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 time scales, 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 1063cm −1 spanning 56cm −1 . The high-finesse bow-tie cavity provided a 285 m effective path length, 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 , when considering the active measurement time. Measurements of the ν 8 fundamental band of methanol determined concentrations as low as 1.3 ppm in a single shot, which were captured in a 15 ms sweep of the cavity PZT. The detection limit after averaging 30 measurements was 20 ppb, which took 450 ms in measurement time and 70 s in wall time. This demonstrates the potential of cavity-enhanced dual-comb spectroscopy in challenging applications such as free radical kinetics and environmental monitoring.