Tunable frequency combs based on dual microring resonators

TL;DR

This paper introduces a dual-microring resonator system with tunable coupling and dispersion, enabling efficient and adjustable frequency comb generation in integrated photonic devices.

Contribution

It presents a novel dual-cavity microresonator design with integrated microheaters for tunable coupling and dispersion, enhancing frequency comb efficiency and control.

Findings

Achieved 110-fold improvement in comb generation efficiency

Demonstrated wide extinction tunability via integrated microheaters

Enabled dispersion engineering through coupled cavity design

Abstract

In order to achieve efficient parametric frequency comb generation in microresonators, external control of coupling between the cavity and the bus waveguide is necessary. However, for passive monolithically integrated structures, the coupling gap is fixed and cannot be externally controlled, making tuning the coupling inherently challenging. We design a dual-cavity coupled microresonator structure in which tuning one ring resonance frequency induces a change in the overall cavity coupling condition. We demonstrate wide extinction tunability with high efficiency by engineering the ring coupling conditions. Additionally, we note a distinct dispersion tunability resulting from coupling two cavities of slightly different path lengths, and present a new method of modal dispersion engineering. Our fabricated devices consist of two coupled high quality factor silicon nitride microresonators, where the extinction ratio of the resonances can be controlled using integrated microheaters. Using this extinction tunability, we optimize comb generation efficiency as well as provide tunability for avoiding higher-order mode-crossings, known for degrading comb generation. The device is able to provide a 110-fold improvement in the comb generation efficiency. Finally, we demonstrate open eye diagrams using low-noise phase-locked comb lines as a wavelength-division multiplexing channel.