PURPOSE: A thermo-optic tunable attenuator is provided, which has a high linearity and a light attenuation area above 35dB and an operation speed of milliseconds by preventing power consumption according to the nonlinearity of a light attenuation variation as to an applied external power. CONSTITUTION: An optical waveguide of a Mach-Zehnder interferometer structure includes two single mode channel waveguides(20a,20b) coupling signals incident from an optical fiber into a single mode and two symmetric Y-branches(21a,21b) branching a signal being output from the single mode channel waveguides into two beams. Two metal heat lines(30a,30b) are located on the waveguide(21) and varies a refractive index of the waveguide. The first gold electrode(41) supplies a power by connecting two metal heat lines serially at a center of the Mach-Zehnder interferometer waveguide. The second metal electrodes(42a,42b) supply a power by being connected to the metal heat lines respectively.
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