Optical modulator

1. An optical modulator comprising:

a substrate;

first and second optical waveguides each formed of a ridge-shaped electro-optic material film and disposed so as to be mutually adjacent on the substrate;

a buffer layer covering upper surfaces of the first and second optical waveguides;

an insulating layer formed above the buffer layer;

an electrode layer formed on the insulating layer and including first and second signal electrodes provided above the buffer layer so as to be opposed respectively to the first and second optical waveguides; and

a dielectric layer covering at least one of a part of an exposed surface of the first signal electrode and a part of an exposed surface of the second signal electrode, and a part of an upper surface of the buffer layer, wherein

the first signal electrode includes a first lower layer part opposed to the first optical waveguide through the buffer layer and a first upper layer part provided above the first lower layer part,

the second signal electrode includes a second lower layer part opposed to the second optical waveguide through the buffer layer and a second upper layer part provided above the second lower layer part,

a width of a lower surface of each of the first and second lower layer parts is smaller than a width of each of the first and second upper layer parts,

a width of the first upper layer part extends from the first lower layer part to a side opposite to the second signal electrode,

a width of the second upper layer part extends from the second lower layer part to the side opposite to the first signal electrode,

the first and second upper layer parts are formed in the electrode layer,

the first and second lower layer parts are each embedded in openings formed in the insulating layer, and

differential signals are applied to the first and second signal electrodes.

2. The optical modulator as claimed in claim 1 , wherein a gap between the first and second upper layer parts is equal to or larger than a gap between the first and second lower layer parts.

3. The optical modulator as claimed in claim 1 , wherein at least part of the insulating layer existing at the lower portion of an electrode isolation area between the first and second signal electrodes may be removed.

4. The optical modulator as claimed in claim 1 , wherein

the dielectric layer is formed on upper and side surfaces of each of the first and second signal electrodes and an upper surface of the insulating layer in an area not overlapping the first and second signal electrodes in a plan view.

5. The optical modulator as claimed in claim 1 , wherein

the dielectric layer is formed on the upper surface of the buffer layer, and

the insulating layer is formed on an upper surface of the dielectric layer.

6. The optical modulator as claimed in claim 1 , wherein no ground electrode is provided in an area in the vicinity of the first signal electrode on a side opposite to the second signal electrode with respect to the first signal electrode and in an area in the vicinity of the second signal electrode on a side opposite to the first signal electrode with respect to the second signal electrode.

7. An optical modulator comprising:

a substrate;

first and second optical waveguides each formed of a ridge-shaped electro-optic material film and disposed so as to be mutually adjacent on the substrate;

a buffer layer covering upper surfaces of the first and second optical waveguides;

first and second signal electrodes provided above the buffer layer so as to be opposed respectively to the first and second optical waveguides; and

a dielectric layer covering at least one of a part of an exposed surface of the first signal electrode and a part of an exposed surface of the second signal electrode, and a part of an upper surface of the buffer layer, wherein

a first ground electrode disposed in an area in the vicinity of the first signal electrode on a side opposite to the second signal electrode with respect to the first signal electrode; and

a second ground electrode disposed in an area in the vicinity of the second signal electrode on a side opposite to the first signal electrode with respect to the second signal electrode.

8. An optical modulator comprising:

a substrate;

first and second optical waveguides each formed of a ridge-shaped electro-optic material film and disposed so as to be mutually adjacent on the substrate;

a buffer layer covering upper surfaces of the first and second optical waveguides;

first and second signal electrodes provided above the buffer layer so as to be opposed respectively to the first and second optical waveguides; and

a dielectric layer covering at least one of a part of an exposed surface of the first signal electrode and a part of an exposed surface of the second signal electrode, and a part of an upper surface of the buffer layer, wherein

each of the first and second optical waveguides has at least one linear section and at least one curved section,

the first signal electrode is provided along the linear section and the curved section of the first optical waveguide, and

the second signal electrode is provided along the linear section and the curved section of the second optical waveguide.

9. An optical modulator comprising:

a substrate;

first and second optical waveguides each formed of a ridge-shaped lithium niobate film and disposed so as to be mutually adjacent on the substrate;

a buffer layer covering upper surfaces of the first and second optical waveguides;

first and second signal electrodes provided above the buffer layer so as to be opposed respectively to the first and second optical waveguides; and

a dielectric layer covering at least one of a part of an exposed surface of the first signal electrode and a part of an exposed surface of the second signal electrode, and a part of an upper surface of the buffer layer, wherein

a c-axis of the lithium niobate film is oriented perpendicular to a main surface of the substrate.