Integrated UHQ Silica Resonators

Optical microresonators are essential to a broad range of technologies and scientific disciplines. However, many of their applications rely on discrete devices to attain challenging combinations of ultra-low-loss performance (ultrahigh Q) and resonator design requirements. This prevents access to scalable fabrication methods for photonic integration and lithographic feature control. Indeed, finding a microfabrication bridge that connects ultrahigh-Q device functions with photonic circuits is a priority of the microcavity field. Here, an integrated resonator having a record Q factor over 200 million is presented. Its ultra-low-loss and flexible cavity design brings performance to integrated systems that has been the exclusive domain of discrete silica and crystalline microcavity devices. Two distinctly different devices are demonstrated: soliton sources with electronic repetition rates and high-coherence/low-threshold Brillouin lasers. This multi-device capability and performance from a single integrated cavity platform represents a critical advance for future photonic circuits and systems.

A monolithic microcavity featuring an integrated silicon nitride waveguide is described. The silica ridge resonator provides optical Q factors in excess of 200 million for TM modes. Critically, and as required for new system-on-achip applications, the resonator supports design controls required to realize many device functions previously possible using only discrete (non-waveguide-integrated) devices. To demonstrate its capability, electronics-rate soliton mode locking (15 GHz) is generated at a low pumping power level. Also, as a distinctly different capability, high-coherence stimulated Brillouin laser oscillation is demonstrated. Beyond the necessity of ultra-high-Q factor in these demonstrations, these new devices illustrate resonator dispersion control to support dissipative Kerr soliton generation and precise diameter control to phase match the Brillouin laser process.

The Brillouin process has attracted considerable interest in micro devices . Brillouin laser action has been demonstrated in discrete resonators based on silica and CaF₂. Laser action has also been realized in integrated resonators using silicon and chalcogenide waveguides. Reference sources, microwave synthesizers and Brillouin gyroscopes benefit from the highest possible optical Q factors for generation of narrow-linewidth signals and have so far relied upon discrete devices. Moreover, ultra-high-Q also allows these devices to function at low power despite their increased size. Even more recently, the demonstration of coherently-pumped soliton mode locking in fiber resonators and in micro-resonators has been a major advance for miniature frequency comb devices. Long considered a theoretical possibility, these soliton systems are able to regenerate through Kerr-effect parametric amplification. As mode-locked optical oscillators, soliton operation provides stable repetition rates and reproducible waveforms, which are essential ingredients in all comb applications. These microcavity solitons have opened new perspectives on optical soliton physics, and integrated comb systems on-a-chip.

Brillouin lasers and soliton microcombs using an integrated ultra-high-Q silica resonator

Figure: (a) Optical spectrum of soliton microcomb with pump line indicated. (b) Demonstration of Brillouin lasing in an integrated optical microresonator.