Resolution of overlapping lines by wavelength modulation spectroscopy using semiconductor lasers

Interference from nearby spectral features is encountered quite often when performing absorption experiments involving gaseous species. Closely spaced or overlapping lines are typically avoided when other lines ofthe same species can be probed, or the density of the probed sample is reduced sufficiently such that the lines do not completely overlap. We present an alternative that can be used in any situation, regardless of the experimental circumstances, to resolve the line or lines of interest. Our results show that wavelength modulation spectroscopy employing higher harmonic detection can be used as a means to extract the spectral features conveniently. We show, through an extension of the Rayleigh criterion, that the higher harmonic detection orders (i.e., those harmonic orders greater than the commonly used second) provide superior resolution over direct absorption and the lower harmonic orders. Wavelength modulation spectroscopy is a sensitive, non-intrusive means of probing absorbing media, such as gaseous samples