U.S. patent application number 10/211209 was filed with the patent office on 2003-02-20 for active gain equalization.
Invention is credited to Freeman, Michael J., Islam, Mohammed N..
Application Number | 20030035202 10/211209 |
Document ID | / |
Family ID | 26763357 |
Filed Date | 2003-02-20 |
United States Patent
Application |
20030035202 |
Kind Code |
A1 |
Islam, Mohammed N. ; et
al. |
February 20, 2003 |
Active gain equalization
Abstract
A pump assembly includes a pump source operable to generate at
least one pump signal operable to pump a Raman gain medium of a
Raman amplification stage. The pump assembly further includes an
active gain equalization element operable to manipulate a
wavelength or an intensity of the at least one pump signal to
affect a spectral shape of a Raman gain spectrum associated with
the Raman amplification stage. In one particular embodiment, the
combination of the Raman gain spectrum and a substantially
non-uniform gain spectrum of an existing amplification stage over a
wavelength range .lambda..sub.s results in a substantially uniform
overall gain spectrum over a wavelength range of at least
.lambda..sub.s.
Inventors: |
Islam, Mohammed N.; (Allen,
TX) ; Freeman, Michael J.; (Northville, MI) |
Correspondence
Address: |
BAKER BOTTS L.L.P.
2001 ROSS AVENUE
SUITE 600
DALLAS
TX
75201-2980
US
|
Family ID: |
26763357 |
Appl. No.: |
10/211209 |
Filed: |
August 2, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10211209 |
Aug 2, 2002 |
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10033848 |
Dec 19, 2001 |
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10211209 |
Aug 2, 2002 |
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09550730 |
Apr 17, 2000 |
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6370164 |
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10211209 |
Aug 2, 2002 |
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09110696 |
Jul 7, 1998 |
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6052393 |
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10211209 |
Aug 2, 2002 |
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08773482 |
Dec 23, 1996 |
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5778014 |
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60310147 |
Aug 3, 2001 |
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60080317 |
Apr 1, 1998 |
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Current U.S.
Class: |
359/334 |
Current CPC
Class: |
H01S 5/4087 20130101;
H01S 3/302 20130101; H01S 5/4012 20130101; H01S 3/094061 20130101;
H01S 3/094096 20130101; H01S 5/4025 20130101; H01S 3/094003
20130101 |
Class at
Publication: |
359/334 |
International
Class: |
H01S 003/00 |
Claims
What is claimed is
1. A pump assembly, comprising a pump source operable to generate
at least one pump signal operable to pump a Raman gain medium of a
Raman amplification stage; and an active gain equalization element
operable to manipulate a wavelength or an intensity of the at least
one pump signal to affect a spectral shape of a Raman gain spectrum
associated with the Raman amplification stage; and wherein the
combination of the Raman gain spectrum and a substantially
non-uniform gain spectrum of an existing amplification stage over a
wavelength range .lambda..sub.s results in a substantially uniform
overall gain spectrum over a wavelength range of at least
.lambda..sub.s.
2. The pump assembly of claim 1, wherein the pump source comprises
a plurality of laser diodes, each operable to generate at least one
pump wavelength.
3. The pump assembly of claim 1, wherein the pump source comprises
a pump assembly operable to generate a plurality of output pump
wavelengths from a single input pump wavelength.
4. The pump assembly of claim 1, wherein the active gain
equalization element is coupled between the pump source and the
Raman gain medium.
5. The pump assembly of claim 1, wherein the pump source comprises
the active gain equalization element.
6. The pump assembly of claim 1, wherein the active gain
equalization element comprises a device selected from the group
consisting of a Mach-Zehnder filter, a dielectric filter, a lattice
device, and a long-period grating.
7. The pump assembly of claim 1, wherein the active gain
equalization element comprises a controller operable to affect a
current driving the at least one pump source.
8. The pump assembly of claim 1, wherein the Raman gain medium
comprises a high-dispersion gain fiber.
9. The pump assembly of claim 8, wherein at least a portion of the
pump signal traverses the high-dispersion gain fiber in
substantially the same direction as at least one wavelength of a
multiple wavelength optical signal for at least a portion of the
Raman gain medium.
10. The pump assembly of claim 8, wherein at least a portion of the
pump signal traverses the high-dispersion gain fiber in a
substantially opposite direction from at least one wavelength of a
multiple wavelength optical signal for at least a portion of the
Raman gain medium.
11. The pump assembly of claim 1, wherein the Raman gain spectrum
comprises a gain profile that is approximately complimentary a gain
spectrum of the existing amplifier.
12. The pump assembly of claim 1, wherein the substantially uniform
gain over the wavelength range of at least .lambda..sub.s comprises
a gain variation of five (5) decibels or less over the wavelength
range of at least .lambda..sub.s.
13. The pump assembly of claim 1, wherein the Raman amplification
stage comprises a distributed Raman amplifier.
14. The pump assembly of claim 1, wherein the overall gain profile
of the Raman amplification stage combined with the existing
amplification stage comprises a substantially uniform gain profile
over a wavelength range greater than .lambda..sub.s.
15. The pump assembly of claim 1, wherein the wavelength range
.lambda..sub.s comprises a difference between a longest wavelength
and a shortest wavelength of fifteen (15) nanometers or more.
16. A Raman amplification stage, comprising: a Raman gain medium
operable to receive a multiple wavelength optical signal; at least
one pump source operable to generate at least one pump signal for
introduction to the Raman gain medium; an active gain equalization
element operable to manipulate a wavelength or an intensity of the
at least one pump signal generated by the pump source to affect a
spectral shape of a Raman gain spectrum; and a coupler operable to
couple the Raman amplification stage to an existing amplification
stage operable to provide a gain spectrum over a wavelength range
.lambda..sub.s; wherein the combination of the Raman gain spectrum
and the gain spectrum of the existing amplification stage results
in a substantially uniform overall gain spectrum over a wavelength
range of at least .lambda..sub.s.
17. The Raman amplification stage of claim 16, wherein the at least
one pump source comprises a laser diode pump source.
18. The Raman amplification stage of claim 17, wherein the laser
diode pump source comprises: a plurality of laser diodes each
capable of generating a lasing wavelength; and at least one
wavelength combiner operable to combine the plurality of lasing
wavelengths generated by the plurality of laser diodes into at
least one pump signal; wherein at least some of the plurality of
lasing wavelengths generated by the plurality of laser diodes
comprise wavelengths centered between 1270 nanometers and 1350
nanometers.
19. The Raman amplification stage of claim 16, wherein the at least
one pump source comprises a broadband Raman oscillator capable of
wavelength shifting the pump signal to a desired Raman cascade
order.
20. The Raman amplification stage of claim 19, wherein the
broadband Raman oscillator comprises at least one control element
coupled to one end of a Raman gain fiber.
21. The Raman amplification stage of claim 20, wherein the at least
one control element comprises a device selected from the group
consisting of a broadband grating, a dielectric filter, and a
wavelength division multiplexed filter.
22. The Raman amplification stage of claim 16, wherein the at least
one pump source comprises a pump assembly operable to generate a
plurality of output pump wavelengths from a single input pump
wavelength.
23. The Raman amplification stage of claim 16, wherein the active
gain equalization element is coupled between the pump source and
the Raman gain medium.
24. The Raman amplification stage of claim 16, wherein the pump
source comprises the active gain equalization element.
25. The Raman amplification stage of claim 16, wherein the active
gain equalization element comprises a device selected from a group
consisting of a Mach-Zehnder filter, a dielectric filter, a lattice
device, and a long-period grating.
26. The Raman amplification stage of claim 16, wherein the active
gain equalization element comprises a controller operable to affect
a current driving the at least one pump source.
27. The Raman amplification stage of claim 16, wherein the
manipulated pump signal traverses the Raman gain medium counter to
the multiple wavelength optical signal.
28. The Raman amplification stage of claim 16, wherein at least a
portion of the Raman gain medium comprises a dispersion
compensating fiber.
29. The Raman amplification stage of claim 16, wherein the Raman
gain medium comprises a high-dispersion gain fiber.
30. The Raman amplification stage of claim 29, wherein the
high-dispersion gain fiber comprises a magnitude of dispersion of
greater than two (2) picoseconds per nanometer-kilometer.
31. The Raman amplification stage of claim 29, wherein at least a
portion of the pump signal traverses the high-dispersion gain fiber
in substantially the same direction as at least one wavelength of
the multiple wavelength optical signal for at least a portion of
the Raman gain medium.
32. The Raman amplification stage of claim 16, wherein the Raman
gain medium comprises at least a portion of a transmission
link.
33. The Raman amplification stage of claim 16, wherein the
manipulated pump signal produces the Raman gain spectrum that
generates a spectrally tailored output signal from the Raman
amplification stage.
34. The Raman amplification stage of claim 33, wherein the
spectrally tailored output signal comprises a gain profile that is
approximately complimentary to the gain spectrum of the existing
amplifier.
35. The Raman amplification stage of claim 16, wherein the
substantially uniform gain over the wavelength range of at least
.lambda..sub.s comprises a gain variation of five (5) decibels or
less over the wavelength range of at least .lambda..sub.s.
36. The Raman amplification stage of claim 16, wherein the
substantially uniform gain over the over the wavelength range of at
least .lambda..sub.s comprises a gain variation of three (3)
decibel or less over over the wavelength range of at least
.lambda..sub.s.
37. The Raman amplification stage of claim 16, wherein the
substantially uniform gain over the over the wavelength range of at
least .lambda..sub.s comprises a gain variation of one-half (1/2)
decibel or less over over the wavelength range of at least
.lambda..sub.s.
38. The Raman amplification stage of claim 16, wherein the gain
spectrum of the existing amplification stage comprises a
substantially uniform gain spectrum.
39. The Raman amplification stage of claim 16, wherein the gain
spectrum of the existing amplification stage comprises a
substantially non-uniform gain spectrum.
40. The Raman amplification stage of claim 16, wherein the overall
gain profile of the Raman amplification stage combined with the
existing amplification stage comprises a substantially uniform gain
over a wavelength range greater than .lambda..sub.s.
41. The Raman amplification stage of claim 16, wherein the existing
amplification stage comprises a multiple-stage amplifier.
42. The Raman amplification stage of claim 41, wherein the
manipulated pump signal operates to pump an intermediate stage of
the multiple-stage amplifier.
43. The Raman amplification stage of claim 16, wherein the Raman
amplification stage comprises a distributed Raman amplification
stage.
44. The Raman amplification stage of claim 16, wherein the existing
amplification stage comprises an amplifier selected from the group
consisting of a discrete Raman amplifier, a distributed Raman
amplifier, an erbium doped amplifier, a rare earth doped amplifier,
and a hybrid amplifier.
45. The Raman amplification stage of claim 16, wherein the
wavelength range .lambda..sub.s comprises a difference between a
longest wavelength and a shortest wavelength of fifteen (15)
nanometers or more.
46. A Raman amplification stage, comprising: a Raman gain medium
operable to receive a multiple wavelength optical signal; at least
one pump source operable to generate at least one pump signal for
introduction to the Raman gain medium; and a coupler operable to
introduce the at least one pump signal to the Raman gain medium to
facilitate amplification of at least a portion of the multiple
wavelength optical signal; wherein at least a wavelength or an
intensity of the at least one pump signal is manipulated to affect
a shape of a gain spectrum associated with a Raman amplification
stage.
47. The Raman amplification stage of claim 46, wherein the Raman
gain medium comprises a distributed gain medium.
48. The Raman amplification stage of claim 46, wherein at least a
portion of the Raman gain medium comprises a dispersion
compensating fiber.
49. The Raman amplification stage of claim 46, wherein the at least
one pump source operates to generate a plurality of pump
wavelengths.
50. The Raman amplification stage of claim 49, wherein the
wavelength or the intensity of at least one of the plurality of
pump wavelengths is manipulated to affect the shape of the gain
spectrum associated with the Raman amplification stage.
51. The Raman amplification stage of claim 49, further comprising a
controller operable to select one or more of the plurality of pump
wavelengths for application to the Raman gain medium.
52. The Raman amplification stage of claim 46, further comprising a
coupling device operable to couple the Raman amplification stage to
an existing amplification stage operable to provide a gain spectrum
over a wavelength range .lambda..sub.s; wherein the combination of
the Raman gain spectrum and the gain spectrum of the existing
amplification stage results in a substantially uniform overall gain
spectrum over a wavelength range of at least .lambda..sub.s.
53. The Raman amplification stage of claim 52, wherein the Raman
gain spectrum comprises a gain profile that is approximately
complimentary to the gain spectrum of the existing amplification
stage.
54. The Raman amplification stage of claim 52, wherein the
substantially uniform gain over the wavelength range of at least
.lambda..sub.s comprises a gain variation of five (5) decibels or
less over the wavelength range of at least .lambda..sub.s.
55. A Raman amplification stage, comprising: a Raman gain medium
operable to receive a multiple wavelength optical signal and to be
coupled to an existing amplification stage having a substantially
non-uniform gain over a wavelength range .lambda..sub.s; a pump
source operable to generate at least one pump signal and to
manipulate a wavelength or an intensity of the at least one pump
signal to affect a spectral shape of a Raman gain spectrum; and
wherein the combination of the Raman gain spectrum and the
non-uniform gain spectrum of the existing amplification stage
results in a substantially uniform overall gain spectrum over a
wavelength range of at least .lambda..sub.s.
56. The Raman amplification stage of claim 55, wherein the at least
one pump source comprises a plurality of laser diodes, each
operable to generate at least one pump wavelength.
57. The Raman amplification stage of claim 55, wherein the at least
one pump source comprises a pump assembly operable to generate a
plurality of output pump wavelengths from a single input pump
wavelength.
58. The Raman amplification stage of claim 55, wherein the at least
one pump source comprises a broadband Raman oscillator capable of
wavelength shifting the pump signal to a desired Raman cascade
order.
59. The Raman amplification stage of claim 55, wherein at least a
portion of the Raman gain medium comprises a dispersion
compensating fiber.
60. The Raman amplification stage of claim 55, wherein the Raman
gain spectrum comprises a gain profile that is approximately
complimentary to the gain spectrum of the existing optical
amplifier.
61. The Raman amplification stage of claim 55, wherein the
substantially uniform gain over the wavelength range of at least
.lambda..sub.s comprises a gain variation of five (5) decibels or
less over the wavelength range of at least .lambda..sub.s.
62. The Raman amplification stage of claim 55, wherein the overall
gain profile of the Raman amplification stage combined with the
existing amplification stage comprises a substantially uniform gain
over a wavelength range greater than .lambda..sub.s.
63. The Raman amplification stage of claim 55, wherein the
wavelength range .lambda..sub.s comprises a difference between a
longest wavelength and a shortest wavelength of fifteen (15)
nanometers or more.
64. The Raman amplification stage of claim 55, wherein the Raman
amplification stage comprises a distributed Raman amplification
stage.
65. A Raman amplification stage, comprising: a Raman gain medium
operable to receive a multiple wavelength optical signal; at least
one pump source operable to generate at least one pump signal for
introduction to the Raman gain medium; and a coupler operable to
introduce the at least one pump signal to the Raman gain medium to
facilitate amplification of at least a portion of the multiple
wavelength optical signal; wherein at least a wavelength or an
intensity of the at least one pump signal is manipulated to
generate a gain tilt operable to at least partially compensate for
an inter-channel Raman effect of the multiple wavelength optical
signal received by the Raman gain medium.
66. The Raman amplification stage of claim 65, wherein the Raman
gain medium comprises a distributed gain medium.
67. The Raman amplification stage of claim 65, wherein the Raman
gain medium comprises an optical fiber.
68. The Raman amplification stage of claim 65, wherein the at least
one pump source operates to generate a plurality of pump
wavelengths.
69. The Raman amplification stage of claim 65, wherein the gain
tilt comprises a negative gain tilt wherein shorter signal
wavelengths have a larger magnitude of gain than longer signal
wavelengths.
70. A method of amplifying an optical signal, comprising:
generating at least one pump signal; manipulating a wavelength or
an intensity of the at least one pump signal to affect a spectral
shape of a Raman gain spectrum; and combining the Raman gain
spectrum with a gain spectrum of an existing amplification stage
having a substantially non-uniform gain over a wavelength range
.lambda..sub.s resulting in a substantially uniform overall gain
spectrum over a wavelength range of at least .lambda..sub.s.
71. The method of claim 70, wherein the at least one pump signal is
generated by a plurality of laser diodes, each operable to generate
at least one pump wavelength.
72. The method of claim 70, wherein the at least one pump signal is
generated by a broadband Raman oscillator capable of wavelength
shifting the at least one pump signal to a desired Raman cascade
order.
73. The method of claim 70, wherein the at least one pump signal is
generated by a pump assembly operable to generate a plurality of
output pump wavelengths from a single input pump wavelength.
74. The method of claim 70, wherein the at least one pump signal is
manipulated by an active gain equalization element.
75. The method of claim 70, wherein manipulating the wavelength or
the intensity of the at least one pump signal comprises adjusting
an amplitude of at least one lasing wavelength associated with the
at least one pump signal.
76. The method of claim 70, wherein the spectral shape of the Raman
gain spectrum is approximately complimentary to the gain spectrum
of the existing amplification stage.
77. The method of claim 70, wherein the substantially uniform gain
over the wavelength range of at least .lambda..sub.s comprises a
gain variation of five (5) decibels or less over the wavelength
range of at least .lambda..sub.s.
78. The method of claim 70, wherein the overall gain profile of the
Raman amplification stage combined with the existing amplification
stage comprises a substantially uniform gain over a wavelength
range greater than .lambda..sub.s.
79. The method of claim 70, further comprising coupling the
spectrally tailored pump signal to a Raman amplification stage.
80. A method of upgrading a pre-existing optical amplifier,
comprising: generating at least one pump signal; manipulating a
wavelength or an intensity of the at least one pump signal to
affect a spectral shape of a Raman gain spectrum of a Raman
amplification stage; coupling the Raman amplification stage to an
existing amplification stage having a substantially uniform gain
over a wavelength range .lambda..sub.s; and combining the Raman
gain spectrum with the substantially uniform gain spectrum of the
existing amplification stage resulting in a substantially uniform
overall gain spectrum over a wavelength range greater than
.lambda..sub.s.
81. The method of claim 80, wherein the at least one pump signal is
generated by a plurality of laser diodes, each operable to generate
at least one pump wavelength.
82. The method of claim 80, wherein the at least one pump signal is
generated by a broadband Raman oscillator capable of wavelength
shifting the at least one pump signal to a desired Raman cascade
order.
83. The method of claim 80, wherein the at least one pump signal is
generated by a pump assembly operable to generate a plurality of
output pump wavelengths from a single input pump wavelength.
84. The method of claim 80, wherein the at least one pump signal is
manipulated by an active gain equalization element.
85. The method of claim 80, wherein manipulating the wavelength or
the intensity of the at least one pump signal comprises adjusting
an amplitude of at least one lasing wavelength associated with the
at least one pump signal.
86. The method of claim 80, wherein the spectral shape of the Raman
gain spectrum is approximately complimentary to the gain spectrum
of the existing amplification stage.
87. The method of claim 80, wherein the substantially uniform gain
over the wavelength range greater than .lambda..sub.s comprises a
gain variation of five (5) decibels or less over the wavelength
range greater than .lambda..sub.s.
88. A method of controlling the shape of a gain spectrum of a Raman
amplification stage, comprising: receiving a multiple wavelength
signal at a gain medium of a Raman amplification stage; selecting
at least a wavelength or an intensity of a pump signal; and
introducing the pump signal to the gain medium to facilitate
amplification of at least a portion of the multiple wavelength
signal over at least a portion of the gain medium; wherein the
shape of the gain spectrum for a Raman amplification stage is
determined at least in part based on the selection of the
wavelength or intensity of the pump signal.
89. The method of claim 88, wherein the pump signal comprises a
multiple wavelength pump signal comprising a plurality of pump
wavelengths.
90. The method of claim 89, wherein selecting at least a wavelength
or an intensity of the pump signal comprises selecting at least a
wavelength or an intensity of at least one of the plurality of pump
wavelengths.
91. The method of claim 89, wherein a wavelength or an intensity of
at least one of the plurality of pump wavelengths is manipulated to
affect the shape of the gain spectrum associated with the Raman
amplification stage.
92. The method of claim 88, wherein at least a portion of the gain
medium comprises a dispersion compensating fiber.
93. The method of claim 88, further comprising: coupling the Raman
amplification stage to an existing amplification stage having a
gain spectrum over a wavelength range .lambda..sub.s; and combining
the Raman gain spectrum with the gain spectrum of the existing
amplification stage resulting in a substantially uniform overall
gain spectrum over a wavelength range greater than
.lambda..sub.s.
94. The method of claim 93, wherein the spectral shape of the Raman
gain spectrum is approximately complimentary to the gain spectrum
of the existing amplification stage.
95. The method of claim 93, wherein the substantially uniform gain
over the wavelength range greater than .lambda..sub.s comprises a
gain variation of five (5) decibels or less over the wavelength
range greater than .lambda..sub.s.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority under 35
U.S.C. .sctn.119(e) of U.S. Provisional Application Serial No.
60/310,147, filed Aug. 3, 2001.
[0002] This application claims priority to U.S. patent application
Ser. No. 10/033,848, filed Dec. 19, 2001 and entitled "BROADBAND
SAGNAC RAMAN AMPLIFIERS AND CASCADE LASERS," which is a divisional
of U.S. patent application Ser. No. 09/550,730 filed Apr. 11, 2000,
now U.S. Pat. No. 6,370,164.
[0003] U.S. Pat. No. 6,370,164 is a divisional of U.S. patent
application Ser. No. 09/110,696, filed Jul. 7, 1998, now U.S. Pat.
No. 6,052,393. U.S. Pat. No. 6,052,393 claims priority to U.S.
patent application Ser. No. 08/773,482 filed Dec. 23, 1996, now
U.S. Pat. No. 5,778,014, and to U.S. provisional patent application
Serial No. 60/080,317 filed Apr. 1, 1998.
TECHNICAL FIELD OF THE INVENTION
[0004] The present invention relates generally to the field of
communications systems and, more particularly, to active gain
equalization and applications thereof.
[0005] Overview
[0006] To improve gain flatness of optical signals amplified in an
optical amplifier, optical amplifiers used in optical communication
systems typically utilize gain equalization elements that introduce
spectrally dependent loss in the path of the amplified optical
signal. Introducing a gain equalization element in the path of the
optical signal, however tends to result in a reduction in the
efficiency and noise figure of the optical amplifier, and a
reduction in the signal to noise ratio for the system or
device.
SUMMARY OF EXAMPLE EMBODIMENTS
[0007] The present invention provides an improved apparatus and
method for optical amplification using an active gain equalization
technique to spectrally shape a gain spectrum of a Raman amplifier.
In accordance with the present invention, an apparatus and method
for optical amplification are provided that reduce or eliminate at
least some of the shortcomings associated with prior
approaches.
[0008] In one embodiment, a pump assembly comprises a pump source
operable to generate at least one pump signal operable to pump a
Raman gain medium of a Raman amplification stage. The pump assembly
comprises an active gain equalization element operable to
manipulate a wavelength or an intensity of the at least one pump
signal to affect a spectral shape of the Raman gain spectrum
associated with the Raman amplification stage. In one particular
embodiment, the combination of the Raman gain spectrum and a
substantially non-uniform gain spectrum of an existing
amplification stage over a wavelength range .lambda..sub.s results
in a substantially uniform overall gain spectrum over a wavelength
range of at least .lambda..sub.s.
