Soliton Microcomb Generation Dynamics in Pockels Microring Resonator

Date of Award

Spring 2022

Document Type


Degree Name

Doctor of Philosophy (PhD)


Electrical Engineering (ENAS)

First Advisor

Tang, Hong


Optical frequency comb, an array of equidistant coherent laser lines, has revolutionized time/frequency metrology. Recent progress in microresonator-based Kerr soliton generation promises chip-scale comb sources for fieldable applications beyond lab research. To date, several nanophotonic platforms have been developed to support Kerr soliton generation, among which $\mathrm{Si_{3}N_{4}}$ photonics is the most mature one and has proved viable for producing broadband self-referenceable combs. However, it lacks second-order ($\chi^{(2)}$) nonlinearity and relies on off-chip optical components for comb self-referencing, which hinders the chip integration of comb. On the other hand, the emerging aluminum nitride (AlN) and lithium niobate (LN) thin-film platforms bring renewed potential to microcomb development. Their strong $\chi^{(2)}$ nonlinearity not only extends soliton operation regimes but also opens a pathway to on-chip comb locking. In this thesis, I will outline our work on developing both platforms for Kerr soliton generation and dynamics control via the $\chi^{(2)}$ nonlinearity. I will begin with a review of the optical frequency comb technology and soliton formation dynamics before detailing our approach. The first part of the thesis focuses on soliton generation in AlN microrings. To begin with, I will highlight how we overcome cavity opto-thermal effects for high-fidelity soliton generation. We then leverage phase-matched $\chi^{(2)}$ quadratic couplings to realize dual-band solitons, and investigate the impact of strong quadratic couplings on soliton formation. Guidance for engineering dual-band mode-locked comb sources on other $\chi^{(2)}$ platforms is also provided. Aside from soliton operation, I will briefly discuss AlN platform's potential for making wide-spanning parametric oscillators. The second part of this thesis explores soliton generation and control on LN thin film. LN is highly Raman active. The intracavity low-threshold Raman lasing poses a significant challenge to soliton formation. First, we propose and implement photonic dissipation control to favor soliton formation over Raman lasing. Second, we further optimize microring dispersion to realize broadband self-referenceable solitons. At last, we demonstrate a hybrid Kerr and electro-optic ($\chi^{(2)}$) microcombs generation circuit for chip-based photonic frequency division and soliton repetition rate locking. At the end of the thesis, I will discuss future avenues for microcomb research based on $\chi^{(2)}$-$\chi^{(3)}$ material platforms.

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