Optical mode-converter structure

1. An apparatus, comprising:

an optical-mode-converter structure, including:

a tapered optical core on a planar substrate;

an optical cladding layer covering said tapered optical core;

a mode-expanding layer directly contacting said tapered optical core, wherein:

said optical cladding layer directly contacts said mode-expanding layer that directly contacts said tapered optical core,

said mode-expanding layer has a refractive index that is in-between a refractive index of said tapered optical core and a refractive index of said optical cladding layer, and

said mode-expanding layer directly contacting said tapered optical core has a uniform thickness; and

a second mode-expanding layer is located in-between said mode-expanding layer and said optical cladding layer, said second mode-expanding layer having a refractive index that is in-between said refractive index of said mode-expanding layer and said refractive index of said optical cladding layer.

2. The apparatus of claim 1 , wherein said mode-expanding layer covers said tapered optical core on all sides except a side adjacent to said substrate.

3. The apparatus of claim 1 , wherein said tapered optical core and said optical cladding layer have a percent difference in refractive index of at least about 20 percent.

4. The apparatus of claim 1 , wherein said tapered optical core is substantially composed of silicon, said mode-expanding layer is substantially composed of silicon nitride, and said optical cladding layer is substantially composed of silicon oxide.

5. The apparatus of claim 1 , wherein said refractive index of said mode-expanding layer is at least about 10 percent lower than said refractive index of said tapered optical core, and, said refractive index of said mode-expanding layer is at least about 10 percent greater than said refractive index of said optical cladding layer.

6. The apparatus of claim 1 , wherein said thickness of said mode-expanding layer is less than about 1300 nanometers.

7. The apparatus of claim 1 , wherein said thickness of said mode-expanding layer is less than about 650 nanometers.

8. The apparatus of claim 1 , wherein said thickness of said mode-expanding layer is in a range from about 5 to 50 nanometers.

9. The apparatus of claim 1 , wherein said thickness of said mode-expanding layer has a thickness variation of about +10 percent or less.

10. The apparatus of claim 1 , wherein said refractive index of tapered optical core ranges from about 3 to 4, and said refractive index of said mode-expanding layer ranges from about 1.8 to 2.1, and said refractive index of said optical cladding layer ranges from about 1.4 to 1.5.

11. The apparatus of claim 1 , further including:

an optical planar waveguide on said planar substrate wherein a core of said waveguide is directly coupled to a wide end of said tapered optical core; and

an optical fiber wherein a core of said optical fiber is directly coupled to a narrow end of said tapered optical core.

12. The apparatus of claim 11 , wherein said optical fiber is a standard single mode fiber having an optical mode size of about 8 to 10 microns.

13. The apparatus of claim 1 , further including an optical planar waveguide on said substrate and directly coupled to said optical-mode-converter structure and an integrated circuit on said substrate, said integrated circuit coupled to said optical planar waveguide.

14. The apparatus of claim 1 , wherein said optical-mode-converter structure is part of the apparatus configured as an optoelectronic device in an optical fiber communication system.

15. An apparatus, comprising:

an optical-mode-converter structure, including:

a tapered optical core on a planar substrate;

an optical cladding layer covering said tapered optical core;

a mode-expanding layer covering said tapered optical core and located in-between said tapered optical core and said optical cladding layer, wherein:

said mode-expanding layer has a refractive index that is in-between a refractive index of said tapered optical core and a refractive index of said optical cladding layer; and

a second mode-expanding layer is located in-between said mode-expanding layer and said optical cladding layer, said second mode-expanding layer having a refractive index that is in-between said refractive index of said mode-expanding layer and said refractive index of said optical cladding layer.

16. The apparatus of claim 15 , wherein said tapered optical core is continuous with a waveguide core and said optical cladding layer is continuous with a waveguide cladding and said mode-expanding layer covers both said tapered optical core and said waveguide core.

17. A method for operating an apparatus, comprising:

sending an optical signal through a core of an optical fiber having a first mode size; and

coupling said optical signal between said optical fiber core and a planar waveguide core having a second mode size, including expanding said second mode size to substantially match said first mode size by passing said optical signal through an optical-mode-converter structure, said optical-mode-converter structure including:

a tapered optical core on a planar substrate;

an optical cladding layer covering said optical core;

a mode-expanding layer covering said tapered optical core and located in-between said tapered optical core and said optical cladding layer, said mode-expanding layer having a refractive index that is in-between a refractive index of said tapered optical core and a refractive index of said optical cladding layer; and

a second mode-expanding layer is located in-between said mode-expanding layer and said optical cladding layer, said second mode-expanding layer having a refractive index that is in-between said refractive index of said mode-expanding layer and said refractive index of said optical cladding layer.

18. The method of claim 17 , wherein said first mode size ranges from about 8 to 10 microns and said second mode size ranges from about 0.5 to 1 microns.

19. The method of claim 17 , wherein said thickness of said mode-expanding layer has a thickness variation of about ±10 percent or less.

20. A method for manufacturing an apparatus, comprising:

fabricating an optical-mode-converter structure, including:

providing a planar substrate having a first material layer on a second material layer, wherein said first material layer has a higher refractive index than said second material layer;

patterning said first material layer to form a tapered optical core;

coating said tapered optical core with a third material to form a mode-expanding layer of a uniform thickness;

coating said mode expanding layer with a fourth material to form a second mode-expanding layer;

covering said tapered optical core, said mode-expanding layer and said second mode expanding layer with a fifth material to form an optical cladding layer, wherein:

said mode-expanding has a refractive index that is in-between said refractive index of said tapered optical core and a refractive index of said optical cladding layer, and

said second mode-expanding layer has a refractive index that is in-between said refractive index of said mode-expanding layer and said refractive index of said optical cladding layer.

21. The method of claim 20 , wherein said patterning further includes patterning said first material layer to form a waveguide optical core of a waveguide optically coupled to said optical-mode-converter structure, and said coating further includes coating said waveguide optical core with said third material.

22. The method of claim 20 , wherein said thickness of said mode-expanding layer has a thickness variation of about ±10 percent or less.