Design of Erbium-Doped Tellurium Oxide Optical Amplifiers on a Low-Loss Silicon Nitride Waveguide Platform Design of Erbium-Doped Tellurium Oxide Optical Amplifiers on a Low-Loss Silicon Nitride Waveguide Platform By CHENGLIN ZHANG B.Eng (HIT, Harbin, China) 2016 A Thesis Submitted to the School of Graduate Studies in Partial Fulfillment of the Requirements for the Degree Master of Applied Science McMaster University © Copyright by Chenglin Zhang, September 2018 MASTER OF APPLIED SCIENCE (2018) (Department of Engineering Physics) McMASTER UNIVERSITY Hamilton, Ontario TITLE: Design of Erbium-Doped Tellurium Oxide Optical Amplifiers on a Low-Loss Silicon Nitride Waveguide Platform AUTHOR: Chenglin Zhang, B.Eng. (Harbin Institute of Technology, China) THESIS SUPERVISOR: Dr. Jonathan Bradley NUMBER OF PAGES: XCV, 95 ii Abstract Research interest in optical amplifiers has remained intensive over the years due to their widespread application in high-speed telecommunication systems. The requirement for amplification arises from the need to strengthen weakened signals as they propagate through optical fibers or through waveguides on chips and in order to meet strict system power budgets. Erbium-doped fiber amplifiers offer high, broadband and low-noise gain for wavelength-division multiplexed systems. Nevertheless, fiber amplifiers are bulky and many applications, including the development of compact transceiver modules for the data center, require an integrated on-chip solution. Erbium-doped waveguide amplifiers (EDWAs), which are integrated onto silicon chips, could potentially replace EDFAs in many instances. Of prospective materials for EDWAs, erbium-doped tellurium dioxide has a range of advantages including its high refractive index for compact waveguides and bends, large erbium light emission bandwidth, high Er3+ ion solubility and high gain. However, fabrication of erbium-doped tellurium oxide waveguides has proved challenging, and methods developed to date are not compatible with standard silicon- based photonic integration platforms. Meanwhile, silicon nitride is a standard high- quality light guiding platform due to its fairly high refractive index, high transmission throughout the visible and infrared spectrum and the compatibility of silicon nitride fabrication process with standard CMOS fabrication lines. The combination of erbium- doped tellurium oxide with silicon nitride technology could lead to compact and high performance optical amplifiers that are scalable and compatible with existing photonic iii integration platforms. This thesis describes research on different designs of erbium-doped tellurium oxide optical amplifiers integrated on a low-loss silicon nitride waveguide platform. Both theoretical and experimental work is described in the context of improving the amplitude, capacity and stability of erbium-doped tellurium oxide optical amplifiers for communications. Specifically, Chapter 2 studies the theory and background of optical waveguides and rare earth dopants and their transitions. Chapter 3 presents the finite element method waveguide mode solver, tellurium oxide-coated silicon nitride waveguide design and amplifier model. In Chapter 4, simulation results on the optimization of the tellurium dioxide-coated silicon nitride waveguides are presented. Modelling and comparisons between Er-doped Al2O3 and TeO2 waveguides are also described using a MATLAB code that provides predictive results for both of their performance. Fabrication and initial measurements results are generally shown in this chapter as well. iv Acknowledgments First of all, I would like to offer my deepest gratitude to my supervisor Dr. Jonathan Bradley whose door was always open whenever I ran into a trouble spot or had a question about my research or writing. He consistently helped me in my simulation, lab work, and thesis which gave me a better understanding of silicon photonics field. I would like to thank him for his professional knowledge, kindness, patience, and politeness. Also, I would like to thank Dr. Andy Knights who continuously offered help in the by weekly optical amplifier group meeting. In the cooperation between Dr. Knights' and Dr. Bradley's groups, I learned how to work in different teams and met many amazing team members. I also would like to thank Dr. Peter Mascher, Dr. Jonathan Bradley, Dr. Chang-qing Xu, and Dr. Ayse Turak who offered me great classes during these two years on various subjects. I would also want to say think you to all the group members in Dr. Bradley group: Mengyuan, Dawson, Henry, Khadijeh, Jeremy, Cameron, Josh, Arthur, Caitlin, Daniel and many other people for their assistance and friendships over these two years. Last but not least, I would like to thank my family far away in China who offered unconditional support and care when I lived alone in another country. Thanks to my mum, dad, and sister who skyped with me every week. Thanks to my friends in China, Canada, and Australia who talked to me once in a while offering me helpful advice and opinions v about study and life. vi Contents List of Figures...................................................................................... x Index of Tables ................................................................................. xiii Chapter 1 Introduction ...................................................................... 