Mid-infrared fibre-based optical comb

An optical frequency synthesizer (OFS), based on difference-frequency generation (DFG), is demonstrated at 3 ?m. By mixing a near-infrared (IR) OFS and a tunable continuous-wave (CW) laser in a periodically poled lithium-niobate (PPLN) crystal, a new comb is created from 2.9 to 3.5 ?m (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 fibre-based set-up opens real perspectives for the realization of transportable mid-IR frequency standards. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.