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

Abstract

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 1.8×10⁻⁹cm−1Hz⁻¹/², when considering the active measurement time. Measurements of the ᵥ₈ 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.

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