Cross-phase modulation induced microcomb generation in the normal-dispersion region

Microcomb generation in the normal-dispersion region usually requires specially designed microresonators with mode interactions, increasing the complexity of device design and control. Here we demonstrate a novel, to the best of our knowledge, scheme of frequency comb generation by bidirectionally pumping an ordinary normal-dispersion microresonator. The cross-phase modulation from the counter-propagating light reshapes the cavity response, facilitating the emergence of modulational instability for comb initiation. By properly adjusting the pump power ratio and frequency detuning in two directions, frequency combs can be formed at any pumped resonance. The proposed method provides a universal pathway to flexible microcomb generation in the normal-dispersion regime.