Patent ID: 7085446

Claim:
An optical multi/demultiplexer comprising a substrate and an optical circuit formed on the substrate; the optical circuit comprising (a) a circuit having a wavelength dependency and (b) a Y-junction, wherein: the circuit having a wavelength dependency is a Mach-Zehnder interferometer (hereinafter referred to as an MZI); the MZI comprises: at least two directional couplers; a first optical waveguide; and a second optical waveguide which is optically coupled with the first optical waveguide through the at least two directional couplers; the MZI has at least one arm waveguide section, said at least one arm waveguide section represents the waveguide section composed of the first and second optical waveguides lying between adjacent directional couplers and has different optical paths; the at least three directional couplers are a first directional coupler, a second directional coupler, and a third directional coupler provided in this order from the first and second input/output ends toward the Y-junction; a second arm waveguide section represents the waveguide section composed of the first and second optical waveguides lying between the second directional coupler and the third directional coupler; and an optical-path difference, ΔL 2 , in the second arm waveguide section has a magnitude of λ/1.45 or less, where λ is one of the wavelengths in the en-tire wavelength band for signal transmission.