Abstract

The near infrared continuous-wave cavity ring-down spectroscopy (CW-CRDS) technique is used to determine the isotope ratio of atmospheric methane. A narrow linewidth distributed feedback (DFB) diode laser was used to achieve high sensitivity detection with a high finesse cavity [1]. By changing temperature and current of the DFB diode laser, the output wavelength was tuned across the absorption line peaks of ¹²CH₄ and ¹³CH₄ around 6046.954 cm⁻¹ and 6049.121 cm⁻¹, respectively. The isotope ratio of atmospheric CH₄ (-45.7±1.4) ‰ was obtained. We achieved CH₄ absorption spectra with peak signal-to-noise ratios as high as 470:1. The current system has a detection limit of 8.4×10⁻¹¹ cm⁻¹. For performing concentration measurements the sample gas was admitted to vacuum chamber. To detect a trace gas concentration, it is necessary to increase light and gas interaction time as much as possible [2]. Therefore, we used 172 cm long chamber and were able to achieve a decay time of 460 μs and a path length of 138 km. Experiments were conducted at different pressures. The experimentally measured CH₄ absorbance spectra plotted versus wavenumber show a very good agreement with calculations from HITRAN database [3]. The results are presented in Fig. 1.

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