CLADDING-PUMPED OPTICAL WAVEGUIDE

• US 20110228382A1
• Filed: 11/30/2009
• Published: 09/22/2011
• Est. Priority Date: 11/28/2008
• Status: Active Grant

• Current Assignee: NKT Photonics A/S (Hamamatsu Photonics KK)
• Original Assignee: NKT Photonics A/S (Hamamatsu Photonics KK)
• Inventors: Hansen, Kim Per, Mattsson, Kent
• Granted Patent: US 9,001,414 B2
• Time in Patent Office: Days
• US Class Current: 359/341.3
• CPC Class Codes: H01S 3/06716 Fibre compositions per se C03C13/04or doping with active elements lasing materials in general H01S3/14
• CPC Class Codes: H01S 3/0672 Non-uniform radial doping
• CPC Class Codes: H01S 3/06729 Peculiar transverse fibre profile
• CPC Class Codes: H01S 3/06733 Fibre having more than one cladding
• CPC Class Codes: H01S 3/0675 Resonators including a grating structure, e.g. distributed Bragg reflectors [DBR] or distributed feedback [DFB] fibre lasers
• CPC Class Codes: H01S 3/06754 Fibre amplifiers H01S3/06708 takes precedence
• CPC Class Codes: H01S 3/094003 the pumped medium being a fibre
• CPC Class Codes: H01S 3/094007 Cladding pumping, i.e. pump light propagating in a clad surrounding the active core
• CPC Class Codes: H01S 3/094053 Fibre coupled pump, e.g. delivering pump light using a fibre or a fibre bundle
• CPC Class Codes: H01S 3/09408 Pump redundancy
• CPC Class Codes: H01S 3/09415 the pumping beam being parallel to the lasing mode of the pumped medium, e.g. end-pumping
• CPC Class Codes: H01S 3/1618 ytterbium
• CPC Class Codes: H01S 3/1695 germanium
• CPC Class Codes: H01S 3/2316 Cascaded amplifiers
• CPC Class Codes: Y10T 29/49826 Assembling or joining

Abstract

The invention relates to a high power amplifier waveguide for amplifying an optical signal wherein photo darkening due to high optical flux is reduced considerably. This is achieved by providing a cladding pumped amplifier waveguide wherein the optical mode overlap to the active material of the waveguide is low and/or wherein the active material is distributed over a large cross sectional region of the waveguide.

Claims

1. A cladding pumped amplifier waveguide for amplifying an optical signal, said cladding pumped amplifier waveguide comprising

a) a signal core adapted to guide an optical signal, said signal core having an effective refractive index n_core-1;

b) an inner cladding region for guiding pump light, said inner cladding region comprising a first inner cladding having an effective refractive index n_inner-clad-1, said first inner cladding comprising a material doped with at least one rare earth element;

where n_core-1>

n_inner-clad-1, and where

the area of said signal core is larger than 25 μ

m², and/or

said signal core comprises at least one core feature, and/or

said inner cladding region comprises a plurality of inner cladding features.

2. A waveguide according to claim 1, wherein the area of said signal core is in the range from about 25 μ

m² to about 50000 μ

m², such as in the range from about 50 μ

m² to about 50000 μ

m², in the range from about 75 μ

m² to about 50000 μ

m², in the range from about 80 μ

m² to about 50000 μ

m², in the range from about 100 μ

m² to about 50000 μ

m², in the range from about 300 μ

m² to about 50000 μ

m², in the range from about 500 μ

m² to about 50000 μ

m², in the range from about 1000 μ

m² to about 50000 μ

m².

3. A waveguide according to claim 1 or 2, further comprising an outer cladding region surrounding said inner cladding region, said outer cladding region comprising at least a first outer cladding having an effective refractive index n_outer-clad-1.

4. A waveguide according to any of claims 1 to 3, said waveguide comprising a second inner cladding surrounding said first inner cladding, said second inner cladding having an effective refractive index n_inner-clad-2.

5. A waveguide according to claim 4, where n_outer-clad-1<

n_inner-clad-2.

6. A waveguide according to claim 4 or 5, wherein n_inner-clad-1<

n_inner-clad-2.

7. A waveguide according to claim 4 or 5, wherein n_inner-clad-1>

n_inner-clad-2.