[0009] In another embodiment, a Raman amplification stage comprises
a Raman gain medium operable to receive a multiple wavelength
optical signal. The amplification stage also comprises at least one
pump source operable to generate at least one pump signal for
introduction to the Raman gain medium. In addition, the
amplification stage comprises an active gain equalization element
operable to manipulate a wavelength or an intensity of the at least
one pump signal generated by the pump source to affect a spectral
shape of a Raman gain spectrum. The amplification stage further
comprises a coupler operable to couple the Raman amplification
stage to an existing amplification stage operable to provide a gain
spectrum over a wavelength range .lambda..sub.s. In one particular
embodiment, the combination of the Raman gain spectrum and the gain
spectrum of the existing amplification stage results in a
substantially uniform overall gain spectrum over a wavelength range
of at least .lambda..sub.s.
[0010] In yet another embodiment, a Raman amplification stage
comprises a Raman gain medium operable to receive a multiple
wavelength optical signal. The Raman amplification stage also
comprises at least one pump source operable to generate at least
one pump signal for introduction to the Raman gain medium. In
addition, the Raman amplification stage comprises a coupler
operable to introduce the at least one pump signal to the Raman
gain medium to facilitate amplification of at least a portion of
the multiple wavelength optical signal. In one particular
embodiment, at least a wavelength or an intensity of the at least
one pump signal is manipulated to affect a shape of a gain spectrum
associated with a Raman amplification stage
[0011] In another embodiment, a Raman amplification stage comprises
a Raman gain medium operable to receive a multiple wavelength
optical signal and to be coupled to an existing amplification stage
having a substantially non-uniform gain over a wavelength range
.lambda..sub.s. The amplification stage comprises a pump source
operable to generate at least one pump signal and to manipulate a
wavelength or an intensity of the at least one pump signal to
affect a spectral shape of a Raman gain spectrum. In one particular
embodiment, the combination of the Raman gain spectrum and the
non-uniform gain spectrum of the existing amplification stage
results in a substantially uniform overall gain spectrum over a
wavelength range of at least .lambda..sub.s.
[0012] In yet another embodiment, a Raman amplification stage
comprises a Raman gain medium operable to receive a multiple
wavelength optical signal. The Raman amplification stage also
comprises at least one pump source operable to generate at least
one pump signal for introduction to the Raman gain medium. In
addition, the Raman amplification stage comprises a coupler
operable to introduce the at least one pump signal to the Raman
gain medium to facilitate amplification of at least a portion of
the multiple wavelength optical signal. In one particular
embodiment, at least a wavelength or an intensity of the at least
one pump signal is manipulated to generate a gain tilt operable to
at least partially compensate for an inter-channel Raman effect of
the multiple wavelength optical signal received by the Raman gain
medium.
[0013] In a method embodiment, a method of amplifying an optical
signal comprises generating at least one pump signal. The method
also comprises manipulating a wavelength or an intensity of the at
least one pump signal to affect a spectral shape of a Raman gain
spectrum. The method further comprises combining the Raman gain
spectrum with a gain spectrum of an existing amplification stage
having a substantially non-uniform gain over a wavelength range
.lambda..sub.s resulting in a substantially uniform overall gain
spectrum over a wavelength range of at least .lambda..sub.s.
[0014] In another method embodiment, a method of upgrading a
pre-existing optical amplifier comprises generating at least one
pump signal. The method also comprises manipulating a wavelength or
an intensity of the at least one pump signal to affect a spectral
shape of a Raman gain spectrum of a Raman amplification stage. In
addition, the method comprises coupling the Raman amplification
stage to an existing amplification stage having a substantially
uniform gain over a wavelength range .lambda..sub.s. The method
further comprises combining the Raman gain spectrum with the
substantially uniform gain spectrum of the existing amplification
stage resulting in a substantially uniform overall gain spectrum
over a wavelength range greater than .lambda..sub.s.
[0015] In yet another method embodiment, a method of controlling
the shape of a gain spectrum of a Raman amplification stage
comprises receiving a multiple wavelength signal at a gain medium
of a Raman amplification stage. The method also comprises selecting
at least a wavelength or an intensity of a pump signal. In
addition, the method comprises introducing the pump signal to the
gain medium to facilitate amplification of at least a portion of
the multiple wavelength signal over at least a portion of the gain
medium. In one particular embodiment, the shape of the gain
spectrum for a Raman amplification stage is determined at least in
part based on the selection of the wavelength or intensity of the
pump signal.
[0016] Depending on the specific features implemented, particular
embodiments of the present invention may exhibit some, none, or all
of the following technical advantages. Various embodiments enable
the formation of a spectrally tailored or manipulated pump signal.
Some embodiments may be capable of generating a spectrally shaped
gain spectrum of a Raman amplification stage that is approximately
complimentary to a gain spectrum of another optical amplification
stage resulting in a substantially uniform overall gain profile for
the combination of amplifier stages. Other embodiments may be
capable of generating an approximately uniform gain over a spectral
range of an optical signal.
[0017] Other technical advantages will be readily apparent to one
skilled in the art from the following figures, descriptions and
claims. Moreover, while specific advantages have been enumerated
above, various embodiments may include all, some or none of the
enumerated advantages.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] For a more complete understanding of the present invention,
and for further features and advantages thereof, reference is now
made to the following description taken in conjunction with the
accompanying drawings, in which:
[0019] FIG. 1 is a block diagram showing at least a portion of an
exemplary optical communication system operable to facilitate
communication of one or more multiple wavelength signals;
[0020] FIGS. 2A through 2D are block diagrams illustrating
exemplary embodiments of a laser diode pump comprising a plurality
of laser diodes;
[0021] FIG. 3 is a chart illustrating three exemplary Raman cascade
orders starting from laser diodes centered at approximately the
1310 nm wavelength and exemplary applications for each order;
[0022] FIGS. 4A through 4C are block diagrams illustrating example
embodiments of wavelength shifters capable of generating a multiple
wavelength output signal from a single wavelength input signal;
[0023] FIG. 5 is a graph illustrating example wavelength spectra
generated by a wavelength shifter from a single pump input
wavelength;
[0024] FIGS. 6A and 6B are block diagrams illustrating exemplary
embodiments of broadband Raman oscillators;
[0025] FIGS. 7A through 7C are block diagrams illustrating
exemplary embodiments of broadband Raman oscillators implementing
Sagnac Raman cavities;
[0026] FIG. 8 is a block diagram illustrating an exemplary
embodiment of a broadband Raman oscillator implementing a Sagnac
Raman mirror;
[0027] FIG. 9 is a block diagram illustrating an exemplary
embodiment of a broadband Raman oscillator implementing a
circulator loop cavity;
[0028] FIGS. 10A through 10C are graphs illustrating exemplary
Raman gain spectra for laser diode pumps;
[0029] FIGS. 11A through 11D are block diagrams illustrating
example optical amplifiers capable of varying the amplifier gain
spectrum by controlling pump wavelength power levels;
[0030] FIG. 12 is a graph illustrating example gain spectra of an
optical amplifier generated by varying the power levels of the
wavelengths of a multiple wavelength pump signal;
[0031] FIGS. 13A and 13B are block diagrams illustrating exemplary
embodiments of Raman amplifiers implementing active gain
equalization;
[0032] FIGS. 14A through 14C illustrate an example multiple stage
amplifier with a plurality of gain profiles associated with various
amplification stages and an overall gain profile for the
amplifier;
[0033] FIGS. 15A through 15C illustrate a high pump efficiency
embodiment of a multiple stage wide band amplifier including
example gain profiles associated with various amplification stages
and an overall gain profile for the amplifier;
[0034] FIG. 16 is a graph illustrating how the use of spectrally
tailored pump signals generates a substantially flat gain
output;
[0035] FIG. 17 is a chart illustrating an exemplary formula for
selecting the appropriate active gain equalization necessary to
achieve an approximately uniform gain over the desired spectral
range;
[0036] FIG. 18 is a graph comparing a sinusoidal filter function to
a delta filter function for active gain equalization elements;
[0037] FIG. 19 is a block diagram illustrating an exemplary
embodiment of an active gain equalized Raman amplification stage
implemented in a pre-existing multiple-stage amplifier;
[0038] FIGS. 20A and 20B are block diagrams illustrating exemplary
embodiments of active gain equalization pump sources implemented to
upgrade pre-existing optical amplifiers;
[0039] FIGS. 21A and 21B are block diagrams illustrating exemplary
embodiments of implementing distributed Raman amplifiers to at
least partially counteract losses in relatively high loss
systems;
[0040] FIGS. 22A and 22B are block diagrams illustrating exemplary
embodiments of Raman amplifiers implementing a high-dispersion
Raman gain fiber; and
[0041] FIGS. 23A through 23C are block diagrams illustrating
exemplary embodiments of Raman amplifiers implementing a laser
diode pump to various wavelength ranges.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0042] I. Example System Architecture
[0043] FIG. 1 is a block diagram showing at least a portion of an
exemplary optical communication system 10 operable to facilitate
communication of one or more multiple wavelength signals 16. Each
multiple wavelength signal 16 comprises a plurality of optical
wavelength signals (or channels) 15a-15n, each comprising a center
wavelength of light. In some embodiments, each optical signal
15a-15n can comprise a center wavelength that is substantially
different from the center wavelengths of other signals 15. As used
throughout this document, the term "center wavelength" refers to a
time-averaged mean of the spectral distribution of an optical
signal. The spectrum surrounding the center wavelength need not be
symmetric about the center wavelength. Moreover, there is no
requirement that the center wavelength represent a carrier
wavelength.
[0044] In this example, system 10 includes a transmitter assembly
12 operable to generate the plurality of optical signals (or
channels) 15a-15n. Transmitters 12 can comprise any devices capable
of generating one or more optical signals. Transmitters 12 can
comprise externally modulated light sources, or can comprise
directly modulated light sources.
[0045] In one embodiment, transmitter assembly 12 comprises a
plurality of independent pairs of optical sources and associated
modulators, each pair being operable to generate one or more
wavelength signals 15. Alternatively, transmitter assembly 12 could
comprise one or more optical sources shared by a plurality of
modulators. For example, transmitter assembly 12 could comprise a
continuum source transmitter including a mode-locked source
operable to generate a series of optical pulses and a continuum
generator operable to receive a train of pulses from the
mode-locked source and to spectrally broaden the pulses to form an
approximate spectral continuum of optical signals. In that
embodiment, a signal splitter receives the continuum and separates
the continuum into individual signals each having a center
wavelength. In some embodiments, transmitter assembly 12 can also
include a pulse rate multiplexer, such as a time division
multiplexer, operable to multiplex pulses received from the mode
locked source or the modulator to increase the bit rate of the
system.
[0046] Transmitter assembly 12 may, in some cases, comprise a
portion of an optical regenerator. That is, transmitter assembly 12
may generate optical signals 15 based on electrical representations
of electrical or optical signals received from other optical
communication links. In other cases, transmitter assembly 12 may
generate optical signals 15 based on information received from
sources residing locally to transmitters 12. Transmitter assembly
12 could also comprise a portion of a transponder assembly (not
explicitly shown), containing a plurality of transmitters and a
plurality of receivers.
[0047] In the illustrated embodiment, system 10 also includes a
combiner 14 operable to receive wavelength signals 15a-15n and to
combine those signals into a multiple wavelength signal 16. As one
particular example, combiner 14 could comprise a wavelength
division multiplexer (WDM). The terms wavelength division
multiplexer and wavelength division demultiplexer as used herein
may include equipment operable to process wavelength division
multiplexed signals and/or equipment operable to process dense
wavelength division multiplexed signals.
[0048] In this example, system 10 includes one or more booster
amplifiers 18 operable to receive and amplify wavelengths of signal
16 in preparation for transmission over a communication medium
20.
[0049] System 10 communicates multiple wavelength signal 16 over an
optical communication medium 20. Communication medium 20 can
comprise a plurality of spans 20a-20n of fiber. Fiber spans 20a-20n
could comprise standard single mode fiber (SMF), dispersion-shifted
fiber (DSF), non-zero dispersion-shifted fiber (NZDSF), dispersion
compensating fiber (DCF), or another fiber type or combination of
fiber types. In various embodiments, communication medium 20 can
comprise up to five (5) fiber spans, seven (7) fiber spans, ten
(10) fiber spans, fifteen (15) fiber spans, twenty (20) fiber
spans, or more.
[0050] Two or more spans of communication medium 20 can
collectively form an optical link. In the illustrated example,
communication medium 20 includes a single optical link 25
comprising numerous spans 20a-20n. System 10 could include any
number of additional links coupled to link 25. For example, optical
link 25 could comprise one optical link of a multiple link system,
where each link is coupled to other links through, for example,
optical regenerators.
[0051] Optical communication link 25 could comprise, for example, a
unidirectional link or a bi-directional link. Link 25 could
comprise a point-to-point communication link, or could comprise a
portion of a larger communication network, such as a ring network,
a mesh network, a star network, or any other network
configuration.
[0052] Where communication system 10 includes a plurality of fiber
spans 20a-20n, system 10 can also include one or more in-line
amplifiers 22a-22m. In-line amplifiers 22 couple to one or more
spans 20a-20n and operate to amplify multiple wavelength signal 16
as it traverses communication medium 20. Optical communication
system 10 can also include one or more preamplifiers 24 operable to
amplify signal 16 received from a final fiber span 20n. Amplifiers
18, 22, and 24 could each comprise, for example, one or more stages
of discrete Raman amplification stages, distributed Raman
amplification stages, rare earth doped amplification stages, such
as erbium doped or thulium doped stages, semiconductor
amplification stages or a combination of these or other
amplification stage types. In some embodiments, amplifiers 18, 22,
and 24 could each comprise bi-directional Raman amplifiers.
[0053] Throughout this document, the term "amplifier" denotes a
device or combination of devices operable to at least partially
compensate for at least some of the losses incurred by signals
while traversing all or a portion of optical link 25. Likewise, the
terms "amplify" and "amplification" refer to offsetting at least a
portion of losses that would otherwise be incurred.
[0054] An amplifier may, or may not impart a net gain to a signal
being amplified. Moreover, the terms "gain" and "amplify" as used
throughout this document, do not (unless explicitly specified)
require a net gain. In other words, it is not necessary that a
signal experiencing "gain" or "amplification" in an amplifier stage
experience enough gain to overcome all losses in the amplifier
stage or in the fiber connected to the amplifier stage. As a
specific example, distributed Raman amplifier stages typically do
not experience enough gain to offset all of the losses in the
transmission fiber that serves as a gain medium. Nevertheless,
these devices are considered "amplifiers" because they offset at
least a portion of the losses experienced in the transmission
fiber.
[0055] Although optical link 25 is shown to include one or more
booster amplifiers 18 and preamplifiers 24, one or more of the
amplifier types could be eliminated in other embodiments.
[0056] In some cases, multiple wavelength signal 16 can carry
wavelength signals 15a-15n ranging across a relatively wide
bandwidth. In some implementations, wavelength signals 15a-15n may
even range across different communications bands (e.g., the short
band (S-band), the conventional band (C-band), and/or the long band
(L-band)). Depending on the amplifier types chosen, one or more of
amplifiers 18, 22, and/or 24 could comprise a wide band amplifier
operable to amplify all signal wavelengths 15a-15n received.
[0057] Alternatively, one or more of those amplifiers could
comprise a parallel combination of narrower band amplifier
assemblies, wherein each amplifier in the parallel combination is
operable to amplify a portion of the wavelengths of multiple
wavelength signal 16. In that case, system 10 could incorporate
signal separators and/or signal combiners surrounding the parallel
combinations of amplifier assemblies to facilitate amplification of
a plurality of wavelength groups of wavelengths prior to combining
or recombining the wavelengths for communication through system
10.
[0058] System 10 may further include one or more access elements
27. For example, access element 27 could comprise an add/drop
multiplexer, a cross-connect, or another device operable to
terminate, cross-connect, switch, route, process, and/or provide
access to and from optical link 25 and another optical link or
communication device. System 10 may also include one or more lossy
elements (not explicitly shown) coupled between spans 20a-20n of
link 25. For example, the lossy element could comprise a signal
separator, a signal combiner, an isolator, a dispersion
compensating element, or a gain equalizer.
[0059] In this example, system 10 includes a separator 26 operable
to separate individual optical signal 15a-15n from multiple
wavelength signal 16 received at the end of link 25. Separator 26
can communicate individual signal wavelengths or ranges of
wavelengths to a bank of receivers 28 and/or other optical
communication paths. Separator 26 may comprise, for example, a
wavelength division demultiplexer (WDM).
[0060] In one embodiment, at least one amplifier 18, 22, or 24 of
system 10 comprises an amplifier that utilizes a gain fiber pumped
by a laser diode based pump. Power amplification typically places a
relatively large burden on the pump associated with the amplifier,
in terms of both power levels and pump wavelength range. This is
particularly true for Raman amplifiers. This disclosure recognizes
that a laser diode based pump can provide a desired power level and
pump wavelength range by combining the power of a plurality of
grating-tuned laser diodes centered, for example, approximately at
1310 nm. For example, a plurality of laser diodes can be combined
to produce power levels of 400 mW or more and can provide the
desired pump wavelength range. In addition, a low noise and highly
efficient pump source can be achieved by combining a plurality of
grating-tuned laser diodes and operating the pump in the low-loss
window of the transmission fiber used in fiber spans 20a-20n. As
used throughout this document the phrase "low-loss window" refers
to an operating characteristic of an optical transmission fiber
that has an intrinsic fiber (e.g., excluding losses attributable to
coupling, splicing, aging, etc.) loss of 0.4 decibels per kilometer
or less for a given wavelength.
[0061] One aspect of this disclosure recognizes that combining a
plurality of grating-tuned laser diodes approximately centered on
the 1310 nm wavelength to pump the gain fiber enables a broadening
of the gain spectrum. Broadening the gain spectrum of the amplifier
allows the amplifier to amplify a wider range of wavelength
signals, in particular in a Raman amplifier. As used throughout
this document, the phrase "approximately centered on the 1310 nm
wavelength" refers to a wavelength centered in the wavelength range
between 1270 nm and 1350 nm. In some embodiments, the laser diode
based pump centered at approximately the 1310 nm wavelength is
capable of providing Raman gain through at least a substantial
portion of the low-loss window of the optical transmission fiber.
For example, the laser diode based pump centered at approximately
the 1310 nm wavelength can provide gain in the 1400 nm window, the
"violet" window (e.g., 1430-1525 nm) and the EDFA window (e.g.,
1525-1610 nm).
[0062] In some embodiments, the combination of a plurality of
grating-tuned laser diodes can produce an un-polarized broadband
laser diode pump, particularly useful in Raman amplifiers.
Implementing an un-polarized pump source provides the advantage of
minimizing polarization dependent gain effects within the Raman
amplifier. In one embodiment, the lasing wavelength of the
plurality of laser diodes are combined using polarization
multiplexers and wavelength multiplexers. As used throughout this
document, the term "lasing wavelength" refers to the center
wavelength of the signal generated by a pump source and/or a laser
diode. Lasing wavelength is not intended to suggest that a range of
wavelengths cannot be used.
[0063] In particular embodiments, the laser diode based pump can be
used to directly pump the gain fiber of a Raman amplifier. The
Raman amplifier may comprise a discrete Raman amplifier or a
distributed Raman amplifier. Using a 1310 nm laser diode pumped
Raman amplifier is advantageous in providing Raman gain in the 1400
nm window and the "violet" window of the optical fiber. In one
particular embodiment, the laser diode based pump can be used to
pump a distributed Raman amplifier, resulting in improved system
margin. Improvements to system margin are typically desired, for
example, in soliton-based systems and in systems with transmission
fiber operating windows that have a relatively high loss
coefficient.
[0064] In other embodiments, the laser diode based pump can be
wavelength shifted using a broadband oscillator. In one particular
embodiment, the broadband oscillator comprises a broadband Raman
oscillator. The broadband oscillator can comprise, for example, one
or more chirped fiber gratings in a linear cavity, one or more
Sagnac Raman cavities, or one or more circulator loop cavities.
Using a 1310 nm laser diode pumped broadband oscillator is
advantageous in pumping a gain fiber in the 1400 nm window and the
"violet" window of the transmission fiber. Using higher order
cascades, the wavelength shifted 1310 nm laser diode pump can also
be used to pump Raman or rare-earth doped gain fibers in the EDFA
window.
[0065] In one particular embodiment, the laser diode pump
implements a plurality of pairs of laser diodes, each pair centered
on a different wavelength within the 1280 nm to 1340 nm wavelength
range. Implementing pairs of laser diodes centered on a different
wavelength allows additional control of the gain spectrum width and
the gain flatness associated with the broadband Raman oscillator.
In alternate embodiments, the laser diode pumped broadband Raman
oscillator can be used as the pump for a distributed Raman
amplifier in systems requiring additional system margin.
[0066] Whether implementing laser diode based pumps or not, some
embodiments of system 10 can comprise at least one Raman amplifier
that implements active gain equalization. As used throughout this
document, the term "gain equalization" refers to a process of
equalizing the gain response of an amplified optical signal so that
substantially all amplified wavelength signals receive an
approximately uniform gain over a particular spectral range.
Conventional gain equalization techniques typically introduce a
spectrally dependent loss in the path of the amplified optical
signal. These techniques, referred to generally as "in-path"
equalization techniques, seek to introduce a spectral loss profile
that is approximately complimentary to the gain profile or spectrum
of the optical amplifier. The substantially complimentary profiles,
when added together, generate a substantially uniform gain over a
desired spectral range.
[0067] The generation of spectral loss approximately complimentary
to the gain spectrum of the amplifier has, in previous designs,
typically required implementation of a gain equalizing filter
in-line with the optical amplifier and the optical signal. For
example, where the optical amplifier is a single stage amplifier,
then the gain equalizing element was placed downstream of the
optical amplifier in the optical signal path. Introducing gain
equalization elements into the transmission path typically results
in a reduction in the efficiency and noise figure of the amplifier,
and results in a decrease in the signal-to-noise ratio for the
optical communication system.
[0068] One aspect of the present disclosure recognizes that
providing active gain equalization (AGEQ) to achieve an
approximately uniform gain over a specified spectral range improves
the efficiency and noise figure of the optical amplifier, and the
signal-to-noise ratio of the system. As used throughout this
document, the term "active gain equalization" or "AGEQ" refers to a
gain equalization technique that tailors the pump signal spectrum
to produce an approximately uniform gain over a specified spectral
range. Thus, in active gain equalization, adding the tailored pump
signal spectrum and the gain spectrum of the optical amplifier
and/or amplifiers generates an approximately uniform gain over the
desired spectral range. In some embodiments, active gain
equalization can be used in combination with a laser diode pump
approximately centered on the 1310 nm wavelength.
[0069] Implementing active gain equalization allows a reduction in
the loss or the removal of the gain equalization element from the
transmission path of the optical signal, if desired. This provides
gain equalization while reducing spectrally dependent loss in the
signal. In various embodiments, active gain equalization can be
implemented to tailor the gain spectrum of the pump signal by
utilizing a gain equalization element between the Raman gain fiber
and a pump source or a broadband Raman oscillator. In some
embodiments, active gain equalization can be implemented by
utilizing a gain equalization element within a broadband Raman
oscillator assembly to control the output spectrum for pumping the
Raman gain fiber. In other embodiments, active gain equalization
can be implemented by tailoring the amplitudes and/or power of the
lasing wavelengths combined to form the pump signal.
