A new line-shape asymmetry model for wavelength modulation spectroscopy in gaseous flows

This communication reports technical notes on the development and application of an automated line-shape fitting procedure for wavelength modulation spectroscopy (WMS). Near-infrared transitions of carbon dioxide (CO₂) around 1573 nm were measured in vertical cold (nonreacting) flow of CO₂ at atmospheric pressure using WMS with demodulation at second harmonic frequency. Semi-empirical model based on the set of so-called Gabor functions was developed and parameters of Lorentzian line-shape profile and its asymmetry resulting from simultaneous frequency and amplitude response of the current-modulated semiconductor laser were determined. Nonlinear least-square fitting procedure employing differential evolution algorithm was successfully utilized for performing this task. Line-shape fitting procedure enabling efficient signal de-noising and background subtraction of wavelength modulation spectra was implemented into an open-source code.