Trim the leads if necessary so that in conjunction with adjusting the height of the platform, the beam can be approximately centered vertically in the AP.

    4. Adjust the position of the AP so that the transmitted and reflected beam intensities are approximately equal. This will be near or at the higher-density end of the AP. Perfect balance may not be possible since the maximum reflectance of the AP is not 50 percent, so the remaining adjustment will need to be done with the Gain trim-pots (substituting for the fixed resistors, or later when the interface circuit is built).

    5. Install the second PD (Channel B or 2) to intercept the reflected beam. Note that the AP glass is slightly tilted so it may be necessary to trim the PD's leads or bend them over so its height will be correct.

    6. The PDs should be reverse biased with 5 to 15 VDC (+ to cathode, the right leg facing front). A load resistor of approximately 100K ohms should result in a decent signal amplitude for a ~1 mW laser. If too high or too low using only the laser, no need to change until using the actual interferometer output. To accomodate lower power lasers and/or losses in the interferometer optics, 1M ohm trim-pots are included in the kits for this purpose. The typical wiring is shown below.

             R-Protect    PD1      Yellow
         +-----/\/\----+--|<|---<<-----------------------+----------o Scope Channel 1
         |             |                                 |
         |             |  PD2      Blue                  |
         |             +--|<|---<<---------------------------+------o Scope Channel 2
         |                                               |   |
         |                                               /   /
         |                                       R-Load1 \   \ R-Load2
         |                                               /   /
         |                                DC Power       \   \
         |                          Red    +| |-   Black |   |
         +-----------------------<<---------||||---------+---+-------o Scope Ground
                                            | |
         |<--- SBB or QDx PCB --->|<----- Scope / Power Wiring ------>

    PD Pins: Facing Front of PD with Legs Down: Anode on left, Cathode on right.

    R-Protect: 250-1K ohms typical. R-Load: 10K-1M ohms typical.

    In the photo, above, one piece of CP sheet is stuck directly to the channel 1. PD. The other piece of CP sheet is simply propped in front of the channel 2 PD with the sticky side facing out. Long term, that side should be protected with some 5 Minute Epoxy or a microscope cover slip. The AP is just sitting on the SB. Dabs of 5 Minute Epoxy, wire loops, or other means can be used to secure them more permanently. If using wire loops, take care not to short out anything that shouldn't be connected. ;)

    The following must be done using the output of the interferometer that has been properly aligned so that interference can be seen on a white screen if a linear polarizer is placed in the output beam at 45 degrees.

    Make sure you hands are clean or use a pair of latex surgical gloves when handling the pieces of CP.

    1. Cut a pair of 0.5x0.5cm or larger pieces of the CP sheet aligned with the original edges.

    2. Carefully remove the protective film from both sides of the cut pieces. One side is simply bare plastic; the other side has a sticky adhesive.

    3. The adhesive-side of the CP is the QWP with its optical axis at 0/90 degrees (aligned) with the edges; the other side is the LP with its axis at 45 degrees. This can be confirmed by testing with a linearly polarized laser or a random polarized laser with a separate linear polarizer in front of it:

      • When the CP's LP-side faces the laser, rotating it will result in the transmitted intensity going very close to 0.

      • With the CP-side facing the laser, there will be little or no change in intensity.

    4. Channel A: Attach the adhesive side of one of the pieces of CP directly to the front of the Channel A PD making sure to keep it aligned with the horizontal and vertical axes. This will result in its LP-side facing the AP oriented so the polarization axis is at 45 degrees (edges aligned with the X and Y axes). The result should be a strong signal as the micrometer stage is moved or whatever is convenient to change the PLD.

    5. Channel B: Attach the other piece of CP in the reflected beam so that its QWP-side faces the AP and is oriented at 0/90 degrees (edges also aligned with the X and Y axes). This is best done by using a drop of 5 Minute Epoxy to stick it directly to the Channel B PD, but if large enough, it can just be placed in front of the PD with a wire loop or something similar to stabilize it. Just make sure that the wire loop doesn't short out something important. ;) The result should be a strong signal as the micrometer stage is moved or whatever is convenient to change the PLD. Once correct behavior is confirmed, coat the adhesive-side with a thin layer of 5 Minute Epoxy to prevent it from attracting contamination like dirt and dust, or stick it to a microscope cover slip.

