Optical coupler

1. An optical coupler comprising:

a substrate;

a first optical waveguide positioned on the substrate;

a second optical waveguide positioned on the substrate, the second optical waveguide extending adjacent to and parallel with the first optical waveguide for at least one effective coupling length, the at least one length having a first end and a second end, the second optical waveguide coupled with the first optical waveguide such that an optical signal applied only to the first optical waveguide couples to the second optical waveguide between said first end and said second end;

a first phase shift section forming a portion of the first optical waveguide, the first phase shift section located at a first predetermined distance from the first end of the length; and

a second phase shift section forming a portion of the second optical waveguide, the second phase shift section located at a second predetermined distance from the second end of the length,

wherein the first predetermined distance is approximately equal to the second predetermined distance, and

wherein the second optical waveguide does not have a phase shift section forming a portion of the second optical waveguide and located at the first predetermined distance from the first end of the length.

2. The optical coupler of claim 1 , wherein the first predetermined distance is in a range of about 5% to about 7% of the length.

3. The optical coupler of claim 1 , further comprising:

a third phase shift section forming a portion of the first optical waveguide, the third phase shift section located at a second predetermined distance from the first end of the length; and

a fourth phase shift section forming a portion of the second optical waveguide located at a fourth predetermined distance from the second end of the length.

4. The optical coupler of claim 3 , wherein the first predetermined distance is in a range of about 5% to about 7% of the length and the second predetermined distance is in a range of about 18% to about 20% of the length.

5. The optical coupler of claim 4 , wherein the first predetermined distance from the first end of the length is equal to the third predetermined distance from the second end of the length.

6. The optical coupler of claim 5 , wherein the second predetermined distance from the first end of the length is equal to the fourth predetermined distance from the second end of the coupling length.

7. The optical coupler of claim 1 , wherein the first optical waveguide comprises a ridge waveguide.

8. The optical coupler of claim 7 , wherein at least one of the first phase shift section and the second phase shift section is defined by a change in a width of a ridge of the ridge waveguide for a specified length.

9. The optical coupler of claim 7 , wherein at least one of the first phase shift section and the second phase shift section comprises at least one of a diffused material and an implanted material for a specified length.

10. The optical coupler of claim 9 , wherein the diffused material comprises lithium niobate.

11. An optical coupler comprising:

a substrate;

a first optical waveguide positioned on the substrate;

a second optical waveguide positioned on the substrate, the second optical waveguide arranged to couple with the first optical waveguide, the second optical waveguide extending adjacent to and parallel with the first optical waveguide for at least one coupling length, the at least one coupling length having a first end and a second end;

a phase shift section forming a portion of the first optical waveguide, the phase shift section located midway between the first end of the length and the second end of the length; and

an electrode arranged to adjust a refractive index of said first optical waveguide independently from said phase shift section;

wherein the second optical waveguide does not have any phase shift sections.

12. The optical coupler of claim 11 , wherein the first optical waveguide comprises a ridge waveguide.

13. The optical coupler of claim 12 , wherein the phase shift section is defined by a change in a width of a ridge of the ridge waveguide for a specified length.

14. The optical coupler of claim 11 , wherein the phase shift section comprises at least one of a diffused material and an implanted material for a specified length.

15. An optical coupler comprising:

a first optical waveguide;

a second optical waveguide configured to couple with the first optical waveguide such that an optical signal applied only to the first optical waveguide couples to the second optical waveguide, said second optical waveguide coupled to said first optical waveguide for at least one effective coupling length, the at least one length having a first end and a second end; and

an even number of phase shift sections, wherein the phase shift sections are located in two or more non-propagation sections of the first optical waveguide and the second optical waveguide.

16. The coupler of claim 15 , wherein the even number of phase shift sections are evenly divided between the two or more non-propagation sections.

17. The coupler of claim 15 , wherein the first optical waveguide is coupled to a first optical input and the second optical waveguide is coupled to a second optical input.

18. The optical coupler of claim 1 , wherein the first optical waveguide is coupled to a first optical input and the second optical waveguide is coupled to a second optical input.