Optical communication systems using raman repeaters and components therefor

• US 4,616,898 A
• Filed: 09/28/1983
• Issued: 10/14/1986
• Est. Priority Date: 03/31/1980
• Status: Expired due to Term

• Current Assignee: JDS Uniphase Corporation (Lumentum Holdings Inc.)
• Original Assignee: Polaroid Corporation (PLR IP Holdings LLC)
• Inventors: Hicks, John W. Jr.
• Primary Examiner(s): Sikes, William L.
• Assistant Examiner(s): Gonzalez, Frank
• Application Number:
  US06/536,655
• Time in Patent Office: 1,112 Days
• Field of Search: 350/96.15, 350/96.16, 350/96.19, 350/96.20, 350/96.29, 350/96.30, 350/96.32, 350/96.33, 372/6, 370/1, 370/3, 370/4, 455/601, 455/610, 455/611, 455/612
• US Class Current: 385/24
• CPC Class Codes: G02B 6/2821 using lateral coupling between contiguous fibres to split or combine optical signals
• CPC Class Codes: G02B 6/2835 formed or shaped by thermal treatment, e.g. couplers
• CPC Class Codes: G02B 6/29332 Wavelength selective couplers, i.e. based on evanescent coupling between light guides, e.g. fused fibre couplers with transverse coupling between fibres having different propagation constant wavelength dependency non wavelength-selective light guide couplers G02B6/28
• CPC Class Codes: G02B 6/29337 Cavities of the linear kind, e.g. formed by reflectors at ends of a light guide
• CPC Class Codes: G02B 6/29358 Multiple beam interferometer external to a light guide, e.g. Fabry-Pérot, etalon, VIPA plate, OTDL plate, continuous interferometer, parallel plate resonator G02B6/29347, G02B6/29349, G02B6/2935, G02B6/29361 take precedence; resonator evanescently coupled to light guide G02B6/29335
• CPC Class Codes: G02B 6/2938 for multiplexing or demultiplexing, i.e. combining or separating wavelengths, e.g. 1xN, NxM
• CPC Class Codes: H01S 3/094076 Pulsed or modulated pumping H01S3/1024 takes precedence
• CPC Class Codes: H01S 3/09408 Pump redundancy
• CPC Class Codes: H01S 3/302 in an optical fibre
• CPC Class Codes: H01S 5/0607 by varying physical parameters other than the potential of the electrodes, e.g. by an electric or magnetic field, mechanical deformation, pressure, light, temperature
• CPC Class Codes: H01S 5/0622 Controlling the frequency of the radiation
• CPC Class Codes: H01S 5/50 Amplifier structures not provided for in groups H01S5/02 - H01S5/30
• CPC Class Codes: H04B 10/2916 using Raman or Brillouin amplifiers

Abstract

An optical fiber communication system for transmitting information-bearing optical signals over an optical fiber transmission line includes a signal generator coupled to the transmission line for introducing information-bearing optical signals thereonto, preferably as wavelength modulated signals, and one or more optical amplifiers for periodically, collectively amplifying the transmitted signals to overcome the effects of attenuation. Wavelength modulation is achieved by altering the resonant characteristics of an optical resonator, optically coupled to a gain medium so as to provide an output that varies between a first and second wavelength. The optical amplification is achieved by injecting, preferably via lateral optical fiber couplings, pumping beams into the optical fiber so as to effect an increase in the amplitude of the information-bearing signals by stimulated Raman scattering.

Claims

1. An optical fiber communication system, said system comprising:

an optical fiber transmission line including a core fabricated of a given material;

means, coupled to said optical fiber transmission line, for introducing a plurality of information-bearing optical signals thereonto for transmission therealong, each of said information-bearing optical signals being carried in a distinct channel of predetermined wavelength; and

means coupled to said optical fiber transmission line at a predetermined location, downline of said means for introducing information-bearing optical signals thereonto, for effecting collective optical amplification of all of said information-bearing optical signals, said optical amplification effecting means comprising means for introducing optical energy into said optical fiber transmission line at a plurality of preselected wavelengths relative to the wavelengths of said information-bearing optical signals to collectively amplify all of said information-bearing optical signals by stimulated Raman scattering, said plurality of said preselected wavelengths of said optical energy including an initial wavelength and other wavelengths separated from each other and said initial wavelength wherein adjacent wavelengths thereof are separated in wavenumber by an amount sufficient to minimize the effects of any Brillouin backscattering tending to interfere with said information-bearing optical signals.