8. A waveguide according to claim 4 or 5, wherein n_inner-clad-1 is substantially equal to n_inner-clad-2.

9. A waveguide according to any of claims 4 to 8, wherein n_core-1<

n_inner-clad-2.

10. A waveguide according to any of claims 4 to 8, wherein n_core-1>

n_inner-clad-2.

11. A waveguide according to any of claims 4 to 8, wherein n_core-1 is substantially equal to n_inner-clad-2.

12. A waveguide according to any of claims 1 to 11, wherein said signal core comprises a silica material of a composition so that n_core-1 is substantially equal to the refractive index of pure silica.

13. A waveguide according to any of claims 1 to 12, wherein the refractive index of said first inner cladding, n_inner-clad-1, is below that of pure silica.

14. A waveguide according to any of claims 1 to 13, adapted to guide an optical signal at a signal wavelength, λ

_signal.

15. A waveguide according to any of claims 1 to 14, wherein said signal core comprises at least one photosensitive region comprising at least one photosensitive element.

16. A waveguide according to claim 15, wherein said at least one photosensitive region is surrounded by a down-doped silica region.

17. A waveguide according to claim 15, wherein said signal core comprises a plurality of photosensitive regions each of which are surrounded by a down-doped silica region.

18. A waveguide according to any of claims 15 to 17, wherein said photosensitive element is selected from the group of Germanium (Ge), Boron (B), or combinations thereof.

19. A waveguide according to any of claims 15 to 18, where the largest cross sectional dimension of one photosensitive region is less than 0.5 micrometer, such as less than 0.2 micrometer, such as less than 0.1 micrometer.

20. A waveguide according to any of claims 15 to 18, where the largest cross sectional dimension of one photosensitive region is less than said signal wavelength λ

_signal divided by 2, such as less than λ

_signal divided by 5, such as less than λ

_signal divided by 10.

21. A waveguide according to any of claims 1 to 20, wherein said first inner cladding comprises at least one rare earth doped silica region.

22. A waveguide according to claim 21, wherein said at least one rare earth doped silica region comprises an annular shaped region surrounding said signal core.

23. A waveguide according to any of claims 1 to 21, wherein said first inner cladding comprises a plurality of rare earth doped silica regions.

24. A waveguide according to any of claims 21 to 23, wherein at least part of said rare earth doped silica regions are surrounded by down-doped silica regions.

25. A waveguide according to claim 24, wherein n_inner-clad-1 is given by the average refractive index of said rare earth doped silica regions and said down-doped silica regions.

26. A waveguide according to any of claims 13 to 25, wherein said down-doped silica comprises an index lowering element selected from the group of Fluorine (F), Boron (B), or combinations thereof.

27. A waveguide according to any of claims 14 to 26, wherein the signal wavelength, λ

_signal, of said optical signal is within the interval from 970 nm to 1250 nm, such as within the interval 1020 nm to 1120 nm, such as within the interval 1050 nm to 1090 nm.

28. A waveguide according to claim 27, wherein the signal core is adapted to guide an optical signal at a wavelength, λ

_signal, of 1064 nm in a single mode condition.

29. A waveguide according to any of claims 21 to 28, where the largest cross sectional dimension of the individual rare earth doped silica regions is less than said optical signal wavelength λ

_signal divided by 2, such as less than λ

_signal divided by 5, such as less than λ

_signal divided by 10.

30. A waveguide according to any of claims 4 to 29, wherein said second inner cladding is doped with at least one rare earth element.

31. A waveguide according to claim 30, wherein the level of rare earth element doping is higher in said first inner cladding than in said second inner cladding.

32. A waveguide according to any of claims 1 to 31, wherein n_core-1−

n_inner-clad-1 is larger than 1·

10<sup>−

5</sup>, such as larger than 1·

10<sup>−

4</sup>, such as larger than 2·

10<sup>−

4</sup>, such as larger than 5·

10<sup>−

4</sup>, such as larger than 1·

10<sup>−

3</sup>.

33. A waveguide according to any of claims 6 to 32, wherein n_inner-clad-1−

n_inner-clad-2 is less than −

1·

10<sup>−

4</sup>, such as less than −

2·

10<sup>−

4</sup>

34. A waveguide according to any of claims 7 to 32, wherein n_inner-clad-1−

n_inner-clad-2 is less than 1·

10<sup>−

3</sup>, as less than 5·

10<sup>−

4</sup>, as less than 2·

10<sup>−

4</sup>, as less than 1·

10<sup>−

4</sup>, as less than 1·

10<sup>−

5</sup>.

