A trace methane gas sensor using mid-infrared quantum cascaded laser at 7.5 μm

Presented is a compact instrument developed for in situ high-stable and sensitive continuous measurement of trace gases in air, with results shown for ambient methane (CH4) concentration. This instrument takes advantage of recent technology in thermoelectrically cooled pulsed Fabry-Perot (FP) quantum cascaded (QC) laser driving in a pulse mode operating at 7.5 mu m to monitor a well-isolated spectral line near the nu 4 fundamental band of CH4. A high-quality liquid nitrogen cooled mercury cadmium telluride mid-infrared detector with time discriminating electronics is used along with a total reflection coated gold ellipsoid mirror offering 20 cm single pass optical absorption in an open-path cell to achieve stability of 5.2 x 10(-3) under experimental condition of 200 ppm measured ambient CH4. The instrument operates continuously, and integrated software for laser control using direct absorption provides quantitative trace gas measurements without calibration. One may substitute a QC laser operating at a different wavelength to measure other gases. The instrument can be applied to field measurements of gases of environmental concern.