Robust cepstral analysis at variable wavelength scan depth for narrowband tunable laser absorption spectroscopy

Abstract

This work focuses on optimizing the modified free induction decay (m-FID) method, or cepstral analysis, for accuracy, precision, and robustness in performing laser absorption spectroscopy at conditions of variable baseline distortion where laser tuning range is of a similar order of magnitude as the targeted spectral linewidth. The optimum selections of the initial and final time of the m-FID signal were assessed at variable scan indices (ratio of laser scan range to spectral linewidth), and guidance is offered in parameter selection based on scan index and noise characteristics of the optical system. The sensitivities of the m-FID method to multiple types of baseline distortion were also analyzed to reflect typical experimental factors such as beam attenuation, background radiation, and laser stability. Experimental demonstration of the technique was performed with an interband cascade laser targeting the CO2 transition (0001 ← 0000) R(60) near 4.2 micron to measure gas concentration at variable scan indices in a static cell by using both the m-FID and direct absorption spectroscopy (DAS) method while varying injection current amplitude. An estimated baseline was determined by the measured laser output and superimposed with different types and levels of distortion. Under conditions of signal distortion, the m-FID method is shown to demonstrate superior accuracy and precision with smaller deviations in time than that rendered by typical DAS, especially at smaller scan depths, showing its potential for measurements at elevated pressures or at a high scan frequency where distributed feedback lasers typically yield narrower tuning range.

A selective laser-based sensor for fugitive methane emissions

“…This was done by computationally adjusting the decay rate of the FID signal to account for collisional broadening of cross-sections. Recently, Li et al optimized this method by careful selection of scan index and initial and final time of the fitting 47 . This method was demonstrated by targeting a CO 2 transition near 4.2 µm.…”

“…Li et al observed that for a small scan index (ratio of laser tuning range to spectral linewidth), most of the m-FID signal is concentrated in the early time period, i.e., it decays rapidly, which makes it difficult to separate the molecular response from the baseline intensity 47 . This means that higher scan index leads to longer decay time of the m-FID signal, which is desirable to minimize interference effects of the fast-decaying signals.…”

A selective laser-based sensor for fugitive methane emissions

“…This was done by computationally adjusting the decay rate of the FID signal to account for collisional broadening of cross-sections. Recently, Li et al optimized this method by careful selection of scan index and initial and final time of the fitting [50]. This method was demonstrated by targeting a CO2 transition near 4.2 µm.…”

“…Li et al observed that for a small scan index (ratio of laser tuning range to spectral linewidth), most of the m-FID signal is concentrated in the early time period, i.e., it decays rapidly, which makes it difficult to separate the molecular response from the baseline intensity [50]. Thus, the tuning range of the laser was maximized to achieve a scan index of ~10.…”

Interference-Free Methane Laser Sensor Using Cepstral Analysis

“…A year later, Goldenstein et al developed an improved model by predicting the baseline intensity [2]. In 2021, Li et al optimized this method by careful selection of scan index and initial and final time of the fitting [3]. However, interference from absorbing species has not been studied using this method.…”