The longitudinal mode spacing of an external cavity semiconductor laser is dramatically reduced if a high-reflectivity etalon is inserted in the external cavity. By measuring the reduction in the longitudinal mode beat frequency using a microwave spectrum analyzer, a quick and precise method for determining the coating reflectivity is demonstrated.
Chin B. Su,
"High reflectivity measurements using the beat frequency of longitudinal modes of external cavity semiconductor lasers," Optical Engineering 39(6), (1 June 2000). https://doi.org/10.1117/1.602533