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Huang, Po-Chun
Peckerar, Martin
Nanophotonic phased array (NPA) technology has been an active topic of research for many years. This is due to its widespread applicability in the emerging fields of virtual reality (VR) and light detection and ranging (LIDAR). This thesis describes an integrated NPA system consisting of an optical phased array and its electrical driver circuit. These two components are realized in two separate “chips”: an optical chip (OC) for generating and routing light; and, a “motherboard chip” (MB) for creating the current drivers for the individual elements of the array. The driver circuit sources voltage or current to modulate the phase or amplitude (or both) of the optical output beams of each unit in the phased array. The output beams interfere with each other either constructively or destructively in such a way as to replicate the light field of a portrayed scene (forming a VR image) or to form a steerable beam (LIDAR). This dissertation centers on the realization of a scalable thermo-optic based NPA system. The thermo-optic based system changes the phase of the output beam emerging from an individual emitter by locally heating the optical path through which the beam emerges from the array. To realize a large NPA system with pixel-level independent phase control, the size of a driver circuit unit must be matched to the size of the individual phased array element, or pixel. This must be accomplished, while at the same time avoiding interconnection congestion issues. This poses a critical design challenge for the driver circuit limiting its functionality. Also, a large amount of heat is generated in the thermo-optic system. Unintentional spreading of this heat through the array (known as proximity effect) not only introduces phase errors across the array, but it also causes reliability issues in the densely integrated electronic elements of the array. To overcome these issues, my thesis was divided into the following tasks.First, I conducted a comprehensive simulation based thermal study of our proposed integrated NPA system using the COMSOL finite element method (FEM) solver. The study includes detailed single pixel simulations characterizing the thermo-electrical properties of the system. This helped guide the driver circuit design. It further enabled small array simulation for quantifying thermal spread blurring (proximity effect) and phase errors. The thesis includes transient simulations to show the response speed of the system. I show that our system requires less than 50 µs to reach a target temperature. I introduce a model simplification method to reduce the computation resource requirement of system-level simulations. These calculations tell us how large the array can be without incurring thermal damage. This thesis further discloses a unique broken-loop feedback control system to achieve pixel-level temperature regulation for phase error minimization. The control system uses an integrated thin-film thermocouple/heater device to sense the temperature feedback signal and to source a current for providing the required phase shift. This device requires but a single contact post between each phase array pixel and its control circuit. In this way, the OC and the MB chips can be integrated by most available flip-chip bonding technologies. Two design implementations of the driver circuit sourcing 4.8 mW per pixel from a 2.5 V supply voltage are provided. One design can be realized in an area of 15 µm x 15 µm per pixel with pixel-level independent phase control using the TSMC 65 nm technology node. This exactly matches the size of the NPA pixel. The other design can meet the same area constraint using a more advanced technology node. This thesis also provides an experimental characterization of the driver circuit designed and fabricated on the TSMC foundry’s 65nm product line. Experimental results of characterizing each component of the driver circuit are provided. The broken-loop feedback control method was electrically evaluated independent of the optical system by using a resistor to generate a simulated feedback signal. The circuit achieves a maximum 3.6% (0.07π) and average 1% (0.02 π) introduced by a ±20% variation of the load resistance. I provide a comparison of the performance of both VR image quality and LIDAR steering accuracy using either the direct control method or our broken loop feedback control method. This was done using the structure similarity index (SSIM) method. This method ranks image quality in a range from 0 to 1 (0 the poorest image and 1 the best image.) On average, the images studied improved their SSIM index from 0.45 to 0.9 using the broken-loop method. In beam steering, our feedback control method achieves less than 0.05° angle deviation and constant main beam intensity as compared to a 0.9° angle deviation and more than 90% reduction in main beam intensity using direct control. This demonstrates that our feed-back controlled driver circuit is essential for NPA systems to achieve high performance.