[0070] Another aspect of this disclosure recognizes that active
gain equalization can be used to upgrade existing optical
amplifiers. As optical communication system designers continue to
increase the capacity of optical communication systems, existing
optical amplifiers can require upgrading to handle the increased
capacity. In various embodiments, existing optical amplifiers can
be upgraded to provide increased gain and output power, gain
equalization, dispersion compensation, or a combination of these or
other amplification upgrades. These upgrades can be implemented in
either single-stage or multiple-stage optical amplifiers. In
addition, active gain equalization can be used in distributed Raman
amplifiers to upgrade high-loss systems, soliton transmission
systems, and/or bi-directional transmission links.
[0071] In some embodiments, active gain equalization may be used to
upgrade multiple stage amplifiers by implementing an active gain
equalized pump source and a Raman gain fiber. In some cases, the
Raman gain fiber can comprise a dispersion compensating fiber. In
these embodiments, the dispersion compensating fiber and the active
gain equalized pump source can be implemented at any intermediate
stage in a multiple stage amplifier. In other embodiments, active
gain equalization may be used to upgrade single stage optical
amplifiers by implementing an active gain equalized pump source to
create a distributed Raman amplifier. In these embodiments, the
distributed Raman amplifier may comprise, for example, a low-noise
pre-amplifier to the existing amplifier.
[0072] Regardless of whether system 10 implements laser diode pump
sources or active gain equalization, in some embodiments, at least
one amplifier 18, 22, and/or 24 can comprise a forward pumped or
bi-directionally pumped Raman amplifier that implements a
relatively high-dispersion gain fiber and/or a low-noise pump
source. In various embodiments, the forward pumped or
bi-directionally pumped Raman amplifier comprises a relatively
high-dispersion gain fiber that provides adequate walk off to
minimize cross talk and inter-channel interference between adjacent
wavelength channels. In other embodiments, the Raman amplifier
comprises a low-noise pump. Implementing a low-noise pump provides
the advantage of minimizing cross talk between the pump signal and
the amplified optical signals during the period the signals are
co-propagating. In some embodiments, the low noise pump comprises a
relatively low relative intensity noise (RIN) pump source. As used
in this document the term "low noise pump" refers to a pump source
capable of generating a noise fluctuation of no more than a
twenty-two (22) percent in the at least one pump signal prior to
propagation within the Raman gain medium. U.S. Pat. No. 5,778,014
provides one example of a low noise pump source. The low RIN pump
source can be selected, for example, as a function of the number of
spans in the multiple span communications link and the dispersion
acquired by optical signal 16 while traversing system 10.
[0073] In one particular embodiment, system 10 comprises a
dispersion managed system capable of reducing the system penalties
associated with chromatic dispersion and relative intensity noise
(RIN). As used throughout this document, the term "dispersion
managed" refers to a system that compensates for chromatic
dispersion in multiple spans of the communication system. In this
example, the dispersion managed system includes at least one
in-line Raman amplifier and at least one dispersion compensating
element coupled between spans 20a-20n of system 10.
[0074] In some embodiments, the dispersion acquired by optical
signal 16 while traversing system 10 can comprise a residual
dispersion and/or a local dispersion. As used throughout this
document, the term "residual dispersion" refers to dispersion
introduced into the span from a previous span in system 10. The
term "local dispersion" refers to dispersion acquired by optical
signal 16 while traversing a particular span of system 10.
[0075] In one particular embodiment, the dispersion compensating
element comprises a maximum dispersion compensation level that
provides only partial compensation for dispersion acquired by
optical signal 16 while traversing a span of system 10. Providing
only partial dispersion compensation in each span allows system 10
to maintain a sufficiently high dispersion. Maintaining a
sufficiently high dispersion throughout system 10 is advantageous
in providing adequate walk off between the pump signal and optical
signal 16, which tends reduce the RIN system penalty.
[0076] II. Stimulated Raman Scattering and Raman Cascading
[0077] To provide a better understanding of the amplification
mechanisms, stimulated Raman scattering and Raman cascading are
first described. Stimulated Raman scattering is the result of
third-order non-linearities that occur when a dielectric material
such as an optical fiber is exposed to intense light. The
third-order non-linear effect (e.g., Raman gain) is proportional to
the instantaneous light intensity passing through a gain
medium.
[0078] Stimulated Raman scattering is an important nonlinear
process that turns optical fibers into amplifiers and tunable
lasers. Raman gain results from the interaction of intense light
with optical phonons in glass fibers, and Raman scattering leads to
a transfer of energy from one optical beam (e.g., the pump) to
another optical beam (e.g., the optical signal). The signal is
downshifted in frequency (or upshifted in wavelength) by an amount
determined by the vibrational modes and composition of a given
glass fiber. The Raman gain coefficient g.sub.r for typical silica
fibers extends over a relatively large frequency range (e.g., up to
40 THz) with a broad peak centered at approximately 13.2 THz (this
corresponds to 440 cm.sup.-1). The relatively large frequency range
results from the amorphous nature of the silica glass and enables
stimulated Raman scattering to be used in broadband amplifiers. The
Raman gain depends at least in part on the composition of the fiber
core and can vary the with core dopant concentration.
[0079] Raman amplification provides some advantageous features.
First, Raman gain can be used to upgrade existing fiber optic links
because Raman gain is based at least in part on the interaction of
the pump signal with the optical phonons in the existing
transmission fibers. Second, Raman gain provides minimal loss in
the absence of pump power, which is an important consideration for
system reliability. Third, the gain band associated with Raman gain
is set by the pumping wavelengths. Fourth, peak Raman gain is
proportional to pump power. Consequently, bandwidth can be
increased by increasing the pump power.
[0080] Cascading is the mechanism by which optical energy at the
pump wavelength is transferred, through a series of nonlinear
polarizations, to an optical signal at a longer wavelength. Each
nonlinear polarization of the dielectric produces a molecular
vibrational state corresponding to a wavelength that is offset from
the wavelength of the light that produced the stimulation. The
nonlinear polarization effect is distributed throughout the
dielectric, resulting in a cascading series of wavelength shifts as
energy at one wavelength excites a vibrational mode that produces
light at a longer wavelength. This process can cascade through
numerous orders. Because the Raman gain profile has a peak centered
at 13.2 THz in silica fibers, one Raman order can be arranged to be
separated from the previous order by 13.2 THz. In other words, each
Raman order shifts by a photon energy of 13.2 THz, which
corresponds to about 80 nm, 90 nm, and 100 nm, at approximately the
1310 nm, 1400 nm, and 1460 nm wavelengths, respectively.
[0081] The effects of cascading results in stimulated Raman
scattering amplifiers providing a beneficial effect in optical
communication systems. Raman amplification can be used to amplify
multiple wavelengths or short optical pulses because the Raman gain
spectrum is relatively broad, for example, a bandwidth of greater
than 5 THz around the 13.2 THz peak. Moreover, cascading enables
Raman amplification over a wide range of different wavelengths. In
some cases, Raman gain can be provided over the entire
telecommunications window (e.g., 1300 nm to 1600 nm) by varying the
pump wavelength or by using cascaded orders of Raman gain from a
single or a few pump wavelengths. Thus, cascaded Raman orders
provide an efficient means by which to accomplish frequency
shifting from a pump signal to any desired wavelength on a longer
wavelength side of the pump signal.
[0082] The effective gain bandwidth of Raman amplification is the
convolution of the bandwidth of the pump with the Raman gain curve.
In other words, broadening the bandwidth amplified by Raman
amplification can occur because of the Raman amplification property
that the gain spectrum follows the pump spectrum. Thus, as the pump
spectrum changes, the Raman gain spectrum changes as well. This can
be useful in both extending the wavelength range of a pump source,
and in amplifying various wavelength signals through Raman transfer
from the pump source.
[0083] III. Laser Diode Pump Sources
[0084] FIGS. 2A through 2D are block diagrams illustrating
exemplary embodiments of laser diode pumps implementing a
combination of laser diodes centered approximately at 1310
nanometers.
[0085] FIG. 2A depicts a pump source 200 including four laser
diodes 202a-202d, each centered approximately at 1310 nanometers.
Although this example uses four (4) laser diodes 202a-202d, any
number of laser diodes can be used without departing from the scope
of the present disclosure. Using laser diodes centered
approximately at 1310 nanometers provides an advantage of
facilitating use of simpler ternary material, such as ternary
InGaAs, compared to more complex designs requiring quarternary
materials. Use of laser diodes formed from quarternary materials
is, however, also within the scope of this disclosure. Although
these examples use laser diodes centered at approximately the 1310
nanometer wavelength, the laser diodes can be centered on any other
desired wavelength without departing from the scope of the present
disclosure. For example, in various embodiments, laser diodes
202a-202d can be centered at a wavelength between 1395 nanometers
and 1510 nanometers.
[0086] Each laser diode generates an optical signal 203a-203d,
respectively, which ranges over several nanometers of bandwidth. In
order to more narrowly focus the wavelength range output from each
laser diode, the illustrated embodiment implements wavelength
selecting elements 204a-204d, coupled to outputs of laser diodes
202a-202d, respectively. In this particular example, wavelength
selecting elements 204 each comprise an external fiber grating
device. To enable the fiber grating device to control the lasing
wavelength, this example avoids distributed feedback and/or a
distributed Bragg reflector type laser diodes. In some embodiments,
one or more anti-reflective coatings may be located between laser
diode 202 and wavelength selecting element 204.
[0087] In various embodiments, each wavelength selecting element
204 is capable of reflecting at least five (5) percent of the
desired lasing center wavelength 212. In some embodiments, each
wavelength selecting element 204 is capable of tuning the lasing
center wavelength 212 at least 30 nm from the gain peak of laser
diode 202 coupled to wavelength selecting element 204. Maintaining
a desired lasing wavelength when it is within 30 nm of the gain
peak of a laser diode is advantageous in ensuring wavelength and
output power stability over changes in drive current and/or
temperature.
[0088] In some embodiments, wavelength selecting elements 204 are
capable of maintaining the polarization of lasing wavelengths 212.
For example, in the illustrated embodiment, polarization
maintaining fibers 201 couple wavelength selecting elements 204 to
laser diodes 202. Although not required in any embodiments, using
polarization maintaining fiber to couple laser diodes 202 and
wavelength selecting elements 204 can ensure substantially linearly
polarized signals exiting wavelength selecting elements 204. In
this example, the fiber pigtails of laser diodes 202 comprise
polarization maintaining fibers 201 which are coupled to or further
comprise wavelength selecting elements 204. In some embodiments,
polarization maintaining fibers 201 can comprise one or more
gratings. Where polarization maintaining fiber is not used,
polarization can be substantially maintained by keeping the length
of the external cavity sufficiently short. The external cavities
formed between laser diodes 202 and wavelength selecting elements
204.
[0089] In this example, signals output from wavelength selecting
elements 204a-204b comprise wavelengths of approximately 1310
nanometers, while signals output from wavelength selecting elements
204c-204d comprise wavelengths of approximately 1330 nanometers.
Although this example uses the 1310 nm and 1330 nm lasing
wavelengths, any other desired lasing wavelengths and spacing
between the wavelengths can be used without departing from the
scope of the present disclosure.
[0090] In the illustrated embodiment, pairs of outputs from laser
diodes 202 are combined using polarization combiners 206.
Polarization combiners 206 could comprise, for example,
polarization beam splitters, polarization multiplexers, or
birefringent elements. Using a pair of outputs from laser diodes
202a and 202b to generate a specific lasing wavelength is
advantageous in generating a un-polarized pump, which minimizes
polarization dependent gain effects within Raman amplifiers.
[0091] First polarization combiner 206a operates to combine lasing
wavelengths 212a and 212b received from fiber wavelength selecting
elements 204a and 204b, respectively, to generate a first
un-polarized pump signal 214a. In a similar manner, second
polarization combiner 206b operates to combine lasing wavelengths
212c and 212d and to generate a second un-polarized pump signal
214b.
[0092] In this example, pump 200 further includes a wavelength
combiner 208 operable to combine un-polarized pump signals 214a and
214b into an un-polarized multiple wavelength pump signal 210.
Wavelength combiner 208 may comprise any device capable of
combining a plurality of wavelength signals into a multiple
wavelength signal, such as, for example, one or more wavelength
division multiplexers (WDM).
[0093] FIG. 2B depicts a pump source 250 including eight laser
diodes 252a-252h, each centered approximately at 1310 nanometers.
Although this example utilizes eight laser diodes 252a-252h, any
number of laser diodes can be used without departing from the scope
of the present disclosure. Each laser diode 252a-252h generates an
optical signal 253a-253h, respectively, which ranges over several
nanometers of bandwidth. The structure and function of each laser
diode 252a-252h can be substantially similar to laser diodes
202a-202d of FIG. 2A.
[0094] In this embodiment, pump source 250 includes a plurality of
wavelength selecting elements 254a-254h coupled to outputs of laser
diodes 252a-252h and each operable to more narrowly focus the
wavelength range from each laser diode. In this particular example,
wavelength selecting elements 254 each comprise an external fiber
grating device. To enable the fiber grating device to control the
lasing wavelength, this example avoids distributed feedback and/or
a distributed Bragg reflector type laser diodes. In some
embodiments, one or more anti-reflective coatings may be located
between laser diode 252 and fiber wavelength selecting element
254.
[0095] In various embodiments, each wavelength selecting element
254 is capable of reflecting at least five (5) percent of the
desired lasing center wavelength 262. In some embodiments, each
wavelength selecting element 254 is capable of tuning the lasing
center wavelength 262 at least 30 nm from the gain peak of laser
diode 252 coupled to wavelength selecting element 254. Maintaining
a desired lasing wavelength when it is within 30 nm of the gain
peak of a laser diode is advantageous in ensuring wavelength and
output power stability over changes in drive current and/or
temperature.
[0096] In some embodiments, wavelength selecting elements 254 are
capable of maintaining the polarization of lasing wavelengths 262.
For example, in the illustrated embodiment, polarization
maintaining fibers 251 couple wavelength selecting elements 254 to
laser diodes 252. Although not required in any embodiments, using
polarization maintaining fiber to couple laser diodes 252 and
wavelength selecting elements 254 can ensure substantially linearly
polarized signals exiting wavelength selecting elements 254. In
this example, the fiber pigtails of laser diodes 252 comprise
polarization maintaining fibers 251 which are coupled to or further
comprise wavelength selecting elements 254. In some embodiments,
polarization maintaining fibers 251 can comprise one or more
gratings. Where polarization maintaining fiber is not used,
polarization can be substantially maintained by keeping the length
of the external cavity sufficiently short. The external cavities
formed between laser diodes 252 and wavelength selecting elements
254.
[0097] In this example, signals output from wavelength selecting
elements 254a and 254b comprise wavelengths of approximately 1295
nm, while signals output from selecting elements 254c and 254d
comprise wavelengths of approximately 1310 nm. In addition, output
signals from wavelength selecting elements 254e and 254f comprise
wavelengths of approximately 1325 nm, while signals output from
selecting elements 254g and 254h comprise wavelengths of
approximately 1340 nm. Although this example uses the 1295 nm, 1310
nm, 1325 nm, and 1340 nm lasing wavelengths, any other desired
lasing wavelengths and wavelength spacing can be used without
departing from the scope of the present disclosure.
[0098] In the illustrated embodiment, pairs of outputs from laser
diodes 252 are combined using polarization combiners 256.
Polarization combiners 256 could comprise, for example,
polarization beam splitters, polarization multiplexers, or
birefringent elements. Using a pair of outputs from laser diodes
252a and 252b to generate a specific lasing wavelength is
advantageous in generating an un-polarized pump, which minimizes
polarization dependent gain effects within Raman amplifiers.
[0099] In this example, pump 250 includes four (4) polarization
combiners 256a-256d operable to combine a pair of lasing
wavelengths 262 received from its respective wavelength selecting
element 254 and to generate an un-polarized pump signal 264. For
example, first polarization combiner 256a operates to combine
lasing wavelengths 262a and 262b received from wavelength selecting
elements 254a and 254b, respectively, to generate a first
un-polarized pump signal 264a.
[0100] In this example, pump source 250 further includes a
wavelength combiner 258 operable to combine un-polarized pump
signals 264a, 264b, 264c, and 264d into an un-polarized multiple
wavelength pump signal 260. Wavelength combiner 258 may comprise
any device capable of combining a plurality of wavelength signals
into a multiple wavelength signal, such as, for example, one or
more wavelength division multiplexers (WDM).
[0101] FIGS. 2C and 2D illustrate two embodiments of techniques to
create polarization diversity pumping. A pump laser 280, such as a
cladding-pumped fiber laser, can be linearly polarized. As depicted
in FIG. 2C, pump laser 280 is coupled to a length of polarization
maintaining fiber 282, where the light is coupled at forty-five
(45) degrees to the fiber axes, and the birefringence of the
polarization maintaining fiber splits the two polarizations. The
fiber can be rotated to accommodate the launch angle, or a quarter
or half-wave retarder 290 can be used at the entrance end of the
polarization maintaining fiber to adjust the polarization.
Alternately, as shown in FIG. 2D, a 50:50 coupler 284 is used to
split the pump light into two beams. The polarization of one of the
beams is then rotated by a quarter or half-wave retarder 286, and
the two beams are then combined using a polarization beam splitter
288. Once separated along two axes, the pump light is then
delivered to the Raman amplifier or laser configurations. The
polarization diversity scheme can be combined with other
improvements disclosed in the specification.
[0102] FIG. 3 is a chart illustrating three exemplary Raman cascade
orders starting from laser diodes centered at approximately the
1310 nm wavelength and exemplary applications for each order. In
this example, the pump comprises a plurality of laser diodes
centered approximately at the 1310 nm wavelength. The 1310 nm laser
diode pump is capable of directly pumping a Raman amplifier or
capable of use in a broadband Raman oscillator. The broadband Raman
oscillator operates to wavelength shift the Raman gain spectrum to
the desired wavelength window. In this example, each of the
plurality of laser diodes is combined with another laser diode to
form a pair of laser diodes. In this example, the pump comprises
five (5) pairs of laser diodes each pair of laser diodes generates
a lasing wavelength that spaced approximately 15 nm from the
adjacent lasing wavelength.
[0103] As illustrated in this chart, each Raman order shifts by a
photon energy of approximately 13.2THz. This shift corresponds
roughly to about 80 nm, about 90 nm, and about 100 nm at
approximately the 1310 nm, 1400 nm, and 1460 nm wavelengths,
respectively. In this example, the 1310 nm laser diode pump can be
used to directly pump a Raman amplifier for the 1400 nm window or
as a Raman oscillator pump for the "violet" window. The second
cascade order can be used as a Raman amplifier for the "violet"
window or a Raman oscillator pump for supplementing the EDFA window
with either active gain equalization, dispersion compensating
fiber, or distributed Raman amplification. Finally the third
cascade order can be used for an amplifier to supplement the EDFA
band, either with discrete or distributed gain, using Raman or
other types of amplification.
[0104] IV. Wavelength Shifters
[0105] FIGS. 4A through 4C are block diagrams illustrating example
embodiments of wavelength shifters capable of generating a multiple
wavelength output signal from a single wavelength input signal.
Although FIGS. 4A through 4C describe particular examples of
wavelength shifters, other wavelength shifter designs can be
implemented without departing from the scope of the present
disclosure. The wavelength shifter designs described in FIGS. 4A
through 4C are for illustrative purposes only. In various
embodiments, the wavelength shifters illustrated in FIGS. 4A
through 4C can control the power level for each wavelength in the
output signal. Generating a multiple wavelength output signal from
a single pump input signal and controlling the output power of each
wavelength advantageously enables tailoring of the gain spectrum of
an optical amplifier coupled to the wavelength shifter.
[0106] FIG. 4A is a block diagram illustrating an example
embodiment of a wavelength shifter 400 capable of generating a
multiple wavelength output signal 416 from a single pump input
signal 414. In this particular embodiment, wavelength shifter 400
operates to generate multiple wavelength output signal 416 with at
least 1314 nanometer and 1324 nanometer wavelengths. In this
example, wavelength shifter 400 includes pump source 401 operable
to generate a pump input signal 414. In this particular embodiment,
pump source 401 comprises fiber laser 402 capable of generating an
1117 nanometer pump signal, and a Raman resonator 406 capable of
shifting pump signal 414 at least one Raman cascade orders. Raman
resonator 406 may comprise any device capable of shifting pump
signal 414 at least one Raman cascade order. In this particular
embodiment, Raman resonator 406 operates to shift the 1117
nanometer pump signal two Raman cascade orders (e.g., to 1240
nanometers). In some embodiments, varying the current supplied to
fiber laser 402 can operate to control the power level for each
wavelength in output signal 416.
[0107] In this example, wavelength shifter 400 includes at least a
first selecting element 408a and a second selecting element 408b.
Selecting elements 408a and 408b can comprise any device, such as,
for example, a dielectric grating or one or more Fabry Perot
filters. Each selecting element 408a and 408b operates to transmit
a portion of a desired wavelength to be output from wavelength
shifter 400. In addition, each selecting element 408a and 408b
operates to at least partially reflect a desired wavelength to a
gain medium 410 to allow wavelength shifter 400 to continue lasing
at the desired wavelength or wavelengths. In this example, elements
408a and 408b comprise partially transmitting gratings
approximately centered on the 1314 and 1324 nanometer wavelengths,
respectively.
[0108] Raman gain fiber 410 operates to shift the frequency
response of pump input signal 414 to one or more desired
wavelengths to form multiple wavelength output signal 416. Gain
fiber 410 may comprise any fiber type capable of wavelength
shifting pump input signal 414 to a different Raman cascade order.
In various embodiments, gain fiber 410 may comprise, for example, a
dispersion compensating fiber or dispersion shifted fiber. In one
particular embodiment, gain fiber 410 comprises a dispersion
shifted fiber with a fiber length of approximately two kilometers.
Wavelength shifter 400 also includes a reflector 412 operable to at
least partially reflect pump input signal 414. In this particular
embodiment, reflector 412 comprises a Sagnac mirror with a 50:50
coupler. Selecting elements 408 and reflector 412 form optical
cavities where output signals 416 are generated within gain medium
410.
[0109] In this example, wavelength shifter 400 includes a
wavelength separator 404 capable of de-coupling the desired pump
wavelengths received from selecting elements 408. Separator 404
could comprise, for example, a wavelength division demultiplexer or
an optical coupler.
[0110] FIG. 4B is a block diagram illustrating an example
embodiment of a wavelength shifter 430 capable of generating a
multiple wavelength output signal 446 from a single pump input
signal 444. In this example, wavelength shifter 430 is similar in
structure and function to wavelength shifter 400 of FIG. 4A. Like
shifter 400, wavelength shifter 430 includes a fiber laser 432, a
Raman gain fiber 440, a wavelength separator 434, and a reflector
442. In this particular example, gain fiber 440 comprises a
dispersion compensating fiber with a fiber length of approximately
one-half kilometer, while reflector 442 comprises a reflective
mirror. In this particular embodiment, wavelength shifter 430
operates to generate multiple wavelength output signal 446 with at
least 1396 nanometer and 1421 nanometer wavelengths.
