U.S. patent number RE41,610 [Application Number 11/150,606] was granted by the patent office on 2010-08-31 for gain equalization system and method.
Invention is credited to Michael H. Eiselt, Mark Shtaif.
United States Patent |
RE41,610 |
Eiselt , et al. |
August 31, 2010 |
Gain equalization system and method
Abstract
A power equalization system and method for use in an optical
transmission system are provided. The power equalization system
includes an optical line including at least one transmission
channel and a management line. The transmission system further
includes a plurality of amplifiers, a plurality of Optical spectrum
analyzers and a plurality of equalizers. The plurality of
amplifiers are coupled to the optical line, spaced periodically
throughout the optical transmission system. As information is sent
through the optical transmission system, the plurality of
amplifiers boost the power of each channel of the optical signal. A
plurality of optical spectrum analyzers are also coupled to the
optical lien and are spaced periodically throughout the optical
transmission system and are co-located with a first portion of the
amplifiers coupled to the optical line. A plurality of equalizers
are also coupled to the optical line and are spaced periodically
throughout the optical transmission system and equalize the power
on each channel of the optical line. The plurality of equalizers
are co-located with a second portion of the plurality of amplifiers
and at least one of the plurality of Optical spectrum analyzers is
not co-located with one of the plurality of equalizers. As optical
information is transmitted over the optical transmission system,
the Optical spectrum analyzers provide analysis data via the
management line to the non co-located equalizers for use by the
equalizers in equalizing the power of the channels of the optical
line at that point. The analysis data generated by the Optical
spectrum analyzer identifies the analysis data at the point of the
Optical spectrum analyzer which is not co-located with the
equalizer.
Inventors: |
Eiselt; Michael H. (99334
Kirchheim, DE), Shtaif; Mark (Even Yehuda,
IL) |
Family
ID: |
21815966 |
Appl.
No.: |
11/150,606 |
Filed: |
June 10, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
10023579 |
Dec 14, 2001 |
06577788 |
Jun 10, 2003 |
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Current U.S.
Class: |
385/24; 359/885;
359/337 |
Current CPC
Class: |
H04B
10/07955 (20130101); H04B 10/2935 (20130101); H04J
14/0221 (20130101); H04B 10/077 (20130101); H04B
10/25073 (20130101) |
Current International
Class: |
G02B
6/28 (20060101); H04J 14/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wong; Tina M
Attorney, Agent or Firm: Woodcock Washburn LLP
Claims
What is claimed is:
1. A power equalization system for use in an optical transmission
system, the system comprising: an optical line, wherein the optical
line includes at least one transmission channel; a plurality of
amplifiers coupled to the optical line, wherein the amplifiers are
spaced .[.periodically throughout.]. .Iadd.along .Iaddend.the
optical line; a plurality of .[.Optical.]. .Iadd.optical
.Iaddend.spectrum analyzers coupled to the optical line, wherein
the .[.Optical.]. .Iadd.optical .Iaddend.spectrum analyzers are
spaced .[.periodically throughout.]. .Iadd.along .Iaddend.the
optical line and wherein the .[.Optical.]. .Iadd.optical
.Iaddend.spectrum analyzers generate analysis data; and a plurality
of equalizers coupled to the optical line, wherein the equalizers
are spaced .[.periodically throughout.]. .Iadd.along .Iaddend.the
optical line and wherein the equalizers equalize the power on the
channels; a management .[.line.]. .Iadd.channel .Iaddend.for
transmitting management data coupled to the pluralities of
amplifiers, .[.Optical.]. .Iadd.optical .Iaddend.spectrum analyzers
and equalizers; wherein the plurality of .[.Optical.].
.Iadd.optical .Iaddend.spectrum analyzers are .[.collocated.].
.Iadd.co-located .Iaddend.with a portion of the plurality of
amplifiers and wherein the plurality of equalizers are
.[.collocated.]. .Iadd.co-located .Iaddend.with a second portion of
the plurality of amplifiers and wherein at least one of the
plurality of .[.Optical.]. .Iadd.optical .Iaddend.spectrum
analyzers is not .[.collocated.]. .Iadd.co-located .Iaddend.with
the plurality of equalizers; and .[.whereby.]. .Iadd.wherein
.Iaddend.analysis data generated by the .[.Optical.]. .Iadd.optical
.Iaddend.spectrum analyzers is transmitted via the management
channel to the equalizers for use by the equalizers in equalizing
the power of the channels of the optical line.
2. The system of claim 1, wherein the amplifiers .Iadd.are spaced
periodically along the optical line and .Iaddend.include in-line
amplifiers.
3. The system of claim 1, wherein the equalizers .Iadd.are spaced
periodically along the optical line and are configured to
.Iaddend.equalize the power of each channel individually.
4. The system of claim 1, wherein the equalizers .Iadd.are spaced
periodically along the optical line and .Iaddend.include dynamic
gain equalizers.
5. The system of claim 1, wherein the analysis data transmitted by
the .[.Optical.]. .Iadd.optical .Iaddend.spectrum analyzer is
transmitted upstream.
6. The system of claim 1, wherein the plurality of amplifiers
include: a plurality of erbium-doped fiber amplifiers coupled to
the optical line and the management .[.line.].
.Iadd.channel.Iaddend., wherein the erbium-doped fiber amplifiers
adjust the power of the optical line to counteract gain tilt;
.[.whereby.]. .Iadd.wherein .Iaddend.the erbium-doped fiber
amplifiers counteract gain tilt through the adjustment of the power
of all channels collectively at the amplifiers.
