Abstract

Four different forms of modulation spectroscopy were used to study water vapor absorption around 816 nm using GaAlAs semiconductor lasers. They are wavelength modulation at 1 kHz and 100 MHz, single-tone frequency modulation (FM) at 1 GHz, and two-tone FM at 1 GHz ± 10 MHz. Extrapolating from measurements on a 1.4% absorption we find that two-tone frequency modulation is sensitive to an absorption of 2.4 × 10−6. This is at least an order of magnitude more sensitive than single-tone FM and wavelength modulation.

© 1988 Optical Society of America

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