Introduction

This document contains guidelines and links to other manuals for the construction of a homodyne or heterodyne interferometer displacement measuring system using a HeNe laser and a microcontroller-based measurement readout. There are kits providing various levels of performance from really basic to something similar to a commercial system.

The brief summary is that the heterodyne kit (there is only one version) has the best performance and would be useful in a serious application. The homodyne kits have very limited slew rate unless augmented with high speed front-end electronics, which are not provided here at least.

Since the lasers and measurement readouts are also available as separate kits, links will be provided to their detailed manuals. Only information not available elsewhere will be elaborated on in this document.

The heterodyne and homodyne kits consist of 4 parts:

• HeNe laser: The lasers use similar tubes though the ones sent will have been tested to assure they are compatible with single frequency or two-frequency (Zeeman) stabilization, or simply well behaved, mode-wise depending on the specific requirements. The main difference in the setup is that the Zeeman adds a set of magnets to split the frequency and a Quarter WavePlate (QWP) to convert the circularly polarized Zeeman modes to orthogonal linearly polarized modes. The power supply, heater, (and for the stabilized lasers) the Arduino controller are identical.

• Interferometer optics: Basic components are provided which can be configured in various ways to create linear, plane mirror, and other interferometers.

• Detector: For heterodyne these are kits using the SG-OR3 custom PCB to provide functionality similar to that of the HP/Agilent 10780 optical receivers. For homodyne, parts for a Quad-Sin-Cos decoder and basic interface are provided. This is sufficient to get started with limited bandwidth. In the future, parts for a higher performance decoder may be available.

• Readout: µMD0, µMD1, or µMD2 convert the detector outputs to displacmeent information that is displayed using a Windows GUI.

These parts of the system can be constructed and tested separately.

For a general introduction to this technology, see LIPM: An Inexpensive Laser Interferometer-Based Precision Measurement System. This is highly recommended reading prior to construction.

There are several variations on this kit:

Standard Systems with µMD2:

• Heterodyne interferometer using two frequency Zeeman HeNe laser: This is based on the same principles as systems from HP/Agilent/Keysight, Excel, Zygo, and others.

  The typical parts are shown below:

  

  Links:

  • Stabilized Zeeman HeNe Laser Kit 1 with or without Arduino. The heterodyne kit uses the Arduino version.

  • Detectors for heterodyne known as "optical receivers" for these are SG-OR3 which provide functionality similar to that of the HP/Agilent 10780. SG-OR3 uses a relatively simple PCB so construction is straightforward and Hathkit™-style assembly instructions are provided. See Optical Receivers for Heterodyne Interferometers for more info. Unlike the commercial optical receivers, these will run on 12 VDC, so no DC-DC boost converter will be required. (The optical receivers for the "Hewlett Packard (HP) Interferometer Measurement System-Hobbyist's Special 1,2,3" kits will continue to be commercial parts.)

  • Micro Measurement Display 2 (µMD2).

• Homodyne interferometer using Single Frequency (SF) / Single Longitudinal Mode (SLM) laser: This is based on the same principles as systems from Teletrac/Axsys/General Dynamics/MotionX, Renishaw, and others.

  The typical parts are shown below:

  

  Links:

  • Stabilized HeNe Laser Kit 1 with Arduino

  • Construction Guidelines for Basic Quadrature-Sin-Cos Decoder and Quad-A-B Interface Kits.

  • Micro Measurement Display 2 (µMD2)

  The detector in all the homodyne kits is quite primitive consisting of a pair of biased photodiodes with a Quarter WavePlate (QWP) to shift the phase of one of the signals by 90 degrees. Thus the bandwidth is quite limited. But a better one could be constructed relatively easily.

  Parts for a simple Quad-Sin-Cos to Quad-A-B converter with RS422 drivers for µMD2 are included. The schematic is shown in the above graphic. There is no PCB for this but a Perf. board is provided on which it can be constructed. A solderless breadboard could also be used. available for a couple bucks on eBay. The RS422 drivers at the output can be dispensed with if µMD2 is set up for single-ended inputs as described in its assembly manual.

• Homodyne interferometer using Multi-Longitudinal Mode (MLM) / laser: This does away with the stabilization electronics but the path length difference then needs to be less than a few cm.

  The typical parts are shown below:

  

  Links:

  • Stabilized HeNe Laser Kit 1 with Arduino

  • Construction Guidelines for Basic Quadrature-Sin-Cos Decoder and Quad-A-B Interface Kits.

  • Micro Measurement Display 2 (µMD2)

  The detector in all the homodyne kits is quite primitive consisting of a pair of biased photodiodes with a Quarter WavePlate (QWP) to shift the phase of one of the signals by 90 degrees. Thus the bandwidth is quite limited. But a better one could be constructed relatively easily.

  Parts for a simple Quad-Sin-Cos to Quad-A-B converter with RS422 drivers for µMD2 are included. The schematic is shown in the above graphic. There is no PCB for this but a Perf. board is provided on which it can be constructed. A solderless breadboard could also be used. available for a couple bucks on eBay. The RS422 drivers at the output can be dispensed with if µMD2 is set up for single-ended inputs as described in its assembly manual.

• Homodyne kit using LP (linear polarized) laser head: This replaces the glass laser tube with a common unstabilized linear polarized HeNe laser head so issues of shocking experiences due to high voltage are mostly eliminated and it is more robust. It's also simpler to construct than those with bare laser tubes. Being unstabilized, the path length difference between the two arms of the interferometer is limited to a several cm.

