U.S. patent application number 10/033848 was filed with the patent office on 2002-08-29 for broadband sagnac raman amplifiers and cascade lasers.
Invention is credited to Islam, Mohammed N..
Application Number | 20020118709 10/033848 |
Document ID | / |
Family ID | 26763357 |
Filed Date | 2002-08-29 |
United States Patent
Application |
20020118709 |
Kind Code |
A1 |
Islam, Mohammed N. |
August 29, 2002 |
Broadband sagnac raman amplifiers and cascade lasers
Abstract
This invention describes new developments in Sagnac Raman
amplifiers and cascade lasers to improve their performance. The
Raman amplifier bandwidth is broadened by using a broadband pump or
by combining a cladding-pumped fiber laser with the Sagnac Raman
cavity. The broader bandwidth is also obtained by eliminating the
need for polarization controllers in the Sagnac cavity by using an
all polarization maintaining configuration, or at least using loop
mirrors that maintain polarization. The polarization maintaining
cavities have the added benefit of being environmentally stable and
appropriate for turn-key operation. The noise arising from sources
such as double Rayleigh scattering is reduced by using the Sagnac
cavity in combination with a polarization diversity pumping scheme,
where the pump is split along two axes of the fiber. This also
leads to gain for the signal that is independent of the signal
polarization. Finally, a two-wavelength amplifier for 1310 nm and
1550 nm can be implemented by using a parallel combination of Raman
amplifiers with shared pump lasers or by combining Raman amplifiers
with erbium-doped fiber amplifiers. Combinations of the above
improvements can be used advantageously to meet specifications for
broad bandwidth, polarization independence, noise performance and
multi-wavelength operation.
Inventors: |
Islam, Mohammed N.; (Allen,
TX) |
Correspondence
Address: |
WILSON SONSINI GOODRICH & ROSATI
650 PAGE MILL ROAD
PALO ALTO
CA
943041050
|
Family ID: |
26763357 |
Appl. No.: |
10/033848 |
Filed: |
December 19, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10033848 |
Dec 19, 2001 |
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09550730 |
Apr 17, 2000 |
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6370164 |
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09550730 |
Apr 17, 2000 |
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09110696 |
Jul 7, 1998 |
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6052393 |
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09110696 |
Jul 7, 1998 |
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08773482 |
Dec 23, 1996 |
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5778014 |
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60080317 |
Apr 1, 1998 |
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Current U.S.
Class: |
372/3 ; 359/334;
372/69 |
Current CPC
Class: |
H01S 3/094003 20130101;
H01S 3/094096 20130101; H01S 3/094061 20130101; H01S 5/4012
20130101; H01S 5/4087 20130101; H01S 5/4025 20130101; H01S 3/302
20130101 |
Class at
Publication: |
372/3 ; 372/69;
359/334 |
International
Class: |
H01S 003/30; H01S
003/09; H01S 003/00 |
Claims
What is claimed is:
1. A broadband Sagnac Raman amplifier comprising: a first
reflector; a second reflector comprising a Sagnac loop mirror
fabricated from a distributed gain medium and a coupling means
connected to two ends of said Sagnac loop mirror, said first
reflector and said second reflector forming an optical resonator
therebetween; an input port for inputting an optical signal to said
distributed gain medium; a pumping means for generating a pumping
light to pump said distributed gain medium; and an output port for
outputting said optical signal from said distributed gain
medium.
2. The broadband Sagnac Raman amplifier according to claim 1,
wherein said pumping means is a broadband pump comprising: a pump
laser having an output port; and a bandwidth adding mirror
connected to said output port to generate a broadened pump
spectrum.
3. The broadband Sagnac Raman amplifier according to claim 2,
wherein said bandwidth adding mirror comprises a Sagnac loop
mirror.
4. The broadband Sagnac Raman amplifier according to claim 3,
wherein said bandwidth adding mirror further comprises a coupler
with an unequal ratio.
5. The broadband Sagnac Raman amplifier according to claim 4,
wherein said coupler has a ratio of f:(1-f), and
0.ltoreq.f.ltoreq.1.
6. The broadband Sagnac Raman amplifier according to claim 3,
wherein said bandwidth adding mirror further comprises a
polarization controller located within said Sagnac loop mirror.
7. The broadband Sagnac Raman amplifier according to claim 3,
wherein said bandwidth adding mirror further comprises a phase
modulator asymmetrically located within said Sagnac loop
mirror.
8. The broadband Sagnac amplifier according to claim 3, wherein
said bandwidth adding mirror further comprises an amplitude
modulator asymmetrically located within said Sagnac loop
mirror.
9. The broadband Sagnac Raman amplifier according to claim 2,
wherein said pump laser is a cladding pumped fiber laser.
10. The broadband Sagnac Raman amplifier according to claim 9,
wherein said cladding pumped fiber laser is driven by a modulated
pump drive.
11. The broadband Sagnac Raman amplifier according to claim 9,
wherein a mechanical modulation is applied to said cladding pumped
fiber.
12. The broadband Sagnac Raman amplifier according to claim 1,
wherein said distributed gain medium comprises polarization
maintaining fibers cross-spliced at a joint in said Sagnac loop
mirror and said coupling means is polarization maintaining, and
said polarization maintaining fibers interchange polarization axes
at the cross-splicing joint.
13. The broadband Sagnac Raman amplifier according to claim 12,
wherein said coupling means is a coupler having a ratio of
50:50.
14. The broadband Sagnac Raman amplifier according to claim 12,
wherein said coupling means is a bulk 50:50 beam splitter.
15. The broadband Sagnac Raman amplifier according to claim 12,
wherein said pumping means is a broadband pump comprising a pump
laser and a bandwidth adding mirror attached thereto.
16. The broadband Sagnac Raman amplifier according to claim 12,
wherein said input and output ports are polarization maintaining
WDMs.
17. The broadband Sagnac Raman amplifier according to claim 1,
wherein said pumping means comprises: a pump laser generating a
linearly polarized pumping light; and a polarization maintaining
fiber, said pumping light being launched at a 45 degree angle into
said polarization maintaining fiber to produce a beam having two
polarization directions.
18. The broadband Sagnac Raman amplifier according to claim 1,
wherein said pumping means comprises: a pump laser generating a
linearly polarized pumping light; a polarization maintaining fiber;
and a quarter wavelength plate located between said pumping means
and said polarization maintaining fiber such that said polarization
maintaining fiber produces a beam having two polarization
directions.
19. The broadband Sagnac Raman amplifier according to claim 1,
wherein said pumping means comprises: a pump laser generating a
linearly polarized pumping light; a 50:50 coupler dividing said
pumping light into a first beam and a second beam; a retarder
located in the path of said first beam to change the polarization
direction of said first beam; and a polarization beam splitter for
combining said first beam and said second beam to produce a beam
having two polarization directions.
20. The broadband Sagnac Raman amplifier according to claim 19,
wherein said retarder is a half-wave plate.
21. The broadband Sagnac Raman amplifier according to claim 19,
wherein said retarder is a quarter-wave plate.
22. The broadband Sagnac Raman amplifier according to claim 1,
wherein said pumping means comprises: a pump laser generating a
pumping light; a cladding-pumped fiber having two ends, one end
being pumped by said pumping light; a polarization maintaining
fiber spliced at a 45 degree angle to the other end of said
cladding-pumped fiber to output from said polarization maintaining
fiber a beam having two polarization directions.
23. A broadband Sagnac Raman cascade laser comprising: a first
reflector; a second reflector comprising a Sagnac loop mirror
fabricated from a distributed gain medium and a coupling means
connected to two ends of said Sagnac loop mirror, said first
reflector and said second reflector forming an optical resonator
therebetween; a pumping means for generating a pumping light to
pump said distributed gain medium; and an output port for
outputting an optical signal from said distributed gain medium.
24. The broadband Sagnac Raman cascade laser according to claim 23,
wherein said pumping means is a broadband pump comprising: a pump
laser having an output port; and a bandwidth adding mirror
connected to said output port to generate a broadened pump
spectrum.
25. The broadband Sagnac Raman cascade laser according to claim 24,
wherein said bandwidth adding mirror comprises a Sagnac loop
mirror.
26. The broadband Sagnac Raman cascade laser according to claim 25,
wherein said bandwidth adding mirror further comprises a coupler
with an unequal ratio.
27. The broadband Sagnac Raman cascade laser according to claim 26,
wherein said coupler has a ratio of f:(1-f), and
0.ltoreq.f.ltoreq.1.
28. The broadband Sagnac Raman cascade laser according to claim 25,
wherein said bandwidth adding mirror further comprises a
polarization controller located within said Sagnac loop mirror.
29. The broadband Sagnac Raman cascade laser according to claim 25,
wherein said bandwidth adding mirror further comprises a phase
modulator asymmetrically located within said Sagnac loop
mirror.
30. The broadband Sagnac Raman cascade laser according to claim 25,
wherein said bandwidth adding mirror further comprises an amplitude
modulator asymmetrically located within said Sagnac loop
mirror.
31. The broadband Sagnac Raman cascade laser according to claim 24,
wherein said pump laser is a cladding pumped fiber laser.
32. The broadband Sagnac Raman cascade laser according to claim 31,
wherein said cladding pumped fiber laser is driven by a modulated
pump drive.
33. The broadband Sagnac Raman cascade laser according to claim 31,
wherein a mechanical modulation is applied to said cladding pumped
fiber.
34. The broadband Sagnac Raman cascade laser according to claim 23,
wherein said distributed gain medium comprises polarization
maintaining fibers cross-spliced at a joint in said Sagnac loop
mirror and said coupling means is polarization maintaining, and
said polarization maintaining fibers interchange polarization axes
at the cross-splicing joint.
35. The broadband Sagnac Raman cascade laser according to claim 34,
wherein said coupling means is a coupler having a ratio of
50:50.
36. The broadband Sagnac Raman cascade laser according to claim 34,
wherein said coupling means is a bulk 50:50 beam splitter.
37. The broadband Sagnac Raman cascade laser according to claim 34,
wherein said pumping means is a broadband pump comprising a pump
laser and a bandwidth adding mirror attached thereto.
38. The broadband Sagnac Raman cascade laser according to claim 34,
wherein said output port is a polarization maintaining WDM.
39. A broadband pump, comprising: a pump laser having an output
port; and a bandwidth adding mirror connected to said output port
to generate a broadened pump spectrum.
40. The broadband pump according to claim 39, wherein said
bandwidth adding mirror comprises a Sagnac loop mirror.
41. The broadband pump according to claim 40, wherein said
bandwidth adding mirror further comprises a coupler with an unequal
ratio.
42. The broadband pump according to claim 41, wherein said coupler
has a ratio of f:(1-f), and 0.ltoreq.f.ltoreq.1.
