Reaching low temperatures is of great interest for precision spectroscopy, as it simplifies rovibrationnal spectra of large molecules. Efficient cooling can be achieved via the supersonic expansion of a gas through a nozzle, which can be probed using cavity-enhanced spectroscopy [1]–[3]. We demonstrate comb-based Fourier transform spectroscopy (FTS) of acetylene in a supersonic jet. The experimental setup is shown on Fig. 1(a): the comb was an amplified Er:fiber source coupled to a 300-finesse enhancement cavity mounted perpendicular to the gas chamber, and the cavity transmission was analyzed using FTS. The comb was locked to the cavity using the two-point Pound-Drever-Hall scheme, and the repetition rate was stabilized by acting on the cavity length with a piezo actuator [4]. The comb filtered by the cavity had a , and two-burst interferograms were acquired, yielding comb mode resolution [4]. A mixture of 90% Ar and 10% C2H2 was expanded from a reservoir at 16 Torr into a chamber at 0.35 Torr through an aerospike nozzle. The isentropic core of the resulting supersonic jet exhibited a rotational temperature of 140 K and was surrounded by shear layers and residual gas at room temperature.
(a) Setup. osc, oscillator; EOM, electro-optic modulator; PD1,2, photodiodes; DBM, double-balanced mixer; DDS, direct digital synthetizer; DAQ, acquisition board. (b) Experimental data (red) vs cavity enhanced model (blue, inverted). (c) Zoom at 6590.6 cm−1, (d) Allan-Werle plot of the baseline standard deviation.