Optical waveguide and arrayed waveguide grating

A technique that does not increase the circuit size, does not make the circuit design and manufacturing difficult, and can reduce insertion loss when light enters from a slab waveguide toward an arrayed waveguide or when the light enters from the arrayed waveguide toward the slab waveguide. An optical waveguide provided with a slab waveguide in which a grating is formed therein at a distance from an end, and an arrayed waveguide whose end is connected to an end of the slab waveguide at a position where a constructive interference portion of a self-image of the grating is formed. An arrayed waveguide grating provided with a first input/output waveguide, the above-mentioned optical waveguide where an end of the slab waveguide on the opposite side of the arrayed waveguide is connected to an end of the first input/output waveguide, a second slab waveguide connected to an end of the arrayed waveguide on the opposite side of the slab waveguide, and a second input/output waveguide connected to an end of the second slab waveguide on the opposite side of the arrayed waveguide.