U.S. patent application number 10/293703 was filed with the patent office on 2003-05-22 for system and method for wide band raman amplification.
This patent application is currently assigned to Xtera Communications, Inc., a Delaware corporation. Invention is credited to Dewilde, Carl A., Freeman, Michael J., Islam, Mohammed N..
Application Number | 20030095324 10/293703 |
Document ID | / |
Family ID | 25205566 |
Filed Date | 2003-05-22 |
United States Patent
Application |
20030095324 |
Kind Code |
A1 |
Islam, Mohammed N. ; et
al. |
May 22, 2003 |
System and method for wide band Raman amplification
Abstract
A multi-stage Raman amplifier includes a first Raman amplifier
stage having a first sloped gain profile operable to amplify a
plurality of signal wavelengths, and a second Raman amplifier stage
having a second sloped gain profile operable to amplify at least
most of the plurality of signal wavelengths after those wavelengths
have been amplified by the first stage. The second sloped gain
profile is approximately complementary slope to the slope of the
first sloped gain profile. The combined effect of the first and
second Raman stages contributes to an approximately flat overall
gain profile over the plurality of signal wavelengths.
Inventors: |
Islam, Mohammed N.; (Allen,
TX) ; Dewilde, Carl A.; (Richardson, TX) ;
Freeman, Michael J.; (Canton, MI) |
Correspondence
Address: |
BAKER BOTTS L.L.P.
2001 ROSS AVENUE
SUITE 600
DALLAS
TX
75201-2980
US
|
Assignee: |
Xtera Communications, Inc., a
Delaware corporation
|
Family ID: |
25205566 |
Appl. No.: |
10/293703 |
Filed: |
November 12, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10293703 |
Nov 12, 2002 |
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09811103 |
Mar 16, 2001 |
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6532101 |
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Current U.S.
Class: |
359/337.1 ;
359/334 |
Current CPC
Class: |
H04B 10/2935 20130101;
H04B 10/2916 20130101; H04B 2210/003 20130101 |
Class at
Publication: |
359/337.1 ;
359/334 |
International
Class: |
H04B 010/12 |
Claims
What is claimed is:
1. An optical amplifier, comprising: a plurality of serially
coupled Raman amplifier stages, at least some of the Raman stages
having sloped gain profiles operable to contribute to an overall
gain profile of the amplifier; wherein the overall gain profile of
the amplifier is approximately flat over at least 60 nanometers and
wherein an effective noise figure of at least one of the amplifier
stages is no greater than 6 decibels over the at least 60
nanometers.
2. The amplifier of claim 1, wherein at least two of the plurality
of Raman amplifier stages comprise approximately complementary gain
profiles.
3. The amplifier of claim 1, wherein the plurality of Raman
amplifier stages comprise: a first Raman stage wherein a majority
of shorter signal wavelengths are amplified more than a majority of
longer signal wavelengths; and a second Raman stage wherein a
majority of the longer signal wavelengths are amplified more than a
majority of the shorter signal wavelengths.
4. The amplifier of claim 1, wherein the plurality of Raman
amplifier stages comprise: a first Raman stage wherein a majority
of longer signal wavelengths are amplified more than a majority of
shorter signal wavelengths; and a second Raman stage wherein a
majority of the shorter signal wavelengths are amplified more than
a majority of the longer signal wavelengths.
5. The amplifier of claim 1, wherein an overall gain profile of the
amplifier is approximately flat over at least 70 nanometers of the
plurality of signal wavelengths.
6. The amplifier of claim 1, wherein the overall gain profile of
the amplifier without the use of a gain flattening filter would
vary by less than five decibels over at least 60 nanometers.
7. An optical amplifier, comprising: a plurality of serially
coupled Raman amplifier stages each comprising a gain fiber, at
least some of the Raman stages having sloped gain profiles operable
to contribute to an overall gain profile of the amplifier; wherein
the overall gain profile of the amplifier is approximately flat
over at least 60 nanometers and wherein a difference between signal
to noise ratios measured at an input to and at an output from at
least one of the gain fibers of the amplifier is no greater than 6
decibels over the at least 60 nanometers.
8. The amplifier of claim 7, wherein at least one of the sloped
gain profiles comprises a nonlinear slope.
9. The amplifier of claim 7, wherein at least two of the plurality
of Raman amplifier stages comprise approximately complementary gain
profiles.
10. The amplifier of claim 7, wherein the plurality of Raman
amplifier stages comprise: a first Raman stage wherein a majority
of shorter signal wavelengths are amplified more than a majority of
longer signal wavelengths; and a second Raman stage wherein a
majority of the longer signal wavelengths are amplified more than a
majority of the shorter signal wavelengths.
11. The amplifier of claim 7, wherein the plurality of Raman
amplifier stages comprise: a first Raman stage wherein a majority
of longer signal wavelengths are amplified more than a majority of
shorter signal wavelengths; and a second Raman stage wherein a
majority of the shorter signal wavelengths are amplified more than
a majority of the longer signal wavelengths.
12. The amplifier of claim 7, wherein an overall gain profile of
the amplifier is approximately flat over at least 80 nanometers of
the plurality of signal wavelengths.
13. The amplifier of claim 7, wherein the overall gain profile of
the amplifier without the use of a gain flattening filter would
vary by less than five decibels over at least 60 nanometers.
14. The amplifier of claim 7, wherein the overall gain profile of
the amplifier without the use of a gain flattening filter would
vary by less than one decibel over at least 60 nanometers.
15. The amplifier of claim 7, wherein the gain profiles of the
Raman amplifier stages are each determined at least in part by at
least some of a plurality of pump wavelength signals, and wherein
the plurality of pump wavelength signals comprise a shortest pump
wavelength and a longest pump wavelength.
16. The amplifier of claim 15, wherein a highest level of gain
supplied by the longest pump wavelength is supplied in a last Raman
amplifier stage of the amplifier.
17. The amplifier of claim 15, wherein an initial Raman stage of
the amplifier operates to apply a higher gain level to a signal
wavelength closest to the longest pump wavelength than a gain
applied to a signal furthest from the longest pump wavelength.
18. The amplifier of claim 15, wherein the longest pump wavelength
that provides Raman gain to at least a portion of the signal
wavelengths comprises a wavelength at least 5 and no more than 50
nanometers shorter than the shortest wavelength of the plurality of
signal wavelengths.
19. The amplifier of claim 7, wherein an increase in noise figure
of the amplifier due to phonon stimulated noise comprises no more
than four decibels.
20. The amplifier of claim 7, wherein at least one of the Raman
amplifier stages imparts a net gain to at least a portion of the
plurality of signal wavelengths.
21. The amplifier of claim 7, further comprising a gain flattening
filter coupled to the amplifier, the gain flattening filter
operable to further flatten the gain profile of the amplifier.
22. An optical amplifier, comprising: a plurality of serially
coupled Raman amplifier stages, at least some of the Raman stages
having gain profiles varying as a function of wavelength and
operable to contribute to an overall gain profile of the amplifier;
wherein the overall gain profile of the amplifier is approximately
flat over at least 60 nanometers and wherein an effective noise
figure of at least one of the amplifier stages is no greater than 6
decibels over the at least 60 nanometers.
23. The amplifier of claim 22, wherein at least two of the
plurality of Raman amplifier stages comprise approximately
complementary gain profiles.
