Real-time monitoring of environmental methane and other gases with semiconductor lasers: a review

We have developed a gas-monitoring system using distributed-feedback semiconductor lasers in the 1.3-1.7 mu m wavelength region, The principle, performance and applications of this system are reviewed in this paper, together with recent improvements achieved in a sturdy postable model. By wavelength modulation of the single-mode semiconductor lasers, in combination with a second-harmonic detection technique, a minimum detectable absorption of as low as 3 x 10(-6) is attained, which corresponds to a concentration-pathlength product of 70 ppb m for methane. This high sensitivity and the portability of the system have made various real-time sensing measurements of methane and other molecules possible, such as the precise monitoring of atmospheric gases, field measurements of methane emission rates, itinerant detection of leaks from pipelines and isotope-ratio analyses, We also investigate theoretically the effects of the temperature and pressure of a gas sample on the signal intensity in second-harmonic detection, so that we can precisely determine the gas concentration from a measurement.