  A Silicon-on-insulator (SOI) perfectly vertical fibre-to-chip grating coupler
is proposed and designed based on engineered subwavelength structures. The high
directionality of the coupler is achieved by implementing step gratings to
realize asymmetric diffraction and by applying effective index variation with
auxiliary ultra-subwavelength gratings. The proposed structure is numerically
analysed by using two-dimensional Finite Difference Time Domain (2D FDTD)
method and achieves 76% (-1.19 dB) coupling efficiency and 39 nm 1-dB
bandwidth.
