A key figure of merit for a Fabry-Perot interferometer is the Finesse.

Finesse = (full width @ half maximum of the intensity maxima) / (separation of maxima). Since the Finesse relates to the intensity maxima we have to square the voltage measured by the diode: it measures amplitude in the present configuration, where it acts as a half wave rectifier. An intensity graph is plotted below, from which it is possible to determine the width and position of the peaks.

Doing so we find the following data:

Finally, we are able to determine an average Finesse of 66, for these peaks. It is an interesting quantity since it relates to power reflection coefficient from the mirrors and also to how well it acts as a spectrometer, given by its resolving power. More of all this in a following log entry.