Self-injection-locked bright and dark-pulse microcomb generation in ultrahigh-Q crystalline microresonators

We demonstrate self-injection locking (SIL)-based bright and dark-pulse microcomb generation in an ultrahigh-Q crystalline magnesium fluoride (MgF₂) microresonator in the telecom C-band. By simple tuning of the laser injection current, both a single bright soliton and a dark-pulse state are obtained in the same resonator. We further perform a direct comparison of SIL-induced linewidth reduction using the same laser chip with a 10-GHz-FSR MgF₂ resonator and a 100-GHz-FSR silicon nitride microring resonator. The MgF₂ resonator exhibits substantially stronger noise suppression, reaching a hertz-level Lorentzian linewidth of 1.78 Hz. These results highlight ultrahigh-Q crystalline resonators as a promising platform for ultralow-noise microwave-rate coherent comb generation.