Sensitive density-fluctuation measurements using wavelength-modulation spectroscopy with high-order-harmonic detection

Abstract

We discuss experimental and theoretical results which show that when wavelength-modulation spectroscopy is used to monitor concentration fluctuations of gaseous species, greater sensitivity may be obtained if one uses high-order detection. We also show that, depending on the ambient concentration being monitored, there are regions in which the commonly used second derivative would show a negligible variation of signal magnitude with concentration fluctuations, whereas measurement with a higher harmonic would result in a much improved signal. Theoretical results for the measurements of any transition that can be described by the Voigt profile are given. The technique discussed is illustrated by presenting the results of measurements of wavelength-modulation spectroscopy of lines in the oxygenA band. Different detection harmonic orders are suitable for different ambient concentrations, and a related criterion that helps in the determination of a suitable detection harmonic order is given.