Broadband coherent cavity-enhanced dual-comb spectroscopy

Cavity-enhanced spectroscopic instruments impact a broad range of scientific fields because they are sensitive, self-contained, and can be packaged for use by nonspecialists. Infrared-laser-based cavity-enhanced instruments are typically limited to measuring a few small molecules because they rely on narrowly tunable lasers that each measure a molecular transition. Broadband dual-comb spectroscopy (DCS) is a spectroscopic technique capable of simultaneously probing molecular absorption on thousands of narrowly spaced wavelengths using a single high-speed photo-detector. With coherent averaging, DCS can measure multiple species, including large molecules, with a high signal-to-noise ratio (SNR). Here, we lock a high-finesse optical cavity to a broadband mode-locked frequency comb with an approach that enables a high SNR, broadband cavity-enhanced DCS with coherent averaging. With a 7.5 cm optical cavity, we attain a > 12,000x path-length-enhancement factor, 60 nm bandwidth near 1660 nm (3.5% fractional bandwidth), and > 27 dB SNR in 160 s. We measure the dense rovibrational spectrum of ethane in a background of overlapping H2O, CH4, and CO2 absorption. By combining the broadband, high-resolution, and single-detector nature of DCS with a compact optical cavity, we enable future self-contained instruments that are able to simultaneously detect a variety of molecules in realistic ambient conditions. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement