Integrated Lithium Niobate Photonics for Spectrometry and Modulation

Abstract

Photonic devices for creation, manipulation and detection of light are omnipresent in the modern society of the 21st century. While the impact of certain photonic components such as LED light sources or image sensors is quite obvious in everyday life, others go unnoticed. Various optoelectronic devices, including laser diodes, electro-optic modulators and photodiodes, are essential parts of an immense global fiber-optic communication infrastructure, facilitating today’s information society. Optical sensors and spectrometers are relevant in medical and environmental applications, for example as diagnostic tools for detecting biomarkers or for monitoring greenhouse gases in the atmosphere. Likewise as the invention of integrated circuits (ICs) revolutionized the world of electronics, integrated photonics holds the promise to reduce size, weight, energy consumption and cost of optical components, while increasing robustness and reliability at the same time. Furthermore, the small footprints of individual components enable large-scale photonic integrated circuits (PICs) combining several functionalities on a single chip. The most utilized materials for PICs are silicon, silicon nitride and indium phosphide, partly owing to the already existing nanofabrication infrastructure and the compatibility with mature CMOS process flows. Over the last decade, thanks to advancements in the challenging nanofabrication, a new photonic platform based on a well-established optical material has emerged as an alternative: Thin-film lithium niobate on insulator (LNOI) enables low-loss integrated circuits and multi-functional devices using the electro-optic and nonlinear properties of lithium niobate. This thesis discusses the design, fabrication and experimental characterization of integrated electro-optic devices in LNOI for spectrometry and light modulation. We introduce our developed process flows for the fabrication of monolithic and hybrid LNOI waveguides, the fundamental building blocks for larger integrated circuits. The sub-wavelength optical mode confinement and material properties of LNOI waveguides enable high-efficiency linear electro-optic interactions, an effect which is not present in the established integrated photonics platforms. Apart from the electro-optic properties of lithium niobate, we also make use of the wide transparency window and demonstrate integrated devices operating from the visible-near-infrared (780 nm) to the optical communication bands (∼1600 nm). For the first device, we present a novel concept for an integrated Fourier-transform spectrometer. The proposed working principle combines a spatial sampling technique of the stationary wave resulting from counterpropagative interference with active electro-optic tuning of the optical path difference. Thereby, bandwidth limitations due to undersampling are avoided and the demonstrated proof-of-concept device exhibits an operational wavelength range of 500 nm, only limited by the single-mode condition of the waveguide. To extend the existing portfolio of photonic building blocks, we studied various types of waveguide Bragg gratings in sidewall corrugated monolithic LNOI waveguides. We show that by control of the design parameters, high optical extinction ratios (53.8 dB), narrowband transmission filters (12.5 pm) and spectral responses including multiple periodic stopbands are achievable. As a possible application of LNOI Bragg filters, we present a compact electro-optic modulator with intrinsic single sideband suppression for 100 Gbit/s on-off keying data transmission and a footprint of 10 × 400 µm2 . Lastly, we investigate more conventional travelling-wave Mach-Zehnder modulators in LNOI for operation at 1550 nm and 780 nm. In the C-band (1535-1560 nm), a 3-dB electro-optic bandwidth beyond 67 GHz is demonstrated by optimizing the microwave transmission line while the extinction ratios and half-wave voltages match the state-of-the-art metrics for LNOI devices. At visible-near-infrared wavelengths, integrated electro-optic modulators are presently less developed than at telecom wavelengths. However, LNOI is a promising platform for integrated photonics in the visible-near-infrared and we present the design procedure for Mach-Zehnder modulators at 780 nm with low voltage-length products of 1.27 V·cm capable of transmitting 40 Gbit/s on-off keying signals. A considerable part of this thesis is devoted to put the presented findings and developments into context with the rapidly evolving fields of integrated photonics and, in particular, LNOI photonics. We discuss the potential future developments of the presented devices and look at the broader perspective for LNOI to become a major platform for versatile photonic integrated circuits.