An optical waveguide propagates a laser beam. Main diffraction grating and sub-diffraction grating couple light propagating in the optical waveguide. The main diffraction grating and the sub-diffraction grating couple the light in such a manner that propagation in a second order transverse mode of the light propagating in the optical waveguide when both the main diffraction grating and the sub-diffraction grating are disposed, is suppressed more than propagation in the second order transverse mode of the light propagating in the optical waveguide when only the main diffraction grating is disposed.