Multiple-core planar optical waveguides and methods of fabrication and use thereof

1. An optical waveguide comprising:

a substantially planar waveguide substrate;

a lower waveguide core layer;

an upper waveguide core layer;

a waveguide core between the upper and lower waveguide core layers;

lower cladding between the substrate and the lower waveguide core layer;

upper cladding above the upper waveguide core layer; and

middle cladding between the upper and lower waveguide core layers substantially surrounding the waveguide core,

wherein:

the lower cladding has a refractive index less than refractive indices of the lower waveguide core layer, the upper waveguide core layer, and the waveguide core;

the middle cladding has a refractive index less than refractive indices of the lower waveguide core layer, the upper waveguide core layer, and the waveguide core;

the upper cladding has a refractive index less than refractive indices of the lower waveguide core layer, the upper waveguide core layer, and the waveguide core;

a width of the waveguide core is substantially larger than a thickness thereof;

an upper surface of the waveguide core is substantially flat;

the upper and lower waveguide core layers and the waveguide core are arranged so as to together support at least one propagating optical mode supported by the optical waveguide;

the upper and lower waveguide core layers extend bilaterally substantially beyond the lateral extent of the supported optical mode; and

the upper and lower waveguide core layers and the waveguide core are arranged so that the supported optical mode is substantially confined bilaterally by the waveguide core, from below by the lower waveguide core layer, and from above by the upper waveguide core layer.

2. The waveguide of claim 1 wherein an upper surface of the middle cladding is substantially planar, and the upper waveguide core layer is substantially planar.

3. The waveguide of claim 1 wherein:

an upper surface of the middle cladding layer is non-planar and comprises a raised substantially flat portion above the waveguide core; and

the upper waveguide cladding layer is non-planar, and comprises a raised substantially flat portion above the waveguide core.

4. The waveguide of claim 1 further comprising a second waveguide core between the upper and lower waveguide core layers, wherein:

a refractive index of the second waveguide core is greater than the refractive indices of the upper cladding, the middle cladding, and the lower cladding;

a width of the second waveguide core is substantially larger than a thickness thereof;

an upper surface of the second waveguide core is substantially flat; and

the first waveguide core and the second waveguide core are arranged one-above-the-other within the middle cladding.

5. The waveguide of claim 4 wherein the waveguide cores are in contact with one another.

6. The waveguide of claim 4 wherein:

index contrast between the upper core layer and the upper and middle claddings is less than about 5%;

index contrast between the lower core layer and the lower and middle claddings is less than about 5%;

index contrast between the first waveguide core and the middle cladding is less than about 5%; and

index contrast between the second waveguide core and the middle cladding is greater than about 5%.

7. The waveguide of claim 6 wherein the second waveguide core is arranged so as to substantially confine at least one propagating optical mode along at least a portion of the length of the optical waveguide.

8. The waveguide of claim 6 wherein:

the upper, middle, and lower claddings comprise silica or doped silica;

the upper and lower cores layers and the first waveguide core comprise doped silica; and

the second waveguide core comprises silicon nitride or silicon oxynitride.

9. The waveguide of claim 8 wherein:

the upper and lower core layers are each between about 0.3 μm and about 2 μm thick;

the first waveguide core is between about 0.3 μm and about 1 μm thick and between about 3 μm and about 12 μm wide;

the second waveguide core is less than about 2 μm wide and less than about 200 nm thick;

the middle cladding between the lower core layer and the first waveguide core is between about 1 μm and about 3 μm thick; and

the middle cladding between the upper core layer and the first waveguide core is between about 1 μm and about 3 μm thick.

10. The waveguide of claim 1 further comprising an optical fiber or a second planar optical waveguide, wherein the optical waveguide terminates at a waveguide end face, the waveguide core and the upper and lower waveguide core layers each reach the waveguide end face, and the optical waveguide is optically end-coupled through the end face to the optical fiber or to the second planar optical waveguide.

11. The waveguide of claim 10 wherein end-coupled optical fiber or planar optical waveguide is longitudinally spaced-apart from the waveguide end face, and diffractive optical coupling loss is less than about 0.3 dB for a longitudinal spacing from the waveguide end-face between about 10 μm and about 25 μm.

12. The waveguide of claim 1 wherein:

index contrast between the upper core layer and the upper and middle claddings is less than about 5%;

index contrast between the lower core layer and the lower and middle claddings is less than about 5%; and

index contrast between the waveguide core and the middle cladding is less than about 5%.

13. The waveguide of claim 12 wherein:

the upper and lower waveguide core layers comprise doped silica between about 0.3 μm thick and about 2 μm thick;

the waveguide core comprises doped silica between about 0.3 μm thick and about 1 μm thick and between about 3 μm wide and about 12 μm wide;

the lower, middle, and upper claddings comprise silica or doped silica;

the middle cladding between the lower core layer and the first waveguide core is between about 1 μm and about 3 μm thick; and

the middle cladding between the upper core layer and the first waveguide core is between about 1 μm and about 3 μm thick.