REAL-TIME MONITORING OF ENVIRONMENTAL METHANE AND OTHER GASES WITH SEMICONDUCTOR-LASERS - A REVIEW

We have developed a gas-monitoring system using distributed-feedback s emiconductor lasers in the 1.3-1.7 mu m wavelength region, The princip le, performance and applications of this system are reviewed in this p aper, together with recent improvements achieved in a sturdy postable model. By wavelength modulation of the single-mode semiconductor laser s, in combination with a second-harmonic detection technique, a minimu m detectable absorption of as low as 3 x 10(-6) is attained, which cor responds to a concentration-pathlength product of 70 ppb m for methane . This high sensitivity and the portability of the system have made va rious real-time sensing measurements of methane and other molecules po ssible, such as the precise monitoring of atmospheric gases, field mea surements of methane emission rates, itinerant detection of leaks from pipelines and isotope-ratio analyses, We also investigate theoretical ly the effects of the temperature and pressure of a gas sample on the signal intensity in second-harmonic detection, so that we can precisel y determine the gas concentration from a measurement.