1 1.1Erbium-Doped Optical Amplifiers for Telecommunications ........................ 1 1.2 Integrated Erbium-Doped Amplifiers ........................................................... 3 1.3 Erbium-Doped Tellurium Oxide ................................................................... 6 1.4 Low-Loss Silicon Nitride Waveguide Platform ........................................... 7 1.5 Thesis Outline ............................................................................................... 8 Chapter 2 Theory and Background ................................................ 10 2.1 Waveguide Theory ...................................................................................... 10 2.1.1 Planar Waveguides ............................................................................... 12 2.1.2 Ridge Waveguides ............................................................................... 21 2.2 Material Properties ...................................................................................... 24 2.2.1 Rare Earth Dopants and Their Optical Properties ................................ 24 2.2.2 Optical Properties of Erbium ............................................................... 24 2.2.3 Erbium-Doped TeO2 Film Properties .................................................. 27 vii 2.2.4 Si3N4 Waveguide Properties ................................................................. 28 2.3 Optical Amplifier Theory ........................................................................... 29 2.3.1 Loss Mechanisms in Optical Waveguides ........................................... 29 Chapter 3 Design and Modeling ...................................................... 35 3.1 Waveguide Design ...................................................................................... 35 3.1.1 Finite Element Method Waveguide Mode Solver................................ 35 3.1.2 Optimization of TeO2-Coated Si3N4 Waveguides for Optical Amplifiers39 3.2 Erbium-Doped Amplifier Model ................................................................ 40 Chapter 4 Results .............................................................................. 43 4.1 Waveguide Simulation Results ................................................................... 43 4.2 Amplifier Modeling Results ....................................................................... 62 3+ 4.2.1 Application of Amplifier Model to Er -doped Al2O3 and TeO2 Waveguides from Literature .............................................................................................. 62 3+ 4.2.2 Amplifier Modeling Based on TeO2:Er -Coated Si3N4 Waveguide Design in this Thesis ..................................................................................................... 67 4.3 Silicon Nitride Mask Layout ....................................................................... 70 4.4 Fabrication Results ...................................................................................... 75 4.5 Measurements ............................................................................................. 77 Chapter 5 Conclusions and Future Work ...................................... 80 viii Appendix: Amplifier Simulation Code ........................................... 82 References .......................................................................................... 91 ix List of Figures: Figure 1.1: Amplification in an EDFA …………………………………………………3 Figure 1.2: Er-doped waveguide amplifiers and lasers for on-chip applications ………………...……………………………….……………………...……5 Figure 2.1: Planar waveguide……………………………………………………………12 Figure 2.2: Reflection and refraction of light at an interface……………………………13 Figure 2.3: One-dimensional confinement of light propagating in a step-index planar waveguide…………………………………………………………………………....…...14 Figure 2.4: A typical ridge waveguide…………………………………………………..21 Figure 2.5: A typical strip-loaded waveguide…………………………………………...22 Figure 2.6: Energy level diagram and selected transitions for erbium ions relevant to the 3+ 980-nm pumped TeO2:Er doped amplifier……………………………………………..25 Figure 2.7: The radial electric field of a bent waveguide…………..………….………...32