2. The system of claim 1 wherein said adjacent wavelengths are separated in wavenumber by an amount equal to or greater than the wavenumber difference corresponding to the region at which Brillouin backscattering occurs for said core material.

3. The system of claim 2 wherein said adjacent wavelengths are separated by at least 1.0 Angstrom.

4. The system of claim 3 wherein said core material comprises fused silica or doped fused silica.

5. The system of claim 2 wherein said optical energy at said initial wavelength and at said other wavelengths is provided by separate sources each having a wavelength corresponding to each of the wavelengths of said additional optical energy.

6. The system of claim 1 wherein said adjacent wavelengths are separated by an amount sufficient to substantially broaden the bandwidth over which said channels can be uniformly amplified whereby more channels than otherwise can be collectively amplified on said system.

7. The system of claim 6 wherein said core material has a characteristic Raman gain curve which varies as a function of the difference in wavenumber between that of said optical energy and the wavenumbers corresponding to said information-bearing optical signals, peaks at a predetermined wavenumber difference between one of the wavelengths of said optical energy and the wavelength of a selected one of said channels carrying said information-bearing optical signals, gradually decreases on either side of said peak gain, and has a range in wavenumber difference that encompasses all of said channels and wherein said adajacent wavelengths of said optical energy are separated by no more than the range of wavenumber difference under said Raman gain curve corresponding to plus or minus 25 percent of said peak gain of said Raman gain curve.

8. The system of claim 1 wherein all of said wavelengths of said optical energy are provided by a single source modulated to operate at all of said wavelengths of said optical energy.

9. An optical fiber communication system, said system comprising:

an optical fiber transmission line including a core fabricated of a given material;

means coupled to said optical fiber transmission line for introducing a plurality of information-bearing optical signals thereonto for transmission therealong, each of said information-bearing optical signals being carried on a distinct channel of predetermined wavelength; and

means laterally coupled to said optical fiber transmission line at a predetermined location, downline of said means for introducing said information-bearing optical signals thereonto, for effecting collective optical amplification of all of said information-bearing optical signals, said amplification effecting means comprising at least one non-rotationally symmetric optical fiber tap for introducing optical energy onto said optical fiber transmission line at a preselected wavelength relative to the wavelengths of said information-bearing optical singals to collectively amplify all of said information-bearing optical signals by stimulated Raman scattering.

10. The system of claim 9 wherein said means for introducing optical energy onto said optical fiber transmission line includes at least one laser diode as an optical energy source.

11. The system of claim 9 where said means for introducing optical energy onto said optical fiber transmission line comprises a first and at least one other laser diode having spectral outputs of different wavelengths.

12. The system of claim 11 further comprising means coupled to said optical fiber transmission line for removing power level information therefrom and coupled to said first and at least one other laser diode for controlling the power level outputs thereof in a manner responsive to the power level information removed from said transmission line.

13. The system of claim 12 wherein said means for controlling the power level output of said first and at least one other laser diode comprises:

means for removing optical energy from the transmission line;

means coupled to said means for removing energy from the transmission line for detecting the level of energy removed therefrom; and

means connected between said detector means and said first and at least one other laser diode to control the power outputs thereof in a manner responsive to the optical energy removed from said transmission line.

14. The system of claim 12 further comprising a wideband filter inserted upline of said means for removing optical energy from said optical fiber transmission line.

15. The system of claim 9 further comprising a first laser diode of given wavelength and a first lateral coupler, said first laser being coupled to said optical fiber tap via said first lateral couple, said system further comprising at least one other laser diode of longer wavelength than that of said first laser diode and at least one other lateral coupler, said at least one other laser diode being laterally coupled to said optical fiber tap via said at least one other lateral coupler.

16. The system of claim 15 wherein said respective lateral couplings of said first and other laser diodes are wavelength selective to prevent optical energy from one of said laser diodes from entering the other of said laser diodes.

17. The system of claim 16 wherein the optical power output of said longer wavelength laser diode is adjusted to compensate for a gain increase caused by Raman amplification effected by the output of said given wavelength laser diode.

18. The system of claim 15 further comprising means coupled to said optical fiber transmission line for removing power level information therefrom, and for conveying a signal representative of said removed power level information through said optical fiber transmission line to said first and at least one other laser diode for controlling the respective power outputs thereof in a manner responsive to the power level information removed from said transmission line.

