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