U.S. patent application number 09/906195 was filed with the patent office on 2002-02-07 for method and apparatus for extending fiber transmission distance with multiple pre-emphases in optically amplified dwdm system.
This patent application is currently assigned to SYCAMORE NETWORKS, INC.. Invention is credited to Azizoglu, Murat, Barry, Richard A., Swanson, Eric A., Zhou, Jianying.
Application Number | 20020015201 09/906195 |
Document ID | / |
Family ID | 26914293 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020015201 |
Kind Code |
A1 |
Zhou, Jianying ; et
al. |
February 7, 2002 |
Method and apparatus for extending fiber transmission distance with
multiple pre-emphases in optically amplified DWDM system
Abstract
A Dense Wavelength Division Multiplexed (DWDM) optical
transmission system that compensates for unequal channel
performance over an extended fiber transmission distance. The DWDM
optical transmission system includes optical transmitters at a
transmitter end of a transmission path for transmitting respective
channels of information at different wavelengths, a first
pre-emphasis device for performing a first pre-emphasis technique
on the respective channels, an optical multiplexor for combining
the respective channels into a multi-wavelength optical signal for
transmission on a single transmission fiber, at least one optical
amplifier for amplifying the optical signal along the path, at
least one second pre-emphasis device disposed along the path for
performing a second pre-emphasis technique on the respective
channels, an optical de-multiplexor for separating the optical
signal into its component channels, and optical receivers at a
receiver end of the path for receiving and detecting the
information carried by the respective channels. The first and
second pre-emphasis techniques performed by the respective first
and second pre-emphasis devices compensate for unequal channel
performance along the transmission path over a desired fiber
transmission distance.
Inventors: |
Zhou, Jianying; (Acton,
MA) ; Barry, Richard A.; (Brookline, MA) ;
Azizoglu, Murat; (North Billerica, MA) ; Swanson,
Eric A.; (Acton, MA) |
Correspondence
Address: |
WEINGARTEN, SCHURGIN, GAGNEBIN & LEBOVICI LLP
TEN POST OFFICE SQUARE
BOSTON
MA
02109
US
|
Assignee: |
SYCAMORE NETWORKS, INC.
|
Family ID: |
26914293 |
Appl. No.: |
09/906195 |
Filed: |
July 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60219830 |
Jul 21, 2000 |
|
|
|
60261564 |
Jan 12, 2001 |
|
|
|
Current U.S.
Class: |
398/79 ;
398/14 |
Current CPC
Class: |
H04B 10/25073 20130101;
H04J 14/0221 20130101; H04B 2210/258 20130101 |
Class at
Publication: |
359/124 ;
359/110 |
International
Class: |
H04B 010/08; H04J
014/02 |
Claims
What is claimed is:
1. A method of compensating for unequal channel performance in a
wavelength division multiplexed optical transmission system,
comprising the steps of: providing a transmission path for carrying
a multi-wavelength optical signal, the transmission path including
a transmitter end and a receiver end, the multi-wavelength optical
signal comprising a plurality of channels of information;
transmitting the plurality of channels by respective optical
transmitters at the transmitter end; performing a first
pre-emphasis technique on the respective channels by a first
pre-emphasis device coupled along the transmission path between the
transmitter end and the receiver end; performing at least one
second pre-emphasis technique on the respective channels by at
least one second pre-emphasis device coupled along the transmission
path between the first pre-emphasis device and the receiver end;
and receiving and detecting the information carried by the
plurality of channels by respective optical receivers at the
receiver end, wherein the first pre-emphasis technique performed in
the first performing step includes compensating for unequal channel
performance along the transmission path between the transmitter end
and the first pre-emphasis device, and wherein the second
pre-emphasis technique performed in the second performing step
includes compensating for unequal channel performance along the
transmission path between the second pre-emphasis device and the
receiver end.