19. An optical fiber communication system, said system comprising:

an optical fiber transmission line;

means coupled to said optical fiber transmission line for introducing a plurality of information-bearing optical signals thereonto for transmission therealong, each of said information-bearing optical signals being carried on a distinct channel of predetermined wavelength; and

means coupled to said optical fiber transmission line, downline of said means for introducing said information-bearing optical signals thereonto, for effecting collective optical amplification of all of said information-bearing optical signals, said means for effecting optical amplification comprising means for introducing optical energy onto said optical fiber transmission line at a preselected wavelength relative to the wavelengths of said information-bearing optical signals to collectively amplify all of said information-bearing optical signals by stimulated Raman scattering, said means for introducing optical energy into said optical fiber transmission line comprising a gain medium, a Fabry-Perot cavity, and an optical fiber coupling line wherein said gain medium is coupled to said Fabry-Perot cavity via said optical fiber coupling line.

20. The system of claim 19 wherein said coupling line is laterally coupled to said Fabry-Perot cavity.

21. The system of claim 20 wherein said gain medium and Fabry-Perot cavity oscillate to provide an output having multiple wavelengths.

22. An optical fiber communication system, said system comprising:

an optical fiber transmission line;

means coupled to said optical fiber transmission line for introducing a plurality of information-bearing optical signals thereonto for transmission therealong, said means for introducing information-bearing optical signals encoding information in the form of pulses having a given pulse width and wherein each of said information-bearing optical signals is carried on a distinct channel of predetermined wavelength; and

means coupled to said optical fiber transmission line, downline of said means for introducing information-bearing optical signals, for effecting collective optical amplification of all of said information-bearing optical signals, said amplification effecting means comprising a plurality of pump sources for introducing optical energy onto said optical fiber transmission line at preselected wavelengths relative to the wavelengths of said information-bearing optical signals to collectively amplify all of said information-bearing optical signals by stimulated Raman scattering, the number of said pump sources being at least equal to the number of photons in an information pulse prior to amplification divided by 20 times the number of electromagnetic states within said given pulse width.

23. An optical fiber communication system, said system comprising:

an optical fiber transmission line;

means coupled to said optical fiber transmission line for introducing a plurality of information-bearing optical signals thereonto for transmission therealong, each of said information-bearing optical signals being carried on a distinct channel of predetermined wavelength, said means for introducing information-bearing optical signals comprising at least one signal generator including;

(a) an elongated gain medium;

(b) means for introducing energy into said gain medium to cause a population inversion of its atomic population such that said gain medium operates to spontaneously emit optical energy of different wavelengths in a random manner characteristic of its material composition; and

(c) means external of said gain medium and optically coupled thereto for receiving optical energy therefrom, oscillating said energy at a preselected wavelength and feeding said energy at said preselected wavelength back through said medium for amplification thereby by stimulated emission whereby said signal generator operates to provide as an output a coherent electromagnetic wave at said preselected wavelength, said external means including a resonant fiber cavity structured to oscillate optical energy at said preselected wavelength and being substantially isolated from said gain medium to be relatively insensitive to conditions within said gain medium which would otherwise destabilize the wavelength output of said signal generator, and means coupled to said optical fiber transmission line at a predetermined location, downline of said means for introducing information-bearing optical signals thereonto, for effecting collective optical amplification of all of said information-bearing optical signals.

24. The system of claim 23 wherein said external means includes a mirror on one end of said gain medium.

25. The system of claim 23 wherein said external means includes a transfer fiber segment coupled to one end of said gain medium to receive optical energy therefrom and wherein said resonant fiber cavity is coupled to said transfer fiber segment.

26. The system of claim 25 wherein said resonant fiber cavity is a Fabry-Perot cavity laterally coupled to said transfer fiber segment.

27. The system of claim 26 wherein a portion of said resonant fiber cavity is laterally coupled to said fiber transmission line to transfer said signal generator output thereonto.

28. The system of claim 23 wherein said signal generator includes means for changing the characteristics of said external means to provide said output with at least two distinct wavelength outputs.

29. The system of claim 23 wherein said external means includes means for suppressing unwanted resonant orders of said resonant fiber cavity.

30. The system of claim 29 wherein said means for suppressing unwanted resonant orders comprises a second resonant fiber cavity coupled to said gain medium at an end thereof opposite the end to which said first-mentioned resonant fiber cavity is coupled thereto.

