Wavelength-scanned cavity ring down spectroscopy based on Fourier transform

Abstract

Continuous wave cavity ring down spectroscopy (CW-CRDS) method with using cavity length scanning is ideal for accurately characterizing the low pressure spectra and measuring the small spectral parameters (such as the Dicke narrowing coefficient and the speed dependent collision broadening coefficient). However, the laser of any wavelength can be coupled to the cavity due to the cavity scan, so the spectral noise caused by the laser wavelength fluctuations cannot be ignored. This noise is non-uniformly distributed in the spectrum (especially on both wings on the spectral line) and is difficult to eliminate even with long-term averaging. Unlike the complex laser frequency locking techniques or the optical frequency combs or the better lasers, in this paper, a simple, easy to operate, fast wavelength-scanned CRDS method is proposed based on Fourier transform. The laser wavelength is continuously tuned across the absorption line to measure the periodic ring-down time. A reconstruction algorithm is developed to precisely recover the absorbance by extracting the characteristic frequencies of the periodic ring-down time after the Fourier transform. An etalon, instead of the wavelength meter, is used to calibrate the relative laser wavelength. This method effectively eliminates the non-uniform spectral noise caused by laser wavelength fluctuation in traditional CW-CRDS and significantly improves the measurement accuracy of spectral line parameters (especially line parameters in complex line shapes, such as speed dependent Voigt line shape) at low pressure. In addition, the measuring system, in which no wavelength meter is used, is simpler, more economical than CW-CRDS. The smaller residuals of the Galatry profile fit to the measured CO transitions at R(5) 6371.299 cm<sup>–1</sup> and R(6) 6374.406 cm<sup>–1</sup> show that the noise on both wings of the spectra, caused by laser wavelength fluctuation, is effectively reduced and the spectral SNR is then improved. The measured N<sub>2</sub> perturbed collision broadening coefficient of the Voigt profile fit for CO is consistent with that from the classical CW-CRDS method and is in good agreement with the HITRAN2016 database. The measured N<sub>2</sub> perturbed Dicke narrowing coefficient of the Rautian and Galatry profile and speed dependent collision broadening coefficient of the speed dependent Voigt profile have very good linear relationship with pressure, and have smaller uncertainties than the results from the CW-CRDS method.