Directional semiconductor waveguide coupler

1. A directional coupler for optically coupling waveguides, the directional coupler comprising:

a first input;

a second input;

a first output;

a second output;

a shoulder disposed on a substrate, wherein:

the shoulder extends from the first input to the first output, from the second input to the second output, from the first input to the second output, and from the second input to the first output; and

the shoulder comprises a material;

a first ridge is disposed on the shoulder and extends from the first input to the first output, wherein the first ridge comprises the material; and

a second ridge is disposed on the shoulder and extends from the second input to the second output, wherein:

the second ridge comprises the material; and

the second ridge is separate from the first ridge;

and wherein the directional coupler further comprises:

a first region where the shoulder tapers;

a second region where the first ridge and the second ridge taper;

a third region where the first ridge and the second ridge taper in an opposite direction than in the second region; and

a fourth region where the shoulder tapers in an opposite direction than in the first region.

2. The directional coupler for optically coupling waveguides as recited in claim 1 , wherein;

the shoulder tapers in the first region to increase a width of the shoulder; and

the shoulder extends beyond an outside edge of the first ridge and beyond an outside edge of the second ridge.

3. A directional coupler for optically coupling waveguides, the directional coupler comprising:

a first input;

a second input;

a first output;

a second output;

a shoulder disposed on a substrate, wherein:

the shoulder extends from the first input to the first output, from the second input to the second output, from the first input to the second output, and from the second input to the first output; and

the shoulder comprises a material;

a first ridge, disposed on the shoulder and extending from the first input to the first output, wherein the first ridge comprises the material; and

a second ridge, disposed on the shoulder and extending from the second input to the second output, wherein the second ridge comprises the material and the second ridge is separate from the first ridge;

wherein the directional coupler further comprises:

a first region where the shoulder tapers;

a second region where the first ridge and the second ridge taper;

a third region where the first ridge and the second ridge taper in an opposite direction than in the second region; and

a fourth region where the shoulder tapers in an opposite direction than in the first region;

and wherein:

a width of the first ridge decreases in the second region in a longitudinal direction from the first input to the first output; and

a width of the first ridge increases in the third region in the longitudinal direction from the first input to the first output.

4. The directional coupler for optically coupling waveguides as recited in claim 3 , wherein:

the first ridge has an outside edge; and

the width of the first ridge decreases in the second region by tapering the outside edge; and

the width of the first ridge increases in the third region by tapering the outside edge.

5. The directional coupler for optically coupling waveguides as recited in claim 1 , wherein:

the substrate comprises silicon dioxide; and

the material is crystalline silicon.

6. The directional coupler for optically coupling waveguides as recited in claim 1 , wherein the material is a dielectric.

7. The directional coupler for optically coupling waveguides as recited in claim 1 , wherein:

the shoulder has a thickness between 0.5 and 1.2 μm; and

the first ridge and the second ridge each have a height between 0.2 and 0.65 μm.

8. The directional coupler for optically coupling waveguides as recited in claim 1 , wherein the directional coupler is symmetric along a longitudinal axis between the first ridge and the second ridge.

9. The directional coupler for optically coupling waveguides as recited in claim 1 , wherein:

the first ridge has a minimum width; and

the second ridge has a minimum width.

10. The directional coupler for optically coupling waveguides as recited in claim 9 , wherein the minimum width of the first ridge is different from the minimum width of the second ridge.

11. The directional coupler for optically coupling waveguides as recited in claim 1 , wherein:

a direction from the first input to the first output, and from the second input to the second output, defines a direction of beam propagation; and

the first, second, third and fourth regions are located in sequence along the direction of beam propagation.

12. The directional coupler for optically coupling waveguides as recited in claim 11 , wherein:

each of the first and second ridges defines an inner lateral edge and an outer lateral edge, wherein:

the inner lateral edges of the first and second ridges face one another;

the outer lateral edges of the first and second ridges face away from the directional coupler;

the inner lateral edge of the first ridge forms a straight line along the direction of beam propagation between the first input and the first output;

the inner lateral edge of the second ridge forms a straight line along the direction of beam propagation between the second input and the second output;

the shoulder defines lateral edges;

the lateral edges of the shoulder at the first input, the second input, the first output and the second output are coincident with the lateral edges of the first ridge and the second ridge;

in the first region, the lateral edges of the shoulder taper by deviating from the inner lateral edges of the ridges so as to narrow a gap in the material of the shoulder along the direction of beam propagation, until the gap in the material of the shoulder disappears in the direction of beam propagation; and

in the fourth region, the lateral edges of the shoulder taper by forming a gap in the material of the shoulder at a beginning of the fourth region, wherein the gap widens in the fourth region along the direction of beam propagation until the lateral edges of the shoulder are coincident with the lateral edges of the first ridge and the second ridge.

13. The directional coupler for optically coupling waveguides as recited in claim 12 , wherein in the second region, the first ridge tapers along the direction of beam propagation such that the outer lateral edge of the first ridge becomes closer to the inner lateral edge of the first ridge, such that the first and second ridges form a coupler waist at an end of the second region.

14. The directional coupler for optically coupling waveguides as recited in claim 13 , wherein in the second region, the second ridge also tapers along the direction of beam propagation, such that the outer lateral edge of the second ridge becomes closer to the inner lateral edge of the second ridge.