Tuning the index of a waveguide structure

• US 7,120,338 B2
• Filed: 09/10/2002
• Issued: 10/10/2006
• Est. Priority Date: 09/10/2001
• Status: Active Grant

• Current Assignee: California Institute of Technology
• Original Assignee: California Institute of Technology
• Inventors: Gunn, Lawrence Cary III
• Primary Examiner(s): Healy, Brian
• Assistant Examiner(s): Wong, Eric
• Application Number:
  US10/242,318
• Publication Number:
  US 20030068134A1
• Time in Patent Office: 1,491 Days
• Field of Search: 385/16, 385/27, 385/40, 385/32, 385 8- 9, 385/30
• US Class Current: 385/50
• CPC Class Codes: B82Y 20/00 Nanooptics, e.g. quantum optics or photonic crystals
• CPC Class Codes: G02B 2006/12038 Glass (SiO2 based materials)
• CPC Class Codes: G02B 2006/12061 Silicon
• CPC Class Codes: G02B 2006/12097 Ridge, rib or the like
• CPC Class Codes: G02B 2006/12109 Filter
• CPC Class Codes: G02B 2006/12116 Polariser; Birefringent
• CPC Class Codes: G02B 2006/12145 Switch
• CPC Class Codes: G02B 2006/12147 Coupler
• CPC Class Codes: G02B 6/10 of the optical waveguide type G02B6/02, G02B6/24 take precedence; devices or arrangements for the control of light by electric, magnetic, electro-magnetic or acoustic means G02F1/00; transferring the modulation of modulated light G02F2/00; optical logic elements G02F3/00; optical analogue/digital converters G02F7/00
• CPC Class Codes: G02B 6/12004 Combinations of two or more optical elements
• CPC Class Codes: G02B 6/12007 forming wavelength selective elements, e.g. multiplexer, demultiplexer
• CPC Class Codes: G02B 6/122 Basic optical elements, e.g. light-guiding paths
• CPC Class Codes: G02B 6/1223 high refractive index type, i.e. high-contrast waveguides
• CPC Class Codes: G02B 6/1225 comprising photonic band-gap structures or photonic lattices
• CPC Class Codes: G02B 6/132 by deposition of thin films
• CPC Class Codes: G02B 6/136 by etching
• CPC Class Codes: G02F 1/011 in optical waveguides, not otherwise provided for in this subclass
• CPC Class Codes: G02F 1/0147 based on thermo-optic effects G02F1/132 takes precedence
• CPC Class Codes: G02F 1/025 in an optical waveguide structure G02F1/017, G02F1/2257 take precedence
• CPC Class Codes: G02F 1/3133 the optical waveguides being made of semiconducting materials
• CPC Class Codes: G02F 2202/10 : semiconductor
• CPC Class Codes: G02F 2203/15 : involving resonance effects, e.g. resonantly enhanced interaction

Abstract

The index of refraction of waveguide structures can be varied by altering carrier concentration. The waveguides preferably comprise semiconductors like silicon that are substantially optically transmissive at certain wavelengths. Variation of the carrier density in these semiconductors may be effectuated by inducing an electric field within the semiconductor for example by apply a voltage to electrodes associated with the semiconductor. Variable control of the index of refraction may be used to implement a variety of functionalites including, but not limited to, tunable waveguide gratings and resonant cavities, switchable couplers, modulators, and optical switches.

Claims

1. An optical waveguide coupler having a tunable coupling coefficient, said coupler comprising:

a first waveguide and a second waveguide comprising first and second cores respectively, said first and second cores separated by cladding in an evanescent coupling region, said first and second waveguides supporting respective first and second spatial modes, wherein said first and second waveguides are comprised of semiconductor having a distribution of free carriers, and said cladding comprises a dielectric material that electrically isolates the first and second waveguides from each other, said first waveguide comprising an optical path in a resonant cavity;

a first electrode electrically connected to said first waveguide;

a second electrode electrically connected to said second waveguide; and

a variable voltage source connected between first and second electrodes;

wherein the distributions of free carriers are responsive to application of said voltage such that index of refraction variations can be introduced in said first waveguide to increase or decrease confinement of said first spatial mode and alter overlap of said first and second spatial modes in said cladding of said coupling region thereby changing the coupling coefficient between said waveguides from a first value to a second value.

