  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.
Our results show for the first time that mode interactions can be programmably
tuned to facilitate broadband normal-dispersion microcombs. The proposed scheme
increases freedom in microresonator design 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.
