Integrated Lithium Niobate Microwave Photonics

Abstract

Microwave photonics (MWP) is an intriguing field that leverages photonic technologies for the generation, transmission, manipulation, and measurement of microwave signals. This interdisciplinary field combines both the strengths of microwave engineering and photonic technology, featuring broad bandwidth, low loss, parallel processing, and immunity to electromagnetic interference. The extensive capabilities of MWP technology enhance the performance of optoelectronic devices and systems, enabling the realization of key functionalities such as MWP filters, microwave signal processing, generation of ultra-broadband signals, and photonics-enhanced radar systems. These advancements have subsequently positioned MWP technology as a primary solution to the bottleneck of the underlying radio frequency (RF) systems, encompassing operation bandwidth, processing speed, and power consumption. Recent advances in photonic integration technologies have further led to a dramatic reduction in the size, weight, and power of MWP systems with enhanced robustness and functionalities, termed integrated MWP. To date, most MWP applications have been demonstrated on silicon (Si), silicon nitride (SiN), and indium phosphide (InP) platforms, while the unsatisfactory link performance, operation bandwidth, and energy consumption have restricted the applications in future large-scale MWP systems. The recently emerged thin-film lithium niobate (LN) platform is a promising candidate for enhancing the performances of MWP systems owing to its unique electro-optic (EO) properties, low optical loss, and excellent scalability.

In this thesis, we explore the implementation of diverse MWP applications on thin-film LN platform through the combination of superior material properties and an optimized state-of-the-art nanofabrication technology.

In Chapter 1, we briefly introduce the fundamental characteristics and figures of merit of the MWP system. We then make an all-round comparison between the mainstream photonic platforms for integrated MWP systems.

In Chapter 2, we delineate the advantages and uniqueness of the thin-film LN platform for MWP applications, as well as the optimized nanofabrication approaches capable of yielding high-performance building blocks for the MWP system. In particular, we demonstrate a key building block, i.e., ultra-high-linearity EO modulators for faithfully converting analog signals. The linear modulators feature an ultrahigh spurious-free dynamic range (SFDR) of 120.04 dB·Hz^4/5 at 1 GHz, leading to ~ 20 dB improvement over previous results in the thin-film LN platform.

In Chapter 3, we further combine individual device blocks to realize LN MWP integrated circuits (PIC), demonstrating an integrated MWP signal processing engine. We combine the high-speed EO modulation block and low-loss functional photonic network on the same LN chip, enabling multi-purpose tasks with processing bandwidths up to 67 GHz at CMOS-compatible voltages. We achieve ultrafast analog computation, i.e., temporal integration and differentiation, at sampling rates up to 256 billion samples per second, and deploy these functions to showcase three proof-of-concept applications, namely, ordinary differential equation solving, ultra-wideband signal generation, and edge detection of images. We further leverage the image edge detector to realize a photonic-assisted image segmentation model that could effectively outline the boundaries of melanoma lesions in medical diagnostic images.

In Chapter 4, we demonstrate another MWP application by utilizing high-resolution spectrum analysis of microwave domain to demonstrate a miniaturized optical vector analysis (OVA) system. Building on a broadband single sideband (SSB) modulator, the OVA system could provide a direct probe of both amplitude and phase responses of photonic devices with kHz-level resolution and tens of terahertz measurement bandwidth. We then perform in-situ characterizations of various integrated optical devices fabricated on the same chip as the OVA, unfolding their intrinsic loss and coupling states unambiguously.

In Chapter 5, we realize an integrated optoelectronic oscillator (OEO) on LN platform to generate high-quality microwave and millimeter-wave signals. The unique EO properties of LN platform empower the OEO loop with record-high frequency components up to 88.8 GHz. The excellent synergy of the broad bandwidth modulators and high-quality microring resonators on LN platform enables a narrowband MWP filter to flexibly choose oscillation mode, exhibiting broadband frequency tunability from 1.6 GHz to 88.8 GHz. Additionally, a low phase noise level close to -110 dBc/Hz at 10 kHz offset frequency is achieved for all microwave frequencies, illustrating the potential of the approach for the generation of stable high oscillation.

In Chapter 6, we demonstrate a first-ever experimental observation of backward stimulated Brillouin scattering (SBS) in LN waveguides, unlocking the possibility of developing high-performance MWP devices and systems, such as ultra-narrow bandwidth filters and SBS lasers. The peak Brillouin gain coefficient of the z-cut LN waveguide with a crystal rotation angle of 20° is as high as 84.9 m^-1W^-1, facilitated by surface acoustic waves (SAW) at 7.88 GHz. Subsequently, we combine the LN SBS waveguide with the off-chip modulator to exhibit a narrow bandwidth MWP filter, with rejection of 15 dB and filter bandwidth of 16 MHz utilizing the on-chip SBS gain of 0.45 dB only. The important observation could unlock the SBS-based MWP system on LN platform towards more advanced performance in terms of noise figure and spectral resolution.

In Chapter 7, we summarize this thesis and give an outlook on the future directions of the LN-based MWP system, including comb-driven LN MWP systems, more programmable and cognitive LN MWP systems, and fully integrated LN MWP systems.

Date of Award	9 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Cheng WANG (Supervisor)