24. The amplifier of claim 22, wherein the plurality of Raman
amplifier stages comprise: a first Raman stage wherein a majority
of shorter signal wavelengths are amplified more than a majority of
longer signal wavelengths; and a second Raman stage wherein a
majority of the longer signal wavelengths are amplified more than a
majority of the shorter signal wavelengths.
25. The amplifier of claim 22, wherein the plurality of Raman
amplifier stages comprise: a first Raman stage wherein a majority
of longer signal wavelengths are amplified more than a majority of
shorter signal wavelengths; and a second Raman stage wherein a
majority of the shorter signal wavelengths are amplified more than
a majority of the longer signal wavelengths.
26. The amplifier of claim 22, wherein an overall gain profile of
the amplifier is approximately flat over at least 70 nanometers of
the plurality of signal wavelengths.
27. The amplifier of claim 22, wherein the overall gain profile of
the amplifier without the use of a gain flattening filter would
vary by less than five decibels over at least 60 nanometers.
28. An optical amplifier, comprising: a plurality of serially
coupled Raman amplifier stages each comprising a gain fiber, at
least some of the Raman stages having gain profiles varying as a
function of wavelength and operable to contribute to an overall
gain profile of the amplifier; wherein the overall gain profile of
the amplifier is approximately flat over at least 60 nanometers and
wherein a difference between signal to noise ratios measured at an
input to and at an output from at least one of the gain fibers of
the amplifier is no greater than 6 decibels over the at least 60
nanometers.
29. The amplifier of claim 28, wherein at least two of the
plurality of Raman amplifier stages comprise approximately
complementary gain profiles.
30. The amplifier of claim 28, wherein the plurality of Raman
amplifier stages comprise: a first Raman stage wherein a majority
of shorter signal wavelengths are amplified more than a majority of
longer signal wavelengths; and a second Raman stage wherein a
majority of the longer signal wavelengths are amplified more than a
majority of the shorter signal wavelengths.
31. The amplifier of claim 28, wherein the plurality of Raman
amplifier stages comprise: a first Raman stage wherein a majority
of longer signal wavelengths are amplified more than a majority of
shorter signal wavelengths; and a second Raman stage wherein a
majority of the shorter signal wavelengths are amplified more than
a majority of the longer signal wavelengths.
32. The amplifier of claim 28, wherein an overall gain profile of
the amplifier is approximately flat over at least 80 nanometers of
the plurality of signal wavelengths.
33. The amplifier of claim 28, wherein the overall gain profile of
the amplifier without the use of a gain flattening filter would
vary by less than five decibels over at least 60 nanometers.
34. The amplifier of claim 28, wherein the overall gain profile of
the amplifier without the use of a gain flattening filter would
vary by less than one decibel over at least 60 nanometers.
35. The amplifier of claim 28, wherein at least one of the
plurality of Raman amplifier stages comprises a distributed Raman
amplifier stage and wherein at least one of the plurality of Raman
amplifier stages comprises a discrete Raman amplifier stage.
36. The amplifier of claim 28, wherein the gain profiles of the
Raman amplifier stages are each determined at least in part by at
least some of a plurality of pump wavelength signals, and wherein
the plurality of pump wavelength signals comprise a shortest pump
wavelength and a longest pump wavelength.
37. The amplifier of claim 36, wherein a highest level of gain
supplied by the longest pump wavelength is supplied in a last Raman
amplifier stage of the amplifier.
38. The amplifier of claim 36, wherein an initial Raman stage of
the amplifier operates to apply a higher gain level to a signal
wavelength closest to the longest pump wavelength than a gain
applied to a signal furthest from the longest pump wavelength.
39. The amplifier of claim 36, wherein the longest pump wavelength
that provides Raman gain to at least a portion of the signal
wavelengths comprises a wavelength at least 5 and no more than 50
nanometers shorter than the shortest wavelength of the plurality of
signal wavelengths.
40. The amplifier of claim 28, wherein at least one of the Raman
amplifier stages imparts a net gain to at least a portion of the
plurality of signal wavelengths.
41. The amplifier of claim 28, further comprising a rare earth
doped amplifier stage coupled to at least one of the plurality of
Raman amplifier stages.
42. The amplifier of claim 28, further comprising a gain flattening
filter coupled to the amplifier, the gain flattening filter
operable to further flatten the gain profile of the amplifier.
43. A method of amplifying an optical signal having multiple
wavelengths, the method comprising: amplifying a plurality of
signal wavelengths at a first Raman amplifier stage having a first
gain profile; amplifying at least most of the plurality of signal
wavelengths at a second Raman amplifier stage having a second gain
profile that is different than the first sloped gain profile;
wherein an effective noise figure of at least one of the amplifier
stages is no greater than 6 decibels over the at least 60
nanometers.
44. The method of claim 43, wherein an overall gain profile of the
amplifier is approximately flat over at least 80 nanometers of the
plurality of signal wavelengths.
45. The method of claim 43, wherein the overall gain profile of the
amplifier without the use of a gain flattening filter would vary by
less than five decibels over at least 60 nanometers.
46. The method of claim 43, wherein the first and second Raman
amplifier stages comprise sloped gain profiles.
47. The method of claim 46, wherein the first and second Raman
amplifier stages comprise approximately complementary gain
profiles.
48. A method of amplifying an optical signal having multiple
wavelengths, the method comprising: amplifying a plurality of
signal wavelengths at a first Raman amplifier stage having a first
gain profile; amplifying at least most of the plurality of signal
wavelengths at a second Raman amplifier stage having a second gain
profile that is different than the first sloped gain profile;
wherein a difference between signal to noise ratios measured at an
input to and at an output from at least one of the gain fibers of
the amplifier is no greater than 6 decibels over the at least 60
nanometers.
49. The method of claim 48, wherein at least one of the first and
second gain profiles comprises a nonlinear slope.
50. The method of claim 48, wherein at least two of the plurality
of Raman amplifier stages comprise approximately complementary gain
profiles.
51. The method of claim 48, wherein an overall gain profile of the
amplifier is approximately flat over at least 80 nanometers of the
plurality of signal wavelengths.
52. The method of claim 48, wherein the overall gain profile of the
amplifier without the use of a gain flattening filter would vary by
less than five decibels over at least 60 nanometers.
Description
STATEMENT OF OTHER APPLICATIONS
[0001] This application discloses subject matter that is in some
respects similar to that disclosed in copending application Ser.
No. 09/811,067, entitled Method and System for Reducing Degradation
of Optical Signal to Noise Ratio, filed Mar. 16, 2001., This
application shares a common specification with copending
application Ser. No. ______, filed Nov. 5, 2002, entitled "System
and Method for Wide Band Raman Amplification."
[0002] This application also discloses subject matter that is in
some respects similar to that disclosed in copending application
Ser. No. 09/768,367, entitled All Band Amplifier, filed Jan. 22,
2001. application Ser. No. 09/768,367 is a continuation-in-part of
U.S. application Ser. No. 09/719,591, filed Dec. 12, 2000, which
claims the benefit of copending application serial number
PCT/US99/13551, entitled Dispersion Compensating and Amplifying
Optical Element, Method for Minimizing Gain Tilt and Apparatus for
Minimizing Non-Linear Interaction Between Band Pumps, filed on Jun.
16, 1999, and published on Dec. 23, 1999 as WO 99/66607, which in
turn claims the benefit of U.S. application Ser. No.
60/089,426.
[0003] This application and U.S. Application Ser. Nos. 09/768,367
and 09/811,067 are currently assigned to Xtera Communications,
Inc.
