Raman oscillator including an intracavity filter and amplifiers utilizing same

• US 6,567,430 B1
• Filed: 09/21/1999
• Issued: 05/20/2003
• Est. Priority Date: 09/21/1998
• Status: Expired due to Term

• Current Assignee: Board of Regents of the University of Michigan (University of Michigan), Neptune Subsea IP Ltd. (Xtera Topco Ltd.)
• Original Assignee: Board of Regents of the University of Michigan (University of Michigan), Xtera Communications Incorporated (Xtera Topco Ltd.)
• Inventors: Islam, Mohammed N., Freeman, Michael J., Harris, Hayden H.
• Primary Examiner(s): Leung, Quyen
• Assistant Examiner(s): Rodriguez, Armando
• Application Number:
  US09/400,414
• Time in Patent Office: 1,337 Days
• Field of Search: 372/6, 372/3, 372/21, 372/22, 372/70, 372/71, 372/72, 372/94, 372/99, 385/39
• US Class Current: 372/3
• CPC Class Codes: H01S 3/06791 Fibre ring lasers fibre laser gyrometers G01C19/72
• CPC Class Codes: H01S 3/302 in an optical fibre

Abstract

In one embodiment, a Raman oscillator includes at least one laser cavity and a distributed gain fiber positioned in the at least one laser cavity having a single spatial mode over a pumping wavelength to a signal wavelength. The oscillator also includes a coupler adapted to be coupled to a pumping mechanism to pump the distributed gain fiber at the pumping wavelength to obtain an optical signal wherein distributed gain is provided by Raman amplification over at least one cascade order corresponding to the pumping wavelength. A filter is positioned in the at least one laser cavity and has at least one pass band with a transmission peak placed approximately at the at least one cascade order to filter the optical signal to obtain a filtered optical signal having the signal wavelength. The filter has a substantially continuous sinusoidal filter function over at least one period of the filter function.

Claims

1. A Raman oscillator having high efficiency due to low intracavity loss, the oscillator comprising:

at least one laser cavity;

a distributed gain fiber positioned in the at least one laser cavity, the distributed gain fiber having a single spatial mode over a pumping wavelength to a signal wavelength;

a coupler adapted to be coupled to a pumping mechanism to pump the distributed gain fiber at the pumping wavelength to obtain an optical signal wherein distributed gain is provided by Raman amplification over at least one cascade order corresponding to the pumping wavelength;

a filter positioned in the at least one laser cavity and having at least one pass band with a transmission peak placed approximately at the at least one cascade order to filter the optical signal to obtain a filtered optical signal having the signal wavelength, wherein the filter comprises a substantially continuous sinusoidal filter function over at least one period of the filter function; and

an output port for outputting the filtered optical signal.

2. The oscillator as claimed in claim 1, wherein the filter has an all-glass composition.

3. A The oscillator as claimed in claim 1, wherein the filter includes a Mach-Zehnder filter.

4. The oscillator as claimed in claim 1, wherein the file includes a low-Q etalon.

5. The oscillator as claimed in claim 1, wherein the filter includes a fused fiber coupler.

6. The oscillator as claimed in claim 1, wherein the distributed gain is provided by Raman amplification over a plurality of Raman cascade orders including a final Raman order and wherein the filter has pass bands periodic in frequency with transmission peaks placed at the plurality of cascade orders or multiples of the cascade orders.

7. The oscillator as claimed in claim 6, wherein the transmission peaks placed at the cascade orders are separated by approximately 13.2 THz or some multiple of 13.2 THz or some submultiple of 13.2 THz.

8. The oscillator as claimed in claim 6, wherein the transmission peaks are separated by two cascade orders.

9. The oscillator as claimed in claim 1, wherein the distributed gain is provided by Raman amplification over a plurality of cascade orders and wherein the filter includes a band-splitting filter that separates the at least one cascade order from at least another cascade order.

10. The oscillator as claimed in claim 9, wherein the band-splitting filter separates the pumping wavelength from the signal wavelength.

11. The oscillator as claimed in claim 1, wherein the filter includes a band-splitting filter that separates at least one cascade order from at least another cascade order.

