Dual-comb spectroscopy over a 100 km open-air path

Dual-comb spectroscopy (DCS) provides broadband, high-resolution, high-sensitivity amplitude and phase spectra within a short measurement time, thus holding promises for atmospheric spectroscopy. However, previous research has been limited to measuring over open-air paths of about 20 km. Here, by developing a bistatic set-up using time–frequency dissemination and high-power optical frequency combs, we implement DCS over a 113 km turbulent horizontal open-air path. We successfully measure the absorbance spectra of CO2 and H2O with a 7 nm spectral bandwidth and a 10 kHz frequency accuracy, and achieve a sensing precision of <2 ppm in 5 min and <0.6 ppm in 36 min for CO2. We anticipate our system to find immediate applications in the monitoring of urban greenhouse gas and gaseous pollutants emission. Our technology may also be extended to satellite-based DCS for greenhouse gas monitoring and calibration measurements. Dual-comb spectroscopy with time–frequency dissemination and high-power frequency combs enables sensing CO2 and H2O over a 113 km turbulent open-air path, with a sensing precision as high as 2 parts per million of CO2.