Patent ID: 8463086

Claim:
An optical semiconductor device in which light having a wavelength of 1.25 μm or greater is waveguided, comprising: a first waveguide of embedded type that includes a semiconductor and is lattice-matched with InP, the first waveguide having a region having a first constant width equal to or greater than 1.50 μm and a first region narrower than the region having the first constant width equal to or greater than 1.50 μm; and a second waveguide of embedded type that includes another semiconductor having a refractive index different from that of the first waveguide, the second waveguide having a region having a second constant width smaller than 1.50 μm and a second region wider than said region having the second constant width smaller than 1.50 μm, the first waveguide and the second waveguide being joined at an intermediate waveguide portion, the intermediate waveguide portion including the first region and the second region and a joining plane on which the first region and the second region are joined, and the joining plane having a width equal to or smaller than 1.35 μm.