This thing is a Michelson interferometer,
It can be used to measure relative motion with nanometer precision and accuracy. This is done by counting interference fringes generated by the constructive and destructive interference of coherent light. A transition from constructive to destructive interference corresponds to a movement of half the wavelength of the light used.
It consists of a laser holder, laser, a beam splitter, two mirrors, a diffusing lens an optional photodetector and the framework for fixing the position of all the parts.
Good luck implementing it as a position feedback system on your 3D printer ;-)
It's mostly based off of a kinematic mirror mount http://www.thingiverse.com/thing:30727
There are two images include that show the interferometer mounted vertically. These were an attempt at making a load-displacement testing tool. Unfortunately the vibration in the linear actuator was too great, making displacement measurements erroneous.
Next step is to mount a piezo buzzer behind one of the mirrors to determine its stroke.
Print two laserHolder01 parts to mount your laser pointer using 6 M3 screws. To my delight, all of the laser pointers I've tried have been coherent enough to produce interference patterns.
Print three mm052 kinematic mirror mounts, two for the mirrors and one for the lens. These are based on the thing (http://www.thingiverse.com/thing:30727). They require 2 M3 screws each, two M3 nuts and two 3x3mm magnets. Any highly polished mirror should work, I used pieces of Silicon. The lens I used was salvaged from an old camera, I don't know the specs but almost any should do.
Print one mm035, 45 degree kinematic mirror mount for the beam splitter. Follow the same construction as above. You can purchase 50% beam splitters from optical component manufacturers such as Anchor or Thorlabs. Anchor has one for ~$10 (http://www.anchoroptics.com/catalog/product.cfm?id=258). If you have a sputter deposition tool you can prepare your own half silvered mirror as I did. 20nm of chromium did the trick.
Assemble two pieces of T-slot extrusion into a cross and mount all optical pieces. All pieces were mounted using T-Nut fastener screws (http://www.mcmaster.com/#catalog/118/1106/=jv51zp), these simply have a wide but shallow head allowing it to slide into the t-slot without pulling out. This is probably not the best way to do it but it works.
Follow the diagram on wikipedia for the placement of the optical component, it's very straight forward.. The lens can either be placed before the beam splitter, creating interference rings, or after the beams have re-converged, creating interference lines. The images shown here are interference lines, which seemed to be a little easier for detecting with the photodetector.
Note: do not attempt to debug this while playing loud music. The mirrors can easily pick up the vibrations making mirror alignment difficult.