  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.
