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

1. A planar 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; and

the upper and lower waveguide core layers extend bilaterally substantially beyond the lateral extent of a propagating optical mode supported by the planar optical waveguide, the supported optical mode being substantially confined bilaterally by the waveguide core,

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;

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

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%;

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

the second waveguide core tapers in at least one transverse dimension in a longitudinal direction along a portion of the waveguide;

the tapered portion of the second waveguide core and the first waveguide core are arranged so as to guide a selected propagating optical mode having a transverse spatial profile that evolves along the length of the tapered portion of the second waveguide core; and

the second waveguide core tapers sufficiently gradually so as to substantially avoid optical coupling between the selected propagating optical mode and other optical modes.

2. A planar 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; and

the upper and lower waveguide core layers extend bilaterally substantially beyond the lateral extent of a propagating optical mode supported by the planar optical waveguide, the supported optical mode being substantially confined bilaterally by the waveguide core,

further comprising an optical fiber or a second planar optical waveguide, wherein the planar 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 planar optical waveguide is optically end-coupled through the end face to the optical fiber or to the second planar optical waveguide,

further comprising a tapered segment of the planar optical waveguide near the waveguide end face wherein the upper and lower core layers taper bilaterally toward the waveguide end face, wherein the supported optical mode is substantially confined bilaterally at least in part by the upper and lower core layers near the waveguide end face.

3. The apparatus of claim 2 , wherein the tapered waveguide segment is bounded bilaterally by respective protruding surfaces, and the upper and lower core layers and the claddings are bounded bilaterally along the tapered segment of the planar optical waveguide by the protruding surfaces.

4. The apparatus of claim 3 , wherein the protruding surfaces are substantially covered by a substantially transparent embedding medium or encapsulant.

5. The apparatus of claim 2 , wherein:

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

the upper and lower core layers and the waveguide core comprise doped silica.

6. The apparatus of claim 5 , wherein:

the waveguide core is between about 0.3 μm and about 1 μm thick and between about 3 μm and about 12 μm wide at the waveguide end face;

the upper and lower core layers are each between about 0.3 μm and about 2 μm thick, are each wider than about the width of the waveguide core, and are each less than about 30 μm wider than the waveguide core at the waveguide end face;

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

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

7. A method for forming a planar optical waveguide, comprising:

forming a lower cladding layer on a waveguide substrate;

forming a lower waveguide core layer on the lower cladding layer;

forming a lower portion of a middle cladding layer on the lower core layer;

forming a waveguide core on the lower portion of the middle cladding layer;

forming an upper portion of the middle cladding layer over the waveguide core and on exposed areas of the lower portion of the middle cladding layer;

forming an upper waveguide core layer on the upper portion of the middle cladding layer; and

forming an upper cladding layer on the upper waveguide core layer,

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; and

the upper and lower waveguide core layers extend bilaterally substantially beyond the lateral extent of a propagating optical mode supported by the planar optical waveguide, the supported optical mode being substantially confined bilaterally by the waveguide core.

8. The method of claim 7 , further comprising forming a second waveguide core over the first waveguide core, 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.

9. The method of claim 8 , wherein the waveguide cores are in contact with one another.

10. The method of claim 8 , 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%.

11. The method of claim 10 , wherein:

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

the first waveguide core comprises doped silica and 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 comprises silicon nitride or silicon oxynitride and is less than about 2 μm wide and less than about 200 nm thick;

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

12. The method of claim 10 , wherein:

the second waveguide core tapers in at least one transverse dimension in a longitudinal direction along a portion of the waveguide;

the tapered portion of the second waveguide core and the first waveguide core are arranged so as to guide a selected propagating optical mode having a transverse spatial profile that evolves along the length of the tapered portion of the second waveguide core; and

the second waveguide core tapers sufficiently gradually so as to substantially avoid optical coupling between the selected propagating optical mode and other optical modes.

13. The method of claim 7 , further comprising:

forming a waveguide end face terminating the planar optical waveguide; and

positioning an optical fiber or a second planar optical waveguide for optical end-coupling with the planar optical waveguide through the end face thereof,

wherein the waveguide core and the upper and lower waveguide core layers each reach the waveguide end face.

14. The method of claim 13 , further comprising forming a tapered segment of the planar optical waveguide near the waveguide end face wherein the upper and lower core layers taper bilaterally toward the waveguide end face,

wherein the supported optical mode is substantially confined bilaterally at least in part by the upper and lower core layers at the waveguide end face.

15. The method of claim 14 , further comprising forming bilateral protruding surfaces for bounding the tapered waveguide segment, wherein the upper and lower core layers and the claddings are bounded bilaterally along the tapered segment of the planar optical waveguide by the protruding surfaces.

16. The method of claim 15 , further comprising substantially covering the protruding surfaces with a substantially transparent embedding medium or encapsulant.

17. The method of claim 14 , wherein:

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

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

the waveguide core is between about 0.3 μm and about 1 μm thick and between about 3 μm and about 12 μm wide at the waveguide end face;

the upper and lower core layers are each between about 0.3 μm and about 2 μm thick, are each wider than about the width of the waveguide core, and are each less than about 30 μm wider than the waveguide core at the waveguide end face;

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

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

18. The method of claim 7 , 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%.

19. The method of claim 18 , 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.