Abstract
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.
Highlights
IntroductionKerr optical frequency combs [1,2,3,4,5], based on high-Q whisperinggallery-mode (WGM) resonators [6,7,8,9,10], have significantly stimulated the applications in optical clocks, optical synthesis, optical communications, optical sensing, cavity quantum electrodynamics, and precision navigation [11,12,13,14,15,16,17,18,19]
Crystalline resonators [26,27], suffering less from intrinsic and surface Rayleigh scattering, reported with record high-Q of 300 billion [28], have not yet been implemented with low-loss on-chip optical couplers. Such optical coupling to crystalline resonators should be based on a phase- and mode-matched power exchange between a resonator mode and a wave propagating in a specially engineered coupler
We exploit the principle of evanescent wave coupling under total internal reflection (TIR) condition in traditional prism elements [45,46,47,48] and demonstrate, for the first time to our knowledge, a compact prism-waveguide coupler on a silicon nitride platform
Summary
Kerr optical frequency combs [1,2,3,4,5], based on high-Q whisperinggallery-mode (WGM) resonators [6,7,8,9,10], have significantly stimulated the applications in optical clocks, optical synthesis, optical communications, optical sensing, cavity quantum electrodynamics, and precision navigation [11,12,13,14,15,16,17,18,19]. Crystalline resonators [26,27], suffering less from intrinsic and surface Rayleigh scattering, reported with record high-Q of 300 billion [28], have not yet been implemented with low-loss on-chip optical couplers Such optical coupling to crystalline resonators should be based on a phase- and mode-matched power exchange between a resonator mode and a wave propagating in a specially engineered coupler (a waveguide or a prism). We exploit the principle of evanescent wave coupling under total internal reflection (TIR) condition in traditional prism elements [45,46,47,48] and demonstrate, for the first time to our knowledge, a compact prism-waveguide coupler on a silicon nitride platform This prism-waveguide coupler laterally couples light into and out of an ultrahigh-Q crystalline MgF2 resonator with 1 dB loss and 1.9 × 109 loaded Q, realizing a new method for integration between crystalline resonators and on-chip waveguides [49]. We further present a packaged module containing the prismwaveguide-coupled ultrahigh-Q MgF2 resonator to generate a stable and low-noise optical frequency comb at the 26 GHz repetition rate
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