Mid-infrared fibre-based optical comb

An optical frequency synthesizer, based on difference-frequency generation, is demonstrated at 3 micron. By mixing a near-IR optical frequency synthesizer (OFS) and a tunable continuous-wave laser in a periodically-poled lithium niobate crystal, a new comb is created from 2.9 to 3.5 micron (in 180-nm-wide spans) with a 100-MHz mode spacing. By phase-locking the pump laser to the near-IR OFS, the mid-IR comb is directly linked to the Cs primary standard via a global positioning system- (GPS-) time receiver. Then, the compactness of our fiber-based set-up opens real perspectives for the realization of transportable mid-IR frequency standards.