Re: [CAD_CAM_EDM_DRO] Laser Linear Scales

Chuck Hackett wrote:

>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.
>
>

Yes, this is a real problem. Also, there is the problem of air temperature
changes affecting the accuracy, and thermal expansion of the interferometer
reference-path components. I tried to build one of these years ago, and
was never able to even get any visible fringes due to vibrations. (I don't
have an optical bench supported on air bags at home.)

You should also know that the coherence length of commercial laser diodes
is on the order of a cm, at best. A cheap He-Ne gas laser does a couple
of meters. So, diode lasers, used directly, apparently won't work for
this measurement scale. You could use a DPSS laser, but gas lasers are
pretty cheap. Oh, one other problem with the gas lasers is mode hopping,
where the laser wavelength abruptly changes (by a small amount) as
the laser tube warms up. this will play hob with any long-scale
interferometer.
Of course, the wavelength shift of a diode laser, over a range of a couple
degree C, is hundreds of times worse. During warmup, a visible diode
laser will likely shift a nm or more.

Jon