Optical waveguide element, and optical modulation device and optical transmission apparatus using optical waveguide element

An optical waveguide element comprising:

• a substrate that is X-cut lithium niobate or X-cut lithium tantalate;
• and
• an optical waveguide that is disposed on the substrate,
• wherein the optical waveguide has an effective refractive index change portion in which an effective refractive index of the optical waveguide related to a fundamental mode A parallel to a plane of polarization of a light wave propagated through the optical waveguide changes according to propagation of the light wave,
• the effective refractive index change portion is one portion of the optical waveguide in which a propagation direction of the light wave is different from Y-axis direction of the substrate, and
• in the effective refractive index change portion, a cross-sectional shape of the optical waveguide which is perpendicular to a propagation direction of the light wave is set such that the effective refractive index of the optical waveguide related to the fundamental mode A is higher than an effective refractive index of the optical waveguide related to another fundamental mode B perpendicular to the fundamental mode A.