Apparatus:

Here is (almost) the entire setup.  The Fabre Perrot Interferometer is just off the right of the image, along with some of the electronics.

This is the path that the laser takes.  You can see the saturated absorption spectroscopy setup where the beams intersect in the RB Vapor Cell.

This is the main detector setup.  The most important part are the two photodiodes, which detect separate beams (the probe and reference beam) only about 1 cm apart.   The difference of intensity of these two beams gives the saturated absorption spectrums shown in the results.

This is the interferometer I used to calibrate my results.  The box in the background was a current to voltage converter I built for my photodiodes.