Sensitive Fourier-Transform Cavity Ring Down Spectroscopy Based on a Near-Infrared Frequency Comb

Multiplexed cavity ring-down spectroscopy (CRDS) is challenging to achieve as it requires a detection scheme able to retrieve many undistorted exponential decays simultaneously [1]. We recently demonstrated Fourier transform CRDS (FT-CRDS) based on an optical frequency comb source and a time-resolved Fourier transform spectrometer (FTS) [2]. In this proof-of-concept experiment, the frequency comb was locked to a CRDS cavity using the Pound-Drever-Hall (PDH) technique, to ensure a quasi-continuous comb light transmission through the cavity. The decays were synchronized with the movement of the fast-scanning FTS, allowing to retrieve a 3D spectrum containing the untangled decays at each spectral element. The cavity was characterized by a moderate finesse of 2,000, far below standard reflectivity used for CRDS, and the comb mode absolute frequencies were not stabilized. Here, we revise the locking scheme, as depicted in Fig. 1(a): the offset frequency of the Erbium comb is stabilized to a reference radiofrequency while the repetition rate is phase-locked to a continuous wave laser emitting at 1550 nm, which in turn is locked to a cavity mode using the Pound-Drever-Hall technique. Absolute frequency stability is ensured by actively stabilizing the repetition rate to a tunable reference acting on the piezo-actuator glued to one of the cavity mirrors. The cavity transmission is sent to the FTS where it propagates parallel to a frequency-stabilized HeNe laser whose interferogram is used for optical path difference (OPD) calibration. This reference is also used to trigger the acousto-optic modulator switching off the comb light, allowing retrieval of cavity decays with a controlled OPD step. Fast Fourier transformation of the comb interferograms obtained at different delays after the triggers yields the ring-down events in spectral domain. Finally, these exponential decays are fitted individually to retrieve the CRDS spectrum, which is directly proportional to the absorption coefficient of the analyte, without requiring knowledge of the cavity length or finesse. In addition, as the comb frequencies are stable in time, averaging of successive measurements is now possible.