Differential waveguide pair

• US 6,393,185 B1
• Filed: 11/03/1999
• Issued: 05/21/2002
• Est. Priority Date: 11/03/1999
• Status: Expired due to Term

• Current Assignee: Intel Corporation
• Original Assignee: Sparkolor Corp. (Intel Corporation)
• Inventors: Deacon, David A. G.
• Primary Examiner(s): Sanghavi, Hemang
• Assistant Examiner(s): ROJAS, OMAR R
• Application Number:
  US09/433,830
• Time in Patent Office: 930 Days
• Field of Search: 385/9, 385/12, 385/28, 385/30, 385/40, 385/50, 385/129-132
• US Class Current: 385/50
• CPC Class Codes: G02B 2006/12102 Lens
• CPC Class Codes: G02B 2006/12119 Bend
• CPC Class Codes: G02B 6/12004 Combinations of two or more optical elements
• CPC Class Codes: G02B 6/122 Basic optical elements, e.g. light-guiding paths
• CPC Class Codes: G02B 6/1228 Tapered waveguides, e.g. integrated spot-size transformers for coupling with fibres G02B6/305
• CPC Class Codes: G02B 6/30 for use between fibre and thin-film device
• CPC Class Codes: G02B 6/42 Coupling light guides with opto-electronic elements
• CPC Class Codes: G02B 6/4206 Optical features G02B6/4207, G02B6/421 take precedence
• CPC Class Codes: G02F 1/0147 based on thermo-optic effects G02F1/132 takes precedence
• CPC Class Codes: G02F 1/3132 of directional coupler type
• CPC Class Codes: G02F 2201/302 grating coupler
• CPC Class Codes: G02F 2203/15 involving resonance effects, e.g. resonantly enhanced interaction
• CPC Class Codes: H01S 5/026 Monolithically integrated components, e.g. waveguides, monitoring photo-detectors, drivers stabilisation of output H01S5/06
• CPC Class Codes: H01S 5/1064 varying width along the optical axis
• CPC Class Codes: H01S 5/12 the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers comprising a photonic bandgap structure H01S5/11; surface-emitting lasers H01S5/18
• CPC Class Codes: H01S 5/4025 Array arrangements, e.g. constituted by discrete laser diodes or laser bar H01S5/42 takes precedence
• CPC Class Codes: H01S 5/4087 emitting more than one wavelength

Abstract

An asymmetric waveguide pair (1440) with a differential thermal response has an optical coupling frequency that may be thermo-optically tuned. Tuning may also be accomplished by applying an electric field (1445) across a liquid crystal portion (1442) of the waveguide structure. The waveguide pair may include a grating and be used as a frequency selective coupler for an optical resonator. The differential waveguide pair may also be used as a temperature or electric field sensor, or it may be used in a waveguide array to adjust a phase relationship, e.g. in an arrayed waveguide grating.

Claims

1. A differential waveguide pair device comprising:

a substrate;

a first cladding layer disposed over said substrate;

a first elongated core material region disposed on said first cladding layer;

a second elongated core material region disposed on said first cladding layer horizontally displaced from but proximate to said first elongated core material region for optical coupling therebetween; and

a second cladding layer disposed above said first cladding layer over and around at least one of said elongated core material regions, said second cladding layer having a different thickness in a vicinity of said first elongated core material region relative to a vicinity of said second elongated core material region, said core material regions characterized by higher refractive indices than said cladding layers thereby defining first and second optical waveguides for propagation of optical energy therein, said different second cladding layer thicknesses in the vicinities of said first and second core material regions resulting in different sensitivities of optical propagation constants for said first and second optical waveguides to an externally variable field, and wherein the coupling between said first and second waveguides varies substantially with said changes in said externally variable field.

2. The device of claim 1 wherein said different optical propagation constants for said first and second optical waveguides are temperature dependent, a coupling strength for said optical coupling between said first and second core material regions thereby changing with changes in temperature.

3. The device of claim 2 further comprising a third cladding layer disposed above said core material regions and first and second cladding layers, said third cladding layer characterized by a refractive index having a substantially different temperature dependence from that of said second cladding layer, optical propagation in said first and second optical waveguides characterized by different mode overlap with said third cladding layer.

4. The device of claim 3 wherein mode overlap for one of said optical waveguides is such that optical propagation therein is substantially temperature independent, which mode overlap for the other of said optical waveguides is such that optical propagation therein is temperature dependent.

