Abstract
Kerr frequency combs generated from microresonators are the subject of intense study. 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, for the first time to our knowledge, 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.
Highlights
High quality factor (Q) microresonators have been intensively investigated for optical comb generation
Modulational instability (MI) gain is missing in fibers or waveguides with normal dispersion, when it comes to resonators, the detuning provides an extra degree of freedom which enables MI to take place in the normal dispersion regime, providing a route to comb generation [16, 18, 25]
Significant changes in comb spectra have been observed when pumping different longitudinal modes spaced by only a few free spectral ranges (FSR), both in normal dispersion silicon nitride microring resonators [5] and in the whispering gallery mode resonators of [26]; in the latter case, such effects were attributed to mode interactions
Summary
High quality factor (Q) microresonators have been intensively investigated for optical comb generation. Mode coupling has been suggested as a mechanism enabling comb generation in resonators with normal dispersion [26].
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