35. A waveguide according to any of claims 3 to 34, wherein n_inner-clad-1−

n_outer-clad-1 is less than 1·

10<sup>−

3</sup>, such as less than 5·

10<sup>−

4</sup>, as less than 2·

10<sup>−

4</sup>, as less than 1·

10<sup>−

4</sup>, as less than 1·

10<sup>−

5</sup>, as less than −

1·

10<sup>−

4</sup>, as less than −

2·

10<sup>−

4</sup>.

36. A waveguide according to any of claims 1 to 35, wherein the effective mode area of said optical signal is larger than 50 μ

m², such as 150 μ

m², such as 250 μ

m², such as 350 μ

m².

37. A waveguide according to any of claims 1 to 36, where said signal core and said first inner cladding hold equal centre of mass and the border between the two regions is substantially circular in cross section.

38. A waveguide according to any of claims 1 to 36, where said signal core and said first inner cladding hold equal centre of mass and the border between the two regions is substantially hexagonal in cross section.

39. A waveguide according to any of claims 1 to 38, where the overlap of said guided optical signal to said rare earth doped silica region(s) is 25%, or less, such 20% or less, such as 15% or less, such as 10% or less, such as 5% or less, such as in the range of about 2% to about 25%.

40. A waveguide according to any of claims 1 to 39, where the area of said signal core is larger than about 10 μ

m², such as larger than about 25 μ

m², such as larger than about 50 μ

m², such as larger than about 80 μ

m², such as larger than about 100 μ

m², such as larger than about 200 μ

m², such as larger than about 300 μ

m², such as larger than about 500 μ

m², such as larger than about 700 μ

m², such as larger than about 1000 μ

m².

41. A waveguide according to any of claims 1 to 40, wherein the area of said first inner cladding is larger than about 10 μ

m², such as larger than about 50 μ

m², such as larger than about 100 μ

m², such as larger than about 150 μ

m², such as larger than about 200 μ

m², such as larger than about 300 μ

m², such as larger than about 350 μ

m², such as larger than about 400 μ

m², such as larger than about 500 μ

m², such as larger than about 600 μ

m², such as larger than about 750 μ

m², such as larger than about 1000 μ

m², such as larger than about 2000 μ

m².

42. A waveguide according to any of claims 1 to 41 wherein the ratio between the area of the first inner cladding and the area of the signal core is in the range of about 4 to about 500, such as about 5 to 500, such as about 6 to 500, such as about 8 to 500, such as about 10 to 500, such as about 15 to 500, such as about 20 to 500, such as about 25 to 500, such as about 50 to 500, such as about 100 to 500, such as about 150 to 500, such as about 200 to 500, such as about 300 to 500, such as about 400 to 500.

43. A waveguide according to any of claims 1 to 41 wherein the ratio between the area of the first inner cladding and the area of the signal core is in the range of about 4 to about 500, such as about 4 to 400, such as about 4 to 300, such as about 4 to 200, such as about 4 to 100, such as about 4 to 50, such as about 4 to 25, such as about 4 to 20, such as about 4 to 10.

44. A waveguide according to any of claims 1 to 41 wherein the ratio between the area of the first inner cladding and the area of the signal core is about 4, such as about 4.5, such as about 5, such as about 6, such as about 8, such as about 10, such as about 15, such as about 20, such as about 25, such as about 50, such as about 100, such as about 200, such as about 300, such as about 400, such as about 500.

45. A waveguide according to any of claims 37 to 44, wherein the mode field intensity of said optical signal at the border between the signal core and the inner cladding region is in the range of about 0.5% to 5%, such as in the range of about 1% to 4%, such as in the range of about 1% to 3%, such as in the range of about 1% to 2%.

46. A waveguide according to any of claims 1 to 44, wherein the mode field intensity of said optical signal in the inner cladding region is in the range of about 0.5% to 5%, such as in the range of about 1% to 4%, such as in the range of about 1% to 3%, such as in the range of about 1% to 2%.

47. A waveguide according to any of claims 1 to 46, further comprising a spacer region surrounding said signal core, said spacer region being surrounded by said inner cladding.