17. The optical waveguide coupler of claim 1, wherein said first and second waveguides comprise elements of a directional coupler.

18. The optical waveguide coupler of claim 1, wherein said resonant cavity comprises a closed loop.

19. The optical waveguide coupler of claim 1, wherein said resonant cavity is configured to support a single optical mode.

20. The optical waveguide coupler of claim 1, wherein said first and second waveguides are symmetrical about a centerline disposed therebetween.

21. The optical waveguide coupler of claim 1, wherein said first and second waveguides are monothically fabricated on a single substrate.

22. The optical waveguide of claim 21, further comprising electronics integrated on the same substrate.

23. The optical waveguide coupler of claim 22, further comprising dielectric material over said first and second waveguides, said dielectric arranged to provide electrical isolation for conductive pathways.

24. The optical waveguide coupler of claim 1, wherein application of said voltage induces free carriers of opposite sign to accumulate in the first and second waveguides.

25. The optical waveguide coupler of claim 24, wherein electrons accumulate in the first waveguide and holes accumulate in the second waveguide.

26. The optical waveguide coupler of claim 25, wherein the free carriers are concentrated in the first and second cores adjacent the cladding between the first and second waveguides.

27. The optical waveguide coupler of claim 25, wherein the dielectric cladding material prevents the carriers from freely flowing between the first and second waveguides.

35. The optical waveguide of claim 1, wherein at least one of the first and second waveguides comprises a rib waveguide.

36. The optical waveguide of claim 1, wherein at least one of the first and second waveguides comprises a strip-loaded waveguide.

2. A method of tuning the coupling coefficient of an optical waveguide coupler, comprising:

providing a first waveguide comprising semiconductor containing a distribution of free carriers, said first waveguide comprising an optical path in a resonant cavity;

providing a second waveguide separated from and electrically isolated from said first waveguide, said second waveguide comprising semiconductor containing a distribution of free carriers, said first and second waveguides supporting first and second spatial modes respectively; and

applying a voltage between said first and second waveguides to alter the free carrier distribution in said semiconductor so as to reduce confinement of said first spatial mode in said first waveguide and increase overlap between said first and second spatial modes, thereby changing the coupling coefficient of said optical waveguide coupler from a first value to a second value.

28. The method of claim 2, wherein applying said voltage induces free carriers of opposite sign to accumulate in first and second cores in the first and second waveguides, respectively.

29. The method of claim 28, wherein electrons accumulate in the first waveguide and holes accumulate in the second waveguide.

37. The method of claim 2, wherein at least one of the first and second waveguides comprises a rib waveguide.

38. The method of claim 2, wherein at least one of the first and second waveguides comprises a strip-loaded waveguide.

3. A capacitor comprising:

a first capacitor electrode comprising a first semiconductor optical waveguide;

a second capacitor electrode comprising a second semiconductor optical waveguide, the second semiconductor waveguide comprising a portion of a resonant cavity;

a dielectric comprising cladding disposed between said first and second optical waveguides,

wherein the first and second semiconductor optical waveguides are sufficiently close to permit evanescent optical coupling therebetween, said coupling changeable with application of a voltage between said first and second capacitor electrodes.

4. The apparatus of claim 3, further comprising a variable voltage supply that is electrically connected between the first and second semiconductor optical waveguides.

30. The capacitor of claim 3, wherein application of said voltage induces free carriers of opposite sign to accumulate in the first and second optical waveguides.

31. The capacitor of claim 30, wherein electrons accumulate in the optical waveguide and holes accumulate in the second optical waveguide.

32. The capacitor of claim 30, wherein the free carriers are concentrated in portions of the first and second waveguides adjacent the cladding between the first and second waveguides.

33. The capacitor of claim 30, wherein the dielectric prevents the carriers from freely flowing between the first and second waveguides.

39. The capacitor of claim 3, wherein at least one of the first and second semiconductor optical waveguides comprises a rib waveguide.

