Low-loss waveguide and method of making same

• US 20020031321A1
• Filed: 06/07/2001
• Published: 03/14/2002
• Est. Priority Date: 07/10/2000
• Status: Active Grant

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Kimerling, Lionel C., Wada, Kazumi, Lim, Desmond R., Lee, Kevin K.
• Granted Patent: US 6,850,683 B2
• Time in Patent Office: Days
• US Class Current: 385/129
• CPC Class Codes: G02B 2006/12176 Etching
• CPC Class Codes: G02B 6/122 Basic optical elements, e.g. light-guiding paths
• CPC Class Codes: G02B 6/13 Integrated optical circuits characterised by the manufacturing method
• CPC Class Codes: G02B 6/132 by deposition of thin films

Abstract

A method of reducing the scattering losses that involves smoothing of the core/cladding interface and/or change of waveguide geometry in high refractive index difference waveguides. As an example, the SOI-based Si/SiO₂ waveguides are subjected to an oxidation reaction at high temperatures, after the waveguide patterning process. By oxidizing the rough silicon core surfaces after the patterning process, the core/cladding interfaces are smoothened, reducing the roughness scattering in waveguides.

Claims

1. A method of making a low-loss waveguide having silicon as its core, comprising:

providing a planar strip waveguide having core/cladding interface roughness; and

subjecting said waveguide to one or more reactions that reduce the core/cladding interface root-mean-square (RMS) roughness in order to in turn reduce scattering losses in said waveguide.

2. The method of claim 1, wherein the waveguide core is reduced in size.

3. The method of claim 1, wherein the effective index of the waveguide is reduced.

4. The method of claim 1, wherein one of said reactions produces reaction products with different chemical compositions from that of said core.

5. The method of claim 4, wherein said reaction products are removed after the reaction.

6. The method of claim 4, wherein said reaction products are left between the core and the cladding after the reaction.

7. The method of claim 4, wherein said reaction products have refractive indices that change from that of the core to that of the cladding.

8. The method of claim 4, wherein said reaction products have graded refractive index profiles from that of the core to that of the cladding.

9. The method of claim 1, wherein one of said reactions comprises a wet chemical reaction.

10. The method of claim 9, wherein said wet chemical reaction occurs with one or more anistropic etchants having OH⁻

ions in an aqueous solution.

11. The method of claim 9, wherein said wet chemical reaction occurs with one or more isotropic etchants.

12. The method of claim 1, wherein one of said reactions comprises a thermal reaction at elevated temperatures above 100°

C.

13. The method of claim 1, wherein one of said reactions comprises an oxidation reaction.

14. The method of claim 13, wherein said oxidation reaction comprises reactant species including oxygen in their chemical compositions.

15. The method of claim 13, wherein said oxidation reaction occurs at temperatures above 600°

C.

16. The method of claim 13, wherein said reaction products are removed after the reaction.

17. The method of claim 13, wherein said reaction products are left between the core and the cladding after the reaction.

18. The method of claim 13, wherein the cladding includes a region of air or vacuum.

19. The method of claim 13, wherein the cladding includes a region of air or vacuum before said reactions and no region of air or vacuum after said reactions.

20. The method of claim 13, wherein the cladding includes a region of material that includes silicon in its chemical composition.

21. The method of claim 1, wherein one of said reactions comprises annealing in an ambience other than air at elevated temperatures above 100°

C.

22. The method of claim 1, wherein said strip waveguide has said core surrounded by said cladding:

said cladding comprising one or more materials having different refractive indices than said core.

23. The method of claim 22, wherein the cladding includes a region of silicon dioxide.

24. The method of claim 22, wherein the cladding includes a region of air or vacuum.

25. The method of claim 22, wherein the cladding includes a region of air or vacuum before said reactions and no region of air or vacuum after said reactions.

26. The method of claim 1, wherein the cladding includes a region of material that includes silicon in its chemical composition.

28. The method of claim 27, wherein the difference in the effective refractive indices of the core and the cladding of said high index difference waveguide is greater than or equal to 0.1.

29. The method of claim 27, wherein the single-mode cutoff dimension of said high index difference waveguide is less than 2.5 times the wavelength in the core.

30. The method of claim 27, wherein the waveguide core is reduced in size.

31. The method of claim 27, wherein the effective index of the waveguide is reduced.

32. The method of claim 27, wherein one of said treatments is a reaction that produces reaction products with different chemical compositions from that of the core.

33. The method of claim 32, wherein said reaction products are removed after the reaction.

34. The method of claim 32, wherein said reaction products are left between the core and the cladding after the reaction.

35. The method of claim 32, wherein said reaction products have refractive indices that change from that of the core to that of the cladding.

36. The method of claim 32, wherein said reaction products have graded refractive index profile from that of the core to that of the cladding.

37. The method of claim 27, wherein one of said treatments involves wet chemical reaction.

38. The method of claim 27, wherein one of said treatments involves thermal reaction at elevated temperatures above 100°

C.

39. The method of claim 27, wherein one of said treatments involves oxidation reaction.

40. The method of claim 39, wherein said oxidation reaction comprises the reactant species including oxygen in their chemical compositions.

41. The method of claim 39, wherein said oxidation reaction occurs at temperatures above 600°

C.

42. The method of claim 27, wherein one of said treatments comprises annealing in an ambience other than air at elevated temperature above 100°

C.

43. The method of claim 27, wherein the core includes silicon in its chemical composition.

44. The method of claim 27, wherein the cladding is a region or regions surrounding the core with lower effective refractive index than that of the core.

45. The method of claim 44, wherein the cladding includes a region of silicon dioxide.

46. The method of claim 44, wherein the cladding includes a region of air or vacuum.

47. The method of claim 44, wherein the cladding includes a region of air or vacuum before said treatments and no region of air or vacuum after said treatments.

27. A method of making a low-loss high index difference waveguide, comprising:

providing a planar waveguide containing core/cladding interface roughness; and

subjecting said waveguide to one or more treatments that reduce the core/cladding interface root-mean-square (RMS) roughness in order to in turn reduce scattering losses in said waveguide.