31. The system of claim 23 wherein said external means includes a transfer fiber coupled to one end of said gain medium to receive optical energy therefrom wherein said resonant fiber cavity is coupled to said transfer fiber, a second transfer fiber coupled to the other end of said gain medium to receive optical energy therefrom, and a second resonant fiber cavity coupled to said second transfer fiber.

32. The system of claim 31 wherein both said first and second resonant fiber cavities are Fabry-Perot resonant cavities.

33. The system of claim 32 wherein said first and second resonant fiber cavities are structured so that their frequency responses are coincident at a single dominant wavelength.

34. The system of claim 32 wherein said first resonant fiber cavity includes means for resonating at two desired wavelengths and wherein said second resonant fiber cavity has a finesse such that both of said resonant wavelengths of said first resonant fiber cavity lie within a common resonant line width thereof.

35. The system of claim 23 wherein said signal generator includes means for modulating its output so that said output has at least two distinct wavelengths.

36. The system of claim 35 wherein said modulating means includes a piezoelectric crystal mechanically coupled to said resonant fiber cavity.

37. An optical fiber communication system, said system comprising:

an optical fiber transmission line;

means coupled to said optical fiber transmission line for introducing a plurality of information-bearing optical signals thereonto for transmission therealong, each of said information-bearing optical signals being carried on a distinct channel of predetermined wavelength, said means for introducing information-bearing optical signals comprising at least one signal generator including;

(a) a light gain element including an elongated gain medium having a mirror at one end and means for energizing said gain medium;

(b) a transfer optical fiber coupled to the other end of said gain medium;

(c) an optical fiber Fabry-Perot resonant cavity laterally coupled to said transfer optical fiber and a portion of said optical fiber transmission line, said optical fiber Fabry-Perot resonant cavity being structured and arranged with respect to said transfer fiber and said gain medium to provide feedback to said gain medium via said transfer fiber so that said signal generator lases to provide as an output a coherent electromagnetic wave of predetermined wavelength and bandwidth and wherein said optical fiber Fabry-Perot resonant cavity is structured and arranged with respect to said portion of said fiber transmission line so that said output is coupled onto said fiber transmission line; and

means coupled to said optical fiber transmission line at a predetermined location, downline of said information-bearing optical signal introducing means, for effecting collective optical amplification of all of said information-bearing optical signals.

38. An amplifier apparatus for use in an optical fiber transmission system in which a plurality of information-bearing optical signals are each carried on an optical fiber transmission line in a distinct channel of predetermined wavelength and wherein the optical fiber transmission line has a core composed of a given material, said amplifier apparatus comprising:

means adapted to be coupled to said coupled fiber transmission line at a predetermined location, downline of the location at which the information-bearing optical signals are introduced thereonto, for effecting collective optical amplification of all of said information-bearing optical signals, said optical amplification effecting means comprising means for introducing optical energy into said optical fiber transmission line at a plurality of preselected wavelengths relative to the wavelengths of said information-bearing optical signals to collectively amplify all of said information-bearing optical signals, said plurality of said preselected wavelengths of said optical energy including an initial wavelength and other wavelengths separated from each other and said initial wavelength wherein adjacent wavelengths thereof are separated in wavenumber by an amount sufficient to minimize the effects of any Brillouin backscattering tending to interfere with said information-bearing optical signals.

39. The apparatus of claim 38 wherein said adjacent wavelengths are separated in wavenumber by an amount equal to or greater than the wavenumber difference corresponding to the region at which Brillouin backscattering occurs for said core material.

40. The apparatus of claim 39 wherein said adjacent wavelengths are separated by at least 1.0 Angstrom.

41. The apparatus of claim 39 wherein said optical energy at said initial wavelength and at said other wavelengths is provided by separate sources each having a wavelength corresponding to each of the wavelengths of said additional optical energy.

42. The apparatus of claim 40 wherein said core material comprises fused silica or doped fused silica.

43. The apparatus of claim 38 wherein said adjacent wavelengths are separated by an amount sufficient to substantially broaden the bandwidth over which the channels can be uniformly amplified whereby more channels than otherwise can be collectively amplified on the system.

