Extending the optical comb synthesizer to the infrared: from He at 1.083 um to CO2 at 4.2 um

We report what is, to the best of our knowledge, the first extension of an optical frequency comb synthesizer (OFS) to the infrared, around 4.2 ?m, by combining it with a difference-frequency spectrometer. We show preliminary results on absolute frequency measurements of molecular CO2 lines. Moreover, we report the first absolute frequency measurements, with uncertainties up to 7 × 10-12, on atomic helium at 1.083 ?m wavelength, by using the OFS system.