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

An optical waveguide device comprising:

• an optical waveguide formed on a substrate;
• and
• a spot size converter that changes a mode field diameter of a light wave propagating through the optical waveguide in at least one end of the optical waveguide, wherein the optical waveguide is a rib type waveguide having a protruding shape in the substrate,
• a height of the rib type waveguide is set to 1 μm or lower,
• the optical waveguide includes a modulation portion to which an electric field is applied to modulate a phase of the light wave propagating through the optical waveguide,
• an insulating layer that covers an upper surface and side surfaces of the optical waveguide is provided, and
• the insulating layer is continuously disposed from the modulation portion to the spot size converter and is arranged linearly along the optical waveguide in plan view.