7. The system of claim 6, wherein the plurality of erbium-doped
fiber amplifiers adjust the power of the optical line to counteract
the gain tilt from Stimulated Raman Scattering.
8. The system of claim 6, wherein the plurality of erbium-doped
fiber amplifiers adjust the power of the optical line to counteract
the gain tilt from non-uniform fiber loss.
9. The system of claim 6, wherein the plurality of erbium-doped
fiber amplifiers adjust the power of the optical line to .[.counter
act.]. .Iadd.counteract .Iaddend.the gain tilt based upon the
analysis data transmitted via the management line.
10. The system of claim 1, wherein the plurality of equalizers
.Iadd.are spaced periodically along the optical line and are
configured to .Iaddend.equalize the power on the channels so the
average power over a periodic spacing is zero.
11. The system of claim 1, wherein the management .[.line.].
.Iadd.channel .Iaddend.includes an optical supervisory channel.
12. The system of claim 11, wherein the optical supervisory channel
includes one of the transmission channels of the optical line.
13. The system of claim 1, wherein the management channel includes
a public telephone network.
14. The system of claim 1, wherein the management channel includes
the Internet.
15. A method of gain equalization of an optical transmission
system, the method comprising the steps of: transmitting an optical
signal over at least one optical channel of the optical
transmission system; transmitting a management signal over a
management .[.line.]. .Iadd.channel .Iaddend.of the optical
transmission system; amplifying the optical signal at
.[.predetermined.]. .Iadd.amplifying .Iaddend.positions in the
optical transmission system; analyzing the optical signal at a
first portion of the .[.predetermined.]. amplifying positions;
determining an optical spectrum gain from the optical signal
analysis; transmitting an optical spectrum gain signal including
the optical spectrum gain to an equalizer; and equalizing the
optical signal based upon the received optical spectrum gain at a
second portion of the .[.predetermined.]. amplifying positions;
wherein at least one position of the first portion of
.[.predetermined.]. amplifying positions is not .[.collocated.].
.Iadd.co-located .Iaddend.with a .[.predetermined amplifying.].
position of the second portion.
16. The method of claim 15, wherein .Iadd.the .Iaddend.step of
transmitting an optical signal over at least one optical channel
includes transmitting optical signals over a plurality of optical
channels.
17. The method of claim 16, wherein the steps of analyzing the
optical signal and determining the optical spectrum gain includes
analyzing each optical signal channel and determining the optical
spectrum gain for each optical signal channel.
18. The method of claim 17, wherein the step of equalizing the
optical signal includes equalizing each channel of the optical
signal based upon the optical spectrum gain of each channel.
19. The method of claim 15, wherein the optical spectrum gain
average over a predetermined span is zero.
20. The method of claim 15, wherein the equalizing of the optical
signal includes dynamic gain equalization.
21. The method of claim 15, wherein the transmission of the optical
spectrum gain signal is upstream.
22. The method of claim 15 further comprising: determining an
amount of gain tilt in the optical signal based upon the optical
signal analysis; transmitting a gain tilt signal including the
amount of gain tilt of the optical signal to .[.an.]. .Iadd.one or
more .Iaddend.erbium-doped fiber amplifiers; adjusting the power of
the optical signal with the erbium-doped fiber amplifiers based
upon the amount of gain tilt, wherein the erbium-doped fiber
amplifiers counteract gain tilt through the adjustment of the power
of all channels collectively at the amplifiers.
.Iadd.23. An optical transmission system, comprising: a plurality
of optical transmission channels coupled at a first end to a first
terminal and at a second end to a second terminal; an optical
spectrum analyzer coupled to the optical transmission channels at a
first intermediate location between the first end and the second
end; an equalizer coupled to the optical transmission channels at a
second intermediate location between the first end and the second
end, wherein the equalizer is spaced apart from the optical
spectrum analyzer; wherein the system is configured to transmit
analysis data generated by the optical spectrum analyzer to the
equalizer for use by the equalizer in reducing power differences in
the optical transmission channels. .Iaddend.
.Iadd.24. An optical transmission system as recited in claim 23,
further comprising a management channel, wherein said management
channel is configured to carry said analysis data from the optical
spectrum analyzer to the equalizer. .Iaddend.
.Iadd.25. An optical transmission system as recited in claim 23,
further comprising a first amplifier co-located with the optical
spectrum analyzer and a second amplifier co-located with the
equalizer, the first and second amplifiers being coupled to the
optical transmission channels and configured to amplify optical
signals in said channels. .Iaddend.
.Iadd.26. An optical transmission system as recited in claim 23,
further comprising a management channel, wherein said management
channel is coupled to said first and second amplifiers such that
commands from said optical spectrum analyzer are communicable via
said management channel to said first and second amplifiers.
.Iaddend.
.Iadd.27. The optical transmission system of claim 23, further
comprising an amplification location, wherein the first and second
intermediate locations are not co-located with the amplification
location. .Iaddend.
.Iadd.28. The optical transmission system of claim 27, wherein the
amplification location is between the first intermediate location
and the second intermediate location. .Iaddend.
.Iadd.29. A method for use in an optical transmission system,
comprising: amplifying an optical signal at one or more amplifying
positions in the optical transmission system; analyzing the optical
signal at a first position and determining an optical spectrum
gain; transmitting an optical spectrum gain signal to an equalizer
at a second position spaced apart from said first position; and
reducing spectral component power differences in the optical signal
at said first second position based upon the received optical
spectrum gain. .Iaddend.
