Ultrasensitive dual-beam absorption and gain spectroscopy: applications for near-infrared and visible diode laser sensors

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Abstract
A dual-beam detection strategy with automatic balancing is described for ultrasensitive spectroscopy. Absorbances of 2 × 10−7 Hz−1/2 in free-space configurations and 5 × 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 AlGaInP single-longitudinal-mode diode Nlasers. Applications to trace measurements of NO2, O2, and H2O are described by the use of simple, inexpensive laser and detector systems. Small signal gain measurements on optically pumped I2 with a sensitivity of 10−5 are also reported.