Frequency comb solutions for driven X(2) optical microresonators

Frequency combs in X(2) optical microresonators caused by cascaded second-order nonlinear processes currently attract great research interest. In contrast to X(3) resonators, two light amplitudes relevant to the first and second harmonics (FH and SH), two dispersion coefficients, and a considerable difference of the FH and SH group velocities (the walk-off) must be taken into account to investigate localized coherent structures (solitons) propagating with a common velocity. Finding such comb solutions taking into account external pumping is a crucial step toward the X(2) combs. We report on two new families of driven soliton-comb solutions for X(2) microresonators. They are strongly localized, corresponding to spectrally broad combs, and possess well-defined FH and SH frequency detunings, propagation velocities, and, generally, nonzero spatial backgrounds.