48. A waveguide according to claim 47, wherein said spacer region is arranged to reduce the intensity of an optical signal guided in said signal core in said first inner cladding region.

49. A waveguide according to any of claims 1 to 48, wherein said first inner cladding comprises a first inner cladding background material, and wherein said plurality of inner cladding features comprises first inner cladding features arranged in said first inner cladding, said first inner cladding features having a refractive index profile different from the refractive index of the said first inner cladding background material.

50. A waveguide according to any of claims 4 to 49, wherein said second inner cladding comprises a second inner cladding background material, and wherein said plurality of inner cladding features comprises second inner cladding features arranged in said second inner cladding, said second inner cladding features having a refractive index profile different from the refractive index of said second inner cladding background material.

51. A waveguide according to any of claims 1 to 50, wherein said inner cladding features comprise regions with refractive index below that of the first and/or second inner cladding background material, such as voids.

52. A waveguide according to any of claims 1 to 51, wherein said inner cladding features are arranged in a lattice.

53. A waveguide according to claim 52, wherein said inner cladding features are arranged in a hexagonal lattice.

54. A waveguide according to any of claims 49 to 53, wherein said signal core is defined by replacing at least one of said inner cladding features with at least one core feature comprising a core material of an refractive index profile different from the refractive index profile of said first inner cladding features.

55. A waveguide according to claim 54, wherein said signal core is formed by replacing one of said inner cladding features and the six nearest inner cladding features with core features, such replacing one of said inner cladding features and the eighteen nearest inner cladding features with core features.

56. A waveguide according to any of claims 49 to 55, wherein said first inner cladding features comprises a rare earth doped material.

57. A waveguide according to claim 56, wherein said first inner cladding features comprising a rare earth doped material are arranged to form at least one chain surrounding said signal core.

58. A waveguide according to any of claim 56 or 57, wherein said first inner cladding features comprises rare earth doped silica tubes.

59. A waveguide according to any of claims 1 to 58, wherein said first inner cladding comprises at least one annular distribution of a rare earth doped silica glass.

60. A waveguide according to any of claims 1 to 55, wherein at least a part of said first inner cladding background material comprises rare earth doped silica.

61. A waveguide according to any of claims 49 to 60, wherein said first inner cladding features comprises a material with an effective refractive index above that of said first inner cladding background material, whereby a Photonic Bandgap is formed enabling guidance of the optical signal in the signal core by the Photonic Bandgap effect.

62. A waveguide according to any of claims 50 to 61, wherein said second inner cladding features comprises a material with an effective refractive index above that of said second inner cladding background material, whereby a Photonic Bandgap is formed enabling guidance of the optical signal in the signal core by the Photonic Bandgap effect.

63. A waveguide according to any of claims 1 to 62, wherein the signal core and the first inner region comprises a uniform distribution of rare earth elements

64. A waveguide according to any of claims 1 to 62, wherein said signal core comprises a inner core region comprising a high phonon velocity material, wherein phonons has a higher velocity than in pure silica glasses

65. A waveguide according to claim 63, wherein the high phonon velocity material comprises Aluminium doped silica glass.

66. A waveguide according to any of claims 1 to 65, wherein said first inner cladding feature and optionally said seconds inner cladding features are arranged so that the waveguide can be operated as an anti-resonant reflecting optical waveguide.

67. A waveguide according to 66, wherein said signal core is surrounded on opposite sides by first inner cladding features, said first inner cladding features being surrounded by second inner cladding features arranged on opposite sides of said signal core.

68. A waveguide according to claim 1, wherein said first inner cladding region comprises a ring of first inner cladding features, said inner cladding features comprising a low index region surrounded by a rare earth doped region.

69. A waveguide according to claim 68, wherein said low index material comprises a void.

70. A waveguide according to any of claims 3 to 69, wherein said first outer cladding comprises an air cladding.

71. A waveguide according to any of claims 3 to 70, wherein said outer cladding comprises a polymer coating

72. A waveguide according to any of claims 1 to 71, wherein said rare earth element is selected from the group of Ytterbium (Yb), Erbium (Er), Praseodymium (Pr), Neodynium (Nd), Holmium (Ho), Thulium (Tm), Dysprosium (Dy), or combinations thereof.

73. A waveguide according to any of claims 1 to 72, wherein said rare earth doped silica material is co-doped with Aluminum (Al), Phosphorous (P), Cesium (Cs) or combinations thereof.

