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.　©,　2014,　Chinese　Optical　Society.　All　right　reserved.