<title>Frequency-modulation absorption spectroscopy for trace species detection: theoretical and experimental comparison among methods</title>

Abstract

Theoretical and practical limits for detection of trace concentrations of gas phase species using frequency modulation spectroscopy are described. A variety of frequency modulation schemes are examined, including wavelength modulation (harmonic detection) spectroscopy (WMS) and one-tone and two-tone frequency modulation spectroscopy (FMS). The distinctions among these methods are mostly semantic and all of these techniques can be described by a single theory. The goal of this research is to define guidelines useful for implementing the optimum modulation technique for specific measurement needs. Applying this formalism, expected sensitivities for each method are compared for selected absorption systems. The results suggest that the choice among techniques is most strongly driven by the individual laser tuning characteristics, the absorption linewidth and the detection bandwidth; no individual method is a priori superior. Results of experimental diode laser measurements which confirm these calculations are presented. Predicted minimum detectable concentrations for a representative variety of gas phase species are also shown.