5. The device of claim 3 wherein said mode overlaps for said optical waveguides are selected such that optical propagation therein are characterized by equal and opposite temperature dependence.

6. The device of claim 3 further comprising a heater electrode structure disposed on said third cladding layer over at least one of said optical waveguides.

7. The device of claim 1 further comprising a third electro-optic cladding layer disposed above said core material regions and first and second cladding layers and an electric-field-applying electrode structure disposed on said third cladding layer, said third cladding layer characterized by a refractive index that is dependent upon applied electric field.

8. The device of claim 1 further comprising a grating structure optically coupled to said first and second waveguides, said grating structure characterized by a coupling frequency.

9. The device of claim 1 wherein said first and second elongated core material regions are substantially adjacent and parallel to one another over a length.

10. The device of claim 1 wherein said second cladding layer has zero thickness and is substantially absent in a vicinity of said first elongated core material region.

11. A differential waveguide device comprising:

a first waveguide segment having a waveguide core and clad material surrounding the core, the first waveguide segment characterized by a first lowest order mode profile and a first rate of change in a first effective index of refraction with increases in applied field;

a second waveguide segment having a waveguide core and clad material surrounding the core, the second waveguide segment horizontally displaced from but proximate to said first waveguide for coupling optical energy and characterized by a second lowest order mode profile and a second rate of change in a second effective index of refraction with increases in applied field, the clad material surrounding the core of the first waveguide segment having a different thickness than the clad material surrounding the core of the second waveguide segment;

an actuator electrode disposed to apply a field that is substantially identical in said first and second waveguide segments;

wherein the magnitude of the first rate is substantially different from the magnitude of the second rate, and wherein a difference between the first and second effective indices may be modified by varying the applied field.

12. The device of claim 11 wherein the applied field is an electric field, and said actuator electrode is actuated with an applied voltage.

13. The device of claim 11 wherein said first and second waveguides are aligned end to end along an optical path.

14. The device of claim 13 wherein the second waveguide segment is characterized by a second optical length, the second rate is substantially zero, and further comprising:

a grating disposed in said first waveguide in overlapping relation with the first mode profile and characterized by a Bragg reflection frequency for providing optical feedback, wherein actuating said electrode modifies the feedback frequency without changing the second optical length.

15. The device of claim 14 further comprising:

an optical resonator wherein said grating is a reflector in said resonator, said second waveguide is an intracavity waveguide in said resonator, and actuating said electrode selects a longitudinal mode from a plurality of longitudinal modes of said resonator without tuning a frequency of the longitudinal mode.

16. The device of claim 11 wherein said second waveguide is disposed substantially parallel to said first waveguide, and said coupling is directional coupling.

17. The device of claim 16 wherein said coupling is modified by actuating said electrode.

18. The device of claim 11 wherein the applied field is a temperature field, said actuator electrode is resistive and actuated with an applied current, and wherein the difference between the first and second effective indices may be varied by varying the applied current.

19. The device of claim 11 further comprising:

a grating disposed in overlapping relation with the first and second mode profiles.

20. The device of claim 11 wherein said first mode profile has a first overlap with a material with high magnitude of rate of change of index with increase of applied field, and said second mode profile has a second overlap with the high rate material, the second overlap being substantially smaller than the first overlap, and wherein the magnitude of the first rate is substantially larger than the magnitude of the second rate.

21. The device of claim 20 wherein said second overlap is substantially equal to zero.

22. The device of claim 20 wherein the high rate material is a cladding material.

23. The device of claim 20 wherein said first and second effective indices are substantially equal at an operating temperature.

24. The device of claim 11 wherein said first and second waveguides are characterized by different refractive index contrast values between core material of the respective waveguides and surrounding cladding material.