    6. Fine tune interferometer alignment to maximize signal amplitude and adjust the values of the PD load resistors so that the signal amplitudes are between 3 and 5 V p-p.

    7. Observe the Channel A and B signals on the oscillscope. They should be in quadrature (or very close). This means the phase shift between them should flip from +90 to -90 degrees depending on the direction of motion as in the photos, above.

      Note: Even if the peak-peak amplitudes are made equal, it may not be possible to avoid an offset on one channel. In that case, the scope vertical position can be set to superimpose them on the screen, and later, the Threshold trim-pots can be set appropriately.

      There can be a number of reasons for this offest. What might they be? Hint: Think polarization.

    Micro Measurement Display 0 (µMD0)

    µMD0 is a very inexpensive system for precision readout of displacement (change in position), angle, straightness, and more in metrology applications using (homodyne) interferometry with single frequency HeNe lasers. Where the path length difference is limited to a few cm, it may also be used with (unstabilized) multi-longitudinal mode HeNe lasers. µMD0 may be used with devices like linear and rotary encoders which produce Quad-Sin-Cos or Quad-A/B signals.

    For the purposes of these interferometer kits, µMD0 consists of three parts:

    1. Interface from Quad-Sin-Cos analog signals to Quad-A-B digital signals, consisting of very simple circuitry to provide gain and thresholding.
    2. Atmega 328 Nano 3.0 (or similar Arduino-compatible) microcomputer board, which runs the µMD0 firmware.
    3. Micro Measurement Display (µMD) Graphical User Interface which typically runs on a Windows PC or laptop.

    The general organization of a typical system is shown below (though the one implemented in this kit differs in some subtle details):

    Typical Homodyne Interferometer Measurement Setup using µMD0

    To convert the analog sin and cos signals to something for a low cost microprocessor with adequate performance requires a simple interface which provides gain and threshold adjustments. (While it has analog inputs, their conversion rate is way too slow.)

    Referring to the schematic, the trim-pots on the left are the load resistors for the quadrature detector. The 100K value should be satisfactory to resultin a signal of a few volts p-p using a laser with an output power of around 1 mW and no beam expander. (The expanded beam may slightly exceeds the dimensions of the photodiodes so the sensitivity will be reduced.) For a lower power laser or a laser with a beam expander, larger values may be required. Or for finer control, fixed resistors can be added in series with the trim-pots. The trim-pots on the right adjust the comparator threshold for the Sin and Cos signals from the Quad decoder, with the feedback resistors providing some hysteresis. The Atmega 328P Nano 3.0 board runs firmware that is compatible with the µMD GUI. Of course, no high tech system would be complete without indicator lights, so LEDs are added to monitor the A and B inputs. ;-)

    The minimal implementation is shown below along with a shot of the laptop screen while twiddling the linear stage micrometer:


    µMD0 and Interface Schematic (left), Impementation on Solderless Breadboard (Middle), µMD GUI Display while changing Displacemet (right)

    A schematic with slightly more detail like pin numbers may be found at: µMD0 Sin-Cos Analog and RS422 Digital Interfaces.

    A simple assembled PCB for quad decoder is included. Blank PCBs are available for the µMD0 and interface, but should not be required for a student project. ;-)

    The ptototype setup is shown below, but with the dual Thorlabs DET110 detectors and interface without gain adjustments, sorry. ;-)

    Overall Setup showing Interferometer, Scope, µMD0 with Interface, and µMD GUI Display

    The bandwidth of the photodiode + resistor combination is quite limited, probably a few thousand counts/second, if that. But it is sufficient to track the movement of the micrometer stage, though not if it's pushed back and forth by hand without the micrometer or being moved on the rail. That would require a proper transimpedance pre-amp circuit. µMD0 has a maximum slew rate believed to be above 125,000 counts/second, or around 1 cm/s with the Linear Interferometer.

    For more information, see the Laser FAQ chapter Laser Instruments and Applications, sections starting with "Interferometers for Precision Measurement in Metrology Applications". And the Micro Measurement Display 0 (µMD0) Installation and Operation Manual.

