re:Laser Linear Scales

Chuck,
I've designed and built interferometers for about 30 years now. They are
great for many appications, including metrology.

99% of problem with using laser diodes in metrology is that their wavelength
is strongly dependant on temperature and drive current. Wavelength is the
scale, if it is not constant you don't have much accuracy.
For example, a typical laser diode could have a 0.25 nm/C wavelength shift,
this would be 0.03%/C for a 780 nm LD. Way too much.
Wavelength also changes as the LD ages.

To stabilize a laser diode, it is run attached to a TE cooler. Some means
of measuring wavelength must be provided. Gas absorbtion lines are
typically used, these are precise and not too hard to make.

If you do a search for "chip scale atomic clock" you'll find some references
to this work.
A Cs vapor source can work just fine. The target size and power draw for
these is a few cm^3 and 30 mW.

Other sorts of wavelength standards can be imagined, but are usually not
stable enough over long periods of time and environmental variations.

There is a strong effort being made now to develop stabilized diode laser
sources. There are some in the telcom field, these are quite expensive.

The mis-statement has been often repeated that the coherence length of diode
lasers is not adequate for long distance measurements. This was true ten
years ago for the cheapest laser diodes, however technology changes have
made even the cheapest laser pointer have a long coherence length, several
meters in most cases.

There are many references for this.

BTW, a laser diode can be frequency modulated by modulating the drive
current. A superheterodyne detection scheme can therefore be used.

Another problem with using laser diodes in metrology is that the measured
distance is the optical path, which is equal to the index of refraction of
air times the distance. The index of refraction of air varies with
temperature, pressure, and humidity. Typically a reference interferometer
is required to compensate for these factors. Sometimes a little weather
station is positioned near the interferometer to collect data and allow
compensation.

For measuring short distance (a few mm) a neon tube source (not a laser)
works well as an interferometer source if the neon is made from one isotope.

In short, scales are easier for most cases. and work well enough for most
applications.
It is very difficult in same cases to eliminate Abbe offset error with
scales, for these the interferometer may be the only way to go.

regards-
Elliot

Message: 1
Date: Mon, 31 May 2004 19:01:48 -0500
From: "Chuck Hackett" <egroupscdh@...>
Subject: Laser Linear Scales

This may have been covered before but ...

With the availability of good quality laser diodes these days wouldn't it be
possible to build a low cost/high precision linear position sensor based on
Laser Interferometry?

... I assume not, or I'd see them everywhere. What's the sticking point?

I suspect that one of the problems might be the ability to track fringes
caused
by normal machine vibration.

Cheers,

Chuck Hackett


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.686 / Virus Database: 447 - Release Date: 5/14/2004