25. A thermally adjustable differential waveguide coupler device comprising:

a substrate;

a first cladding material layer disposed on said substrate and characterized by a first refractive index;

a second cladding material layer disposed on said substrate and characterized by a second refractive index;

a third cladding material layer disposed adjacent said second cladding and characterized by a temperature dependent third refractive index;

a first elongated region of a core material characterized by a core refractive index larger than the first and second refractive indices interposed between said first and second claddings to form a first waveguide, said first waveguide characterized by a first effective index of refraction having a first rate of change with increases in temperature, said first waveguide further characterized by a first overlap factor of a first mode profile with said third cladding layer;

a second elongated region of the core material interposed between said first and second claddings to form a second waveguide disposed proximate to and substantially parallel to said first waveguide, said second waveguide characterized by a second effective index of refraction having a second rate of change with increases in temperature, said second waveguide further characterized by a second overlap factor of a second mode profile with said third cladding layer; and

a heater electrode thermally coupled to said first and second waveguides;

wherein said second cladding layer is thicker in a vicinity of said second waveguide than in a vicinity of said first waveguide, the first overlap factor is larger than the second overlap factor, and the magnitude of the first rate is substantially different from the magnitude of the second rate; and

wherein an optical coupling between said first and second waveguides is modified by actuating said heater electrode thereby modifying the difference between the first and second effective indices.

26. The coupler of claim 25 wherein the actuation of said heater electrode is adjustable such that substantially all of an input optical power in one of said first and second waveguides is coupled into the other of said first and second waveguides.

27. The coupler of claim 25 further comprising:

a grating disposed in overlapping relation with the first and second mode profiles of said first and second waveguides, said grating characterized by a coupling frequency and a first optical length, whereby the coupler is a grating-assisted coupler.

28. The coupler of claim 27 wherein the grating assisted coupler is a codirectional coupler and actuating said heater electrode tunes the coupling frequency.

29. The coupler of claim 28 wherein said first and second rates of change of said effective refractive indices are substantially equal and opposite, and the first optical length is substantially independent of the tuning.

30. The coupler of claim 25 wherein the second waveguide segment is characterized by a second optical length, the second rate of change of said second effective refractive index is substantially zero, and the actuation of said heater electrode modifies the optical coupling without substantial change in the second optical length.

31. The coupler of claim 25 wherein said second cladding layer has zero thickness and is substantially absent in a vicinity of said first waveguide.

32. The coupler of claim 25 wherein the core material is disposed on top of the first cladding, the second cladding is disposed on top of the core material, and the third cladding is disposed on top of the second cladding.

33. The coupler of claim 25 wherein said first and second cladding materials are silica and said core material is doped silica.

34. The coupler of claim 25 wherein said first and second waveguides are single mode waveguides.

35. The coupler of claim 25 wherein the second and third cladding layers have substantially equal refractive indices for at least one operating temperature of the coupler, and the first effective index of the first waveguide is substantially equal to the second effective index of the second waveguide.

36. The coupler of claim 25 wherein said third cladding layer is disposed directly on top of said second cladding.

37. The coupler of claim 25 wherein the heater electrode is an electrically conductive structure actuated by passing a current through the electrode.

38. The coupler of claim 25 wherein said third cladding material is a polymer.

39. The coupler of claim 25 wherein the effective refractive index for the first waveguide has a larger magnitude of rate of change with temperature than the effective refractive index of the second waveguide.

40. The coupler of claim 25 wherein the third cladding material is air, the second cladding material is characterized by a temperature dependent second refractive index, and the magnitude of the second rate is larger than the magnitude of the first rate.

41. The coupler of claim 25 wherein the third cladding material layer comprises an uppermost region of said substrate, with said second cladding layer being disposed over said uppermost region of said substrate, the first and second core material regions disposed on top of the second cladding layer and the first cladding disposed over the core material regions.

42. The coupler of claim 25 wherein the substrate is a composite structure.

43. A field adjustable differential waveguide coupler device comprising:

a substrate;

a first cladding material layer disposed on said substrate and characterized by a first refractive index;

a second cladding material layer disposed on said substrate and characterized by a second refractive index;

a third cladding material layer disposed adjacent said second cladding and characterized by a third refractive index that is dependent upon an applied field;

a first elongated region of a core material characterized by a core refractive index larger than the first and second refractive indices interposed between said first and second claddings to form a first waveguide, said first waveguide characterized by a first effective index of refraction having a first rate of change with increases in applied field, said first waveguide further characterized by a first overlap factor of a first lowest order mode profile with said third cladding layer;

a second elongated region of the core material interposed between said first and second claddings to form a second waveguide disposed proximate to said first waveguide for optical coupling therebetween, said second waveguide characterized by a second effective index of refraction having a second rate of change with increases in applied field, said second waveguide further characterized by a second overlap factor of a second lowest order mode profile with said third cladding layer; and

an actuator electrode disposed to apply a field in said first and second waveguide;

wherein said second cladding layer is thicker in a vicinity of said second waveguide than in a vicinity of said first waveguide, the first overlap factor is larger than the second overlap factor, and the magnitude of the first rate is substantially different from the magnitude of the second rate, and wherein a difference between the first and second effective indices may be modified by varying the applied field.

