Coherence properties of a mid-infrared frequency comb produced by a degenerate optical parametric oscillator

We show that a broadband mid-infrared frequency comb produced by a degenerate optical parametric oscillator (OPO) is stable and phase locked to the pump. Moreover, the OPO can operate in two phase and two frequency states. In this experiment, two identical OPOs were synchronously pumped by a single 70-fs fiber laser at 1560 nm and produced broadband (>;1000 nm-wide) frequency combs centered at 3120 nm. We observed stable interference fringes between the outputs of the OPOs. Also, by fine-tuning the OPO cavity length, the whole frequency comb could be shifted by steps of half the repetition rate. These results prove that sub-harmonic OPOs are suitable candidates for generation of phase-locked frequency combs in mid-IR.