We investigate frequency comb generation in silicon nitride ring resonators by using a pump subject to a weak amplitude modulation. We show that a partial locking is obtained when the external modulation frequency differs from the resonator free-spectral-range by up to hundreds of MHz.
The generation of dissipative Kerr solitons is experimentally investigated in ring resonators with optical feedback. This new double-resonator geometry allows generating frequency combs with smooth solitonic spectral shape over much broader spectral bandwidths if compared with the standard ring resonator architecture. By using an amplitude modulated pump, the repetition rate of the generated frequency comb is locked to the external modulation and exhibits a stability comparable to the modulating radio frequency signal, i.e. the repetition rate linewidth is very narrow (20 Hz). Furthermore, the energy conversion efficiency (pump-to-frequency comb) can be up to 60%, being a record for microresonators.