High frequency modulation spectroscopy: a sensitive detection technique for atmospheric pollutants
Authors: Werle Peter W.
Autors Affiliation: Fraunhofer Inst. fuer, Atmosphaerische Umweltforschung,, Garmisch-Partenkirchen, Ger
Abstract: Sensitivity, specificity, high time resolution, and cost-effective simultaneous measurements of several components using operational systems are the main requirements for atmospheric research and air-pollution-monitoring instruments. Tunable diode lasers absorption spectroscopy is increasingly being used to measure atmospheric trace gas concentrations down to low ppb-levels (10-9 volume mixing ratio). This optical 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-IR spectral region, where the most important atmospheric constituents have strong rotational vibrational absorption bands. The application of high-frequency modulation (FM) schemes can further improve sensitivity and detection speed of modern instrumentation. With this technique, the absorption of a narrow spectral feature is measured by detecting the heterodyne beat signal that occurs when the balance of the FM optical spectrum of the laser is distorted by a molecular absorption line of the target gas. In this paper the FM technique will be reviewed and the predicted and current performance in terms of the detection limit will be discussed.
KeyWords: Absorption spectroscopy; Air pollution; Atmospheric composition; Frequency modulation; Monitoring, Atmospheric pollutants; Detection limit; Detection technique; High frequency modulation spectroscopy; Trace gas analysis, Spectroscopic analysisDOI: 10.1117/12.171229