Optical EngineeringSuppression of the etalon fringe in absorption spectrometry with an infrared tunable diode laser
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A polarization control method is applied to suppress the e´ talon fringe, and experimental results are described. It is found that this method can suppress an e´ talon fringe of the order of 102-6 in terms of the equivalent absolute absorption value, 1/100 of the off-controlled condition. This technique enables our tunable diode laser absorption spectrometry (TDLAS) system to measure atmospheric methane of about 2 ppmv concentration with ambiguity within 102-4 ppmv (0.1 ppbv). This scheme is useful for the portable TDLAS systems because of its simplicity.