Optical waveguide structure and optical waveguide device

An optical waveguide structure comprising a base waveguide comprising a taper section whose width becomes continuously narrower from one side toward another side, and a narrow-width section that is consecutively connected to a narrow-width side of the taper section and extends toward the another side, wherein

• at least three-layered upper waveguides each of which has a planar shape smaller than the taper section and includes a planar-view-roughly-wedge-shaped section whose width becomes continuously narrower from the one side toward the another side at least on a tip side are stacked above the taper section of the base waveguide in such a manner that the planar shape becomes successively smaller from the base waveguide side, and
• a maximum width of an uppermost layer among the at least three-layered upper waveguides is narrower than a maximum width of the base waveguide,
• wherein when an upper surface of the base waveguide is defined as a reference surface, as viewed from a side,
• an imaginary line connecting all of cusps of tips of the at least three-layered upper waveguides with each other has a convex shape that protrudes above an imaginary straight line connecting a cusp of tip of a lowermost upper waveguide among the at least three-layered upper waveguides with a cusp of tip of an uppermost upper waveguide.