Optical waveguide element
In order to provide an optical waveguide element that is capable of reducing coupling loss at a coupling portion with an optical fiber and of reducing propagation loss in an optical waveguide, the optical waveguide element comprises a supporting substrate and a waveguide layer consisting of a material having an electro-optic effect stacked on the supporting substrate, wherein a rib portion for forming an optical waveguide is provided protruding on an upper surface of the waveguide layer; a groove portion is formed on an upper surface of the supporting substrate directly below a part of the rib portion; and the groove portion is filled with a material having an effective refractive index comparable to that of the waveguide layer.
1. An optical waveguide element comprising:
a supporting substrate; and
a waveguide layer consisting of a material having an electro-optic effect stacked on the supporting substrate, wherein:
a rib portion for forming an optical waveguide is provided protruding on an upper surface of the waveguide layer;
a groove portion is formed on an upper surface of the supporting substrate directly below a part of the rib portion;
the groove portion is filled with a material having an effective refractive index comparable to that of the material constituting the waveguide layer; and
a dimension in the width direction of the rib portion is greater than half and less than twice of a dimension in the height direction from a top surface of the rib portion to a bottom surface of the groove portion, in a region where a dimension in the height direction of the groove portion is greater than zero.
2. The optical waveguide element according to claim 1 , wherein:
a cross-sectional area of the groove portion varies continuously along a direction of extension of the rib portion.
3. The optical waveguide element according to claim 2 , wherein:
the cross-sectional area of the groove portion varies so that the cross-sectional area of the groove portion continuously decreases with respect to the direction of propagation of a light wave propagating through the optical waveguide.
4. The optical waveguide element according to claim 2 , wherein:
the cross-sectional area of the groove portion varies so that the cross-sectional area of the groove portion continuously increases with respect to the direction of propagation of a light wave propagating through the optical waveguide.
5. The optical waveguide element according to claim 3 , wherein:
a dimension in the width direction of the groove portion varies continuously with respect to the direction of propagation of the light wave propagating through the optical waveguide.
6. The optical waveguide element according to claim 3 , wherein:
a dimension in the height direction of the groove portion varies continuously with respect to the direction of propagation of the light wave propagating through the optical waveguide.
7. The optical waveguide element according to claim 1 , wherein:
at a coupling portion with an input side optical fiber that introduces the light wave into the optical waveguide, and at a coupling portion with an output side optical fiber that outputs the light wave from the optical waveguide, a dimension in the width direction of the groove portion is almost the same as a dimension in the width direction of the rib portion; and a dimension in the height direction from a top surface of the rib portion to a bottom surface of the groove portion is almost the same as a dimension in the width direction of the rib portion.
8. The optical waveguide element according to claim 7 , having a modulation unit for modulating the light wave propagating through the optical waveguide, wherein:
a cross-sectional area of the groove portion varies continuously so that the cross-sectional area of the groove portion is continuously reduced in a portion of the optical waveguide from the coupling portion with the input side optical fiber to the modulation unit; and
the cross-sectional area of the groove portion varies continuously so that the cross-sectional area of the groove portion is increased in the optical waveguide from the modulation unit to the coupling portion with the output side optical fiber.
9. The optical waveguide element according to claim 1 , wherein:
a dimension in the height direction of the rib portion is greater than half and less than twice of a dimension in the height direction of the groove portion.
10. The optical waveguide element according to claim 1 , wherein:
the material constituting the waveguide layer is lithium niobate; and
the material occupying the groove portion is either lithium niobate, silicon nitride having an effective refractive index about the same as that of lithium niobate, or a resin adjusted to an effective refractive index about the same as that of lithium niobate.
11. The optical waveguide element according to claim 1 , wherein:
the waveguide layer is bonded to the supporting substrate.
12. The optical waveguide element according to claim 4 , wherein:
a dimension in the width direction of the groove portion varies continuously with respect to the direction of propagation of the light wave propagating through the optical waveguide.
13. The optical waveguide element according to claim 4 , wherein:
a dimension in the height direction of the groove portion varies continuously with respect to the direction of propagation of the light wave propagating through the optical waveguide.