Improvement of a Pound-Drever-Hall Technique to Measure Precisely the Free Spectral Range of a Fabry-Perot Etalon

Abstract

We examine the principle of a modified Pound-Drever-Hall (PDH) technique to measure the free spectral range of a Fabry-Perot etalon (FPE). The FPE's periodic transmission of phase-modulated light allows us to adopt a sampling theorem to develop a new relationship for the PDH error signal. This leads us to find the key parameters governing the measurement accuracy: the phase modulation index β and the FPE finesse. Without any additional complexity for background noise reduction, we achieve a measurement accuracy of 0.5 ppm. The improvement is mainly attributed to the wide-band phase modulation approaching β = 10, and partly to the use of both reflected and transmitted light from the FPE and good FPE finesse.