.Iadd.30. A method as recited in claim 29, wherein said spectrum
gain signal is transmitted over a management channel to said
equalizer. .Iaddend.
.Iadd.31. A method as recited in claim 30, further comprising
transmitting a command signal over the management channel to
control one or more amplifiers. .Iaddend.
.Iadd.32. A method as recited in claim 29, wherein said second
position corresponds to one of said amplifying positions.
.Iaddend.
.Iadd.33. A method as recited in claim 32, wherein the optical
signal is amplified at first and second amplifying positions, said
first position corresponds to one of said amplifying positions, and
said second position corresponds to a different one of said
amplifying positions. .Iaddend.
.Iadd.34. A system for use in an optical transmission system,
comprising: at least one amplifier configured to amplify an optical
signal at one or more amplifying positions in the optical
transmission system; at least one equalizer; an analyzer configured
to analyze the optical signal at a first position, to determine a
spectrum gain, and to transmit a spectrum gain signal to said
equalizer; wherein said equalizer is located at a second position
spaced apart from said first position, and is configured to reduce
channel-to-channel power differences in the optical signal at said
first second position based upon the received spectrum gain.
.Iaddend.
.Iadd.35. A system as recited in claim 34, wherein said gain signal
is transmittable over a channel to said equalizer. .Iaddend.
.Iadd.36. A system as recited in claim 35, wherein said at least
one amplifier is configured to be controlled via a command signal
received over said channel. .Iaddend.
.Iadd.37. A system as recited in claim 34, wherein said second
position corresponds to one of said amplifying positions.
.Iaddend.
.Iadd.38. A system as recited in claim 37, wherein the optical
signal is amplifiable at first and second amplifying positions,
said first position corresponds to one of said amplifying
positions, and said second position corresponds to a different one
of said amplifying positions. .Iaddend.
.Iadd.39. A system as recited in claim 34, comprising a plurality
of equalizers spaced periodically along an optical line of said
system. .Iaddend.
.Iadd.40. A system as recited in claim 34, wherein said equalizer
comprises a dynamic gain equalizer. .Iaddend.
.Iadd.41. An optical transmission system, comprising: an optical
channel; a management channel; at least one amplifier stationed at
an amplifying position along said optical channel; at least one
equalizer stationed at an equalizing position along said optical
channel; an amplifier stationed at an analyzing position along said
optical channel and configured to generate a spectrum gain signal
and to transmit said spectrum gain signal via said management
channel to said at least one equalizer. .Iaddend.
.Iadd.42. A system as recited in claim 41, further comprising at
least one equalizer, wherein said equalizer is located at an
equalizing position spaced apart from said analyzing position, and
is configured to reduce channel-to-channel power difference in an
optical signal at said equalizing position based upon the spectrum
gain signal. .Iaddend.
.Iadd.43. A system as recited in claim 42, wherein said equalizing
position corresponds top said amplifying position. .Iaddend.
.Iadd.44. A system as recited in claim 43, wherein said equalizer
comprises a dynamic gain equalizer. .Iaddend.
.Iadd.45. A method for use in an optical transmission system,
comprising: amplifying an optical signal at one or more amplifying
positions in the optical transmission system; analyzing the optical
signal at a first position and determining an optical spectrum
gain; transmitting an optical spectrum gain signal via a management
channel to a remote location. .Iaddend.
.Iadd.46. A method as recited in claim 45, wherein said spectrum
gain signal is transmitted over said management channel to an
equalizer at a second position spaced apart from said first
position. .Iaddend.
.Iadd.47. A method as recited in claim 46, further comprising
reducing spectral component power differences in the optical signal
said second position based upon the optical spectrum gain.
.Iaddend.
.Iadd.48. A method as recited in claim 25, further comprising
transmitting a command signal over the management channel to
control one or more amplifiers. .Iaddend.
.Iadd.49. A method as recited in claim 46, wherein said second
position corresponds to one of said amplifying positions.
.Iaddend.
.Iadd.50. A method as recited in claim 46, wherein the optical
signal is amplified at first and second amplifying positions, said
first position corresponds to one of said amplifying positions, and
said second position corresponds to a different one of said
amplifying positions. .Iaddend.
Description
CROSS--REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH/DEVELOPMENT
Not Applicable.
FIELD OF THE INVENTION
This invention relates to an equalization system, and more
particularly, to a system for use in an optical network to correct
for unequal gain of power in the channels of the optical
signal.
BACKGROUND OF THE INVENTION
The transmission, routing and dissemination of information has
occurred over computer networks for many years via standard
electronic communication lines. These communication lines are
effective, but place limits on the amount of information being
transmitted and the speed of the transmission. With the advent of
light-wave technology, a large amount of information is capable of
being transmitted, routed and disseminated across great distances
at a high transmission rate over fiber optic communication
lines.
When information is transmitted over fiber optic communication
lines, impairments of the pulse of light carrying the information
can occur, including pulse broadening (dispersion) and attenuation
(energy loss). In-line amplifiers spaced throughout the fiber optic
communication system boosts the power of each channel of the
optical signal to assist in the compensation of the energy lost
during transmission. The in-line amplifiers boost each channel of
the optical signal with the same amount of power. However, as
different wave lengths of light are used over the different
channels of the fiber optic communication system, the amount of
energy lost per channel is not consistent. As the in-line amplifier
boosts the energy across all channels of the optical signal
transmitted over the fiber optic communication system, the power
gain of any specific channel may fail to meet or exceed the desired
power gain. Further, energy loss caused by polarization dependent
loss (PDL) lead to further nonuniform power gain over the multiple
channels of the optical signal transmitted over the fiber optic
communication system.
