OPTICAL MULTIPLEXER/DEMULTIPLEXER

PROBLEM TO BE SOLVED: To attain miniaturization in an optical multiplexer/demultiplexer.SOLUTION: An optical multiplexer/demultiplexer includes: a slab 1; an input waveguide 2 connected to the slab 1; an output waveguide 3 connected to the slab 1; a concave diffraction grating 4 formed on the end surface of the slab 1; a first reflection surface 5 which is arranged on the locus 101 of a principal beam of light to be made incident from the input waveguide 2 to the concave diffraction grating 4 so as to reflect the light to be made incident from the input waveguide 2 toward the concave diffraction grating 4; and a second reflection surface 6 which is arranged on the locus 101 of the principal beam of light proceeding from the concave diffraction grating 4 to the output waveguide 3 so as to reflect the light proceeding from the concave diffraction grating 4 toward the output waveguide 3.