ABSTRACT
A. N. Dharamsi and Y. Lu; App. Phys. B.; 62,
273, 1996
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 results of measurements of wavelength
modulation spectroscopy of lines in the Oxygen A 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.
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Modulation Spectroscopy