Integrated LIDAR with Optical Phased Arrays in Silicon Photonics by Christopher Vincent Poulton B.S., University of Colorado Boulder (2014) Submitted to the Department of Electrical Engineering and Computer Science in partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering and Computer Science at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY September 2016 © Massachusetts Institute of Technology 2016. All rights reserved. Author .............................................................. Department of Electrical Engineering and Computer Science August 31, 2016 Certified by.......................................................... Michael R. Watts Associate Professor Thesis Supervisor Accepted by ......................................................... Leslie A. Kolodziejski Chair, Department Committee on Graduate Students 2 Integrated LIDAR with Optical Phased Arrays in Silicon Photonics by Christopher Vincent Poulton Submitted to the Department of Electrical Engineering and Computer Science on August 31, 2016, in partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering and Computer Science Abstract Light detection and ranging (LIDAR) has become an ubiquitous ranging technology. LIDAR systems are integral to almost all autonomous vehicles and robotics. Most LIDAR systems today use discrete free-space optical components and utilize a mechanical apparatus for beam steering. Apart from the relative high cost of the system, this mechanical apparatus limits the scan rate of the LIDAR system while increasing both size and complexity. This leads to concerns about long-term reliabil- ity, especially in harsh environments. In this thesis, the design and experimental results of an integrated chip-scale frequency-modulated continuous-wave LIDAR system are presented. This system has the capability of measuring both distance and velocity simultaneously with a 20mm resolution and a 2m range. Its functionality is then extended by utilizing optical phased arrays as a transmitter and receiver for solid-state beam steering. The phased array utilized has a grouped cascaded phase shifter architecture and is shown to have a steering range of 46∘×36∘. This is the first integrated coherent LIDAR system based on optical phased arrays. In order to have a viable LIDAR system with optical phased arrays, high beam powers and large aperture sizes are needed. A silicon nitride distribution network is used to enable high on-chip power because of the low material nonlinearities. An ultra-high main beam power of 520mW is reported. A phased array is demonstrated with an ultra-large aperture size of 4×4mm2, achieving a record-small and near diffraction limited spot size of 0.021∘×0.021∘ with a side lobe suppression of 10dB. This is the largest optical phased array to date by an order of magnitude and shows the scalability of optical phased arrays. Finally, an optical phased array at a visible wavelength of 635nm is shown with an aperture size of 0.5×0.5mm2 and a spot size of 0.064∘×0.074∘. This demonstration moves large-scale integrated photonics into the visible spectrum and has potential applications in bathymetric LIDAR. Thesis Supervisor: Michael R. Watts Title: Associate Professor 3 4 Acknowledgments I first want to thank my advisor, Prof. Michael Watts, who has given me the opportu- nity to work on this extremely impactful project in a growing field. There were always resources to make it better, including expensive equipment and dedicated fabrication runs, and for that I am extremely thankful. The mixture of academic freedom and practical pressure I have been given has allowed me to grow to be an independent researcher. I also want to thank Dr. Joshua Conway, the DARPA program manager for this project, for his technical and funding support throughout the project. I am humbled to be his DARPA Riser and grateful that we were included in the DARPA Demo Day. It’s been a wonderful experience being involved in the larger DARPA community. Manan Raval has been one of my best friends in Boston that has not only been a person to bounce ideas off, but also to talk to during the bad times that occurred during this work. He has been extremely supportive and I’ve enjoyed the many nights at Lord Hobo talking about photonics, the lab, and life in general. In addition, I want to thank Matthew Byrd for the all of his design and exper- imental effort that helped propel this work, specifically on the silicon nitride based devices. Much of it would not have been possible without him. It was a wonderful experience to mentor him in the beginning and see him grow into one of the best photonic designer and experimenter I’ve ever met. Diedrik Vermeulen was crucial for the success of the demo of this work at the DARPA Demo Day. Without his device packaging knowledge and effort, there might not have been a demo. The many late nights we had in the lab gluing fibers to chips will not be a memory I will forget, for better or for worse. It’s been an honor to continue Ami Yaacobi’s Ph.D work, and his initial teachings on phased arrays allowed for this thesis to be written. His personality is greatly missed in the lab. Dave Cole was also monumental in getting the first on-chip LIDAR measurement that took this work off the ground. The general day-to-day mentorship of Zhan Su and Erman Timurdogan taught 5 me the technical details of so many different aspects in silicon photonics in the past two years. I thank them both for all of their time and dedication. Because of them, I feel that I have the knowledge to excel at almost anything in silicon photonics. Finally, I want to thank my parents, Susie and Jonathan Poulton, who sacrificed so much to allow for me to be here, and my brother, Matthew Poulton, who will always be my number one bro. I love you guys, this thesis is dedicated to you. 6 Contents 1 Introduction 17 1.1 Background on LIDAR . . . . . . . . . . . . . . . . . . . . . . . . . . 17 1.2 LIDAR with Optical Phased Arrays . . . . . . . . . . . . . . . . . . . 20 1.2.1 Phased Array Mathematics and Basic Concepts . . . . . . . . 