A Fabry-Perot cavity (or interferometer, or etalon) is an optical system that has two partially
transmitting mirrors facing each other, between which light coming in through one mirror is reflected
multiple times before it exits the opposite mirror. These cavities may be designed in various ways. The
Model 240 Spectrum Analyzer is a confocal scanning Fabry-Perot interferometer. It consists of two
concave partially-reflecting mirrors separated by their radius of curvature, one of which is mounted on a
piezoelectric material. (For some reason, the system is drawn somewhat differently in the brochure by
Coherent, Inc.) Light entering through one mirror makes four passes through the cavity before exiting.
Where it exits in phase, it produces an interference maximum, and where is it out of phase, there is a
minimum. The result is a circular fringe pattern. If the light entering the cavity is a beam much
narrower than the diameter of the first off-axis fringe, then it will not survive the trip through the cavity
unless it exits in phase with itself. For the geometry of the spectrum analyzer in this experiment, this
happens when nλ = 4d, where d is the distance between the mirrors