Investigation of mode coupling in normaldispersion silicon nitride microresonators for Kerr frequency comb generation

Kerr frequency combs generated from microresonators are the subject of intense study for potential applications ranging from short pulse generation to frequency metrology to radiofrequency signal processing. Most research employs microresonators with anomalous dispersion, for which modulation instability is believed to play a key role in initiation of the comb. Comb generation in normal-dispersion microresonators has also been reported but is less well understood. Here, we report a detailed investigation of few-moded, normal-dispersion silicon nitride microresonators, showing that mode coupling can strongly modify the local dispersion, even changing its sign. We demonstrate a link between mode coupling and initiation of comb generation by showing experimentally the pinning of one of the initial comb sidebands near a mode-crossing frequency. Associated with this route to comb formation, we observe direct generation of coherent, bandwidth-limited pulses at repetition rates down to 75 GHz, without the need to first pass through a chaotic state. Our results with normal-dispersion devices highlight mode interactions as a beneficial tool for comb initiation and pulse formation.