Patent ID: 8831385

Claim:
A mach-zehnder optical modulator comprising a substrate having electrooptical effect, wherein: the substrate has an inputting waveguide portion, two intermediate waveguide portions, an outputting waveguide portion and a sub optical waveguide portion, the inputting waveguide portion being branched and coupled to the two intermediate waveguide portions, the two intermediate waveguide portions being combined and coupled to the outputting waveguide portion, the sub optical waveguide portion being provided on at least one side of the outputting optical waveguide portion and having a given distance from the outputting optical waveguide portion, the sub optical waveguide portion guiding an off-light emitted from a combining portion of the two intermediate waveguide portions; an end face of the sub optical waveguide portion at a starting point is at a non-perpendicular angle with respect to the outputting waveguide portion and is configured so that a distance between the sub optical waveguide portion and the outputting waveguide portion increases in a transmission direction of an on-light; and a width of the end face of the sub optical waveguide portion is larger than that of the outputting waveguide portion.