TECHNICAL FIELD OF THE INVENTION
[0004] The present invention relates to the field of communication
systems, and more particularly to a system and method operable to
facilitate wide band optical amplification while maintaining
acceptable noise figures.
BACKGROUND OF THE INVENTION
[0005] 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.
[0006] To exploit 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 in 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, broadband optical amplifier systems that
permit simultaneous amplification of many WDM channels are a key
enabler for utilizing the full fiber bandwidth.
[0007] Traditionally, amplification of signals having a broad range
of wavelengths has required separating the signals into subsets of
wavelengths, and amplifying each subset with a separate amplifier.
This approach can be complex and expensive. Using separate
amplifiers for each subset requires additional hardware, additional
laser pumps for each amplifier, and additional power to launch the
additional pumps.
[0008] Although a more efficient approach would be to amplify the
entire signal using a single amplifier for at least some amplifiers
in the system, unfortunately, no acceptable single amplifier
approach has been developed. For example, erbium doped-amplifiers
are an inherently bad choice for wide band amplification if the
ultimate goal is to provide an amplifier that can operate over the
entire telecommunications spectrum. For example, for wavelengths
shorter than about 1525 nanometers, erbium-atoms in typical glasses
will absorb more than they amplify. Even with use of various
dopings, such as, aluminum or phosphorus, the absorption peak for
the various glasses is still around 1530 nanometers. This leaves a
large gap in the short communications band (S-Band) unreachable by
erbium doped fiber amplifiers.
[0009] Raman amplifiers provide a better solution in terms of
broadband amplification potential, but conventional Raman
amplifiers have suffered from other shortcomings. For example,
Raman amplifiers have traditionally suffered from high noise
figures when used in wide band applications. In addition, Raman
amplifiers suffer from gain tilt introduced when longer wavelength
signals rob energy from shorter wavelength signals. This effect
becomes increasingly pronounced as amplifier launch power and
system bandwidth increases. Wide band Raman amplifiers operating at
high launch powers on a wide range of wavelengths can be
particularly vulnerable to this effect.
[0010] Masuda, et al. (see e.g., U.S. Pat. No. 6,172,803 B1 and
related research papers) have attempted to improve the bandwidth of
erbium doped amplifiers by cascading with the erbium doped
amplifier a Raman amplifier with an approximately complementary
gain profile. Masuda, et al, however, consistently require the
presence of an erbium doped amplifier (which relies on different
physics for amplification and does not suffer from the same noise
problems as Raman amplifiers do) to provide virtually all
amplification to signal wavelengths close in spectrum to the pump
wavelengths. Indeed, Masuda, et al. concede that the noise figures
they report ignore the effect of the Raman portion of their
amplifier.
SUMMARY OF THE INVENTION
[0011] The present invention recognizes a need for a method and
apparatus operable to facilitate wide band Raman amplification
while maintaining an approximately flat gain profile and an
acceptable noise figure.
[0012] In accordance with the present invention, a system and
method for providing wide band Raman amplification are provided
that substantially reduce or eliminate at least some of the
shortcomings associated with prior approaches. In one aspect of the
invention, a multi-stage Raman amplifier comprises a first Raman
amplifier stage having a first sloped gain profile operable to
amplify a plurality of signal wavelengths, and a second Raman
amplifier stage having a second sloped gain profile operable to
amplify at least most of the plurality of signal wavelengths after
those wavelengths have been amplified by the first stage. The
second sloped gain profile has an approximately complementary slope
to the slope of the first sloped gain profile. The combined effect
of the first and second Raman stages contributes to an
approximately flat overall gain profile over the plurality of
signal wavelengths.
[0013] In another aspect of the invention, a method of amplifying
an optical signal having multiple wavelengths comprises amplifying
a plurality of signal wavelengths at a first Raman amplifier stage
having a first sloped gain profile, and amplifying at least most of
the plurality of signal wavelengths at a second Raman amplifier
stage after those signal wavelengths have been amplified by the
first stage. The second stage has a second sloped gain profile
comprising an approximately complimentary gain profile to the first
gain profile. The combined effect of the first and second Raman
stages contributes to an approximately flat overall gain profile
over the plurality of signal wavelengths.
[0014] In still another aspect of the invention, a multi-stage
Raman amplifier comprises a plurality of cascaded Raman amplifier
stages each having a gain profile, wherein the gain profile of at
least some of the Raman stages is sloped. At least two of the
sloped gain profiles comprise approximately complimentary gain
profiles, wherein the combined effect of the gain profiles of the
amplification stages results in an approximately flat overall gain
profile over a plurality of signal wavelengths amplified by the
amplifier.
[0015] In yet another aspect of the invention, a method of
amplifying multiple-wavelength optical signals comprises applying a
first sloped gain profile to a plurality of signal wavelengths at a
first stage of a Raman amplifier, and applying a second sloped gain
profile to at least most of the plurality of signal wavelengths at
a second stage of the Raman amplifier. The second gain profile.
comprises an approximately complementary gain profile of the first
sloped gain profile. The combined effect of the first and second
sloped gain profiles contributes to an approximately flat overall
gain profile over the plurality of signal wavelengths.
[0016] In another aspect of the invention, a multi-stage Raman
amplifier comprises a plurality of cascaded Raman amplifier stages
each operable to amplify a plurality of signal wavelengths and each
having a gain profile determined at least in part by one or more
pump wavelengths applied to the amplifier stage. The plurality of
amplifier stages comprise a first Raman stage operable to apply a
higher gain level to a signal wavelength closest to a longest pump
wavelength than a gain applied to a signal wavelength furthest from
the longest pump wavelength.
[0017] In still another aspect of the invention, a method of
amplifying an optical signal having multiple wavelengths comprises
receiving a plurality of signal wavelengths at a plurality of
cascaded Raman amplifier stages having at least a first stage and a
last stage, where each stage is operable to amplify a plurality of
signal wavelengths and each stage has a gain profile determined at
least in part by one or more pump wavelengths applied to the
amplifier stage. The method further includes applying a highest
level of gain supplied by the longest pump wavelength in the last
Raman stage of the amplifier.
[0018] In yet another aspect of the invention, a multi-stage Raman
amplifier comprises a plurality of cascaded Raman amplifier stages,
at least some of the Raman stages having sloped gain profiles
operable to contribute to a combined gain profile of the amplifier.
The combined gain profile of the amplifier is approximately flat
across a bandwidth of at least eighty nanometers and comprises a
small signal noise figure no greater than eight decibels.
[0019] In another aspect of the invention, a method of amplifying
an optical signal having multiple wavelengths comprises amplifying
a plurality of signal wavelengths at a first Raman amplifier stage
having a first sloped gain profile, and amplifying at least most of
the plurality of signal wavelengths at a second Raman amplifier
stage having a second sloped gain profile that is different than
the first sloped gain profile. The combined gain profile of the
amplifier is approximately flat across a bandwidth of at least
eighty nanometers and comprises a small signal noise figure no
greater than eight decibels.
[0020] In another aspect of the invention, an optical pre-amplifier
operable to be coupled to an optical communication link carrying
optical signals having a plurality of wavelengths comprises a first
Raman stage having a gain profile where a majority of shorter
signal wavelengths are amplified more than a majority of longer
signal wavelengths. The preamplifier further comprises a second
Raman stage operable to receive at least most of the signal
wavelengths after they have been amplified by the first stage, the
second stage having a gain profile where a majority of longer
signal wavelengths are amplified more than a majority of shorter
signal wavelengths. In this embodiment, the gain profiles of the
first and second Raman stages are operable to combine to contribute
to an approximately flat combined gain profile over the plurality
of signal wavelengths.
