An optical waveguide structure formed over a substrate defines an optical waveguide for guiding light along a direction parallel to the substrate surfaces, and biasing a light intensity distribution of transverse modes of guided wave toward a first side of the optical waveguide path. A main diffraction grating is disposed at least on a second side opposite to the first side, and coupled with the guided wave propagating along the optical waveguide. A subsidiary diffraction grating is disposed on the first side, and diffracts the guided wave coupled with the main diffraction grating and propagating along the optical waveguide, to a direction different from an extending direction of the optical waveguide.