[0111] Wavelength shifter 430 also includes at least a first
selecting element 438a and a second selecting element 438b.
Although this example includes two selecting elements 438a and
438b, any number of selecting elements can be used without
departing from the scope of the present disclosure. The structure
and function of selecting elements 438a and 438b can be
substantially similar to selecting elements 408a and 408b of FIG.
4A. In this example, selecting element 438a comprises a grating
that is approximately twenty (20) percent reflective and is
approximately centered on the 1396 nanometer wavelength. Selecting
element 408b comprises a grating that is approximately forty (40)
percent reflective and is approximately centered on the 1421
nanometer wavelength.
[0112] The example, shown in FIG. 4B differs from the example shown
in FIG. 4A in that wavelength shifter 430 implements a plurality of
reflective gratings 436a-436c each centered on a different
wavelength of a reflection band. Although this example includes
three gratings 436a-436c, any number of gratings can be used
without departing from the scope of the present disclosure.
Gratings 436a-436c can comprise any device, such as, for example, a
high-reflectivity dielectric grating. In this particular example,
each of gratings 436a-436c comprises a grating with a reflectivity
between ninety-five (95) to one hundred (100) percent at the center
wavelength. Gratings 436a-436c operate to facilitate cascading of
pump input signal 444 to a desired lasing wavelength. In this
example, gratings 436a, 436b, and 436c are approximately centered
on the 1175, 1240, and 1311 nanometer wavelengths,
respectively.
[0113] In some embodiments, varying the current supplied to fiber
laser 432 can operate to control the power level for each
wavelength in output signal 446. An illustrative example of how
varying the current supplied to fiber laser 432 can affect the
power level of each output wavelength in output signal 446 is set
forth in Table 1.
1 TABLE 1 1396 nm 1421 nm Total Pump Wavelength Wavelength Drive
Current Signal Power Power Level Power Level 14.0 amps 369 mW 57.8%
38.0% 14.5 amps 414 mW 47.6% 48.5% 15.0 amps 460 mW 38.6% 57.5%
16.0 amps 560 mW 22.8% 73.3%
[0114] FIG. 4C is a block diagram illustrating an example
embodiment of a wavelength shifter 460 capable of generating a
multiple wavelength output signal 476 from a single pump input
signal 474. In this example, wavelength shifter 460 is similar in
structure and function to wavelength shifter 430 of FIG. 4B. Like
shifter 430, wavelength shifter 460 includes a fiber laser 462, a
wavelength separator 464, a Raman gain fiber 470, reflectors 472,
selecting elements 468a-468b, and a plurality of reflective
gratings 466a-466c. Gratings 466a, 466b1, 466b2, and 466c are
approximately centered on the 1175, 1240, 1240, and 1311 nanometer
wavelengths, respectively. In this particular embodiment,
wavelength shifter 460 operates to generate multiple wavelength
output signal 476 with at least 1396 nanometer and 1421 nanometer
wavelengths.
[0115] The example shown in FIG. 4C differs from the example shown
in FIG. 4B in that wavelength shifter 460 implements separator 464
within the laser cavity. In this example, the laser cavity
comprises that portion of wavelength shifter 460 residing between
reflective gratings 466a and 466b1 on one end and reflectors 472 on
the other end. Separator 464 can comprises any device capable of
de-coupling multiple Raman cascade orders and the desired pump
wavelengths from wavelength shifter 460. In this particular
example, separator 464 comprises a wavelength division multiplexer
coupler with a sinusoidal filter function.
[0116] FIG. 5 is a graph illustrating example output spectra
generated by a wavelength shifter from a single pump input
wavelength. In this particular embodiment, the wavelength shifter
comprises at least a first grating approximately centered on the
1314 nanometer wavelength and a second grating approximately
centered on the 1324 nanometer wavelength. In various embodiments,
the structure and function of the wavelength shifter can be
substantially similar to any the wavelength shifters depicted in
FIGS. 4A through 4C. In this example, each of lines 502, 504, and
506 represents an output spectrum that was generated from a single
pump input wavelength. Line 502 represents the output spectrum
generated while the fiber laser is receiving a current of
approximately nine (9) amps. Line 504 represents the output
spectrum generated while the fiber laser is receiving a current of
approximately twelve (12) amps. Line 506 represents the output
spectrum generated while the fiber laser is receiving a current of
approximately eighteen (18) amps. The horizontal axis represents
the output lasing wavelength, while the vertical axis represents
the output signal power.
[0117] This graph illustrates that modifying the current supplied
to the fiber laser can result in a change of the power of each of
the desired output wavelengths generated by the wavelength shifter.
In most cases, the wavelength shifter causes the shorter wavelength
to lase within the laser cavity at a lower fiber laser power than
the longer wavelength. As illustrated by output spectrum 502, only
the 1314 nanometer wavelength lases within the laser cavity while
the fiber laser is receiving a current of approximately nine (9)
amps. As the current supplied to the fiber laser increases, the
wavelength shifter causes the longer wavelength to lase and
increases the overall power level of the shorter wavelength. Output
spectrum 504 shows that the 1324 nanometer wavelength lases and the
power level of the 1314 nanometer wavelength increases by
approximately ten (10) decibels after the current supplied to the
fiber laser has been increased to twelve (12) amps.
[0118] This graph further illustrates that at some point the longer
wavelength signal begins to deplete the shorter wavelength signal
as the current supplied to the fiber laser is increased. For
example, output spectrum 506 shows that the power level associated
with the 1324 nanometer wavelength is approximately six (6)
decibels higher than the power level associated with the 1314
nanometer wavelength. Modifying the power levels associated with
each of the lasing wavelengths can advantageously enable tailoring
of the gain spectrum of an optical amplifier coupled to the
wavelength shifter.
[0119] V. Broadband Raman Oscillators as Wavelength Shifters
[0120] FIGS. 6A and 6B are block diagrams illustrating exemplary
embodiments of broadband Raman oscillators. In these embodiments,
each broadband Raman oscillator 600 and 650 comprises a pump 602
and 652, respectively, capable of generating a relatively low noise
pump signal. In various embodiments, the structure and function of
pumps 602 and 652 can be substantially similar to any of the pump
sources in FIGS. 2A through 2D. Broadband Raman oscillator 600 and
650 each operate to wavelength shift the lasing wavelengths
generated by pumps 602 and 652, which produces an oscillator output
signal 614 and 664, respectively, at a desired frequency for Raman
amplification.
[0121] In these embodiments, each broadband Raman oscillator 600
and 650 comprises at least one wavelength control element 604 and
654 coupled to one end of a Raman gain fiber 606 and 656,
respectively. Oscillators 600 and 650 also include a reflector 608
and 658, such as, for example, a Sagnac mirror coupled to the other
end of Raman gain fiber 606 and 656, respectively. Gain is provided
by a Raman gain fiber that is pumped by pump 602 and 652.
[0122] FIG. 6A is a block diagram illustrating an exemplary
embodiment of a broadband Raman oscillator 600 implementing a pump
602. In this example, oscillator 600 includes pump 602 operable to
generate a pump signal 612 and a Raman gain fiber 606 operable to
facilitate shifting the frequency response of pump signal 612. Pump
signal 612 can comprise, for example, a multiple wavelength signal
generated by a plurality of laser diodes or a single pump source as
in FIGS. 4A through 4C. In one particular example, pump signal 612
comprises at least one wavelength approximately centered at the
1310 nm wavelength. Gain fiber 606 may comprise any fiber type
capable of wavelength shifting pump signal 612 to a different Raman
cascade order. In various embodiments, gain fiber 606 may comprise,
for example, an optical fiber comprising a high germanium content,
a large core-cladding index difference, and a small effective area.
In one particular embodiment, gain fiber 606 comprises a fiber
length of at least one (1) kilometer.
[0123] In this embodiment, oscillator 600 includes a wavelength
combiner 610 capable of coupling pump signal 612 to oscillator 600
and transmitting all other Raman cascade orders. Wavelength
combiner 610, may comprise any device, such as, for example, a
wavelength division multiplexer. Oscillator 600 also includes a
reflector 608 operable to at least partially reflect pump signal
612 and a plurality of wavelength control elements 604. In this
particular embodiment, reflector 608 comprises a Sagnac mirror with
a 50:50 coupler. Although this example uses two control elements
604a and 604b, any number of elements may be used without departing
from the scope of the present disclosure. Control elements 604 may
comprise any device, such as, for example, a broadband grating, a
dielectric filter, or a wavelength division multiplexer filter. In
this example, elements 604a and 604b are partially transmitting
gratings. Control elements 604a and 604b operate to communicate the
desired Raman cascade order from broadband Raman oscillator 600. In
this example, elements 604a and 604b are centered on the desired
Raman cascade order.
[0124] FIG. 6B is a block diagram illustrating an exemplary
embodiment of a broadband Raman oscillator 650 implementing a pump
652. In this example, oscillator 650 includes pump 652 operable to
generate a pump signal 662 and a plurality of wavelength control
elements 654 operable to couple one or more selected wavelengths of
pump signal 662 to oscillator 650. Pump signal 662 can comprise,
for example, a multiple wavelength signal generated by a plurality
of laser diodes or a single pump source as in FIGS. 4A through 4C.
In one particular example, pump signal 662 comprises at least one
wavelength approximately centered at the 1310 nm wavelength.
Although this example uses two control elements 654a and 654b, any
number of elements may be used without departing from the scope of
the present disclosure. In this example, elements 654a and 654b
also operate to reject all other Raman cascade orders. Control
elements 654a and 654b enable pump signal 662 to pass and
substantially reflect the cascaded signals generated within
oscillator 650.
[0125] In this embodiment, oscillator 650 also includes a Raman
gain fiber 656 operable to shift the frequency response of pump
signal 662. Gain fiber 656 may comprise any fiber type capable of
wavelength shifting pump signal 662 to a different Raman cascade
order. In various embodiments, gain fiber 656 may comprise, for
example, an optical fiber comprising a high germanium content, a
large core-cladding index difference, and a small effective area.
In one particular embodiment, gain fiber 656 comprises a fiber
length of at least one (1) kilometer.
[0126] In this embodiment, oscillator 650 also includes a reflector
658 operable to at least partially reflect pump signal 662, and at
least one wavelength division multiplexer 660 capable of
de-coupling the desired Raman cascade order. In this particular
embodiment, high reflector 658 comprises a Sagnac mirror with a
50:50 coupler.
[0127] FIGS. 7A through 7C are block diagrams illustrating
exemplary embodiments of broadband Raman oscillators implementing
Sagnac Raman cavities. In these embodiments, each broadband Raman
oscillator 700, 730, and 760 comprises a pump 702, 732, and 762,
respectively, capable of generating a relatively low noise pump
signal. In various embodiments, the structure and function of pumps
702, 732, and 762 can be substantially similar to any of the pump
sources in FIGS. 2A through 2D. Each broadband Raman oscillator
700, 730, and 760 operates to wavelength shift the lasing
wavelengths generated by pumps 702, 732, and 762 to the desired
Raman cascade order. Each broadband Raman oscillator 700, 730, and
760 is capable of producing an oscillator output signal 718, 748,
and 778, respectively, at a desired lasing wavelength or
wavelengths.
[0128] In these embodiments, each broadband Raman oscillator 700,
730, and 760 comprises a Sagnac Raman mirror 704, 734, and 764.
Each Sagnac Raman mirror comprises a gain fiber 708, 738, and 768
operable to facilitate shifting the pump signals received from
pumps 702, 732, and 762. In these examples, the ends of each Sagnac
Raman mirror are coupled to an optical coupler 706, 736, and 766.
Optical coupler 706, 736, and 766 may comprise an approximately
50:50 coupler from the pump signal wavelength to the output signal
wavelength. Although this example uses a 50:50 coupler to connect
the ends of the Sagnac Raman mirror, any other coupler may be used
without departing from the scope of the present disclosure.
[0129] In some embodiments, each Sagnac Raman mirror 704, 734, and
764 may include a polarization controller 710, 740, and 770
operable to control a polarization state of the desired Raman
cascade order. In other embodiments, each gain fiber 708, 738, and
768 may comprise a polarization maintaining fiber operable to
control a polarization state of the desired Raman cascade order.
Using Sagnac Raman mirrors 704, 734, and 764 provides the advantage
of generating a passive noise dampening property, which tends to
lead to relatively quiet cascading of various Raman orders.
[0130] FIG. 7A is a block diagram illustrating an exemplary
embodiment of a broadband Raman oscillator 700 implementing a
Sagnac Raman mirror 704. In this example, oscillator 700 comprises
at least a first wavelength combiner 714a and a second wavelength
combiner 714b. Although this example uses two wavelength combiners
714a and 714b, any other number of combiners may be used without
departing from the scope of the present disclosure. Wavelength
combiners 714a and 714b may comprise any device capable of coupling
and/or de-coupling one or more wavelength signals to and/or from
oscillator 700, for example, a wavelength division multiplexer. In
this example, first combiner 714a operates to couple a particular
Raman cascade order pump signal 716 to oscillator 700 and to
transmit all other Raman cascade orders. In one particular
embodiment, pump signal 716 comprises at least one lasing
wavelength approximately centered at the 1310 nm wavelength. Second
combiner 714b operates to de-couple oscillator output signal 718
from oscillator 700 and to transmit all other wavelength signals.
Oscillator output signal 718 may comprise, for example, a desired
Raman cascade order.
[0131] In this particular embodiment, broadband Raman oscillator
includes a reflector 712 operable to substantially reflect all
wavelength signals contained within oscillator 700. Reflector 712
may comprise any device capable of reflecting a wide range of
wavelength signals, such as, for example, a mirror.
[0132] FIG. 7B is a block diagram illustrating an exemplary
embodiment of a broadband Raman oscillator 730 implementing a
Sagnac Raman mirror 734. In this example, oscillator 730 includes a
laser diode pump 732 coupled to a port 750 of optical coupler 736.
Port 750 of optical coupler 736 operates to couple a pump signal
746 to oscillator 730. In one particular embodiment, pump signal
746 comprises at least one lasing wavelength approximately centered
at the 1310 nm wavelength.
[0133] In the illustrated embodiment, oscillator 730 includes a
wavelength combiner 744 operable to de-couple oscillator output
signal 748 and to transmit all other Raman cascade orders.
Wavelength combiner 744 may comprise any device capable of
de-coupling one or more wavelength signals from oscillator 730, for
example, a wavelength division demultiplexer. Oscillator output
signal 748 may comprise, for example, a desired Raman cascade
order.
[0134] In this particular embodiment, broadband Raman oscillator
730 includes a reflector 742 operable to substantially reflect all
wavelength signals contained within oscillator 730. Reflector 742
may comprise any device capable of reflecting a wide range of
wavelength signals, such as, for example, a mirror.
[0135] FIG. 7C is a block diagram illustrating an exemplary
embodiment of a broadband Raman oscillator 760 implementing a
Sagnac Raman mirror 764. In this example, oscillator 760 includes a
laser diode pump 762 coupled to a port 774 of optical coupler 766.
Port 774 of optical coupler 766 operates to couple a pump signal
776 to oscillator 760. In one particular embodiment, pump signal
766 comprises at least one lasing wavelength approximately centered
at the 1310 nm wavelength.
[0136] In the illustrated embodiment, oscillator 760 includes a
wavelength control element 772 operable to de-couple oscillator
output signal 778 and to reflect all other Raman cascade orders and
wavelength signals. Control element 772 may comprise any device,
such as, for example, a broadband grating, a dielectric filter, or
a wavelength division multiplexer filter. In this example, element
772 comprises a partially transmitting grating. In this example,
element 772 is centered on the desired Raman cascade order.
[0137] FIG. 8 is a block diagram illustrating an exemplary
embodiment of a broadband Raman oscillator 800 implementing a
Sagnac Raman mirror 802. In this example, broadband Raman
oscillator 800 includes at least a first pair of laser diodes 804a
and a second pair of laser diodes 804b. Although this example
includes two pairs of laser diodes 804a and 804b, any additional
number of laser diode pairs can be used without departing from the
scope of the present disclosure. Each pair of laser diodes 804a and
804b is capable of producing an un-polarized pump signal 830 and
832, respectively, at a desired lasing wavelength or wavelengths.
In one particular embodiment, each pair of laser diodes 804 is
capable of generating a lasing wavelength centered at approximately
1310 nm. In some embodiments, the lasing wavelengths can be chosen
to provide a desired Raman gain spectrum.
[0138] In this embodiment, broadband Raman oscillator 800 includes
at least a first pair of fiber grating devices 806a and a second
pair of fiber grating devices 806b. Although this example includes
two pairs of fiber grating devices 806a and 806b, any additional
number of grating device can be used without departing from the
scope of the present disclosure. The structure and function of each
fiber grating device within pairs of fiber grating devices 806a and
806b can be substantially similar to wavelength selecting elements
204a-204d of FIG. 2A.
[0139] In this embodiment, broadband Raman oscillator 800 includes
at least a first polarization combiner 808a and a second
polarization combiner 808b. Although this example includes two
polarization combiners 808a and 808b, any additional number of
polarization combiners may be used without departing from the scope
of the present disclosure. The structure and function of
polarization combiners 808a and 808b can be substantially similar
to polarization combiners 206 of FIG. 2A. In this particular
embodiment, first polarization combiner 808a operates to generate
pump signal 830, while second polarization combiner 808b operates
to generate pump signal 832.
[0140] In this embodiment, broadband Raman oscillator 800 includes
a Sagnac Raman mirror 802 operable to wavelength shift pump signals
830 and 832 generated by laser diode pairs 804a and 804b to the
desired Raman cascade order. Sagnac Raman mirror 802 also operates
to combine pump signals 830 and 832. Sagnac Raman mirror 802
comprises a gain fiber 812 operable to wavelength shift pump
signals 830 and 832 to the desired Raman cascade order.
[0141] In some embodiments, Sagnac Raman mirror 800 may include a
polarization controller 814 operable to control a polarization
state of the desired Raman cascade order. In other embodiments,
gain fiber 812 may comprise a polarization maintaining fiber,
operable to control a polarization state of the desired Raman
cascade order. Using Sagnac Raman mirror 802 provides the advantage
of generating a passive noise dampening property, which tends to
lead to relatively quiet cascading of various Raman orders.
[0142] In this embodiment, broadband Raman oscillator 800 includes
an optical coupler 810 coupled to the ends of Sagnac Raman mirror
802 and operable to combine pump signals 830 and 832 to form a
multiple wavelength pump signal. Optical coupler 810 may comprise
any coupling device, such as, for example, an approximately 50:50
coupler from the wavelengths of pump signals 830 and 832 to the
wavelengths of oscillator output signal 834. Although this example
uses a 50:50 coupler, any other coupler may be used without
departing from the scope of the present disclosure.
[0143] In this embodiment, broadband Raman oscillator 800 includes
a wavelength combiner 816 operable to de-couple oscillator output
signal 834 and to transmit all other Raman cascade orders and pump
signals 830 and 832. Wavelength combiner 816 may comprise any
device capable of de-coupling one or more wavelength signals from
oscillator 800, for example, a wavelength division demultiplexer.
Oscillator output signal 834 may comprise, for example, a desired
Raman cascade order.
[0144] FIG. 9 is a block diagram illustrating an exemplary
embodiment of a broadband Raman oscillator 900 implementing a
circulator loop cavity 902. In this example, broadband Raman
oscillator 900 includes a laser diode pump 904 capable of
generating a relatively low noise pump signal 920. In one
particular embodiment, pump signal 920 comprises at least one
lasing wavelength approximately centered at the 1310 nm wavelength.
In various embodiments, the structure and function of laser diode
pump 904 can be substantially similar to any of the pump sources in
FIGS. 2A through 2D. In this example, pump signal 920 circulates
within circulator cavity 902 in a clockwise direction.
[0145] In this embodiment, broadband Raman oscillator 900 includes
circulator cavity 902 operable to wavelength shift the lasing
wavelengths generated by laser diode pump 904 to the desired Raman
cascade order. Circulator cavity 904 comprises a circulator 908, a
gain fiber 906 coupled between port 908c and 908a of circulator
908, and a wavelength control element 910 coupled to port 908b of
circulator 908. Although in this example circulator 908 comprises
three ports 908a-908c, any other number of ports may be used
without departing from the scope of the present disclosure.
[0146] Circulator 908 may comprise any non-reciprocal device
capable of sequentially transmitting wavelength signals from one
port to another port. For example, port 908a can only transmit a
wavelength signal to port 908b, while port 908b can only transmit a
wavelength signal to port 908c. Port 908a and port 908c are
typically isolated from one another. In various embodiments,
circulator 908 can comprise a polarization independent device with
a relatively low insertion loss. In other embodiments, circulator
908 can comprise a broadband device capable of transmitting the
desired Raman cascade order and the lasing wavelengths of pump
signal 920.
[0147] In the illustrated embodiment, circulator cavity 902
includes a wavelength control element 910 operable to reflect the
desired Raman cascade order. In this embodiment, control element
910 operates to provide broadband reflection at selected Raman
cascade orders. Control element 910 may comprise any device, such
as, for example, a chirped fiber grating, a wavelength control
filter, a combination of these or other devices.
[0148] In one particular embodiment, control element 910 comprises
a plurality of chirped Bragg gratings. Chirped Bragg gratings
comprise a length and a variation in periodicity along that length
that allows the gratings to control the bandwidth of the output
Raman cascade order. In some embodiments, the Bragg condition at
the center of chirped grating can coincide with the maximum gain
wavelengths of the desired Raman cascade order. The use of chirped
Bragg gratings as element 910 is advantageous in providing a
mechanism to tailor the bandwidth of the desired Raman cascade
order. In addition, chirped Bragg gratings also tend to reduce
fiber insertion loss. In another embodiment, wavelength control
element 910 can comprise a plurality of serially coupled gratings
each comprising a different center wavelength.
[0149] Coupling wavelength control element 910 to port 908b enables
pump signal 920 and/or a Raman cascade order to partially
counter-propagate with oscillator output signal 922. In this
example, oscillator output signal 922 comprises a desired Raman
cascade order.
[0150] In this example, circulator cavity 902 comprises at least a
first wavelength combiner 912a and a second wavelength combiner
912b. Although this example uses two wavelength combiners 912a and
912b, any other number of combiners may be used without departing
from the scope of the present disclosure. Wavelength combiners 912a
and 912b may comprise any device capable of coupling and/or
de-coupling one or more wavelength signals from cavity 902, for
example, a wavelength division demultiplexer. In this example,
first combiner 912a operates to couple a pump signal 920 to cavity
902 and to transmit all other Raman cascade orders. Second combiner
912b operates to de-couple oscillator output signal 922 from cavity
902 and to transmit all other wavelength signals. Oscillator output
signal 922 may comprise, for example, a desired Raman cascade
order.
[0151] VI. Broadening the Output Bandwidth from a BBRO
[0152] FIGS. 10A through 10C are graphs illustrating exemplary
Raman gain spectra for laser diode pumps. In these embodiments, the
structure and function of the laser diode pumps can be
substantially similar to any of the pump sources in FIGS. 2A
through 2D.
[0153] In various embodiments, the laser diode pump can be
wavelength shifted in a broadband Raman oscillator. In those
embodiments, the structure and function of the Raman oscillator can
be substantially similar to any of the Raman oscillators of FIGS. 6
through 9. In some cases, the broadband Raman oscillator can
achieve an increased bandwidth by selecting a gain fiber that has
an appropriate zero dispersion wavelength. For example, where the
zero dispersion wavelength of the gain fiber is in close wavelength
proximity to the pump signal, say within 30 nm, the pump will
spectrally broaden through phase matching and/or four-wave mixing.