[0021] In still another aspect of the invention, an optical booster
amplifier operable to be coupled to an optical communication link
carrying optical signals having a plurality of wavelengths
comprises a first Raman stage having a gain profile where a
majority of longer signal wavelengths are amplified more than a
majority of shorter signal wavelengths. The booster amplifier also
comprises a second Raman stage operable to receive at least most of
the signal wavelengths after they have been amplified by the first
stage, the second stage having a gain profile where a majority of
shorter signal wavelengths are amplified more than a majority of
longer signal wavelengths. The gain profiles of the first and
second Raman stages are operable to combine to contribute to an
approximately flat combined gain profile over the plurality of
wavelengths.
[0022] In yet another aspect of the invention, a Raman amplifier
assembly comprises a preamplifier coupled to an optical
communication link. The preamplifier includes a first Raman stage
having a gain profile wherein a majority of shorter wavelengths are
amplified more than a majority of longer wavelengths, and a second
Raman stage having a gain profile approximately complementary to
the first gain stage. The amplifier assembly also includes a
booster amplifier coupled to the optical communication link. The
booster amplifier comprises a first Raman stage having a gain
profile wherein a majority of longer wavelengths are amplified more
than a majority of shorter wavelengths, and a second Raman stage
having a gain profile approximately complementary to the first gain
stage.
[0023] In another aspect of the invention, an optical communication
system operable to facilitate communication of multiple signal
wavelengths comprises a transmitter bank operable to generate a
plurality of signal wavelengths, and a multiplexer operable to
combine the plurality of signal wavelengths into a single multiple
wavelength signal for transmission over a transmission medium. The
system further comprises an amplifier coupled to the transmission
medium and operable to amplify the multiple wavelength signal prior
to, during, or after the multiple wavelength signal's transmission
over the transmission medium, the amplifier comprising a
multi-stage Raman amplifier. The amplifier includes a first Raman
amplifier stage having a first sloped gain profile operable to
amplify a plurality of signal wavelengths and a second Raman
amplifier stage having a second sloped gain profile operable to
amplify at least most of the plurality of signal wavelengths after
those wavelengths have been amplified by the first stage. The
second sloped gain profile has an approximately complementary slope
to the slope of the first sloped gain profile, and the combined
effect of the first and second Raman stages contributes to an
approximately flat overall gain profile over the plurality of
signal wavelengths. In one embodiment, the system further includes
a demultiplexer operable to receive the multiple wavelength signal
and to separate the signal wavelengths from the multiple wavelength
signal, and a receiver bank operable to receive the plurality of
signal wavelengths.
[0024] Depending on the specific features implemented, particular
embodiments of the present invention may exhibit some, none, or all
of the following technical advantages. For example, one aspect of
the invention facilitates optical amplification of a wide bandwidth
of wavelengths while maintaining an approximately flat gain profile
and an acceptable noise figure.
[0025] In a particular embodiment, one aspect of the invention
reduces the noise figure associated with the amplifier by
amplifying in a first Raman stage a majority of shorter wavelengths
more than a majority of longer wavelengths. In this way, shorter
wavelengths (which are often closest to the pump wavelength) are
amplified to overcome any effects that might be caused by
phonon-stimulated noise. As a further enhancement, the amplifier
could be designed so that the longest pump wavelength is at least
ten nanometers below the shortest signal being amplified.
[0026] In addition to yielding an acceptable noise figure, this
approach can produce an approximately flat gain tilt, for example,
by cascading a second Raman amplifier stage having a gain profile
that amplifies a majority of longer wavelengths more than a
majority of shorter wavelengths. In a particular embodiment, the
second gain profile can be approximately complementary to the first
gain profile. In some applications, the second gain profile can
have an approximately equal (although opposite) slope from the
first gain profile.
[0027] Another aspect of the invention results in increased
efficiency in a multi-stage Raman amplifier. This aspect of the
invention involves applying, in at least one Raman stage, a first
gain profile that amplifies a majority of longer wavelengths more
than a majority of shorter wavelengths; and applying, in a later
cascaded Raman stage, a second gain profile that amplifies a
majority of shorter wavelengths more than a majority of longer
wavelengths. This embodiment facilitates allowing longer pump
wavelengths in the first stage to accept energy from shorter pump
wavelengths in the later Raman stage. This effect, in turn,
facilitates using smaller pump wavelengths and/or fewer pump
wavelengths in the first stage than would otherwise be required,
thereby increasing the efficiency of the device. In a particular
embodiment, the gain profiles of the first and later Raman stages
can be approximately complimentary, contributing to an
approximately flat overall gain profile for the amplifier. The
noise figure can be reduced, for example, by performing a majority
of the amplification of wavelengths closest to the pump wavelengths
in one of the final amplifier stages, or in the last amplifier
stage.
[0028] Other aspects of the invention facilitate cascading multiple
amplifier stages to realize advantages of low noise and high
efficiency in a multiple stage Raman amplifier. Moreover, cascaded
stages can provide mid-stage access to the amplifier to facilitate,
for example, optical add/drop multiplexing of WDM signals while
maintaining an acceptable noise figure and an approximately flat
gain profile, both at the mid-stage access point and across the
entire amplifier.
[0029] Other technical advantages are readily apparent to one of
skill in the art from the attached figures, description, and
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] For a more complete understanding of the present invention,
and for further features and advantages thereof, reference is now
made to the following description taken in conjunction with the
accompanying drawings, in which:
[0031] FIG. 1 is a block diagram showing an exemplary optical
communication system operable to facilitate communication of wide
band optical signals constructed according to the teachings of the
present invention;
[0032] FIG. 2 is a graphical illustration of the phonon-stimulated
optical noise figure;
[0033] FIG. 3a is a block diagram of an exemplary embodiment of a
multiple stage Raman amplifier constructed according to the
teachings of the present invention;
[0034] FIG. 3b-3c show gain profiles associated with various
amplification stages and an overall gain profile for the amplifier
shown in FIG. 3a, respectively, constructed according to the
teachings of the present invention;
[0035] FIG. 4a is a block diagram of an exemplary embodiment of a
multiple stage Raman amplifier constructed according to the
teachings of the present invention;
[0036] FIGS. 4b-4c show gain profiles associated with various
amplification stages and an overall gain profile for the amplifier
shown in FIG. 4a, respectively, constructed according to the
teachings of the present invention;
[0037] FIG. 5a is a block diagram of an exemplary embodiment of a
three stage Raman amplifier constructed according to the teachings
of the present invention;
[0038] FIGS. 5b-5c show gain profiles associated with various
amplification stages and an overall gain profile for the amplifier
shown in FIG. 5a, respectively, constructed according to the
teachings of the present invention;
[0039] FIGS. 6a is a block diagram of an exemplary embodiment of a
four stage Raman amplifier constructed according to the teachings
of the present invention;
[0040] FIGS. 6b-6c show gain profiles associated with various
amplification stages and an overall gain profile for the amplifier
of FIG. 6a, respectively, constructed according to the teachings of
the present invention;
[0041] FIG. 7 is a flow chart illustrating one example of a method
of amplifying a plurality of wavelengths using a multi-stage Raman
amplifier according to the teachings of the present invention;
[0042] FIGS. 8a-8b show simulated gain and noise profiles for one
embodiment of a multi-stage hybrid Raman amplifier constructed
according to the teachings of the present invention; and
[0043] FIGS. 9a-9b show simulated gain and noise profiles for one
embodiment of a multi-stage discrete Raman amplifier constructed
according to the teachings of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0044] FIG. 1 is a block diagram showing an exemplary optical
communication system 10 operable to facilitate communication of
wide band optical signals. System 10 includes a transmitter bank 12
operable to generate a plurality of wavelength signals 16a-16n.
