Grating coupler using angle-polished optical fiber

In recent years, there has been tremendous development in photonic integrated circuits (PICs) because of the growing demand for computational power and the slowing down of transistor shrinkage. PICs are seen as a promising technology for developing next-generation technologies including the Internet of Things, on-chip data routers, and optical quantum computers due to their compatibility with the long-established complementary metal-oxide-semiconductor fabrication technology and making use of common materials such as silicon and silicon dioxide. As the size of the PICs is shrinking and considering different aspects such as modal size mismatch, fabrication/packaging cost, it has become increasingly difficult to couple light efficiently in-plane or out-of-plane among different photonic elements such as waveguides and fibers. In this study, we propose a two-layer grating coupler using a horizontally placed angle-polished single-mode optical fiber. We used the finite-difference time-domain method and optimization tools including the inverse design technique to investigate the design parameters for performance enhancement of light coupling in silicon-on-insulator (SOI) integrated circuits. We achieved a coupling efficiency of -1.54 dB (70.15%) for fiber-to-SOI chip coupling and a coupling efficiency of -0.97 dB (80%) for chip-to-fiber coupling over a wide bandwidth.