Cavity-enhanced dual-comb spectroscopy in the molecular fingerprint region using free-running quantum cascade lasers

Cavity-enhanced dual-comb spectroscopy promises broadband, high-resolution, and highly sensitive spectroscopic measurements on sub-millisecond timescales, making it highly attractive for trace gas monitoring. In this work, we demonstrate cavity-enhanced dual-comb spectroscopy in the molecular fingerprint region using two quantum cascade lasers (QCLs) operating as optical frequency combs centered at 1063 cm^-1 spanning 56 cm^-1. The high-finesse bow-tie cavity provided a 285 m effective pathlength, and the high power-per-mode of the QCL combs granted a strong multi-heterodyne signal of the swept-cavity transmission. This ultimately resulted in a noise equivalent absorption per spectral element of 1.8×10^-9 cm^-1 Hz^-1/2. Measurements of the ν₈ fundamental band of methanol determined concentrations as low as 1.3 ppm in a single shot, with a detection limit of 20 ppb after 30 averages. This demonstrates the potential of cavity-enhanced dual-comb spectroscopy in challenging applications such as free radical kinetics and environmental monitoring.