2. The method of claim 1 wherein the first pre-emphasis technique
performed in the first performing step further includes receiving a
first plurality of OSNR values (OSNR1_M) of the respective channels
measured at the second pre-emphasis device, and adjusting
pre-emphasis attenuation or gain of the respective channels based
on the first plurality of measured OSNR values (OSNR1_M) to achieve
a first plurality of designated OSNR values (OSNR1_D) of the
respective channels at the second pre-emphasis device.
3. The method of claim 1 wherein the second pre-emphasis technique
performed in the second performing step further includes receiving
a second plurality of OSNR values (OSNR2_M) of the respective
channels measured at the receiver end, and adjusting pre-emphasis
attenuation or gain of the respective channels based on the second
plurality of measured OSNR values (OSNR2_M) to achieve a second
plurality of designated OSNR values (OSNR2_D) of the respective
channels at the receiver end.
4. The method of claim 1 further including the step of amplifying
the multi-wavelength optical signal by at least one optical
amplifier coupled along the transmission path between the first
pre-emphasis device and the second pre-emphasis device.
5. The method of claim 1 further including the step of amplifying
the multi-wavelength optical signal by at least one optical
amplifier coupled along the transmission path between the second
pre-emphasis device and the receiver end.
6. The method of claim 2 further including the step of measuring
the first plurality of OSNR values of the respective channels by a
first measurement device at the second pre-emphasis device.
7. The method of claim 3 further including the step of measuring
the second plurality of OSNR values (OSNR2_M) of the respective
channels by a second measurement device at the receiver end.
8. The method of claim 4 wherein the amplifying step includes
operating the at least one optical amplifier under a constant gain
control mode.
9. The method of claim 5 wherein the amplifying step includes
operating the at least one optical amplifier under a constant gain
control mode.
10. The method of claim 7 wherein the adjusting step of the second
pre-emphasis technique includes in the event the OSNR2_M value of
one of the respective channels in not within a range OSNR2_D+TOL,
adjusting the pre-emphasis attenuation or gain of the respective
channel at the second pre-emphasis device by an amount OSNR2_ADJ
defined by OSNR2_ADJ=OSNR2_M-OSNR2_D, and repeating the measuring
and adjusting steps of the second pre-emphasis technique until the
OSNR2_M value of the respective channel is within the range
OSNR2_D+TOL.
11. The method of claim 10 wherein the adjusting step of the second
pre-emphasis technique includes adjusting the pre-emphasis
attenuation or gain of the respective channel using at least one
dispersion component or at least one channel power control
element.
12. The method of claim 10 wherein the measuring step of the second
pre-emphasis technique includes measuring out-of-band ASE noise of
the respective channel, and using the measured out-of-band ASE
noise to estimate the OSNR2_M value of the respective channel.
13. The method of claim 10 wherein the measuring step of the second
pre-emphasis technique includes turning-off optical power of the
respective channel, measuring in-band ASE noise of the respective
channel, and using the measured in-band ASE noise to estimate the
OSNR2_M value of the respective channel.
14. The method of claim 10 wherein the measuring step of the second
pre-emphasis technique includes monitoring respective channel
powers at each amplifier input by applying a dither signal to the
respective channel, detecting the applied dither signal at inputs
of respective optical amplifiers coupled along the transmission
path between the second pre-emphasis device and the receiver end,
using the detected dither signals to estimate optical power of the
respective channel, and using the estimated optical power to
estimate the OSNR2_M value of the respective channel.
15. The method of claim 1 wherein the steps of the first and second
pre-emphasis techniques are performed under control of a network
management system.
16. A wavelength division multiplexed optical transmission system,
comprising: a transmission path configured to carry a
multi-wavelength optical signal, the transmission path including a
transmitter end and a receiver end, the multi-wavelength optical
signal comprising a plurality of channels of information; a
plurality of optical transmitters at the transmitter end configured
to transmit the respective channels; a first pre-emphasis device
coupled along the transmission path between the transmitter end and
the receiver end and configured to perform a first pre-emphasis
technique on the respective channels; at least one second
pre-emphasis device coupled along the transmission path between the
first pre-emphasis device and the receiver end and configured to
perform a second pre-emphasis technique on the respective channels;
and a plurality of optical receivers at the receiver end configured
to receive and detect the information carried by the respective
channels, wherein the first and second pre-emphasis techniques are
performed to compensate for unequal channel performance along the
transmission path.
