  Optical parametric oscillation (OPO) in Kerr microresonators can efficiently
transfer near-infrared laser light into the visible spectrum. To date, however,
chromatic dispersion has mostly limited output wavelengths to >560 nm, and
robust access to the whole green light spectrum has not been demonstrated. In
fact, wavelengths between 532 nm and 633 nm, commonly referred to as the "green
gap", are especially challenging to produce with conventional laser gain.
Hence, there is motivation to extend the Kerr OPO wavelength range and develop
reliable device designs. Here, we experimentally show how to robustly access
the entire green gap with Kerr OPO in silicon nitride microrings pumped near
780 nm. Our microring geometries are optimized for green-gap emission; in
particular, we introduce a dispersion engineering technique, based on partially
undercutting the microring, which not only expands wavelength access but also
proves robust to variations in resonator dimensions, in particular, the
microring width. Using just two devices, we generate >100 wavelengths evenly
distributed throughout the green gap, as predicted by our dispersion
simulations. Moreover, we establish the usefulness of Kerr OPO to coherent
applications by demonstrating continuous frequency tuning (>50 GHz) and narrow
optical linewidths (<1 MHz). Our work represents an important step in the quest
to bring nonlinear nanophotonics and its advantages to the visible spectrum.
