Patent ID: 7340115

Claim:
An optical wavelength tunable filter comprising: a substrate having an acousto-optic effect; an optical waveguide formed on said substrate; a surface acoustic wave generating unit that generates a surface acoustic wave on said substrate; and a single surface acoustic waveguide, to one end of which a surface acoustic wave from said surface acoustic wave generating unit is provided, so as to propagate said surface acoustic wave along said optical waveguide, wherein said optical waveguide comprises: a separation section for separating a light input to an input port into two polarized waves; two parallel arm portions respectively provided with the light separated by said separation section; and a mode conversion section for converting the polarization mode of the selected light which has propagated on said respective arm portions, and outputting the selected light from an output port, said surface acoustic waveguide is positioned symmetrically with respect to a straight line as an axis passing through the middle of the respective arm portions of said optical waveguide, and has a pair of low sound-velocity areas, said respective low sound-velocity areas having a curved shape with widths corresponding to a gap between said pair of low sound-velocity areas, and said surface acoustic wave propagating in said surface acoustic waveguide includes an area in which a mode distribution of a propagating surface acoustic wave changes symmetrically with respect to a straight line as an axis passing through the middle of the respective arm portions of said optical waveguide, while keeping the sound velocity of the surface acoustic wave constant, so that the intensity of the surface acoustic wave interacting with the light propagating on the respective arm portions of said optical waveguide changes with respect to a propagation direction of the light.