Patent ID: 7496127

Claim:
An optical device comprising: an optical waveguide for propagating a laser beam; a main diffraction grating coupling light propagating in the optical waveguide; and a sub-diffraction grating coupling the light propagating in the optical waveguide, wherein 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, wherein the light propagating in the optical waveguide in the second transverse mode and coupling the main diffraction grating is diffracted by the sub-diffraction grating and re-couples the optical waveguide at a first re-coupling degree, the light propagating in the optical waveguide in the fundamental transverse mode and coupling the main diffraction grating is diffracted by the sub-diffraction grating and re-couples the optical waveguide at a second re-coupling degree, and the first re-coupling degree is lower than the second re-coupling degree.