12. The oscillator as claimed in claim 1, further comprising:

at least one of a splitter and circulator for splitting the pump wavelength from the signal wavelength outside the at least one laser cavity.

13. The oscillator as claimed in claim 12, wherein the coupler and the output port are combined outside the at least one laser cavity.

14. The oscillator as claimed in claim 1, wherein the at least one laser cavity is a linear laser cavity with first and second ends, and the second end is at least partially reflective.

15. The oscillator as claimed in claim 1, wherein the filter includes the coupler.

16. The oscillator as claimed in claim 1, further comprising:

a second coupler coupled to the coupler and the output port and having different periodicity wherein the pumping wavelength and the signal wavelength are separated by a number of cascade orders outside the at least one laser cavity.

17. The oscillator as claimed in claim 1, further comprising at least one grating positioned in the at least one laser cavity.

18. The oscillator as claimed in claim 17, further comprising a second coupler coupled to the coupler and the output port and having different periodicity wherein the pumping wavelength and the signal wavelength are separated by a number of cascade orders outside the at least one laser cavity.

19. The oscillator as claimed in claim 1, wherein the at least one laser cavity is a ring laser cavity closed on itself.

20. The oscillator as claimed in claim 19, wherein the filter includes an inline periodic filter.

21. The oscillator as claimed in claim 20, wherein the ring laser cavity has an arm and wherein the oscillator further comprises fine-tuning elements positioned in the arm of the ring laser cavity for fine frequency tuning the filtered optical signal.

22. The oscillator as claimed in claim 19, further comprising an isolator positioned in the ring laser cavity to make the ring laser cavity operate unidirectionally.

23. The oscillator as claimed in claim 1, wherein the filter includes a many-order periodic filter.

24. The oscillator as claimed in claim 1, further comprising at least one tuning element positioned in the at least one laser cavity for fine frequency tuning the filtered optical signal.

25. The oscillator as claimed in claim 24, wherein the at least one tuning element comprises a grating.

26. The oscillator as claimed in claim 24, wherein the at least one tuning element comprises a Fabry-Perot interferometer.

27. The oscillator as claimed in claim 24, wherein the at least one tuning element comprises a dielectric structure.

28. The oscillator of claim 1, wherein the at least one pass band comprises a bandwidth of at least ten (10) nanometers.

29. The oscillator of claim 1, wherein the filter comprises at least three (3) ports, and wherein at least two (2) of the at least three (3) ports are positioned within the at least one laser cavity.

30. The oscillator of claim 1, wherein the at least one laser cavity comprises no more than one filter with a transmission peak centered on the at least one cascade order.

31. The oscillator of claim 1, wherein the Raman amplification comprises distributed Raman amplification.

32. The oscillator of claim 1, wherein the Raman amplification comprises discrete Raman amplification.

33. In a fiber optic transmission system including at least one transmission link, an optical amplifier comprising:

an amplifier input port for receiving an input optical signal;

an amplifier distributed gain medium connected to the amplifier input port to amplify the input optical signal;

a Raman oscillator operable to pump the amplifier distributed gain medium at a pumping level sufficiently high so that the input optical signal experiences a gain, the Raman oscillator comprising;

at least one laser cavity;

an oscillator distributed gain fiber positioned in the at least one laser cavity, the oscillator distributed gain fiber having a single spatial mode over a pumping wavelength to a signal wavelength;

a coupler adapted to be coupled to a pumping mechanism to pump the oscillator distributed gain fiber at the pumping wavelength to obtain an oscillator optical signal wherein distributed gain is provided by Raman amplification over at least one cascade order corresponding to the pumping wavelength;

a filter positioned in the at least one laser cavity and having at least one pass band with a transmission peak placed approximately at the at least one cascade order to filter the oscillator optical signal to obtain a filtered oscillator optical signal having the signal wavelength, wherein the filter comprises a substantially continuous sinusoidal filter function over at least one period of the filter function;

an output port for outputting the filtered oscillator optical signal; and

an amplifier output port for outputting an amplified input optical signal.

