Two-tone frequency modulation spectroscopy has been used in conjunction with InGaAsP lasers in the 1.3-microm region to monitor weak water vapor absorptions in a long path White cell. Detection electronics that reduce the effect of Johnson noise are described. The system was capable of detecting optical densities corresponding to <1.7 x 10(-6) in a 1-Hz bandwidth. Factors limiting the difference between observed and shot noise limited performance for these types of laser and condition are discussed.
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Johnson et al. (1991) studied this question.
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