Frequency combs in a microring optical parametric oscillator

We report the soliton frequency comb generation in microring optical parametric oscillators operating in the downconversion regime and with the simultaneous presence of the χ⁽²⁾ and Kerr nonlinearities. The combs are studied considering a typical geometry of a bulk LiNbO₃ toroidal resonator with the normal group velocity dispersion spanning an interval between the pump and the downconverted signal. We have identified critical power signaling a transition between the relatively low pump power predominantly χ⁽²⁾ combs and the high pump power ones shaped by the competition between the χ⁽²⁾ and Kerr nonlinearities.