Remote Sensing of Trace Gases with Tunable Diode Lasers

Abstract

Tunable diode lasers (TDLs) are often used in point monitoring instruments for the detection of atmospheric trace gases and pollutants. For this application the gas sample flows through a multipass cell at a reduced pressure (~30 Torr), and the resulting narrow infrared absorption lines are detected with very high sensitivity. Several problems must be overcome before similar sensitivities are attained in remote sensing applications under atmospheric conditions. One must contend with broader absorption lines, overlapping and interference from other molecular species, and the presence of atmospheric turbulence. An important system constraint on remote sensing with TDLs is imposed by the low output powers of the TDLs (typically < 1 mW). A cooperative target such as a retroreflector must be employed at the remote site to define the measurement volume. However, if such a target is available, a TDL-based sensing system is extremely versatile and has the potential for higher sensitivity than other infrared remote sensing technologies.