An arrayed waveguide grating optical multiplexer/demultiplexer includes an arrayed
waveguide connected to at least one first optical waveguide via a first slab waveguide,
a plurality of second optical waveguides connected to the arrayed waveguide via
a second slab waveguide. At least one expanding width waveguide has a first end
portion and a second end portion having a second width larger than a first width
of the first end portion. Each first end portion is connected to each first optical
waveguide. The second end portion is connected to the first slab waveguide. The
first width of the first end portion is larger than a first optical waveguide width
of the at least one first optical waveguide. The first width of the first end portion
satisfies a single mode condition. A width of the expanding width waveguide increases
from the first end portion toward the second end portion.
