  Optical parametric oscillators are widely used to generate coherent light at
frequencies not accessible by conventional laser gain. However, chip-based
parametric oscillators operating in the visible spectrum have suffered from
pump-to-signal conversion efficiencies typically less than 0.1 %. Here, we
demonstrate efficient optical parametric oscillators based on silicon nitride
photonics that address frequencies between 260 THz (1150 nm) and 510 THz (590
nm). Pumping silicon nitride microrings near 385 THz (780 nm) yields
monochromatic signal and idler waves with unprecedented output powers in this
wavelength range. We estimate on-chip output powers (separately for the signal
and idler) between 1 mW and 5 mW and conversion efficiencies reaching
approximately 15 %. Underlying this improved performance is our development of
pulley waveguides for broadband near-critical coupling, which exploits a
fundamental connection between the waveguide-resonator coupling rate and
conversion efficiency. Finally, we find that mode competition reduces
conversion efficiency at high pump powers, thereby constraining the maximum
realizable output power. Our work proves that optical parametric oscillators
built with integrated photonics can produce useful amounts of visible laser
light with high efficiency.
