High Sensitivity Short-Path Monitoring of Trace Gases Employing PbSnTe Tunable Diode Laser

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Abstract
A system for atmospheric trace gas monitoring is described. The absorption spectrum in the infrared region of a specific gas is involved. A lead-salt diode laser is employed for the tunable light source and the short optical path of less than one meter allows the equipment to be portable. Fast measurement can be performed thanks to be built-in microcomputer and specially developed electronic system. Reliability is achieved because a correlational algorithm is calculated by the computer avoiding the ambiguity of the absolute laser frequency. Gas-density data is obtained every few seconds. Experiments were executed for methane and a sensitivity of 0.3 ppm·m with S/N=1 was attained being limited by parasitic etalon fringes. A higher sensitivity down to 0.01 ppm·m is possible provided that a transversally mono-modal laser is employed.