Dissipative Kerr solitons in microring resonators have the potential to enable precision measurements with optical frequency combs in an integrated photonics package. However, the small volume of these resonators makes them highly susceptible to thermorefractive effects. We present a system of two coupled resonators that reduces the effect of thermal shifts on comb states through controllable mode crossings and fast detuning control of the pump laser. Both methods aim to beat the thermal timescale of the resonator to stabilize a dispersive Kerr soliton. Using soliton lifetime as a metric, we find increased resilience to thermorefractive effects when the pump laser is swept faster than the thermalization timescale of the cavity and when mode frequencies in the auxiliary resonator at or near the pumped mode are tuned towards degeneracy with the main resonator (i.e. the resonator with the soliton). The lifetime near an auxiliary resonator-main resonator mode crossing shows a three order of magnitude increase from the native soliton lifetime set by the thermalization time of a single resonator. These results suggest that both methods can be used to reduce thermorefractive phase noise in soliton microcombs.
Chip-based photonic microresonators are attractive for a multitude of applications owing to their small form factors and compatibility with photonic integration and standard CMOS fabrication. Within the last decade, the ring resonator geometry has gained widespread adoption in the application of optical frequency comb generation. However, these devices often require waveguides several hundreds of nanometers thick, posing a challenge for subtractive fabrication processes where the desired pattern must be chemically etched with the use of a protective “mask” pattern. Here, we demonstrate two procedures for subtractive processing of thick SiN waveguides based on both a polymer-based “soft” photoresist mask and a chromium metallic “hard” mask as etch templates. Optical characterization of our devices fabricated with both soft mask and hard mask techniques demonstrate quality factors of 320k ± 100k and 500k ± 180k., respectively. Furthermore, this work details two reliable pathways for achieving high quality optical microring resonators and illustrates the benefits and drawbacks of these two techniques.
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