Phase matching generates long and short side-band wavelengths that
spectrally broaden of the pump signal. In turn, this broadened pump
signal leads to a broadening of the gain band through the Raman
process.
[0154] In some embodiments, the laser diode pump itself can broaden
the output bandwidth from the broadband Raman oscillator. The gain
spectrum of Raman gain tends to follow the pump spectrum and
broadens at each consecutive Raman cascade order. Thus, if the
laser diode pump implements a series of laser diodes at varying
wavelengths or if a single pump source is used to generate multiple
pump wavelengths, then the Raman gain will correspondingly
broaden.
[0155] FIG. 10A is a graph illustrating an exemplary Raman gain
spectrum 1002 for a laser diode pump implementing four equal
amplitude pairs of laser diodes. In this particular example, laser
diode pump 1002 generates a multiple wavelength signal comprising
the 1391 nm, 1406 nm, 1421 nm, and 1436 nm wavelengths. Although
this example includes four wavelengths each separated by 15 nm, any
additional number of wavelengths or different wavelength spacing
can be used without departing from the scope of the present
invention.
[0156] This graph shows that the spectral width of gain spectrum
1002 at full width half maximum comprises approximately 84 nm at a
wavelength of approximately 1500 nm. In contrast, a typical
fused-silica optical fiber has a peak gain at approximately 13.2
THz, while the spectral width at full width half maximum comprises
approximately 7 THz or approximately 53 nm at a wavelength of 1500
nm. Thus, the gain spectral width has broadened by an amount equal
to the square root of the sum of squares of the pump signal width
and the Raman gain width.
[0157] FIG. 10B is a graph illustrating an exemplary Raman gain
spectrum 1026 for a laser diode pump implementing two pairs of
laser diodes with different relative intensities. In this example,
the laser diode pump comprises at least a first pair of laser
diodes and a second pair of laser diodes. The first pair of laser
diodes generates a pump signal centered at approximately the 1400
nm wavelength. In addition, the first pair of laser diodes
comprises a relative intensity of about 1. Similarly the second
pair of laser diodes generates a pump signal centered at
approximately the 1425 nm wavelength and comprises a relative
intensity of about 4.7.
[0158] This graph shows that the uniformity of Raman gain spectrum
1026 can be improved by tailoring the amplitudes and/or power of
the different wavelengths and the wavelength spacing. For example,
gain spectrum 1026 comprises a relatively flat gain between
approximately 1497 nm and 1535 nm wavelengths. In this particular
example, the spectral width of gain spectrum 1026 at full width
half maximum comprises approximately 68 nm.
[0159] FIG. 10C is a graph illustrating an exemplary Raman gain
spectrum 1052 for a laser diode pump implementing a plurality of
pairs of laser diodes with different relative intensities. In this
example, the laser diode pump comprises three pairs of laser
diodes. The first pair of laser diodes generates a pump signal
centered at approximately the 1380 nm wavelength and comprises a
relative intensity of about 1. The second pair of laser diodes
generates a pump signal centered at approximately the 1397.5 nm
wavelength and comprises a relative intensity of about 0.9. The
third pair of laser diodes generates a pump signal centered at
approximately the 1430 nm wavelength and comprises a relative
intensity of about 2.5.
[0160] This graph shows that the uniformity of Raman gain spectrum
1052 can be improved by tailoring the amplitudes and/or intensities
of the different pump wavelengths as well as the wavelength
spacing. As used in this document, the terms "amplitude" and
"intensity" at a pump wavelength refer to the pump output power at
that wavelength. Likewise, the terms "amplitude" and "intensity" of
laser or pump source refer to the output power of that laser or
pump source. For example, gain spectrum 852 comprises a relatively
flat gain between approximately 1475 nm and 1540 nm wavelengths. In
this particular example, the spectral width of gain spectrum 1052
at full width half maximum comprises approximately 100 nm.
[0161] VII. Gain Control by Varying Pump Powers
[0162] FIGS. 11A through 11D are block diagrams illustrating
example optical amplifiers capable of varying the amplifier gain
spectrum by controlling pump wavelength power levels.
[0163] FIG. 11A is a block diagram illustrating an example optical
amplifier 1100 capable of varying a gain spectrum associated with
amplifier 1100 by controlling pump wavelength power levels. In this
example, amplifier 1100 includes a pump source 1102 operable to
generate a multiple wavelength pump signal 1114. Pump source 1102
may comprise any device capable of generating the desired pump
signal 1114, such as, for example, a plurality of laser diodes, a
laser diode combined with a wavelength shifter, or a broadband
Raman oscillator. In this particular embodiment, pump source 1102
generates a multiple wavelength pump signal 1114 operable to
control a gain spectrum of amplifier 1100. In various embodiments,
pump source 1102 controls the gain spectrum of amplifier 1100 by
modifying a power level and/or amplitude of one or more wavelengths
of pump signal 1114.
[0164] In this particular embodiment, pump source 1102 comprises a
plurality of laser diodes, each operable to generate a pump
wavelength. In other embodiments, the structure and function of
pump source 1102 can be substantially similar to any one of
wavelength shifters depicted in FIGS. 4A through 4C, to any of the
pump sources in FIGS. 2A through 2D, or to any of the broadband
Raman oscillators illustrated in FIGS. 6 through 9. In this
example, pump source 1102 operates to generate multiple wavelength
pump signal 1114 comprising at least the 1396 nanometer and 1421
nanometer wavelengths.
[0165] In this embodiment, multiple wavelength pump signal 1114
amplifies optical signal 1110 in a gain medium 1106. In various
embodiments, gain medium 1106 may comprise a gain fiber or at least
a portion of a fiber span or transmission link. In some
embodiments, at least a portion of gain medium 1106 may comprise a
dispersion compensating fiber. Implementing a dispersion
compensating fiber as at least a portion of gain medium 1106 is
advantageous in enabling dispersion compensation.
[0166] In this example, amplifier 1100 includes an input coupler
1104a and an output coupler 1104b. Couplers 1104a and 1104b can
comprise any device capable of coupling and/or de-coupling optical
signal 1110 to and/or from amplifier 200. In this particular
embodiment, coupler 1104a comprises a wavelength division
multiplexer, while coupler 1104b comprises a wavelength division
demultiplexer. Input coupler 1104a operates to introduce an input
signal 1110 for amplification and output coupler 1104b operates to
remove signal 1110 from amplifier 1100 after amplification.
[0167] FIG. 11B is a block diagram illustrating an exemplary
two-stage optical amplifier 1130 capable of varying a gain spectrum
associated with amplifier 1130 by controlling pump wavelength power
levels. In this example, amplifier 1130 includes a pump source 1132
operable to generate a multiple wavelength pump signal 1144. In
various embodiments, the structure and function of pump source 1132
can be substantially similar to pump source 1102 of FIG. 11A. In
some embodiments, the structure and function of pump source 1132
can be substantially similar to any one of wavelength shifters
depicted in FIGS. 4A through 4C. In other embodiments, the
structure and function of pump source 1132 can be substantially
similar to any of the pump sources in FIGS. 2A through 2D. In some
embodiments, the structure and function of pump source 1132 can be
substantially similar to any of the broadband Raman oscillators
illustrated in FIGS. 6 through 9.
[0168] In this particular embodiment, pump source 1132 comprises a
plurality of laser diodes 1146a-1146d, each pair approximately
centered on a particular desired wavelength. Although this example
uses four (4) laser diodes, any number of laser diodes can be used
without departing from the scope of the present disclosure. In this
example, pump source 1132 generates a multiple wavelength pump
signal 1144 operable to control a gain spectrum of amplifier 1130.
In various embodiments, pump source 1132 controls the gain spectrum
of amplifier 1130 by modifying a power level and/or amplitude of
each wavelength generated by laser diodes 1146a-1146d.
[0169] In this particular example, wavelength signals generated by
laser diodes 1146a and 1146b comprise wavelengths of approximately
1396 nanometers, while wavelength signals generated by laser diodes
1146c and 1146d comprise wavelengths of approximately 1421
nanometers. Although this examples uses 1396 and 1421 nanometer
wavelengths, any other desired wavelengths can be used without
departing from the scope of the present disclosure.
[0170] In this embodiment, pump source 1132 includes at least a
first polarization combiner 1148a and a second polarization
combiner 1148b. Although this example uses two polarization
combiners 1148a and 1148b, any number of polarization combiners can
be used without departing from the scope of the present disclosure.
Polarization combiners 1148a and 1148b could comprise, for example,
polarization beam splitters, polarization multiplexers, or
birefringent elements. Using a pair of outputs from laser diodes
1146a and 1146b to generate a specific lasing wavelength is
advantageous in generating an un-polarized pump signal, which
minimizes polarization dependent gain effects within Raman
amplifiers.
[0171] First polarization combiner 1148a operates to combine lasing
wavelengths 1147a and 1147b received from laser diodes 1146a and
1146b, respectively, to generate a first un-polarized pump signal
1149a. In a similar manner, second polarization combiner 1148b
operates to combine lasing wavelengths 1147c and 1147d received
from laser diodes 1146c and 1146d, respectively, to generate a
first un-polarized pump signal 1149b.
[0172] In this example, pump source 1132 further includes a
wavelength combiner 1150 operable to combine un-polarized pump
signals 1149a and 1149b into an un-polarized multiple wavelength
pump signal 1144. Wavelength combiner may comprise any device
capable of combining a plurality of wavelength signals into a
multiple wavelength signal, such as, for example, a wavelength
division multiplexer. In this particular embodiment, multiple
wavelength pump signal 1144 operates to control the gain spectrum
of amplifier 1130.
[0173] In this embodiment, multiple wavelength pump signal 1144
amplifies an optical signal 1140 in at least a first gain medium
1136a and a second gain medium 1136b. Although this example
includes two gain media 1136a and 1136b, any additional number of
gain media can be used without departing from the scope of the
present disclosure. The type and length of optical fiber used to
form gain medium 1136a and 1136b depends at least in part on the
desired gain spectrum of amplifier 1130.
[0174] In this particular embodiment, first gain medium 1136a
comprises dispersion compensating fiber with a length of
approximately three (3) kilometers, while second gain medium 1136b
comprises dispersion compensating fiber with a length of
approximately five (5) kilometers. Although this example implements
dispersion compensating fibers with lengths of three and five
kilometers, any desired optical fiber of a sufficient length can be
used without departing from the scope of the present
disclosure.
[0175] In this embodiment, pump signal 1144 propagates in each gain
medium 1136a and 1136b counter to optical signal 1140. A
counter-propagating pump signal is advantageous in substantially
minimizing the coupling fluctuations in pump signal 1144 to optical
signal 1140. In addition, counter-propagating pump signals tend to
minimize cross-talk between closely spaced wavelength channels.
[0176] Amplifier 1130 also includes an isolator 1138 capable of
minimizing pump signal feedback and amplifier multi-path
interference. In this example, amplifier 1130 includes at least a
first coupler 1134a, a second coupler 1134b, and a third coupler
1134c. Although this example uses three couplers 1134a-1134c, any
number of couplers may be used without departing from the scope of
the present disclosure. Couplers 1134a-1134c can comprise any
device capable of coupling and/or de-coupling an optical signal to
and/or from amplifier 1130. In this particular embodiment, coupler
1134a operates to couple multiple wavelength pump signal 1144 to
amplifier 1130, while couplers 1134b and 1134c operate to couple
and de-couple multiple wavelength optical signal 1140,
respectively, to/from amplifier 1130. In this example, couplers
1134a and 1134b comprise wavelength division multiplexers, while
coupler 1134c comprises a wavelength division demultiplexer.
[0177] FIG. 11C is a block diagram illustrating an exemplary
two-stage optical amplifier 1160 capable of varying a gain spectrum
associated with amplifier 1160 by controlling pump wavelength power
levels. In this example, amplifier 1160 is similar in structure and
function to amplifier 1130 of FIG. 11B. Like amplifier 1130,
amplifier 1160 includes a pump source 1162, a plurality of couplers
1164a-1164c, and a plurality of gain media 1166a and 1166b. In this
particular embodiment, pump source 1162 operates to vary the gain
spectrum of amplifier 1160 by changing the current supplied to pump
source 1162.
[0178] The example shown in FIG. 11C differs from the example shown
in FIG. 11B in that amplifier 1160 includes an isolator 1168b
between gain medium 1166a and gain medium 1166b. Isolator 1166b
operates to minimize optical signal feedback.
[0179] FIG. 11D is a block diagram illustrating an exemplary
two-stage optical amplifier 1175 capable of varying a gain spectrum
associated with amplifier 1175 by controlling pump wavelength power
levels. In this example, amplifier 1175 includes at least a first
pump source 1176a and a second pump source 1176b. Although this
example includes two pump sources 1176a and 1176b, any number of
pump sources can be used without departing from the scope of the
present disclosure. In various embodiments, the structure and
function of pump sources 1176a and 1176b can be substantially
similar to pump source 1102 of FIG. 11A. In some embodiments, the
structure and function of pump sources 1176a and 1176b can be
substantially similar to any one of wavelength shifters depicted in
FIGS. 4A through 4C. In other embodiments, the structure and
function of pump sources 1176a and 1176b can be substantially
similar to any of the pump sources in FIGS. 2A through 2D. In some
embodiments, the structure and function of pump sources 1176a and
1176b can be substantially similar to any of the broadband Raman
oscillators illustrated in FIGS. 6 through 9.
[0180] In this example, first pump source 1176a generates a first
multiple wavelength pump signal 1190a and second pump source 1176b
generates a second multiple wavelength pump signal 1190b. Each of
pump signals 1190a and 1190b can control a gain spectrum of
amplifier 1175. In various embodiments, pump sources 1176a and
1176b control the gain spectrum of amplifier 1175 by modifying a
power level and/or amplitude of each wavelength generated by laser
diodes 1182 and 1184.
[0181] In one example (not explicitly shown), first pump source
1176a and second pump source 1176b can each include a plurality of
laser diodes each capable of generating a lasing wavelength. In
that example, the lasing wavelengths generated by the plurality of
laser diodes of first pump source 1176a can be combined to form
first multiple wavelength pump signal 1190a. Similarly, the lasing
wavelengths generated by the plurality of laser diodes of second
pump source 1176b can be combined to form second multiple
wavelength pump signal 1190b.
[0182] In this particular embodiment, first pump source 1176a
includes at least a first pair of laser diodes 1182a and 1182b and
a second pair of laser diodes 1184a and 1184b. Similarly, second
pump source 1176b includes at least a first pair of laser diodes
1182c and 1182d and a second pair of laser diodes 1184c and 1184d.
Although each pump source 1176a and 1176b includes two pairs of
laser diodes, any number of laser diodes can be used without
departing from the scope of the present disclosure.
[0183] In this particular example, wavelength signals generated by
laser diodes 1182a-1182d comprise wavelengths of approximately 1395
nanometers, while wavelength signals generated by laser diodes
1184a-1184d comprise wavelengths of approximately 1420 nanometers.
Although this examples uses 1395 and 1420 nanometer wavelengths,
any other desired wavelengths can be used without departing from
the scope of the present disclosure. Using pairs of outputs from
laser diodes 1182 and 1184 to generate a specific lasing wavelength
is advantageous in generating an un-polarized pump signal, which
minimizes polarization dependent gain effects within Raman
amplifiers.
[0184] In this embodiment, first pump source 1176a includes at
least a first polarization combiner 1186a and a second polarization
combiner 1186b. Similarly, second pump source 1176b includes at
least a third polarization combiner 1186c and a fourth polarization
combiner 1186d. Although each pump source 1176a and 1176b includes
two polarization combiners, any number of polarization combiners
can be used without departing from the scope of the present
disclosure. Polarization combiners 1186a-1186d could comprise, for
example, polarization beam splitters, polarization multiplexers,
wavelength division multiplexers, or birefringent elements.
[0185] First polarization combiner 1186a operates to combine the
lasing wavelengths generated by laser diodes 1182a and 1182b, while
second polarization combiner 1186b operates to combine the lasing
wavelengths generated by laser diodes 1184a and 1184b. Similarly,
third polarization combiner 1186c operates to combine the lasing
wavelengths generated by laser diodes 1182c and 1182d, while fourth
polarization combiner 1186d operates to combine the lasing
wavelengths generated by laser diodes 1184c and 1184d.
[0186] In this example, first pump source 1176a further includes a
first wavelength combiner 1188a operable to combine substantially
un-polarized pump signals 1187a and 1187b generated by polarization
combiners 1186a and 1186b, respectively, into a substantially
un-polarized multiple wavelength pump signal 1190a. Similarly,
second pump source 1176b further includes a second wavelength
combiner 1188b operable to combine substantially un-polarized pump
signals 1187c and 1187d generated by polarization combiners 1186c
and 1186d, respectively, into a substantially un-polarized multiple
wavelength pump signal 1190b. Wavelength combiners 1188a and 1188b
may comprise any device capable of combining a plurality of
wavelength signals into a multiple wavelength signal, such as, for
example, a wavelength division multiplexer. In this particular
embodiment, multiple wavelength pump signals 1190a and 1190b
operate to control the gain spectrum of amplifier 1175.
[0187] In this embodiment, amplifier 1175 includes at least a first
wavelength combiner 1178a and a second wavelength combiner 1178b.
Although this example implements two wavelength combiners 1178a and
1178b, any additional number of combiners can be used without
departing from the scope of the present disclosure. In this
example, first wavelength combiner 1178a operates to couple
spectrally tailored pump signal 1190a to a first Raman gain medium
1180a. In a similar manner, second wavelength combiner 1178b
operates to couple spectrally tailored pump signal 1190b to a
second Raman gain medium 1180b. Wavelength combiners 1178a and
1178b may comprise any device, such as, for example, a wavelength
division multiplexer or optical circulator.
[0188] In this embodiment, multiple wavelength pump signals 1190a
and 1190b amplify an optical signal 1192 in at least first Raman
gain medium 1180a and second Raman gain medium 1180b, respectively.
Although this example includes two gain media 1180a and 1180b, any
additional number of gain media can be used without departing from
the scope of the present disclosure. The type and length of optical
fiber used to form gain medium 1180a and 1180b depends at least in
part on the desired gain spectrum of amplifier 1175. First Raman
gain medium 1180a and second Raman gain medium 1180b can comprise,
for example, a distributed transmission fiber, a discrete fiber, or
a combination of these or other fiber types.
[0189] In this embodiment, first pump signal 1190a propagates
within first Raman gain medium 1180a counter to optical signal
1192. Similarly, second pump signal 1190b propagates within second
Raman gain medium 1180b counter to optical signal 1192. A
counter-propagating pump signal is advantageous in substantially
minimizing the coupling of fluctuations in pump signals 1190 to
optical signal 1192. In addition, counter-propagating pump signals
tend to minimize cross-talk between closely spaced wavelength
channels of optical signal 1192.
[0190] In operation, spectrally tailored pump signal 1190a
generates a spectrally shaped gain spectrum within first Raman gain
medium 1180a. The spectrally shaped gain spectrum of Raman gain
medium 1180a operates on optical signal 1192 to form a spectrally
tailored output optical signal 1193. Spectrally tailored pump
signal 1190b generates a spectrally shaped gain spectrum within
second Raman gain medium 1180b. The spectrally shaped gain spectrum
of Raman gain medium 1180b operates on signal 1193 output from
first Raman gain medium 1180a to form a spectrally tailored output
optical signal 1194. Through appropriate control of the gain
spectra of amplifier 1175, output signal 1194 can experience an
approximately flat overall gain profile.
[0191] FIG. 12 is a graph illustrating example gain spectra of an
optical amplifier generated by varying the power levels of the
wavelengths of a multiple wavelength pump signal. In this
particular embodiment, the structure and function of the optical
amplifier can be substantially similar to amplifier 1100 of FIG.
11A. In this example, line 1202 represents the gain spectrum
generated by the optical amplifier while the pump source receives a
current of approximately fourteen and one-half (14.5) amps. Line
1204 represents the gain spectrum generated by the optical
amplifier while the pump source receives a current of approximately
fifteen (15) amps. The horizontal axis represents the optical
signal wavelengths, while the vertical axis represents the gain
generated by the amplifier.
[0192] In this example, the multiple wavelength pump signal
comprises a 1396 nanometer and a 1421 nanometer wavelength. This
graph illustrates that varying the power levels of each wavelength
of the multiple wavelength pump signal results in a change to the
gain spectrum of an optical amplifier. Varying the power levels
associated with each of the pump signal wavelengths can
advantageously enable tailoring of the gain spectrum of the optical
amplifier.
[0193] VIII. Active Gain Equalization
[0194] FIGS. 13A and 13B are block diagrams illustrating exemplary
embodiments of Raman amplifiers implementing active gain
equalization. In various embodiments, Raman amplifiers 1300 and
1350 can be implemented as a stage of an existing multiple-stage
amplifier or as a pre-amplifier for an existing single stage
amplifier. In some embodiments, a Raman gain spectrum of Raman
amplifiers 1300 and 1350 combines with the gain spectrum of the
existing amplifier to generate a substantially uniform gain over
the spectral range of an amplified optical signal.
[0195] FIG. 13A is a block diagram illustrating an exemplary
embodiment of a Raman amplifier 1300 implementing an active gain
equalization element 1304. In this example, Raman amplifier 1300
includes a pump source 1302 operable to generate a pump signal
1312. Pump source 1302 may comprise any device capable of
generating the desired pump signal, such as, for example, a laser
diode pump, a wavelength shifter, or broadband Raman oscillator. In
one particular embodiment, the structure and function of pump
source 1302 can be substantially similar to any of the pump sources
in FIGS. 2A through 2D. In various embodiments, the structure and
function of pump source 1302 can be substantially similar to any of
the wavelength shifters illustrated in FIG. 4. In other
embodiments, the structure and function of pump source 1302 can be
substantially similar to any of the broadband Raman oscillators
illustrated in FIGS. 6 through 9. In some embodiments, pump source
1302 may comprise at least one pump wavelength generated by a laser
diode approximately centered on the 1310 nm wavelength.
[0196] In this embodiment, Raman amplifier 1300 includes active
gain equalization element 1304 operable to spectrally tailor the
wavelength spectrum of pump signal 1312 received from pump source
1302 and to generate a spectrally tailored pump signal 1310. Active
gain equalizing element 1304 may comprise any device capable of
spectrally tailoring pump signal 1312, by controlling the
wavelength and/or the power of one or more pump signals. For
example, to control pump signal wavelengths active gain equalizing
element 1304 may comprise a Mach-Zehnder type filter, a dielectric
filter, a lattice device, or a long-period grating. Pump signal
powers can be controlled, for example, by regulating laser drive
current or using variable attenuators on the pump signals. Although
this example depicts active gain equalization filter 1304 as
residing externally to pump source 1302, in other embodiments
active gain equalization filter 1304 can be integrated into pump
source 1302.
