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

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 lowloss 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 × 10 9. 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.

Emerging material systems for integrated optical Kerr frequency combs

“…[84][85][86][87] As a result, numerous approaches for coupling light into optical cavities have been developed with differing degrees of loss. [34,85,86,[88][89][90][91][92][93] Currently, the lowest loss methods are the angle-polished fiber [94], a tapered fiber [95], a coupling prism [96,97], and an integrated bus waveguide [98] (Fig. 2).…”

“…The first three nearly lossless couplers can be integrated with on-chip devices using thermal or UV-curable polymers or "pick-and-place" techniques. [89,99,100] However, because these methods are unable to be lithographically patterned, the total density of devices is fundamentally limited. The drive to increase density inspired research into alternative coupling methods, such as the bus waveguide, as well as innovation in device design.…”

Resonator-free sub-MHz spectral dip in a twisted gain medium

“…Also, a 6-fold increase in the sensitivity of the Brillouin ring laser gyroscope was reported when an ultrahigh Q silica wedge resonator (Q > 100 million) was being used [16]. Such favourable performance scaling has stimulated phenomenal progress in obtaining narrow resonances through platforms like gas-phase atomic systems [18,19], photonic crystal cavities [11,[20][21][22], whispering gallery mode (WGM) [23,24], and microring resonators [25][26][27], slow-light fiber Bragg gratings (FBGs) [2], and phase-shifted FBGs [28]. However, despite significant progress, there remain many challenges with these platforms.…”

“…Narrow resonances can be realized in gas-phase atomic systems using electromagnetically induced transparency (EIT) [18,19], but they typically require low temperatures to obtain sub-MHz resonances. Photonic crystal cavities [11,[20][21][22], WGM [23,24], and microring resonators [25][26][27] usually require complex and costly fabrication steps like chemical-mechanical polishing and high temperature anneals to obtain sub-MHz resonances [29]. Impedance-matched resonator based on -phase-shifted FBGs have exhibited narrow resonances [28], but their resonance linewidth is limited to a few MHz due to their intrinsic losses.…”

Integrated photonics for space applications

“…Color online) (a) Ultrahigh-Q resonator based on MgF 2 ; (b) schematic of packaged Kerr optical frequency comb source[36].…”