Patent ID: 8264919

Claim:
A spot size converter, comprising: a second core that operates to introduce a light beam and to taper light from the light beam, a first core that is linked to the second core, and a clad that surrounds a periphery of a core coupling body formed by linking the first and second cores, wherein when the refractive index of the second core is n C2 , the refractive index of the first core is n C1 , and the refractive index of the clad is n ad , the condition of n C1 >n C2 >n ad is satisfied; the second core has a rectangular cross-sectional area that gradually decreases as the second core advances internally from the optical incident end surface, and that has a taper portion that is a triangular plate-like part when viewed from a level plane; the first core has a tapered coupling part that is integrated within the taper portion of the second core, and a waveguide core part that is linked with the tapered coupling part and that propagates primarily in a single mode, and the tapered coupling part is structured such that its shape viewed from a level plane is a triangular plate-like coupling part, and the level plane in which the rectangular cross-sectional area gradually increases in relation to an advancing direction of the light.