Referencing mid-IR radiation to an optical frequency comb

In this paper, we report on recent results of experiments carried out to reference mid-IR coherent radiation to an optical frequency comb (OFC) based on a Ti:sapphire femtosecond laser covering the visible/near-IR spectrum. In a first approach we have measured the absolute frequency of a 4.5-mum source obtained with a difference-frequency-generation (DFG) nonlinear process, with the OFC referenced to a Rb/GPS clock. DFG radiation at 4.5 mum is coupled to a high-finesse Fabry-Perot resonator to perform cavity ring-down (CRD) spectroscopy of ro-vibrational molecular transitions of CO₂ rare isotopologues.