This value is extremely close to theoretical values. Below we show a few sample readouts and the theoretical and experimental frequency separation. Using the data taken from all trials, we were able to create a graph of Frequency Separation vs. Cavity Length. As the graph above shows, our experimental data correlates very nicely with the theoretical equation. As expected, the frequency separation decreases as the cavity length increases.
Curved mirrors make aligning the laser somewhat easier, but are much more expensive to produce.
Using one flat and one curved mirror makes for a good trade between cost and usability. In order for the laser to lase at all, there must be optical resonance between the two mirrors. The setup will lase at any cavity length less than or equal to twice the radius of curvature for the mirrors. Looking at the representation of the Gaussian beam propagating, the positions marking the phase fronts represent possible positions for reflectors to cause resonance. So, if two symmetrical mirrors will lase at any cavity length less than or equal to twice the radius of curvature, the semi-confocal configuration will lase at any cavity length less than or equal to the radius of curvature.