To perform an optical coupling more efficiently by using an optical waveguide which transforms spot size.

The optical waveguide is characterized in that a second core 103 is tapered toward the beginning end (on the side of waveguide region 111) of a spot size transformation region 112. Because the second core 103 is tapered toward the beginning end of the spot size transformation region 112, the reflection and the loss of guided light occurring at the boundary between the waveguide region 111 and the spot size transformation region 112 are reduced. As a result, the optical coupling is more efficiently performed.

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