Developments in the theory of frequency modulation spectroscopy

Abstract

Frequency-modulation (FM) spectroscopy is proving to be a versatile tool in applications where high-sensitivity absorption measurements are required. In this paper we would like to generalize the theory of FM spectroscopy by considering the effect of two commonly used approximations, those of assuming a small modulation index and weak absorption and phase shift. We present a closed form solution for the FM signal when the modulation index is arbitrarily large. This solution shows that an optimum value for the modulation exists. We also show that the clean-phase separation of absorptive and dispersive FM signals only occurs when absorption is weak. © 1983 Optical Society of America