Electro-optical device

1 . An electro-optical device comprising:

a substrate;

an optical waveguide composed of an electro-optic material film formed in a ridge shape on the substrate;

a buffer layer configured to cover the optical waveguide; and

an upper electrode provided on the optical waveguide through the buffer layer,

wherein:

the buffer layer has a curved recess that is bent toward the optical waveguide, and the upper electrode is provided on the curved recess of the buffer layer,

a width of the upper electrode is smaller than a width of the curved recess of the buffer layer,

the buffer layer contacts an entire upper surface of a layer of the optical waveguide, and

an upper surface of the buffer layer outside the curved recess is entirely a flat surface.

2 . The electro-optical device according to claim 1 , wherein an amount of the curved recess of the buffer layer is 1000 Angstrom to 10000 Angstrom.

3 . The electro-optical device according to claim 1 , wherein an amount of the curved recess of the buffer layer is 1000 Angstrom to 2000 Angstrom.

4 . The electro-optical device according to claim 1 , wherein the curved recess covers the optical waveguide as a whole.

5 . An electro-optical device comprising:

a substrate;

first and second optical waveguides adjacent to each other, composed of an electro-optic material film formed in a ridge shape on the substrate;

a buffer layer configured to cover the first and second optical waveguides and to be buried between the first and second optical waveguides; and

first and second electrodes arranged to be opposite to the first and second optical waveguides above the buffer layer,

wherein:

the buffer layer has one or more curved recesses that are bent toward the first and second optical waveguides, and the first and second electrodes are provided on the one or more curved recesses of the buffer layer,

a width of the first and second electrodes is smaller than a width of the one or more curved recesses, and

the buffer layer provided between the first and second optical waveguides comprises a first buffer layer and a second buffer layer, each of which has a respective curved recess bent toward the first and second optical waveguides and covering the first and second optical waveguides as a whole, above the first and second optical waveguides.

6 . The electro-optical device according to claim 5 , wherein the first and second optical waveguides are Mach-Zehnder optical waveguides.

7 . The electro-optical device according to claim 5 , wherein the substrate is a single crystal substrate, and the electro-optic material film is a lithium niobate film.

8 . The electro-optical device according to claim 5 , wherein an amount of each of the one or more curved recesses is 1000 Angstrom to 2000 Angstrom.

9 . The electro-optical device according to claim 5 , wherein

the buffer layer covers an entire upper surface of a layer of the first and second optical waveguides, and

the buffer layer has a flat upper surface, and the one or more curved recesses are formed from the flat upper surface toward the first and second optical waveguides.

10 . An electro-optical device comprising:

a substrate;

first and second optical waveguides adjacent to each other, composed of an electro-optic material film formed in a ridge shape on the substrate;

a buffer layer configured to cover the first and second optical waveguides and to be buried between the first and second optical waveguides; and

first and second electrodes arranged to be opposite to the first and second optical waveguides above the buffer layer,

wherein:

the buffer layer has one curved recess that is bent toward the first and second optical waveguides, and the first and second electrodes are provided on the one curved recess of the buffer layer, and

the one curved recess provided in the buffer layer covers the first and second optical waveguides as a whole as well as a portion of the buffer layer between the first and second optical waveguides, with the first and second electrodes being provided on the one curved recess.