Transmitter bank 12 may include, for example, a plurality of laser
diodes or semiconductor lasers. Each wavelength signal 16a-16n
comprises at least one wavelength of light unique from wavelengths
carried by other signals 16.
[0045] System 10 also includes a combiner 14 operable to receive
multiple signal wavelengths 16a-16n and to combine those signal
wavelengths into a single multiple wavelength signal 16. As one
particular example, combiner 14 could comprise a wavelength
division multiplexer (WDM). The term wavelength division
multiplexer as used herein may include conventional wavelength
division multiplexers or dense wavelength division
multiplexers.
[0046] In one particular embodiment, system 10 may include a
booster amplifier 18 operable to receive and amplify wavelengths of
signal 16a prior to communication over a transmission medium 20.
Transmission medium 20 can comprise multiple spans 20a-20n of
fiber. As particular examples, fiber spans 20 could comprise
standard single mode fiber (SMF), dispersion-shifted fiber (DSF),
non-zero dispersion-shifted fiber (NZDSF), or other fiber type or
combinations of fiber types.
[0047] Where communication system 10 includes a plurality of fiber
spans 20a-20n, system 10 can include one or more inline amplifiers
22a-22m. Inline amplifiers 22 reside between fiber spans 20 and
operate to amplify signal 16 as it traverses fiber 20.
[0048] Optical communication system 10 can also include a
preamplifier 24 operable to receive signal 16 from a final fiber
span 20n and to amplify signal 16 prior to passing that signal to a
separator 26. Separator 26 may comprise, for example, a wavelength
division demultiplexer (WDM), which can operate on wavelength
division multiplexed signals or dense wavelength division
multiplexed signals. Separator 26 operates to separate individual
wavelength signals 16a-16n from multiple wavelength signal 16.
Separator 26 communicates individual signal wavelength 16a-16n to a
bank of receivers 28.
[0049] At least one amplifier in system 10 comprises a wide band
multi-stage Raman amplifier operable to receive a wide bandwidth of
wavelength signal 16. In a particular embodiment, the amplifier can
process over 80 nanometers of bandwidth, and in some cases over 100
nanometers of bandwidth while maintaining an approximately flat
gain profile over the bandwidth of amplified signal wavelengths
16.
[0050] Throughout this document, the term "approximately flat"
describes a condition where the maximum signal gain differs from
the minimum signal gain by an no more than amount suitable for use
in telecommunication systems. The deviation between minimum and
maximum signal gains may comprise, for example five decibels prior
to application of any gain flattening filters. Particular
embodiments of the invention may achieve gain flatness of
approximately three decibels prior to application of any gain
flattening filters.
[0051] Some amplifiers in system 10 could comprise a plurality of
individual amplifiers working in conjunction, each amplifying a
subset of the bandwidth processed by the single wide band
amplifier. Alternatively, all amplifiers in system 10 could
comprises wide bandwidth amplifiers. Depending on the overall
bandwidth communicated by system 10, one or more amplifier
locations in system 10 could comprise a plurality of wide band
amplifiers operating in conjunction to handle a total bandwidth
significantly in excess of 100 nanometers. In other cases, a single
wide band amplifier could process all traffic at a given location
in system 10.
[0052] Wide band amplifiers within system 10 comprise multi-stage
Raman amplifiers having at least two stages with approximately
complimentary gain profiles. A combination of the complimentary
gain profiles, in cooperation with any other gain stages in the
wide band amplifier, results in approximately flat gain profile for
the amplifier.
[0053] Throughout this description, the phrase "approximately
complementary" refers to a situation where, at least in general,
signal wavelengths 116 that are highly amplified in the first stage
are less amplified in the second stage, and signal wavelengths 116
that are highly amplified in the second stage are less amplified in
the first stage. Two gain profiles said to be "approximately
complementary" need not have equal and opposite slopes. Moreover,
equal amplification of any particular wavelengths in both gain
profiles does not preclude those gain profiles from being
"approximately complementary."
[0054] Conventional designs of multi-stage Raman amplifiers have
been unable to process bandwidths in excess of 80 nanometers while
maintaining approximately flat gain profiles and acceptable noise
figures. One aspect of this invention recognizes that a major
culprit in noise figures associated with conventional multi-stage
Raman amplifiers is the phonon-stimulated optical noise created
when wavelength signals being amplified reside spectrally close to
pump wavelengths used for amplification. One aspect of the
invention reduces adverse effect of this noise by enhancing the
Raman amplification of signal wavelengths near the pump wavelengths
to overcome the effects of the noise, and applying an approximately
complementary Raman gain profile in another stage to result in an
approximately flat overall gain profile.
[0055] FIG. 2 graphically illustrates the phonon-stimulated optical
noise figure increase as the spectral spacing between signal
wavelengths and pump wavelengths decreases. As shown in FIG. 2,
phonon-stimulated noise increases dramatically as signal wavelength
get close to the pump wavelengths.
[0056] One aspect of the invention significantly reduces adverse
effects associated with phonon-stimulated noise by providing
multiple stages of Raman gain having approximately complimentary
gain profiles acting on substantially the same bandwidth of
signals. While best results are obtained by applying approximately
complimentary gain profiles to all or nearly all of the same signal
wavelengths, some portion of wavelengths can be omitted from one
gain profile and included in the other gain profile without
departing from the scope of this invention.
[0057] FIG. 3a is a block diagram of an exemplary embodiment of a
multiple stage Raman amplifier 110 including gain profiles 30 and
40 associated with various amplification stages and an overall gain
profile 50 for the amplifier. In this example, amplifier 100
comprises a two-stage amplifier having a first stage 112 and a
second stage 114 cascaded with first stage 112. As will be further
discussed below, the invention is not limited to a particular
number of amplifier stages. For example, additional amplification
stages could be cascaded onto second stage 114. Moreover, although
the illustrated embodiment shows second stage 114 cascaded directly
to first stage 112, additional amplification stages could reside
between first stage 112 and second stage 114 without departing from
the scope of the invention.
[0058] Amplifier 100 could comprise a distributed Raman amplifier,
a discrete Raman amplifier, or a hybrid Raman amplifier which
comprises both discrete and distributed stages. Each stage 112, 114
of amplifier 100 includes an input operable to receive a multiple
wavelength optical input signal 116. As a particular example,
optical input signal 116 could include wavelengths ranging over one
hundred nanometers.
[0059] Each stage 112, 114 also includes distributed gain media
120, 121. Depending on the type of amplifier being implemented,
media 120, 121 may comprise, for example a transmission fiber, or a
gain fiber such as a spooled gain fiber. In a particular
embodiment, media 120, 121 may comprise a dispersion compensating
fiber.
[0060] Each stage 112, 114 further includes one or more wavelength
pumps 122. Pumps 122 generate pump light 124 at specified
wavelengths, which are pumped into distributed gain media 120, 121.
Raman gain results from the interaction of intense light from the
pumps with optical phonons in silica fibers. The Raman effect leads
to a transfer of energy from one optical beam (the pump) to another
optical beam (the signal). Pumps 122 may comprise, for example, one
or more laser diodes. Although the illustrated embodiment shows the
use of counter propagating pumps, under some circumstances using a
relatively quiet pump, co-propagating pumps could also be used
without departing from the scope of the invention.
