  A necessary condition for generation of bright soliton Kerr frequency combs
in microresonators is to achieve anomalous group velocity dispersion (GVD) for
the resonator modes. This condition is hard to implement in visible as well as
ultraviolet since the majority of optical materials are characterized with
large normal GVD in these wavelength regions. We overcome this challenge by
borrowing ideas from strongly dispersive coupled systems in solid state physics
and optics. We show that photonic compound ring resonators can possess large
anomalous GVD at any desirable wavelength, even if each individual resonator is
characterized with normal GVD. Based on this concept we design a mode locked
frequency comb with thin-film silicon nitride compound ring resonators in the
vicinity of Rubidium D1 line (794.6nm) and propose to use this optical comb as
a flywheel for chip-scale optical clocks.