44. The apparatus of claim 43 wherein said core material has a characteristic Raman gain curve which varies as a function of the difference in wavenumber between that of said optical energy and the wavenumbers corresponding to said information-bearing optical signals, peaks at a predetermined wavenumber difference between one of the wavelengths of said optical energy and the wavelength of a selected one of said channels carrying said information-bearing optical signals, gradually decrease on either side of said peak gain, and has a range in wavenumber difference that encompasses all of said channels and wherein said adjacent wavelengths of said optical energy are separated by no more than the range of wavenumber difference under said Raman gain curve corresponding to plus or minus 25 percent of said peak gain of said Raman gain curve.

45. The apparatus of claim 38 wherein all of said wavelengths of said optical energy are provided by a single source modulated to operate at all of said wavelengths of said optical energy.

46. An amplifier apparatus for use in an optical fiber transmission system in which a plurality of information-bearing optical signals are each carried on an optical fiber transmission line on a distinct channel of predetermined wavelength and wherein said optical fiber transmission line includes a core fabricated of a given material, said apparatus comprising:

means laterally coupled to said optical fiber transmission line at a predetermined location, downline of the location where information-bearing optical signals are introduced thereonto, for effecting collective optical amplification of all of said information-bearing optical signals, said amplification effecting means comprising at least one non-rotationally symmetric optical fiber tap for introducing optical energy onto said optical fiber transmission line at a preselected wavelength relative to the wavelengths of said information-bearing optical signals to collectively amplify all of said information-bearing optical signals by stimulated Raman scattering.

47. The apparatus of claim 46 wherein said means for introducing optical energy onto said optical fiber transmission line includes at least one laser diode as an optical energy source.

48. The apparatus of claim 46 where said means for introducing optical energy onto said optical fiber transmission line comprises a first and at least one other laser diode having spectral outputs of different wavelengths.

49. The apparatus of claim 48 further comprising means structured for coupling to the optical fiber transmission line for removing power level information therefrom and coupled to said first and at least one other laser diode for controlling the power level outputs thereof in a manner responsive to the power level information removed from the optical fiber transmission line.

50. The apparatus of claim 49 wherein said means for controlling the power level outputs of said first and at least one other laser diode comprises:

means structured for removing optical energy from the transmission line;

means coupled to said means for removing energy from the transmission line for detecting the level of energy removed therefrom; and

means connected between said detector means and said first and at least one other laser diode to control the power outputs thereof in a manner responsive to the optical energy removed from the transmission line.

51. The apparatus of claim 49 further comprising a wideband filter structured for insertion upline of said means for removing optical energy from the optical fiber transmission line.

52. The apparatus of claim 46 further comprising a first laser diode of given wavelength and a first lateral coupler, said first laser diode being coupled to said optical fiber tap via said first lateral couple, said apparatus further comprising at least one other laser diode of longer wavelength than that of said first laser diode and at least one other lateral coupler, said at least one other laser diode being laterally coupled to said optical fiber tap via said at least one other lateral coupler.

53. The apparatus of claim 52 wherein said respective lateral couplings of said first and other laser diodes are wavelength selective to prevent optical energy from one of said laser diodes from entering the other of said laser diodes.

54. The apparatus of claim 53 wherein the optical power output of said longer wavelength laser diode is adjusted to compensate for a gain increase caused by Raman amplification effected by the output of said given wavelength laser diode.

55. The apparatus of claim 52 further comprising means structured for coupling to the optical fiber transmission line for removing power level information therefrom, and for conveying a signal representative of said removed power level information through the optical fiber transmission line to said first and at least one other laser diode for controlling the respective power outputs thereof in a manner responsive to the power level information removed from the optical fiber transmission line.

56. An amplifier apparatus for use in an optical fiber transmission system in which a pluraltiy of information-bearing optical signals are each carried on an optical fiber transmission line on a distinct channel of predetermined wavelength and wherein said optical fiber transmission line includes a core fabricated of a given material, said apparatus comprising:

means optically coupled to said optical fiber transmission line, downline of the location where said information-bearing optical signals are introduced thereonto, for effecting collective optical amplification of all of said information-bearing optical signals, said means for effecting optical amplification comprising means for introducing optical energy onto said optical fiber transmission line at a preselected wavelength relative to the wavelengths of said information-bearing optical signals to collectively amplify all of said information-bearing optical signals by stimulated Raman scattering, said means for introducing optical energy into said optical fiber transmission line comprising a gain medium, a Fabry-Perot cavity, and an optical fiber coupling line wherein said gain medium is coupled to said Fabry-Perot cavity via said optical fiber coupling line.

