Mode transformation and loss reduction in silicon waveguide structures utilizing tapered transition regions

• US 20040202440A1
• Filed: 04/05/2004
• Published: 10/14/2004
• Est. Priority Date: 04/11/2003
• Status: Active Grant

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Sioptical Inc. (Cisco Systems, Inc.)
• Inventors: Ghiron, Margaret, Yanushefski, Katherine A., Pathak, Soham, Montgomery, Robert Keith, Patel, Vipulkumar, Gothoskar, Prakash, Shastri, Kalpendu
• Granted Patent: US 6,980,720 B2
• Time in Patent Office: Days
• US Class Current: 385/129
• CPC Class Codes: G02B 2006/12097 Ridge, rib or the like
• CPC Class Codes: G02B 6/1228 Tapered waveguides, e.g. integrated spot-size transformers for coupling with fibres G02B6/305

Abstract

A low loss coupling arrangement between a slab/strip waveguide and a rib waveguide in an optical waveguiding structure formed on a silicon-on-insulator (SOI) platform utilizes tapered sections at the input and/or output of the rib waveguide to reduce loss. Optical reflections are reduced by using silicon tapers (either vertical tapers, horizontal tapers, or two-dimensional tapers) that gradually transition the effective index seen by an optical signal propagating along the slab/strip waveguide and subsequently into and out of the rib waveguide. Loss can be further reduced by using adiabatically contoured silicon regions at the input and output of the rib waveguide to reduce mode mismatch between the slab/strip waveguide and rib waveguide. In a preferred embodiment, concatenated tapered and adiabatic sections can be used to provide for reduced optical reflection loss and reduced optical mode mismatch.

Claims

1. An optical waveguiding structure comprising

a slab/strip single crystal silicon waveguide including a top major surface;

a silicon rib waveguide disposed over a portion of the top major surface of the slab/strip single crystal silicon waveguide, the silicon rib waveguide defined as comprising an input face and an output face such that an optical signal propagating along the silicon slab/strip waveguide will be coupled through the silicon rib input face to thereafter propagate along both the slab/strip and the rib, and subsequently exit the silicon rib waveguide along the output face and then propagate along the slab/strip silicon waveguide; and

at least one silicon taper disposed along a portion of the slab/strip waveguide top major surface so as to be contiguous with at least one of the silicon rib waveguide input face and output face, the silicon taper used to improve optical coupling between the silicon slab/strip waveguide and the silicon rib waveguide.

2. An optical waveguiding structure as defined in claim 1 wherein the at least one silicon taper comprises a pair of tapers, a first taper disposed contiguous with the input face of the silicon rib waveguide and a second taper disposed contiguous with the output face of the silicon rib waveguide.

3. An optical waveguiding structure as defined in claim 1 wherein the structure further comprises a relatively thin dielectric layer disposed between the slab/strip silicon waveguide and the silicon rib waveguide.

4. An optical waveguiding structure as defined in claim 1 wherein the at least one silicon taper comprises a vertically disposed taper.

5. An optical waveguiding structure as defined in claim 4 wherein the vertically disposed taper comprises a single taper disposed contiguous with the silicon rib waveguide input face, said vertically disposed taper increasing in height from the top major surface of the slab/strip silicon waveguide so as to be essentially the same height as the silicon rib waveguide at said input face.

6. An optical waveguiding structure as defined in claim 4 wherein the vertically disposed taper comprises a single taper disposed contiguous with the silicon rib waveguide output face, said vertically disposed taper decreasing in height from the silicon rib waveguide output face to the top major surface of the slab/strip silicon waveguide.

7. An optical waveguiding structure as defined in claim 4 wherein the vertically disposed taper comprises a pair of tapers, an input taper disposed contiguous with the silicon rib waveguide input face, said input taper increasing in height from the top major surface of the slab/strip silicon waveguide so as to be essentially the same height as the silicon rib waveguide at said input face, and an output taper disposed contiguous with the silicon rib waveguide output face, said vertically disposed taper decreasing in height from the silicon rib waveguide output face to the top major surface of the slab/strip silicon waveguide.

