Diode laser sensor for resolution of congested spectra by modulation spectroscopy

Wavelength modulation spectroscopy is a well-established method for sensitive measurements of the properties of gaseous media. The technique is becoming widely used because advances in semiconductor technology have made available diode lasers, which operate in single mode, over a wide range of the visible and near- infrared spectrum. Often the spectral features are congested and in many cases the spectra of different species overlap. In the presence of pressure broadening and/or modulation broadening the overlap can be quite severe. We show that wavelength modulation spectroscopy, performed with higher (than the commonly-used second) harmonic detection provides a means to counter the loss of spectral resolution due to such overlap. A theoretical framework is given. Experimental results which bear out the predictions of the theory are presented. These experimental results show that nearly overlapping lines with highly disparate oscillator strengths can be resolved with higher harmonic detection. The technique allows the simultaneous measurement of several gaseous species, using only one laser.