Optical frequency combs (OFCs), which consist of equally spaced lines in the optical frequency domain, play an unrivaled role in science and engineering. Though first used as rulers in frequency space that measure large optical frequency differences in frequency metrology, OFCs gradually find applications in optical clocks, long-haul optical communications, light detection and ranging (LiDAR), spectroscopy and sensing. Recent years have witnessed the rapid development of chip-scale integrated OFCs technologies, allowing the above functions to be achieved compactly for future photonic integrated circuits (PICs). To date, most on-chip OFCs are based on semiconductor mode-locked lasers (SMLLs), nonlinear Kerr effect, or electro-optic (EO) effect, the latter of which stands out due to the high optical powers, widely tunable repetition rates and central wavelength. Among plenty of photonic platforms, lithium niobate (LiNbO₃, LN) shows suitable features for integrated EO comb sources due to its unique optical properties, such as high refractive index (~2.2), significant EO coefficient (r₃₃ ~31 pm/V), wide optical transparency window (0.4 μm - 5 μm), and low optical propagation loss. Traditional LN EO comb sources, usually via one or multiple discrete EO phase/amplitude modulators, suffer from low EO efficiency and substantial radio frequency (RF) power consumption, since the optical waveguides of bulk LN modulators are mainly defined by titanium-diffusion or proton-exchange methods, leading to a low refractive index contrast (Δn ~ 0.02), weak optical confinement, and subsequently weak overlap between electrical and optical modes. Recent technological advances in LN-on-insulator (LNOI) photonic platform have inspired a renaissance for LN devices to be achieved in chip scales that are more efficient, compact, and cost less. This thesis describes the realization of integrated EO comb generators based on LN nanophotonics by the combination of advanced properties of the LNOI platform, an optimized wafer-scale top-down nanofabrication technology, as well as novel device and circuit designs. Leveraging mode-space multiplexing, strong EO efficiency, and low-loss LN waveguides, EO combs could be generated with an enhancement of modulation index by ~3.9 times, and a subsequent reduction of electrical power consumption by ~15 times. Moreover, using a particular design, EO combs with flat-top spectrum can be expected, making EO combs more practical for future applications.

In Chapter 1, we briefly introduce the development of OFCs and widely used integrated OFCs technologies based on SMLLs, the nonlinear Kerr effect and the EO effect.

In Chapter 2, we introduce LN material properties and the LNOI platform, where a sub-micron-thick LN thin film is bonded on top of a SiO₂ dielectric substrate, resulting in a higher refractive index contrast (Δn ~0.7), much better light confinement, and substantially improved EO modulation efficiencies and performances compared with their bulk counterparts.

In Chapter 3, we discuss the nanofabrication technology used in this thesis. Three different fabrication processes of LN are investigated and compared, including metallic-mask-based, HSQ-based and UV-stepper-lithography-based processes. We show that micro-resonators with high quality-factors (Q-factors) over one million can be achieved using these optimized LN fabrication processes, corresponding to optical propagation losses less than 0.3 dB/cm, which lays the foundations for the subsequent low-loss optical components and integrated devices on LNOI platform.

Chapter 4 demonstrates several basic integrated LN building blocks using the above fabrication processes, including passive elements (e.g., waveguides, micro-resonators, and interferometers) and active elements (e.g., modulators). Besides, an ultra-high-linearity LN modulator is demonstrated, which is realized by combining a Mach-Zehnder interferometer (MZI) and a micro-resonator. Leveraging this linearization configuration and the intrinsically linear EO response of LN, we could fully suppress the cubic terms of third-order intermodulation distortions (IMD3) without any active feedback controls, leading to an 18 dB improvement over previous results in the thin-film LN platform. This ultra-high-linearity modulator can faithfully convert analog microwave signals into optics, which is crucial to ideal chip-scale microwave photonic systems.

Chapter 5 demonstrates an integrated power-efficient EO comb generator based on a non-resonant LN waveguide structure. This power-efficient characteristic benefits from passing optical signals through the modulation electrodes for a total of 4 round trips with the assistance of multiple low-loss TE₀/TE₁ mode conversion processes, leading to a substantially lowered radio frequency (RF) phase-modulation voltage-length product (V_π∙L) of 1.90 V∙cm at 25 GHz. Besides, we show the generation of broadband optical frequency combs with 47 comb lines at a 25-GHz repetition rate using a moderate RF driving power of 28 dBm, while maintaining excellent tunability in both the repetition rate (1.4 GHz) and operation wavelength (1500-1630 nm).

Chapter 6 shows EO comb generation in a resonant LN microring. Leveraging the high Q-factor and substantial microwave-optic field overlap, optical signals can pass through the EO modulation area many times, leading to a strong and cascaded sideband generation process, and in turn a broadband comb span. Besides, dispersion engineering can be realized on LN waveguides, which could further broaden the spectral span of these microring-based EO combs. Moreover, mode-splitting can be induced by combining photonic crystal and micro-resonator, which serve as efficient boundaries in the frequency crystal, and lead to flat comb generation if external RF signals are applied. Due to the anisotropy of LN crystal, the mechanism of photonic crystal ring (PhCR) resonators on x-cut and z-cut LN chips are well studied. We show that controllable mode-splitting can be induced by finely designing waveguide width, geometric modulation period and strength, while maintaining Q-factors on the level of millions.

Chapter 7 summarizes this thesis and gives an outlook on LN photonics, which mainly focuses on applications based on EO comb, including optical computing, optical communications, LiDAR, and spectroscopy/sensing.