  • Back to Sam's Educational Michelson Homodyne Interferometer Project Manual Table of Contents.


    Where Does the Missing Light Go?

    This question inevitably comes up with interferometers where visible fringes are produced. Where does the light go that would be in the dark spaces? In principle, with perfect alignment, a visible display could be totally bright or totally dark. It is possible to demonstrate this here by viewing the fringe display directly on a screen behind a linear polarizer oriented at 45 degrees. And by rotating the LP between +45 degrees and -45 degrees, it would go between light and dark.

    The analog with the signal is that it can be high or low. But since the laser is still lasing and with no polarizer in front of the detector, the detected signal is more or less constant. So when the signal is low (or the spot is dark), where is the missing power? I'll save you some of your brain cycles and state that with the LP film, it's lost in the plastic and actually increases its temperature a miniscule amount.

    But what about a Polarizing Beam-Splitter (PBS) like the large one that is the heart of this setup? They have negligible losses and it would be simple in principle to use one in place of the LP film. To do that, either the PBSC would need to be rotated 45 degrees or the polarization of the beam would need to be rotated 45 degrees so that the PBS can generate the two polarized outputs. Rotating the polarization of the beam can be done with a Half WavePlate, but there is none in the kit, believe it or not. ;-) However, it can be simulated using two pieces of LP film and the Attenuator Plate (AP) as a Non-Polarizing Beam-Splitter (NPBS). This isn't quite identical because there will still be losses in the LPs, but if oriented at ±45 degrees, the effect will be the same.

    Dual Polarization Detector using LPs at ±45 Degrees

    This uses the same CP as the quad decoder but for these experiments, the QWP portion is totally irrelevant and the QWP (sticky) side goes toward the photodiodes for both.

    It should be pretty obvious what is going to happen, but seeing it is not quite the same as theory. ;-)

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  • Back to Sam's Educational Michelson Homodyne Interferometer Project Manual Table of Contents.

    Future Options

    Transparent Laser Enclosure with Heater

    Modified laser head cylinders may be available made of transparent aluminum (a.k.a., acrylic or Plexiglas). While there isn't a huge amount of action that would be visible, it would add a cheery glow to the setup. And at the same time (and more importantly), a thin-film heater can be added to the laser tube to control cavity length. The photo on the left is a closeup of the laser while the one on the right is the rail setup with Thermal Expandsion experiment using the glowing laser.


    HeNe Laser Tube in Transparent Acrylic Cylinder with Heater (left), Scope Display of Fringe Signal while Heating Glass Block in Arm 1 (right)

    Something like this would be used to house the single frequency or two frequency lasers described above. The heater would also be useful to accelerate mode sweep for experiments dealing with the interferometer response particularly for random polarized lasers.


    Linear Polarized versus Random Polarized HeNe Laser

    There are two basic types of HeNe lasers with respect to polarization of the output beam: Random polarized and linearly polarized. With just a pair of mirrors and HeNe gas, the result will usually random polarized. To construct a linearly polarized HeNe laser, a Brewster-angle plate is added inside the laser cavity. (Search for: "Brewster Angle".) This introduces an orientation dependent loss with respect to polarization and virtually 100 percent of the photons will then be linearly polarized with the orientation with minimal loss. The Brewster plate on the types of HeNe laser tubes used here are buried within one of the mirror mount stems and are thus not readily visible.

    Although the experiments are described with a linear polarized HeNe laser, it is possible to use a random polarized laser instead if one is more readily available. However, its output will need to be converted to linear polarization in a very specific way with a linear polarizer to behave the same. But a random polarized laser can also demonstrate some interesting interferometer behavior not available with a linear polarized laser.

    Most random polarized lasers do not actually have polarization varying, well, at random. :) The term "random polarized" with respect to to HeNe laser simply means that nothing special is done to control the polarization. (I.e., no Brewster plate.) In the case of many red (633 nm) HeNe lasers, that means:

    1. There are two polarization axes instead of one.
    2. They are orthogonal to each-other.
    3. Their orientation cannot be predicted without testing but remains fixed for the life of the tube.
    4. Adjacent longitudinal modes have orthogonal polarization.