34. The amplifier as claimed in claim 33, wherein the amplifier distributed gain medium is a gain fiber.

35. The amplifier as claimed in claim 34, wherein the gain fiber is pumped in a counter-propagating fashion by the Raman oscillator.

36. The amplifier as claimed in claim 34, wherein the optical amplifier is a distributed amplifier and wherein the at least one transmission link includes the gain fiber of the amplifier so that the distributed amplifier serves as a low-noise amplifier.

37. The amplifier as claimed in claim 33, wherein the optical amplifier is a discrete Raman amplifier.

38. The amplifier as claimed in claim 37, further comprising at least one isolator and wherein the discrete Raman amplifier is positioned within the at least one transmission link.

39. In a fiber optic transmission system, a discrete optical amplifier comprising:

an amplifier input port for receiving an input optical signal;

an amplifier distributed gain medium coupled to the amplifier input port and operable to amplify the input optical signal;

a Raman oscillator to pump the amplifier distributed gain medium, the Raman oscillator comprising;

at least one laser cavity;

a distributed gain fiber positioned in the at least one laser cavity, the distributed gain fiber having a single spatial mode over a pumping wavelength to a signal wavelength;

a coupler adapted to be coupled to a pumping mechanism to pump the distributed gain fiber at the pumping wavelength to obtain an optical signal wherein distributed gain is provided by Raman amplification over at least one cascade order corresponding to the pumping wavelength;

a filter positioned in the at least one laser cavity and having at least one pass band with a transmission peak placed approximately at the at least one cascade order to filter the optical signal to obtain a filtered optical signal having the signal wavelength, wherein the at least one pass band comprises a bandwidth of at least ten (10) nanometers; and

an amplifier output port for outputting an amplified input optical signal.

40. The transmission system of claim 39, wherein the filter comprises a sinusoidal filter function.

41. The transmission system of claim 39, wherein the filter comprises at least three (3) ports, and wherein at least two (2) of the at least three (3) ports are positioned within the at least one laser cavity.

42. The transmission system of claim 39, wherein the at least one laser cavity comprises no more than one filter with a transmission peak centered on the at least one cascade order.

43. The transmission system of claim 39, wherein the bandwidth of the at least one pass band is measured at a half-power power point of the pass band.

44. In a fiber optic transmission system, an optical amplifier comprising:

an amplifier input port for receiving an optical signal; and

a Raman oscillator, comprising;

at least one laser cavity;

an oscillator distributed gain fiber positioned in the at least one laser cavity, the oscillator distributed gain fiber having a single spatial mode over a pumping wavelength to a signal wavelength;

a coupler adapted to be coupled to a pumping mechanism to pump the oscillator distributed gain fiber at the pumping wavelength to obtain an oscillator optical signal wherein distributed gain is provided by Raman amplification over a plurality of Raman cascade orders including a final Raman order;

a filter positioned in the at least one laser cavity and having pass bands periodic in frequency with transmission peaks placed approximately at the plurality of cascade orders or multiples of the cascade orders to filter the oscillator optical signal to obtain a filtered oscillator optical signal having the signal wavelength, wherein the filter comprises a substantially continuous sinusoidal filter function over at least one period of the filter function;

an output port for outputting the filtered oscillator optical signal.

45. The system of claim 44, wherein the Raman oscillator has a pair of interconnected ring cavities and wherein the distributed gain medium is positioned in each of the ring cavities and wherein the Raman oscillator pumps the distributed gain medium so that the optical signal experiences a gain and to provide for cascaded Raman wavelength shifting and wherein each cascade order is counter-propagated relative to an adjacent cascade order, and wherein the output port of the Raman oscillator outputs the filtered and amplified output signal.

46. A Raman oscillator having high efficiency due to low intracavity loss, the oscillator comprising:

at least one ring laser cavity closed on itself,

a distributed gain medium positioned in the at least one ring laser cavity;

a coupler adapted to be coupled to a pumping mechanism to pump the distributed gain medium at a pumping wavelength to obtain an optical signal wherein distributed gain is provided by Raman amplification over at least one cascade order corresponding to the pumping wavelength;

a filter positioned in the at least one ring laser cavity and having at least one pass band with a transmission peak placed approximately at the at least one cascade order to filter the optical signal to obtain a filtered optical signal having a signal wavelength, wherein the filter comprises a substantially continuous sinusoidal filter function over at least one period of the filter function; and

an output port for outputting the filtered optical signal.

