Modeling of Scanned Wavelength Modulation Spectroscopy Considering Nonlinear Behavior of Intensity Response

In scanned wavelength modulation spectroscopy (WMS), the nonlinear behavior of the intensity response will produce a periodic ramp-like background signal in the absence of absorption. It always seriously obscures the second harmonic signal in trace gas detection, which is a significant limitation to improving detection sensitivity and limits. In this paper, we propose a theoretical model of scanned WMS based on the power-current relation of the WMS system, which illustrates the mechanism of background generation in a relatively simple way. The model can also characterize the time-domain asymmetry of the
$2{f}$
signal in a reciprocal cycle of wavelength scanning. Simulated results based on the model are further compared with experimental results and exhibit good agreement. The model is instructive for studying the
$2{f}$
signal characteristics of scanned WMS systems.