ULTRASENSITIVE DUAL-BEAM ABSORPTION AND GAIN SPECTROSCOPY - APPLICATIONS FOR NEAR-INFRARED AND VISIBLE DIODE-LASER SENSORS

dual-beam detection strategy with automatic balancing is described for ultrasensitive spectroscopy. Absorbances of 2 x 10(-7) Hz-(1/2) in free-space configurations and 5 x 10(-6) Hz(-1/2) in fiber-coupled configurations are demonstrated. With the dual-beam technique, atmospherically broadened absorption transitions may be resolved with InGaAsP, ALGaAs, and AIGaInP single-longitudinal-mode diode lasers. Applications to trace measurements of NO2, O-2, and H2O are described by the use of simple, inexpensive laser and detector systems. Small signal gain measurements on optically pumped It with a sensitivity of 10(-5) are also reported.