Instructions for DIY Interferometer Displacement Measuring System Kits
Version 1.04 (17-Oct-21)
Introduction
UNDER CONSTRUCTION
This document contains guidelines and links to other manuals for
the construction of a homodyne or heterodyne interferometer displacement
measuring system based on a bare HeNe laser
tube and a measurement readout based on a microcontroller. There are
kits providing various levels of performance from really basic to
something similar to a commercial system.
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 have been tested to assure they are compatible with single frequency or
two-frequency (Zeeman) stabilization. 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 currently a pair of HP/Agilent or
Excel optical receivers (yes I know it's cheating slightly). So in the near
future, these will be the SG-OR3 custom PCB. 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 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.
There are several variations on this kit:
- Heterodyne interferometer using two frequency Zeeman HeNe
laser: This is based on the same principles as systems from
HP/Agilent/Keysight and Excel.
- Stabilized Zeeman HeNe Laser Kit 1 with or
without Arduino. The DIY Displacement kits use the Arduino version.
- Micro Measurement Display 2 (µMD2).
Previously, the µMD1 kit was used, they are no longer available.
Should that be preferred, it is still possible to construct µMD1
using the SG-µMD1 PCB with programmed PIC, and buying
most of the other parts through Digikey. See item # 303980234586 .
When combined with the heterodyne combined kits here, the cost would be
reduced by $50.
The detectors for heterodyne known as "optical receivers" may be of two
types:
- As of now, these are either HP/Agilent 10780A/B/Cs or 10780F/Us,
or Excel 1031As or 1031Fs. They are all electrically and
functionally identical. But the 10780A/B/Cs and 1031As include a lens to
focus the beam onto the detector and a linear polarizer at 45 degrees to
combine the H and V polarized components of the beam. If one or both of
the optical receivers are 10780F/Us or 1031Fs, they have no lens or
polarizer. In that case, polarizers will be provided either already
in or attached to the front of the optical receiver, or separately
for you to install. This is DIY kit after all. ;-) The 10780s and
1031s require 15 VDC. Since the heater normally runs on 12 VDC, a
DC-DC boost converter is included.
- Starting in late 2021 or early 2022, the optical receivers for these
"DIY Interferometer Displacement Measuring System Kits" ONLY
will be the SG-OR3, and thus cheating by using commercial parts for those at
least will no longer be possible. :) SG-OR3 uses a relatively simple PCB so
assembly is straightforward and Hathkit™-style assembly
instructions will be 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.)
- Homodyne interferometer using Single Frequency (SF) /
Single Longitudinal Mode (SLM) laser: This is
based on the same principlse as systems from Teletrac/Axsys/General
Dynamics/MotionX and Renishaw.
The detector in the kits for homodyne is quite primitive consisting of
pair of biased photodiodes and with a
Quarter WavePlate (QWP) to shift the phase of one of the signals by
90 degrees and the bandwidth is quite limited. But a better one could
be constructed relatively easily.
- 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, though there is currently no
interpolation so native resolution for heterodyne is only
80 nm with a plane mirror interferometer.
- Minimal homodyne kit using SF / SLM laser:
The slew rate is further limited by the entry-level µMD0 display.
- Minimal homodyne kit using unstabilized Multi-Longitudinal Mode
(MLM) laser: This uses a common unstabilized linearly
polarized HeNe tube, which limits the path length difference between
the two arms of the interferometer to a few cm.
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.
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