High-precision methanol spectroscopy with a widely tunable SI-traceable frequency-comb-based mid-infrared QCL

There is an increasing demand for precise molecular spectroscopy, in particular in the mid-infrared (MIR) fingerprint window that hosts a considerable number of vibrational signatures, whether it be for modeling our atmosphere, interpreting astrophysical spectra, or testing fundamental physics. We present a high-resolution MIR spectrometer traceable to primary frequency standards. It combines a widely tunable ultra-narrow quantum cascade laser (QCL), an optical frequency comb, and a compact multipass cell. The QCL frequency is stabilized onto a comb controlled with a remote near-infrared ultra-stable laser, transferred through a fiber link. The resulting QCL frequency stability is below 10−15 from 0.1 to 10 s, and its frequency uncertainty of 4×10−14 is given by the remote frequency standards. Continuous tuning over ∼400  MHz is reported. We use the apparatus to perform saturated absorption spectroscopy of methanol in the low-pressure multipass cell and demonstrate a statistical uncertainty at the kilohertz level on transition center frequencies, confirming its potential for driving the next generation technology required for precise spectroscopic measurements.

R. Santagata, D. B. A. Tran, B. Argence, O. Lopez, S. K. Tokunaga, F. Wiotte, H. Mouhamad, A. Goncharov, M. Abgrall, Y. Le Coq, H. Alvarez-Martinez, R. Le Targat, W. K. Lee, D. Xu, P.-E. Pottie, B. Darquié, and A. Amy-Klein
Optica 6, p 411 (2019). Here is the  link for the full article (or on arxiv).

Here is another recent work from the same team:
– Two-Branch Fiber Link for International Clock Networks.
D. Xu, E. Cantin, F. Frank, N. Quintin, F. Meynadier, P. Tuckey, A. Amy-Klein , O. Lopez, and P. Pottie
IEEE Trans. Instrum . Meas. 68(6), p 2195-2200, 2019. Here is the  link for the full article (or on arxiv)

Experimental setup. The NIR reference signal (frequency 𝜈refνref ) developed at LNE-SYRTE (Paris) is transferred to LPL (Villetaneuse) through a 43-km-long optical-fiber link with active noise compensation (not shown). At LNE-SYRTE, its absolute frequency is measured against primary frequency standards using an OFC. At LPL, a local laser diode (frequency 𝜈OLO) is used as an optical local oscillator (OLO). An electro-optic modulator (EOM) driven at frequency 𝑓EOM with a phase-jump-free synthesizer generates sidebands tunable over 9 GHz in the OLO signal. The beat note signal between one sideband and the reference signal provided by LNE-SYRTE is used to phase-lock the former to the latter, with an offset frequency 𝛿1 (using PLL1). This allows the carrier frequency to be used as a tunable ultra-stable local oscillator. The repetition rate 𝑓rep of an OFC is then phase-locked to the OLO carrier frequency after removal of the comb carrier-envelope offset frequency (via PLL2). A QCL (frequency 𝜈QCL ) is finally phase-locked to the stabilized OFC by performing sum-frequency generation in a AgGaSe2 crystal and processing the beat note signal between the resulting beam and the OFC (using PLL3). Tunability is thus transferred from the local reference to the OFC and finally the QCL. The stabilized and tunable QCL beam is then used to perform saturated absorption spectroscopy in a multipass cell. The QCL is frequency modulated, and the signal is recorded after detection in a lock-in amplifier. PD, photodetector; PLL, phase-lock loop; FM, frequency modulation; OC, optical coupler; OSA, optical spectrum analyzer. Padlocks symbolize phase-lock loops.