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

Abstract

The absorbance is of great importance in the tunable diode laser absorption spectroscopy (TDLAS) as it contains information of both gas properties and spectroscopic parameters. A novel, calibration-free wavelength modulation-direct absorption spectroscopy (WM-DAS) is proposed and experimentally verified in this two-part paper. This method combines the capability of absorbance measurement from DAS and the advantages of enhanced noise rejection and high sensitivity from WMS. In this Part I, we focus on the full theoretical basis and procedures of this method from the following three aspects: the high-accuracy characterizations of laser frequency and intensity, noise rejection ability by extracting the characteristic spectra through the fast Fourier transform (FFT) of the light intensity, and the simultaneous fitting strategy for both baseline and absorbance. The preliminary validation experiment of CO transition at 4300.6999 cm−1 in a static gas cell shows the high accuracy of the proposed method.

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

“…The best-fit parameters including the collisional broadening widths, the integrated absorbance and the peak absorbance for both edges are also listed in Figure 4. The inferred collisional broadening widths and integrated absorbance agreed within 0.6% and 1.2% of those from the noise-free experiment in Part I ( Figure 7) [33], which reveals the good noise rejection ability of the proposed WM-DAS method. In addition, the best-fit residual is one order of magnitude smaller than that in [20] with traditional DAS, and the accordance of the inferred collisional broadening widths and integrated absorbance for both sides is also better than that in [20].…”

“…Figure 3c shows the spectrum of the detected signal derived from FFT. As discussed in Equation (9) from Part I, [33] theoretically, the spectrum of transmitted intensity distributes only at the integer multiples of the modulation frequency, i.e., kf (k = 0, 1, 2 . .…”

“…. ) frequencies into the recovery formula and combining other processes in Part I [33] including the time-frequency transformation and simultaneous fitting, the absorbance obtained from V 1 V 2 and V 1 V 3 edges are shown in Figure 4. The best-fit parameters including the collisional broadening widths, the integrated absorbance and the peak absorbance for both edges are also listed in Figure 4.…”

“…Considering the importance of the noise rejection ability, SNR and the temporal resolution of TDLAS, in Part I [33] we proposed a WM-DAS method based on the FFT analysis of a sine wave scan. This method possesses the advantages of both DAS and WMS, allowing the direct measurement on the absorbance and further gas properties.…”

“…This method possesses the advantages of both DAS and WMS, allowing the direct measurement on the absorbance and further gas properties. The analysis in Part I [33] theoretically demonstrated that enhanced SNR and noise robustness can be obtained by extracting the characteristic spectra of the transmitted light intensity. In addition, the accuracy of the proposed method is improved by using a more accurate description of laser FM and IM response as well as the simultaneous fitting algorithm for the baseline and absorbance.…”