43. The broadband pump according to claim 40, wherein said
bandwidth adding mirror further comprises a polarization controller
located within said Sagnac loop mirror.
44. The broadband pump according to claim 40, wherein said
bandwidth adding mirror further comprises a phase modulator
asymmetrically located within said Sagnac loop mirror.
45. The broadband pump according to claim 40, wherein said
bandwidth adding mirror further comprises an amplitude modulator
asymmetrically located within said Sagnac loop mirror.
46. The broadband pump according to claim 39, wherein said pump
laser is a cladding pumped fiber laser.
47. The broadband pump according to claim 46, wherein said cladding
pumped fiber laser is driven by a modulated pump drive.
48. The broadband pump according to claim 46, wherein a mechanical
modulation is applied to said cladding pumped fiber.
49. A broadband Sagnac Raman cascade laser comprising: a first
reflector; a second reflector comprising a Sagnac loop mirror
fabricated from a distributed gain medium and a coupler connected
to two ends of said Sagnac loop mirror, said first reflector and
said second reflector forming an optical resonator therebetween; a
cladding-pumped fiber located in said optical resonator; a pumping
means for generating a pumping light to pump said cladding-pumped
fiber; and an output port for outputting an optical signal from
said distributed gain medium.
50. The broadband Sagnac Raman cascade laser according to claim 49,
wherein said coupler has a ratio of 50:50.
51. The broadband Sagnac Raman cascade laser according to claim 49,
wherein said pumping means is a diode pump array.
52. The broadband Sagnac Raman cascade laser according to claim 49,
further comprising a polarization controller.
53. The broadband Sagnac Raman cascade laser according to claim 49,
wherein said output port is a WDM.
54. A broadband Sagnac Raman amplifier comprising: a first
reflector; a second reflector comprising a Sagnac loop mirror
fabricated from a distributed gain medium and a coupler connected
to two ends of said Sagnac loop mirror, said first reflector and
said second reflector forming an optical resonator therebetween; an
input port for inputting an optical signal to said distributed gain
medium; a cladding-pumped fiber located in said optical resonator;
a pumping means for generating a pumping light to pump said
cladding-pumped fiber; and an output port for outputting said
optical signal from said distributed gain medium.
55. The broadband Sagnac Raman amplifier according to claim 54,
wherein said coupler has a ratio of 50:50.
56. The broadband Sagnac Raman amplifier according to claim 54,
wherein said pumping means is a diode pump array.
57. The broadband Sagnac Raman amplifier according to claim 54,
further comprising a polarization controller.
58. The broadband Sagnac Raman amplifier according to claim 54,
wherein said input port and output port are WDMs.
59. The broadband Sagnac Raman amplifier according to claim 54,
further comprising a gain flattening element connected to said
output port.
60. A broadband Sagnac Raman amplifier comprising: a first
reflector; a second reflector comprising a Sagnac loop mirror
fabricated from a distributed gain medium and a coupler connected
to two ends of said Sagnac loop mirror, said first reflector and
said second reflector forming an optical resonator therebetween; an
input port for inputting an optical signal to said distributed gain
medium; a pumping means for generating a pumping light to pump said
distributed gain medium, said pumping means being connected to one
of said two ends of said Sagnac loop mirror; and an output port for
outputting said optical signal from said distributed gain
medium.
61. The broadband Sagnac Raman amplifier according to claim 60,
wherein said coupler has a ratio of 50:50.
62. The broadband Sagnac Raman amplifier according to claim 60,
wherein said pumping means is a pump laser.
63. The broadband Sagnac Raman amplifier according to claim 60,
further comprising a polarization controller.
64. The broadband Sagnac Raman amplifier according to claim 60,
wherein said input port and output port are WDMs.
65. A broadband Sagnac Raman cascade laser comprising: a first
reflector; a second reflector comprising a Sagnac loop mirror
fabricated from a distributed gain medium and a coupler connected
to two ends of said Sagnac loop mirror, said first reflector and
said second reflector forming an optical resonator therebetween; a
pumping means for generating a pumping light to pump said
distributed gain medium, said pumping means being connected to one
of said two ends of said Sagnac loop mirror; and an output port for
outputting an optical signal from said distributed gain medium.
66. The broadband Sagnac Raman cascade laser according to claim 65,
wherein said coupler has a ratio of 50:50.
67. The broadband Sagnac Raman cascade laser according to claim 65,
wherein said pumping means is a pump laser.
68. The broadband Sagnac Raman cascade laser according to claim 65,
further comprising a polarization controller.
69. The broadband Sagnac Raman cascade laser according to claim 65,
wherein said output port is a WDM.
70. A broadband Sagnac Raman amplifier comprising: a first
reflector; a second reflector comprising a Sagnac loop mirror
fabricated from a polarization maintaining fiber and a coupler
connected to two ends of said Sagnac loop mirror, said first
reflector and said second reflector forming an optical resonator
therebetween; a Raman gain fiber located in said optical resonator;
an input port for inputting an optical signal to said distributed
gain medium; a pumping means for generating a pumping light to pump
said Raman gain fiber; and an output port for outputting said
optical signal from said distributed gain medium.
71. The broadband Sagnac Raman amplifier according to claim 70,
wherein said coupler has a ratio of 50:50.
72. The broadband Sagnac Raman amplifier according to claim 70,
wherein said Sagnac loop mirror comprises a dispersion-shifted
polarization maintaining fiber.
73. The broadband Sagnac Raman amplifier according to claim 70,
wherein said pumping means is a broadband pump comprising a pump
laser having an output port; and a bandwidth adding mirror attached
to said output port.
74. The broadband Sagnac Raman amplifier according to claim 70,
wherein said input port and output port are polarization
maintaining WDMs.
75. The broadband Sagnac Raman amplifier according to claim 70,
wherein said first reflector comprises a Sagnac loop mirror
fabricated from a polarization maintaining fiber and a coupler
connected to two ends of said Sagnac loop mirror.
76. The broadband Sagnac Raman amplifier according to claim 75,
wherein said coupler of said first reflector has a ratio of
50:50.
77. The broadband Sagnac Raman amplifier according to claim 75,
wherein said Sagnac loop mirror of said first reflector comprises a
dispersion-shifted polarization maintaining fiber.
78. A polarization diversity pumping system, comprising: a pumping
means generating a linearly polarized pumping light; and a
polarization maintaining fiber, said pumping light being launched
at a 45 degree angle into said polarization maintaining fiber to
produce a beam having two polarization directions.
79. The polarization diversity pumping system according to claim
78, wherein said pumping means is a laser.
80. A polarization diversity pumping system, comprising: a pumping
means generating a linearly polarized pumping light; a polarization
maintaining fiber; and a quarter wavelength plate located between
said pumping means and said polarization maintaining fiber such
that said polarization maintaining fiber produces a beam having two
polarization directions.
81. The polarization diversity pumping system according to claim
80, wherein said pump means is a laser.
82. A polarization diversity pumping system, comprising: a pumping
means generating a linearly polarized pumping light; a 50:50
coupler dividing said pumping light into a first and second beams;
a retarder located in the path of said first beam to change the
polarization direction of said first beam; and a polarization beam
splitter for combining said first and second beams to produce a
beam having two polarization directions.
83. The polarization diversity pumping system according to claim
82, wherein said pump means is a laser.
84. The polarization diversity pumping system according to claim
82, wherein said retarder is a half-wave plate.
85. The polarization diversity pumping system according to claim
82, wherein said retarder is a quarter-wave plate.
86. A polarization diversity pumping system, comprising: a pumping
means generating a pumping light; a cladding-pumped fiber having
two ends, one end being pumped by said pumping light; a
polarization maintaining fiber spliced at a 45 degree angle to the
other end of said cladding-pumped fiber to output from said
polarization maintaining fiber a beam having two polarization
directions.
87. A two-wavelength broadband Sagnac Raman amplifier, comprising:
a separating means for separating an optical signal to be amplified
into a first and second beams, said first beam having a different
wavelength from said second beam; a first broadband Sagnac Raman
amplifier comprising a first reflector; a second reflector
comprising a Sagnac loop mirror fabricated from a distributed gain
medium and a coupling means connected to two ends of said Sagnac
loop mirror, said first reflector and said second reflector forming
an optical resonator therebetween; an input port for inputting said
first beam to said distributed gain medium; and an output port for
outputting said first beam from said distributed gain medium; a
second broadband Sagnac Raman amplifier comprising a first
reflector; a second reflector comprising a Sagnac loop mirror
fabricated from a distributed gain medium and a coupling means
connected to two ends of said Sagnac loop mirror, said first
reflector and said second reflector forming an optical resonator
therebetween; an input port for inputting said second beam to said
distributed gain medium; and an output port for outputting said
second beam from said distributed gain medium; a pumping means for
generating a pumping light to pump said distributed gain mediums of
said first and second broadband Sagnac Raman amplifiers; and a
combining means for combining said first and second beams.
88. The two-wavelength broadband Sagnac Raman amplifier according
to claim 87, wherein said first broadband Sagnac Raman amplifier is
operated at a wavelength of approximately 1310 nm.
89. The two-wavelength broadband Sagnac Raman amplifier according
to claim 87, wherein said second broadband Sagnac Raman amplifiers
is operated at a wavelength of approximately 1550 nm.
90. A two-wavelength broadband Sagnac Raman amplifier, comprising:
a separating means for separating an optical signal to be amplified
into a first and second beams, said first beam having a different
wavelength from said second beam; a broadband Sagnac Raman
amplifier comprising a first reflector; a second reflector
comprising a Sagnac loop mirror fabricated from a distributed gain
medium and a coupling means connected to two ends of said Sagnac
loop mirror, said first reflector and said second reflector forming
an optical resonator therebetween; an input port for inputting said
first beam to said distributed gain medium; and an output port for
outputting said first beam from said distributed gain medium; an
erbium-doped fiber amplifier having an input port and an output
port, said input port receiving said second beam; a Sagnac Raman
cascade laser for pumping said erbium-doped fiber amplifier
comprising a first reflector; a second reflector comprising a
Sagnac loop mirror fabricated from a distributed gain medium and a
coupling means connected to two ends of said Sagnac loop mirror,
said first reflector and said second reflector forming an optical
resonator therebetween; and an output port for outputting a pumping
beam from said distributed gain medium, wherein said pumping beam
pumping said erbium-doped fiber amplifier; a pumping means for
generating a pumping light to pump said distributed gain media of
said broadband Sagnac Raman amplifier and said Sagnac Raman cascade
laser; and a combining means for combining said first beam and said
second beam from said distributed gain medium of said broadband
Sagnac Raman amplifier and said erbium-doped fiber amplifier
respectively.
91. The two-wavelength broadband Sagnac Raman amplifier according
to claim 90, wherein said broadband Sagnac Raman amplifier is
operated at a wavelength of approximately 1310 nm.
