FM Spectroscopy at 1.3 microns 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¬,um 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.