74. A waveguide according to any of claims 1 to 73, wherein the concentration of said rare earth element in said rare earth doped silica material is in the range of about 1×

10¹⁹ cm<sup>−

3</sup> to about 5×

10²¹ cm<sup>−

3</sup>.

98. A fibre laser or amplifier comprising an optical waveguide according to any one of claims 1 to 97.

99. A high power amplifier waveguide system for amplifying an optical signal comprising:

a) a diode bar array pump laser which operates at a wavelength λ

_pump and with a pump power exceeding 100 W;

b) a coupling device;

c) a cladding pumped amplifier waveguide according to any of claims 1 to 97;

d) an output delivery fibre;

where the wavelength λ

_pump is resonant with an absorption band of said rare earth doping.

100. A system according to claim 99, wherein between said coupling device and said cladding pumped amplifier waveguide is formed a first fibre Bragg grating, and wherein between said cladding pumped amplifier waveguide and said output delivery fibre is formed a second fibre Bragg grating.

101. A system according to claim 99 or 100, where said first and second fibre Bragg grating are formed by fusion splicing sections of fibre comprising Bragg gratings to the amplifier waveguide.

102. A system according to any of claims 99 to 101, wherein said Bragg gratings are written directly into said amplifier waveguide

114. A method for fabricating a cladding pumped amplifier waveguide according to any of claims 1 to 112, comprising arranging precursor elements in a perform and drawing said perform into an amplifier waveguide

115. A method according to claim 114, wherein said precursor elements comprises a plurality of inner cladding precursor elements arranged to provide inner cladding features in the drawn amplifier waveguide.

116. The method according to claim 115, wherein at least a part of said inner cladding precursor elements comprises at least one region doped with an active element, such as a Rare Earth element.

117. The method according to claim 116, wherein at least a part of said active element doped regions are individually surrounded by down-doped region.

118. The method according to any of claims 116 to 117, wherein the number of regions doped with an active element per inner cladding precursor elements is more than about 10, such as more than about 25, such as more than about 50, such as more than about 100, such as more than about 125, such as more than about 150, such as more than about 300, such as more than about 500, such as more than about 1000, such as more than about 1500, such as more than about 2000.

119. The method according to any of claims 114 to 117 , wherein the number of said inner cladding features may be less than about 20, such as less than about 15, such as less than about 12, such as less than about 10, such as less than about 8, such as less than about 6, such as less than about 4, such as less than about 3, such as less than about 2.

75. A cladding pumped amplifier waveguide for amplifying an optical signal,

said cladding pumped amplifier waveguide comprising

a) a signal core adapted to guide an optical signal, said signal core having an effective refractive index n_core-1;

b) an inner cladding region for guiding pump light, said inner cladding region comprising a first inner cladding having an effective refractive index n_inner-clad-1 and a second inner cladding having an effective refractive index n_inner-clad-2 and surrounding said first inner cladding;

wherein at least said first inner cladding is doped with at least one rare earth element;

c) a first outer cladding having an effective refractive index n_outer-clad-1;

where n_core-1>

n_inner-clad-1>

p_outer-clad-1

76. A waveguide according to claim 75, wherein said signal core comprises at least one photosensitive region, comprising a photosensitive element, such as Ge-doped silica region, surrounded by at least one down-doped silica region, where said effective refractive index of said signal core is given by the average refractive index of said at least one photosensitive region and said down-doped silica regions and where the largest cross sectional dimension of one photosensitive region is less than 0.5 micrometer, such as less than 0.2 micrometer, such as less than 0.1 micrometer.

77. A waveguide according to claim 75 or 76, wherein said signal core is adapted to guide light at a signal wavelength λ

_signal, said-signal core comprising at least one photosensitive region, comprising a photosensitive element, such as Ge-doped silica region, surrounded by at least one down-doped silica region, where said effective refractive index of said signal core is given by the average refractive index of said at least one photosensitive region and said down-doped silica regions and where the largest cross sectional dimension of one photosensitive region is less than said signal wavelength λ

_signal divided by 2, such as less than λ

_signal divided by 5, such as less than λ

_signal divided by 10.

78. A waveguide according to claim 76 or 77, wherein said down-doped silica comprises F-doped silica.

79. A waveguide according to any of claims 75 to 78, wherein said first inner cladding comprises a plurality of rare earth doped silica regions.

