Direct Frequency Comb Cavity Ring-Down Spectroscopy Using Vernier Filtering

We present direct frequency comb cavity ring-down spectroscopy with Vernier filtering as a straightforward approach to sensitive and multiplexed trace gas detection. The high finesse cavity acts both to extend the interaction length with the sample and as a spectral filter, alleviating the need for dispersive elements or an interferometer. In this demonstration, a free running interband cascade laser was used to generate a comb centered at 3.3 μm covering ∼10 cm^-1 (300 GHz), which was coupled into a high-finesse cavity, providing 0.9 km of effective path length. The Vernier configuration transmitted the comb modes in sequence, and ring-down decays could be initiated either with an acousto-optic modulator or by scanning the cavity length beyond the adiabatic limit, with trade-offs between duty cycle and sensitivity. This implementation achieved a figure of merit of 4.3 × 10^-8 cm^-1 Hz^-1/2 per spectral element, and measurements of toluene demonstrated its ability to observe trace concentrations of a continuum absorber. This method does not require anything beyond a conventional single-mode cavity ring-down instrument other than an optical frequency comb, making it readily accessible to a wide range of applications.