Department of Electrical and Computer Engineering Ph.D. Public Defense

Integrated Silicon Photonic Optical Phased Array for Free-Space Optical Interconnect

Francis A. Smith

Supervised by Professor Hui Wu

Tuesday, August 20, 2019
1 p.m.

Computer Studies Building, Room 426

Integrated optical phased arrays (OPAs) are rapidly becoming one of the most promising technologies for future light detection and ranging (LIDAR) applications, thanks to their advantages in size, weight, and power consumption as com- pared to conventional solutions. Recently, a 1024-element integrated OPA has been   implemented   using an  SOI CMOS technology.  Large-scale   OPA s can  leverage high integration densities of electronic-photonic integrated circuit technology to generate finer beamwidth,  better  beamsteering   control,  and  higher  optical  power. Free-space   optical  interconnects (FSOI)  are  emerging  as  an  attractive  alternative to planar electrical interconnects for inter- and intra-chip data communications. By leveraging the third dimension above the chip surface, FSOI solutions can offer higher bandwidth density compared to on-chip, waveguide-based solutions. Through 3-D integration, low cost, CMOS-compatible, silicon photonic FSOI  systems  for  high  performance  data  center  communications  are  within  reach. The versatility of integrated OPAs can address the accurate alignment requirements of such systems to ensure robust link performance.

At optical frequencies, the conventional design of phased arrays relies on the Finite-Difference Time Domain (FDTD) method. The FDTD method explicitly calculates the evolution of the array pattern from a fi discretized spatial representation of the entire array over a short time scale. Thus, computation accuracy is directly traded  for  simulation time. In  addition,  the  modeling techniques to manage  this tradeoff  do not accommodate  asymmetric  arrays  or allow array analysis at the system level.

In this work, I propose an OPA circuit design based on the synthesis method, which significantly relaxes the computational cost, time, and accuracy tradeoffs of OPA design at optical frequencies by using the radiation pattern of a single emitter element to synthesize that of the whole array. Instead of modeling an entire emitter array using the FDTD method, the synthesis approach requires only   a  single  emitter  to  be  simulated  using 3-D FDTD.   A   design  flow based on the proposed  phased  array  synthesis  allows  accurate  and  robust  modeling  of  arbitrary 1-D and 2-D OPAs, and enables optimization of device parameters and array coefficients across the device and system levels.

The  synthesis  method  is  used to develop an OPA chip prototype in a standard silicon photonic technology. A diagonally asymmetric  array  geometry  is  designed to potentially increase orthogonal free-space optical beam steering range com- pared to conventional rectangular grid OPA geometries. A  technique is  developed to leverage the natural static bias of the optical waveguide channels to reduce the power required for beamsteering control. Further, multi-function optical beam control is explored through subarray beam splitting. The fabricated chip prototype  is  packaged  and  tested  in  a  custom free-space  imaging  test  bench.  We  use a closed-loop, component-to-system optimization  in  both circuit  design and testing. The measured beamwidth of the sparse OPA is 2.13o in φ and 0.67o in θ. The measured grating lobe free beamsteering range in φ is 20o (±10o), and in θ is 24o (±12o). The measurement  results  agree  with  the  simulations  and  are successfully verified by the design.