DIRECTIONAL COUPLER AND MULTIPLEXER/DEMULTIPLEXER DEVICE

摘要

PROBLEM TO BE SOLVED: To provide a directional coupler which has a wide band and a high signal separation degree and can be small-sized, and a multiplexer/demultiplexer device.SOLUTION: The directional coupler includes: grating points 12A which are cyclically disposed within a two-dimensional photonic crystal slab 12 and diffract light waves, terahertz waves or millimeter waves in a photonic band gap band of a photonic band structure in order to prohibit the presence within a plane of the two-dimensional photonic crystal slab; a first two-dimensional photonic crystal waveguide 14formed by a line defect of the grating points; a second two-dimensional photonic crystal waveguide 14that can be mode-coupled with the first waveguide 14; and directional coupling parts 50 which are disposed in two lines between the first waveguide 14and the second waveguide 14and include inter-waveguide grating points of which the radius is smaller than that of the grating points. A width of the second waveguide 14is narrowed in such a manner that an entire diffusion curve of the directional coupling parts moves to a high frequency side in order to match an operation band with the first waveguide 14at an input port side from the directional coupling parts.