92. The two-wavelength broadband Sagnac Raman amplifier according
to claim 90, wherein said Sagnac Raman cascade laser produces a
pumping beam having a wavelength of approximately 1480 nm.
93. A two-wavelength broadband Sagnac Raman amplifier, comprising:
a separating means for separating an optical signal to be amplified
into a first beam and a second beam, said first beam having a
different wavelength from said second beam; a broadband Sagnac
Raman amplifier comprising a first reflector; a second reflector
comprising a Sagnac loop mirror fabricated from a distributed gain
medium and a coupling means connected to two ends of said Sagnac
loop mirror, said first reflector and said second reflector forming
an optical resonator therebetween; an input port for inputting said
first beam to said distributed gain medium; and a pumping means for
generating a pumping light to pump said distributed gain medium; an
output port for outputting said first beam from said distributed
gain medium; an erbium-doped fiber amplifier having an input port
and an output port, said input port receiving said second beam; and
a combining means for combining said first beam and said second
beam from said distributed gain medium of said broadband Sagnac
Raman amplifier and said erbium-doped fiber amplifier
respectively.
94. The two-wavelength broadband Sagnac Raman amplifier according
to claim 93, wherein said broadband Sagnac Raman amplifier is
operated at a wavelength of approximately 1310 nm.
Description
RELATED APPLICATION
[0001] This is a continuation-in-part application of U.S. patent
application Ser. No. 08/773,482 filed Dec. 23, 1996, entitled
"Sagnac Raman Amplifiers and Cascade Lasers," now allowed. The
present application also claims the priority of U.S. provisional
patent application No. 60/080,317 filed Apr. 1, 1998.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to Sagnac Raman
amplifiers and lasers for telecommunications, cable television
(CATV), and other fiber-optics applications. More particularly, the
invention relates to broadband Sagnac Raman amplifiers and lasers
that have substantially improved bandwidth and noise
performance.
BACKGROUND OF THE INVENTION
[0003] Because of the increase in data intensive applications, the
demand for bandwidth in communications has been growing
tremendously. In response, the installed capacity of
telecommunication systems has been increasing by an order of
magnitude every three to four years since the mid 1970s. Much of
this capacity increase has been supplied by optical fibers that
provide a four-order-of-magnitude bandwidth enhancement over
twisted-pair copper wires.
[0004] To exploit further the bandwidth of optical fibers, two key
technologies have been developed and used in the telecommunication
industry: optical amplifiers and wavelength-division multiplexing
(WDM). Optical amplifiers boost the signal strength and compensate
for inherent fiber loss and other splitting and insertion losses.
WDM enables different wavelengths of light to carry different
signals parallel over the same optical fiber. Although WDM is
critical in that it allows utilization of a major fraction of the
fiber bandwidth, it would not be cost-effective without optical
amplifiers. In particular, a broadband optical amplifier that
permits simultaneous amplification of many WDM channels is a key
enabler for utilizing the full fiber bandwidth.
[0005] With the advent of erbium-doped fiber amplifiers (EDFAs)
around 1990 to replace electronic repeaters, the capacity of
telecommunication systems has since been increased by almost two
orders of magnitude. Although EDFAs have had a significant impact
in the past five years, they are not without problems. As shown in
FIG. 1a, there are two main low-loss telecommunications windows in
silica-based optical fibers at wavelengths of 1.3 .mu.m and 1.55
.mu.m. EDFAs work only in the 1.55 .mu.m window. Yet, most of the
terrestrial fibers installed in the United States during the 1970s
and up through the mid 1980s are designed for operation at 1.3
.mu.m, and thousands of miles of 1.3 .mu.m terrestrial fibers have
already been laid. This presents major difficulties in upgrading to
the higher bandwidth EDFA technology. In the prior art, some have
sought to combine EDFAs with dispersion compensators in an effort
to correct the wavelength mismatch. However this approach does not
permit further upgrading based on wavelength-division-multiplexing,
and therefore is not seen as the best solution. Others are
experimenting with new glass formulations that might provide the
advantages of EDFAs at the shorter 1.3 .mu.m wavelength. However,
currently no glass formulation has proven to be commercially
viable.
[0006] Aside from the wavelength mismatch, EDFAs are also
inherently prone to signal loss when the pump laser fails. EDFA is
a system of the type known as a "three-level" system that does not
allow the optical signal to pass through unless its pump laser is
operative. Reliance on the "three-level" system could have
catastrophic consequences for the reliability of fiber
networks.
[0007] Stimulated Raman scattering amplifiers are advantageous over
EDFAs because they can operate in both optical communication
windows and, in fact, over the entire transparency window of
optical fibers. Moreover, the stimulated Raman scattering amplifier
is a "four-level" system that simply provides no gain when its pump
laser is off, but otherwise allows the optical signal to pass
through the system. Stimulated Raman scattering amplifiers are
based on nonlinear polarization of the dielectric silica host, and
are capable of cascading to higher Raman orders or longer
wavelengths. However, there is a significant problem with Raman
amplifiers that has not heretofore been really overcome. Virtually
every light source or pump produces some intensity fluctuation.
When Raman amplifiers are allowed to cascade through several
orders, the pump source intensity fluctuations are combinatorially
multiplied, and very rapidly result in enormous intensity
fluctuations that have made systems virtually unusable. Compounding
this problem, the gain produced by this nonlinear response is
proportional to instantaneous pump intensity. Thus there is no
opportunity to "average out" intensity fluctuations over time.
Moreover, the gain produced by Raman scattering is, itself, an
exponential effect. All of these properties have lead most to
conclude that stimulated Raman scattering amplifiers and cascade
lasers are not suitable in general-purpose telecommunication
applications.
[0008] Aside from the fluctuation problems above, several other
issues also need to be addressed in order to achieve usable
broadband stimulated Raman scattering amplifiers. In the prior art,
a cladding-pumped fiber laser has been used as a pump source for
Raman amplifiers. A commercial unit delivers 9W of
single-transverse-mode output at 1100 nm with a spectral width of 4
nm. The fiber used in this laser is a rare-earth-doped, double-clad
fiber. As depicted in FIG. 14a, cavity mirrors are applied to the
fiber ends. The mirror applied to the input end is highly
reflective at the lasing wavelength of 1100 nm, while a
low-reflectivity mirror or grating is applied to the output end of
the fiber. The gain band for ytterbium doped fiber is roughly
between 1030 nm and 1160 nm, but using a grating at the fiber end
to select one particular wavelength yields a bandwidth of about 4
nm.
[0009] While this cladding-pumped fiber laser is already quite
broad in bandwidth because of multiple longitudinal modes in the
cavity, it would be desirous to further broaden the pump wavelength
range to achieve broadband Raman gain. The broader pump bandwidth
is also advantageous to avoid reflections associated with
stimulated Brillouin scattering in the gain fiber of the Raman
amplifier.
[0010] Polarization controllers (PCs) are used in almost all Raman
amplifiers to regulate polarization states. A fiber based PC is
typically constructed using quarter-wave loops of optical
single-mode fiber mounted in such a way as to allow precise
rotation of the loops about a common tangential axis. Each loop is
designed to function as a quarter-wave retarder for the wavelength
range of interest. By rotating a loop about its tangential axis,
the loop's birefringence is rotated. Combining three or four loops
in series increases the wavelength range and adjustment range of
the controller and enables complete and continuous polarization
adjustability. However, as the temperature changes, the fiber
birefringence changes and the mechanical setting of the PCs may
also be perturbed. As a result, the PCs may ruin the "turn-key"
operation of the amplifier because they could require periodic
readjustment with changing environmental conditions.
[0011] In the past attempts at applying Raman amplifiers to analog
signal amplification, it was discovered that a major limitation
arises from the noise associated with Double Rayleigh Scattering
(DRS). Stimulated Rayleigh scattering refers to light scattering
due to induced density variations of a material system. More
specifically, stimulated Rayleigh refers to the scattering of light
from isobaric density fluctuations.
[0012] Stimulated Rayleigh scattering gives rise to a backward
traveling wave that is at the same center frequency as the signal
input, somewhat broadened by the Rayleigh linewidth (defined as a
reciprocal to characteristic decay time of the isobaric density
disturbances that give rise to Rayleigh scattering). For example,
J. L. Gimlett, et al., IEEE Photonics Technology Letters, Vol. 2,
p.211 (March 1990) disclosed that the Rayleigh scattering can be
modeled as a Rayleigh mirror with a prescribed reflectivity. DRS
refers to a second stimulated Rayleigh scattering event that
scatters the backward traveling wave back into the original signal,
thereby leading to interference with the original signal,
cross-talk, and increased uncertainty of the amplitude (i.e.,
noise). Also, the DRS is proportional to the pump intensity, the
signal intensity, and the length of the gain fiber. Therefore, the
DRS noise source is a direct consequence of requiring high pump
powers and long interaction lengths due to the inefficiency of the
Raman amplification process.
[0013] Prior art has shown that insertion of an optical isolator
midway through the amplifier and the use of two WDMs to guide the
pump radiation around the isolator can reduce the DRS effect. In
effect, the amplifier is split into two parts and the net gain is
accumulated through both sections, but the isolator reduces the DRS
in half. Although this technique has been used for high gain EDFAs
and in ring designs of Raman amplifiers, it increases the
complexity and cost of the amplifier considerably due to the need
for two additional WDMs and one isolator.
[0014] As shown from the attenuation curve for fibers in FIG. 1a,
there are two low-loss windows for telecommunications. In the prior
art, EDFA technology has been developed to make full use of the 1.5
.mu.m window. Since Raman amplification can be obtained over the
entire transparency range for optical fibers, Raman amplification
can be applied to both the 1.3 .mu.m and 1.5 .mu.m windows. Because
future communication applications will demand the broadest
bandwidth available over the existing fiber base, to fully utilize
optical fiber's bandwidth, it is desirable to have an amplifier
which will use both telecommunications windows and operate with WDM
simultaneously.
SUMMARY OF THE INVENTION
[0015] It is an object of the present invention to provide a Sagnac
Raman amplifier and cascade laser which is operable in both 1.3
.mu.m and 1.5 .mu.m windows.
[0016] It is another object of the present invention to provide a
broadband Sagnac Raman amplifier and cascade laser which is
operable in both 1.3 .mu.m and 1.5 .mu.m windows.
[0017] It is another object of the present invention to provide a
broadband pump for use in a broadband Sagnac Raman amplifier and
cascade laser.
[0018] It is another object of the present invention to remove
environment-sensitive elements from the cavity of the Sagnac Raman
amplifier and cascade laser.
[0019] It is another object of the present invention to provide a
polarization independent Sagnac Raman amplifier and cascade
laser.
[0020] It is another object of the present invention to improve
noise performance of the Sagnac Raman amplifier and cascade
laser.