80. A waveguide according to claim 79, wherein said rare earth doped silica regions are surrounded by F-doped silica regions.

81. A waveguide according to claim 80, wherein n_inner-clad-1 is given by the average refractive index of said rare earth doped silica regions and said F-doped silica regions.

82. A waveguide according to any of claims 78 to 81, where the largest cross sectional dimension of said individual rare earth doped silica regions is less than said signal wavelength λ

_signal divided by 2, such as less than λ

_signal divided by 5, such as less than λ

_signal divided by 10.

83. A waveguide according to any of claims 75 to 82, wherein said second inner cladding is doped with at least one rare earth element.

84. A waveguide according to claim 83, wherein the level of rare earth element doping is higher in said first inner cladding than in said second inner cladding.

85. A waveguide according to any of claims 75 to 84, wherein n_core-1−

n_inner-clad-1 is larger than 1·

10<sup>−

3</sup>, such as larger than 5·

10<sup>−

4</sup>, such as larger than 2·

10<sup>−

4</sup>, such as larger than 1·

10<sup>−

4</sup>, such as larger than 1·

10<sup>−

5</sup>.

86. A waveguide according to any of claims 75 to 85, wherein n_inner-clad-1−

n_outer-clad-1 is less than 1·

10<sup>−

3</sup>, such as less than 5·

10<sup>−

4</sup>, such as less than 2·

10<sup>−

4</sup>, such as less than 1·

10<sup>−

4</sup>, such as less than 1·

10<sup>−

5</sup>, such as less than −

1·

10<sup>−

4</sup>, such as less than −

2·

10<sup>−

4</sup>

87. A waveguide according to any of claims 75 to 86, wherein the effective mode area of said optical signal is larger than 50 μ

m², such as 150 μ

m², such as 250 μ

m², such as 350 μ

m².

88. A waveguide according to any of claims 75 to 87, wherein said signal wavelength λ

_signal of said optical signal is within the interval from 970 nm to 1250 nm, such as within the interval 1020 nm to 1120 nm, such as within the interval 1050 nm to 1090 nm.

89. A waveguide according to any of claims 75 to 88, where said signal core and said first inner cladding hold equal centre of mass and the border between the two regions is substantially circular in cross section.

90. A waveguide according to any of claims 75 to 89, where said signal core and said first inner cladding hold equal centre of mass and the border between the two regions is substantially hexagonal in cross section.

91. A waveguide according to any of claims 75 to 90, where said guided optical signal overlap to said rare earth doped silica region(s) is about 25%, or less, such about 20% or less, such as about 15% or less, such as about 10% or less, such as about 5% or less.

92. A waveguide according to any of claims 75 to 91, where the area of said signal core is larger than about 10 μ

m², such as larger than about 25 μ

m², such as larger than about 50 μ

m², such as larger than about 80 μ

m², such as larger than about 100 μ

m², such as larger than about 200 μ

m², such as larger than about 300 μ

m², such as larger than about 500 μ

m², such as larger than about 700 μ

m², such as larger than about 1000 μ

m².

93. A waveguide according to any of claims 75 to 92, wherein the area of said first inner cladding is larger than about 10 μ

m², such as larger than about 50 μ

m², such as larger than about 100 μ

m², such as larger than about 150 μ

m², such as larger than about 200 μ

m², such as larger than about 300 μ

m², such as larger than about 350 μ

m², such as larger than about 400 μ

m², such as larger than about 500 μ

m², such as larger than about 600 μ

m², such as larger than about 750 μ

m², such as larger than about 1000 μ

m², such as larger than about 2000 μ

m².

94. A waveguide according to any of claims 75 to 93, further comprising a spacer region surrounding said signal core, said spacer region being surround by said inner cladding.

95. A waveguide according to claim 94, wherein said spacer region is arranged to reduce the intensity of an optical signal in said first inner cladding region.

96. A high power amplifier waveguide comprising:

a) a core region containing at least one sub-region with increased refractive index relative to two or more cladding regions, and

b) wherein at least one of said core and cladding regions contains an optically active gain media, and

c) wherein said region is adapted to guide a light signal of wavelength λ

_signal, and

d) wherein said signal to said gain media overlap is 25% or less.

