Normal-dispersion microcombs enabled by controllable mode interactions

We demonstrate a scheme incorporating dual-coupled microresonators through which mode interactions are intentionally introduced and controlled for Kerr frequency comb (microcomb) generation in the normal-dispersion region. Microcomb generation, repetition rate selection, and mode locking are achieved with coupled silicon nitride microrings controlled via an on-chip microheater. The proposed scheme shows for the first time a reliable design strategy for normal-dispersion microcombs and may make it possible to generate microcombs in an extended wavelength range (e.g. in the visible) where normal material dispersion is likely to dominate.