Wavelength modulation spectroscopy for recovering absolute absorbance

Abstract

A highly sensitive, calibration-free method, wavelength modulation - direct absorption spectroscopy (WM-DAS), is proposed by extracting the characteristic frequencies of modulated laser intensity. This method combines the advantages of measuring absolute absorbance profile from calibration-free DAS with the enhanced noise rejection and high sensitivity of WMS. The preliminary validation experiment of HO transitions shows that the WM-DAS method improves the accuracy of the absorbance profile measurement with a best-fit standard residual of 4.9 × 10. This new method might inspire new approaches to high-precision measurement of spectral line parameters and gas information with weak absorptions or industrial applications.

Highly Sensitive, Calibration-Free WM-DAS Method for Recovering Absorbance—Part I: Theoretical Analysis

“…The reassigned relative wavelength points are shown by the pink "○" symbols together with its best-fit curve with Equation (4). As illustrated in Figure 2b, the best-fit residual that considers only the linear response, marked by the red line, presents pronounced sinusoidal structure at twice the modulation frequency, indicating the non-ignorable 2ω nonlinear term [37]. In contrast, the best-fit residual with Equation (4), shown by the blue line, is reduced by more than one order of magnitude with a standard deviation as small as 1.15 × 10 −4 cm −1 .…”

“…The nominal FSR value was experimentally checked by measuring the To obtain the absorbance, the relationship between the laser wavelength (Equation (1)) and intensity (Equation (2)) is essential in WM-DAS. Here, we define variable x as [37]…”

“…Inspired by the above-mentioned works, in reference [37], we first proposed the preliminary idea of the WM-DAS method, which directly measures the absorbance by extracting the characteristic frequencies of the modulated light intensity based on the FFT analysis. However, the accuracy of the laser light intensity and frequency model in [37] is still limited. Meanwhile, the baseline in Ref.…”

“…Meanwhile, the baseline in Ref. [37] has to be experimentally measured, which is not practical in some conditions. In this paper, we further developed the WM-DAS method from the following two aspects.…”

“…Second, a simultaneous fitting algorithm was developed to deduce both baseline and the absorbance based on the accurate model of baseline. Therefore, although the preliminary idea of this method has been reported from our group [37], this paper is still significant for generalizing and developing it toward a fully-fledged method. To provide a better understanding and an easy implementation of the WM-DAS method to readers, several crucial equations that have been reported in reference [37] are also included in this manuscript.…”