[0197] In this embodiment, Raman amplifier 1300 generates a
spectrally tailored pump signal 1310 operable to shape the gain
spectrum of Raman amplifier 1300. In some embodiments, the
spectrally tailored pump signal 1310 can operate to increase the
bandwidth amplified by amplifier 1300. In one particular
embodiment, spectrally tailored pump signal 1310 can shape the gain
spectrum of the Raman amplifier to be approximately complimentary
to a gain spectrum of an existing optical amplifier. In various
embodiments, the spectrally shaped Raman gain spectrum can be used
to augment an existing amplifier to generate an approximately
uniform gain over a desired spectral range. Spectrally tailoring
the gain spectrum of Raman amplifier 1300 is advantageous in
improving the efficiency and/or noise figure of Raman amplifier
1300 and possibly the signal to noise ratio of the optical
system.
[0198] In this embodiment, spectrally tailored pump signal 1310
amplifies multiple wavelength optical signal 1316 in a Raman gain
medium 1306. In various embodiments, Raman gain medium 1306 may
comprise a gain fiber or at least a portion of a fiber span or
transmission link. In some embodiments, at least a portion Raman
gain medium 1306 may comprise a dispersion compensating fiber.
Implementing a dispersion compensating fiber as at least a portion
of Raman gain medium 1306 is advantageous in enabling gain
equalization and dispersion compensation. In other embodiments,
Raman gain medium 1306 may comprise a high germanium content, large
core-cladding index difference, and small effective area, which can
advantageously enhance Raman gain.
[0199] In this embodiment, pump signal 1310 propagates gain medium
1306 counter to a multiple wavelength signal 1316. A
counter-propagating pump signal is advantageous in substantially
minimizing the coupling of fluctuations in pump signal 1310 to
multiple wavelength signal 1316. In addition, counter-propagating
pump signals tend to minimize cross-talk between closely spaced
wavelength channels.
[0200] In this embodiment, Raman amplifier 1300 includes a
wavelength combiner 1308. Wavelength combiner 1308 may comprise any
device capable of coupling one or more wavelength signals to Raman
gain medium 1306, such as, for example, a wavelength division
multiplexer.
[0201] FIG. 13B is a block diagram illustrating an exemplary
embodiment of a Raman amplifier 1350 implementing a spectrally
tailored pump signal 1360. In this example, the structure and
function of Raman amplifier 1350 can be substantially similar to
Raman amplifier 1300 of FIG. 13A. In this particular embodiment,
Raman amplifier 1350 includes a pump source 1352 operable to
generate pump signal 1360 and to spectrally tailor the wavelength
spectrum of pump signal 1360.
[0202] In various embodiments, pump source 1352 includes an active
gain equalization element operable to generate spectrally tailored
pump signal 1360. The active gain equalizing element may comprise
any device capable of spectrally tailoring pump signal 1360 to a
pump signal wavelength profile that spectrally shapes the gain
spectrum of Raman amplifier 1350. For example, the active gain
equalizing element may comprise a Mach-Zehnder type filter, a
dielectric filter, a lattice device, or a long-period grating.
[0203] In other embodiments, pump source 1352 comprises a plurality
of laser diodes capable of tailoring the wavelength spectrum of
pump signal 1360 by manipulating the output power of each of the
plurality of laser diodes. In this example, each of the plurality
of laser diodes is grating tuned and operates to generate a
specific lasing center wavelength. In one particular embodiment,
each of the plurality of laser diodes generates a lasing center
wavelength that is approximately 1310 nm.
[0204] FIGS. 14 and 15 illustrate example embodiments of amplifiers
implementing active gain equalization and capable of amplifying
relatively large bandwidths. In various embodiments, system 10 of
FIG. 1 may implement one or more of the amplifiers described below.
Although FIGS. 14 and 15 describe particular examples of wider band
amplifiers, other amplifier designs can be implemented without
departing from the scope of this disclosure. The amplifier designs
described with respect to FIGS. 14 and 15 are for illustrative
purposes only. Moreover, although these examples depict single
amplifiers operable to amplify all signal wavelengths received, a
plurality of these wider band amplifiers could be used in parallel
to further increase the amplifying bandwidth of the system.
[0205] FIGS. 14A through 14C illustrate an example multiple stage
amplifier 1400 with a plurality of gain profiles 1430 and 1432
associated with various amplification stages and an overall gain
profile 1434 for the amplifier. In this particular example,
amplifier 1400 is capable of amplifying at least 180 channels
spanning 100 nanometers or more of bandwidth, while maintaining an
acceptable signal-to-noise ratio and an approximately flat gain
profile. Any other number of channels and/or bandwidths may be used
without departing with the scope of this disclosure.
[0206] Conventional designs of multi-stage amplifiers have
experienced difficulties in attempting to process wide bandwidths
with a signal amplifier while maintaining approximately flat and/or
uniform gain profiles, acceptable noise figures, or acceptable bit
error rates. For example, in Raman amplifiers, a major culprit in
noise figures is the phonon-stimulated optical noise created when
wavelength signals being amplified reside spectrally close to pump
wavelengths used for amplification. The embodiment shown in FIG.
14A reduces adverse effects of this noise by enhancing the Raman
amplification of signal wavelengths near the pump wavelengths to
overcome the effects of the noise. This embodiment applies an
approximately complementary gain profile in another stage of the
amplifier to result in an approximately flat and/or uniform overall
gain profile with a reduced noise figure.
[0207] In this document, the phrase "approximately complementary"
refers to a situation where, at least in general, wavelength
signals that are more highly amplified in a first stage are less
amplified in a second complementary stage, and wavelength signals
that are more highly amplified in the second stage are less
amplified in the first stage. Note that the use of the terms
"first" and "second" to describe the amplifier stages here is not
meant to specify any-particular order of stages in the amplifier.
Two gain profiles said to be "approximately complementary" need not
have equal and opposite slopes. Moreover, equal amplification of
any particular wavelengths in both gain profiles does preclude
those gain profiles from being "approximately complementary."
[0208] Approximately complementary gain profiles may have one or
more slopes associated with each gain profile. For example,
approximately complementary gain profiles could comprise a "W"
shaped profile followed by an "M" shaped profile, or an "M" shaped
profile followed by a "W" shaped profile. Furthermore, the
approximately complementary gain profiles may become approximately
complementary only after traversing all or a portion of the
transmission medium. In those cases, the gain profiles launched at
the beginning of the amplifier stage may not be approximately
complementary, but may become approximately complementary after
signals traverse all or a portion of the transmission medium.
[0209] While better results could be obtained by applying
approximately complimentary gain profiles to all or nearly all of
the same signal wavelengths, some portion of wavelengths can be
omitted from one gain profile and included in the other gain
profile without departing from the scope of this disclosure.
[0210] In this example, amplifier 1400 comprises a two-stage
amplifier having a first stage 1412 and a second stage 1414
cascaded with first stage 1412. There is no limit to a particular
number of amplifier stages. For example, additional amplification
stages could be cascaded onto second stage 1414. Moreover, although
the illustrated embodiment shows second stage 1414 cascaded
directly to first stage 1412, additional amplification stages could
reside between first stage 1412 and second stage 1414 without
departing from the scope of this disclosure.
[0211] Amplifier 1400 could comprise a distributed Raman amplifier,
a discrete Raman amplifier, a hybrid Raman amplifier having both
discrete and distributed stages, a rare earth doped amplifier, a
semiconductor optical amplifier, or another amplifier type or
combination of amplifier types. Each stage 1412, 1414 of amplifier
1400 includes an input operable to receive a multiple wavelength
optical input signal 1416. As particular examples, signal 1416
could include wavelengths ranging over 32, 60, 80, or 100
nanometers.
[0212] Each stage 1412 and 1414 also includes a gain medium 1420
and 1421, respectively. Depending on the type of amplifier being
implemented, media 1420 and 1421 may comprise, for example, a gain
fiber or a transmission fiber. In some embodiments, all or portions
of media 1420, 1421 may comprise dispersion compensating
fibers.
[0213] Each stage 1412, 1414 further includes one or more
wavelength pumps 1422. Pumps 1422 generate pump light 1424 at
specified wavelengths, which are pumped into gain media 1420, 1421.
Pumps 1422 may comprise, for example, one or more laser diodes, a
wavelength shifter, or a broadband Raman oscillator. Although the
illustrated embodiment shows the use of counter propagating pumps,
under at least some circumstances using a relatively quiet pump,
co-propagating pumps could also be used without departing from the
scope of the disclosure.
[0214] In one particular embodiment, pump wavelengths 1424 can be
selected so that the longest wavelength of pump signal 1424 has a
wavelength that is shorter than the shortest wavelength of signal
1416. As one specific example, the longest wavelength of pump light
1424 could be selected to be, for example, at least ten (10)
nanometers shorter than the shortest wavelength of signal 1416. In
this manner, amplifier 1400 can help to avoid phonon stimulated
noise that otherwise occurs when pump wavelengths interact with
wavelengths of the amplified signal.
[0215] Couplers 1418b and 1418c couple pump wavelengths 1424a and
1424b to gain media 1420 and 1421, respectively. Couplers 1418
could comprise, for example, wavelength division multiplexers or
optical couplers. A lossy element 1426 can optionally reside
between amplifier stages 1412 and 1414. Lossy element 1426 could
comprise, for example, an isolator, an optical add/drop
multiplexer, or a gain equalizer.
[0216] The number of pump wavelengths 1424, their launch powers,
their spectral and spatial positions with respect to other pump
wavelengths and other wavelength signals, and the bandwidth and
power level of the signal being amplified can all contribute to the
shape of the gain profile for the respective amplifier stage.
[0217] FIG. 14B shows example gain profiles 1430 and 1432 for the
first stage 1412 and the second stage 1414, respectively, of
amplifier 1400. Gain profile 1430 shows the overall gain of first
stage 1412 of amplifier 1400 for a bandwidth ranging from the
shortest wavelength of signal 1416 (.lambda..sub.sh) to the longest
wavelength of signal 1416 (.lambda..sub.lg). Gain profile 1432
shows the overall gain of second stage 1414 of amplifier 1400 for a
bandwidth ranging from the shortest wavelength of signal 1416
(.lambda..sub.sh) to the longest wavelength of signal 1416
(.lambda..sub.lg). Each of gain profiles 1430 and 1432 reflects the
effects of the other gain profile acting upon it.
[0218] In this example, gain profile 1430 of first stage 1412 has
primarily a downward slope, where a majority of the shorter signal
wavelengths 1416 are amplified more than a majority of the longer
signal wavelengths 1416. Gain profile 1432 of second stage 1414 is
approximately complimentary to gain profile 1430 of first stage
1412. In this case, gain profile 1432 exhibits primarily an upward
slope where a majority of the longer signal wavelengths 1416 are
amplified more than a majority of the shorter signal wavelengths
1416.
[0219] Although gain profiles 1430 and 1432 are, for simplicity,
depicted as each having substantially one slope, the slope of each
gain profile may change numerous times. Moreover, it is not
necessary that the entire slope of gain profile 1430 be negative,
or that the entire slope of gain profile 1432 be positive. Each
profile may exhibit any number of peaks and valleys over the
amplified bandwidth.
[0220] Gain profile 1434 represents an example overall gain profile
of amplifier 1400 resulting from the application of gain profiles
1430 and 1432 to signal 1416. Overall gain profile 1434 is
approximately flat over at least substantially all of the bandwidth
of wavelengths within signal 1416.
[0221] This particular example provides a significant advantage in
reducing the peak noise figure associated with the amplifier using
complementary gain profiles. The complementary gain profiles reduce
the peak noise figure by amplifying signals closest to the pump
wavelengths at higher levels the signals at wavelengths far from
the pump wavelengths. In addition, the noise figure is reduced by
amplifying longer wavelength signals in a later amplifier stage. In
a discrete amplifier embodiment, using this type of configuration,
the noise figure of amplifier 1400 in the small signal limit can be
reduced to less than eight decibels, in some cases 7 decibels, even
where the bandwidth of signal 1416 exceeds 100 nanometers.
[0222] Complementary gain profiles can also be used to reduce the
pump power requirements for a given amplifier, thus creating a high
efficiency amplifier.
[0223] FIGS. 15A through 15C illustrate a high pump efficiency
embodiment of a multiple stage wide band amplifier 1500 including
example gain profiles 1530 and 1532 associated with various
amplification stages and an overall gain profile 1534 for the
amplifier. In this example, amplifier 1500 is capable of amplifying
at least 180 channels spanning 100 nanometers or more of bandwidth
while maintaining an acceptable signal-to-noise ratio and an
approximately flat gain profile.
[0224] Amplifier 1500 shown in FIG. 15A is similar in structure and
function to amplifier 1400 shown in FIG. 14A. Like amplifier 1400
shown in FIG. 14A, amplifier 1500 of FIG. 15A includes a first
amplification stage 1512 and a second amplification stage 1514.
Each of stages 1512 and 1514 includes a gain medium 1520 and 1521,
respectively, which is operable to receive multiple wavelength
input signal 1516 and pump wavelengths 1524a and 1524b,
respectively.
[0225] Each amplifier stage 1512 and 1514 operates to amplify
wavelengths of signal 1516 according to gain profiles 1530 and 1532
as shown. In this example, at least first stage 1512 comprises a
Raman amplification stage. Second stage 1514 could comprise a Raman
amplification stage, or another type of amplification stage.
[0226] The example shown in FIG. 15 differs from the example shown
in FIG. 14 in that gain profile 1530 (shown in FIG. 15B) of first
stage 1512 exhibits primarily an upward slope where a majority of
longer wavelengths of signal 1516 are amplified more than the
majority of shorter wavelengths of signal 1516. Conversely, gain
profile 1532 of second stage 1514 comprises an approximately
complementary gain profile to first gain profile 1530 of first
stage 1512. Profile 1532 applies a higher gain to a majority of
shorter signal wavelengths 1516 than the gain applied to the
majority of longer signal wavelengths 1516. In addition, in this
embodiment, the power of pumps 1522a driving first gain profile
1530 can be reduced.
[0227] The Raman scattering effect transfers energy from shorter
wavelength signals 1516 to longer wavelength signals 1516. This
embodiment leverages that fact to allow the longer pump wavelengths
of Raman first stage 1512 to accept energy from the shorter pump
wavelengths of second stage 1514. In a particular embodiment,
amplifier 1500 may include a shunt 1560 between second gain medium
1521 and first gain medium 1520 to facilitate the longer pump
wavelengths of first stage 1512 accepting power from the shorter
pump wavelengths of second stage 1514. The combined effects of
first stage 1512 and second stage 1514 result in an overall gain
profile 1534 (FIG. 15C) of the amplifier that remains approximately
flat.
[0228] This embodiment provides significant advantages in terms of
efficiency by allowing the use of fewer wavelength pumps 1522a in
the first stage 1512, and/or also by allowing each pump 1522a to
operate at a lower launch power. By selecting signal launch powers
with reference to the noise figure of the amplifier, this
embodiment enjoys the further efficiency of reduced overall
launched signal power.
[0229] The embodiment shown in FIG. 15A can also provide
improvements for the noise figure of the amplifier. For example,
phonon stimulated noise is created in Raman amplifiers where
wavelengths being amplified spectrally reside close to a wavelength
of pump signals 1524. By spectrally separating pump wavelengths
1524 from signal wavelengths 1516, phonon stimulated noise can be
reduced.
[0230] In a particular embodiment, pump wavelengths 1524 are
selected to have wavelengths at least ten (10) nanometers shorter
than the shortest wavelength in signal 1516 being amplified.
Moreover, in a particular embodiment, second stage 1514, where a
majority of the gain to short wavelengths of signal 1516 is
applied, comprises the last stage of amplifier 1500.
[0231] The amplifiers depicted in FIGS. 14 and 15 can comprises
wide band amplifiers operable to receive and amplify a relatively
large bandwidth of wavelength signals 15. In particular
embodiments, the amplifiers can process wavelengths ranging over
32, 60, 80, or 100 nanometers of bandwidth while maintaining an
approximately flat overall gain profile over the bandwidth of
amplified signal wavelengths.
[0232] In this document, the term "approximately flat" and/or
"uniform overall gain profile" describes a condition where the
maximum signal gain at the output of the amplifier differs from the
minimum signal gain at the output of the amplifier by no more than
an amount suitable for use in telecommunication systems over an
operational bandwidth of information carrying channels. Deviation
of the maximum and minimum signal gain over one or two of several
channels is not intended to be outside of the scope of an
approximately flat overall gain profile. The deviation between
minimum and maximum signal gains may comprise, for example, five
(5) decibels or less over an operational bandwidth of, for example,
32 nanometers or more. Particular embodiments may achieve gain
flatness of approximately three (3) decibels or less over an
operational bandwidth.
[0233] Although the embodiments shown in FIGS. 14 and 15 show two
complementary amplification stages, additional complementary
amplification stages could also be implemented.
[0234] IX. Upgrading Optical Amplifiers with AGEQ
[0235] FIG. 16 is a graph illustrating how a spectrally tailored
signal 1604 can be used to generate a substantially uniform overall
gain output 1606. In this example, line 1602 represents an
amplifier gain spectrum generated by an existing amplifier. Line
1604 represents a spectrally shaped Raman gain spectrum generated
through active gain equalization. In this particular example, the
gain spectrum of spectrally shaped Raman gain spectrum 1604 is
approximately complimentary to gain spectrum 1602 of the existing
amplifier. In various embodiments, spectrally tailored signal 1604
can be created by one of the active gain equalization techniques
illustrated in FIGS. 13A and 13B.
[0236] This graph shows that an approximately uniform gain output
1606 can be achieved by adding amplifier gain spectrum 1602 and the
gain spectrum of spectrally tailored signal 1604. Adding spectrally
tailored signal 1604 with amplifier gain spectrum 1602 is
advantageous in improving the efficiency and noise figure of the
Raman amplifier, and the signal to noise ratio of the optical
system.
[0237] FIG. 17 is a chart illustrating an exemplary formula for
selecting the appropriate active gain equalization necessary to
achieve an approximately uniform gain over the desired spectral
range. In this particular example, a pump signal spectrum generated
by a pump source is convolved with a Raman gain spectrum associated
with a Raman amplifier. Convolving the pump signal and the Raman
gain spectrum generates a spectrally tailored amplifier output
signal. In various embodiments, the spectrally tailored pump signal
can be created by one of the active gain equalization techniques
illustrated in FIGS. 13A and 13B.
[0238] The profile of the spectrally tailored output signal can be
designed to produce an approximately uniform gain over a desired
spectral range when the output signal is added to an existing
optical amplifier spectrum. In various embodiments, the
approximately uniform gain can comprise a gain variation of five
(5) decibels or less, three (3) decibels or less, one (1) decibel
or less, or one-half (1/2) decibel or less over the desired
spectral range.
[0239] The above methodology for selecting an active gain
equalization element comprises a first order iteration. Spectral
tailoring of the output signal for wide band amplification may make
further iterations desirable to select the appropriate spectral
response of the gain equalization element. In some embodiments,
spectral tailoring of the output signal may entail an accounting
for the variation in fiber loss over the pump spectrum and the
Raman gain spectrum. In other embodiments, spectral tailoring of
the output signal may involve modifying the spectral response to
account for potential skewing of the pump spectrum to the longer
wavelength side due to pump interactions in the amplifier gain
medium.
[0240] FIG. 18 is a graph comparing a sinusoidal filter function
1802 to a delta filter function 1804 for active gain equalization
elements. In this example, line 1802 represents the filter function
of a sinusoidal filter. The sinusoidal filter may comprise any
device capable of generating a sinusoidal filter response, such as,
for example a Mach-Zehnder filter. In this example, the sinusoidal
filter comprises a peak every 10 nm. Line 1804 represents the
filter function of a delta filter. In this example, the delta
filter comprises discrete wavelengths every 10 nm. In this example,
each filter function 1802 and 1804 is applied to a pump signal
comprising a wavelength spectrum of 1380 nm to 1480 nm.
[0241] This graph shows that sinusoidal filter function 1802 yields
a less varying gain spectrum than a similar delta filter function
1804. Sinusoidal function 1802 comprises a spectral width at full
width half maximum of approximately 126 nm with a gain that is
approximately uniform over 62 nm. In this example, the gain
spectrum varies within 0.46 decibels over 62 nm. In contrast, delta
filter function 1804 comprises a similar spectral width at full
width half maximum, but yields a gain spectrum that varies more
than 0.46 decibels over 62 nm.
[0242] FIG. 19 is a block diagram illustrating an exemplary
embodiment of an active gain equalized Raman amplification stage
1901 implemented in a pre-existing multiple-stage amplifier 1900.
In this example, multiple-stage amplifier 1900 comprises at least a
first amplification stage 1906a and a second amplification stage
1906b. Although this example uses two amplification stages, any
additional number of amplification stages can be used without
departing from the scope of the present disclosure. The structure
and function of first amplification stage 1906a and second
amplification stage 1906b can be substantially similar to amplifier
22 of FIG. 1. In this particular example, first amplification stage
1906a comprises a low-noise pre-amplification stage, while second
amplification stage 1906b comprises a high-gain power amplification
stage.
[0243] As optical communication system designers continue to
increase the capacity of optical communication systems, existing
optical amplifiers will require either upgrading or replacement to
account for the increased capacity. In this embodiment,
multiple-stage amplifier 1900 is upgraded to include an active gain
equalized Raman amplification stage 1901. In this embodiment, Raman
amplification stage 1901 comprises an intermediate
amplification-stage of amplifier 1900. In other embodiments, Raman
amplification stage 1901 can comprise a pre-amplifier or a final
amplification stage of multiple-stage amplifier 1900. Raman
amplification stage 1901 may comprise a discrete Raman amplifier or
a distributed Raman amplifier.
[0244] Raman amplification stage 1901 includes a pump source 1902
capable of generating a spectrally tailored pump signal 1910. Pump
source 1902 may comprise any device capable of generating and
spectrally tailoring the wavelength spectrum of pump signal 1910.
In various embodiments, the structure and function of pump source
1902 can be substantially similar to pump source 1352 of FIG. 13B.
In other embodiments, the structure and function of pump source
1902 can be substantially similar to the combination of pump source
1302 and active gain equalization element 1304 of FIG. 13A. In some
embodiments, pump source 1902 can generate a pump signal capable of
pumping amplification stage 1906b and Raman amplification stage
1901. In this particular embodiment, pump source 1902 generates
pump signal 1910 operable to pump a Raman gain fiber 1908
associated with Raman amplification stage 1901.
[0245] In this embodiment, Raman amplification stage 1901 includes
Raman gain fiber 1908 operable to at least partially compensate for
losses experienced by a multiple wavelength signal 1916. Raman gain
fiber 1908 may comprise any optical fiber capable of transferring
gain from pump signal 1910 to optical signal 1916. In one
particular embodiment, upgrading amplifier 1900 to include Raman
gain fiber 1908 advantageously enables amplifier 1900 to amplify a
larger number of channels in multiple wavelength signal 1916.
[0246] In various embodiments, Raman amplification stage 1901 can
include a dispersion compensating element coupled to gain fiber
1908 and operable to at least partially compensate for chromatic
dispersion that would otherwise be associated with a multiple
wavelength signal 1916. Implementing a dispersion compensating
element advantageously enables amplifier 1900 to transmit multiple
wavelength signal 1916 at a higher rate, for example, at a rate of
9.5 gigabits per second or more.
[0247] The dispersion compensating element may comprise any device
capable of at least partially counteracting the chromatic
dispersion associated with a communication medium traversed by
multiple wavelength signal 1916. In one particular embodiment, gain
fiber 1908 comprises a length of dispersion compensating fiber
having a slope of dispersion that is approximately equal to and
opposite from the slope of chromatic dispersion associated with the
communication medium traversed by multiple wavelength signal 1916.
