Do you mean a Laser Interferometer?- these are used to measure the profile of a surface and usualy have an output beam size of around 6 inches. They operate at various wavelengths (usualy 633nm in the visible spectrum and 10.6um for the infra- red).
I've built a few of these and if set up properly you can measure surface "form" (ie departure from perfect) down to a twentieth of a wavelength of light. At 633nm (Helium Neon Laser) this equates to about 32nm (yep- that's 32 nanometres)- it's difficult to measure lower than this because light starts diffracting at less than a twentieth of a wavelength.
Anyway, I may be off at a tangent, but if it is interferometers you are talking about Armstrong Optical here in the UK are good, WYKO in the States make nice ones, Precision-Optical Engineering in the UK used to do them, but I don't think they do any more. I think Lambda Photometrics do them too.
Hope this helps. Shout if you need more info