Tunable Dark-Pulse Frequency Combs by Beat-Frequency Pumps in Silicon Nitride Microring Resonators With Normal Dispersion

In this paper, we propose a feasible approach to generate a tunable dark-pulse comb with high coherence by injecting a beat-frequency pump into a standard normal-dispersion microring resonator. The optimized dark-pulse comb has a time duration of 262 fs, a bandwidth of 25 THz at −80 dB level and a pump-to-comb conversion efficiency of 17%. The comb line spacing is strongly correlated with the modulation frequency of the beat-frequency pump and can be tuned by the modulated pump. The dark-pulse comb can be formed with a high repeatability even in a high averaged noise power background of −80 dB, indicating the high coherence of this dark-pulse comb generation. Our results deepen the understanding of the dark-pulse comb generation in microring resonators with normal dispersion and open a new way to generate tunable frequency comb.