8. An optical waveguiding structure as defined in claim 4 wherein the structure further comprises a relatively thin dielectric layer disposed between the slab/strip waveguide and the silicon rib waveguide.

9. An optical waveguiding structure as defined in claim 1 wherein the at least one silicon taper comprises a horizontally disposed taper.

10. An optical waveguiding structure as defined in claim 9 wherein the horizontally disposed taper comprises a single taper disposed contiguous with the silicon rib waveguide input face, said horizontally disposed taper increasing in width along the top major surface of the slab/strip silicon waveguide so as to be essentially the same width as the input face of the silicon rib waveguide.

11. An optical waveguiding structure as defined in claim 9 wherein the horizontally disposed taper comprises a single taper disposed contiguous with the silicon rib waveguide output face, said horizontally disposed taper decreasing in width along the top major surface of the slab/strip silicon waveguide from a width essentially the same as the width of the output face of the silicon rib waveguide until coming to a termination.

12. An optical waveguiding structure as defined in claim 9 wherein the horizontally disposed taper comprises a pair of tapers, an input taper disposed contiguous with the silicon rib waveguide input face, said input taper increasing in width along the top major surface of the slab/strip silicon waveguide so as to be essentially the same width as the input face of the silicon rib waveguide, and an output taper disposed contiguous with the silicon rib waveguide output face, said output taper decreasing in width along the top major surface of the slab/strip silicon waveguide from a width essentially the same as the width of the output face of the silicon rib waveguide until coming to a termination.

13. An optical waveguiding structure as defined in claim 9 wherein the structure further comprises a relatively thin dielectric layer disposed between the slab/strip waveguide and the silicon rib waveguide.

14. An optical waveguiding structure as defined in claim 1 wherein the at least one silicon taper comprises a two-dimensional structure, tapered in both the horizontal and vertical directions.

15. An optical waveguiding structure as defined in claim 14, wherein the at least one silicon taper comprises an input two-dimensional taper, disposed contiguous with the input face of the silicon rib waveguide and configured to increase in height and width from the top major surface of the slab/strip silicon waveguide so as to be essentially the same height and width as the silicon rib waveguide at said input face.

16. An optical waveguiding structure as defined in claim 14, wherein the at least one silicon taper comprises an output two-dimensional taper, disposed contiguous with the output face of the silicon rib waveguide and configured to decrease in height and width from the output face of the silicon rib waveguide along the top major surface of the silicon slab/strip waveguide until coming to a termination.

17. An optical waveguiding structure as defined in claim 14, wherein the at least one silicon taper comprises a pair of tapers, an input two-dimensional taper, disposed contiguous with the input face of the silicon rib waveguide and configured to increase in height and width from the top major surface of the slab/strip silicon waveguide so as to be essentially the same height and width as the silicon rib waveguide at said input face, and an output two-dimensional taper, disposed contiguous with the output face of the silicon rib waveguide and configured to decrease in height and width from the output face of the silicon rib waveguide along the top major surface of the silicon slab/strip waveguide until coming to a termination.

18. An optical waveguiding structure as defined in claim 14 wherein the structure further comprises a relatively thin dielectric layer disposed between the slab/strip waveguide and the silicon rib waveguide.

19. An optical waveguiding structure as defined in claim 1 wherein the at least one silicon taper comprises an adiabatically contoured mode matching taper.

20. An optical waveguiding structure as defined in claim 19 wherein the at least one adiabatically contoured mode matching taper comprises an adiabatic input taper disposed contiguous with the input face of the silicon rib waveguide, said adiabatic input taper providing optical mode matching between the silicon slab/strip waveguide and said silicon rib waveguide.

21. An optical waveguiding structure as defined in claim 19 wherein the at least one adiabatically contoured mode matching taper comprises an adiabatic output taper disposed contiguous with the output face of the silicon rib waveguide, said adiabatic output taper providing optical mode matching between the silicon rib waveguide and the silicon slab/strip waveguide.

