Fabrication and optical frequency comb generation in high-quality factor silicon oxide microcavity (Invited)

Based on ultra-high quality factor(Q) and nonlinear optical microcavities, optical microcombs(microcavity optical frequency comb) have enabled a variety of important applications including high volume optical communications, optical data center, photonic neuromorphic computation and massive parallel LIDAR. Whispering gallery mode (WGM) microcavities stand for an important platform for studying the microcavity optical frequency comb technology, particularly having record ultra-high Q factors as well as the ultra-high finesse. It can realize ultra-narrow linewidth lasers and optical frequency combs, and photonic microwaves for synthesizing ultra-low noise. Here we developed high Q WGM microcavities from a silica (SiO₂) rod fused and shaped with the CO₂ laser. The quality factor is above 10⁸ with a free spectrum range at the level of 10 GHz. The cavity resonances as well as the coupling ideality have been characterized, where a degradation of Q factors in a humid environment was observed and recovered with a second annealing process. Moreover, Kerr comb generation was demonstrated in such SiO₂ microcavities, which at the moment is mostly in a noisy state governed by the modulation instability regime. Yet the footprint of the cavity soliton state was experimentally observed as a “soliton step” signal. The results indicate that a low-noise and fully coherent soliton microcomb is potentially accessible in home developed SiO₂ microcavities, and is readily for comb-related applications.