  Anomalous group velocity dispersion is a key parameter for generating bright
solitons, and thus wideband Kerr frequency combs. Extension of frequency combs
to visible wavelength in conventional photonic materials and structures has
been a major challenge due to strong normal material dispersion at the relevant
frequencies. Extension of frequency combs toward the normal dispersion region
is possible via dispersive waves through soliton-induced Cherenkov radiation.
However, this potentially powerful technique has not been used for extending
frequency combs to the visible spectrum. In this paper, we demonstrate a new
microresonator structure formed by an over-etched silicon nitride waveguide
that enables the use of soliton-induced Cherenkov radiation to extend the
bandwidth of the Kerr-combs. Furthermore, we show that by careful dispersion
engineering in a coupled microring resonator structure we can optimize the
properties (e.g., wavelength, and amplitude) of the generated dispersive wave
to further extend the Kerr frequency combs to the visible spectrum while
increasing the total Kerr-comb bandwidth as well.