[0061] In one particular embodiment, pump wavelengths 124 can be
selected so that the longest wavelength pump signal 124 has a
wavelength that is shorter than the shortest wavelength of signal
116. As one specific example, the longest wavelength of pump light
124 could be selected to be, for example, at least ten nanometers
shorter than the shortest wavelength of signal 116. In this manner,
amplifier 100 can help to avoid phonon stimulated noise that
otherwise occurs when pump wavelengths interact with wavelengths of
the amplified signal.
[0062] Couplers 118b and 118c couple pump wavelengths 124a and 124b
to gain distributed media 120 and 121, respectively. Couplers 118
could comprise, for example, wave division multiplexers (WDM) or
optical couplers. A lossy element 126 can optionally reside between
amplifier stages 112 and 114. Lossy element 126 could comprise, for
example, an isolator, an optical add/drop multiplexer, or a gain
equalizer.
[0063] The number of pump wavelengths 124, their launch powers,
their spectral and spatial positions with respect to other pump
wavelengths and other wavelength signals, and the bandwidth and
power level of the signal being amplified can all contribute to the
shape of the gain profile for the respective amplifier stage. FIG.
3b shows exemplary gain profiles for first stage 112 and second
stage 114. Gain profile 30 shows the overall gain of first stage
112 of amplifier 100 for a bandwidth ranging from the shortest
wavelength of signal 116 (.lambda..sub.sh) to the longest
wavelength of signal 116 (.lambda..sub.lg). Gain profile 40 shows
the overall gain of second stage 112 of amplifier 100 for a
bandwidth ranging from the shortest wavelength of signal 116
(.lambda..sub.sh) to the longest wavelength of signal 116
(.lambda..sub.lg). Each of gain profiles 30 and 40 reflects the
effects of the other gain profile acting upon it.
[0064] In this example, gain profile 30 of first stage 112 has a
downward slope, where a majority of the shorter signal wavelengths
116 are amplified more than a majority of the longer signal
wavelengths 116. Conversely, gain profile 40 of second stage 114 is
approximately complimentary to gain profile 30 of first stage 112.
Gain profile 40 exhibits an upward slope where a majority of the
longer signal wavelengths 116 are amplified more than a majority of
the shorter signal wavelengths 116.
[0065] Gain profile 50 (shown in dotted lines in FIG. 3c)
represents an exemplary composite gain profile of amplifier 100
resulting from the application of gain profiles 30 and 40 to
optical signal 116. Gain profile 50 is approximately flat over at
least substantially all of the bandwidth of wavelengths within
signal 116.
[0066] In operation, amplifier 100 receives optical input signal
116 at distributed gain medium 120 of first stage 112. Distributed
gain medium 120 could comprise, for example, a dispersion
compensating Raman gain fiber, a transmission fiber, a high
non-linearly fiber, a segment of transmission fiber, or combination
thereof. Pumps 122(a) generate pump wavelengths 124(a) and apply
them to distributed gain medium 120 through coupler 118(b). Pump
wavelengths 124 interact with signal wavelengths 116, transferring
energy from the pump wavelengths 124 to the signal wavelengths 116.
In this example, shorter signal wavelengths 116 are amplified more
than longer signal wavelengths 116 in first stage 112.
[0067] Amplified wavelengths of signal 116 are communicated to
distributed gain medium 121 of second stage 114. Wavelengths of
signal 116 are amplified in second stage 114 by interacting with
pump wavelengths 124b generated at pumps 122b. In this example,
pump wavelengths 124b operate to result in gain profile 40 where
longer wavelengths of signal 116 are amplified more than shorter
wavelengths of signal 116.
[0068] The combined effect of amplification in first stage 112 and
second stage 114 of amplifier 100 results in approximately flat
gain profile 50 across wavelengths of optical signal 116. This
particular example provides a significant advantage in reducing the
noise figure associated with the amplifier. Using this
configuration, the small signal noise figure of amplifier 100 can
be reduced to less than eight decibels, in some cases 7 decibels,
even where the bandwidth of signal 16 exceeds 100 nanometers.
[0069] FIG. 4a is a block diagram of another embodiment of a
multiple stage Raman amplifier 110 including exemplary gain
profiles 130 and 140 associated with various amplification stages
and an overall gain profile 150 for the amplifier. Amplifier 110
shown in FIG. 4 is similar in structure and function to amplifier
100 shown in FIG. 1. Like amplifier 100 shown in FIG. 1, amplifier
110 of FIG. 4 includes a first Raman amplification stage 112 and a
second Raman amplification stage 114. Each of stages 112 and 114
includes a distributed gain medium 120, 121, respectively, which is
operable to receive multiple wavelength input signal 116 and pump
wavelengths 124a and 124b, respectively. Each amplifier stage 112
and 114 operates to amplify wavelengths of signal 116 according to
gain profiles 130 and 140 as shown.
[0070] The example shown in FIG. 4 differs from the example shown
in FIG. 3 in that gain profile 130 (shown in FIG. 4b) of first
stage 112 exhibits an upward slope where a majority of longer
wavelengths of signal 116 are amplified more than the majority of
shorter wavelengths of signal 116. Conversely, gain profile 140 of
second stage 114 comprises an approximately complementary gain
profile to first gain profile 130 of first stage 112. In profile
140 applies a higher gain to a majority of shorter wavelengths than
the gain applied to the majority of longer signal wavelengths 116.
In addition, in this embodiment, the launch power of pumps 122a
driving first gain profile 130 can be reduced.
[0071] This aspect of the invention recognizes that due to the
Raman scattering effect, longer wavelength signals tend to rob
energy from shorter wavelength signals. This aspect of the
invention leverages that fact to allow the longer pump wavelengths
of wavelengths 124a to rob energy from the shorter pump wavelengths
of wavelengths 124b. In a particular embodiment, amplifier 110 may
include a shunt 160 between second distributed gain medium 121 and
first distributed gain medium 120 to facilitate the longer pump
wavelengths of wavelengths 124a accepting power from the shorter
pump wavelengths of wavelengths 124b. The effects result in an
overall gain profile 130 for first stage 112 that remains
approximately complimentary to the gain profile of second stage
140. As a result, the composite gain profile 150 (FIG. 4c) of the
amplifier remains approximately flat.
[0072] This embodiment provides significant advantages in terms of
efficiency by allowing the use of fewer wavelength pumps 122a in
the first stage 112, and/or also by allowing each pump 122a to
operate at a lower launch power.
[0073] The embodiment shown in FIG. 4a can also provide
improvements for the noise figure of the amplifier. For example,
phonon stimulated noise is created in Raman amplifiers where
wavelengths being amplified spectrally reside close to a wavelength
of pump signals 124. One aspect of this invention recognizes that
by spectrally separating pump wavelengths 124 from signal
wavelengths 116, phonon stimulated noise can be reduced.
[0074] In a particular embodiment, pump wavelengths 124 are
selected to have wavelengths at least 10 nanometers shorter than
the shortest wavelength in optical signal 116 being amplified.
Moreover, in a particular embodiment, second stage 114 where a
majority of the gain to short wavelength of signal 116 is applied
comprises the last stage of amplifier 110.
