Optical waveguide structure and optical waveguide device
The present invention provides a small optical waveguide structure capable of converting the spot size of light, and capable of reducing the conversion loss when compared under the condition of the same waveguide length and performing an optical conversion with high efficiency. An optical waveguide structure ( 100 ) includes a base waveguide ( 110 ) including a taper section ( 111 ) whose width becomes continuously narrower from one side toward another side, and a narrow-width section ( 112 ) that is consecutively connected to a narrow-width side of the taper section ( 111 ) and extends toward the another side. In the optical waveguide structure ( 100 ), at least three-layered upper waveguides ( 121 to 123 ) each of which has a planar shape smaller than the taper section ( 111 ) 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 ( 111 ) of the base waveguide ( 110 ) in such a manner that the planar shape becomes successively smaller from the base waveguide side ( 110 ).
1. 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.
2. The optical waveguide structure according to claim 1 , wherein an end face of the base waveguide is flush with an end face on a wide-width side of the at least three-layered upper waveguides.
3. The optical waveguide structure according to claim 2 , wherein the at least three-layered upper waveguides are stacked so that a waveguide length becomes successively shorter from the base waveguide side.
4. The optical waveguide structure according to claim 1 , wherein when an upper surface of the base waveguide is defined as a reference surface, as viewed from a side,
the imaginary line gently connecting all of the cusps of tips of the at least three-layered upper waveguides with each other is a convex parabola that protrudes above the imaginary straight line connecting the cusp of tip of the lowermost upper waveguide with the cusp of tip of the uppermost upper waveguide.
5. The optical waveguide structure according to claim 1 , wherein a waveguide length of the taper section of the base waveguide is equal to or less than 2 mm.
6. The optical waveguide structure according to claim 1 , wherein in the at least three-layered upper waveguides, when attention is paid to any given one upper waveguide layer other than an uppermost upper waveguide, light propagates through this upper waveguide with a smaller spot size than a spot size of light propagating through an upper waveguide immediately above this upper waveguide.
7. The optical waveguide structure according to claim 1 , wherein a value of a refractivity of the upper waveguides is equal to or greater than a value of a refractivity of the base waveguide.
8. An optical waveguide device comprising an optical waveguide structure according to claim 1 .
9. The optical waveguide structure according to claim 1 , wherein the at least three-layered upper waveguides are stacked in such a manner that a maximum width becomes successively narrower from the base waveguide side.