Optical waveguide device

1. An optical waveguide device, comprising:

a substrate having an electro-optic effect;

an optical waveguide formed on said substrate;

an electrode section adapted to supply an electric field for carrying out phase modulation, synchronized with a clock signal to said optical waveguide and having a phase modulation amplitude of π, to light propagating through the optical waveguide; and

a Mach-Zehnder interferometer connected to the downstream side of said optical waveguide and including two branching waveguides having optical lengths different from each other by an optical length corresponding to a half wavelength of the clock signal, a branching portion adapted to branch propagation light from said optical waveguide into two propagation lights and connect the two propagation lights individually to said two branching waveguides, a coupling portion adapted to couple said branching waveguides and generate a clock modulation light having a light intensity that is switched on and off in a cycle equal to twice a frequency of the clock signal, and a single-mode waveguide connected to said coupling portion and adapted to propagate the clock modulation light.

2. The optical waveguide device as claimed in claim 1 , wherein said two branching waveguides of said Mach- Zehnder interferometer have a difference in waveguide length therebetween by which the difference between the optical lengths is provided.

3. The optical waveguide device as claimed in claim 1 , wherein said two branching waveguides of said Mach-Zehnder interferometer have a refraction index difference therebetween by which the difference between the optical lengths is provided.

4. An optical waveguide device, comprising:

a substrate having an electro-optic effect;

a clock modulation section formed on said substrate and adapted to carry out clock modulation for input light; and

a data modulation section adapted to carry out data modulation for the light modulated by said clock modulation section; and wherein said clock modulation section includes:

an optical waveguide formed on said substrate;

an electrode section adapted to supply an electric field for carrying out phase modulation synchronized with a clock signal to said optical waveguide; and

a Mach-Zehnder interferometer connected to the downstream side of said optical waveguide and including two branching waveguides having an optical length difference therebetween which corresponds to a half wavelength of the clock signal for clock modulation.

5. The optical waveguide device as claimed in claim 4 , wherein said two branching waveguides which form the Mach-Zehnder interferometer are configured as waveguides curved and folded back to one direction, and an outside one of the two curved and folded back branching waveguides is configured as a branching waveguide having an optical length greater than that of the inside one of the two curved and folded back branching waveguide.

6. The optical waveguide device as claimed in claim 5 , wherein a groove is formed on the outer periphery side of each of said branching waveguides.

7. The optical waveguide device as claimed in claim 5 , wherein the outside curved and folded back branching waveguide includes an S-shaped waveguide provided on each of the upstream and downstream sides thereof, and further includes a curved waveguide which connects the S-shaped waveguides on the upstream and downstream sides to each other and has a curve diameter substantially equal to that of the inside curved and folded back branching waveguide.

8. The optical waveguide device as claimed in claim 4 , further comprising a control electrode adapted to apply a control electric field to said two branching waveguides which form said Mach-Zehnder interferometer.

9. The optical waveguide device as claimed in claim 8 , wherein said control electrode includes three first voltage application electrodes and three second voltage application electrodes disposed alternately, and said first voltage application electrodes and said second voltage application electrodes are disposed such that electric fields having directions reversed from each other are applied to said two branching waveguides.

10. The optical waveguide device as claimed in claim 8 , further comprising:

a light reception device adapted to monitor output light from said Mach-Zehnder interferometer; and

a control section adapted to control an electric field to be applied through said control electrode based on a result of the monitoring by said light reception device.

11. The optical waveguide device as claimed in claim 10 , wherein said Mach-Zehnder interferometer includes, in addition to said two branching waveguides, a branching portion adapted to branch propagation light from said optical waveguide into two propagation lights and connect the two propagation lights to said two branching waveguides and a coupling portion adapted to couple the two branching waveguides and guide light from said branching waveguides through two output paths thereof, and

one of said two output paths of said coupling portion is connected to said data modulation section and the other one of said two output paths is optically connected to said light reception device.

12. The optical waveguide device as claimed in claim 11 , wherein said coupling portion is a multimode interference type coupler having two inputs and two outputs.

13. The optical waveguide device as claimed in claim 4 , further comprising:

a clock signal source adapted to generate the clock signal to be supplied to said electrode portion; and wherein

the clock signal has a frequency corresponding to ½ or ¼ a bit rate of said data modulation section.

14. The optical waveguide device as claimed in claim 4 , wherein a bandwidth of said clock modulation section is narrower than that of said data modulation section.