Disclosed are grating couplers having a high coupling efficiency for optical communications. In one embodiment, an apparatus for optical coupling is disclosed. The apparatus includes: a substrate; a grating coupler comprising a plurality of coupling gratings over the substrate, wherein each of the plurality of coupling gratings extends in a first lateral direction and has a cross-section having a middle-raised shape in a second lateral direction, wherein the first and second lateral directions are parallel to a surface of the substrate and perpendicular to each other in a grating plane; and a cladding layer comprising an optical medium, wherein the cladding layer is filled in over the grating coupler.