Low-loss prism-waveguide optical coupling for ultrahigh-Q low-index monolithic resonators

While compact and low-loss optical coupling to ultrahigh-quality-factor (Q) crystalline resonators is important for a wide range of applications, the major challenge for achieving this coupling stems from the relatively low refractive index of the crystalline resonator host material compared to those of the standard waveguide coupling materials. We report the first demonstration of a single-mode waveguide structure (prism-waveguide coupler) integrated on a low-loss compact silicon nitride platform resulting in low-loss and efficient coupling to magnesium fluoride crystalline resonators by achieving the phase-matched and the mode-matched evanescent wave coupling. The coupling is characterized with 1 dB loss at 1550 nm wavelength. We further present a photonic integrated chip containing a pair of waveguides successfully coupling light into and out of the resonator, demonstrating a planar-waveguide-coupled crystalline resonator with a loaded Q of 1.9×109. We assemble this waveguide-coupled resonator and a distributed-feedback-laser chip into a butterfly package to realize a miniature Kerr optical frequency comb source using self-injection locking of the distributed feedback laser to the waveguide-coupled crystalline resonator.