Students in our upper-division/graduate physical optics laboratory course assemble a high-finesse Fabry–Perot interferometer (FPI) from components, mode-match it to a helium–neon (HeNe) laser, and examine some of the FPI system’s properties and uses. Here, we specify the necessary equipment and describe experimental procedures. For example, the experiments use the FPI’s high spectral resolution to monitor the laser’s behavior as it warms up; in another experimental step, they confront the issue of the photodetection system’s electronic bandwidth. We also provide a review of Gaussian beam formulas and detail their use in mode-matching calculations.