[0021] It is yet another object of the present invention to provide
a parallel optical amplification apparatus having a combination of
the Sagnac Raman amplifier and EDFA for the 1.3 .mu.m and 1.5 .mu.m
low-loss windows of optical fibers.
[0022] The present invention attacks the intensity fluctuation
problem with Raman amplifiers by recognizing that higher order
intensity fluctuations are a distributed effect (everywhere present
in the distributed gain medium that produces the optical signal
gain) that can be significantly reduced by a reflector structure
that rejects intensity fluctuations originating in this distributed
effect. The present invention employs a reflector structure that
defines two optical paths within the distributed gain medium,
configured to support both common mode and difference mode optical
signals. By choosing a configuration that propagates higher order
intensity fluctuations in the difference mode, much of the unwanted
amplification of pump fluctuations is rejected.
[0023] Although numerous configurations are possible, one
embodiment employs a Sagnac interferometer as one of the two
optical resonator reflectors. The Sagnac interferometer employs an
optical coupler with both ends of a fiber loop (a distributed gain
medium) connected to its light splitting ports. The coupler thus
establishes two optical paths, a clockwise path and a
counterclockwise path. Signals are compared at this optical
coupler, with common mode signals being substantially reflected and
difference mode signals being at least partially rejected through a
rejection port associated with the optical coupler. Although
intensity fluctuations originating at the pump (at the pump
wavelength) are amplified, any intensity fluctuations resulting
from higher order stimulation of the distributed gain medium are at
least partially rejected as difference mode signals.
[0024] This specification describes inventions leading to a
broadband Raman amplification that would be compatible with WDM
technologies. Four improvements over the original Sagnac Raman
amplifier and laser are discussed.
[0025] First, broad bandwidth is achieved by using a broadband
laser or amplifier cavity combined with a broadband pump. The
broadband pump has a pump laser and a bandwidth adding mirror
connected thereto to generate a broadened pump spectrum. The
bandwidth adding mirror can be a Sagnac loop mirror with an unequal
ratio coupler. It further has a phase/amplitude modulator
asymmetrically located within the Sagnac loop mirror. The pump
laser is a cladding pumped fiber laser. In one preferred
embodiment, the broadband pump is incorporated directly into the
laser or amplifier cavity.
[0026] Second, turn-key operation is obtained by minimizing the
need for polarization controllers through use of a polarization
maintaining cavity. In one embodiment, the Sagnac loop mirror of
the broadband Sagnac Raman amplifier is fabricated from
polarization maintaining fiber cross-spliced at the middle of the
loop mirror. In another embodiment, the Sagnac loop mirror is made
of polarization maintaining fiber and the Raman gain fiber is
separated from the Sagnac loop mirror. Input and output ports of
the amplifier are polarization maintaining WDMs.
[0027] Third, the noise performance is improved and protection
against double Rayleigh scattering is provided by using a
polarization diversity pumping system. In one embodiment of the
polarization diversity pumping system, the pumping light is
launched at a 45 degree angle into the polarization maintaining
fiber to produce a beam having two polarization directions. Such
angle is achieved by either rotating the fiber or using a quarter
wavelength plate. In another embodiment, the pumping light is first
divided by a 50:50 coupler into two beams. One beam travels through
a retarder to change its polarization direction. Then a
polarization beam splitter combines the two beams In yet another
embodiment, the polarization maintaining fiber is spliced at a 45
degree angle to the cladding-pumped fiber to output a beam having
two polarization directions.
[0028] Finally, two-wavelength operation is achieved between two
parallel amplifiers for two separate windows. In one embodiment,
both 1310 nm and 1550 nm amplifications are performed by the
broadband Sagnac Raman amplifiers. Moreover, the two amplifiers
share a common pump laser. In another embodiment, a combination of
Raman amplifiers and EDFAs are used, the Sagnac Raman amplifier is
used to amplify the 1310 nm signal, while the 1550 nm signal is
amplified by the EDFA. The EDFA may be pumped by another Sagnac
Raman cascade laser.
[0029] For a more complete understanding of the invention, its
objects and advantages, reference may be had to the following
specification and to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1a is a graph of the loss or attenuation in typical
single-mode optical fibers. The solid curve is a measured loss
profile, and the dashed curve shows the intrinsic loss profile
resulting from Rayleigh scattering and absorption in pure
silica.
[0031] FIG. 1b is a graph depicting Raman gain as a function of
frequency shift for fused silica at a pump wavelength of 1
.mu.m;
[0032] FIG. 2 is a diagrammatic illustration of a first embodiment
of a Sagnac Raman amplifier in accordance with the present
invention.
[0033] FIGS. 3a and 3b are illustrations that accompany the
equations used to generate values presented in Table I.
[0034] FIG. 4 is a diagrammatic illustration of a second embodiment
of a Sagnac Raman amplifier of the present invention employing
dichroic couplers for wavelength discrimination.
[0035] FIG. 5 is a diagrammatic illustration of a third embodiment
of a Sagnac Raman amplifier of the present invention employing one
or more grating reflectors.
[0036] FIG. 6 is a diagrammatic illustration of a fourth embodiment
of a Sagnac Raman amplifier of the present invention employing an
uneven coupler.
[0037] FIG. 7a is a diagrammatic illustration of a fifth embodiment
of a Sagnac Raman amplifier of the present invention employing a
Fabry-Perot filter;
[0038] FIG. 7b depicts in detail the Fabry-Perot filter employed in
the embodiment of FIG. 7a.
[0039] FIG. 8 is a diagrammatic illustration of a first embodiment
of the Sagnac Raman cascade laser of the invention.
[0040] FIG. 9 is a diagrammatic illustration of a second embodiment
of the Sagnac Raman cascade laser of the invention providing
bidirectional output with a single coupler.
[0041] FIG. 10 is a diagrammatic illustration of a third embodiment
of the Sagnac Raman cascade laser of the invention employing a
dichroic coupler.
[0042] FIG. 11 is a diagrammatic illustration of a fourth
embodiment of the Sagnac Raman cascade laser of the invention
employing a dichroic mirror at the cavity end.
[0043] FIG. 12 illustrates the general principle of using a
bandwidth adding mirror to a pump laser to broaden the bandwidth of
the pump laser.
[0044] FIG. 13 depicts the spectral broadenings possible due to the
nonlinearity in optical fibers.
[0045] FIG. 14a depicts a prior art pump configuration where the
wavelength of the pump laser is selected by placing a grating at
the fiber output at the desired wavelength.
[0046] FIG. 14b is a diagrammatic illustration of a first
embodiment of the broadband pump of the invention employing a
Sagnac loop as the cavity end mirror.
[0047] FIG. 14c is a diagrammatic illustration of a second
embodiment of the broadband pump of the invention modulating either
the Sagnac loop or the drive to the cladding-pumped fiber
laser.
[0048] FIG. 15a is a diagrammatic illustration of a first
embodiment of a broadband Sagnac Raman amplifier where the
amplifier spectrum is broadened by using a broadened pump laser
such as one of those of FIGS. 14b and 14c.
[0049] FIG. 15b is a diagrammatic illustration of a second
embodiment of the broadband Sagnac Raman amplifier.
[0050] FIG. 16a is a diagrammatic illustration of a preferred
embodiment of the broadband Sagnac Raman cascade laser combining
the broadband pump with the Sagnac Raman cascade laser.
[0051] FIG. 16b is a diagrammatic illustration of a preferred
embodiment of the broadband Sagnac Raman amplifier combining the
broadband pump with the Sagnac Raman amplifier.
[0052] FIG. 17 is a diagrammatic illustration of the broadband
Raman amplifier with a gain flattening element at the output.
[0053] FIG. 18a is a diagrammatic illustration of one embodiment of
a Sagnac Raman amplifier using an all-polarization-maintaining
cavity.
[0054] FIG. 18b illustrates an alternate embodiment of FIG. 18a
using a bulk 50:50 beam splitter instead of the PM coupler.
[0055] FIGS. 19a is a diagrammatic illustration of an embodiment in
which a Sagnac loop mirror made of a short length PM fiber is
used.
[0056] FIGS. 19b is a diagrammatic illustration of an embodiment in
which two Sagnac loop mirrors made of a short length PM fiber are
used.
[0057] FIG. 20a is a diagrammatic illustration of a first
embodiment for polarization diversity pumping of the Raman
amplifier or laser in with the use of a length of polarization
maintaining fiber.
[0058] FIG. 20b is a diagrammatic illustration of a second
embodiment for polarization diversity pumping of the Raman
amplifier or laser with the use of two optical paths.
[0059] FIG. 21 illustrates one embodiment of the combined pump
laser and Sagnac Raman amplifier cavity for polarization diversity
pumping.
[0060] FIG. 22a is a diagrammatic illustration of a parallel
combination of Sagnac Raman amplifiers for amplifying both
wavelengths while using a common pump laser.
[0061] FIG. 22b is a diagrammatic illustration of one Sagnac Raman
amplifier for 1310 nm, and another Sagnac Raman laser operating at
1480 nm for pumping of an EDFA at 1550 nm. Both the Sagnac
amplifier and laser share a common pump laser.
[0062] FIG. 22c illustrates the use of a Sagnac Raman amplifier for
1310 nm in parallel with an EDFA for 1550 nm amplification.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0063] The present invention provides a structure that combines
Sagnac interferometer technology with Raman amplifier technology to
achieve performance improvements that neither technology, by
itself, has heretofore been able to deliver. More specifically, the
preferred embodiments relates to broadband Sagnac Raman amplifiers
and lasers that have substantially improved bandwidth and noise
performance. To provide a better understanding of the amplification
mechanism at work in the present invention, some knowledge of the
Raman effect will be helpful.
[0064] 1. Stimulated Raman Scattering
[0065] Stimulated Raman scattering is an important nonlinear
process that can turn optical fibers into amplifiers and tunable
lasers. Raman gain results from the interaction of intense light
with optical phonons in the glass, and the Raman effect leads to a
transfer of energy from one optical beam (the pump) to another
optical beam (the signal). An interesting property of Raman gain is
that the signal is downshifted in frequency (upshifted in
wavelength) by an amount determined by the vibrational modes of the
glass. FIG. 1b depicts the Raman gain coefficient g.sub.r for
silica fibers. Notably, the gain g.sub.r extends over a large
frequency range (up to 40 terahertz [THz]), with a broad peak
centered at 13.2 THz (corresponding to a wavelength of 440
cm.sup.-1) This broad behavior is due to the amorphous nature of
the silica glass and means that the Raman effect can be used to
make broadband amplifiers. The Raman gain depends on the
composition of the fiber core and can vary with different dopant
concentrations.
