PURPOSE: To assure stable uniaxial mode oscillation without causing oscillation of a TM mode even with anti-reflecting coating by forming a metal film on part of an optical waveguide including an active layer in the vicinity of the wavelength.

CONSTITUTION: After a first order diffraction grating 12 including a phase shift region 10 is formed on an n type InP board 13, on which there are successively grown a guide layer 14, an active layer 1, a cladding layer 2, and a cap layer 3. In a mode filter region 9 for restricting a TM mode, the cap layer 3 and part of the cladding layer 2 are etched a and after a SiO2 film 4 is formed on the region 9, a first electrode 5 is attached to the region 9. The region further has an anti-reflecting coating film 8 on both ends. In the mode filter region 9, since the electrode 5 is located in the close vicinity of the active layer 1, the TM mode with an electric component perpendicular to the electrode 5 among modes propagating in the optical waveguide 7 is attenuated. Further, since a TE mode is parallel to the electrode 5, it can propagate through the optical waveguide 7 substantially without any attenuation. Thus, the TE mode has very high gain and hence an optical output 11 assures stable uniaxial TE mode oscillation.