Optical directional coupler

a, aaaadadaAn optical directional coupler comprises first and second straight optical waveguides having respective cores extending longitudinally close to each other to form an optical coupler portion, and a tapered optical waveguide having a core connected to the core of the first straight waveguide. The core of the first straight waveguide has a centerline. A width of the core of the tapered waveguide becomes narrow toward the first straight waveguide and a profile of the tapered waveguide is asymmetric relative to the centerline of the core of the first straight waveguides.