[0066] The present invention employs a distributed gain medium
comprising a material that produces optical signal gain due to
third order nonlinearities in the material, in which the gain is
proportional to the intensity of the light passing through the
medium. By way of background, the response of any dielectric to
light becomes nonlinear for intense electromagnetic fields, and
optical fibers are no exception. This nonlinear response is related
to anharmonic motion of bound electrons under the influence of an
applied field. The induced polarization P from the electric dipoles
is not linear in the electric field E. Rather, it satisfies the
more general relationship described in equation (1)
P=.di-elect
cons..sub.0[.chi..sup.(1).multidot.E+.chi..sup.(2):EE+.chi..su-
p.(3)EEE . . . ] (1)
[0067] where .di-elect cons..sub.0 is the vacuum permitivity and
.chi..sup.(j) (j=1, 2 , . . . ) is the jth order susceptibility. To
account for the light polarization effects, .chi..sup.(j) is a
tensor of rank j+1. The linear susceptibility .chi..sup.(1)
represents the dominant contribution to P. Its effects are included
through the refractive index n and the attenuation coefficient a.
The second order susceptibility .chi..sup.(2) is responsible for
such nonlinear effects as second harmonic generation and
sum-frequency generation. However, this second order susceptibility
is nonzero only for media that lack an inversion symmetry at the
molecular level. Since silicon dioxide is a symmetric molecule,
.chi..sup.(2) vanishes for silica glasses. As a result, optical
fibers do not normally exhibit second order nonlinear effects.
Nevertheless, dopants introduced inside the fiber core can
contribute to second harmonic generation under certain
conditions.
[0068] The third order susceptibility .chi..sup.(3), which is
responsible for phenomena such as third harmonic generation,
four-wave mixing and nonlinear refraction, is present in optical
fibers. It is this third order nonlinearity that is operative in
the present invention. These third order nonlinear effects are
identifiable as being variable in proportion to the intensity of
the light.
[0069] From a functional standpoint, stimulated Raman scattering
amplifiers can be pumped at any wavelength because there is no pump
absorption band, while the signal gain characteristics are
determined by the optical phonon spectra. This means that
stimulated Raman scattering amplifiers are capable of cascading to
higher Raman orders or longer wavelengths. 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 the longer signal 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. The ability to cascade makes stimulated Raman
scattering amplifiers very attractive, for it allows operation over
a wide range of different wavelengths.
[0070] Hence, Raman amplification has a number of attractive
features. First, Raman gain exists in every fiber; Raman gain is a
good candidate for upgrading existing fiber optic links. Second,
unlike EDFAs, there is no excessive loss in the absence of pump
power, an important consideration for system reliability. Third,
the gain spectrum is very broad (bandwidth of greater than 5 THz
around the peak at 13.2 THz), so that it can be used to amplify
multiple wavelengths (as in wavelength division multiplexing) or
short optical pulses. Also, Raman amplification can be used for
distributed amplification, which may be especially valuable for
ultra-high-bit-rate systems. Finally, by varying the pump
wavelength or by using cascaded orders of Raman gain, the gain can
be provided over the entire telecommunications window between 1.3
.mu.m and 1.6 .mu.m.
[0071] 2. Embodiments of Sagnac Raman Amplifiers
[0072] FIG. 2 illustrates a first embodiment of the Sagnac Raman
amplifier 20 that comprises at least two reflectors 22, 24 and a
port 28 for coupling to a source of light. Specifically, reflector
22 may be any reflective structure such as a mirror. Reflector 24
is a loop reflector such as a Sagnac interferometer. In this
embodiment, these two reflectors form therebetween an optical
resonator. The light source 26 is a pumped fiber laser coupled
through a WDM port 28 to the optical resonator. An optical signal
is injected into the optical resonator and this signal is then
amplified by the optical energy introduced by the light source 26.
An optical signal input WDM port 42 is provided to allow the
optical resonator to be used as an optical amplifier. The optical
signal then exits from an optical signal output WDM port 44.
[0073] The light source 26 can be any suitable source of optical
energy. Because the Raman effect relies upon intense optical
energy, high power semiconductor or cladding-pumped fiber lasers
are presently preferred. A suitable high power source is available
from Spectra Diode Lasers, Inc., San Jose, Calif. The wavelength of
the optical energy from light source 26 will, of course, be chosen
to match the desired application. By way of example, in an
embodiment designed for 1.3 .mu.m telecommunication applications,
the light source 26 provides light at a wavelength of 1117 nm. This
light is introduced through the wavelength division multiplexing
(WDM) coupler 28. The optical signal to be amplified, injected
through WDM coupler 42, may be at a wavelength of 1300 nm to 1310
nm. The injected signal propagates in the clockwise direction
around loop 30 and is then removed using WDM coupler 44. Due to the
frequency downshift (wavelength upshift) of the Raman effect, the
wavelength of the light source 26 is upshifted to match that of the
signal. Although a 1.3 .mu.m amplifier example is presented here,
the configuration illustrated in FIG. 2 and the embodiments
described elsewhere in this specification can be configured to work
at other wavelengths as well. Thus the light source 26 can be any
suitable wavelength to match the application (not necessarily at
1117 nm) and the two WDM couplers 42 and 44 can be designed for any
desired signal wavelengths (not necessarily between 1300 nm and
1310 nm).
[0074] The resonant cavity of the embodiment illustrated in FIG. 2
lies between reflector 22 and reflector 24. In the illustrated
embodiment the optical fiber disposed between these two reflectors
serves as the light transmissive medium. The Sagnac reflector 24 is
fabricated using a distributed gain medium comprising a material
that produces optical signal gain through third order
nonlinearities in the material, characterized by a gain that is
proportional to the intensity of the light passing through the
medium. Although reflector 22 is shown as a discrete mirror in the
embodiment, it will be appreciated that reflector 22 could be any
form of reflector, including a simple metallic coating evaporated
onto the fiber end. Thus the invention can be implemented as an all
fiber configuration. Some of the embodiments yet to be described
use other forms of reflectors for reflector 22.
[0075] The Sagnac interferometer that serves as reflector 24 is
fabricated from a length of optical fiber that may be suitably
coiled to accommodate the physical packaging requirements. The
Sagnac interferometer comprises a fiber loop 30, typically a
kilometer or more in length. The fiber loop is established using a
coupler such as 50:50 coupler 32. The 50:50 coupler defines two
signal paths, such that half of the light from light source 26
travels around loop 30 in a clockwise direction and half of the
light from light source 26 travels around loop 30 in a
counterclockwise direction. These two optical paths support both
common mode and difference mode optical signals. To illustrate,
assume that a continuous wave burst of light is injected via WDM 28
from light source 26. The CW burst enters the Sagnac reflector 24;
half of the energy propagates in a clockwise direction and half of
the energy propagates in a counterclockwise direction. After
propagating through the Sagnac reflector, the continuous wave burst
is then reflected back in the direction of WDM 28, where the burst
then reflects from reflector 22 and is again transmitted to the
Sagnac reflector, where the cycle repeats. The CW burst thus
resonates between the two reflectors 22 and 24, growing in energy
at the resonant frequency. This is the common mode signal path. The
system is designed to reflect the common mode signal between
reflectors 22 and 24, whereby the optical amplification occurs.
[0076] Now consider a noise burst signal that originates at some
random location along fiber loop 30. For purposes of the
illustration, assume that the noise burst is injected at a location
designated by N in FIG. 2. Some of the energy of the noise burst
(that which propagates in the clockwise direction) passes out
through rejection port 46 where it is not returned to the system.
The remainder (propagating in the counterclockwise direction) is
reflected within the system and therefore retained. Because the
signal paths of the noise burst are unbalanced (difference mode), a
portion of the noise burst energy (approximately half of the
energy) is lost, thus lowering the noise level within the system.
The noise burst originating in the fiber loop travels in a
difference mode, in which one optical path is retained within the
system and the other optical path is discharged through rejection
port 46. This is how the invention is able to reduce higher order
amplification of pump source fluctuation. The higher orders
originate (through the Raman effect) within the fiber loop and are
thus treated as difference mode signals.
[0077] One advantage of using the Sagnac reflector 24 is its
inherent broadband properties. Unlike some other systems that are
restricted by the laws of physics to operate at a single resonant
frequency dictated by doping, the present invention operates over a
broad range of frequencies, the operating frequency being dependent
principally upon the frequency of the input signal. Of course, if
desired, frequency-selective gratings or frequency-selective
filters can be employed within the laser cavity if precise
wavelength control is desired.
[0078] One significant advantage of the invention results from the
union of the Sagnac loop mirror with the Raman amplifier
technology. Conventionally, a large source of amplitude jitter in
Raman lasers arises from the pump fluctuations that become greatly
amplified in the highly nonlinear cascaded Raman process.
Advantageously, the Sagnac loop mirror results in a quieter
amplifier (and also a quieter laser) due to its difference mode
noise rejection properties. The Sagnac loop tends to dampen noise
at frequencies larger than the inverse round-trip time of the loop
cavity. For example, for a 2 kilometer (km) long fiber loop, noise
at frequencies larger than 100 kilohertz (kHz) will be partially
rejected via the rejection port 46. Also, spurious signals and
noise injected at some arbitrary point along the loop are also
attenuated.
[0079] As previously noted, the Raman amplifier is capable of
cascading through multiple orders. With each cascade order there is
a corresponding shift in optical wavelength. The wavelength shift
corresponds to a predetermined Stokes wavelength. Thus to achieve a
1310 nm signal wavelength four cascaded orders of Stokes shift
would be employed, namely: 1117 nm to 1175 nm to 1240 nm to 1310
nm. Similarly, a fifth Stokes shift, based on the previous cascaded
orders, would produce an output wavelength at 1480 nm.
[0080] Cascading is a desirable property; it allows the system
designer to shift the pump wavelength to any number of different
desired signal wavelengths. Thus commercially available,
high-powered pumps can be wavelength shifted to match the
wavelength of the signal being amplified. However, cascading comes
at a price. Pump fluctuations are amplified combinatorially, as the
examples of Table I demonstrate. Table I shows how a 10% intensity
fluctuation at the pump cascades exponentially with each cascaded
order. Table I compares two cases. Case 1 assumes a 10% fluctuation
introduced in the first step, using a simple Fabry-Perot (linear)
cavity so that there is no rejection of the fluctuation burst. Case
2 assumes a 10% fluctuation introduced in the second step, using a
Sagnac Raman laser cavity with a 50% rejection of the fluctuation
burst. Thus Case 2 shows the improvement achieved using the
principles of the invention.
1 TABLE I Case 1 Case 2 Fluctuation Fluctuation Initial Fluctuation
10% 10% First Reflection 10% 5% from Sagnac Mirror After First
Stage 26% 12% (10x Gain) After Second Stage 52% 22% (5x Gain) After
Third Stage 61% 22% (2.5x Gain)
[0081] In the specific example illustrated in Table I we are
considering only one noise burst, entered in the first step. The
fluctuation is reduced to 1/3 by using the invention as is
demonstrated by comparing the 61% fluctuation in Case 1 with the
22% in Case 2.
