Re: [CAD_CAM_EDM_DRO] re: DRO computer

> Has anyone checked on Laser Interferometry lately? Last I knew, it
> was the way the ultra precision photo tools used to make IC's know
> where they are. It used to be really expensive but with cheap laser
> diodes and semiconductor integration, it has the potential to be far
> cheaper than all the other approaches while being far more accurate
> and immune to just about all the negatives of other approaches. I'm
> just not sure how far the price has come down at this point.

When I checked a bit over a year ago, it was still many $thousand per
axis. The cheap laser diodes won't work, because they aren't "phase
coherent". The laser, considered as an oscillator, puts glitches in
its output waveform as often as every half an inch. This ruins it for
interferometry work. Cheap lasers also have several "modes", which
means several paths in the cavity are oscillating at once, and the
waveforms are overlapped in the output. You can lower the gain on
these paths to eliminate them, but then you need a more powerful laser
so what's left is sufficient.

To get a one-mode laser that's phase coherent for six feet, you need
adaptive power supplies, active cooling, and a laser mechanism other
than a diode. This is why the price is high.

Somebody is producing measuring devices, based on the Doppler effect I
think, which use the cheaper laser diodes. The resolution was midway
in resolution between glass scales and interometry. The price was
still a couple $thou per axis, but it might go down with competition.


A member of the League for Programming Freedom (LPF) http://lpf.ai.mit.edu
-------------------------------------------------------------------------------
Brian Bartholomew - bb@... - www.wv.com - Working Version, Cambridge, MA