17. The wavelength division multiplexed optical transmission system
of claim 16 wherein the first pre-emphasis device performing the
first pre-emphasis technique is configured to receive a first
plurality of OSNR values of the respective channels measured at the
second pre-emphasis device, and to adjust pre-emphasis attenuation
or gain of the respective channels based on the first plurality of
measured OSNR values to achieve a first plurality of designated
OSNR values of the respective channels at the second pre-emphasis
device.
18. The wavelength division multiplexed optical transmission system
of claim 16 wherein the second pre-emphasis device performing the
second pre-emphasis technique is configured to receive a second
plurality of OSNR values of the respective channels measured at the
receiver end, and to adjust pre-emphasis attenuation or gain of the
respective channels based on the second plurality of measured OSNR
values to achieve a second plurality of designated OSNR values of
the respective channels at the receiver end.
19. The wavelength division multiplexed optical transmission system
of claim 17 further including a first measurement device configured
to measure the first plurality of OSNR values of the respective
channels at the second pre-emphasis device.
20. The wavelength division multiplexed optical transmission system
of claim 18 further including a second measurement device
configured to measure the second plurality of OSNR values of the
respective channels at the receiver end.
21. The wavelength division multiplexed optical transmission system
of claim 16 further including at least one optical amplifier
coupled along the transmission path between the first pre-emphasis
device and the second pre-emphasis device and configured to amplify
the multi-wavelength optical signal.
22. The wavelength division multiplexed optical transmission system
of claim 16 further including at least one optical amplifier
coupled along the transmission path between the second pre-emphasis
device and the receiver end and configured to amplify the
multi-wavelength optical signal.
23. The wavelength division multiplexed optical transmission system
of claim 16 wherein the first pre-emphasis device comprises at
least one variable optical attenuator.
24. The wavelength division multiplexed optical transmission system
of claim 16 wherein the second pre-emphasis device comprises an
optical equalization node.
25. The wavelength division multiplexed optical transmission system
of claim 16 wherein the second pre-emphasis device comprises a
dynamical gain flatness filter.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority of U.S. Provisional patent
application Ser. No. 60/219,830 filed Jul. 21, 2000 entitled METHOD
AND APPARATUS FOR EXTENDING FIBER TRANSMISSION DISTANCE WITH
MULTIPLE PRE-EMPHASES IN OPTICALLY AMPLIFIED DWDM SYSTEM.
[0002] This application claims priority of U.S. Provisional patent
application Ser. No. 60/261,564 filed Jan. 12, 2001 entitled A
SYSTEM AND METHOD OF POWER EQUALIZATION AND DISPERSION COMPENSATION
IN FIBER OPTIC NETWORKS.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
N/A
BACKGROUND OF THE INVENTION
[0003] The present invention relates generally to the field of
optical transmission systems, and more specifically to a dense
wavelength division multiplexed optical transmission system
providing enhanced control of end-to-end channel performance to
extend fiber transmission distance.
[0004] Dense Wavelength Division Multiplexed (DWDM) optical
transmission systems have been widely deployed in optical networks
to increase network speed and capacity. A conventional DWDM optical
transmission system comprises a plurality of optical transmitters
configured to transmit respective channels of information at
different wavelengths, an optical multiplexor configured to combine
the respective channels into a multi-wavelength optical signal for
transmission on a single transmission fiber, a plurality of
serially connected optical amplifiers configured as repeaters to
amplify the multi-wavelength optical signal at intervals along a
transmission path, an optical de-multiplexor configured to separate
the multi-wavelength optical signal into its component channels,
and a plurality of optical receivers configured to receive and
detect the information carried by the respective channels.
