Re: Laser Linear Scales

About the coherence length, this should be a problem, however....
I was on one of the hologram sites recently and the guys are all
excited about making holograms several feet deep using Laser Diodes!
even though the short coherence length is supposed to be the limiting
factor.
So go figure.
I've done a lot of experimenting using helium/neon, the vibration
would be a real killer.

Here is a thought...
Instead of aiming the Laser beam down the length of the Scale, turn
it 90 degrees and use a long mirror along the length with about a
0.001"/in slope. The distance between the Laser and Mirror could be
kept very small, so now your measuring a total of .030" instead of
30". This (I would think) would help out the coherence and vibration
problem about 1000:1.
Any irregularity in the Mirror and Laser travel could be calibrated
out.

Problem still would be providing proper long term Laser/Mirror
supports, thermal problems, tiny optics and more.
May even require a dual beam or dual Laser/Mirror setup (two
interferometers) to get a differential output.

Would be a fun project and interesting to see what accuracy one could
get.

Also... Is there such a thing as a sound wave interferometer? (
Ultrasonic )

Tom M.