As the optical signal is transmitted across the fiber optic
communication system, the gain differences on a channel-by-channel
basis accumulate. These gain differences can cause distortions of
the optical signal shape and therefore lead to performance
degradation. Current systems allow for the optical signals' power
deviations to accumulate before they are compensated by the gain
equalizer after analysis by the optical spectrum analyzer ("OSA").
Inherent in these systems is a process which allows a large amount
of gain differences to accumulate prior to equalization. Prior to
the gain equalization of the channels of the optical signal, the
optical signal performance beings to degrade and thus the overall
performance of the fiber optic communications system is
degraded.
To compensate for gain differences in the multiple channels of the
optical signal, gain equalizers are provided, spaced periodically,
throughout the fiber optic communication lines (FIG. 1). the gain
equalizers equalize the power at the in-line amplifiers on a
channel-by-channel basis throughout the optical signal. To
determine the amount of gain on a channel-b-channel basis, an
optical spectrum analyzer is co-located with the gain equalizer.
The Optical measures the power level associated with each channel
of the optical signal and compares this power level with the
desired power level for each channel and provides this information
to the gain equalizer which is co-located with the Optical spectrum
analyzer at an in-line amplifier within the fiber optic
communication system. The gain equalizer then equalizes the power
of each channel based upon the analysis performed by the Optical
spectrum analyzer at this in-line amplifier location. As can be
seen in FIG. 2, the gain equalizer zeroes out the gain difference
throughout the channels at the point in the fiber optic
communication system where the gain equalizer and Optical spectrum
analyzer are located. Therefore, any advancement in the ability to
lower the amount of gain difference accumulated during the
transmission of information over a fiber optic communication system
would be advantageous.
SUMMARY OF THE INVENTION
A power equalization system and method for use in an optical
transmission system are provided. The power equalization system
includes an optical line including at least one transmission
channel and a management line. The transmission system further
includes a plurality of amplifiers, a plurality of Optical spectrum
analyzers and a plurality of equalizers. The plurality of
amplifiers are coupled to the optical line, spaced periodically
throughout the optical transmission system. As information is sent
through the optical transmission system, the plurality of
amplifiers boost the power of each channel of the optical signal. A
plurality of Optical spectrum analyzers are also coupled to the
optical lien and are spaced periodically throughout the optical
transmission system and are co-located with a first portion of the
amplifiers coupled to the optical line. A plurality of equalizers
are also coupled to the optical line and are spaced periodically
throughout the optical transmission system and equalize the power
on each channel of the optical line. The plurality of equalizers
are co-located with a second portion of the plurality of amplifiers
and at least one of the plurality of Optical spectrum analyzers is
not co-located with one of the plurality of equalizers. Thus, as
the optical information is transmitted over the optical
transmission system, the Optical spectrum analyzers provide
analysis data via the management line to the non co-located
equalizers for use by the equalizers in equalizing the power of the
channels of the optical line at that point. The analysis data
generated by the Optical spectrum analyzer identifies the analysis
data at the point of the Optical spectrum analyzer which is not
co-located with the equalizer.
DETAILED DESCRIPTION OF THE DRAWINGS
A better understanding of the invention can be obtained from the
following detailed description of one exemplary embodiments as
considered in conjunction with the following drawings in which:
FIG. 1 is a block diagram depicting an optical transmission system
according to the prior art;
FIG. 2 is a graphical representation of the accumulated gain of an
optical signal being transmitted over an optical transmission
system according to the prior art;
FIG. 3 is a graphical representation of the loss per kilometer of
different wavelengths of an optical signal transmitted through an
optical transmission system;
FIG. 4 is a block diagram depicting the optical transmission system
according to the present invention;
FIG. 5 is a graphical representation of the optical spectrum
analysis of the gain per channel of an optical signal transmitted
over the optical transmission system according to the present
invention;
FIG. 6 is a graphical representation of the accumulated gain per
channel of an optical signal transmission across the optical
transmission system according to the present invention; and
FIG. 7 is a graphical representation of the accumulated gain per
channel of the optical signal after the gain equalization has
occurred according to the optical transmission system according to
the present invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
In the descriptions which follows, like parts are marked throughout
the specification and drawings with the same numerals,
respectively. The drawing figures are not necessarily drawn to
scale and certain figures may be shown in exaggerated or
generalized form in the interest of clarity and conciseness.
In an optical transmission sytem, the optical signal is transmitted
over an optical line. In certain optical transmission systems,
including dense wavelength division multiplexed systems (DWDM
systems), the optical signal that is transmitted includes multiple
channels where each channel of the optical signal is transmitted at
a different wavelength. As originally transmitted, the optical
signal does not contain enough energy to complete a long haul or
ultra long haul transmission therefore, the optical signal must be
amplified periodically throughout the optical line. The amplifiers
are used to replace energy loss due to attenuation with an equal
amount of replacement energy per wavelength (or per channel) in the
optical signal. There are multiple wavelengths or channels in each
optical signal transmitted over the optical transmission system. In
one disclosed embodiment, forty wavelengths are capable of being
transmitted through one optical signal. As the amount of power
attenuation varies among the channels of the optical signal, some
channels experience an excessive amount of gain caused by the
amplifier, some experience the correct amount of gain and some
experience too little gain. These gain differences experienced by
different channels of the optical signal are accumulated and are
caused by the wavelength profile of the amplifiers or by effects
such as polarization dependent loss (PDL) in the various components
located in the optical system. The losses per wavelength are not
perfectly flat. The number of energy lost between any two points
along the optical network will typically remain nearly constant if
the lost power is caused by the amplification of the amplifiers. A
random gain deviation can be experienced intermittently and is
typically not consistent.
