DOPPLER-LIMITED BROADBAND DUAL-COMB SPECTROSCOPY AT 3 μm

\begin{wrapfigure}{r}{0pt} \includegraphics[scale=1]{spec12.eps} \end{wrapfigure} We present a new scheme of mid-infrared dual-comb Fourier transform spectroscopy. It provides broad-spectral bandwidth absorption and dispersion spectra at a resolution limited by the Doppler width of the molecular profiles, with a negligible instrumental lineshape and self-calibration of the wavenumber scale within the accuracy of an atomic clock. The averaging times can be arbitrarily long and the spectra do not involve any phase corrections or other types of processing that could generate systematic effects or artifacts. Such results build on our concept of feed-forward dual-comb spectroscopy\footnote{Z. Chen, M. Yan, T. W. H\"{a}nsch, and N. Picqu\'{e}, A phase-stable dual-comb interferometer, preprint at arXiv:1705.04214 (2017).}, first demonstrated in the near-infrared 1.5 $\mu$m region. Here, we illustrate our latest results around 3000 cm$^{-1}$ measured at a signal-to-noise ratio that exceeds 1000 and a resolution of 3 10$^{-3}$ \wn. A spectrum with resolved comb lines of acetylene in the region of the $\nu_3$ band, measured within 35 minutes, is shown over its entire span in Fig.a. In an expanded view (Fig.b), the \textit{P}(13) line of the $\nu_3$ band of $^{12}$C$_2$H$_2$ is sampled by the individual comb lines of perfect cardinal sine instrumental line shape. Our technique opens up novel opportunities for broadband and precise investigation of molecular profiles