Spectroscopic trace gas analysis using semiconductor diode lasers
Sensitivity, specificity, high time resolution, cost effective simultaneous measurements of several species and operational systems are the main requirements for in situ trace gas analysis in atmospheric research and process monitoring applications. Tunable diode laser absorption spectroscopy is increasingly being used to measure atmospheric trace gas concentrations down to low ppb-levels and the technique fulfills the requirements for trace gas analysis in the atmosphere for most of the smaller molecules with resolved absorption spectra. Semiconductor lead-salt diode-lasers give 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 further improves sensitivity and detection speed of modern instrumentation. In this paper the FM technique will be reviewed with emphasis to the work, which has been done in our laboratory and findings from r ecent investigations, as well as novel approaches will be discussed. Some applications related to quality control and ISO calibration in addition to measurement challenges for fast and high sensitivity applications will be addressed.