PRECISION CAVITY-ENHANCED DUAL-COMB SPECTROSCOPY: APPLICATION TO THE GAS METROLOGY OF CO2, H2O, and N2O.

With inherent simplicity, mutual phase coherence, and a high degree of user control, electro-optic frequency combs are amenable to both dual-comb spectroscopyfootnote{I. Coddington et al., textit{Optica} textbf{3,} 414 (2016)} and cavity-enhanced comb spectroscopy.footnote{B. Bernhardt et al., textit{Nat. Photonics} textbf{4,} 55 (2010)} This combination of fast, multiplexed spectroscopy, with an effective absorption pathlength $>$1 km, is used here to perform line-by-line metrology of the gas-phase absorption spectra of CO$_2$, H$_2$O, and N$_2$O in the near-infrared. We report absolute transition frequency with precision better than 1 MHz in 1 s of spectral acquisition per transition using a comb with an instantaneous optical bandwidth of 6 GHz, tunable over the entire 6240-6370 wn range. A full model for the electric field transmitted through the enhancement cavity (even in the presence of strong molecular absorption and dispersion) will be discussed