Integrated Photonic Circuits Including Modulators on the Thin Film Lithium Niobate Platform
Creators: name: Huang, Haijin
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400605
subject: Optical fibre communication systems and technologies
classificationCode: 510204
subject: Photonics, optoelectronics and optical communications
classificationCode: 400908
subject: Microelectronics

description: The primary objective of this thesis is to exploit the capabilities of the thin-film lithium niobate platform to establish a comprehensive optical circuit with synergistic functioning of various components. The high electro-optic coefficient and strong optical confinement offered by thin-film lithium niobate on insulator (LNOI) platform makes it a promising platform for integrated optical circuits. The overall question of this thesis is trying to explore will the ability to construct more complex, integrated circuits on a Lithium Niobate on an Insulator platform changed the way of the design and usage of electro-optic modulators.

The need for high-quality high-speed modulators in optical fibres systems has driven the development of lithium niobate as a material for photonics. Chapter 2 delves into the potential of the LNOI platform to engineer high-speed electro-optic modulators with improved efficiency and data rates as a critical building block of integrated photonic systems on a single chip. The development of an 80 Gbps Mach-Zehnder interferometer-based EO modulator and a monolithic photonic integrated circuit (PIC) with a total data capacity of 280 Gbps marks a significant milestone in realizing large-capacity on-chip optical interconnections on the LNOI platform.

Having shown that excellent modulator components can be realised in LNOI Chapter 3 explores how these modulator components can be used as part of an integrated circuit. Specifically, Chapter 3 presents an integrated optical isolator, which can be used to reduce the impact of unwanted reflections in optical systems, thereby increasing their stability and reliability. The design features two identical traveling wave phase modulator sections separated by a delay line, fabricated in the LNOI waveguide platform. Experimental outcomes depict an optical isolation of 27 dB, signifying one of the highest isolation ratios achieved on LNOI platforms using spatiotemporal modulation. This relatively simple demonstration illustrates the potential for more complex integrated circuits for photonic signal processing that can be realised by integrating multiple modulators together as part of a single chip system. 

Having shown the potential for building integrated circuits incorporating multiple modulators on the LNOI platform, Chapter 4 explores whether integration can help to create new forms of modulator. Specifically, this chapter introduces a novel concept of recirculating light multiple times through the same phase modulator, using a multimode waveguide to enhance the modulation efficiency. The proposed recirculating phase modulator design demonstrates that the modulation efficiency can be linearly improved with the number of recirculations, which also means the power consumption can be reduced quadratically with number of loops. When the modulators have the same electrode configuration, the experimental findings indicate that a three-time light recirculation can reduce the microwave signal power by 9 times compared to a standard single-pass phase modulator, marking an important advancement in power-efficient optical modulators.

In summary, the thesis has shown some of the potential that the emerging LNOI platform can provide for realisation of integrated circuits as a significant advancement over simple devices. From realizing an 80 Gbps modulator and a monolithic PIC with a total data capacity of 280 Gbps to designing an integrated optical isolator and a highly efficient recirculating phase modulator, the work emphasizes the potential of the LNOI platform to manifest as truly widely used integrated optical circuit platform.