In a conventional optical networks 100, as can be seen in FIG. 1,
gain deviation is corrected through the use of optical spectrum
analyzers 108 and dynamic gain equalizers 110. The conventional
optical network 100 includes terminals 102 and 104. The terminals
102 and 104 connect the remaining conventional optical network 100,
which is typically a dense wavelength division multiplexed (DWDM)
network to the local optical network (not shown). The terminal 102
receives an optical signal from the local optical network and
transmits this optical signal across the conventional optical
network 100 to terminal 104. For the sake of illustration, the
conventional optical network 100 is shown operating in only one
direction; however, it is well known to those skilled in the art
that an optical network can function bi-directionally.
The optical signal 101 is typically comprised of twenty to forty
channels. Each channel of the optical signal 101 is a separate
wavelength of the optical signal. The optical signal 101 is
typically transmitted in wavelengths between 1530 and 1610 nm. As
the optical signal 101 is transmitted from the terminal 102 over
the optical network 100, the optical signal 101 begins to
experience attenuation or power loss. therefore, an in-line
amplifier 106 is provided at fixed intervals to boost the power of
the optical signal 101.
The in-line amplifier 106 boosts the power of the optical signal
101 according to a predetermined value or according to a value that
can be adjusted during operation. The In-line amplifier 106 boosts
the power of the optical signal 101 across every channel, applying
the same power boost to every channel regardless of the channels
current power level. Therefore, as gain differences are experienced
in the multiple channels of the optical signal 101, these gain
differences are accumulated through the amplification of the
optical signal 101 from the In-line amplifiers 106. As the optical
signal 101 is transmitted from In-line amplifier 106a to In-line
amplifier 106b, the optical signal 101 experiences attenuation and
again must be amplified.
However, prior to amplification at In-line amplifier 106b, an
optical spectrum analyzer 108a evaluates each channel of the
optical signal 101 to determine its current power deviation from
the expected value. The optical signal analyzer 108a then transmits
this power or gain deviation information to a dynamic gain
equalizer 110a which then adjusts the optical signal 101 on a
channel-by-channel basis to optimize the power in each channel.
Thus, any differences in gain accumulated through the transmission
of the optical signal 101 because of random gain or through
amplification by the In-line amplifiers 106 are corrected based
upon the evaluation of the Optical spectrum analyzer 108a and
implemented by the Dynamic gain equalizer 110a.
The process repeats as the optical signal 101 is transmitted
through In-line ampolfiers 106c, 106d, 106e, 106f and 106g. The
optical signal 101 is analyzed by the optical spectrum analyzers
108b and 108c once the signal reaches In-line amplifiers 106d. The
optical spectrum analyzer 108b and 108c identify any gain
differences on a channel-by-channel basis of the optical signal 101
before transmitting these gain differences to the dynamic gain
equalizers 110b and 110c. The dynamic gain equalizers 110b and 110c
then correct for gain differences accumulated, regardless if the
gain differences are caused by the In-line amplifiers 106 or
through polarization dependent loss.
The number of In-line amplifiers 106 is determined based upon the
distance the conventional optical network 100 must cover. Thus,
whether the conventional optical network 100 is a long haul or
ultra long haul optical network, the frequency in quantity of
In-line amplifiers 106 must be determined. Further, the fiber
characteristics, the characteristics of the amplifiers, and
characteristics of the power necessary for the optical signal 101
determine the quantity of amplifiers 106 needed throughout the
conventional optical network 100 and also determine the quantity of
analyzers 108 and equalizers 110 necessary in the conventional
optical network 100. Therefore, the optical spectrum analyzer 108
and the dynamic gain equalizers 110 may only repeat every nth
In-line amplifier 106 where n will be determined based upon the
characteristics described above. In the example shown in FIG. 1,
the Optical spectrum analyzers 108 and the dynamic gain equalizers
110 are provided at every two In-line amplifiers 106. However, the
frequency could be increased or decreased based upon need of the
system.
One constant of the conventional optical network 100 is that the
Optical spectrum analyzer 108 and the dynamic gain equalizer 110
are co-located at the same in-line amplifier 106. Therefore, the
analysis and equalization of the optical signal 101 occurs at one
specific point in the optical network and is typically located at
an in-line amplifier 106 site.
Turning to FIG. 2, a graph, according to the prior art, of the
accumulated gain of an exemplary channel of the optical signal
versus a distance is shown. The graph of FIG. 2 illustrates an
exemplary channel of the optical signal 101; graphs of other
channels of the optical signal 101 may form different slopes,
however, the graph of accumulated gain versus distance of all
channels of the optical signal 101 will have a common element of
reducing the accumulated gain to zero at a specific distance. The
graph of FIG. 2 demonstrates that at every distance x, the
accumulated gain of an exemplary channel of the optical signal 101
is reduced to zero. Therefore, referring to the embodiment shown in
FIG. 1, the distance x would represent the distance between three
in-line amplifiers, for example In-line amplifiers 106b-106d. As
can be seen from the graph of FIG. 2, the gain of one channel of
the optical signal 101 is linear over distance. Therefore, as is
seen in FIG. 2, the gain of the exemplary channel of the optical
signal 101 increases two decibels through every span where the span
equals the distance between two adjacent in-line amplifiers 106.