    And note that this only applies to red HeNes, not even other color HeNes, let alone most other lasers. Murphy must have taken a day off when the HeNe laser was invented because these attributes end up being quite useful - in fact fundamentally important - for many applications.

    A laser tube with these characteristics would be considered "well behaved". if the longitudinal modes move smoothly through the neon gain curve without abrupt changes in amplitude and not all are like this. :( :) A "flipper" will have "polarization switching" whereby at some point or points during mode sweep, the two sets of longitudinal modes will swap their polarization axes, usually instantly. For some truly nasty tubes, this will happen continuously, somewhat well, at random. Flippers are often not suitable for an interferometer because in the region of the flip, there may be excessive optical noise, possibly due to even the smallest amount of back-reflection, which will show up as large oscillations in the fringe signal. (While that in itself may be interesting, it will also be confusing.)

    The simplest way to test for a well behaved random polarized laser is to put the output through a linear polarizer and monitor it on a graphing laser power meter or photodiode and data acquisition system. Adjust the orientation of the polarizer for the maximum amplitude of the mode sweep variation. That aligns it with one of the polarization axes. Then inspect the plot over a couple minutes (from a cold start to get the fastest mode sweep) for abrup changes in amplitude. There should be none.

    The following animation shows the mode sweep of a random polarized HeNe laser similar to the JDSU 1107 or 1108. The red and blue lines represent the amplitudes of the orthogonal polarized outputs. To actually view these live with a similar display requires an instrument called a Scanning Fabry-Perot Interferometer (SFPI) with a dual polarization detector. While commercial SFPIs cost several thousand dollars, an SFPI with these capabilities can be built as a nice student project at modest cost. It's all done with mirrors. ;-)

    Mode Sweep of Short Random Polarized HeNe Laser

    A linearly polarized HeNe laser of similar length would have both modes be the same polarization and same color. :)

    The rate at which the modes pass through the neon gain curve will depend on how fast the tube is expanding from heating of the gas discharge, so it will slow down as it reaches thermal equilibrium from a cold start.

    Plots of the two polarized modes for a well behaved tube (non-flipper) would look something like:

    Mode Amplitude Plots of Well Behaved Short Random Polarized HeNe Laser

    The plots cover the time range from a cold start to close to thermal equilibrium. Note how both the red and blue plots are continuous. A full mode sweep cycle at the start is a few seconds while at the end it is a few minutes. After that it would be irregular as just ambient air moving around will have a significant effect.

    (For a linearly polarized tube with similar physical characteristics, the amplitude of the output would be the sum of the red and blue plots.)

    The plots of a typical flipper might look like the following (zoomed in to a few mode sweep cycles to show details):

    Mode Amplitude of Short Random Polarized "Flipper" HeNe Laser

    (The shape of the curves differ due to the tube not being the same model.) The vertical green line is the instant of the flip, which occurs quite close to the same location during each mode sweep cycle. However, experience shows that in the interferometer, there may be nasty stuff going on around that region and it won't be confined to an instantaneous event. The detected signal may be very noisy.

    For an academic challenge, a random polarized flipper is probably the most interesting type of tube to study. But a good understanding of what's going on is necessary to not to go insane attempting to decipher the behavior.

    The next most interesting tube would be one that is random polarized and well behaved. One or two of the kits put together to date includes that type of laser. A random polarized flipper can be provided in place of or in addition to the linear polarized or random polarized well behaved tube if interested. :-)

  • Back to Sam's Educational Michelson Homodyne Interferometer Project Manual Table of Contents.

    Homodyne V2.1 Summary of Features and Options

    Here is assorted information on what is for sale or will be for sale, directly from me or through eBay. V2.1 is virtually identical to V2.0 except for the size of some posts and spacers. V2.0 is functionally virtually identical to V1.0 and V1.5 except for the sizes of the breadboards, but a few other parts have changed, and thus the assembly procedures differ.

    There are three versions of the V2.1 Homodyne setups, though customization is possible.

    The Basic version really has enough for an introduction to interferometry and can be upgraded to the Deluxe version by adding the appropriate parts from me or elsewhere.

