An ultra-stable, widely tunable and Cs-traceable mid-IR coherent source

Most of the fundamental (and strongest) ro-vibrational transitions of molecular species of atmospheric interest lie in the mid-IR region of the spectrum. With this motivation, in our laboratory we have been applying a difference-frequency-generated (DFG) coherent source, emitting in this spectral region, to high-precision and highsensitivity molecular spectroscopy [1,2]. The availability of optical frequency combs (OFCs) in the visible/near-IR region combined with frequency mixing processes in nonlinear crystals makes it possible to reference such mid-IR radiation to the Cs primary frequency standard, thus extending the present metrological potential of OFCs to even longer wavelengths.