The linear line 200 represents the total gain of the exemplary
channel of the optical signal 101 between equalization points,
which is represented as distance x. Thus, the gain of the channel
equals four decibels over this span. The distance x 204 is the
distance between two in-line amplifiers 106, for example, the
distance between In-line amplifier 106b and In-line amplifier
106d.
As the gain increases on any channel of the optical signal 101, the
integrity of the signal is reduced. Therefore, the greater the area
defined by the triangle formed by lines 200, 204 and 206 the
greater than accumulated gain 202 and the greater the distortion of
the optical signal 101. The graph of FIG. 2 is exemplary of the
accumulated gain caused by the effect of the In-line amplifiers 106
providing a constant power boost across the multiple channels of
the optical signal 101, and for illustrative purposes does not
represent any gain caused by polarization dependent loss or any
other random gain accumulation. However, one skilled in the art
would recognize that the graphical representation of the
accumulated gain versus the distance may be shown with varying
accumulated gain over specific distances or may be shown with a
non-linear curve representing the accumulated gain over a specific
distance.
Referring now to FIG. 3, a graphical representation of the amount
of loss attributed to wavelengths commonly implemented in optical
transmission systems decibels (dBs) is shown. A curve 304 is shown
in FIG. 3 representing the loss in dBs per kilometer versus the
specific wavelengths of the channels of the optical signal
transmitted over an optical network. The curve 304 is non-linear
and represents the varying amount of loss experienced by specific
channels of an optical signal as it is transmitted over the optical
network. Thus, in the C-band range 300, the amount of loss
decreases generally as the wavelength is increased from 1530 to
1560 nms. However, as the wavelength increases from 1570 to 1610
nms to the L-band range 302, the loss begins to generally increase.
Therefore when a constant power boost is applied by an In-line
amplifier to all channels of the optical signal, where each channel
is comprised of a different wavelength, the amount of gain per
channel is not uniform and gain differences are propagated through
the optical network.
Referring now to FIG. 4, an exemplary embodiment of an optical
network according to the present invention is shown. Terminals 400
and 402 are provided and connect the optical network 401 to local
optical networks (not shown). An optical signal 403 is transmitted
from the terminal 400 along an optical line 410. For illustrative
purposes only, the optical network 401 is shown in a unidirectional
manner. However, both of terminals 400 and 402 can act as
transmission and/or receiving terminals and it is expected that the
optical network 401 can be implemented as a bi-directional optical
network allowing the transmission of optical signals from either
terminal 400 or terminal 402.
After a predetermined distance, an in-line amplifier 404 is
connected to the optical line 410. The repeating predetermined
distance of the in-line amplifier 404 is determined as discussed
previously. In one embodiment of the invention, erbium-doped fiber
amplifiers are implemented as the in-line amplifiers 404, however,
a wide range of amplifiers can be implemented without detracting
from the spirit of the invention. Once the optical signal 403
travels across this predetermined distance, the In-line amplifier
404a boosts the power of each channel of the optical signal 403. In
one embodiment, at start up of the optical network 401, the in-line
amplifier 404 do not initially boost the optical signal 403 as the
optical signal 403 is transmitted along the optical network 401 as
no prior analysis of the optical signal 403 has been completed. An
optical spectrum analysis 406 located within the optical system 401
analyzes the optical signal 403 to determine the amount of energy
loss experienced by the optical signal 403 through transmission
over the optical network 401. The optical spectrum analyzer 406
then transmits this analysis data to the in-line amplifiers 440 and
commands the in-line amplifiers 404 to boost the channels of the
optical signal 403 by an amount necessary to maintain a constant
power level of the optical signal 403 across the optical network
401. This fine turning of the optical network 401 occurs when the
optical network is initially initiated, however, the optical
spectrum analyzer 406 continuously monitors the strength of the
optical signal 403 and can periodically direct a modification to
the amounts of power boosted by a specific In-line amplifier 404
during continuous use of the optical network 401 to correct this
gain tilt. In another exemplary embodiment the in-line amplifiers'
404 output power is monitored by each in-line amplifier. This
self-monitoring allows for the average power out of each in-line
amplifier 404 to remain approximately constant. As an additional
step to the self-monitoring, a measurement at the Optical spectrum
analyzer 406 determines the need for fine turning of the in-line
amplifiers 404 output power/average gain of the in-line amplifier
404.
When the optical signal 403 has its power boosted by the In-line
amplifier 404a, each channel of the optical signal 403 has the
power boosted with a consistent amount from the In-line amplifier
404a. In one disclosed embodiment there are forty channels present
in the optical signal 403. All forty channels of the optical signal
403 are boosted by the same power level from the In-line amplifier
404a. As was disclosed with FIG. 3, the power loss per channel
varies according to the wavelength of that particular channel.
Therefore, the per channel loss varies and the gain difference is
amplified and propagated by the In-line amplifier 404a as it is
transmitted from In-line amplifier 404a to In-line amplifier 404b.