57. The apparatus of claim 56 wherein said coupling line is laterally coupled to said Fabry-Perot cavity.

58. The apparatus of claim 57 wherein said gain medium and said Fabry-Perot cavity oscillate to provide an output having multiple wavelengths.

59. An amplifier apparatus for use in an optical fiber transmission system in which a plurality of information-bearing optical signals are each carried in an optical fiber transmission line in a distinct channel of predetermined wavelength and wherein the information contained in said signals is encoded therein in the form of pulses having a given pulse width, said apparatus comprising:

means optically coupled to said optical fiber transmission line, downline of the location where the information-bearing optical signals are introduced thereonto, for effecting collective optical amplification of all of said information-bearing optical signals, said amplification effecting means comprising a plurality of pump sources for introducing optical energy onto said optical fiber transmission line at preselected wavelengths relative to the wavelengths of said information-bearing optical signals to collectively amplify all of said information-bearing optical signals by stimulated Raman scattering, the number of said pump sources being at least equal to the number of photons in an information pulse prior to amplification divided by 20 times the number of electromagnetic states within said given pulse width.

60. An optical signal generator comprising:

(a) an elongated gain medium;

(b) means for introducing energy into said gain medium to cause a population inversion of its atomic population such that said gain medium operates to spontaneously emit optical energy of different wavelengths in a random manner characteristic of its material composition; and

(c) means external of said gain medium and optically coupled thereto for receiving optical energy therefrom, oscillating said energy at a preselected wavelength and feeding said energy at said preselected wavelength back through said medium for amplification thereby by stimulated emission whereby said signal generator operates to provide as an output a coherent electromagnetic wave at said preselected wavelength, said external means including a resonant fiber cavity structured to oscillate optical energy at said preselected wavelength and being substantially isolated from said gain medium to be relatively insensitive to conditions within said gain medium which would otherwise destabilize the wavelength output of said signal generator, said external means further including a transfer fiber segment coupled to one end of said gain medium to receive optical energy therefrom and wherein said resonant fiber cavity is laterally coupled to said transfer fiber segment.

61. The signal generator of claim 60 wherein said resonant fiber cavity is a Fabry-Perot cavity laterally coupled to said transfer fiber segment.

62. The signal generator of claim 61 wherein a portion of said resonant fiber cavity is adapted to be laterally coupled to a fiber transmission line to transfer said signal generator output thereonto.

63. An optical signal generator comprising:

(a) an elongated gain medium;

(b) means for introducing energy into said gain medium to cause a population inversion of its atomic population such that said gain medium operates to spontaneously emit optical energy of different wavelengths in a random manner characteristic of its material composition; and

(c) means external of said gain medium and optically coupled thereto for receiving optical energy therefrom, oscillating said energy at a preselected wavelength and feeding said energy at said preselected wavelength back through said medium for amplification thereby by stimulated emission whereby said signal generator operates to provide as an output a coherent electromagnetic wave at said preselected wavelength, said external means including a resonant fiber cavity structured to oscillate optical energy at said preselected wavelength and being substantially isolated from said gain medium to be relatively insensitive to conditions within said gain medium which would otherwise destabilize the wavelength output of said signal generator, said external means further including a transfer fiber coupled to one end of said gain medium to receive optical energy therefrom wherein said resonant fiber cavity is coupled to said transfer fiber, a second transfer fiber coupled to the other end of said gain medium to receive optical energy therefrom, and a second resonant fiber cavity coupled to said second transfer fiber.

64. The signal generator of claim 63 wherein both said first and second resonant fiber cavities are Fabry-Perot resonant cavities.

65. The signal generator of claim 64 wherein said first and second resonant fiber cavities are structured so that their frequency responses are coincident at a single dominant wavelength.

66. The signal generator of claim 64 wherein said first resonant fiber cavity includes means for resonating at two desired wavelengths and wherein said second resonant fiber cavity has a finesse such that both of said resonant wavelengths of said first resonant fiber cavity lie within a common resonant line width thereof.

