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

Abstract

There is an increasing demand for precise molecular spectroscopy, in\nparticular in the mid-infrared fingerprint window that hosts a considerable\nnumber of vibrational signatures, whether it be for modeling our atmosphere,\ninterpreting astrophysical spectra or testing fundamental physics. We present a\nhigh-resolution mid-infrared spectrometer traceable to primary frequency\nstandards. It combines a widely tunable ultra-narrow Quantum Cascade Laser\n(QCL), an optical frequency comb and a compact multipass cell. The QCL\nfrequency is stabilized onto a comb controlled with a remote near-infrared\nultra-stable laser, transferred through a fiber link. The resulting QCL\nfrequency stability is below 10-15 from 0.1 to 10s and its frequency\nuncertainty of 4x10-14 is given by the remote frequency standards. Continuous\ntuning over ~400 MHz is reported. We use the apparatus to perform saturated\nabsorption spectroscopy of methanol in the low-pressure multipass cell and\ndemonstrate a statistical uncertainty at the kHz level on transition center\nfrequencies, confirming its potential for driving the next generation\ntechnology required for precise spectroscopic measurements.\n