Kerr frequency combs in lithium niobate

Yang He

Supervised by Professor Qiang Lin

Computer Studies Building, Room 601

On-chip generation of optical frequency combs via the Kerr nonlinearity has attracted significant interest in recent years and the use of these devices for comb systems on a chip is being studied across a wide range of applications including spectroscopy, communication, ranging, frequency synthesis and optical clocks. The recent realihation of stable mode locking of Kerr microcombs, wherein the comb lines are mutually phase locked to form a stable soliton pulse train in time, is crucial for these applications. The wide  range  of  functions  that  are possible with lithium niobate (LN) waveguide devices, including phase and intensity modulation,  second-harmonic  generation  and  difference-frequency  generation,  make it attractive as a potential microcomb material. This thesis focuses on the generation of Kerr soliton combs in high-quality LN microresonators.

This thesis starts with dispersion-engineering in LN microring resonators, whose group velocity dispersion (GVD) can be precisely controlled in the normal and anomalous dispersion regimes, while simultaneously exhibiting high optical qs greater than one million. Both small anomalous dispersion and high q are essential prerequisites for Kerr frequency comb generation. By employing such type of devices, broadband soliton microcombs are produced in the telecom-band. Direct frequency doubling as well as a self-frequency shift resulting from the Raman effect are observed in the meanwhile. The LN soliton mode locking process also self-starts and allows bi-directional switching of soliton states, effects that are shown to result from the LN photorefractive effect. Based on those unique properties, this thesis continues to present on-demand generation of perfect soliton crystals, where we are able to continuously tune the comb line spacing from 1 to 11 times of the resonator free spectra range. The further exploration of observed Raman and second harmonic effects are also demonstrated.   Finally,   in order to engineer GVD in over multi-octave spanning spectral range, simple double-cladded and multi-cladded waveguide structures are proposed, which are not only easy to fabricate but also offer significant flexibility in dispersion engineering.