  These lasers produce a narrow beam (less thatn 1 mm in diameter), but because the PLD needs to be small, the diameter at the detector should be similar for both the reference and measurement beams. However, if desired, a beam expander can be added. Ask.

  Note that while this kit has the µMD2 measurement display, the performance is limited mostly by the frequency response of the Quad-Sin-Cos detector, which is quite limited. So it may be no better than the minimal version using µMD0.

  The typical parts are shown below:

  

  Plug the high voltage "Alden" connectors of the laser head and power supply together securely if not already attached. The power supply will probably have a DC connector so it just needs to be plugged into the included wall adapter.

  Orient the laser head so that the polarization axis is at 45 degrees. It will probably be marked but if not, is easily determined with a linear polarizer.

  Links:

  • Construction Guidelines for Basic Quadrature-Sin-Cos Decoder and Quad-A-B Interface Kits.

  • Micro Measurement Display 2 (µMD2)

• Combined heterodyne and homodyne system (but not at the same time): This includes the overlap of most of the parts in the kits above but with only µMD2 since that supports both.

  Since switching between Het and Hom requires reconfiguring the laser itself, among other things, the combined kit is not really recommended and currently disabled in the eBay listing. But if really interested, contact me.

Minimal Systems with µMD0:

• Minimal homodyne kit using SF / SLM laser:

  This is similar to the homodyne system above but but with the µMD0 display.

  

  Links:

  • Stabilized HeNe Laser Kit 1 with Arduino

    Most of the tubes have a high divergence beam - around 8 mR. Therefore a collimating lens is included, probably in the bag with the optics. It has a focal lengh of 3-4 inches. Placing it close to the output bracket of the tube will result a beam that remains parallel enough for a short range interferometer (probably 1 or 2 feet). For longer distances, HP/Agilent beam expanders are available - ask. These result in a 3 or 4 mm beam which would remain fairly parallel over several feet.

  • Construction Guidelines for Basic Quadrature-Sin-Cos Decoder and Quad-A-B Interface Kits.

  • Micro Measurement Display 0 (µMD0)

• Minimal homodyne kit using unstabilized Multi-Longitudinal Mode (MLM) laser: This uses a common unstabilized random polarized HeNe tube, which limits the change in path length difference between the two arms of the interferometer to a few cm but see more below.

  The typical parts are shown below:

  

  Links:

  • Instructions for Red HeNe Laser Kit 1 (Including Zeeman)

    Set up the laser as it would be for stabilization EXCEPT that the polarization axes of the tube should be oriented at ±45 degrees, NOT 0/90 degrees. More on this below.

  • Construction Guidelines for Basic Quadrature-Sin-Cos Decoder and Quad-A-B Interface Kits.

  • Micro Measurement Display 0 (µMD0)

  The tubes provided in the minimal homodyne kits is well behaved (so it could be used in a stabilized laser as another project!). These have two axes of polarization at right angles to one-another whose orientation is fixed for the life of the tube. The amplitude of the power in each axis varies during mode sweep. A linear polarizer could be installed at 45 degrees to the polarization axes to convert the tube in effect to be very similar to a linearly polarized laser, but at the loss of more than one half the power. A better option is to orient the tube so that the polarization axes are at 45 degrees. The PBS cube in the interferometer will than see one half the power in both polarization axes and no power will be lost. However, interestingly, the Path Length Difference (PLD) for optimum performance will be at one tube cavity length, not at 0. For the ~6 inch tube and Linear Interferometer (LI), that is around 2.75 inches; for the Plane Mirror Interferometer (PMI) it is around 1.375 inches. If a PLD of 0 is used, the behavior is, well, very interesting. ;-)

  In case you're curious, the reason there isn't an equivalent "Minimal Heterodyne Kit with Unstabilized Zeeman Laser" is that such a laser would generally not produce a split/beat frequency during part of mode sweep. For homodyne, as long as there is always at least one longitudinal mode present (with no gaps due to mode hops), the interferometer will work properly subject to the constraint that the path length difference is less than a few cm.

• Minimal homodyne kit using linear polarized laser head: This uses a common unstabilized linear polarized HeNe laser head so issues of shocking experiences due to high voltage are mostly eliminated and it is more robust. It's also the simplest of all of these kits to construct. However, upgrades are not really possible partially due to µMD0's limited performance. Being unstabilized and therefore NOT single frequency, the path length difference between the two arms of the interferometer is limited to a several cm.

  These laser produce a narrow beam (less thatn 1 mm in diameter), but because PLD needs to be small, the diameter at the detector should be similar for both the reference and measurement beams. However, if desired, a beam expander can be added. Ask.

  The conversion from Quad-Sin-Cos to µMD0 is all done on the µMD0 PCB.

  Some of the photos show µMD0 constructed on a solderless breadboard, but all these kits now come with the SG-µMD0 PCB instead. If you would still like to use a solderless breadboard (at least initially, or you are not equipped for fine soldering) and one isn't included, they are only a couple bucks on eBay.

  The typical parts are shown below:

  

  Plug the high voltage "Alden" connectors of the laser head and power supply together securely if not already attached. The power supply will probably have a DC connector so it just needs to be plugged into the included wall adapter.

  Orient the laser head so that the polarization axis is at 45 degrees. It will probably be marked but if not, is easily determined with a linear polrizer.

  Links:

  • Construction Guidelines for Basic Quadrature-Sin-Cos Decoder and Quad-A-B Interface Kits.

  • Micro Measurement Display 0 (µMD0)