[0082] The values in Table I are based on the following model.
Assume that the systems compared in both cases start with a pump
and then cascade three orders (e.g., 1117 nm pump, cascade to 1175
nm, 1240 nm and then 1310 nm). We can specify the gain at each
successive order to be 1/2 of the previous order. A gain in the
first step of 10 dB=10.times. has been assumed. In this model the
gain in the earlier stages is higher than in the later stages,
because the earlier stages are robbed of power by the later stages
during the cascading process. In general, the gain required at each
stage for lasing is going to be such that the gain balances the
loss. Thus, pumping higher orders corresponds to a loss and earlier
stages must therefore have more gain. For simplicity, pump
depletion and the resulting gain saturation have been neglected.
Case 1 illustrates how a 10% noise fluctuation grows to a 61%
fluctuation after three stages. Case 2 shows how that same noise
fluctuation is amplified only 22% due to the 50% rejection in the
Sagnac mirror for the higher stages. In Table I, note that the
initial 10% fluctuation is reduced to 5% upon first reflection from
the Sagnac mirror. This corresponds to 50% of the difference mode
energy being rejected through the rejection port.
[0083] The equations used to generate the values shown in Table I
will now be described with reference to FIGS. 3A and 3B. In FIG. 3A
two optical signal paths are shown being fed into and out from a
50:50 coupler. The input signals E.sub.1 and E.sub.2 produce output
signals E.sub.3 and E.sub.4, respectively according to the
following equations: 1 E 3 = 1 2 E 1 + j 1 2 E E 4 = j 1 2 E 1 + 1
2 E 2
[0084] In the above equations j={square root}{square root over
(-1)}, corresponding to the phase of 2 2 .
[0085] Propagation through a fiber of length L is given by the
following expression:
E.sub.1e.sup.j.phi.,
[0086] in which .phi. corresponds to the following phase shift
calculation: 3 = 2 n L .
[0087] FIG. 3b shows the signal propagation within a Sagnac loop
mirror that comprises a 50:50 coupler. The input electric field
E.sub.in is split at the coupler, propagating in clockwise and
counterclockwise directions, corresponding to electric fields
E.sub.3 and E.sub.4. These fields are related to the input field
E.sub.in according to the following equations: 4 E 3 = 1 2 E in E 4
= j 1 2 E in
[0088] The effect of the Sagnac loop mirror is to produce a
reflected field E.sub.ref that corresponds to the common mode of
propagation, and to produce a rejected field E.sub.out that
corresponds to the difference mode of propagation. The common mode
and difference mode signals are thus described by the following
equations: 5 E ref = 1 2 j { E clockwise + E counterclockwise }
-> common mode reflection E out = 1 2 { E clockwise - E
counterclockwise } -> difference mode reflection
[0089] As the above Table shows, even a modest pump fluctuation (in
this example a 10% fluctuation) is multiplied again and again
through each cascaded order. This is why Raman amplifiers have not
been considered generally useful in the past. However, the
invention overcomes this problem by adopting a structure that
places the distributed gain medium in a difference mode signal
path, such that higher order pump fluctuations are at least
partially rejected.
[0090] FIG. 4 shows a second embodiment of the invention. A
resonant cavity is formed between reflector 22 and reflector 24.
Reflector 24 is a Sagnac interferometer including a dichroic
coupler 32b and a fiber loop. Dichroic coupler 32b is used to
provide frequency selectivity. The dichroic coupler provides
nominally 50:50 coupling over the cascade Raman order wavelengths,
but a ratio that is closer to 100:0 for the signal wavelength.
Thus, for a 1.3 .mu.m system the 50:50 coupling would be provided
for wavelengths less than 1300 nm and the 100:0 coupling would be
provided for wavelengths greater than 1300 nm. The advantage of
this configuration is that it is easier to make a balanced Sagnac
interferometer, and the fiber in the Sagnac interferometer may be
packaged more simply. One possible disadvantage of this
configuration is that the dichroic coupler may be more difficult or
expensive to implement. The signal input WDM port 42b is positioned
in the cavity at a location between the two reflectors 22 and 24
and adjacent a WDM coupler 28. Polarization controllers 46 and 48
are used in the cavity and fiber loop, respectively. Polarization
controllers may also be used in a similar fashion in the embodiment
illustrated in FIG. 2.
[0091] FIG. 5 illustrates a third embodiment of the invention in
which reflector 22 of FIG. 2 has been replaced by a series of
grating reflectors 50 and 52. The grating filters may be selected
to provide 100 percent reflection at selected wavelengths, such as
at 1175 nm and 1240 nm. The advantage of the configuration of FIG.
5 is that a narrow pump line width can be achieved. The
disadvantage is that the configuration is more complicated and more
expensive to fabricate.
[0092] FIG. 6 illustrates yet a fourth embodiment in which the
Sagnac reflector 24 is constructed using a coupler 32c having an
unequal coupling ratio, for instance 60:40. By unbalancing the
Sagnac reflector the system will tend to further reject noise
bursts that randomly occur in the loop. This will serve to dampen
out any mode locking or Q-switching tendencies. However, the
unequal coupling leads to a leakage at various wavelengths, so that
higher pump powers may be required to account for the reduced
efficiency.
[0093] FIG. 7a depicts a fifth embodiment of the invention which
employs a Fabry-Perot wavelength filter 54 to narrowly select the
Raman pump orders. In other respects the embodiment is the same as
that of FIG. 2.
[0094] A detailed depiction of the Fabry-Perot filter is shown in
FIG. 7b. The fiber is split into two segments 56 and 58 and
separated to define an air gap 60. The cleaved ends of the fiber
segments are coated at 62 with a nominally high selectivity coating
(R>90%) at the wavelengths of interest. The cleaved faces are
aligned parallel to each other and piezoelectric transducers 64 may
be used to adjust the air gap width. Ideally, the air gap width L
can be adjusted so the free-spectral range of the Fabry-Perot
interferometer (.DELTA.f=c/2 nL) will match the reflection at the
various Raman orders (spaced by .DELTA.f=13.2 THz). Thus a single
Fabry-Perot interferometer can be used to replace the multiple
gratings 50 and 52 of the FIG. 5 embodiment, because the
transmission function is a periodic function of frequency. For
example, for an air gap index n=1, the spacing should be 11.36
.mu.m for .DELTA.f=13.2 THz. Alternatively, the spacing may be some
integer multiple of this fundamental width. The fiber Fabry-Perot
interferometer can also be replaced with a bulk interference
filter, which can be rotated to adjust the peak transmission
frequencies.
[0095] 3. Embodiments of Sagnac Raman Cascade Lasers
[0096] The above embodiments focus on using the optical resonator
of the invention as an optical amplifier. Thus in the preceding
examples, a signal input port is provided into which the signal to
be amplified is injected. However, the invention is not limited to
amplifiers. The invention can also be used to develop cascade
oscillators or cascade lasers. Various configurations are now
described for constructing Sagnac Raman cascade lasers.
[0097] FIG. 8 illustrates a first embodiment of the Sagnac Raman
cascade laser of the invention. In the laser, a fraction of the
light at the desired wavelength is extracted from the cavity. The
laser cavity comprises first and second reflectors 22, 24 wherein
the second reflector is a Sagnac interferometer comprising a gain
fiber 30 approximately 1 km long with enhanced Raman cross-section
and a broad-band 50:50 coupler 31 at the base of the Sagnac gain
fiber, and two wavelength-specific couplers 27 and 43 for bringing
in the pump and removing the desired wavelength, respectively. A
high-powered diode-array-cladding-pumped fiber laser 25 operating
around 1.1 .mu.m pumps the Sagnac Raman laser. This is chosen
because commercial units with continuous wave powers approaching 10
W are available where the light launches directly into a single
mode fiber. Then, a Sagnac cavity is used to permit the cascaded
Raman process to downshift the pump at 1.1 .mu.m to the
communications bands around 1.3 .mu.m (through a three-step
cascade) or to 1.55 .mu.m (through a six-step cascade).
[0098] The cascaded Raman process allows for large and varied
wavelength shift between the pump and signal wavelengths. The novel
Sagnac cavity design can dampen the noise fluctuations that would
normally grow during the cascade process because the Sagnac mirror
reflects common mode signals and dampens difference mode noise.
[0099] FIG. 9 illustrates a second embodiment of the Sagnac Raman
cascade laser of the invention. This embodiment is the same as that
of FIG. 8 but includes an intracavity coupler 66 that provides
bidirectional outputs labeled .lambda..sub.out. Advantageously,
port 68 outputs a larger portion of the laser's total output than
port 70 so the output at port 70 may be used for monitoring
purposes.
[0100] FIG. 10 illustrates a third embodiment of the Sagnac Raman
cascade laser of the invention. This embodiment is the same as that
of FIG. 8 but eliminates output coupler 43 by using a dichroic
coupler 32d in the Sagnac loop mirror 24. The output of this
oscillator .lambda..sub.out exits from the external cavity port of
the Sagnac loop mirror 24. The dichroic coupler can be selected to
provide 50:50 coupling over the cascade order and 100:0 coupling at
the .lambda..sub.out wavelength.
[0101] FIG. 11 shows a fourth embodiment of the Sagnac Raman
cascade laser. This embodiment is the same as that of FIG. 8 but
eliminates output coupler 43 by replacing mirror 22 with a dichroic
mirror 72. Note that the dichroic mirror is reflective for cascade
order wavelengths and is partially or completely transmitting for
the .lambda..sub.out wavelength.
[0102] The Sagnac Raman cascade lasers described in the preceding
examples (FIGS. 8-11) may be used in numerous applications,
including upgrading existing fiber links, remote pumping of EDFAs,
or other applications requiring different wavelengths of light. In
this regard, the embodiments illustrated in FIGS. 8-11 are merely
exemplary, and there may be other possible configurations employing
the principles of the invention.
[0103] 4. Broadband Raman Amplifiers Using Broadband Pumps
[0104] The basic idea to further broaden the bandwidth of the
Sagnac Raman amplifier and cascade laser is to take advantage of
the property of Raman amplification that the gain spectrum follows
the pump spectrum so long as there is nothing in the Raman laser
cavity to restrict the bandwidth. Raman laser schemes using either
gratings or wavelength selective couplers cannot exploit this
unique property of Raman amplification. Thus, the broadband cavity
design of the Sagnac Raman amplifier and laser lends itself
naturally to increased bandwidth by tailoring of the pump
spectrum.
[0105] FIG. 12 illustrates a general principle to form a broadband
pump. A bandwidth-adding mirror 110 is attached at the output end
of a pump laser 100. The spectrum of the reflected signal is
broader than the incident signal. Due to the nonlinear
index-of-refraction in the fiber, the spectrum is broadened in the
fiber through processes known as four-wave mixing or self-phase
modulation.
