Optical Circuit

A second core includes a first portion, a second portion, and a bending portion. In the first portion, a wave-guiding direction is a first direction parallel to a plane of a first substrate. In the second portion, a wave-guiding direction is a second direction that is at a predetermined angle with respect to the plane of the first substrate. For example, in the second portion, the wave-guiding direction is the second direction that is at substantially 90 degrees with respect to the plane of the first substrate. The bending portion connects the first portion and the second portion. A relative refractive index difference between the second core and a cladding of the second optical waveguide preferably has a value such that the propagation loss in the bending portion is equal to or smaller than 0.1 dB.