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

TL;DR

This paper introduces a high-precision, frequency-comb-based mid-infrared spectrometer with a tunable quantum cascade laser, enabling ultra-stable, traceable measurements of molecular spectra, exemplified by methanol spectroscopy.

Contribution

It presents a novel, highly stable and tunable mid-infrared spectrometer traceable to primary frequency standards, combining a quantum cascade laser and optical frequency comb.

Findings

Achieved frequency stability below 10^-15 over 0.1 to 10 seconds.

Demonstrated continuous tuning over approximately 400 MHz.

Measured methanol transition frequencies with kHz-level uncertainty.

Abstract

There is an increasing demand for precise molecular spectroscopy, in particular in the mid-infrared 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 mid-infrared 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 10s and its frequency uncertainty of 4x10-14 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 kHz level on transition center frequencies, confirming its potential for driving the next generation technology required for precise spectroscopic measurements.