FREQUENCY-MODULATION SPECTROSCOPY AT 1.3 MU-M USING INGAASP LASERS - A PROTOTYPE FIELD INSTRUMENT FOR ATMOSPHERIC CHEMISTRY RESEARCH

Two-tone frequency modulation spectroscopy has been used in conjunction with InGaAsP lasers in the 1.3-mu-m 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.