Atmospheric trace gas monitoring using high-frequency modulation spectroscopy with semiconductor lasers

Abstract

Absorption spectroscopy with tunable diode lasers (TDLAS) is increasingly being used to monitor atmospheric trace gases down to low ppb-levels (10<SUP>-9</SUP> volume mixing ratio). This optical technique fulfills the requirements for trace gas analysis for most of the smaller molecules in the atmosphere with resolved absorption lines. The use of lead salt diode lasers gives access to the mid infrared spectral region, where the most important atmospheric constituents have strong rotational vibrational absorption bands. The application of high frequency modulation (FM) schemes is a promising tool for further sensitivity and detection speed improvements. With this technique, the absorption of a narrow spectral feature is measured by detecting the heterodyne beat signal that occurs when the FM optical spectrum of the probe wave is distorted by the spectral feature of the target gas. In this paper the basic principles of the FM measurement technique are reviewed, experimental data are discussed and measurements of ambient air are presented.