[0075] FIG. 5a is a block diagram of a three stage Raman amplifier
200 including gain profiles 230, 240, and 245 associated with
various amplification stages, and an overall gain profile 250 for
the amplifier. Amplifier 200 is similar in structure and function
to amplifier 100 of FIG. 3 but includes three cascaded
amplification stages 212, 214, and 215. Each of amplifier stages
212-215 includes a distributed gain medium 220, 221, 223,
respectively, which operate to receive multiple wavelength optical
signal 216 and pump wavelengths 224a-224c from pumps 222a-222c.
Each amplifier stage includes an optical coupler operable to
introduce pump wavelengths 224 to the respective gain media. In
some embodiments, lossy elements 226 may reside between one or more
amplification stages 212-215. Lossy elements 226 may comprise, for
example, optical add/drop multiplexers, isolators, and/or gain
equalizers.
[0076] Amplifier 200 may comprise a discrete Raman amplifier or a
hybrid Raman amplifier. For example, first distributed gain medium
220 may comprise a transmission fiber, a section of transmission
fiber, or a Raman gain fiber. In a particular embodiment, first
distributed gain medium 220 could comprise a dispersion
compensating Raman gain fiber.
[0077] Distributed gain medium 221 of second stage 214 may comprise
a segment of transmission fiber or a Raman gain fiber. Distributed
gain medium 223 of third amplifier phase 215 could comprise, for
example, a Raman gain fiber. In particular embodiments, any or all
of distributed gain mediums 220-223 could comprise a dispersion
compensating Raman gain fiber.
[0078] In operation, amplifier 200 receives signal 216 at first
stage 212 and applies a gain to signal wavelengths 216 according to
gain profile 230 depicted in FIG. 5b. Signal 216 next traverses
second stage 214 where gain profile 240 is applied. Finally, signal
216 is amplified by third stage 215 according to gain profile 245
shown in FIG. 3b. Signal 216 exits amplifier 200 at output 260
having been exposed to a composite gain profile 250 as shown in
FIG. 3c.
[0079] In this particular example, first stage 212 and second stage
214 operate in a similar manner to amplifier 100 shown in FIG. 3a.
In particular, first stage 212 applies a gain profile 230 that
amplifies a majority of shorter signal wavelengths 216 more than it
amplifies a majority of longer signal wavelengths 216. Second stage
214, conversely, applies and approximately complimentary gain
profile 240 to signal 216, where the majority of longer wavelengths
of signal 216 are amplified more than a majority of shorter
wavelengths of signal 216.
[0080] The combination of second stage 214 and third stage 215, on
the other hand, operates similarly to amplifier 110 shown in FIG.
4. While second stage 214 applies gain profile 240 amplifying a
majority of longer signal wavelengths 216 more than a majority of
shorter signal wavelengths 216, third stage 215 applies to gain
profile 245, which amplifies a majority of shorter signal
wavelengths 216 more than a majority of longer signal wavelengths
216. In this particular example, gain profile 240 of second stage
214 is approximately complimentary to both gain profile 230 of
first stage 212 and gain profile 245 of third stage 215. In this
example, the slope of gain profile 240 is significantly steeper
than the slope of gain profiles 230 and 245 to account for the fact
that gain profile 240 is the only profile exhibiting an upward
slope. The composite gain profile 250 (shown in FIG. 5c) resulting
from the combination of amplifications in first, second, and third
amplifier stages of amplifier 200 results in an approximately flat
gain profile.
[0081] This particular example reaps the efficiency benefits
discussed with respect to FIG. 4, and permits use of the noise
figure reduction techniques discussed with respect to FIGS. 3 and
4. For example, efficiency advantages are realized by allowing
longer pump wavelengths 224 of second stage 214 to accept power
from high powered shorter pump wavelengths 224c of third
amplification stage 215. This results from the Raman effect wherein
longer wavelength signals tend to rob energy from shorter
wavelength signals. As a result, second stage 214 can be operated
with fewer wavelength pumps than what otherwise be required, and
also with lower pump launch powers.
[0082] In terms of improvements in noise figure, the gain profiles
of first stage 212 compared to second stage 214 results in high
amplification of shorter wavelengths of signal 216 to overcome
phonon stimulated noise associated with interaction of those
signals with the longer pump wavelengths 224a. In addition,
providing a significant amount of amplification to shorter
wavelengths of signal 216 in the last stage 215 of amplifier 220
helps to minimize the noise figure associated with amplifier
200.
[0083] FIGS. 6a-6c show a block diagram of a four stage Raman
amplifier, gain profiles associated with various stages of the
amplifier, and a composite gain of the amplifier respectively.
Amplifier 300 is similar in structure and function to amplifiers
100 and 110 shown in FIGS. 1 and 2, respectively. In this example,
amplifier 300 includes four Raman amplification stages 312, 314,
315, and 317. Each amplification stage includes a distributed gain
medium 320, 321, 323, and 325, respectively. Distributed gain
medium 320 of first stage 312 may comprise, for example, a
transmission fiber or a Raman gain fiber. Each of distributed gain
medium 312-325 of second, third, and fourth stages 314-317 may
comprise a Raman gain fiber or a segment of transmission fiber. In
particular embodiments, some or all of distributed gain media
320-325 could comprise dispersion compensating Raman gain
fibers.
[0084] Each distributed gain medium 320-325 is operable to receive
a multi wavelength optical signal 316 and amplify that signal by
facilitating interaction between optical signal 316 and pump
wavelengths 324a-324d. Pump wavelengths 324 are generated by pumps
322 and coupled to distributed gain media 320-325 through couplers
318. In this particular example, couplers 318 comprise wave
division multiplexers.
[0085] In the illustrated embodiment, amplifier 300 includes at
least one lossy element 326 coupled between amplifier stages. In
this example, lossy element 326b comprises an optical add/drop
multiplexer coupled between second stage 314 and third stage 315.
Optical add/drop multiplexer 326b facilitates mid-stage access to
amplifier 300 and allows selective addition and/or deletion of
particular wavelengths from signal 316. Other lossy elements, such
as isolators or gain equalizers could alternatively reside between
amplifier stages.
[0086] In operation, signal 316 enters amplifier 300 at coupler
318a, which passes signal 316 to first amplifier stage 312 where a
gain profile at 330, as shown in FIG. 4b, is applied to wavelengths
of signal 316. Signal 316 is then passed to second stage 314 where
a gain profile 335, as shown in FIG. 4b is applied to wavelengths
of signal 316.
[0087] In this particular example, first and second stages 312 and
314 of amplifier 300 operate similarly to amplifier 100 described
with respect to FIG. 3. In particular, first stage 312 applies a
gain profile where a majority of shorter signal wavelengths are
amplified more than a majority of longer signal wavelengths, and
second stage 314 applies an approximately complimentary gain
profile 335 where a majority of longer signal wavelengths are
amplified more than a majority of shorter signal wavelengths. In
this particular embodiment, the composite gain from first stage 312
and second stage 314 results in an approximately flat gain profile
at the output of second stage 314. This design advantageously
facilitates addition and subtraction of particular wavelengths of
signal 316 without the need for further manipulation of the gain.
In addition, first and second gain stages 312 and 314 provide a low
noise figure, reducing the effects of phonon stimulated noise in
shorter wavelength signals closest to the pump wavelengths.
[0088] Continuing with the operational description, particular
wavelengths of signal 316 may be substituted with other wavelengths
at add/drop multiplexer 326b. After processing by add/drop
multiplexer 326b, signal 316 continues to third amplification stage
315, where gain profile 340 is applied as shown in FIG. 6b. Signal
316 is then communicated to fourth stage 317 where gain profile 345
is applied to wavelengths of signal 316. Amplified signal 316 is
then output at output port 365.