44. The device of claim 43 wherein the applied field is an electric field, and said actuator electrode is actuated with an applied voltage.

45. The device of claim 43 wherein said first and second waveguides are aligned end to end along an optical path.

46. The device of claim 45 wherein the second waveguide is characterized by a second optical length, the second rate is substantially zero, and further comprising:

a grating disposed in said first waveguide in overlapping relation with the first mode profile and characterized by a Bragg reflection frequency for providing optical feedback into said second waveguide, wherein actuating said electrode modifies the frequency of the feedback element without changing the second optical length.

47. The device of claim 46 further comprising:

an optical resonator;

wherein said grating is a reflector in said resonator, said second waveguide is an intracavity waveguide in said resonator, and actuating said electrode selects a longitudinal mode from a plurality of longitudinal modes of said resonator without tuning a frequency of the longitudinal mode.

48. The device of claim 43 wherein said second waveguide is disposed substantially parallel to said first waveguide, and said coupling is directional coupling.

49. The device of claim 48 wherein said coupling is modified by actuating said electrode.

50. The device of claim 43 wherein the applied field is a temperature field, said actuator electrode is resistive and actuated with an applied current, and wherein the difference between the first and second effective indices may be varied by varying the applied current.

51. The device of claim 43 further comprising:

a grating disposed in overlapping relation with the first and second mode profiles.

52. An optical temperature sensor element comprising:

a substrate;

a first cladding material layer disposed on said substrate and characterized by a first refractive index;

a second cladding material layer disposed on said substrate and characterized by a second refractive index;

a third cladding material layer disposed adjacent said second cladding and characterized by a temperature-dependent third refractive index; and

a first elongated region of core material characterized by a core refractive index larger than the first and second refractive indices interposed between said first and second cladding layers to form a first waveguide, said first waveguide characterized by a first effective index of refraction, a first rate of change in the first effective index with increases in temperature and a first overlap factor of a first lowest order mode profile with said third cladding layer;

a second elongated region of core material interposed between said first and second cladding layers to form a second waveguide and disposed proximate to and substantially parallel to said first waveguide for coupling of optical energy, said second waveguide characterized by a second effective index of refraction, a second rate of change in the second effective index with increases in temperature and a second overlap factor of a second lowest order mode profile with said third cladding layer; and

wherein the second cladding layer is thicker in a vicinity of said second waveguide than in a vicinity of said first waveguide, the first overlap factor is larger than the second overlap factor, the magnitude of the first rate is substantially different from the magnitude of the second rate, and wherein the optical coupling between said first and second waveguides varies with the temperature dependent difference in the first and second effective indices.

53. The sensor of claim 52 wherein the optical coupling is characterized by a coupling strength and a thermal sensitivity rate of change of the coupling with temperature, and wherein the coupling strength is preselected to produce a desired thermal sensitivity.

54. The sensor of claim 53 wherein a coupling region between said first and second waveguides has been modified by irradiation with an energy beam for changing the index of refraction, thereby modifying the coupling strength until the thermal sensitivity is changed to a desired operating sensitivity.

55. The sensor of claim 52 wherein the coupling is characterized by a linear response over a temperature range of linearity, and wherein the index difference is preselected for the range of linearity to span a desired temperature range.

56. The sensor of claim 55 wherein one of said first and second waveguides has been modified by irradiation with an energy beam for changing the index of refraction, thereby modifying the index difference until a desired operating temperature is included within the range of linearity.

57. The sensor of claim 52 wherein the coupling is characterized by a length over which the coupling is substantially maintained and a coupling ratio equal to an output power in one of said first and second waveguides divided by an input power in the other of said first and second waveguides, and wherein the length is preselected for the coupling ratio to equal a desired value.

58. The sensor of claim 57 wherein a portion of one of said first and second waveguides has been modified by irradiation with an energy beam for changing the index of refraction, thereby modifying the length until the coupling ratio substantially equals 50%.

