Tuning of optical dispersion by using a tunable fiber bragg grating

• US 20010021294A1
• Filed: 04/06/2001
• Published: 09/13/2001
• Est. Priority Date: 12/15/1997
• Status: Active Grant

• Current Assignee: University of Southern California
• Original Assignee: University of Southern California
• Inventors: Feng, Kai-Ming, Peng, Jiangde, Cai, Jinxing, Willner, Alan E., Khosravani, Reza, Lee, Sanggeon
• Granted Patent: US 6,453,095 B2
• Time in Patent Office: Days
• US Class Current: 385/37
• CPC Class Codes: G02B 6/02085 characterised by the grating profile, e.g. chirped, apodised, tilted, helical
• CPC Class Codes: G02B 6/02109 having polarization sensitive features, e.g. reduced photo-induced birefringence
• CPC Class Codes: G02B 6/02138 based on illuminating a phase mask
• CPC Class Codes: G02B 6/022 using mechanical stress, e.g. tuning by compression or elongation, special geometrical shapes such as "dog-bone" or taper G02B6/02204 takes precedence; optical modulation using photo-elastic effect G02F1/0131
• CPC Class Codes: G02B 6/2932 comprising a directional router, e.g. directional coupler, circulator
• CPC Class Codes: G02B 6/29322 Diffractive elements of the tunable type G02B6/02195 takes precedence; optical modulation devices based on a change of the optical properties of the medium G02F1/00
• CPC Class Codes: G02B 6/29394 Compensating wavelength dispersion G02B6/29376 takes precedence; dispersion compensated optical transmission systems H04B10/2507
• CPC Class Codes: G02B 6/29395 configurable, e.g. tunable or reconfigurable switching G02B6/35
• CPC Class Codes: G02B 6/4215 the intermediate optical elements being wavelength selective optical elements, e.g. variable wavelength optical modules or wavelength lockers G02B6/4246 takes precedence
• CPC Class Codes: G02F 1/011 in optical waveguides, not otherwise provided for in this subclass
• CPC Class Codes: G02F 1/0115 in optical fibres
• CPC Class Codes: G02F 1/0126 Opto-optical modulation, i.e. control of one light beam by another light beam, not otherwise provided for in this subclass
• CPC Class Codes: G02F 1/0134 in optical waveguides
• CPC Class Codes: G02F 2201/307 Reflective grating, i.e. Bragg grating
• CPC Class Codes: H01S 3/0057 Temporal shaping, e.g. pulse compression, frequency chirping soliton generation and propagation G02F1/3513, H01S3/063 and H01S3/108
• CPC Class Codes: H04B 10/2519 using Bragg gratings
• CPC Class Codes: H04B 2210/258 treating each wavelength or wavelength band separately

Abstract

Techniques and devices based on a wave-guiding element which has a spatial grating pattern that is an oscillatory variation along its optic axis. The wave-guiding element is configured to receive an input optical signal and to produce an output optical signal by reflection within a Bragg reflection band produced by the spatial grating pattern so as to produce time delays of different reflected spectral components as a nonlinear function of spatial positions along said optic axis at which the different reflected spectral components are respectively reflected. Such a wave-guiding element may be a nonlinearly chirped fiber grating A control unit may be engaged to the wave-guiding element and is operable to change a property of the spatial grating pattern along the optic axis to tune at least relative time delays of the different reflected spectral components nonlinearly with respect to wavelength.

Claims

1. A device, comprising:

a wave-guiding element having (1) an optic axis to transport optical energy along said optic axis and (2) a spatial grating pattern which is an oscillatory variation along said optic axis, said wave-guiding element configured to receive an input optical signal and to produce an output optical signal by reflection within a Bragg reflection band produced by said spatial grating pattern so as to produce time delays of different reflected spectral components as a nonlinear function of spatial positions along said optic axis at which said different reflected spectral components are respectively reflected; and

a control unit engaged to said wave-guiding element and operable to change a property of said spatial grating pattern along said optic axis to tune at least relative time delays of said different reflected spectral components nonlinearly with respect to wavelength.

2. The device as in

claim 1

, wherein said control unit is configured to control a length of said wave-guiding element along said optic axis.

3. The device as in

claim 2

, wherein said control unit includes a piezoelectric element.

4. The device as in

claim 2

wherein said control unit includes a magnetostrictive element that operates in response to a control magnetic field.

5. The device as in

claim 1

, wherein said control unit is configured to generate a varying control electrical field along said optic axis and said wave-guiding element is configured to have an index of refraction that changes in response to said varying control electrical field so as to tune said relative time delays.

