Comparison of approaches to modulation spectroscopy with GaAIAs semiconductor lasers: application to water vapor.

Four different forms of modulation spectroscopy were used to study water vapor absorption around 816 nm using GaAIAs 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 I 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 x 10(-6). This is at least an order of magnitude more sensitive than single-tone FM and wavelength modulation.