47. The oscillator as claimed in claim 46, wherein the distributed gain medium is a distributed gain fiber.

48. A Raman oscillator having high efficiency due to low interactivity loss, the oscillator comprising:

at least one linear laser cavity;

a distributed gain medium positioned in the at least one linear laser cavity;

a coupler adapted to be coupled to a pumping mechanism to pump the distributed gain medium at a pumping wavelength to obtain an optical signal wherein distributed gain is provided by Raman amplification over at least one cascade order corresponding to the pumping wavelength;

a filter positioned in the at least one linear laser cavity and having at least one pass band with a transmission peak placed approximately at the at least one cascade order to filter the optical signal to obtain a filtered optical signal having a signal wavelength, wherein the filter comprises at least three (3) ports, and wherein at least two (2) of the at least three (3) ports are positioned in the at least one laser cavity; and

an output port for outputting the filtered optical signal.

49. The oscillator of claim 48, wherein at least a portion of the at least one linear laser cavity is a distributed gain fiber with at least one radius of curvature.

50. The oscillator of claim 48, wherein the second end is part of the filter.

51. An optical amplifier configured to be included in a fiber optic transmission system that includes at least one transmission link, comprising:

an amplifier input port for receiving an input optical signal;

an amplifier distributed gain medium coupled to the amplifier input port and operable to amplify the input optical signal;

a Raman oscillator to pump the amplifier distributed gain medium, the Raman oscillator comprising;

at least one laser cavity;

an oscillator distributed gain medium positioned in the at least one laser cavity;

a coupler adapted to be coupled to a pumping mechanism to pump the oscillator distributed gain medium at a pumping wavelength to obtain an optical signal wherein distributed gain is provided by Raman amplification over at least one cascade order corresponding to the pumping wavelength;

a filter assembly positioned in the at least one laser cavity and having at least one pass band with a transmission peak placed approximately at the at least one cascade order to filter the optical signal to obtain a filtered optical signal having a signal wavelength; and

an amplifier output port for outputting the amplified input optical signal;

wherein the at least one laser cavity comprises no more than one filter assembly with a transmission peak approximately centered on any given cascade order.

52. The optical amplifier as claimed in claim 51, wherein the oscillator distributed gain medium is a distributed gain fiber.

53. The optical amplifier as claimed in claim 51, wherein the filter comprises a filter selected from the group consisting of an all-glass composition filter, a Mach-Zehnder filter, and a low-Q etalon filter.

54. The optical amplifier as claimed in claim 51, wherein the filter includes a fused fiber coupler.

55. The optical amplifier as claimed in claim 51, wherein the Raman oscillator distributed gain is provided by Raman amplification over a plurality of Raman cascade orders including a final Raman order and wherein the filter has pass bands periodic in frequency with transmission peaks placed at the plurality of cascade orders or multiples of the cascade orders.

56. The optical amplifier as claimed in claim 55, wherein the transmission peaks placed at the cascade orders are separated by approximately 13.2 THz or some multiple of 13.2 THz or some submultiple of 13.2 THz.

57. The optical amplifier as claimed in claim 55, wherein the transmission peaks are separated by two cascade orders.

58. The optical amplifier as claimed in claim 51, wherein the Raman oscillator distributed gain is provided by Raman amplification over a plurality of cascade orders and wherein the filter includes a band-splitting filter that separates the at least one cascade order from at least one other cascade order.

59. The optical amplifier as claimed in claim 58, wherein the band-splitting filter separates the pumping wavelength from the signal wavelength.

60. The optical amplifier as claimed in claim 51, wherein the filter includes a band-splitting filter that separates the at least one cascade order from at least one other cascade order.

