Frequency-comb-assisted mid-infrared spectroscopy

A new class of IR coherent sources and IR frequency combs, that combine optical frequency-comb synthesizers (OFCSs) and optical parametric up/down-conversions, is already available and still progressing at a very fast pace. Peculiar features for IR radiation produced by difference-frequency-generation (DFG) set-ups or quantum-cascade lasers (QCLs) can he achieved when they are phase and frequency controlled by the OFCS. Indeed, their frequency is accurately known against the primary frequency standard and their linewidth is highly narrowed thanks to the transferred OFCS coherence even for laser sources whose frequencies are several THz apart. These features, together with their wide tunability and their small intensity fluctuations (down to the shot-noise limit), make these IR sources well suited for a wide range of applications, in particular for spectroscopic ones. Very high sensitivity for trace-gas detection has been achieved when combined with enhancement absorption techniques as high-finesse Fabry-Perot cavities or multipass cells. Moreover, the large number of fundamental ro-vibrational transitions of many stable and transient molecular species accessible with this spectrometers, make them particularly attractive for environmental applications, especially considering their compactness and ruggedness when a fiber-based set-up is chosen. Their unique capabilities in terms of achievable precision for absolute frequency measurements can he used to create a "natural" grid of secondary frequency standards of IR molecular absorptions, frequency measured with these high-resolution spectrometers. More important, we have directly generated an IR frequency comb around 3 ?m by DFG Conversion of an OFCS. The generated comb can be employed both as a frequency ruler and a direct source for molecular spectroscopy.