Optical multiplexer/demultiplexer

An optical multiplexer/demultiplexer comprising:

• a substrate;
• first and second slab waveguides provided on said substrate, each having a predetermined slab length;
• a waveguide provided on said substrate, said waveguide being at least one input waveguide for guiding light into said first slab waveguide;
• waveguides flatly arrayed on said substrate in a state in which optical input ends of the respective waveguides are connected to an optical output end face of said second slab waveguide, said waveguides being a plurality of output waveguides provided corresponding to respective signals of channel wavelengths set as signal channels at predetermined wavelength intervals;
• waveguides flatly arrayed on said substrate in a state in which optical input ends of the respective waveguides are connected to an optical output end face of the first slab waveguide so as to place said first slab waveguide between said waveguides and said input waveguide and in which optical output ends of the respective waveguides are connected to an optical input end face of the second slab waveguide so as to place said second slab waveguide between said waveguides and said output waveguides, said waveguides being a plurality of channel waveguides of mutually different lengths;
• and
• a waveguide provided between said input waveguide and said first slab waveguide and provided for coupling part of the fundamental mode of light having propagated through the input waveguide, to a higher order mode, said waveguide being a free propagation area comprised of a first portion for broadening an electric field strength distribution of light propagating from an optical output end of the input waveguide toward an optical input end face of said first slab waveguide by diffraction, and a second portion for limiting broadening of the electric field strength distribution of the light having passed through the first portion;
• and
• further comprising a multi-mode interference coupler provided between the optical output end of said input waveguide and an optical input end of the first portion in said free propagation area.