Once the optical signal is received by the In-line amplifier 404b,
the optical signal 403 is again boosted on a channel-by-channel
basis by the In-line amplifier 404b. However, prior to the optical
signal 403 being transmitted to the In-line amplifier 404c, the
Optical spectrum analyzer 406a analyzes the optical signal 403 on a
channel-by-channel basis to determine the accumulated gain
difference from the expected or optimal power level for each
channel.
The optical spectrum analyzer 406 also determines the power boost
to correct gain tilt propagated by the In-line amplifiers 404 by
averaging the power level of all channels and then applying a boost
so that the average level of all channels equal the pre-determined
power level of the optical signal 403 and further determines the
accumulated gain or loss on a channel-by-channel basis of the power
level as compared to the pre-determined optimal power level. The
optical spectrum analyzer 406 transmits the power boost data to the
in-line amplifiers 404 over line 414 via an optical supervising
channel 412. Once the Optical spectrum analyzer 406a determines the
amount of gain differential for each channel of the optical signal
403, the optical signal analyzer 406a transmits these gain
differentials to the dynamic gain equalizer 408 over line 418 via
an optical supervising channel 412. The optical supervising channel
412 is a management channel and supervisory information is
transmitted via this ethernet channel at 100 megabits to 1 gigabit.
The rates of transmission of information over the optical
supervising channel 412 may vary without detracting from the spirit
and scope of the invention. In another embodiment, the management
or supervisory channel can be implemented as a public telephone
network or Internet line without detracting from the spirit of the
invention. The transmission of management or supervisory
information over the management or supervisory channel does not
require the use of fiber and is not necessarily one of the
wavelength channels of the optical line 410. In one disclosed
embodiment, the management and supervisory channel are implemented
as the optical supervising channel 412 which transmits management
supervisory information optically to the devices connected within
the optical network 401. The Optical spectrum analyzer 406a
transmits the gain differential of each channel of the optical
signal 403 to the dynamic gain equalizer 408a which is co-located
with in-line amplifier 404a. The dynamic gain equalizer 408a
receives the gain differentials transmitted by the Optical spectrum
analyzer 406a through the optical supervising channel 412 over
communication line 416. The dynamic gain equalizer 408a and the
Optical spectrum analyzer 406a are not co-located with the same
in-line amplifier 404. The dynamic gain equalizer 408a is located
at a point between Optical spectrum analyzers 406 or between the
transmitting terminal 400 and the first Optical spectrum analyzer
406a. Therefore, the dynamic gain equalizer 408a corrects the
optical signal 403 at the first In-line amplifier 404a based upon
the signal deviation present at the Optical spectrum analyzer 406a
located at In-line amplifier 404b. This methodology is repeated for
dynamic gain equalizers 408b, 408c and optical spectrum analyzers
406b and 406c. Thus, the optical signal 403 is modified at in-line
amplifier 404c by the dynamic gain equalizer 408b based upon
information transmitted to the dynamic gain equalizer 408b by the
optical spectrum analyzer 408b based upon information transmitted
to the dynamic gain equalizer 408b by the optical spectrum analyzer
406b located with in-line amplifier 404d. The dynamic gain
equalizer 408c modifies the optical signal 403 at in-line amplifier
404e based upon data transmitted from the optical spectrum analyzer
408c located with in-line amplifier 404f. Therefore, the dynamic
gain equalizer 408 correct the optical signal 403 at one subset of
in-line amplifier 404 based upon the signal deviation analyzed at
the optical spectrum analyzer 406 located at a second subset of
in-line amplifiers 404.
In one embodiment, the dynamic gain equalizer 408 and the Optical
spectrum analyzer 406 are spaced evenly throughout the optical
network 401. The dynamic gain equalizer 408 are placed
approximately in the center between two adjacent. Optical spectrum
analyzers 406. The dynamic gain equalizer 408 closed to the
transmitting terminal 400 is placed approximately in the center
between the transmitting terminal 400 and the first Optical
spectrum analyzer 406a. A wide variety of alignment schemes can be
implemented without detracting from the spirit of the invention as
long as at least one dynamic gain equalizer is not co-located with
one Optical spectrum analyzer. In one embodiment, the control of
the optical supervisory channel 412 is implemented through the use
of a token which is transmitted to each device throughout the
optical network 401. For example, when the Optical spectrum
analyzer 406a has the token, that Optical spectrum analyzer 406a
controls the gain tuning of the optical network 401 and can then
transit amplification and gain differential information to the
in-line amplifiers 404a and 404b and the dynamic gain equalizers
408a. Once this information has been transmitted and the in-line
amplifiers 404a and 404b and the Dynamic gain equalizer 408a adjust
the power appropriately, the Optical spectrum analyzer 406a sends
the token downstream of the optical spectrum network 401, allowing
another deice to transit the optical supervisory channel 412. The
Optical spectrum analyzers 406a, in one embodiment, are in
communication with the in-line amplifiers 404 and dynamic gain
equalizers 408 which are located prior to the Optical spectrum
analyzer 406. Thus, each Optical spectrum analyzer 406 maintains
communication with the dynamic gain equalizer 408 and the in-line
amplifiers 404 that are immediately preceding it and are not in
communication with any other Optical spectrum analyzers 406.
However, optional communication and control schemes are available
and can be implemented without departing from the spirit of the
invention.
Referring now to FIGS. 4, 5, 6 and 7, the optical signal
manipulation according to the present invention are shown. In FIG.
5, a graphical representation of a typical optical spectrum
analysis conducted by the Optical spectrum analyzer 406 is shown.
