A high-efficiency grating coupler is designed to couple a planar waveguide and a single-mode fiber. An improvement in fiber coupling efficiency is obtained by using nonuniform diffraction gratings whose grating grooves consist of subwavelength structures with different duty cycle, which can better match the diffracted field profile to that of the Gaussian fiber mode. By optimizing the thickness of the buried oxide layer and integrating a gold bottom mirror, the directionality of SOI grating coupler can be increased. The coupler is optimized using eigenmode expansion-based simulations. A peak coupling eficiency of 93.1% at 1550 nm and 3 dB bandwidth of 82 nm are achieved.