A modified wavelength modulation spectroscopy (WMS) based on the self-heating effect of the tunable diode laser when driven in quasi-continuous-wave (QCW) mode is investigated. A near-infrared distributed feedback (DFB) diode laser working at the QCW mode is employed as the QCW light source, and CO<sub>2</sub> is selected as the target gas. The characteristic of the QCW second harmonic (2f) line profile is analyzed through a comparison with that of the traditional CW WMS with the same system. A noise-equivalent absorbance of 3.2×10<sup>-5</sup> Hz<sup>1/2</sup> for CO<sub>2</sub> at 1.58 μm is obtained with 18-m optical path. The QCW WMS lowers the dependence on lasers and expands selectivity, thus verifying the feasibility of the method.
© 2012 Chinese Optics LettersPDF Article