Portable Diode Laser Spectrometor for Trace Gas Analysis

Summary

A Pb-salt diode laser spectrophotometer has been developed in order to detect atmospheric trace-gas. The instrument is pointed to be capable of non-contacting, local and real-time measurement as well as to be portable. The developed system is substantially a double-beam derivative spectrometor having an one-meter open optical path along with a reference gas cell filled with the target gas of atmospheric pressure. Two spectra, the atmospheric and the reference, are correlated by an ad hoc microprocessor system to produce a density based on the notion of “adjoint spectra”. Optical noises limit the sensitivity and the reliability. The correlation procedure and a particular laser-frequency modulation technique are exploited to suppress the optical noise. For atmospheric methane an accuracy of 0.1 ppm (rms) has been achieved for atmospheric methane with 7.6 µm band in an in-door experiment. A portable instrument was built and outdoor experiments have been made.