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.
Highlights
In the past several decades, the tunable diode laser absorption spectroscopy (TDLAS) has played a growing role in the diagnostic of combustion and plasmas, as well as in some industrial applications [1,2,3,4,5,6,7,8,9]
Inspired by the above-mentioned works, in reference [37], we first proposed the preliminary idea of the wavelength modulation-direct absorption spectroscopy (WM-direct absorption spectroscopy (DAS)) method, which directly measures the absorbance by extracting the characteristic frequencies of the modulated light intensity based on the fast Fourier transform (FFT) analysis
In contrast to the triangular wave in DAS or the triangular wave superimposed by a high-frequency sine wave in wavelength modulation spectroscopy (WMS), pure sinusoidal signal with frequency ω is used to scan around the absorption line in WM-DAS
Summary
In the past several decades, the tunable diode laser absorption spectroscopy (TDLAS) has played a growing role in the diagnostic of combustion and plasmas, as well as in some industrial applications [1,2,3,4,5,6,7,8,9]. Absorbance is acquired through a triangular or sawtooth wave-scanned direct absorption spectroscopy (DAS), which is easy to implement and has relatively straightforward data processing [13,14,15] This method suffers from these following problems in practice: first, the uncertainty in the baseline determination reduces the accuracy of the absorbance. In contrast to DAS, wavelength modulation spectroscopy (WMS) using harmonic detection technology can effectively eliminate the influence of baseline uncertainty and work with much higher modulation frequency (as high as several hundred kHz) [20,21,22,23]. 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. The features of the proposed method and its potential application are demonstrated using experimental results and discussed in
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