61. The optical amplifier as claimed in claim 51, further comprising:

at least one of a splitter and circulator for splitting the pump wavelength from the signal wavelength outside the at least one laser cavity.

62. The optical amplifier as claimed in claim 61, wherein the coupler and the output port are combined outside the at least one laser cavity.

63. The optical amplifier as claimed in claim 51, wherein the at least one laser cavity is a linear laser cavity with first and second ends, and the second end is at least partially reflective.

64. The optical amplifier as claimed in claim 51, wherein the filter includes a coupler.

65. The optical amplifier as claimed in claim 51, further comprising:

a second coupler coupled to the coupler and the output port and having different periodicity wherein the pumping wavelength and the signal wavelength are separated by a number of cascade orders outside the at least one laser cavity.

66. The optical amplifier as claimed in claim 51, further comprising at least one grating positioned in the at least one laser cavity.

67. The optical amplifier as claimed in claim 66, further comprising a second coupler coupled to the coupler and the output port and having different periodicity wherein the pumping wavelength and the signal wavelength are separated by a number of cascade orders outside the at least one laser cavity.

68. The optical amplifier as claimed in claim 51, wherein the at least one laser cavity is a ring laser cavity closed on itself.

69. The optical amplifier as claimed in claim 68, wherein the filter includes an inline periodic filter.

70. The optical amplifier as claimed in claim 69, wherein the ring laser cavity has an arm and wherein the optical amplifier further comprises fine-tuning elements positioned in the arm of the ring laser cavity for fine frequency tuning the filtered optical signal.

71. The optical amplifier as claimed in claim 68, further comprising an isolator positioned in the ring laser cavity to make the ring laser cavity operate unidirectionally.

72. The optical amplifier as claimed in claim 51, wherein the filter includes a many-order periodic filter.

73. The optical amplifier as claimed in claim 51, further comprising at least one tuning element positioned in the at least one laser cavity for fine frequency tuning the filtered optical signal.

74. The optical amplifier as claimed in claim 73, wherein the at least one tuning element comprises an element selected from the group consisting of a grating, a Fabry-Perot interferometer, and a dielectric structure.

75. The optical amplifier as claimed in claim 51, wherein the oscillator distributed gain medium has a single spatial mode over the pumping wavelength to the signal wavelength.

76. The optical amplifier as claimed in claim 51, wherein the amplifier distributed gain medium is a gain fiber.

77. The optical amplifier as claimed in claim 76, wherein the gain fiber is pumped in a counter-propagating fashion by the Raman oscillator.

78. The optical amplifier as claimed in claim 76, wherein the optical amplifier is a distributed amplifier and wherein the at least one transmission link includes the gain fiber of the amplifier so that the distributed amplifier serves as a low-noise amplifier.

79. The optical amplifier as claimed in claim 51, wherein the optical amplifier is a discrete Raman amplifier.

80. The optical amplifier as claimed in claim 79, further comprising at least one isolator and wherein the discrete Raman amplifier is positioned within the at least one transmission link.

81. The optical amplifier of claim 51, wherein the filter comprises a sinusoidal filter function.

82. The optical amplifier of claim 51, wherein the at least one pass band comprises a bandwidth of at least ten (10) nanometers.

83. The optical amplifier of claim 51, wherein the filter comprises at least three (3) ports, and wherein at least two (2) of the at least three (3) ports are positioned within the at least one laser cavity.

84. The optical amplifier of claim 51, wherein the filter assembly comprises a plurality of coupled filters.

85. A Raman oscillator having high efficiency due to low intracavity loss, the oscillator comprising:

at least one laser cavity;

a distributed gain medium positioned in the at least one laser cavity;

a coupler adapted to be coupled to a pumping mechanism to pump the distributed gain medium at a pumping wavelength to obtain an optical signal wherein distributed gain is provided by Raman amplification over at least one cascade order corresponding to the pumping wavelength;

a multiple-order periodic filter positioned in the at least one cavity and having at least one pass band with a transmission peak placed approximately at the at least one cascade order to filter the optical signal to obtain a filtered optical signal having a signal wavelength, wherein the at least one pass band comprises a bandwidth of at least ten (10) nanometers; and

an output port for outputting the filtered optical signal.

