Phase-Sensitive Open-Path Dual-Comb Spectroscopy with Free-Running Combs

Open-path dual-comb spectroscopy has emerged as a promising technique for regional multigas monitoring with its conspicuous advantages of broadband spectral coverage, high spectral resolution, and rapid update rate. However, it is challenging to realize its full potential due to the undesirable mutual coherence of the dual-comb source and turbulence in the air path, which hinder it from field-deployed open-path applications. Here, phase-sensitive open-path dual-comb spectroscopy based on free-running combs is reported, in which dual-purpose compensation is proposed to provide immunity against both the time jitter of comb sources and turbulent noise. Broadband and high-fidelity atmospheric amplitude and phase spectra containing gas absorption and dispersion information over a 900-m turbulent air path are acquired. For the rovibrational resonances of ${\mathrm{CO}}_{2}$ and ${\mathrm{H}}_{2}\mathrm{O}$ in 6250--6660 ${\mathrm{cm}}^{\ensuremath{-}1}$, the achieved residual of the amplitude spectrum is no more than 0.01, and the average residual of the phase spectrum is 0.2 mrad, corresponding to about 0.2 as of relative timing noise or a refraction-index change of about 6 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}14}$ over the target path. The precision of concentration retrieval is about 3 ppm for ${\mathrm{CO}}_{2}$ in 30 s. A simulated gas-leakage measurement validates the dynamic monitoring capability of this system. This highly effective noise-compensation method provides the possibility of deploying the portable configuration and holds the potential to propel environmental protection and atmospheric science.