High precision calibration of spectral parameters of CO at 1567 nm based on wavelength modulation-direct absorption spectroscopy

Abstract

<sec> Direct absorption spectrum (DAS) can be used to measure the molecular absorptivity function and determine the spectral parameters of the gas by fitting the measured absorptivity function. Wavelength modulation-direct absorption spectroscopy (WM-DAS) is based on DAS and combines with the idea of harmonic analysis in wavelength modulation spectrum (WMS). The measurement accuracy of absorptivity function can be effectively improved by using Fourier transform. In this paper, the absorptivity function of CO R5–R11 near infrared weak absorption line at 1567 nm is accurately reproduced by using the WM-DAS method combined with long optical path gas absorption cell at room temperature and low pressure. The standard deviation of the fitting residual reaches 3 × 10^–5, and then the spectral parameters such as collision broadening, Dicke narrowing and speed-dependent collision broadening coefficients are measured in high precision.</sec><sec> These parameters are compared with the results from the high sensitivity continuous wave cavity ring down spectroscopy (CW-CRDS). The experimental results show that the signal-to-noise ratio of the absorptivity function measured by CW-CRDS is about 2.5 times that of the long-path WM-DAS, and the measured spectral parameters are highly consistent. The relative errors of the collision broadening coefficients obtained by using the Voigt profiles of the two methods are less than 1%. At the same time, the detection limit of CO at 1567 nm based on the WM-DAS method is about 80 ppb, and the corresponding absorption coefficient is 2 × 10^–10 cm^–1, which is slightly higher than that from the CW-CRDS method. However, the WM-DAS method has the advantages of fast measurement speed, simple system and low cost, and is expected to provide a new method of measuring the weak absorption lines.</sec>