40. The capacitor of claim 3, wherein at least one of the first and second semiconductor optical waveguides comprises a strip-loaded waveguide.

5. A method of optically switching, comprising:

applying a voltage across first and second electrodes of a capacitor comprising respective first and second waveguides separated from each other by an inter-electrode dielectric wherein at least one of the first and second waveguides comprises at least a portion of a resonant cavity;

wherein application of said voltage induces storage of charge on said first and second electrodes thereby altering coupling of optical modes in said first waveguide to said second waveguide.

6. The method of claim 5, wherein said voltage induces storage of opposite charge in the first and second electrodes.

7. The method of claim 5, further comprising causing index of refraction variations by inducing said storage of charge.

8. The method of claim 7, further comprising modifying the propagation constant of light in said first and second waveguides with said index of refraction variations.

9. The method of claim 7, further comprising modifying optical confinement of said first and second waveguides with said index of refraction variation.

10. The method of claim 5, further comprising modifying evanescent coupling between said first and second waveguides by inducing said stored charge.

11. The method of claim 5, further comprising transmitting an optical signal through said first waveguide and varying the fraction of light coupled from said first waveguide into said second waveguide by storage of charge in said capacitor.

12. The method of claim 11, wherein said voltage applied between the first and second electrodes induces storage of opposite charge in the first and second waveguides.

13. The method of claim 11, further comprising using said storage of charge to modify the effective index of at least one of said first and second waveguides.

14. The method of claim 11, further comprising using said storage of charge to modify the optical confinement of at least one of said first and second waveguides.

15. The method of claim 11, further comprising using said storage of charge to modify evanescent coupling between said first and second waveguides.

16. The method of claim 11, further comprising using said storage of charge to modify the propagation constant of light in said first and second waveguides.

34. The method of claim 6, wherein electrons accumulate in the first waveguide and holes accumulate in the second waveguide.

41. The method of claim 5, wherein at least one of the first and second waveguides comprises a rib waveguide.

42. The method of claim 5, wherein at least one of the first and second waveguides comprises a strip-loaded waveguide.

43. An optical waveguide coupler having a tunable coupling coefficient, said coupler comprising:

a first waveguide and a second waveguide comprising first and second cores respectively, said first and second cores separated by cladding in an evanescent coupling region, said first and second waveguides supporting respective first and second spatial modes, wherein said first waveguide is comprised of semiconductor having a distribution of free carriers, and said cladding comprises a dielectric material that electrically isolates the first and second waveguides from each other, said first waveguide comprising an optical path in a resonant cavity;

a first electrode electrically connected to the first waveguide; and

a variable voltage source coupled to the first electrode so that a voltage is applied to the first waveguide via the first electrode;

wherein the distributions of free carriers are responsive to application of said voltage such that index of refraction variations can be introduced in said first waveguide to increase or decrease confinement of said first spatial mode and alter overlap of said first and second spatial modes in said cladding of said coupling region thereby changing the coupling coefficient between said waveguides from a first value to a second value.

44. The optical waveguide of claim 43, wherein at least one of the first and second waveguides comprises a rib waveguide.

45. The optical waveguide of claim 43, wherein at least one of the first and second waveguides comprises a strip-loaded waveguide.

46. The optical waveguide of claim 43, wherein at least one of the first and second waveguides comprises silicon.

47. A method of tuning the coupling coefficient of an optical waveguide coupler, comprising:

providing a first waveguide comprising semiconductor containing a distribution of free carriers, said first waveguide comprising an optical path in a resonant cavity;

providing a second waveguide separated from and electrically isolated from said first waveguide, said first and second waveguides supporting first and second spatial modes respectively; and

applying a voltage to said first waveguide to alter the free carrier distribution in said semiconductor so as to reduce confinement of said first spatial mode in said first waveguide and increase overlap between said first and second spatial modes, thereby changing the coupling coefficient of said optical waveguide coupler from a first value to a second value.

48. The method of claim 47, wherein at least one of the first and second waveguides comprises a rib waveguide.

49. The method of claim 47, wherein at least one of the first and second waveguides comprises a strip-loaded waveguide.

50. The method of claim 47, wherein at least one of the first and second waveguides comprises silicon.