67. An optical signal generator comprising:

(a) an elongated gain medium;

(b) means for introducing energy into said gain medium to cause a population inversion of its atomic population such that said gain medium operates to spontaneously emit optical energy of different wavelengths in a random manner characteristic of its material composition; and

(c) means external of said gain medium and optically coupled thereto for receiving optical energy therefrom, oscillating said energy at a preselected wavelength and feeding said energy at said preselected wavelength back through said medium for amplification thereby by stimulated emission whereby said signal generator operates to provide as an output a coherent electromagnetic wave at said preselected wavelength said external means including a resonant fiber cavity structured to oscillate optical energy at said preselected wavelength and being substantially isolated from said gain medium to be relatively insensitive to conditions within said gain medium which would otherwise destabilize the wavelength output of said signal generator, and means for modulating output of said signal generator so that said output has at least two distinct wavelengths, said modulating means including a piezoelectric crystal mechanically coupled to said resonant fiber cavity.

68. An optical signal generator for use with an optical fiber transmission line, said signal generator comprising:

(a) a light gain element including an elongated gain medium having a mirror at one end and means for energizing said gain medium;

(b) a transfer optical fiber coupled to the other end of said gain medium; and

(c) an optical fiber Fabry-Perot resonant cavity laterally coupled to said transfer optical fiber and structured for coupling with a portion of an optical fiber transmission line, said optical Fabry-Perot resonant cavity fiber being structured and arranged with respect to said transfer fiber and said gain medium to provide feedback to said gain medium via said transfer fiber so that said signal generator lases to provide as an output a coherent electromagnetic wave of predetermined wavelength and bandwidth and wherein said optical fiber Fabry-Perot resonant cavity is structured and arranged to be coupled to a portion of the fiber transmission line to introduce said output onto said fiber transmission line.

69. A single mode optical fiber for use in optical fiber communication system, said optical fiber comprising:

a core composed of an optically transparent material of given index of refraction and having a predetermined thermal coefficient of expansion; and

a cladding formed of a material of lower index than that of said core material, said cladding material being bound to said core only partially about the circumference of said core such that a pair of spaced apart air cavities are formed on opposite portions of said core between said unbound portions of said core and said cladding, said cladding material having a coefficient of thermal expansion different from that of said core so that said cladding exerts a permanent birefringent stress on said core along a principal axis through said core and said cladding whereby said core propagates only one polarization mode parallel to said axis, said core, said cladding, and said cavities being all generally rectangular in cross-sectional shape and wherein said core is offset to one side of the geometric center of said cladding in one direction and centered in said cladding in a direction perpendicular to said one direction to provide said optical fiber with a non-rotationally symmetric geometry to enhance lateral coupling.

70. An optical fiber communications system comprising multiple channel signal generation means operative to encode information in the form of optical pulses having a given pulse width;

a single mode optical fiber transmission line coupled to said signal generation means; and

one or more Raman amplifier means located along said transmission line wherein the number of Raman amplifier means is at least equal to the number of photons in a pulse divided by 20 times the number of states for said pulse width.

71. The system of claim 70 wherein said Raman amplifier means comprise means for producing a pump beam and means for combining said pump beam with a signal beam along a common path.

72. An optical fiber communications system comprising multiple channel signal generation means;

a single mode fiber optical transmission line coupled to said signal generation means; and

one or more Raman amplifier means located along said optical fiber transmission line wherein each amplifier means comprises a laser diode having two separate spectral line outputs which form a pump beam having multiple spectral characteristics and wherein each amplifier means includes means for combining pump beams thereof with a signal beam as it travels down a common path of said optical fiber transmission line, said means for combining each of said pump beams with said signal beam comprising a non-rotationally symmetric fiber coupled to a respective one of said laser diodes and laterally coupled with a selected portion of said optical fiber transmission line.

73. An optical fiber communications system comprising multiple channel signal generation means;

a single mode optical fiber transmission line coupled to said signal generation means; and

one or more Raman amplifier means located along said optical fiber transmission line, said signal generation means comprising a laser diode coupled to a Fabry-Perot modulator and including means for unwanted resonant order suppression of said Fabry-Perot modulator wherein said suppression means comprises a second Fabry-Perot device coupled to said laser diode of said signal generation means.

74. The system of claim 73 wherein said Raman amplifier means comprise means for producing a pump beam and means for combining said pump beam with a signal beam along a common path.

75. The system of claim 74 wherein said means for combining said pump beam and signal beam comprises a portion of said transmission line.

76. The system of claim 74 wherein said means for combining said pump beam with said signal beam includes means for laterally coupling said pump beam onto said transmission line.

