An optical coupler device comprises a substrate having a substantially planar upper surface, and a grating structure on the upper surface of the substrate. In one embodiment, the grating structure comprises a copropagating array of waveguides that are substantially parallel to each other and extend along at least a portion of the upper surface of the substrate. Each of the waveguides has opposing sidewalls, wherein a width of each waveguide is defined by a distance between the opposing sidewalls. The opposing sidewalls each have a periodic structure that produces a sidewall modulation for each of the waveguides. An input port is in optical communication with the grating structure. The input port is configured to direct an input light beam in plane into the grating structure such that the beam propagates along the waveguides. The grating structure is configured to diffract the beam out of plane and into free space.