[0089] Third and fourth amplification stages of amplifier 300 are
similar in structure and function to amplifier 110 described with
respect to FIG. 4. Through the use of this configuration, third and
fourth amplifier stages 315 and 317 provide increased efficiency in
operation. In particular, pump 322 can operate with fewer pump
signals and/or lower launch power as a result of the Raman
scattering effect which allows longer pump wavelengths 324c of
third stage 316 to accept power from highly amplified shorter pump
wavelengths 324d of fourth stage 317. Moreover, third and fourth
amplification stages 315 and 317 assist in maintaining a low noise
figure by applying a significant amount of the gain to the shortest
wavelengths of signal 316 at the last amplifier stage 317.
[0090] FIG. 7 is a flow chart showing one example of a method 400
of amplifying a multi-wavelength optical signal using a multi-stage
Raman amplifier. This particular example uses FIGS. 6a-6c to
illustrate the method. Similar methods could apply to any of the
embodiments described herein. Method 400 begins at step 410 where
first amplifier stage 312 receives signal wavelengths 316 and
applies first gain profile 330 to those wavelengths. Step 420
allows for optional mid-stage access between first stage 312 and
second stage 314. The method continues where second stage 314
applies second gain profile 325 to signal wavelengths 316 at step
430.
[0091] Second gain profile 335 is approximately complimentary to
first gain profile 330. In this particular example, first gain
profile 330 amplifies a majority of shorter signal wavelengths 316
more than a majority of longer signal wavelengths 316, while second
gain profile 325 amplifies a majority of longer wavelength signals
316 more than a majority of shorter wavelength signals 316. Those
gain profiles could be reversed if desired. Moreover, additional
gain profiles could be applied between first stage 312 and second
stage 314 by intervening stages (not explicitly shown). This
particular example shows additional stages beyond first stage 312
and second stage 314. In a particular embodiment, an amplifier
embodying the invention could comprise only two complimentary
stages of Raman gain.
[0092] This example provides optional mid-stage access at step 450.
Mid-stage access could comprise, for example, application of
optical add/drop multiplexing, gain equalization, or the presence
of one or more optical isolators.
[0093] Where amplifier 300 comprises more than two stages of
complimentary Raman amplification, method 400 continues at step 460
where third stage 316 applies gain profile 340 to signal
wavelengths 316. Where amplifier 300 comprises a three stage
amplifier, third gain profile 340 can be complimentary to second
gain profile 335. An example of this operation is shown in FIG. 5.
Where amplifier 300 comprises a four stage amplifier, third stage
315 can apply gain profile at 340 as shown in FIG. 6b, while fourth
stage 317 applies gain profile 345 as shown in FIG. 6b at step
480.
[0094] In this example, third gain profile 340 amplifies a majority
of longer signal wavelengths 316 more than a majority of shorter
signal wavelengths 316 while fourth stage 317 amplifies a majority
of shorter signal wavelengths 316 more than a majority of longer
signal wavelengths 316. In this manner, third and fourth stages of
amplifier 300 can realize efficiency advantages by allowing longer
pump wavelengths 324c from third stage 315 to accept energy from
highly amplified shorter pump wavelengths 324d in fourth stage
317.
[0095] Although this method has described a four stage
amplification process, the method can equally apply to any system
having two or more Raman amplification stages. In addition,
although this particular example described first and second gain
stages having gain profiles 330 and 335 as shown in FIG. 6b, and
third and fourth gain stages having gain profiles 340 and 345 as
shown in FIG. 6b, those gain profiles could be reversed without
departing from the scope of the invention. The particular example
shown provides significant advantages in a four stage amplifier in
that initial stages can be configured to provide a low noise figure
by emphasizing amplification of shorter wavelength signals early in
the amplification process. In addition, third and fourth
amplification stages advantageously realize efficiency gains in
amplifier locations where noise reduction is not as critical a
concern.
[0096] FIGS. 8a-8b are graphs showing simulations of one aspect of
the present invention embodied in a two stage distributed Raman
amplifier. FIGS. 9a-9b are graphs showing simulations of one aspect
of the present invention embodied in a two stage discrete Raman
amplifier.
[0097] The parameters used for the amplifier simulations were as
follows:
1 Distributed Discrete Stage 1 Input Port Loss 0 dB 1.3 dB Stage 1
Gain Fiber 80 km LEAF fiber DK-21 (DCF) Stage 1 Pump Powers: 438 mW
@ 1396 nm 438 mW @ 1416 nm 380 mW @ 416 nm 438 mW @ 1427 nm 380 mW
@ 1427 nm 170 mW @ 1450 nm 220 mW @ 1450 nm 10 mW @ 1472 nm 4 mW @
1505 nm 19 mW @ 1505 nm Mid-Stage Loss 2 dB 1.6 dB Stage 2 Gain
Fiber DK-30 (DCF) DK-19 (DCF) Stage 2 Pump Powers: 380 mW @ 1399 nm
380 mW @ 1472 nm 380 mW @ 1472 nm 380 mW @ 1505 nm 380 mW @ 1505 nm
Stage 2 Output Port Loss 1 dB 1.3 dB
[0098] FIGS. 8a and 9A show first gain profile 30 of first stage
112, second gain profile 40 of second stage 114, and composite gain
profile 50 of Raman amplifier 100 for distributed and discrete
configurations, respectively. As shown in these figures application
of pump wavelengths 124 as shown in Table 1 above results in a
downwardly sloping gain profile 30 for first stage 112, and an
upwardly sloping gain profile 40 for second stage 114. Gain
profiles 30 and 40 are approximately complementary to one another,
although they do not comprise mirror images of one another.
[0099] The composite gain profile 50 of amplifier 100 is
approximately flat across the bandwidth of signal 116 being
amplified. Gain profile 50 represents the gain profile without
application of any gain flattening filters. In this embodiment,
amplifier 100 obtains an overall gain profile that is approximately
flat for over 100 nanometers.
[0100] FIGS. 8b and 9b show the same gain profile 50 and compare
that profile to the noise figure of the amplifier. In the case of
the discrete Raman amplifier simulated in FIG. 9b, the actual noise
FIG. 55 is shown. In the case of the distributed Raman amplifier
simulated in FIG. 8b, the effective noise FIG. 65 is shown.
[0101] An optical amplifier noise figure is defined as
NF=SNRin/SNRout where SNRin is the signal-to-noise ratio of the
amplifier input signal and SNRout is the signal-to-noise ratio of
the amplifier output signal. As defined, NF is always greater then
1 for any realizable amplifier. Effective noise figure for a
distributed optical amplifier is defined as the noise figure a
discrete amplifier placed at the end of the distributed amplifier
transmission fiber would need to have to produce the same final SNR
as the distributed amplifier. It can be, and in practice is, less
than 1 (negative value in dB) for practical distributed amplifiers
over at least a small portion of their operating wavelength
range.
[0102] As shown in FIGS. 8b and 9b, the noise figure in this
embodiment is always less than eight decibels over the entire
bandwidth of signal 116. In fact, for a bandwidth between 1520
nanometers and 1620 nanometers, the noise figure never exceeds 7
decibels.
[0103] Although the present invention has been described in several
embodiments, a myriad of changes, variations, alterations,
transformations, and modifications may be suggested to one skilled
in the art, and it is intended that the present invention encompass
such changes, variations, alterations, transformations, and
modifications as fall within the spirit and scope of the appended
claims.
* * * * *