Optical Waveguide
Provided is an optical waveguide capable of suppressing optical loss in a groove portion into which a wavelength plate is inserted while eliminating polarization dependence. An optical waveguide according to the present disclosure is an embedded optical waveguide formed on a substrate, including a lower cladding; a core; an upper cladding; and a groove formed on one side of a side surface of the core with respect to a light propagation direction and extending in a direction parallel to the core, wherein a refractive index distribution in a plane perpendicular to the light propagation direction is a distribution in which a principal axis of a refractive index ellipse has a rotation amount with respect to a horizontal direction of the substrate.
1 . An optical waveguide, that is an embedded optical waveguide formed on a substrate, comprising:
a lower cladding;
a core;
an upper cladding; and
a groove formed on one side of a side surface of the core with respect to a light propagation direction and extending in a direction parallel to the core,
wherein a refractive index distribution in a plane perpendicular to the light propagation direction near a center of a length of the groove in a direction parallel to the light propagation direction is a distribution that a principal axis of a refractive index ellipse is rotated by a rotation angle with respect to a vertical direction of the substrate plane.
2 . The optical waveguide according to claim 1 , wherein a distance between a side wall of the groove closer to the core and a center of the core is set based on an angle of the principal axis of the refractive index ellipse determined as an angle that a refractive index difference is the largest with respect to two orthogonal polarization states.
3 . The optical waveguide according to claim 1 , wherein the length of the groove in the direction parallel to the light propagation direction is set based on a difference between a minor axis and a major axis of the refractive index ellipse and a retardation.
4 . The optical waveguide according to claim 1 , wherein linear expansion coefficients TCE SB , TCE UC , and TCE OC of the substrate, the lower cladding, and the upper cladding satisfy Equation 1.
TCE
SB
>
TCE
OC
or
TCE
SB
>
TCE
UC
or
TCE
OC
>
TCE
UC
(
Equation
1
)