6. The device as in as in

claim 1

, wherein said control unit is configured to generate a varying control electromagnetic radiation field along said optic axis and said wave-guiding element is configured to have an index of refraction that changes in response to said electromagnetic radiation field so as to tune said relative time delays.

7. The device as in

claim 1

, wherein said control unit includes an acoustic wave generator configured and coupled to produce a frequency-tunable acoustic wave along said optic axis of said wave-guiding element so that said acoustic wave alters a frequency response of said wave-guiding element.

8. The device as in

claim 1

, wherein said control unit is configured to control both a length and a refractive index of said wave-guiding element along said optic axis.

9. The device as in

claim 1

, wherein said control unit is configured to control a refractive index of said wave-guiding element along said optic axis.

10. The device as in

claim 1

, further comprising:

a dispersion monitor unit configured and coupled to monitor information of optical dispersion in said output signal and coupled to inform said control unit of said information,

wherein said control unit is operable to adjust said property of said spatial grating pattern in response to said information.

11. The device as in

claim 1

, wherein said wave-guiding element includes an optical fiber having a fiber core and a fiber cladding surrounding said fiber core.

12. The device as in

claim 1

, wherein said control unit is configured to control a temperature of said wave-guiding element.

13. The device as in

claim 1

, wherein said wave-guiding element includes an optical waveguide formed on a substrate.

14. The device as in

claim 1

, where said wave-guiding element is formed of an optical birefringent material to have two orthogonal principal polarization axes that are substantially perpendicular to said optic axis.

15. The device as in

claim 1

, wherein said wave-guiding element further includes a spatial sampling pattern that spatially overlaps with said spatial grating pattern along said optic axis and includes a periodic modulation with a period greater than a grating period of said spatial grating pattern so that said wave-guiding element is operable to produce a plurality of Bragg reflection bands at different wavelengths.

16. The device as in

claim 1

, wherein said spatial grating pattern has a grating period that is nonlinearly chirped along said optic axis.

17. The device as in

claim 1

, wherein said spatial grating pattern includes a spatial nonlinear chirp in one aspect of an index of refraction of said wave-guiding element along said optic axis.

18. A system, comprising:

a plurality of optical devices connected in series so that an optical transmission output from one optical device is received by another adjacent optical device located in a downstream of said optical output, wherein each optical device is configured to be independently controlled and includes;

a wave-guiding element having (1) an optic axis to transport optical energy along said optic axis and (2) a spatial grating pattern which is an oscillatory variation along said optic axis, said wave-guiding element configured to receive an input optical signal and to produce (1) an output optical signal by reflection within a Bragg reflection band produced by said spatial grating pattern so as to produce time delays of different reflected spectral components as a nonlinear function of spatial positions along said optic axis at which said different reflected spectral components are respectively reflected, and (2) an output transmission optical signal having spectral components that are not reflected, and

a control unit engaged to said wave-guiding element and operable to change a property of said spatial grating pattern along said optic axis to tune at least relative time delays of said different reflected spectral components nonlinearly with respect to wavelength,

wherein different spatial grating patterns in different optical devices are configured to produce Bragg reflection bands at different wavelengths.

19. The system as in

claim 18

, wherein said control unit in each optical device is configured to control a length of said wave-guiding element along said optic axis.

20. The system as in

claim 18

, wherein said control unit in each optical device is configured to control a refractive index of said wave-guiding element along said optic axis.

21. The system as in

claim 18

, wherein said control unit in each optical device is configured to control both a length and a refractive index of said wave-guiding element along said optic axis.

22. The system as in

claim 18

, further comprising:

a dispersion monitor unit configured and coupled to monitor information of optical dispersion in a final optical output signal from said plurality of optical devices and coupled to communicate said information to at least one of said plurality of optical devices,

wherein said control unit in said at least one optical device is operable to adjust said property of said spatial grating pattern of said wave-guiding element in response to said information to alter optical dispersion in said final optical output signal.

23. The system as in

claim 18

, wherein said wave-guiding element in each optical device includes an optical fiber having a fiber core and a fiber cladding surrounding said fiber core, and further comprising interconnecting optical fibers to interconnect said plurality of optical devices.

24. The system as in

claim 18

, wherein said control unit in at least one optical device is configured to control a temperature of said wave-guiding element to achieve said nonlinear tuning.

25. The system as in

claim 18

, wherein said wave-guiding element in at least one optical device includes an optical waveguide formed on a substrate.