[0106] A numerical example can help illustrate the spectral
broadening possible through either four-wave mixing or self-phase
modulation. For instance, assume that we use a length of the fiber
that would be used in a Sagnac Raman laser or amplifier. Typical
fiber parameters are as follows:
[0107] A.sub.eff=15 .mu.m.sup.2 - - - affective area
[0108] n.sub.2/A.sub.eff=2.times.10.sup.-9 W.sup.-1 - - - effective
nonlinearity
[0109] L=1 km
[0110] The nonlinear phase shift in the fiber is given by 6 = k L =
2 n L = 2 L n 2 A eff P .
[0111] Under the condition that the pump power in the fiber is 1W
at the wavelength of 1.1 .mu.m, the resulting phase shift is
.DELTA..PHI.=3.6.pi.. FIG. 13 illustrates exemplary spectra
resulting from self-phase modulation of Gaussian pulses, and
similar spectral broadening can be expected from four-wave mixing.
In particular, the calculated self-phase modulation spectra are
shown for an unchirped Gaussian pulse. The spectra are labeled by
maximum phase shift at the peak of the pulse (after R. H. Stolen
and C. Lin, Physical Review A, Vol. 17, p. 1448, 1978). The
spectral broadening factor is approximately given by the numerical
value of the maximum phase shift. Therefore, in a single-pass
transmission through the fiber, a spectral broadening of up to an
order-of-magnitude might be expected. However, in closed loop
operation in a laser cavity or oscillator, the spectral broadening
will reach some steady-state value given by the counter-balance
from spectral broadening and narrowing forces. Furthermore, such a
large phase shift will not be achieved because the pump intensity
is depleted by the various Raman Stokes orders.
[0112] Therefore, following on the concept of replacing wavelength
restrictive gratings in the Raman amplifier with broadband Sagnac
loop mirrors, to achieve a broadband pump it would be desirable to
place a Sagnac mirror at the output of the cladding-pumped fiber
laser. Moreover, it would also be desirous to have a reflective
element on the cladding-pumped fiber laser that actually adds
bandwidth to the laser. This could be achieved by using a long
fiber length in the Sagnac loop mirror and utilizing the fiber
nonlinearities, as described further below.
[0113] FIG. 14b illustrates a first embodiment of the broadband
pump in which a Sagnac loop mirror 160 is placed at the output of a
cladding-pumped fiber laser 150. Since there are no reflective
surfaces in the loop, the Sagnac mirror can handle the high powers
from the cladding-pumped fiber laser without damage. As long as
coupler 162 at the base of the loop is broadband, the reflectivity
of the loop mirror can also be broadband. In addition, just as
described for the Sagnac Raman amplifier and lasers above, the
Sagnac loop provides some noise rejection properties. To permit
partial transmission of the output, however, the loop mirror must
be imbalanced in one of several ways. First, the coupler 162 at the
base of the Sagnac loop can have a coupling ratio other than 50:50,
which would lead to a certain amount of minimum leakage from the
mirror. In particular, if the splitting ratio is (f:1-f) wherein
0.ltoreq.f.ltoreq.1, then the reflection coefficient is 4f(1-f) and
the transmission coefficient is [1-4f(1-f)]. Also, the unequal
coupling will lead to an output associated with the fiber
nonlinearities. Alternately, the loop can be imbalanced by using a
polarization controller 165.
[0114] Inserting some sort of modulation in the cavity, thus adding
sidebands to the spectrum, could further increase the bandwidth of
the pump. FIG. 14c is a second embodiment of the broadband pump
where the modulation is produced by a phase or amplitude modulator
167 inserted asymmetrically into the Sagnac loop mirror.
Alternately, modulation might be applied to the drive for the pump
laser 155. For example, the current drive to the diode lasers could
be modulated, or different diodes could be excited as a function of
time (i.e., excite one set of diodes alternately from another set).
Finally, a mechanical modulation could be applied to the
cladding-pumped fiber itself to perturb the longitudinal modes and
lead to additional multi-mode bandwidth.
[0115] The broadband pump can be combined with the Sagnac Raman
amplifier to obtain the desired broadband gain spectrum. FIG. 15a
illustrates a first embodiment of the broadband Sagnac Raman
amplifier of the invention that combines the embodiment described
in FIG. 2 with the broadband pump of FIG. 14. By separating the
Raman amplifier from the broadband pump, the bandwidth of each
element can be individually optimized. The broadened bandwidth for
the pump also increases the amount of pump power that can be
injected into the cavity, since the limitations from stimulated
Brillouin scattering will be reduced. The amplifier resonant cavity
is formed between reflector 22 and reflector 24. Reflector 24 is a
Sagnac interferometer including a coupler 32 and a fiber loop 30.
An input WDM port 42 is used to input an optical signal. The
optical signal then exits from an output WDM port 44. The WDM port
28 used to inject the pump, however, must not restrict the
bandwidth of the pump light. That is, a sufficiently broadband WDM
is required. The resulting bandwidth for the Raman gain should be
the convolution of the pump spectrum with the spectrum from the
Sagnac Raman amplifier. For example, if the pump spectrum is 10 nm
wide and the Sagnac Raman amplifier spectrum is also 10 nm, the
gain spectrum resulting from the configuration of FIG. 15a should
be about 20 nm wide.
[0116] As an alternative from the configuration of FIG. 15a, FIG.
15b shows a second embodiment of the broadband Sagnac Raman
amplifier in which a Sagnac mirror is used for both the pump
linewidth broadening and the Raman amplification. Specifically, the
Raman amplifier cavity is formed between the Sagnac mirror 160 and
a reflector 170. A pump laser 150 is connected to the rejection
port of the Sagnac mirror. Instead of using a WDM to introduce the
pump light to the gain medium of the Sagnac Raman cavity, the
Sagnac loop itself is used to bring in the pump light. The
reflector 170 could be a mirror or other equivalents described
earlier. The gain fiber of the Sagnac mirror is single mode not
only at Raman order wavelengths but also at the pump wavelength,
and the coupler at the base of the Sagnac mirror has similar
characteristics for the pump and the Raman cascade orders.
[0117] Note that FIG. 15b is shown by way of example. Any of the
amplifier cavities in FIGS. 2 and 4-7 or the laser cavities of
FIGS. 8-11 can also take advantage of the pumping through the
Sagnac configuration rather than using a separate WDM to bring in
the pump light.
[0118] To accomplish the broadband pump through nonlinear spectral
broadening and to achieve the broadband gain in the Sagnac Raman
amplifier or laser, a more preferred embodiment is to combine
configurations of the Sagnac Raman amplifier and cascade laser with
the broadband pump. FIGS. 16a and 16b schematically illustrate
respectively preferred embodiments of a broadband Sagnac Raman
cascade laser and amplifier. The resonant cavity is formed between
reflector 22 and reflector 24. Reflector 24 is a Sagnac
interferometer including a coupler 32 and a fiber loop 30. An
output WDM port 44 is used to exit an optical signal from the
cavity. The 50:50 coupler 32 should have more-or-less flatband
response over the spectral range from the pump wavelength to the
signal or output wavelength. A cladding-pumped fiber 174 or 179 is
placed between two reflectors 22 and 24. A diode-array pump 172 or
177 pumps the cladding-pumped fiber 174 or 179 respectively. The
diode-array pump is introduced by using mirrors or lenses directly
to one end of the fiber, or the fiber may have a dichroic mirror at
the end and then the pump is injected through the end mirror. The
cladding-pumped fiber generates a pumping light which subsequently
pumps the fiber loop 30.
[0119] Such a cavity has the following advantages, first, broadband
reflector plus spectral broadening from the fiber nonlinearities
adds bandwidth at the pump wavelength. Second, there is a
significant reduction in the number of components, hence a
reduction in the manufacturing cost. Third, the pump laser and the
Raman unit are combined into a smaller unit thereby making smaller
packaging possible.
[0120] One difficulty in implementing a WDM system using Raman
amplifiers may be the wavelength dependent gain (see FIG. 1b ).
This wavelength dependency or nonuniformity of the gain band has
little impact on single-channel transmission. However, it renders
the amplifier unsuitable for multichannel operation through a
cascade of amplifiers. As different wavelengths propagate through a
chain of amplifiers, they accumulate increasing discrepancies
between them in terms of gain and signal-to-noise ratio. However,
using gain-flattening elements can significantly increase the
usable bandwidth of a long chain of amplifiers. FIG. 17 shows that
the Sagnac Raman amplifier in the above embodiments may be followed
by such a gain flattening element 180 to provide gain equalization
for different channels. Alternately, the gain flattening element
could be introduced directly into the Sagnac interferometer
loop.
[0121] Although several technologies have been proposed for gain
equalization, to date the long period gratings appear to be the
most promising candidates. Long period gratings are periodic
structures formed in the core of a photosensitive optical fiber
(c.f. S. K. Juma, `Gain Flattening of EDFA for DWDM Systems,`
FiberOptic Product News, pp. 17-20, June 1997, or A. M. Vengsarkar,
`Long-period fiber gratings shape optical spectra,` Laser Focus
World, pp. 243-248, June 1996). Usually, the refractive index
perturbations have a periodicity much greater than the wavelength
of light, usually of the order of 200 to 400 microns, hence the
`long` in the name. Special design algorithms and simulation tools
exist for modeling the filter response of the gratings. These
filter characteristics can then serve as a basis for fabricating a
combination of gratings to achieve the desired filter response.
[0122] 5. Broadband Sagnac Raman Amplifier and Cascade Laser Using
Polarization Maintaining Fiber
[0123] To obtain the broadest bandwidth from the Sagnac amplifier
cavity, the wavelength-selective elements in the cavity must be
minimized. Thus far, in the cavity designs of the Sagnac Raman
amplifier and cascade laser, the polarization controllers (PC)
still remain wavelength dependent. The best solution for the Sagnac
Raman cavity is to remove the need for PCs entirely.
[0124] One way of achieving this is to use an
all-polarization-maintaining (PM) cavity. FIG. 18a illustrates such
an all-PM Sagnac Raman cavity that requires PM fiber as well as PM
WDMs and couplers. As is apparent, this embodiment is the same as
that of FIG. 2 except that it uses PM fiber and PM WDMs and
couplers. However, the PM fiber through which the signal travels
might introduce polarization mode dispersion onto the channel. This
can be avoided by mid-way through the cavity cross-splicing the
fibers so the slow and fast axes are interchanged. Then, the second
half of the cavity undoes the polarization mode dispersion in the
first half of the cavity. If the polarization mode dispersion in
half of the cavity causes too much signal degradation, then the
cross-splicing can be done at more frequent intervals.
