Planar polarization rotator

1. An apparatus, comprising:

an optical polarization rotator including first and second optical waveguide ribs located along a planar surface of a substrate, the second optical waveguide rib being located farther from the surface than the first optical waveguide rib; and

wherein first segments of the optical waveguide ribs form a vertical stack over the substrate and second segments of the optical waveguide ribs are offset laterally in a direction along the surface; and

wherein the first and second optical waveguide ribs are formed of materials with different bulk refractive indexes.

2. The apparatus of claim 1 , further comprising a spacer layer being located between the first and second optical waveguide ribs and being formed of a material with a different bulk refractive index than the materials of the first and second optical waveguide ribs.

3. The apparatus of claim 2 , wherein the spacer layer is vertically thinner than the first optical waveguide rib.

4. The apparatus of claim 1 , wherein the first optical waveguide rib is formed of a same material as a portion of the planar surface of the substrate.

5. The apparatus of claim 1 , wherein one of the optical waveguide ribs is formed of a semiconductor and the other of the optical waveguide ribs is formed of a dielectric.

6. The apparatus of claim 1 , wherein the optical polarization rotator is configured to rotate a polarization of received linearly polarized light by at least 45 degrees.

7. The apparatus of claim 1 , further comprising a transition region in which the first optical waveguide rib has a width lateral to the surface that monotonically tapers from a larger value at an input planar optical waveguide to a smaller value at the first segment of first optical waveguide rib.

8. The apparatus of claim 7 , wherein the second optical waveguide rib has a width lateral to the surface that monotonically tapers from a smaller value at an end thereof nearer to the input planar optical waveguide to a larger value at the first segment of the second optical waveguide rib.

9. The apparatus of claim 1 , further comprising a polarization beam splitter having a first optical output connected to transmit light to the first optical waveguide rib and having a second optical output connected to an output optical waveguide, the output optical waveguide being located over the substrate lateral to the first optical waveguide rib.

10. The apparatus of claim 9 , further comprising:

another output optical waveguide connected to the first optical output of the polarization beam splitter via the optical polarization rotator; and

wherein the apparatus is configured to transmit light of substantially the same linear polarization to the two output optical waveguides.

11. The apparatus of claim 1 , further comprising

an optical modulator having a 1×2 optical splitter, first and second optical waveguide arms, and a 2×1 optical combiner, each optical waveguide arm connecting a corresponding optical output of the 1×2 optical splitter to a corresponding optical input of the 2×1 optical combiner; and

wherein each optical waveguide arm includes an electro-optical modulator capable of modulating the phase and/or amplitude of light propagating there through responsive to electrical signals received therein; and

wherein an optical polarization rotator is located in one of the optical waveguide arms.

12. A method, comprising:

receiving, at a first end of a segment of a hybrid optical waveguide, linearly polarized light from a planar waveguide; and

propagating the light through the segment of the hybrid optical waveguide to rotate a linear polarization of the light; and

wherein the segment of the hybrid optical waveguide includes one segment in which a second optical waveguide rib is vertically located over a first optical waveguide rib and includes another segment in which the second optical waveguide rib is substantially laterally offset from a corresponding segment of the first optical waveguide rib; and

wherein the first and second optical waveguide ribs are formed of materials having different bulk refractive indexes.

13. The method of claim 12 , wherein one of the first and second optical waveguide ribs is a semiconductor rib and the other of the first and second optical waveguide ribs is a dielectric rib.

14. The method of claim 12 , wherein the segment of the hybrid optical waveguide includes a spacer layer being located between the first and second optical waveguide ribs and being made of a material with a different bulk refractive index than the materials of the optical waveguide ribs.