In an alternative embodiment, the dispersion compensating fiber
could comprise a length of dispersion compensating transmission
fiber coupled to Raman gain fiber 1908.
[0248] In this particular embodiment, spectrally tailored pump
signal 1910 operates to spectrally shape a Raman gain spectrum of
Raman amplification stage 1901. In various embodiments, the
spectrally shaped Raman gain spectrum of Raman amplification stage
1901 can be approximately complimentary to the gain spectrum of
multiple-stage amplifier 1900. In some embodiments, the Raman gain
spectrum of amplification stage 1901 is combined with the gain
spectrum of multiple stage amplifier 1900 to generate an
approximately uniform gain over the spectral range of optical
signal 1916. In other embodiments, the Raman gain spectrum can be
combined with the gain spectrum of multiple stage amplifier 1900 to
generate an approximately uniform gain over an increased spectral
range.
[0249] In this embodiment, Raman amplification stage 1901 comprises
a wavelength combiner 1904 capable of coupling spectrally tailored
pump signal 1910 to Raman gain fiber 1908. Wavelength combiner 1904
may comprise any device, such as, for example, a wavelength
division multiplexer.
[0250] FIGS. 20A and 20B are block diagrams illustrating exemplary
embodiments of active gain equalization pump sources implemented to
upgrade pre-existing optical amplifiers. FIG. 20A is a block
diagram illustrating an exemplary embodiment of an active gain
equalizing pump source 2002 implemented to upgrade a pre-existing
amplifier 2000. In this example, amplifier 2000 includes existing
amplification stage 2008. Existing amplification stage 2008 can
comprise a single-stage amplifier or multiple-stage amplifier
capable of amplifying a multiple wavelength signal 2016. In some
embodiments, the structure and function of amplification stage 2008
can be substantially similar to amplifier 22 of FIG. 1.
[0251] In this embodiment, amplifier 2000 is upgraded to include a
Raman amplification stage 2001. Raman amplification stage 2001 can
comprise a distributed Raman pre-amplifier or a discrete Raman
pre-amplifier. In this embodiment, Raman amplification stage 2001
comprises a distributed pre-amplification stage of amplifier 2000.
Raman amplification stage 2001 includes an active gain equalization
pump source 2002. Pump sources 2002 may comprise any device capable
of spectrally tailoring pump signal 2010. In various embodiments,
the structure and function of pump source 2002 can be substantially
similar to pump source 1352 of FIG. 13B. In other embodiments, the
structure and function of pump source 2002 can be substantially
similar to the combination of pump source 1302 and active gain
equalization element 1304 of FIG. 13A.
[0252] In this embodiment, Raman amplification stage 2001 includes
a Raman gain medium 2006 capable of at least partially compensating
for losses experienced by multiple wavelength signal 2016. In
various embodiments, gain medium 2006 may comprise a Raman gain
fiber coupled to an existing fiber span of an optical communication
system. In other embodiments, Raman gain medium 2006 may comprise
at least a portion of the existing fiber span coupled to existing
amplification stage 2008 prior to the addition of Raman
amplification stage 2001.
[0253] Spectrally tailored pump signal 2010 generates a spectrally
shaped gain spectrum of Raman pre-amplifier 2006. The spectrally
shaped gain spectrum of Raman pre-amplifier 2006 operates to form a
spectrally tailored output optical signal 2012. Spectrally tailored
output signal 2012 comprises a pre-emphasized signal, which results
in an approximately uniform gain over the spectral range of
multiple wavelength signal 2016 when added to the gain spectrum of
existing amplification stage 2008.
[0254] In some embodiments, Raman gain medium 2006 may comprise a
length of dispersion compensating fiber having a slope of
dispersion that is approximately equal to and opposite from the
slope of chromatic dispersion associated with a communication
medium traversed by multiple wavelength signal 2016. In this
embodiment, the dispersion compensating fiber operates to serve as
the gain medium for pre-amplification stage 2001. In an alternative
embodiment, dispersion compensating fiber could comprise a length
of dispersion compensating transmission fiber coupled to a length
of Raman gain fiber. Implementing a dispersion compensating fiber
advantageously generates a low-noise Raman pre-amplifier.
[0255] In this embodiment, amplifier 2000 is upgraded to include a
wavelength combiner 2004 capable of coupling spectrally tailored
pump signal 2010 to gain medium 2006. Wavelength combiner 2004 may
comprise any device, such as, for example, a wavelength division
multiplexer or optical circulator.
[0256] FIG. 20B is a block diagram illustrating an exemplary
embodiment of a plurality of active gain equalizing pump sources
2052a and 2052b implemented to upgrade a pre-existing
multiple-stage amplifier 2050. In this example, multiple-stage
amplifier 2050 comprises at least a first existing amplification
stage 2056a and a second existing amplification stage 2056b.
Although these examples use two existing amplification stages, any
additional number of amplification stages can be used without
departing from the scope of the present disclosure. In these
particular examples, first existing amplification stage 2056a
comprises a low-noise pre-amplifier, while second existing
amplification stage 2056b comprises a high-gain power
amplifier.
[0257] In this embodiment, amplifier 2050 is upgraded to include at
least a first Raman amplification stage 2051a and a second Raman
amplification stage 2051b. Although this example implements two
Raman amplification stages 2051a and 2051b, any additional number
of Raman amplification stages can be implemented without departing
from the scope of the present disclosure. First Raman amplification
stage 2051a can comprise a distributed Raman pre-amplification
stage, a discrete Raman pre-amplification stage, or a hybrid
pre-amplification stage. In various embodiments, the structure and
function of first Raman amplification stage 2051a can be
substantially similar to Raman amplification stage 2001 of FIG.
20A.
[0258] In this example, second Raman amplification stage 2051b
comprises a discrete Raman amplification stage with a Raman gain
spectrum. In various embodiments, the structure and function of
second Raman amplification-stage 2051b can be substantially similar
to intermediate amplification stage 1901 FIG. 19.
[0259] In this example, first Raman amplification stage 2051a
includes a first active gain equalization pump source 2052a and
second Raman amplification stage 2051b includes a second active
gain equalization pump source 2052b. Although this example
implements two pump sources 2052a and 2052b, any additional number
of pump sources can be implemented without departing from the scope
of the present disclosure. Pump sources 2052a and 2052b may
comprise any device capable of spectrally tailoring pump signals
2060a and 2060b, respectively.
[0260] In this embodiment, first Raman amplification stage 2051a
includes a first wavelength combiner 2054a and second Raman
amplification stage 2051b includes a second wavelength combiner
2054b. Although this example implements two wavelength combiners
2054a and 2054b, any additional number of combiners can be used
without departing from the scope of the present disclosure. In this
example, first wavelength combiner 2054a operates to couple
spectrally tailored pump signal 2060a to a first Raman gain medium
2058a. In a similar manner, second wavelength combiner 2054b
operates to couple spectrally tailored pump signal 2060b to a
second Raman gain medium 2058b. Wavelength combiners 2054a and
2054b may comprise any device, such as, for example, a wavelength
division multiplexer.
[0261] In operation, spectrally tailored pump signal 2060a
generates a spectrally shaped gain spectrum within first Raman
amplification stage 2051a. The spectrally shaped gain spectrum of
first Raman amplification stage 2051a operates on input signal 2066
to form a spectrally tailored output optical signal 2063.
Spectrally tailored output signal 2063 comprises a pre-emphasized
input to first existing amplification stage 2056a. First existing
amplification stage 2056a operates on signal 2063 to form signal
2064.
[0262] Spectrally tailored pump signal 2060b of second Raman
amplification stage 2051b generates a spectrally shaped gain
spectrum within second Raman amplification stage 2051b. The
spectrally shaped gain spectrum of second Raman amplification stage
2051b operates on signal 2064 output from first existing
amplification stage 2056a to form a spectrally tailored output
optical signal 2062. Second existing amplification stage 2056b
applies its gain spectrum to signal 2062 to form output signal
2061. Through appropriate control of the gain spectra of Raman
amplification stages 2051a and 2051b, output signal 2061
experiences an approximately flat overall gain profile for
amplifier 2050.
[0263] By using these and similar active gain equalization
techniques, existing amplifiers can be upgraded to flatten
non-uniform gain profiles over a given wavelength range.
Alternatively, these techniques allow for increasing an amplifiers
bandwidth while maintaining, or even improving, the uniformity of
the amplifier's gain profile.
[0264] X. Distributed Amplification in High Loss Systems
[0265] FIGS. 21A and 21B are block diagrams illustrating exemplary
embodiments of distributed Raman amplifiers capable of at least
partially counteracting losses in relatively high loss systems.
Distributed Raman amplifiers typically comprise an improved noise
figure when compared to a similar discrete Raman amplifier. The
improved noise figure enables distributed Raman amplification to be
used to increase system capacity, to account for variations in
fiber specifications, and/or to increase the separation between
amplifiers and/or regenerators. It can be particularly advantageous
to use distributed Raman amplifiers in the first stages of a
cascade of amplification stages because the noise figure of the
first few amplification stages is particularly important in
generating a sufficient noise figure.
[0266] Existing optical communication systems typically comprise
fixed hut spacing (e.g., locations within the system that
amplifiers can be placed, typically every 40-45 km) and fixed
transmission fiber losses. These systems can be upgraded to include
distributed Raman amplification to provide more uniform gain to the
optical signal over the fiber span, which can maintain a relatively
high signal amplitude and account for fiber losses. In addition,
distributed Raman amplification can provide a relatively better
signal-to-noise ratio than discrete amplifiers. Consequently,
communication systems that require maintenance of relatively high
signal amplitude or comprise a relatively high optical fiber loss
can benefit from distributed Raman amplification.
[0267] A soliton-based communication system provides one example of
a system requiring maintenance of a relatively high signal
amplitude. Soliton signals typically comprise pulses that
substantially maintain their shape over a relatively long
communication distance. Conventional soliton systems typically
require maintenance of the signal level within 6 decibels of the
original level. Consequently, in a soliton system comprising
transmission fiber with a loss of 0.2 dB/km, the soliton system
requires hut spacing approximately every 30 km. Another aspect of
this disclosure recognizes that implementing a distributed
amplifier enables maintenance of the relatively high signal level
throughout the propagation of the Soliton signal and allows hut
spacing to be increased to more than 30 km. In various embodiments,
a distributed Raman amplifier can be implemented in a soliton
communication system without decreasing the current hut
spacing.
[0268] An optical system operating with optical wavelengths shorter
than 1430 nm or longer than 1610 nm provide a few examples of
optical systems comprising a relatively high fiber loss. Operating
a Raman amplifier in the 1310 nm window typically results in an
increased optical signal loss. For example, a loss of 0.35 dB/km
can be expected for operation in the 1310 nm window, where a loss
of 0.2 dB/km can typically be expected for operation in the 1550 nm
window. As optical signal loss increases, the communication system
requires additional gain to achieve optical signal transparency.
Consequently, a higher signal level is typically required to
maintain the same signal-to-noise ratio for the amplifier.
Increasing the signal level, however, can lead to limits imposed on
the system from transmission fiber non-linearities.
[0269] One aspect of this disclosure recognizes that implementing a
distributed Raman amplifier comprising a relatively lower noise
figure enables an increased gain level without increasing the
signal level, thus minimizing the impact on signal to noise ratio.
In addition, communication systems operating in the higher loss
wavelength range of optical fibers (i.e., wavelengths below 1430 nm
or above 1610 nm) can maintain the same hut spacing by using
distributed amplification.
[0270] FIG. 21A is a block diagram illustrating an exemplary
embodiment of a distributed Raman amplifier 2100 operating in the
1310 nm operating window. In this example, distributed Raman
amplifier 2100 includes a pump source 2102 capable of generating a
pump signal 2110. Pump signal 2110 may comprise one or more
wavelength signals capable of amplifying one or more optical
wavelength signals 2116. Pump source 2102 may comprise any device,
such as, for example, a broadband Raman oscillator, a laser diode
pump, an active gain equalized pump source, or a combination of
these or other pump sources.
[0271] In one particular embodiment, the structure and function of
pump source 2102 can be substantially similar to active gain
equalization pump source 1352 of FIG. 13B. In other embodiments,
the structure and function of pump source 2102 can be substantially
similar to the combination of pump source 1302 and active gain
equalization element 1304 of FIG. 13A. In various embodiments, the
structure and function of pump source 2102 can be substantially
similar to any one of the wavelength shifters illustrated in FIGS.
4A through 4C. In other embodiments, the structure and function of
pump source 2102 can be substantially similar to any one of the
broadband Raman oscillators illustrated in FIGS. 6 through 9.
[0272] In one particular embodiment, optical signal 2116 comprises
a multiple wavelength signal operating in the 1310 nm window of
transmission fiber 2106. In this example, a compensation technique
may be implemented to account for the inter-channel Raman effect of
the multiple wavelength optical signals. The inter-channel Raman
effect typically causes the shorter wavelength signals of the
multiple wavelength signal to transfer energy to the longer
wavelength signals, which can result in a tilted gain profile of
optical signal 2116. To substantially counteract this resultant
tilted gain profile, a compensation technique can be implemented to
provide more gain to the shorter wavelength signals. In this
example, pump signal 2110 comprises a spectrally tailored multiple
wavelength pump signal capable of compensating for the gain tilt
and generating a relatively flat gain profile for optical signal
2116. In some embodiments, the spectrally tailored pump signal 2110
can apply an approximately linear compensation to the gain profile
of optical signal 2116.
[0273] In this embodiment, distributed Raman amplifier 2100
includes a wavelength combiner 2104 capable of coupling pump signal
2110 to a transmission fiber 2106. Wavelength combiner 2104 may
comprise any device capable of coupling one or more wavelength
signals to transmission fiber 2106, such as, for example, a
wavelength division multiplexer. Transmission fiber 2106 may
comprise any optical fiber type capable of supporting Raman
gain.
[0274] FIG. 21B is a block diagram illustrating an exemplary
embodiment of a distributed Raman amplifier 2150 implemented in a
pre-existing soliton optical communication system. In this example,
the soliton optical communication system includes an amplifier 2158
operable to amplify soliton optical signal 2166. The structure and
function of amplifier 2158 can be substantially similar to
amplifier 22 of FIG. 1.
[0275] In this example, the soliton optical communication system is
upgraded to include a distributed Raman amplifier 2150. Distributed
Raman amplifier includes a pump source 2152 capable of generating a
pump signal 2160. Pump signal 2160 may comprise one or more
wavelength signals capable of amplifying one or more soliton
optical wavelength signals 2166. Pump source 2152 may comprise any
device, such as, for example, a broadband Raman oscillator, a laser
diode pump source, an active gain equalized pump source, or a
combination of these or other pump sources.
[0276] In one particular embodiment, the structure and function of
pump source 2152 can be substantially similar to active gain
equalization pump source 1352 of FIG. 13B. In other embodiments,
the structure and function of pump source 2152 can be substantially
similar to the combination of pump source 1302 and active gain
equalization element 1304 of FIG. 13A. In various embodiments, the
structure and function of pump source 2152 can be substantially
similar to any one of the wavelength shifters illustrated in FIGS.
4A through 4C. In other embodiments, the structure and function of
pump source 2152 can be substantially similar to any one of the
broadband Raman oscillators illustrated in FIGS. 6 through 9.
[0277] In one particular embodiment, optical signal 2166 comprises
a multiple wavelength soliton signal operating with a relatively
high signal level. In this example, a compensation technique may be
implemented to account for the inter-channel Raman effect of the
multiple wavelength optical signals. The inter-channel Raman effect
typically causes the shorter wavelength signals of the multiple
wavelength signal to transfer energy to the longer wavelength
signals, which can result in a tilted gain profile of optical
signal 2166. To substantially counteract this resultant tilted gain
profile, a compensation technique can be implemented to provide
more gain to the shorter wavelength signals. In this example, pump
signal 2160 comprises a spectrally tailored multiple wavelength
pump signal capable of compensating for the gain tilt and
generating a relatively flat gain profile for optical signal 2166.
In some embodiments, the spectrally tailored pump signal 2160 can
apply an approximately linear compensation to the gain profile of
optical signal 2166.
[0278] In this embodiment, distributed Raman amplifier 2150
includes a wavelength combiner 2154 capable of coupling pump signal
2160 to a transmission fiber 2156. Wavelength combiner 2154 may
comprise any device capable of coupling one or more wavelength
signals to transmission fiber 2156, such as, for example, a
wavelength division multiplexer. Transmission fiber 2156 may
comprise any optical fiber type capable of supporting Raman
gain.
[0279] XI. Co-propagating or Bi-Directionally Pumped Amplifiers
[0280] FIGS. 22A and 22B are block diagrams illustrating exemplary
embodiments of co-propagating or bi-directionally pumped Raman
amplifiers implementing a high-dispersion gain fiber. As used
throughout this document, the term "high dispersion gain fiber"
refers to an optical fiber having a magnitude of dispersion greater
than two (2) picoseconds per nanometer-kilometer over the signal
wavelengths of the amplifier. Although this example shows a
bi-directional communication link, Raman amplifiers 2200 and 2250
could alternatively be implemented in a uni-directional optical
communication system.
[0281] In these embodiments, each Raman amplifier 2200 and 2250
comprises at least one pump signal 2210 and 2260 that co-propagates
with at least one of the wavelengths of optical signals 2216 and
2266, respectively. Conventional design approaches that have
implemented Raman amplification typically limit the pump signal to
counter-propagate in relation to the optical signal. As used
throughout this document, the term "counter-propagate" refers to a
pump signal that propagates through the gain medium of an optical
device in a direction counter to, the direction of optical signal
2216. Counter-propagation has typically been used to minimize the
duration of interaction and cross talk between the pump signal and
the optical signal, and to minimize inter-channel interference
between one or more wavelengths of the optical signal.
[0282] Unlike conventional applications in one particular
embodiment, amplifiers 2200 and 2250 implement at least one pump
signal that co-propagates with at least one wavelength of the
optical signal. Throughout this document, the term "co-propagates"
or "co-propagating" refers to a condition where, for at least some
time at least a portion of the pump signal propagates through the
gain medium in the same direction as at least one wavelength of the
optical signal being amplified.
[0283] One aspect of this disclosure recognizes that generating a
sufficient walk off between different wavelength signals can enable
the formation of a co-propagating or a bi-directionally pumped
Raman amplifier that minimizes cross talk and inter-channel
interference. In particular, cross talk between optical signals and
pump signals, and inter-channel interference can be minimized by
ensuring that the different wavelengths walk off relatively
rapidly.
[0284] The rate of walk off for a wavelength depends at least in
part on the group velocity dispersion in an optical fiber. The
group velocity dispersion depends at least in part on the magnitude
of dispersion of the gain fiber and the velocity differences of the
wavelengths traversing the gain fiber. The velocity differences of
the wavelengths depend at least in part on the magnitude of
dispersion (D) of the gain fiber and the difference
(.DELTA..lambda.) between the optical signal wavelengths and/or a
pump signal wavelength. Increasing the length (L) and/or dispersion
of the gain fiber tends to increase the timing difference between
particular wavelengths. Thus, the larger the dispersion and longer
the gain fiber, the larger the timing difference between
wavelengths. In equation form, this relationship is expressed
as:
.DELTA.t=D.times..DELTA..lambda.L (1)
[0285] where .DELTA.t is the difference in transit time between
optical signals with a wavelength difference .DELTA..lambda..
[0286] One aspect of this disclosure recognizes that a Raman
amplifier implementing at least one co-propagating pump can be
implemented that ensures sufficient. walk off between different
wavelength signals by using an adequate length of high-dispersion
Raman gain fiber. The walk-off length (L.sub.wo) can be determined
by: 1 L wo = t D .times. ( 2 )
[0287] where .DELTA.t is the bit period (e.g., inverse of the bit
rate), D is the average group velocity dispersion, and
.DELTA..lambda. is the wavelength separation between the two
signals or signal and pump. This equation enables the determination
of the walk-off length between the optical signal wavelengths and
the pump wavelength (L.sub.wosp) where .DELTA..lambda. comprises
the difference between the center wavelength of the optical signal
and the pump signal wavelength. In addition, this equation enables
the determination of the walk-off length between adjacent channels
of the optical signals (L.sub.woss) where .DELTA..lambda. comprises
the difference between adjacent optical signals. As used in this
document, the term "channel" refers to a center wavelength of an
information carrying signal and/or wavelength. A channel is
adjacent to another channel if there are no information carrying
signals between them.
[0288] To ensure adequate walk-off between the pump signal
wavelengths and the optical signal wavelengths, the length of the
gain fiber (L.sub.amp) should be at least ten (10) times the walk
off length (L.sub.wosp). In equation form, this relationship is
expressed as:
L.sub.amp>10.times.L.sub.wosp (3)
[0289] In addition, to ensure adequate walk off between adjacent
co-propagating optical signals, the length of the gain fiber
(L.sub.amp) should be at least two (2) times the walk off length
(L.sub.woss) for those signals. In equation form, this relationship
is expressed as:
L.sub.amp>2.times.L.sub.woss (4)
[0290] In various embodiments, the optical signal comprises a
magnitude of dispersion acquired by the optical signal while
traversing the optical communication system. In some embodiments,
the dispersion acquired by the optical signal while traversing an
optical communication system can comprise a combination of a
residual dispersion and/or local dispersion. As used throughout
this document, the term "residual dispersion" refers to dispersion
remaining in an optical signal after traversing a dispersion
compensating element. The term "local dispersion" refers to
dispersion acquired by the optical signal while traversing a
particular span or transmission link of the optical communication
system.
[0291] In one particular embodiment, the dispersion compensating
element comprises a maximum dispersion compensation level that is
intentionally selected to provide only partial compensation for
dispersion acquired by the optical signal while traversing a span
of the system. Providing only partial dispersion compensation in
each span allows the system to maintain a sufficiently high
dispersion. Maintaining a sufficiently high dispersion throughout
the system is advantageous in providing adequate walk off between
the pump signal and the optical signal, which tends reduce the RIN
system penalty. In addition, the optical signal noise associated
with the RIN penalty and the residual dispersion at least partially
destructively interact when received by a receiver after traversing
the multiple span communication system, thus reducing the overall
system noise penalty. In various embodiments, each dispersion
compensating element comprises a maximum dispersion compensation
level of ninety-nine (99) percent or less, ninety (90) percent or
less, seventy-five (75) percent or less, or fifty (50) percent or
less.
[0292] FIG. 22A is a block diagram illustrating an exemplary
embodiment of a Raman amplifier 2200 implementing a
uni-directionally pumped Raman gain fiber 2206. In this example,
Raman amplifier 2200 includes a pump source 2202 capable of
generating a pump signal 2210. Pump source 2202 may comprise any
device capable of generating pump signal 2210.
[0293] In some embodiments, pump source 2202 can comprise a
low-noise pump source, such as, for example, a pump source
comprising a relatively low relative noise intensity (RIN) light
source. Implementing a low-noise pump provides the advantage of
minimizing cross talk between pump signal 2210 and multiple
wavelength signal 2216 while at least a portion of the signals are
co-propagating.