77. An optical fiber communications system comprising multiple channel signal generation means;

a single mode fiber transmission line coupled to said signal generation means; and

one or more Raman amplifier means located along said transmission line wherein said signal generation means comprises a laser diode coupled to first and second Fabry-Perot modulator devices and is a pulse wavelength shift generator, said second Fabry-Perot device having a wider resonant line structure than that of said first Fabry-Perot device to allow resonance of the generator to be obtained with respect to only one order of wavelength common to both Fabry-Perot devices.

78. The system of claim 77 wherein said first Fabry-Perot device includes means for producing an output at two desired wavelengths and said second Fabry-Perot device has a finesse such that both said wavelengths lie within a common resonant line width.

79. The system of claim 78 wherein said Raman amplifier means comprise means for producing a pump beam and means for combining said pump beam with a signal beam along a common path.

80. The system of claim 79 wherein said means for combining said pump beam and signal beam comprises a portion of said transmission line.

81. The system of claim 79 wherein said means for combining said pump beam with said signal beam includes means for laterally coupling said pump beam onto said transmission line.

82. An optical fiber communications system comprising multiple channel signal generation means;

a single mode optical fiber transmission line coupled to said signal generation means; and

one or more Raman amplifier means located along said transmission line and wherein said signal generation means comprises a laser diode coupled to first and second Fabry-Perot modulator devices and is an amplitude modulated pulse generator, said second Fabry-Perot device having a wider resonant line structure than that of said first Fabry-Perot device to allow only one order of wavelength common to both Fabry-Perot devices, said first Fabry-Perot device including means for producing an output at two desired wavelengths, said second Fabry-Perot device having a finesse such that both said wavelengths lie within a common resonant linewidth, and a third Fabry-Perot device coupled to said first Fabry-Perot device for filtering out one of said two desired wavelengths.

83. The system of claim 82 wherein said Raman amplifier means comprise means for producing a pump beam and means for combining said pump beam with a signal beam along a common path.

84. The system of claim 83 wherein said means for combining said pump beam and signal beam comprises a portion of said transmission line.

85. The system of claim 83 wherein said means for combining said pump beam with said signal beam includes means for laterally coupling said pump beam onto said transmission line.

86. An optical fiber communications system comprising multiple channel signal generation means;

a single mode fiber transmission line coupled to said signal generation means; and

one or more Raman amplifier means located along said transmission line and wherein said Raman amplifiers comprise a laser diode and said signal generation means comprises means for generating a double line and modulator means for said line generator wherein said double line generator includes a Fabry-Perot device.

87. The system of claim 86 wherein said double line generator comprises means for modulating said laser diode at a wavelength equal to one-half the wavelength shift desired and said Fabry-Perot device has a split resonant line characteristic.

88. The system of claim 86 wherein said Fabry-Perot device has non-split resonant lines and said double line generator comprises means for modulating said Fabry-Perot device with a sine wave of a frequency equal to one-half the wavelength shift required.

89. The system of claim 86 wherein said modulator means comprises a piezoelectric cyrstal.

90. The system of claim 86 wherein said Raman amplifier means comprise means for producing a pump beam and means for combining said pump beam with a signal beam along a common path.

91. The system of claim 90 wherein said means for combining said pump beam and signal beam comprises a portion of said transmission line.

92. The system of claim 90 wherein said means for combining said pump beam with said signal beam includes means for laterally coupling said pump beam onto said transmission line.

93. An optical fiber communications system comprising multiple channel signal generation means;

a single mode fiber transmission line coupled to said signal generation means; and

one or more Raman amplifier means located along said transmission line and wherein said signal generation means comprises a laser diode coupled to a Fabry-Perot modulator and includes means for unwanted resonant order suppression comprising a single mode modulator.

94. The system of claim 93 wherein said single mode modulator comprises a Fabry-Perot device comprising a single mode fiber optic core and cladding bound to said core only partially about the circumference of said core such that a cavity is formed between a portion of said core and said cladding, and a piezoelectric crystal mounted to said cladding.

95. The system of claim 93 wherein said Raman amplifier means comprise means for producing a pump beam and means for combining said pump beam with a signal beam along a common path.

96. The system of claim 95 wherein said means for combining said pump beam and signal beam comprises a portion of said transmission line.

97. The system of claim 95 wherein said means for combining said pump beam with said signal beam includes means for laterally coupling said pump beam onto said transmission line.