As expected, the gain difference varies from channel to channel and
extends to approximately 1.5 dB's for channels 7 and 8 to 4.5 dB's
for channel 26. Thus, depending upon the wavelength selected for
each channel, the gain difference varies from channel to channel.
The Optical spectrum analyzer 406 uses this information to
determine the average gain difference of the optical signal from
the pre-determined optical level and uses this information to
adjust the power boost level of the In-line amplifiers 404 and then
the Optical spectrum analyzer 406 determines on a
channel-by-channel basis the amount of gain (or loss) necessary for
each channel so that each channel's power equals the desired or
optimal power level. This information is then transmitted from the
Optical spectrum analyzer 406a to the Dynamic gain equalizer 408a
through the optical supervising channel 412. The dynamic gain
equalizer 408a the modifies the optical signal 403 at the In-line
amplifier 404a on a channel-by-channel basis to compensate for the
total amount of gain (or loss) that will be accumulated once the
signal reaches the Optical spectrum analyzer 406a at the In-line
amplifier 404b. The dynamic gain equalizer 408a adjusts the power
associated with each channel of the optical signal 403 so the
accumulated gain is zero once the optical signal 403 reaches the
In-line amplifier 404b. To accomplish this task, the dynamic gain
equalizer 408a must adjust the power associated with each channel
of the optical signal 403 below the accumulated gain of zero. This
can be seen in FIG. 6 which graphically represents the accumulated
gain versus distance over an exemplary single channel of the
optical signal 403 as it is transmitted over the optical network
401. For the exemplary channel of the optical signal 403, the
accumulated gain caused by the loss differences propagated through
the amplifiers 404 is shown as a linear gain over distance. The
amount of gain for an exemplary channel of the optical signal 403
increases 2 dB's from the terminal to the first in-line amplifier
404a. The optical signal 403 on a channel-by-channel basis is then
equalized by the dynamic gain equalizer 408a based upon the
information received from the optical spectrum analyzer 406a.
Therefore, in this example, the accumulated gain according to the
spectrum analyzer 406a for this channel was a 4 dB accumulation.
The dynamic gain equalizer 408a adjusts this channel with a 4 dB
difference so the channel possesses the proper power level when the
signal reaches the Optical spectrum analyzer 406a. When the dynamic
gain equalizer 408a modifies this channel of the optical signal 403
at the in-line amplifier 404 location, the accumulated gain of this
channel of the optical signal 403 is a negative 2 dBs. This
equalization is demonstrated by line 608. As the optical signal
403a is transmitted from the in-line amplifier 404a to the in-line
amplifier 404b, the accumulated gain continues to increase a
constant amount until it reaches point x 602 which corresponds with
the in-line amplifier 404b. As expected, the accumulated gain of
this channel at the Optical spectrum analyzer 406a, which is
co-located with the in-line amplifier 404b at a distance x 602, is
zero. The dynamic gain equalizer 408a remembers, through the use of
a memory mechanism located at the dynamic gain equalizer 408a, the
amount of gain difference per channel that the dynamic gain
equalizer 408c has received from the Optical spectrum analyzer
406a. The Optical spectrum analyzer 406a at distance x 602 again
analyzes the optical signal 403 on a channel-by-channel basis to
determine if the pre-equalized values do indeed correctly
compensate the optical signal 403. If the accumulated gain of any
channel is not zero, then the Optical spectrum analyzer 406a
transmits this information via the optical supervisory channel 412
to the dynamic gain equalizer 408a to direct the dynamic gain
equalizer 408a to adjust the amount of gain equalization to those
specific non-zero channels.
It should be noted that during start-up procedures, the integrity
of the optical signal 403 is not maintained until a series of
adjustments are performed on the optical signal 430. Therefore,
there will be a period of time upon the start-up of the
transmission of the optical network 401 in which the data
transmitted over the optical signal 403 will be invalid. However,
once the integrity of the optical signal 403 is established, then
the process described above continues to modify the amount of gain
equalization necessary over time.
A benefit of the present invention occurs because the pre-emphasis
of the gain equalization ensures that the average power over the n
spans, where n equals the number of in-line amplifiers 404 for each
channel of the dense wavelength division multiplexed system, is the
expected or optimal power level. Further, the effects of random
wavelength losses, as well as time varying polarization dependent
losses are reduced by a factor of 2 by compared to the conventional
approach. This improvement allows for twice as large component
tolerances or with the same component parameters induces only half
the penalty on the system margin. This improvement is demonstrated
by the area formed by triangles outlined by lines 600, 608 and the
distance 1/2.times. which define area 606. Area 606 plus area 604
(a negative value) are approximately zero. The system distortion
from the fixed gain deviation has been nearly eliminated. Thus,
when compared to area 202 of the prior art, a large benefit is
realized. As the accumulated gain remain closer to zero (-2 dBs to
2 dBs) without straying as far as -4 dBs to 4 dBs in the
conventional systems, the overall integrity of the optical signal
403 is increased.
FIG. 7 shows the gain difference power level in dB's of the
channels of the optical signal 403 at the point of the optical
spectral analyzer 406 in the optical network 401. Thus, the gain
differential in dB's per channel of the optical signal 403 is
constant is a constant zero. By having an average power deviation
of zero, as is obtained in the embodiment disclosed in FIG. 4
through FIG. 7, the distortions of the optical signal pulse shape
are greatly reduced.
The foregoing disclosure and description of the invention are
illustrative and explanatory thereof of various changes to the
size, shape, materials, components and order may be made without
departing from the spirit of the invention.
* * * * *