An important practical limitation to the sensitivity of frequency-modulation spectroscopy arises because of imperfect phase modulation of the laser beam. This imperfection manifests itself as residual amplitude modulation (RAM), and we present data from a careful series of experiments designed to elucidate its origin. Experimentally, we find two components to the RAM, one that depends for its intensity on the laser frequency and one that does not. We present a model that correctly accounts for the intensity and rf phase and frequency behavior of the laser-frequency-dependent component. We are unable to offer a plausible explanation for the laser-frequency-independent component.
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Whittaker et al. (1985) studied this question.
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