    Other changes compared to V1.0 and V1.5 include:

    1. V2.1 (and V2.0) use an aluminum plate on Thorlab posts rather than a wood block for the PBS and turning mirror mount as in V1.0 and V1.5. Both are very stable, but the plate affair is more in keeping with optical setups and doesn't contribute to the demise of trees. ;-) The wood block also had issues of degradation with multiple cycles of assembly and disassembly.

    2. A custom detector made from discrete parts replaces the Thorlabs DET110 in V1.0 and V1.5. The performance for the purposes of these kits is identical. And it facilitates adding a second channel for the quad-sin-cos decoder or X/Y detector and also simplifies the wiring with a terminal block and/or jumper wires instead of requiring BNC cables and clip leads. Both a solderless breadboard and assembled PCB version will be included.

      However, for the die-hard, a few DET110s are available at a lower cost than the current version of a biased photodiode from Thorlabs.

    3. The adjustable mounts will be either Thorlabs KM100s or Newport U100s. For the purposes of these kits, they are functionally equivalent and the Thorlabs are smaller and sleeker. ;-) The U100s are more precise (and a lot pricier if bought new!), though these versions lack knobs, requiring the use of a 5/64" ball driver, which may be less convenient. But never fear, appropriate Thorlabs knobs will be included in the kit.

    4. The QWPS and mounted CC in the Deluxe and Deluxe+ versions may be custom parts rather than from HP/Agilent. They will be functionally equivalent without issues of availability.

    5. A generic linear stage will be used, though a Parker 3902 may be an option. The generic stage is equally good for positioning here, possibly even more stable due to its wider track spacing.

    6. All versions will have either a Thorlabs RC carrier with RLA rail, or MR15 rail with MGN15-C block.

    Photos of each of the three versions with typical interferometers are shown below. The first is the Basic version with the 6x18" the breadboard, configured with Linear Interferometer and quad decoder PCB attached to the SG-µMD0 PCB:

    The next one is the Deluxe version configured for the High Stability Plane Mirror Interferometer. The laser includes a beam expander but it is otherwise configured the same way.

    The next is the Deluxe+ "Stretch" version on the 8x24" breadboard.

  • Back to Sam's Educational Michelson Homodyne Interferometer Project Manual Table of Contents.

    Sam's Educational Michelson Interferometer Interferometer Kit Parts List

    These are the parts for the Basic, Deluxe, and Deluxe+ setups except as noted.

    Replicating it should be straightforward

    The 8x18" optical breadboard for V1.0 and V1.5 is custom with the dimensions selected to be convenient for the projects while being able to easily ship Worldwide. The 6x18" and 8x24" breadboards for V2.0 and V2.1 are standard. If machining one, fewer than half the standard holes are enough based on all reasonable mounting locations, but that's only worth it if you're paying by the hole. ;-)

    Most other standard opto-mechanical parts are from Thorlabs. The major exceptions are the HP/Agilent PBS cube, QWPs, and mounted CCs (via eBay). But low cost substitutes for those are in the works.

    The JDSU 1107P and 1108P, or similar laser heads from Melles Griot/Pacific Lasertec like the 05-LHR/P-211 are ideal for these experiments. The criteria include being linearly polarized, eye-safe output power, and physical size. Optics companies like Edmunds, Newport, and Thorlabs have suitable lasers (though most are made by JDSU and PLT), but eBay is often a good source at a fraction of the cost. The laser could also be a bare tube safely enclosed, though the preferred linearly polarized variety is not that common at the low power of 0.5 to 1 mW. Random polarized heads and tubes can be used (and actually add some interesting areas to study) if certain criteria are met, primarily that they are not "flippers". See section: Linear Polarized versus Random Polarized HeNe Laser.

    Parts List for V2.1

    Differences between the versions will be noted.