86. The oscillator as claimed in claim 85, wherein the distributed gain medium is a distributed gain fiber.

87. The oscillator as claimed in claim 85, wherein the filter comprises a filter selected from the group consisting of an all-glass composition filter, a Mach-Zehnder filer, and a low-Q etalon filter.

88. The oscillator as claimed in claim 85, wherein the filter includes a fused fiber coupler.

89. The oscillator as claimed in claim 85, wherein the distributed gain is provided by Raman amplification over a plurality of Raman cascade orders including a final Raman order and wherein the filter has pass bands periodic in frequency with transmission peaks placed at the plurality of cascade orders or multiples of the cascade orders.

90. The oscillator as claimed in claim 89, wherein the transmission peaks placed at the cascade orders are separated by approximately 13.2 THz or some multiple of 13.2 THz or some submultiple of 13.2 THz.

91. The oscillator as claimed in claim 89, wherein the transmission peaks are separated by two cascade orders.

92. The oscillator as claimed in claim 85, wherein the distributed gain is provided by Raman amplification over a plurality of cascade orders and wherein the filter includes a band-splitting filter that separates the at least one cascade order from at least one other cascade order.

93. The oscillator as claimed in claim 92, wherein the band-splitting filter separates the pumping wavelength from the signal wavelength.

94. The oscillator as claimed in claim 85, wherein the filter includes a band-splitting filter that separates the at least one cascade order from at least one other cascade order.

95. The oscillator as claimed in claim 85, further comprising:

at least one of a splitter and circulator for splitting the pump wavelength from the signal wavelength outside the at least one laser cavity.

96. The oscillator as claimed in claim 95, wherein the coupler and the output port are combined outside the at least one laser cavity.

97. The oscillator as claimed in claim 85, wherein the at least one laser cavity is a linear laser cavity with first and second ends, and the second end is at least partially reflective.

98. The oscillator as claimed in claim 85, wherein the filter includes the coupler.

99. The oscillator as claimed in claim 85, further comprising:

a second coupler coupled to the coupler and the output port and having different periodicity wherein the pumping wavelength and the signal wavelength are separated by a number of cascade orders outside the at least one laser cavity.

100. The oscillator as claimed in claim 85, further comprising at least one grating positioned in the at least one laser cavity.

101. The oscillator as claimed in claims 100, further comprising a second coupler coupled to the coupler and the output port and having different periodicity wherein the pumping wavelength and the signal wavelength are separated by a number of cascade orders outside the at least one laser cavity.

102. The oscillator as claimed in claim 85, wherein the at least one laser cavity is a ring laser cavity closed upon itself.

103. The oscillator as claimed in claim 102, wherein the filter includes an inline periodic filter.

104. The oscillator as claimed in claim 103, wherein the ring laser cavity has an arm and wherein the oscillator further comprises fine-tuning elements positioned in the arm of the ring laser cavity for fine frequency tuning the filtered optical signal.

105. The oscillator as claimed in claim 102, further comprising an isolator positioned in the ring laser cavity to make the ring laser cavity operate unidirectionally.

106. The oscillator as claimed in claim 85, further comprising at least one tuning element positioned in the at least one laser cavity for fine frequency tuning the filtered optical signal.

107. The oscillator as claimed in claim 94, wherein the at least one tuning element comprises an element selected from the group consisting of a grating, a Fabry-Perot interferometer, and a dielectric structure.

108. The oscillator as claimed in claim 85, wherein the distributed gain medium has a single spatial mode over the pumping wavelength to the signal wavelength.

109. The oscillator of claim 85, wherein the bandwidth of the at least one pass band is measured at a half-power power point of the pass band.