26. The system as in

claim 18

, where said wave-guiding element in at least one optical device is formed of an optical birefringent material to have two orthogonal principal polarization axes that are substantially perpendicular to said optic axis.

27. The system as in

claim 18

, wherein said spatial grating pattern has a grating period that is nonlinearly chirped along said optic axis.

28. The system as in

claim 18

, wherein said spatial grating pattern includes a spatial nonlinear chirp in one aspect of an index of refraction of said wave-guiding element along said optic axis.

29. A system, comprising:

an input optical fiber carry a plurality of optical WDM channels;

a WDM unit coupled to said input optical fiber and configured to spatially separate said WDM channels;

a tunable dispersion module, connected to said WDM unit to receive said WDM channels and operable to change dispersions of said WDM channels to produce modified WDM channels, said dispersion module comprising a plurality of optical devices which are coupled in parallel with respect to one another to said WDM unit to receive said WDM channels and to produce said modified WDM channels, wherein each optical device includes;

a wave-guiding element having (1) an optic axis to transport optical energy along said optic axis and (2) a spatial grating pattern which is an oscillatory variation along said optic axis, said wave-guiding element configured to receive an input optical signal and to produce an output optical signal by reflection within a Bragg reflection band produced by said spatial grating pattern so as to produce time delays of different reflected spectral components as a nonlinear function of spatial positions along said optic axis at which said different reflected spectral components are respectively reflected, and

a control unit engaged to said wave-guiding element and operable to change a property of said spatial grating pattern along said optic axis to tune at least relative time delays of said different reflected spectral components nonlinearly with respect to wavelength,

wherein different spatial grating patterns in different optical devices are configured to produce Bragg reflection bands at different wavelengths.

30. The system as in

claim 29

, wherein said control unit in each optical device is configured to control a length of said wave-guiding element along said optic axis.

31. The system as in

claim 29

, wherein said control unit in each optical device is configured to control a refractive index of said wave-guiding element along said optic axis.

32. The system as in

claim 29

, wherein said control unit in each optical device is configured to control both a length and a refractive index of said wave-guiding element along said optic axis.

33. The system as in

claim 29

, further comprising:

a dispersion monitor unit configured and coupled to monitor information of optical dispersion in said modified WDM channels and coupled to communicate said information to at least one of said plurality of optical devices,

wherein said control unit in said at least one optical device is operable to adjust said property of said spatial grating pattern of said wave-guiding element in response to said information to alter optical dispersion in said modified WDM channels.

34. The system as in

claim 29

, wherein said wave-guiding element in each optical device includes an optical fiber having a fiber core and a fiber cladding surrounding said fiber core, and further comprising interconnecting optical fibers to interconnect said plurality of optical devices to said WDM unit.

35. The system as in

claim 29

, wherein said control unit in at least one optical device is configured to control a temperature of said wave-guiding element to achieve said nonlinear tuning.

36. The system as in

claim 29

, wherein said wave-guiding element in at least one optical device includes an optical waveguide formed on a substrate.

37. The system as in

claim 29

, where said wave-guiding element in at least one optical device is formed of an optical birefringent material to have two orthogonal principal polarization axes that are substantially perpendicular to said optic axis.

38. The system as in

claim 29

, wherein said spatial grating pattern has a grating period that is nonlinearly chirped along said optic axis.

39. The system as in

claim 29

, wherein said spatial grating pattern includes a spatial nonlinear chirp in one aspect of an index of refraction of said wave-guiding element along said optic axis.

40. A system, comprising:

a laser configured to produce a laser beam;

a wave-guiding element having (1) an optic axis to transport optical energy along said optic axis and (2) a spatial grating pattern which is an oscillatory variation along said optic axis, said wave-guiding element positioned to receive said laser beam from said laser and to produce an output laser beam by reflection within a Bragg reflection band produced by said spatial grating pattern so as to produce time delays of different reflected spectral components as a nonlinear function of spatial positions along said optic axis at which said different reflected spectral components are respectively reflected; and

a control unit engaged to said wave-guiding element and operable to change a property of said spatial grating pattern along said optic axis to tune at least relative time delays of said different reflected spectral components nonlinearly with respect to wavelength.

41. The system as in

claim 40

, wherein said laser is a pulsed laser.

42. The system as in

claim 40

, further comprising a substrate, wherein said laser and said wave-guiding element are integrated on said substrate.

43. The system as in

claim 42

, wherein said wave-guiding element is an optical waveguide formed on said substrate.

