Abstract
The wavelength modulation-direct absorption spectroscopy (WM-DAS) was used to measure the spectroscopic parameters for 8 H2O absorption transitions near 1.39 μm at 300–900 K. A reference light path was introduced to eliminate the effect of ambient gas and inherent fluctuation of laser intensity. Experimental results at high temperatures for the transition at 7185.597 cm−1 showed that the WM-DAS enhanced noise rejection by extracting the characteristic frequencies of modulated laser intensity. The SNR was enhanced to 1800 for the peak absorbance of 20% and remained 380 around the peak absorbance of only 0.5% by using this method, showing its huge potential in situ gas sensing in harsh engineering fields. The Voigt, Rautian, Galatry and quadratic speed-dependent Voigt profiles were used to recover the absorbance and determine the best-fit spectroscopic parameters and their temperature dependence. The measured N2-broadening coefficients compare well with the available values listed in the HITRAN/HITEMP database. The Dicke narrowing and speed-dependent coefficients were also determined with analyses of the uncertainties in these measured parameters.
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