22 1.3 Overview of Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2 LIDAR on a Chip 31 2.1 Device and FMCW LIDAR Overview . . . . . . . . . . . . . . . . . . 31 2.2 Balanced Photodetector . . . . . . . . . . . . . . . . . . . . . . . . . 33 2.3 Laser Diode Characterization . . . . . . . . . . . . . . . . . . . . . . 36 2.4 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 2.4.1 Homodyne Doppler Measurements . . . . . . . . . . . . . . . 38 2.4.2 Stationary Target Measurements . . . . . . . . . . . . . . . . 39 2.4.3 Moving Target Measurements . . . . . . . . . . . . . . . . . . 40 2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3 Grouped Cascaded Phased Array for Integrated LIDAR 43 3.1 Phased Array Design . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.1.1 Grouped Cascaded Phase Shifters . . . . . . . . . . . . . . . . 43 3.1.2 Phase Shifter and Splitter Design . . . . . . . . . . . . . . . . 45 3.1.3 Antenna Design . . . . . . . . . . . . . . . . . . . . . . . . . . 47 3.2 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 3.2.1 Phased Array Characterization . . . . . . . . . . . . . . . . . 50 7 3.2.2 LIDAR System with Phased Array . . . . . . . . . . . . . . . 53 3.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4 High Power Silicon Nitride Phased Arrays 57 4.1 Splitter Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 4.2 Antenna Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.2.1 Mono-Layer Antenna Design . . . . . . . . . . . . . . . . . . . 61 4.2.2 Dual-Layer Antenna Design . . . . . . . . . . . . . . . . . . . 64 4.3 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 4.3.1 Splitter Results . . . . . . . . . . . . . . . . . . . . . . . . . . 67 4.3.2 Uni-Directional Antenna Results . . . . . . . . . . . . . . . . 68 4.3.3 Phased Array Results . . . . . . . . . . . . . . . . . . . . . . . 70 4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 5 Ultra-Large-Scale Passive Phased Arrays 75 5.1 Ultra-Large Silicon Phased Arrays . . . . . . . . . . . . . . . . . . . . 76 5.2 Ultra-Large Silicon Nitride Phased Arrays . . . . . . . . . . . . . . . 78 5.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 6 Phased Arrays for Visible Light Applications 83 6.1 On-Chip Coupling and Routing Waveguides . . . . . . . . . . . . . . 85 6.2 Visible Splitters and Antennas . . . . . . . . . . . . . . . . . . . . . . 86 6.3 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 6.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 7 Thesis Conclusion 93 8 List of Figures 1-1 Point cloud from a single scan of a Velodyne HDL-64 LIDAR module. From [5]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 1-2 (a) Internal and (b) external view of a Velodyne HDL-64E LIDAR module. From [10]. . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 1-3 (a-f) Overview of notable optical phased array systems. From [16–21]. 21 1-4 (a) The phase distribution of three different 𝛼 values. (b) The array factors for the three different 𝛼 values showing steering with a change of 𝛼. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 1-5 (a) The array factors for different antenna pitch values. (b) Maximum steering range of a phased array as a function of antenna pitch. . . . 24 1-6 (a)Exampleopticalphasedarrayantennabasedonalongsiliconwave- guide grating. (b) Element factor, 𝐸(𝜃,𝜑), of the antenna. . . . . . . 25 1-7 Schematic of a (a) tree and (b) cascaded phase shifter architecture for one-dimensional phased arrays. . . . . . . . . . . . . . . . . . . . . . 27 1-8 (a) Thermal phase shifter. From [32]. (b) Illustration of a silicon rib waveguide with vertical PN and measured data of the PN junction. . 28 2-1 (a) Illustration of the effective device consisting of silicon photonic chip with collimating lens. Full device described in [38]. (b) Con- cept of frequency-modulated continuous-wave LIDAR with triangular modulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 2-2 Layout rendering of the balanced photodetector consisting of an adia- batic coupler and two germanium photodetectors. . . . . . . . . . . 33 9 2-3 (a) Measured response of a single adiabatic coupler. (b) Measured photodetector responsivity. . . . . . . . . . . . . . . . . . . . . . . . . 35 2-4 (a) Measured laser diode frequency as a function of injection current. (b) Back-calculated current ramp to produce a linear frequency sweep. 36 2-5 Example of time gating the photodetector signal to reduce the effects of the transients. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 2-6 Experimental setup utilizing a 1.5m motion track. . . . . . . . . . . 38 2-7 (a) Concept of homodyne Doppler measurements. (b) Experimental results of the homodyne Doppler measurements with a theoretical line of the speed dependent Doppler shift. (c) Speed error of the data set in (b). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 2-8 (a) Measured beat frequency at various stationary target distances at 10mm intervals. (b) Fourier transform of time domain data in (a) showing resolved frequency peaks. (c) Distance error of the measure- ments shown in (a). . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 2-9 (a) Measured beat frequencies for a moving target while sweeping ve- locity and distance. (b) Calculated velocity of the data set in (a), black lines show actual velocity. (c) Raw time domain data of the 0.5m target measurements at four different velocities. . . . . . . . . 40 2-10 Error of calculated target (a) distance and (b) velocity of the data set in Fig. 2-9(a). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3-1 (a) Schematic of a grouped cascaded phase shifter architecture with three groups. (b) Concept of sub-phased arrays formed by a grouped architecture showing the ability to create multiple beams. . . . . . . 44 3-2 (a) Layout of cascaded phase shifters with alternating directional cou- plers. Inset shows FDTD simulation of the adiabatic bend. (b) FDTD simulation of power coupling as a function of coupling length. . . . . 46 3-3 (a) Rendering of a portion of a full etch based antenna. (b) Perturba- tion strength as a function of perturbation distance. . . . . . . . . . 47 10