44. The system as in

claim 40

, wherein said spatial grating pattern has a grating period that is nonlinearly chirped along said optic axis.

45. The system as in

claim 40

, wherein said spatial grating pattern includes a spatial nonlinear chirp in one aspect of an index of refraction of said wave-guiding element along said optic axis.

46. A method, comprising:

designing a wave-guiding element to have (1) an optic axis to transport optical energy along said optic axis and (2) a spatial grating pattern which is an oscillatory variation along said optic axis so that said wave-guiding element operates to produce an output optical signal by reflection within a Bragg reflection band produced by said spatial grating pattern with time delays of different reflected spectral components as a nonlinear function of spatial positions along said optic axis at which said different reflected spectral components are respectively reflected;

directing an input optical signal into said wave-guiding element to produce said output optical signal; and

controlling said wave-guiding element to change a property of said spatial grating pattern along said optic axis so as to tune at least relative time delays of said different reflected spectral components nonlinearly with respect to wavelength.

47. The method as in

claim 46

, further comprising setting an amount of change in said property of said spatial grating pattern to control dispersion in said output optical signal.

48. The method as in

claim 46

, further comprising setting an amount of change in said property of said spatial grating pattern so that grating-induced dispersion in said output optical signal negates original dispersion present in said input optical signal.

49. The method as in

claim 46

, wherein said property includes a length of said wave-guiding element along said optic axis.

50. The method as in

claim 49

, further comprising engaging a piezoelectric element to said wave-guiding element to control said length.

51. The method as in

claim 49

, further comprising employing a magnetostrictive element to control said length in response to a control magnetic field.

52. The method as in

claim 46

, wherein said wave-guiding element is configured to have an index of refraction that changes in response to a varying control electrical field along said optic axis, and further comprising:

generating said varying control electrical field;

applying said varying control electrical field to said wave-guiding element; and

controlling said varying control electrical field to tune said relative time delays nonlinearly with respect to wavelength.

53. The method as in as in

claim 46

, wherein said wave-guiding element is configured to have an index of refraction that changes in response to a varying control electromagnetic radiation field along said optic axis, and further comprising:

generating said varying control electromagnetic radiation field;

applying said varying control electromagnetic radiation field to said wave-guiding element; and

controlling said varying control electromagnetic radiation field to tune said relative time delays nonlinearly with respect to wavelength.

54. The method as in

claim 46

, wherein said property includes a frequency response of said wave-guiding element, and further comprising:

generating and applying a frequency-tunable acoustic wave along said optic axis of said wave-guiding element; and

controlling said acoustic wave to alter said frequency response of said wave-guiding element to achieve said nonlinear tuning.

55. The method as in

claim 46

, wherein both a length and a refractive index of said wave-guiding element along said optic axis are controlled to achieve said nonlinear tuning.

56. The method as in

claim 46

, wherein a refractive index of said wave-guiding element along said optic axis is controlled to achieve said nonlinear tuning.

57. The method as in

claim 46

, further comprising:

obtaining information of optical dispersion in said output signal; and

adjusting said property of said spatial grating pattern in response to said information.

58. The method as in

claim 57

, further comprising using said information to dynamically adjust grating-induced dispersion produced by said wave-guiding element in response to a time-dependent change in dispersion in said input optical signal.

59. The method as in

claim 46

, wherein said wave-guiding element includes an optical fiber having a fiber core and a fiber cladding surrounding said fiber core.

60. The method as in

claim 46

, wherein said controlling includes controlling a temperature of said wave-guiding element to achieve said nonlinear tuning.

61. The method as in

claim 46

, wherein said wave-guiding element includes an optical waveguide formed on a substrate.

62. The method as in

claim 46

, further comprising:

forming said wave-guiding element by using an optical birefringent material to have two orthogonal principal polarization axes that are substantially perpendicular to said optic axis; and

controlling said wave-guiding element to control a polarization-mode dispersion in said output optical signal.

63. The method as in

claim 46

, further comprising:

designing said wave-guiding element to include a spatial sampling pattern that spatially overlaps with said spatial grating pattern along said optic axis and includes a periodic modulation with a period greater than a grating period of said spatial grating pattern so that said wave-guiding element is operable to produce a plurality of Bragg reflection bands at different wavelengths; and

controlling said wave-guiding element to control dispersion of WDM channels in said input optical signal.

64. The method as in

claim 46

, wherein said spatial grating pattern has a grating period that is nonlinearly chirped along said optic axis.