[0125] Although the optimal cavity would use PM fiber and PM
components, alternate cavity designs can be used if such fibers or
components are either not available or too expensive. The
embodiment of FIG. 18b has the same configuration as that of FIG.
18a except that it uses a bulk 50:50 beam splitter 200. On the
other hand, if the gain fiber cannot be made PM while retaining the
other desired qualities in terms of Raman gain, then cavities such
as FIGS. 19a or 19b can be used. In FIG. 19a, the cavity is formed
between reflector 22 and a Sagnac loop mirror 210. The Sagnac loop
mirror is made of a short length of standard or dispersion-shifted
PM fiber. A Raman gain fiber 202 is placed in the middle of the
cavity of the amplifier. Alternatively, two Sagnac loop mirrors are
used as the two reflectors of the amplifier as depicted in FIG.
19b. Note that similar modifications can be made to the
configurations of FIGS. 8-11, 15 or 16 as have been made in FIGS.
18a, 18b, 19a and 19b to the Sagnac Raman cavity shown in FIG.
2.
[0126] 6. Noise Reduction through Polarization Diversity
[0127] The Sagnac Raman cavity design is also advantageous for
reducing the deleterious effects of DRS. For example, at the 50:50
coupler 32 in FIG. 2 or 8, the pump power is split along the two
directions. Since DRS is proportional to the pump intensity, this
reduction in half of the power along each path reduces the DRS.
Second, when the clockwise and counter-clockwise paths are combined
interferometrically at the 50:50 coupler, the common mode signal
will be reflected while the difference mode noise will be partially
rejected. Hence, the rejection port 46 in FIG. 2 or 8 will also
dampen the DRS.
[0128] An improvement on the Sagnac Raman amplifier design to
further reduce the DRS is to use a polarization diversity pumping
scheme. In such a scheme, the pump power is split between the two
orthogonal polarizations of the fiber, reducing in half the power
per polarization. In addition to reducing DRS, this pumping scheme
will also produce gain from the Raman amplifier that is independent
of the input signal polarization. This polarization independent
gain property is highly desirable for most applications.
[0129] FIGS. 20a and 20b illustrate two embodiments of techniques
to create the polarization diversity pumping. A pump laser 230,
such as the cladding-pumped fiber laser, is normally linearly
polarized. As depicted in FIG. 20a, pump laser 230 is coupled to a
length of polarization maintaining fiber 240, where the light is
coupled at 45 degrees to the fiber axes, and the birefringence of
the PM fiber splits the two polarizations. The fiber can be rotated
to accommodate the launch angle, or a quarter or half-wave retarder
250 can be used at the entrance end of the PM fiber to adjust the
polarization. Alternately, as shown in FIG. 20b, a 50:50 coupler
260 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 270, and the two beams are then combined using a
polarization beam splitter 280. Once separated along two axes, the
pump light is then delivered to the Raman amplifier or laser
configurations.
[0130] The polarization diversity scheme can be combined with other
improvements disclosed earlier in the specification. For example,
if an all-PM cavity is used as described in FIGS. 18a, 18b, 19a and
19b, then the linearly polarized pump light is adjusted to be
launched at 45 degrees to the fiber principal axes. Alternately, if
the integrated pump and Sagnac Raman amplifier is used as described
in FIGS. 16a and 16b, then the axis of the cladding-pumped fiber
174 or 179 can be spliced at 45 degrees to the remainder of the
cavity.
[0131] FIG. 21 shows an embodiment of the polarization diversity
pumping system of the invention in which a length of PM fiber 290
is inserted after a cladding-pumped fiber 295. In particular, the
axis of the cladding-pumped fiber is spliced at 45 degrees to the
fiber axis of the PM Sagnac loop. Alternately, a polarization
maintaining fiber is inserted if the Sagnac loop is not
polarization maintaining.
[0132] 7. Two-Wavelength Amplifiers
[0133] Raman amplifiers as described herein can provide the
necessary element of two-wavelength, broadband amplification. To
illustrate the various orders of Raman amplification that can be
involved in the amplification process, Table II provides exemplary
cascade orders for Raman amplification of signals at either 1310 nm
or 1550 nm. The pump is assumed to be a cladding-pumped fiber
laser, which can be tuned to operate at 1100 nm or 1117 nm.
Amplification of a signal at 1310 nm involves three cascaded Raman
orders, while a signal at 1550 nm involves six cascaded Raman
orders. However, the criteria for using such a device is that there
should not be any degradation of the performance at 1550 nm when
adding the window at 1310 nm, or vice versa.
2TABLE II EXEMPLARY CASCADE ORDERS FOR RAMAN AMPLIFICATION (Peak
Raman Gain at 13.2 THZ) (a) 1310 nm amplification .lambda..sub.p:
1117 nm - pump wavelength from pump to two {close oversize brace}
1st: 1175 nm Raman orders below signal 2nd: 1240 nm } One Raman
order below signal 3rd: 1310 nm -signal (b) 1550 nm amplification
.lambda..sub.p: 1100 nm - pump wavelength from pump to two {close
oversize brace} 1st: 1155 nm Raman orders below signal 2nd: 1218 nm
3rd: 1288 nm {close oversize brace} 4th: 1366 nm 5th: 1455 nm } One
Raman order below signal 6th 1550 nm -signal
[0134] There are several physical effects that should permit
simultaneous support of signals at 1310 nm and 1550 nm. First, as
illustrated in Table II, the amplification of 1310 nm signals
requires three cascaded orders of Raman gain, while the
amplification of 1550 nm signals requires six cascaded orders.
Since there are at least two intermediate orders of Raman gain in
the cascade, the Raman gain or cross-talk penalty between signals
at 1310 .mu.m and 1550 .mu.m should be minimal. However, to avoid
any interaction through pump depletion effects, it would be better
to use Raman cascaded orders in two separate amplifier cavities.
Moreover, the dispersion difference between signals at 1310 .mu.m
and 1550 .mu.m will be quite large, typically about 16 psec/nm-km
in standard or dispersion-shifted fiber. The large amount of
group-velocity dispersion should lead to minimal nonlinear
cross-talk between the two windows. For example, the four-wave
mixing penalty will be minimal because the large dispersion leads
to poor phase-matching. Also, the large dispersion leads to a rapid
walk-off between pulses at the two wavelengths, so the interaction
length will be short.
[0135] The two-wavelength optical fiber amplifier can be
implemented on the improvements described in this application as
well as on the basic framework of the Sagnac Raman amplifier. To
avoid cross-talk through pump depletion, the two wavelengths should
be amplified in parallel. However, since the pump laser is the most
expensive part of the amplifiers, to the extent possible the pump
laser should be shared.
[0136] FIGS. 22a-22c illustrate several preferred embodiments of
the two-wavelength amplifier. In all of these configurations,
1310/1550 nm WDMs are used to separate the incoming signals and
then eventually combine the amplified signals. In FIG. 22a, two
Sagnac Raman amplifiers such as that of FIG. 2 are used in parallel
to form the two-wavelength amplifier. The 1310 nm amplifier
comprises a reflector 308 and a Sagnac loop mirror 310 having a
loop and a coupler attached at the base of the loop. Similarly the
1550 nm amplifier comprises a reflector 314 and a Sagnac loop
mirror 320 having a loop and a coupler attached at the base of the
loop. Both the 1310 nm and 1550 nm amplifiers are powered by a
common pump laser 305 through couplers 306 and 312 respectively. An
input optical signal is divided into two beams by a WDM 330, and
the two beams are sent to input ports of the 1310 nm and 1550 nm
amplifiers respectively. After amplification by these amplifiers,
the output signals from each of the amplifiers are combined by a
WDM 340. Note that to change the operating wavelength of the Sagnac
amplifier, the wavelength specifications for the 50:50 coupler 310
or 320 and the WDM couplers 330 and 340 for injecting and removing
the signal must be adjusted.
[0137] Because of the high efficiency and mature technology of
EDFAs, it may alternately be desirable to use an EDFA to amplify
the signal channel at 1550 nm. FIGS. 22b and 22c illustrate two
exemplary configurations for employing a combination of Raman
amplifiers and EDFAs. In FIG. 22b, a two-wavelength amplifier
comprises a Sagnac Raman amplifier 350 to amplify a 1310 nm signal
and an EDFA 370 to amplify a 1550 nm signal. The Sagnac Raman
amplifier has the same configuration as that depicted in FIG. 2
which primarily comprises two reflectors, one of which is a Sagnac
loop mirror. The EDFA is pumped by a Sagnac Raman cascade laser 360
such as in FIG. 8 that generates a 1480 nm light. The Sagnac Raman
amplifier and cascade laser share a common pump laser 380.
[0138] Finally, FIG. 22c illustrates a two-wavelength amplifier
that comprises a Sagnac Raman amplifier 390 to amplify a 1310 nm
signal and an EDFA 400 to amplify a 1550 nm signal. The Sagnac
Raman amplifier has the same configuration as that depicted in FIG.
2 which primarily comprises two reflectors, one of which is a
Sagnac loop mirror. The EDFA 400 is pumped by its own 980 nm or
1480 nm pump unit 410. Since the pump laser is the single most
expensive element in a typical EDFA or Raman amplifier, one
advantage of the shared pump of FIGS. 22a or 22b will be reduced
cost of the unit. However, the configuration of FIG. 22c may also
be advantageous when upgrading an existing 1550 nm amplifier unit.
It should be clear that the Sagnac Raman amplifiers and lasers
pictured in FIGS. 22a-22c can embody any of the improvements, such
as broadband response and lower noise spectrum, discussed elsewhere
in this patent.
[0139] It should be noted that the gain for the two channels and
the division of the power of the shared pump may be different for
the parallel paths. For example, as shown in the loss curve of FIG.
1a, the loss around 1550 nm is typically about 0.2 dB/km, while the
loss around 1310 nm is typically about 0.35 dB/km. Therefore, for
the same spacing of amplifiers at 1310 nm and 1550 nm in a
fiber-optic link, the amplifier at 1310 nm will have to provide
more gain than the 1550 nm unit. Moreover, since the Raman
amplifier at 1550 nm (six orders) or 1480 nm (five orders) requires
more cascaded orders than when operated at 1310 nm (three orders),
the pump power to achieve the same gain level will be higher as the
wavelength increases. Consequently, adjusting the splitting ratio
of the pump to the two units can satisfy the different pumping
requirements. Alternately, note from Table II that there may be
different optimal pump wavelengths to reach specific end gain
windows. Therefore, a broadband pump such as those described in
FIGS. 14a-14c might be divided spectrally using wavelength
selective elements (i.e., gratings, WDMs or filters) rather than on
the basis of power.
[0140] While the present invention has been described in a number
of different exemplary embodiments, it will be understood that the
principles of the invention can be extended to still further
embodiments and that the embodiments illustrated here are not
intended to limit the scope of the invention as set forth in the
appended claims.
* * * * *