Abstract

High-sensitivity remote measurements of methane concentration in the air with a 1.66 m distributed-feedback diode laser have been demonstrated by laboratory simulations. The laser drive current is modulated at 5 MHz, and the laser center frequency is stabilized on the center of a Q-branch line of the 23 band of methane. The collimated laser radiation thus modulated in both frequency and amplitude, is directed to the probed region and is collected by a receiver after one-way propagation or further backscattering by some topographic targets. The methane absorption is detected from the second-harmonic component in the received radiation intensity, while the received power is measured by using the fundamental component. The ratio of the lock-in detected signals of the two components gives the amount of methane and is independent of the received power. The minimum detectable concentration pathlength product in the one-way scheme was 0.3 ppm-m. In the backscattering scheme the detection limit depended on the distance and the reflectivity of the target; it was 50 ppm-m when a tip of wood was placed at a distance of 5 m. The amount of methane could be determined within an uncertainty of a few percent. The present method will be useful for real-time monitoring of natural gas leaks and for measurements of trace methane in the natural atmosphere.

© 1990 Optical Society of America