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.