[0005] One drawback of the conventional DWDM optical transmission
system is that the optical amplifiers disposed along the
transmission path typically have wavelength dependent gain and
noise profiles, which can cause unequal channel performance. For
example, the performance of the channels in the conventional DWDM
optical transmission system may be characterized by associated
Optical Signal-to-Noise Ratio (OSNR) values. Further, because of
the wavelength dependent gain and noise profiles of the optical
amplifiers, the OSNR values associated with the channels may not be
equal to one another at the receiver end of the transmission path
even though the channels may have the same optical power levels at
the transmitter end of the path.
[0006] One approach to compensating for such unequal channel
performance in the conventional DWDM optical transmission system is
to perform a pre-emphasis technique at the transmitter end of the
transmission path. For example, the OSNR values of the respective
channels may be monitored at the receiver end of the transmission
path by a measurement device such as an optical spectrum analyzer,
and the pre-emphasis attenuation or gain of the respective channels
may be adjusted by varying the optical power levels at the
transmitter end of the path based on the measured OSNR values to
achieve designated OSNR values at the receiver end of the path.
[0007] However, performing such pre-emphasis techniques at the
transmitter end of the transmission path to compensate for unequal
channel performance at the receiver end of the path has its own
drawbacks. For example, pre-emphasis techniques that raise optical
power levels of selected channels at the transmitter end of the
transmission path may increase power levels at the optical
amplifier outputs, which may in turn increase the total power
requirements of the optical transmission system. Having high power
levels in some channels may also introduce transmission impairment
due to fiber non-linearity, especially for channel bit rates of 10
Gbit/s or more. Therefore, such pre-emphasis techniques are
typically only used to compensate for unequal channel performance
over a limited fiber transmission distance.
[0008] Another approach to equalizing channel performance in the
conventional DWDM optical transmission system, which may be used in
conjunction with the above-mentioned pre-emphasis technique, is to
terminate and regenerate the multi-wavelength optical signal on the
transmission path. However, this approach also has drawbacks in
that such optical signal termination/regeneration typically
requires optical-to-electrical and electrical-to-optical
conversions, which are very costly and usually must be performed at
regular intervals along the transmission path.
[0009] It would therefore be desirable to have a DWDM optical
transmission system that compensates for unequal channel
performance. Such a DWDM optical transmission system would be
capable of compensating for unequal channel performance over an
extended fiber transmission distance, thereby reducing the need for
terminating and regenerating multi-wavelength optical signals on an
optical transmission path.
BRIEF SUMMARY OF THE INVENTION
[0010] In accordance with the present invention, a Dense Wavelength
Division Multiplexed (DWDM) optical transmission system is provided
that compensates for unequal channel performance over an extended
fiber transmission distance. The presently disclosed invention
achieves such benefits by way of a multiple pre-emphases technique
that provides enhanced control of the channel performance from a
transmitter end to a receiver end of an optical transmission
path.
[0011] In one embodiment, the DWDM optical transmission system
includes a plurality of optical transmitters at a transmitter end
of a transmission path configured to transmit respective channels
of information at different wavelengths, a first pre-emphasis
device configured to perform a first pre-emphasis technique on the
respective channels, an optical multiplexor configured to combine
the respective channels into a multi-wavelength optical signal for
transmission on a single transmission fiber, at least one optical
amplifier configured to amplify the multi-wavelength optical signal
along the path, at least one second pre-emphasis device disposed
along the path and configured to perform a second pre-emphasis
technique on the respective channels, an optical de-multiplexor
configured to separate the multi-wavelength optical signal into its
component channels, and a plurality of optical receivers at a
receiver end of the path configured to receive and detect the
information carried by the respective channels.
[0012] The first pre-emphasis technique performed by the first
pre-emphasis device includes measuring a first plurality of Optical
Signal-to-Noise Ratio (OSNR) values of the respective channels at
an output of the second pre-emphasis device, and adjusting
pre-emphasis attenuation and gain of the respective channels based
on the first measured OSNR values to achieve a first plurality of
designated OSNR values of the respective channels at the second
pre-emphasis device output. In the first pre-emphasis technique,
the OSNR values of the respective channels are measured by a
measurement device such as an optical spectrum analyzer.
