PROBLEM TO BE SOLVED: To obtain the asymmetrical directional coupler type wavelength filter which has such stepped spectrum characteristics that the transmission wavelength range is flat and the side lobe is low by providing the core of one of two optical waveguides with the width or film thickness tapered along the propagation direction. ;SOLUTION: A channel optical waveguide and an ARROW type optical waveguide are coupled as an upper optical waveguide 1 and a lower optical waveguide 2 at right angles with respect to the substrate surface. The width of the ARROW type optical waveguide is tapered so that the width at the coupling start part of the directional coupling part is wider than that at the coupling end part. The transmission wavelength band ranges from the lower- limit wavelength at which the equivalent refractive index of the width of the coupling start part of the ARROW type optical waveguide is equal to the equivalent refractive index of the width of the channel optical waveguide to the upper- limit wavelength at which the equivalent refractive index of the width of the coupling end part of the ARROW type optical waveguide is equal to the propagation constant of the width of the channel optical waveguide.;COPYRIGHT: (C)1999,JPO
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