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Frequency comb generation in dispersion engineered Si3N4 microresonators and their applications


Optical frequency combs, unique light sources that coherently link optical frequencies with microwave electrical signals, have heralded several scientific frontiers such as frequency metrology, optical clockwork, precision navigation, and high speed communication over the past decades. Parametric oscillation in ultrahigh Q microresonators, facilitated by the high quality factors and the small mode volumes, is an alternative physical process that offers the opportunity of optical frequency comb generation in compact footprints with smaller weight and lower power consumption. In particular, the observation of dissipative Kerr soliton (DKS) formation and soliton-induced Cherenkov radiation offers a reliable route towards self-referenced broadband optical frequency microcomb. DKS is localized attractor where the Kerr nonlinearity is compensated by the cavity dispersion and the cavity loss is balanced by the parametric gain. Thus the cavity dispersion and the pump-resonance detuning are two determining parameters in the existence of DKS in ultrahigh Q microresonators. Among numerous material platforms, Si3N4 planar waveguide system draws great attention for its high nonlinearity, wide transparent window, low propagation loss and its CMOS-compatibility. For Si3N4 microresonators, dispersion is typically engineered by designing waveguide geometry. However, conventional method of using multi-mode waveguide results in additional perturbation to the Kerr frequency comb generation dynamics. Furthermore, despite of all the advantages, DKS suffers from low conversion efficiency, restrict pump-resonance detuning requirement and difficult dispersion design, which could be supplemented by normal dispersion frequency microcomb. In this thesis, I focus on frequency microcomb studies in novel designs of tapered Si3N4 planar waveguide geometry achieving higher-order-mode suppression. I not only work on unravelling the fundamental dynamics of DKS in anomalous dispersion regime, but I also investigate a novel pulse formation mechanism in normal dispersion device. Benefiting from their high intrinsic phase coherence and individual comb line powers, I further extend my research to absolute distance metrology with a single soliton comb that achieves nm-level precision, and terabits/s free-space optical communication with a flat normal dispersion comb.

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