Ultrathin Silicon-on-Insulator Grating Couplers

We demonstrate a novel grating coupler fabricated on a silicon-on-insulator (SOI) wafer operating at 1550 nm, based on an ultrathin 50-nm silicon geometry. The devices are fabricated in a CMOS-compatible process with a single etch step. A low insertion loss of ${-}{\rm 3.7}~{\rm dB}$ is achieved. We also calculate the backreflection loss to be ${-}{\rm 14}~{\rm dB}$ . The devices are likely to be useful in terms of light coupling between optical fibers and ultrathin silicon waveguides.