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.