110. In a fiber optic transmission system including at least one transmission link, an optical amplifier comprising:

an amplifier input port for receiving an input optical signal;

an amplifier distributed gain medium connected to the amplifier input port to amplify the input optical signal;

a Raman oscillator operable to pump the amplifier distributed gain medium at a pumping level sufficiently high so that the input optical signal experiences a gain the Raman oscillator comprising;

at least one laser cavity;

an oscillator distributed gain medium positioned in the at least one laser cavity;

a coupler adapted to be coupled to a pumping mechanism to pump the oscillator distributed gain medium at a pumping wavelength to obtain an oscillator optical signal wherein distributed gain is provided by Raman amplification over at least one cascade order corresponding to the pumping wavelength;

a multiple-order periodic filter positioned in the at least one cavity and having at least one pass band with a transmission peak placed approximately at the at least one cascade order to filter the oscillator optical signal to obtain a filtered oscillator optical signal having a signal wavelength, wherein the at least one pass band comprises a bandwidth of at least ten (10) nanometers;

an output port for outputting the filtered oscillator optical signal; and

an amplifier output port for outputting an amplified input optical signal.

111. The amplifier as claimed in claim 110, wherein the amplifier distributed gain medium is a gain fiber.

112. The amplifier as claimed in claim 111, wherein the gain fiber is pumped in a counter-propagating fashion by the Raman oscillator.

113. The amplifier as claimed in claim 110, wherein the optical amplifier is a discrete Raman amplifier.

114. The amplifier as claimed in claim 113, further comprising at least one isolator and wherein the discrete Raman amplifier is positioned within the at least one transmission link.

115. The amplifier as claimed in claim 110, wherein the optical amplifier is a distributed amplifier and wherein the at least one transmission link includes the amplifier distributed gain medium so that the distributed amplifier serves as a low-noise amplifier.

116. In a fiber optic transmission system, a discrete optical amplifier comprising:

an amplifier input port for receiving an input optical signal;

an amplifier distributed gain medium coupled to the amplifier input port and operable to amplify the input optical signal;

a Raman oscillator to pump the amplifier distributed gain medium, the Raman oscillator comprising;

at least one laser cavity;

a distributed gain medium positioned in the at least one laser cavity;

a coupler adapted to be coupled to a pumping mechanism to pump the distributed gain medium at a pumping wavelength to obtain an optical signal wherein distributed gain is provided by Raman amplification over at least one cascade order corresponding to the pumping wavelength;

a multiple-order periodic filter comprising at least three (3) ports with at least two (2) of the at least three (3) ports positioned in the at least one cavity and having at least one pass band with a transmission peak placed approximately at the at least one cascade order to filter the optical signal to obtain a filtered optical signal having a signal wavelength; and

an amplifier output port for outputting an amplified input optical signal.

117. A Raman oscillator having high efficiency due to low intracavity loss, the oscillator comprising:

at least one laser cavity;

a distributed gain medium positioned in the at least one laser cavity;

a coupler adapted to be coupled to a pumping mechanism to pump the distributed gain medium at a pumping wavelength to obtain an optical signal wherein distributed gain is provided by Raman amplification over at least one cascade order corresponding to the pumping wavelength;

an optical filter comprising at least three (3) ports with at least 2 of the at least three (3) ports positioned in the at least one laser cavity and having at least one pass band with a transmission peak placed approximately at the at least one cascade order to filter the optical signal to obtain a filtered optical signal having a signal wavelength; and

an output port for outputting the filtered optical signal.

118. The oscillator as claimed in claim 117, wherein the distributed gain medium is a distributed gain fiber.

119. A Raman oscillator having high efficiency due to low intracavity loss, the oscillator comprising:

at least one laser cavity;

a distributed gain fiber positioned in the at least one laser cavity, the distributed gain fiber having a single spatial mode over a pumping wavelength to a signal wavelength;

a coupler adapted to be coupled to a pumping mechanism to pump the distributed gain fiber at the pumping wavelength to obtain an optical signal wherein distributed gain is provided by Raman amplification over at least one cascade order corresponding to the pumping wavelength;

a filter positioned in the at least one laser cavity and having at least one pass band with a transmission peak placed approximately at the at least one cascade order to filter the optical signal to obtain a filtered optical signal having the signal wavelength, wherein the at least one pass band comprises a bandwidth of at least ten (10) nanometers; and

an output port for outputting the filtered optical signal.