Investigation of Mode Coupling in Normal Dispersion Silicon Nitride Microresonators for Kerr Frequency Comb Generation

TL;DR

This study investigates how mode coupling in normal dispersion silicon nitride microresonators influences Kerr frequency comb generation, revealing new mechanisms and enabling direct coherent pulse generation without chaos.

Contribution

It provides the first experimental evidence linking mode coupling to comb initiation in normal dispersion microresonators, showing how it can alter local dispersion and enable direct pulse generation.

Findings

Mode coupling can significantly modify local dispersion in microresonators.

Pinning of comb sidebands near mode crossings facilitates comb initiation.

Coherent, bandwidth-limited pulses are generated directly at GHz repetition rates.

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.