my $0.02 and laser interferometers

Message: 25
Date: Thu, 15 Jul 1999 16:58:23 -0500
From: Jon Elson <jmelson@...>
Subject: Re: counts per inch, my $0.02



Elliot Burke wrote:

> From: "Elliot Burke" <elliot@...>
>
> If the DRO is used with a Twyman-Green interferometer using a HeNe laser

at

> 632.8 nm wavelength, a fringe count will occur every 312.4 nm, which comes
> out to 80278 counts/inch.
> Other types of interferometers may have twice this sensitivity.
> Shorter wavelengths (doubled YAG at 532 nm) will have even more

sensitvity.

>
> For those interested in using interferometers to measure distance, a
> suitable laser source costs much less than $100. The optics required

aren't

> too complex. Basic precision can be 0.5 ppm, if the temperature and
> humidity of the air path are compensated for.

I tried to breadboard something like this many years ago. I couldn't get
any fringes. One of the problems is I couldn't afford the 1/4 wave
retardation plate, to convert plane to circular polarization. Diode lasers
should do fine, at $9 for a 5 mW output at 660 nM.

Jon

Something must have been very wrong, it's hard NOT to get fringes with a
coherent source.
There are many interferometers that don't need quarter wave plates (QWP), in
fact, most don't. Look in any university library, Steel's book on
interferometry is a good one, also Malacara's Optical Shop Testing.
The diode lasers are sometimes useful, in a limited way, for interferometry.
There are two limitations:
1. the wavelength changes by about 0.25 nm/�C, or 0.03%/�C. Not very good,
unless you are very well stabilized. Also there is no easy way of knowing
precisely what the wavelength is, since they are all different by 1% or so.
Wavelength is not tightly controlled.
2. the coherence length most most LD's is short. A path difference of more
than a few mm gives very low fringe contrast. There are some LD's with
longer coherence length, but it varys with drive current, age, and other
factors.
Unless you want to be a laser engineer, stay away from metrology with LD's.

The HeNe laser, on the other hand, is an inspired choice for metrology.
NIST has certified its wavelength to be know to 0.5 ppm, so if the index of
refraction of air is corrected for by temp and humidity measurement, a 1 ppm
accuracy is easily obtained.
The coherence length of the HeNe is also very long, many meters, if a single
or double longitudinal mode laser is chosen.
I have obtained microinch stability over 60 inch paths over periods of
hours, on a big surface plate in a temperature controlled room.

Elliot Burke