22. An optical waveguiding structure as defined in claim 19 wherein the at least one adiabatically contoured mode matching taper comprises a pair of tapers, an input taper disposed contiguous with the input face of the silicon rib waveguide, said adiabatic input taper providing optical mode matching between the silicon slab/strip waveguide and said silicon rib waveguide, and an output taper disposed contiguous with the output face of the silicon rib waveguide, said adiabatic output taper providing optical mode matching between the silicon rib waveguide and the silicon slab/strip waveguide.

23. An optical waveguiding structure as defined in claim 19 wherein the structure further comprises a relatively thin dielectric layer disposed between the slab/strip waveguide and the silicon rib waveguide.

24. An optical waveguiding structure as defined in claim 1 wherein the at least one silicon taper comprises a form of silicon selected from the group consisting of:

polysilicon, amorphous silicon, strained silicon, substantially single crystal silicon, and single crystal silicon.

25. An optical waveguiding structure as defined in claim 24 wherein the polysilicon is selected from a group consisting of:

grain-size-enhanced, grain-boundary-passivated, and grain-aligned polysilicon.

26. An optical waveguiding structure as defined in claim 1 wherein the silicon rib waveguide and the at least one silicon taper comprise the same form of silicon.

27. An optical waveguiding structure as defined in claim 26 wherein the at least one silicon taper is formed as integral with the silicon rib waveguide.

28. An optical waveguiding structure as defined in claim 1 wherein the at least one silicon taper comprises a single layer of silicon material.

29. An optical waveguiding structure as defined in claim 1 wherein the at least one silicon taper comprises a plurality of layers of one or more silicon materials.

30. An optical waveguiding structure providing both optical mode matching and optical reflection reduction, said structure comprising

a slab/strip single crystal silicon waveguide including a top major surface;

a silicon rib waveguide disposed over a portion of the top major surface of the slab/strip single crystal silicon waveguide, the silicon rib waveguide defined as comprising an input face and an output face such that an optical signal propagating along the silicon slab/strip waveguide will be coupled through the silicon rib input face to thereafter propagate along both the slab/strip and the rib, and subsequently exit the silicon rib waveguide along the output face and then propagate along the slab/strip silicon waveguide;

an adiabatically-contoured silicon region disposed along the slab/strip waveguide top major surface so as to be contiguous with either one of the silicon rib waveguide input face and output face, the adiabatically-contoured silicon region used to provide optical mode matching between the silicon slab/strip waveguide and the silicon rib waveguide; and

a silicon taper disposed along the slab/strip waveguide top major surface so as to be contiguous with an exposed endface of the adiabatically-contoured silicon region, the silicon taper providing a change in effective index between the silicon slab/strip waveguide and the silicon rib waveguide so as to reduce optical reflections at either one of the silicon rib waveguide input and output faces.

31. The optical waveguiding structure as defined in claim 30 wherein the adiabatically-contoured silicon region and the silicon taper are disposed at the input face of the silicon rib waveguide.

32. The optical waveguiding structure as defined in claim 30 wherein the adiabatically-contoured silicon region and the silicon taper are disposed at the output face of the silicon rib waveguide.

33. The optical waveguiding structure as defined in claim 30 wherein the adiabatically-contoured silicon region and the silicon taper are disposed at the input face of the silicon rib waveguide, and a second adiabatically-contoured silicon region and second silicon taper are disposed at the output face of the silicon rib waveguide.

34. The optical waveguiding structure as defined in claim 30 wherein the silicon taper comprises a horizontally disposed taper.

35. The optical waveguiding structure as defined in claim 30 wherein the silicon taper comprises a vertically disposed taper.

36. The optical waveguiding structure as defined in claim 30 wherein the silicon taper comprises a two-dimensional taper, tapering in both the horizontal and vertical directions.

37. The optical waveguiding structure as defined in claim 30 wherein the structure further comprises a relatively thin dielectric layer disposed between the slab/strip waveguide and the silicon rib waveguide.