[0294] In this particular embodiment, a Raman gain fiber 2206 is
implemented in a bi-directional optical communication system. In
this example, Raman gain fiber 2206 receives at least a first
optical signal 2216a and a second optical signal 2216b. In this
example, first optical signal 2216a comprises one or more
wavelengths propagating in a first direction through gain fiber
2206. Similarly, second optical signal 2216b comprises one or more
wavelengths propagating in a second direction through gain fiber
2206. In one particular embodiment, the first direction of
propagation is approximately complimentary to the second direction
of propagation. In this particular embodiment, at least one
wavelength of pump signal 2210 co-propagates with at least one
wavelength of second optical signal 2216b. In an alternative
embodiment, pump signal 2210 can be introduced to gain fiber 2206
in a manner that enables at least one wavelength of pump signal
2210 to co-propagate with at least one wavelength of first optical
signal 2216a.
[0295] In an alternative embodiment, Raman gain fiber 2206 can be
implemented in an uni-directional optical communication system. In
that example, Raman gain fiber 2206 receives an optical signal
traversing gain fiber 2206 in a first direction of propagation. In
this embodiment, pump signal 2210 is introduced to Raman gain fiber
2206 in a manner that enables at least one wavelength of pump
signal 2210 to co-propagate with at least one wavelength of optical
signal 2216.
[0296] In some embodiments, optical signal 2216a and 2216b
traveling in different directions can comprise distinct sets of
wavelengths. As one particular example, each wavelength of first
optical signal 2216a can be adjacent to one or more wavelengths of
second optical signal 2216b. For example, where first optical
signal 2216a comprises wavelengths .lambda.1, .lambda.3, and
.lambda.5, first optical signal 2216b could comprise at least
wavelengths .lambda.2, .lambda.4, and .lambda.6, which are adjacent
to wavelengths .lambda.1, .lambda.3, and .lambda.5 of signal 2216a.
In this particular example, the first direction of propagation and
the second direction of propagation are approximately complimentary
one another. In various embodiments, optical signals 2216a and
2216b propagate through amplifier 2200 having a data rate of 2.5
gigabits per second or more, or having a data rate of 9.5 gigabits
per second or more.
[0297] In this embodiment, Raman amplifier 2200 includes a
uni-directionally pumped gain fiber 2206 capable of amplifying
optical signals 2216a and 2216b. Implementing a uni-directionally
pumped gain fiber 2206 is particularly advantageous in a Raman
amplifier where changes in pump power are three (3) decibels or
less in gain fiber 2206. In this example, gain fiber 2206 operates
to minimize cross talk and inter-channel interference. Gain fiber
2206 may comprise any optical fiber capable of generating a
relatively high magnitude of dispersion over the range of signal
wavelengths, such as, for example, dispersion shifted fiber,
standard single mode fiber, or dispersion compensating fiber.
[0298] Although the use of high dispersion gain fiber can minimize
inter-channel interference, a system designer should also consider
the impact of Raman cross talk between wavelengths of a multiple
wavelength optical signal. This effect results from shorter
wavelengths transferring energy to longer wavelengths of an optical
signal via the Raman effect. For example, if equal power is
launched in four adjacent multiple wavelength signal channels, at
the end of the gain fiber the channel power profile will typically
be tilted with more power in the longer wavelength channels. In
various embodiments, a pre-emphasis technique can be implemented to
account for Raman cross talk and achieve a substantially uniform
power profile. In other embodiments, active gain equalization can
be implemented to compensate for the Raman cross talk effect.
[0299] In this embodiment, Raman amplifier 2200 includes a
wavelength combiner 2204 capable of coupling pump signal 2210 to
Raman gain fiber 2206. Wavelength combiner 2204 may comprise any
device capable of coupling one or more wavelength signals to Raman
gain fiber 2206, such as, for example, a wavelength division
multiplexer.
[0300] FIG. 22B is a block diagram illustrating an exemplary
embodiment of a Raman amplifier 2250 implementing a
bi-directionally pumped Raman gain fiber 2256. In this example,
Raman amplifier 2250 includes at least a first pump source 2252a
and a second pump source 2252b. Although this example includes two
pump sources 2252a and 2252b, any number of pump sources can be
used without departing from the scope of the present disclosure.
Each pump source 2252a and 2252b may comprise any device capable of
generating pump signal 2260. The structure and function of each
pump source 2252a and 2252b can be substantially similar to pump
source 2202 of FIG. 22A. In one particular embodiment, pump sources
2252a and 2252b each comprise a low-noise pump source, such as, for
example, a pump source comprising a relatively low relative noise
intensity (RIN) light source.
[0301] In an alternative embodiment, Raman amplifier 2250 can
include a first optical isolator and a second optical isolator. The
first optical isolator can be coupled between pump source 2252a and
gain fiber 2256. The first optical isolator operates to minimize
cross talk between pump signal 2260b and 2260a and/or instabilities
generated through feedback. The second optical isolator can be
coupled between pump source 2252b and gain fiber 2256 and operate
to minimize cross talk between pump signal 2260a and 2260b.
[0302] In this particular embodiment, a Raman gain fiber 2256 is
implemented in a bi-directional optical communication system. In
this example, Raman gain fiber 2256 receives at least a first
optical signal 2266a and a second optical signal 2266b. In this
example, first optical signal 2266a comprises one or more
wavelengths propagating in a first direction through gain fiber
2256. Similarly, second optical signal 2266b comprises one or more
wavelengths propagating in a second direction through gain fiber
2256. In one particular embodiment, the first direction of
propagation is approximately complimentary to the second direction
of propagation. In this particular embodiment, at least one
wavelength of pump signal 2260b co-propagates with at least one
wavelength of second optical signal 2266b. In an alternative
embodiment, pump signal 2260a can be introduced to gain fiber 2256
in a manner that enables at least one wavelength of pump signal
2260a to co-propagate with at least one wavelength of first optical
signal 2266a.
[0303] In an alternative embodiment, Raman gain fiber 2256 can be
implemented in an uni-directional optical communication system. In
that example, Raman gain fiber 2256 receives an optical signal 2266
traversing gain fiber 2256 in a first direction of propagation. In
this embodiment, pump signal 2260 is introduced to Raman gain fiber
2256 in a manner that enables at least one wavelength of pump
signal 2260 to co-propagate with at least one wavelength of optical
signal 2266.
[0304] In some embodiments, each wavelength of first optical signal
2266a is adjacent to a wavelength of second optical signal 2266b.
For example, where first optical signal 2266a comprises wavelengths
.lambda.1, .lambda.3, and .lambda.5, second optical signal 2266b
would comprise at least wavelengths .lambda.2, .lambda.4, and
.lambda.6, which are adjacent to wavelengths .lambda.1, .lambda.3,
and .lambda.5 of signal 2266a. In these particular examples, the
first direction of propagation and the second direction of
propagation are substantially opposite one another. In various
embodiments, optical signals 2266a and 2266b propagate through
amplifier 2250 having a data rate of 2.5 gigabits per second or
more, or having a data rate of 9.5 gigabits per second or more.
[0305] In this embodiment, Raman amplifier 2250 includes a
bi-directionally pumped Raman gain fiber 2256 capable of amplifying
multiple wavelength signals 2266a and 2266b. Implementing a
bi-directionally pumped Raman gain fiber 2256 is particularly
advantageous in a Raman amplifier where changes in pump power are
greater than three (3) decibels in gain fiber 2256. In this
example, gain fiber 2256 operates to minimize cross talk between
pump signal 2260 and multiple wavelength signal 2266. In addition,
gain fiber 2256 operates to minimize inter-channel interference
between adjacent signals within multiple wavelength signals 2266.
Gain fiber 2256 may comprise any optical fiber capable of
generating a relatively high magnitude of dispersion, such as, for
example, dispersion shifted fiber, standard single mode fiber, or
dispersion compensating fiber. In one particular embodiment, gain
fiber 2256 comprises a sufficient length of dispersion compensating
fiber with a magnitude of dispersion of at least sixteen (16)
picoseconds per nanometer-kilometer.
[0306] In this embodiment, Raman amplifier 2250 includes at least a
first wavelength combiner 2254a and a second wavelength combiner
2254b. Although this example includes two wavelength combiners
2254a and 2254b, any number of wavelength combiners can be used
without departing from the scope of the present disclosure. Each
wavelength combiner 2254a and 2254b may comprise any device capable
of coupling one or more wavelength signals to Raman gain fiber
2256, such as, for example, a wavelength division multiplexer.
[0307] XII. Examples of Bi-Directionally Pumped Raman
Amplifiers
[0308] This section provides specific examples of bi-directional
Raman amplifiers. In these examples, the bi-directional Raman
amplifiers receive an optical signal band centered at approximately
the 1550 nm wavelength. The Raman amplifiers also receive a pump
signal centered at approximately the 1450 nm wavelength
(.DELTA..lambda.=100 nm for signal and pump). In this example, the
optical signal comprises a multiple wavelength signal with a
channel spacing of approximately 0.8 nm. Each adjacent channel
counter-propagates with the channel spaced approximately 0.8 nm. In
other words, the channel spacing between adjacent co-propagating
channels is approximately 1.6 nm. The multiple wavelength optical
signals propagate at bit rates of approximately 2.5 Gb/s in a first
embodiment, and at 10 Gb/s in a second embodiment. The respective
bit periods of the two embodiments are 400 picoseconds or 100
picoseconds, respectively.
[0309] In a first example, the bi-directional Raman amplifier
implements a gain fiber comprising dispersion compensating fiber
with a length of 8 km in the first embodiment, and 15 km in the
second embodiment. In addition, the dispersion compensating fiber
comprises an average magnitude of dispersion of 90
picoseconds/nm-km and a loss of 0.55 dB/km at the pump signal
wavelength.
[0310] According to equations (1)-(4) discussed above, the walk off
length (L.sub.woss) between adjacent signal channels is:
[0311] L.sub.woss=2.78 km at 2.5 Gb/s; and
[0312] L.sub.woss=0.69 km at 10 Gb/s
[0313] The walk off length (L.sub.wosp) between the pump signal and
the multiple wavelength optical signal is:
[0314] L.sub.wosp=44.4 m at 2.5 Gb/s; and
[0315] L.sub.wosp=11.1 m at 10 Gb/s
[0316] This example illustrates that adequate walk off (L.sub.wosp)
between the pump signal wavelengths and optical signal wavelengths
can be achieved in either embodiment:
[0317] 8 km>10.times.44.4 m at 2.5 Gb/s
[0318] 15 km>10.times.11.1 m at 10 Gb/s
[0319] In addition, adequate walk off (L.sub.woss) between adjacent
signal channels can also be achieved in either embodiment:
[0320] 8 km>2.times.2.78 km at 2.5 Gb/s
[0321] 15 km>2.times.0.69 km at 10 Gb/s
[0322] Therefore, a bi-directional amplifier implementing
dispersion compensating fiber can be made to satisfactorily amplify
multiple wavelength signals propagating at least at 2.5 Gb/s or 10
Gb/s. However, since the loss is more than 3 dB for the pump
signal, it may be advantageous to bi-directionally PUMP the
amplifier, as shown in FIG. 22B.
[0323] Implementing dispersion compensating fiber as the Raman gain
fiber in a bi-directional Raman amplifier provides the advantage of
enabling the gain fiber to function as a dispersion compensating
element. In various embodiments, the gain fiber comprises a
sufficient length of dispersion compensating fiber to at least
partially compensate for chromatic dispersion. In other
embodiments, a bi-directional amplifier comprising dispersion
compensating fiber can be used to upgrade an existing
bi-directional amplifier. In some embodiments, the bi-directional
amplifier can implement active gain equalization to achieve an
approximately uniform gain over a specified spectral range.
[0324] In a second example, the bi-directional Raman amplifier
implements a gain fiber comprising a standard single mode fiber
with a length of 8 km. In addition, the standard single mode fiber
comprises an average magnitude of dispersion of 16
picoseconds/nm-km and a loss of 0.30 dB/km at the pump signal
wavelength.
[0325] According to equations (1)-(4) discussed above, the walk off
length (L.sub.woss) between adjacent signal channels is:
[0326] L.sub.woss=15.6 km at 2.5 Gb/s; and
[0327] L.sub.woss=3.9 km at 10 Gb/s
[0328] The walk off length (L.sub.wosp) between the pump signal and
the multiple wavelength optical signal is:
[0329] L.sub.wosp=0.25 km at 2.5 Gb/s; and
[0330] L.sub.wosp=62.5 m at 10 Gb/s
[0331] This example illustrates that adequate walk off (L.sub.wosp)
between the pump signal wavelengths and optical signal wavelengths
can be achieved in either embodiment:
[0332] 8 km>10.times.0.25 km at 2.5 Gb/s
[0333] 8 km>10.times.62.5 m at 10 Gb/s
[0334] In addition, this example illustrates that adequate walk off
(L.sub.woss) between adjacent signal channels can also be achieved,
but only at a bit rate of 10 Gb/s:
[0335] 8 km<2.times.15.6 km at 2.5 Gb/s (fails)
[0336] 8 km>2.times.3.9 km at 10 Gb/s
[0337] Therefore, a bi-directional amplifier implementing standard
single mode fiber with a fiber length of 8 km can be made to
satisfactorily amplify optical signals propagating at least at 10
Gb/s. In addition, the bi-directional amplifier of this example
cannot satisfy the design criteria of the above equations at a bit
rate of 2.5 Gb/s. However, a bi-directional amplifier implementing
standard single mode fiber can be made to satisfactorily amplify
optical signals propagating at least at 2.5 Gb/s or 10 Gb/s by
selecting the appropriate channel spacing, fiber length, and/or
wavelengths propagated.
[0338] In a third example, the bi-directional Raman amplifier
implements a gain fiber comprising dispersion-shifted fiber with a
length of 8 km. In addition, the dispersion-shifted fiber comprises
an average magnitude of dispersion of 2 picoseconds/nm-km and a
loss of 0.30 dB/km at the pump signal wavelength.
[0339] According to equations (1)-(4) discussed above, the walk off
length (L.sub.woss) between adjacent signal channels is:
[0340] L.sub.woss=125 km at 2.5 Gb/s; and
[0341] L.sub.woss=31.3 km at 10 Gb/s
[0342] The walk off length (L.sub.wosp) between the pump signal and
the multiple wavelength signal is:
[0343] L.sub.wosp=2 km at 2.5 Gb/s; and
[0344] L.sub.wosp=0.5 km at 10 Gb/s
[0345] This example illustrates that adequate walk off (L.sub.wosp)
between the pump signal wavelengths and optical signal wavelengths
can be achieved only at a bit rate of 10 Gb/s:
[0346] 8 km<10.times.2 m at 2.5 Gb/s (fails)
[0347] 8 km>10.times.0.5 m at 10 Gb/s
[0348] In addition, this example illustrates that adequate walk off
(L.sub.woss) between adjacent signal channels can not be achieved
in either embodiment:
[0349] 8 km<2.times.125 km at 2.5 Gb/s (fails)
[0350] 8 km<2.times.31.3 km at 10 Gb/s (Fails)
[0351] Therefore, a bi-directional amplifier implementing
dispersion-shifted fiber with a fiber length of 8 km cannot satisfy
the design criteria of the above equations at bit rates of at least
2.5 Gb/s and 10 Gb/s. However, a bi-directional amplifier
implementing dispersion-shifted fiber can be made to satisfactorily
amplify optical signals propagating at least at 2.5 Gb/s or 10 Gb/s
by selecting the appropriate channel spacing, fiber length, and/or
wavelengths propagated.
[0352] XIII. Laser Diode Pumped BBRO for New Bandwidth Windows
[0353] FIGS. 23A through 23C are block diagrams illustrating
exemplary embodiments of Raman amplifiers implementing a laser
diode pump to various wavelength ranges. In these examples, each
Raman amplifier 2300, 2330, and 2360 includes an isolator 2308,
2338, and 2368 operable to minimize lasing feedback.
[0354] FIG. 23A is a block diagram illustrating an exemplary
embodiment of a Raman amplifier 2300 implementing a laser diode
pump 2302 to amplify wavelengths in the 1400 nm window. In this
example, Raman amplifier 2300 includes laser diode pump 2302
capable of generating a multiple wavelength pump signal 2310. The
structure and function of laser diode pump 2302 can be
substantially similar to any of the pump sources in FIGS. 2A
through 2D. In this particular embodiment, pump 2302 comprises a
plurality of laser diodes each capable of generating a lasing
wavelength centered at approximately the 1310 nm wavelength.
[0355] In this embodiment, Raman amplifier 2300 is coupled to a
fiber span 2312 comprising a low-loss operating window
approximately centered at the 1400 nm wavelength. Fiber span 2312
may comprise any optical fiber capable of generating a low-loss
operating window approximately centered at the 1400 nm wavelength,
such as, for example, the "all-wave" optical fiber.
[0356] In this embodiment, Raman amplifier 2300 includes a
wavelength combiner 2304 capable of coupling pump signal 2310 to
gain fiber 2306. Wavelength combiner 2304 may comprise any device,
such as, for example, a wavelength division multiplexer. Gain fiber
2306 may comprise any optical fiber capable of transferring the
optical energy of pump signal 2310 to multiple wavelength signal
2316. In one particular embodiment, at least a portion of gain
fiber 2306 comprises an appropriate length of dispersion
compensating fiber capable of at least partially counteracting
chromatic dispersion associated with multiple wavelength signal
2316.
[0357] In this particular embodiment, pump signal 2310 traverses
gain fiber 2306 in a direction approximately complimentary to that
of multiple wavelength optical signal 2316. In an alternative
embodiment, pump signal 2310 can be introduced to gain fiber 2306
in a manner that enables at least one wavelength of pump signal
2310 to co-propagate with at least one wavelength of optical signal
2316. In that example, optical signal 2316 traverses gain fiber
2306 in a substantially similar direction of propagation to that of
pump signal 2310. In yet another alternative embodiment, gain fiber
2306 can be bi-directionally pumped.
[0358] In this embodiment, laser diode pump 2302 directly pumps
gain fiber 2306. In an alternative embodiment, Raman amplifier 2300
can include an active gain equalization element coupled between
laser diode pump 2302 and wavelength combiner 2304. The structure
and function of the active gain equalization element can be
substantially similar to active gain equalization element 1304 of
FIG. 13A.
[0359] FIG. 23B is a block diagram illustrating an exemplary
embodiment of a Raman amplifier 2330 implementing a broadband Raman
oscillator 2332 to amplify wavelengths in the violet window (e.g.,
1430 and 1525 nm wavelength range). In this example, Raman
amplifier 2330 includes broadband Raman oscillator 2332 operable to
wavelength shift a laser diode generated pump signal. In various
embodiments, the structure and function of Raman oscillator 2332
can be substantially similar to any of the broadband Raman
oscillators illustrated in FIGS. 6 through 9. In this embodiment,
Raman oscillator 2332 operates to wavelength shift a pump signal
centered at approximately the 1310 nm wavelength to a first Raman
cascade order. In this example, the first Raman cascade order
comprises a 1356 nm to 1424 nm wavelength range.
[0360] In this embodiment, broadband Raman oscillator 2332 also
operates to generate a spectrally tailored pump signal 2340. In one
particular embodiment, broadband Raman oscillator 2332 comprises an
active gain equalization element operable to generate spectrally
tailored pump signal 2340. The active gain equalizing element may
comprise any device capable of spectrally tailoring pump signal
2340. In one particular embodiment, the active gain equalizing
element can generate an approximately uniform gain over the desired
spectral range. For example, the active gain equalizing element may
comprise a Mach-Zehnder type filter, a dielectric filter, a lattice
device, or a long-period grating.
[0361] In another embodiment, broadband Raman oscillator 2332
operates to receive a multiple wavelength pump signal generated by
a plurality of laser diodes capable of tailoring the gain spectrum
of the pump signal. Spectrally tailored pump signal 2340 can be
generated by adjusting the amplitude of each of the plurality of
laser diodes. In this example, each of the plurality of laser
diodes is grating tuned and generates a specific lasing wavelength.
In one particular embodiment, each of the plurality of laser diodes
generates a lasing wavelength centered at approximately the 1310 nm
wavelength.
[0362] In this embodiment, Raman amplifier 2330 includes a
wavelength combiner 2334 capable of coupling pump signal 2340 to
gain fiber 2336. Wavelength combiner 2334 may comprise any device,
such as, for example, a wavelength division multiplexer. Gain fiber
2336 may comprise any optical fiber capable of transferring the
optical energy of pump signal 2340 wavelength to multiple
wavelength signal 2346. In one particular embodiment, at least a
portion gain fiber 2336 comprises an appropriate length of
dispersion compensating fiber capable of at least partially
counteracting chromatic dispersion associated with multiple
wavelength signal 2346.
[0363] In this particular embodiment, pump signal 2340 traverses
gain fiber 2336 approximately complimentary to multiple wavelength
optical signal 2346. In an alternative embodiment, pump signal 2340
can be introduced to gain fiber 2336 in a manner that enables at
least one wavelength of pump signal 2340 to co-propagate with at
least one wavelength of optical signal 2346. In that example,
optical signal 2346 traverses gain fiber 2336 in a substantially
similar direction of propagation to that of pump signal 2340. In
yet another alternative embodiment, gain fiber 2336 can be
bi-directionally pumped.
[0364] In this embodiment, broadband Raman oscillator 2332 includes
an active gain equalization element. In an alternative embodiment,
Raman amplifier 2330 can include an active gain equalization
element coupled between broadband Raman oscillator 2332 and
wavelength combiner 2334. The structure and function of the active
gain equalization element can be substantially similar to active
gain equalization element 1304 of FIG. 13A.
[0365] FIG. 23C is a block diagram illustrating an exemplary
embodiment of a Raman amplifier 2360 implementing a laser diode
pump 2362 to amplify wavelengths in the violet window. In this
example, Raman amplifier 2360 includes laser diode pump 2362
capable of generating a multiple wavelength pump signal 2370. The
structure and function of laser diode pump 2362 can be
substantially similar to any of the pump sources in FIGS. 2A
through 2D. In this particular embodiment, pump 2362 comprises a
plurality of laser diodes each capable of generating a lasing
wavelength centered at approximately the 1310 nm wavelength.
[0366] In this embodiment, Raman amplifier 2360 comprises a Raman
amplification stage 2364 coupled to laser diode pump 2362. Raman
amplification stage 2364 comprises a broadband Raman oscillator
operable to wavelength shift pump signal 2370 received from laser
diode pump 2362. In this example, the broadband Raman oscillator
operates to wavelength shift pump signal 2370 by two Raman cascade
orders. Raman amplification stage 2364 also operates to amplify a
multiple wavelength signal 2376. In this example, a gain fiber
associated with the broadband Raman oscillator is combined with a
gain fiber used to amplify multiple wavelength signal 2376. Raman
amplification stage 2364 may comprise any device, such as, for
example, a Sagnac Raman cavity, a linear cavity with chirped
gratings, or a circulator loop cavity.
[0367] In this embodiment, laser diode pump 2362 directly pumps
Raman amplification stage 2364. In an alternative embodiment, Raman
amplifier 2360 can include an active gain equalization element
coupled between laser diode pump 2362 and Raman amplification stage
2364. The structure and function of the active gain equalization
element can be substantially similar to active gain equalization
element 1304 of FIG. 13A.
[0368] Although the present invention has been described in several
embodiments, a myriad of changes, variations, alterations,
transformations, and modifications may be suggested to one skilled
in the art, and it is intended that the present invention encompass
such changes, variations, alterations, transformations, and
modifications as falling within the spirit and scope of the
appended claims.
* * * * *