High-Density Wafer-Scale 3-D Silicon-Photonic Integrated Circuits

This paper discusses recent advances on the newly developed high-density three-dimensional photonic integrated circuits (3-D PICs). In particular, we introduce our efforts in design, fabrication, and characterization toward a silicon photonic wafer-scale light detection and ranging (LIDAR) system populated by 3-D PIC unit cells. The 3-D PIC unit cell includes vertical U-shaped photonic couplers created by a combination of anisotropic etching, vertical α-silicon via formation, and wafer bonding. Using the U-shaped photonic couplers, the 3-D PIC unit cell forms a 120-channel folded single tile optical phased array (OPA) with 2-μm pitch. Ultracompact vertical U-shaped coupler arrays with 1-μm pitch are also fabricated and tested. The 3-D PIC unit cells will be tiled on a wafer-scale interposer, transmitting and receiving signals through equal-power splitters with pathlength-matched waveguides, and 3-D arbitrarily shaped waveguides or evanescent couplers. Designs of such interposers are shown and low-loss and misalignment tolerant chip-to-interposer coupling using evanescent coupler is demonstrated. Loss values of arbitrarily shaped waveguides fabricated by ultrafast laser inscription on the deposited-SiO2 cladding and their alignment tolerance to conventional silicon nitride edge couplers are reported. A path toward realizing 3-D integrated LIDAR is discussed.