    Parts in Homodyne Kits ONLY

    Quantity Description ------------------------------------------------------------------------------ Baseplate/Optical breadboard: 1x Aluminum optical breadboard 6x18" (Basic and Deluxe versions, BASE Lab Tools SAB0618) or 8x24" (Deluxe+ version, BASE Lab Tools SAB0824). Laser Assembly: 1x Laser consisting of 0.5-1 mW JDSU 1107/P or 1108/P laser head and power supply with DC input brick and universal wall adapter, which may be used Worldwide. Minimum power 0.4 mW. A lab-style HeNe laser power supply may be an option. 1x (Random polarized lasers only): ~1x1" piece of circular polarizer. Consists of linear polarizer (+/-45 degrees) with QWP (0/90 degrees). Cut up as needed. 1x Beam Expander (Deluxe/Deluxe+ only) consisting of: 1x Beam Expander Adapter Plate. The front bezel of the laser has had 3 tapped holes added for this. 1x HP/Agilent 6 mm beam expander, modified, must be glued or press-fit into Beam Expander Adapter Plate. Resulting beam is 3-4 mm in diameter. 3x M2.5 x 5 mm cap-head screws to secure adapter plate. 2x Laser Mounts each consisting of: 1x Small Ring Mount with two 8-32 x 1.5" and two 8-32 x 1" thumbscrews. 1x Thorlabs TR3 post with 8-32 x 1/2" cap-head screw to secure the ring mounts and optional #8 plastic washer to provide compliance if the mounting hole in the rings is threaded (some are) so the orientation can be set correctly. In the latter case, an 8-32 x 1/2" setscrew can substitute for the cap-head screw so as not to impinge on the available space within the rings for the laser head. Custom Single Channel Detector and Quadrature Decoder: 1x Variable attenuator plate to be used as NPBS. 2x 1/2" piece of sheet (may be used as CP or LP), or CP+LP, cut to size. 2x Silicon photodiodes + spare. 1x 1K ohm resistor (PD protection). 2X Load resistors, 100K to 1M typical, or 1M trim-pot 1x Set of jumper wires, etc. 1x Solderless breadboard with adhesive back, 170 tie points. 1x Detector adapter plate with 8-32 or 1/4-20 1/2" setscrew. 1x QD1 PCB (option) with 8-32 1/2" cap-head screw and 1/4" x 1/4" #8 spacer. 2x 4 pole screw terminal block. 1x 1K ohm resistor. 1x 0.1 µF capacitor. 2x Photodiode. 2x 2 pin male to female socket strip. AB2 parts kit. Custom Single Channel Detector and Quadrature Decoder: 1x Variable attenuator plate to be used as NPBS. 2x 1/2" piece of sheet (may be used as CP or LP), or CP+LP, cut to size. 2x Silicon photodiodes + spare. 1x 1K ohm resistor (PD protection). 2X Load resistors, 100K to 1M typical, or 1M trim-pot 1x Set of jumper wires, etc. 1x Solderless breadboard with adhesive back, 170 tie points. 1x Detector adapter plate with 8-32 or 1/4-20 1/2" setscrew. 1x QD1 PCB (option) with 8-32 1/2" cap-head screw and 1/4" x 1/4" #8 spacer. 2x 4 pole screw terminal block. 1x 1K ohm resistor. 1x 0.1 µF capacitor. 2x Photodiode. 2x 2 pin male to female socket strip. Common to both detectors: 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 5/8" cap-head setscrew to attach to base. Micro Measurement Display 0 (µMD0): 1x Atmega 328 Nano 3.0 microcomputer board with pins soldered. 1x Homodyne firmware for Nano (may be preloaded or download from µMD0 Manual). 1x µMD Graphics User Interface (download from µMD0 Manual). 1x Solderless breadboard 3-1/4" x 2-1/4", 25 columns, 400 tie points. 1x LM393P dual voltage comparator. 1x 3 mm Red LED and 1-2K ohm resistor. 1x 3 mm green LED and 27-47K ohm resistor. 2x 100K to 1M ohm resistors or trim-pots. 2x 10K ohm trim-pots. 2x 100K ohm resistor. 2x 470K ohm resistors. 1x SG-µMD0 PCB (option). 1x 30 pin Socket for Nano (may need trimming). 2x 8 pin sockets for LM393 and UA9637. 1x 3 mm blue LED. 1x 10K ohm resistor. Support (Deluxe and Deluxe+): 1x 10K ohm potentiometer wired with 10K ohm current limiting resistor. - Hookup wire and jumper wires.

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