[0013] The second pre-emphasis technique performed by the second
pre-emphasis device includes measuring a second plurality of OSNR
values of the respective channels at the receiver end of the
transmission path, and adjusting pre-emphasis attenuation and gain
of the respective channels based on the second measured OSNR values
to achieve a second plurality of designated OSNR values of the
respective channels at the receiver end of the path.
[0014] In the second pre-emphasis technique, the OSNR values of the
respective channels are measured by one of a plurality of possible
OSNR measurement techniques. Each OSNR measurement technique takes
into account Amplified Spontaneous Emission (ASE) noise at the
output of the second pre-emphasis device, which may have been
modified by dispersion components included in the second
pre-emphasis device. A first OSNR measurement technique includes
measuring out-of-band ASE noise levels of the respective channels
at the second pre-emphasis device output, and estimating actual ASE
noise levels of the respective channels using the measured
out-of-band ASE noise. A second OSNR measurement technique includes
measuring in-band ASE noise levels of the respective channels at
the second pre-emphasis device output by alternately turning
optical signal power "on" and "off". A third OSNR measurement
technique includes measuring optical signal power levels of the
respective channels at the second pre-emphasis device output by
dithering optical signal carriers, and estimating OSNR values at
the respective channels using the measured optical signal
power.
[0015] By employing a plurality of pre-emphasis devices to perform
a multiple pre-emphases technique on the respective channels of
information, the DWDM optical transmission system compensates for
unequal channel performance while reducing the need for terminating
and regenerating the multi-wavelength optical signal on the optical
transmission path.
[0016] Other features, functions, and aspects of the invention will
be evident from the Detailed Description of the Invention that
follows.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0017] The invention will be more fully understood with reference
to the following Detailed Description of the Invention in
conjunction with the drawings of which:
[0018] FIG. 1 is a block diagram depicting a conventional DWDM
optical transmission system capable of performing a single
pre-emphasis technique;
[0019] FIG. 2 is a diagram depicting exemplary optical waveforms at
transmitter and receiver ends of the conventional DWDM optical
transmission system of FIG. 1, the exemplary waveforms illustrating
an OSNR estimation technique employing extrapolation;
[0020] FIG. 3 is a block diagram depicting a DWDM optical
transmission system capable of performing a multiple pre-emphases
technique according to the present invention;
[0021] FIG. 4 is a diagram depicting exemplary optical waveforms at
a transmitter end, a pre-emphasis device, and a receiver end of the
DWDM optical transmission system of FIG. 3, the exemplary waveforms
illustrating an OSNR estimation technique employing out-of-band ASE
noise level measurements;
[0022] FIG. 5 is a diagram depicting exemplary optical waveforms at
a receiver end of the DWDM optical transmission system of FIG. 3,
the exemplary waveforms illustrating an OSNR measurement technique
that includes alternately turning channel power "on" and "off";
and
[0023] FIG. 6 is a block diagram depicting an alternative
embodiment of the DWDM optical transmission system of FIG. 3
capable of performing an automatic multiple pre-emphases
technique.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The entire disclosure of U.S. Provisional patent application
Ser. No. 60/219,830 filed Jul. 21, 2000 is incorporated herein by
reference.
[0025] The entire disclosure of U.S. Provisional patent application
Ser. No. 60/261,564 filed Jan. 12, 2001 is incorporated herein by
reference.
[0026] A Dense Wavelength Division Multiplexed (DWDM) optical
transmission system is disclosed that is capable of compensating
for unequal channel performance over an extended fiber transmission
distance. The presently disclosed DWDM optical transmission system
achieves such compensation by performing a multiple pre-emphases
technique to provide enhanced control of end-to-end channel
performance.
[0027] FIG. 1 depicts a block diagram of a conventional DWDM
optical transmission system 100 that includes a plurality of
optical amplifiers 104.1-104.4 serially coupled along a
transmission path between a transmitter end 102 and a receiver end
106.
