Michelson interferometry with a diode laser

A typical Fabry-Pérot diode laser is used to study the relationship between the laser spectrum and the interference signal obtained from a Michelson interferometer. The interference signal is measured over a range of path length differences of a few centimeters, and the laser spectrum is obtained with a spectrometer of a high resolution of 0.005 nm. Contrary to the laser datasheet, the laser is found to have several longitudinal modes even at high injection currents well above the threshold current. A simple model based on treating the laser's longitudinal modes as pure sinusoidal waves is derived and successfully used in modeling the complex and repetitive interference signal. The spectral spacing and the laser cavity length can be found from the interference signal without the need to use a spectrometer, provided that the interference signal is measured over a long enough range of path length differences.