59. The sensor of claim 52 further comprising:

a first detector disposed to detect optical power in said first waveguide to produce a first proportional electronic output; and

a second detector disposed to detect optical power in said second waveguide to produce a second proportional electronic output.

60. The sensor of claim 59 further characterized by a sensor ratio equal to a difference between the first and second electronic outputs divided by a sum of the first and second electronic outputs, wherein the sensor ratio is linearly proportional to the temperature over a sensor linearity temperature range.

61. The sensor of claim 52 further comprising:

a thermal actuator for providing a thermal input to the sensor; and

an electronic feedback control circuit responsive to an input signal that is a function of the optical coupling, said control circuit controlling said thermal actuator so that a temperature of said sensor is maintained within a tolerance of a set point temperature.

62. An optical temperature sensor comprising:

a first waveguide having a first cladding with a first thickness such that the first waveguide is characterized by a first lowest order mode profile and a first rate of change in a first effective index of refraction with increases in temperature;

a second waveguide horizontally displaced from but proximate to said first waveguide and having a second cladding with a second thickness different from said first thickness such that the second waveguide is characterized by a second lowest order mode profile and a second rate of change in a second effective index of refraction with the increases in temperature;

wherein the magnitude of the first rate is substantially different from the magnitude of the second rate, and wherein a coupling between said first and second waveguides varies with the temperature dependent difference in the first and second effective indices.

63. The sensor of claim 62 further comprising:

an optical material disposed in overlapping relation with said first and second waveguides and characterized by a material index of refraction and a material rate of change of index with increases in temperature;

wherein said optical material has a first overlap factor with said first mode profile and a second overlap factor with said second mode profile, the first and second overlap being substantially different so that the first rate is not equal to the second rate.

64. The sensor of claim 62 wherein the coupling is characterized by a coupling strength and a thermal sensitivity rate of change of the coupling with temperature, and wherein the coupling strength is preselected to produce a desired thermal sensitivity.

65. The sensor of claim 62 wherein the coupling is characterized by a linear response over a temperature range of linearity, and wherein the index difference is preselected for the range of linearity to span a desired temperature range.

66. The sensor of claim 62 wherein the coupling is characterized by a coupling length over which the coupling is substantially maintained and a coupling ratio equal to an output power in one of said first and second waveguides divided by an input power in the other of said first and second waveguides, and wherein the coupling length is preselected for the coupling ratio to equal a desired value.

67. A field sensitive optical device comprising:

a plurality of horizontally displaced but proximate waveguides characterized by a plurality of lowest order mode profiles, a plurality of effective indices, and a plurality of rates of change of effective index of refraction with increases in an applied field;

an optical material layer disposed in overlapping relation with said plurality of waveguides, characterized by a material index of refraction and a material rate of change of index with increases in said applied field, and having a plurality of thicknesses at said plurality of waveguides;

wherein the thickness at an nth one of said plurality of waveguides determines the nth overlap factor with the nth mode profile, and

wherein at least one of the plurality of thicknesses differs substantially from another of the plurality of thicknesses and the plurality of thicknesses are spatially distributed among the plurality of waveguides to produce a desired pattern of the rates of change in effective index of refraction with increases in said applied field.

68. The device of claim 67 wherein said applied field is an electric field.

69. The device of claim 67 wherein said applied field is a thermal field.

70. The device of claim 69 wherein the plurality of waveguides is a pair of waveguides disposed proximate and parallel to each other establishing an optical coupling, the coupling varies with a temperature dependent difference in the effective index of refraction of the pair of waveguides, and the device is a temperature sensor.

71. The device of claim 67 further comprising:

at least one input waveguide;

an input planar coupling region optically coupled to said input waveguide;

at least one output waveguide; and

an output planar coupling region optically coupled to said output waveguide;

wherein said plurality of waveguides forms an array of waveguides disposed between and optically coupled to said input and output planar coupling regions, said array of waveguides is curved, and said device is an arrayed waveguide grating.

72. The device of claim 69 wherein the pattern of thicknesses is characterized by a monotonic variation across the plurality of waveguides from one side of the array to the other side.

73. The device of claim 71 further comprising:

a thermal actuator for providing a thermal input to the device;

wherein heating the actuator adds a skew to a pattern of phase shifts of the arrayed waveguide grating, changing a filter wavelength of said arrayed waveguide grating.