Compact Inversely-Designed Vertical Coupler with Bottom Reflector for Sub-Decibel Fiber-to-Chip Coupling on Silicon-on-Insulator Platform

Inverse design via topology optimization has led to innovations in integrated photonics and offers a promising way for designing high-efficiency on-chip couplers with a minimal footprint. In this work, we exploit topology optimization to design a compact vertical coupler incorporating a bottom reflector, which achieves sub-decibel coupling efficiency on the 220-nm silicon-on-insulator platform. The final design of the vertical coupler yields a predicted coupling efficiency of -0.35 dB at the wavelength of 1550 nm with a footprint of 14 {\mu}m x 14 {\mu}m, which is considerably smaller than conventional grating couplers. Its topology-optimized geometry can be realized by applying one full etch and one 70-nm shallow-etch process and the fabricability is also guaranteed by a minimum feature size around 150 nm. The proposed vertical coupler can further miniaturize photonic integrated circuits and enable highly-efficient networks between optical fibers and other photonic devices.