Normal-dispersion Microcombs Enabled by Controllable Mode Interactions
The paper presents a novel approach to Kerr frequency comb generation within microresonators equipped to reliably operate in the normal dispersion regime. Unlike traditional methods focusing on anomalous dispersion, the authors introduce a system of dual coupled silicon nitride microrings that utilize intentionally controlled mode interactions. This strategic setup overcomes the inherent limitations of normal dispersion, widely considered unsuitable for microcomb generation due to the absence of modulational instability typically essential in anomalous regions.
Key experimental results highlight the ability to achieve microcomb generation using coupled microresonators with precise thermal control via an integrated on-chip microheater. The mechanism permits the programmable tuning of mode interactions, effectively altering the resonance asymmetry factor, which is instrumental in facilitating equivalent anomalous dispersion conditions necessary for comb production. This adjustment spuriously shifts resonant frequencies between device microrings, encoding mode interactions tightly controlled by the thermal tuning process.
A strong point of the study is the demonstration of repetition-rate-selectable comb generation, where the repetition rate could be tuned from 378 GHz to 2.27 THz while sustaining a consistent spectral output. This configurability has potential implications in advancing optical communications and signal processing technologies, offering refinements in bandwidth utility and spectral precision. Notably, the approach enables direct generation of 1-FSR combs, especially advantageous for large microresonators undertaking extended band designs.
Furthermore, the investigation extends to document transitions into mode-locked states characterized by reduced intensity noise, posited as indicative of potential coherence enhancements. This shift, corroborated by empirical autocorrelation measurements of transform-limited pulses, sets a promising stage for broader utilization in precision optical metrology, possibly underpinning next-generation optical clock systems.
In conclusion, the research outlines a compelling design strategy presenting normal-dispersion microcombs with reliable operational feasibility under a broader spectrum of conditions. The microcomb generation methodology discussed avails extended latitude in wavelength selection, suggesting applicability in ranges such as visible light domains, where normal material dispersion predominates. Potential future advancements could iteratively refine this system by integrating more resilient material interfaces and escalated thermal modulation efficiencies to further broaden microcomb applicability across diverse optical applications.