97. In a cladding pumped amplifier waveguide for amplifying an optical signal, said waveguide comprising

a) a signal core arranged to guide said optical signal, said signal core having an effective refractive index n_core;

b) an inner cladding region for guiding pump light, said inner cladding region surrounding said signal core and having a refractive index n_inner-clad;

c) a first outer cladding having an effective refractive index n_outer-clad-1;

wherein n_core>

n_inner-clad-1>

n_outer-clad-1, and wherein said amplifier waveguide is improved by having the highest concentration of said active rare earth elements in said inner cladding region resulting in that photodarkening induced degradation of the amplifier efficiency is mitigated.

103. A cladding pumped amplifier waveguide for amplifying an optical signal, said cladding pumped amplifier waveguide comprising

a) a signal core adapted to guide an optical signal, said signal core having an effective refractive index n_core-1;

wherein said signal core is doped with at least one rare earth element in an average concentration of C_{RE,signal-core};

b) an inner cladding region for guiding pump light, said inner cladding region comprising a first inner cladding having an effective refractive index n_inner-clad-1 and a second inner cladding having an effective refractive index n_inner-clad-2 and surrounding said first inner cladding;

wherein at least said first inner cladding is doped with at least one rare earth element in an average concentration of c_{RE,inner-clad};

c) a first outer cladding having an effective refractive index p_outer-clad;

where n_core-1>

n_inner-clad>

n_outer-clad.

104. A waveguide according to claim 103, wherein c_{RE,signal-core} is substantially identical to c_{RE,inner-clad} or wherein c_{RE,signal-core}<

c_{RE,inner-clad}, such as c_{RE,signal-core}<

0.9*c_{RE,inner-clad}, such as c_{RE,signal-core}<

0.8*c_{RE,inner-clad}, such as c_{RE,signal-core}<

0.7*c_{RE,inner-clad}, such as c_{RE,signal-core}<

0.6*c_{RE,inner-clad}, such as c_{RE,signal-core}<

0.5*c_{RE,inner-clad}, such as c_{RE,signal-core}≦

0.4*c_{RE,inner-clad}, such as c_{RE,signal-core}<

0.3*c_{RE,inner-clad}, such as c_{RE,signal-core}≦

0.2*c_{RE,inner-clad}, such as c_{RE,signal-core}<

0.1*c_{RE,inner-clad}

105. A waveguide according to claim 103 or 104, wherein said first inner cladding comprises a plurality of inner cladding features comprising rare earth co-doped silica.

106. A waveguide according to claim 105, wherein said inner cladding features are surrounded by F-doped silica regions, such as said inner cladding features being individually surrounded by F-doped silica regions.

107. A waveguide according to claim 106, n_inner-clad-1 is given by the average refractive index of said inner cladding features and said F-doped silica regions

108. A waveguide according to any of claims 105 to 107, wherein said optical signal comprises a signal wavelength, λ

_signs, where the largest cross sectional dimension of said individual rare inner cladding features is less than said signal wavelength λ

_signal divided by 2, such as less than λ

_signal divided by 5, such as less than λ

_signal divided by 10.

109. A waveguide according to any of claims 103 to 108, wherein said second inner cladding is doped with at least one rare earth element.

110. A waveguide according to claim 109, wherein the level of rare earth element doping is higher in said first inner cladding than in said second inner cladding.

111. A waveguide according to any of claims 103 to 110, where said signal core and said first inner cladding hold equal centre of mass and the border between the two regions is substantially hexagonal in cross section.

112. In a cladding pumped amplifier waveguide for amplifying an optical signal, said waveguide comprising

a) a signal core arranged to guide said optical signal, said signal core having an effective refractive index n_core;

b) an inner cladding region for guiding pump light, said inner cladding region surrounding said signal core and having an effective refractive index n_inner-clad;

c) a first outer cladding having an effective refractive index n_outer-clad;

wherein n_core>

n_inner-clad>

n_outer-clad, and wherein said amplifier waveguide is improved by having the highest concentration of said active rare earth elements in said inner cladding region resulting in that photodarkening induced degradation of the amplifier efficiency is mitigated.

113. A method for producing a cladding pumped amplifier waveguide, said method comprising

a) arranging at least one signal core precursor elements and a plurality of inner cladding precursor elements in stack

b) wherein said inner cladding precursor elements are arranged in a chain surrounding said core precursor element.