Remote Detection of Methane by Using a 1.7.MU.m Diode Laser.

Abstract

We have demonstrated high sensitivity real-time remote detection of methane in air with a 1.7-μm single-mode diode laser operating at room temperature. The laser current was modulated at 5.35 MHz and the laser center frequency was locked to a methane absorption line at 1.6659 μm. The laser light directed to the probed area was received after one-way transmission or further reflection from a topographic target. The methane absorption was detected by lock-in detection of the 10.7 MHz component in the received optical power. The detection limit in the transmission scheme was 0.3 ppm-m for a signal averaging time of 1.3 s. In the reflection scheme, the sensitivity depended on the range and the reflectance of the target. However, the amount of methane could be measured from the ratio of the 5.35 MHz and 10.7 MHz signal intensities.