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We describe a shallow-etched diffractive waveguide grating coupler which achieves efficient coupling between single-mode fiber and a silicon-on-insulator optical waveguide. By the appropriate choice of waveguide/grating thicknesses and varying the coupling strength of the grating coupler via tailoring its fill factor to optimize the mode matching, a coupling loss of only 1.2 dB was obtained for each fiber/silicon waveguide interface. The coupling strength and effective refractive index of grating with different fill factors were also characterized to confirm the design simulations. The back reflection was suppressed by varying the coupling strength, leading to further enhancement of the coupling efficiency.