Re: [CAD_CAM_EDM_DRO] re:Re: Interferometer designs for DRO or servo control?

Elliot Burke wrote:

>2. the laser wavelength is not completely stable unless you have a (duh)
>stabilized laser. There are two sorts of common lasers used for
>interferometry, the HeNe and the diode. The HeNe has the remarkable
>property that its wavelength is known to 1 ppm, so if care is taken (see 1.
>above) distance measurements of this precision can be made. The HeNe can
>mode hope, which can cause the fringe count to jump even then there is no
>motion, but there are ways to prevent this I won't get into here.
>
>The second common laser is the diode type. A cheap 650 nm laser pointer
>will make a nifty source for your interferometer, but its wavelength is not
>exactly known (can be calibrated) and is unstable with temperature and drive
>current.
>

The other problem with the common diode lasers is that the coherence
length is
just a few mm. Just as with HeNe, it is many times larger than the
cavity length,
but the cavity in the diode laser is just um long!

Jon