An optical waveguide is disclosed. The optical waveguide includes a plate of transparent material comprising opposed first and second surfaces for guiding an optical beam between the surfaces by at least one of reflection or diffraction. A diffraction grating is disposed at the first surface for spreading the optical beam by diffracting portions thereof into a non-zero diffraction order to propagate inside the plate. The first diffraction grating includes an array of parallel grooves structured to provide a spatial variation of optical phase of the portions of the optical beam diffracted by the first diffraction grating into the non-zero diffraction order.