Abstract

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.

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