65. The method as in

claim 46

, wherein said spatial grating pattern includes a spatial nonlinear chirp in one aspect of an index of refraction of said wave-guiding element along said optic axis.

66. The method as in

claim 46

, wherein said input optical signal is a pulsed laser beam from a pulsed laser, and further comprising controlling said wave-guiding element to control a pulse shape of said pulsed laser beam.

67. A method, comprising:

designing a fiber Bragg grating in a fiber to have a spatial grating pattern which is an oscillatory variation along said fiber to produce an output optical signal by reflection within a Bragg reflection band produced by said spatial grating pattern so that time delays of different reflected spectral components are a nonlinear function of spatial positions along said fiber at which said different reflected spectral components are respectively reflected;

directing an input optical signal into said fiber Bragg grating to produce said output optical signal;

controlling a property of said spatial grating pattern of said fiber Bragg grating so as to (1) shift said Bragg reflection in frequency and (2) tune at least relative time delays of said different reflected spectral components nonlinearly with respect to wavelength.

68. The method as in

claim 67

, wherein both a length and a refractive index of said fiber Bragg grating along said fiber are controlled.

69. The method as in

claim 67

, wherein a refractive index of said fiber Bragg grating along said fiber is controlled.

70. The method as in

claim 67

, wherein a length of said fiber Bragg grating along said fiber is controlled.

71. The method as in

claim 67

, further comprising setting an amount of change in said property of said spatial grating pattern to control dispersion in said output optical signal.

72. The method as in

claim 67

, further comprising setting an amount of change in said property of said spatial grating pattern so that grating-induced dispersion in said output optical signal negates original dispersion present in said input optical signal.

73. The method as in

claim 67

, further comprising:

obtaining information of optical dispersion in said output signal; and

adjusting said property of said spatial grating pattern in response to said information.

74. The method as in

claim 73

, further comprising using said information to dynamically adjust grating-induced dispersion produced by said fiber Bragg grating in response to a time-dependent change in dispersion in said input optical signal.

75. The method as in

claim 67

, wherein said spatial grating pattern has a grating period that is nonlinearly chirped along said optic axis.

76. The method as in

claim 67

, wherein said spatial grating pattern includes a spatial nonlinear chirp in one aspect of an index of refraction of said fiber Bragg along said fiber.

77. The method as in

claim 67

, further comprising:

deploying a plurality of additional fiber Bragg gratings connected in series to said fiber Bragg grating to receive a transmitted optical signal from said fiber Bragg grating so that an optical transmission signal from one additional fiber Bragg grating is received by another adjacent additional fiber Bragg located in a downstream of said optical transmission signal, wherein transmission of each fiber Bragging includes spectral components outside a respective Bragg reflection band which are not reflected, and wherein each additional fiber Bragg grating is designed to produce time delays of different reflected spectral components as a nonlinear function of spatial positions along each fiber and to have a unique Bragg reflection band at a center band wavelength different any other fiber Bragg grating; and

controlling each additional fiber Bragg grating to tune at least relative time delays of said different reflected spectral components nonlinearly with respect to wavelength in each respective Bragg reflection band to control dispersion in respectively reflected optical signals.

78. The method as in

claim 67

, further comprising:

separating WDM channels received from an input optical fiber;

providing additional fiber Bragg gratings connected in parallel to said fiber Bragg grating to receive WDM channels from said input optical fiber, respectively, wherein each additional fiber Bragg grating is designed to produce time delays of different reflected spectral components as a nonlinear function of spatial positions along each fiber and to have a unique Bragg reflection band at a center band wavelength different any other fiber Bragg grating;

selecting a first WDM channel as said input optical signal directed into said fiber Bragg grating, wherein said first WDM channel is selected within said Bragg reflection band said fiber Bragg grating;

selecting and directing other WDM channels to said additional fiber Bragg gratings so that each WDM channel is within a Bragg reflection band of a respective Bragg reflection band;

controlling said fiber Bragg grating and each additional fiber Bragg grating to tune at least relative time delays of different reflected spectral components nonlinearly with respect to wavelength in each respective Bragg reflection band to control dispersion in respectively reflected WDM channels;

combining and exporting said reflected WDM channels in an output optical fiber.

79. The method as in

claim 67

, wherein said spatial grating pattern has a grating period that is nonlinearly chirped along said optic axis.

80. The method as in

claim 67

, wherein said spatial grating pattern includes a spatial nonlinear chirp in one aspect of an index of refraction of said wave-guiding element along said optic axis.