[0028] Specifically, the transmitter end 102 includes a plurality
of optical transmitters Tx1-TxN configured to transmit respective
channels of information, in which the respective channels are at
different wavelengths .lambda..sub.1-.lambda..sub.N. The
transmitter end 102 also includes a plurality of Variable Optical
Attenuators (VOAs) 108.1-108.N configured to receive the respective
channels and reduce optical power gain across at least a portion of
the channels. For example, the optical transmitters Tx1-TxN may
have produced non-uniform optical power gains across the channels
at wavelengths near an edge of the available optical transmission
spectrum. Accordingly, the VOAs 108.1-108.N receive the respective
channels and reduce these non-uniform optical power gains to a
uniform level.
[0029] The transmitter end 102 further includes an optical
multiplexor 110 configured to receive the respective channels
having uniform optical power gains and combine the respective
channels into a multi-wavelength optical signal for transmission on
a single transmission fiber. The optical amplifiers 104.1-104.4 are
configured as repeaters to amplify the multi-wavelength optical
signal at intervals along the transmission path.
[0030] The receiver end 106 includes an optical demultiplexor 112
configured to separate the multi-wavelength optical signal into its
component channels, and a plurality of optical receivers Rx1-RxN
configured to receive and detect the information carried by the
respective channels.
[0031] Those of ordinary skill in the art will appreciate that
channel performance in optically amplified DWDM transmission
systems can be characterized by Optical Signal-to-Noise Ratio
(OSNR) values associated with the respective channels. It is noted
that OSNR is defined herein as the difference between the optical
signal power and the power of corresponding Amplified Spontaneous
Emission (ASE) noise within a predetermined bandwidth, e.g., 0.1
nm, of the optical signal.
[0032] The optical amplifiers 104.1-104.4 disposed along the
transmission path typically have wavelength dependent gain and
noise profiles that can cause unequal channel performance at the
receiver end 106. In order to compensate for such unequal
performance across the channels, the conventional DWDM optical
transmission system 100 typically performs a single pre-emphasis
technique at the transmission end 102 of the transmission path.
[0033] For example, the single pre-emphasis technique may include
estimating OSNR values of the respective channels at the receiver
end 106, and adjusting the pre-emphasis attenuation or gain of the
respective channels by way of the VOAs 108.1-108.N based on the
estimated OSNR values to achieve designated OSNR values at the
receiver end 106.
[0034] FIG. 2 depicts a diagram including exemplary optical
waveforms at outputs of the optical transmitters Tx1-Tx4 and at the
inputs of the optical receivers Rx1-Rx4. The exemplary waveforms of
FIG. 2 illustrate an OSNR estimation technique employing
extrapolation, which may be used with the above-described
conventional single pre-emphasis technique.
[0035] Specifically, the diagram of FIG. 2 shows the exemplary
waveforms at the Tx1-Tx4 outputs having non-uniform optical power
gains. For example, the optical transmitters Tx1-Tx4 may produce
the non-uniform power gains across the channels 1-4. Further, the
VOAs 108.1-108.4 may subsequently receive the channels 1-4 and
reduce the non-uniform power gains to a uniform level. The diagram
also shows the exemplary waveforms at the Rx1-Rx4 inputs. Because
the optical amplifiers 104.1-104.4 may have wavelength dependent
gain and noise profiles, the diagram of FIG. 2 shows a non-flat ASE
noise floor across the channels 1-4 at the Rx1-Rx4 inputs.
[0036] Even though the ASE noise floor across the channels 1-4 at
the receiver end 106 is non-flat, particularly across channel 2,
OSNR values of the respective channels 1-4 can be estimated by way
of extrapolation. These estimated OSNR values may then be used in
the conventional single pre-emphasis technique to compensate for
the unequal channel performance.
[0037] For example, to estimate the OSNR value of channel 2, the
optical signal power level of channel 2 "Power_2" is measured.
Next, the ASE noise level at a left shoulder portion "ASE_L_2" and
at a right shoulder portion "ASE_R_2" of the optical signal of
channel 2 are measured. The OSNR value of channel 2 "OSNR_2" is
then estimated using the following equation:
OSNR_2=Power_2-0.5 * (ASE_L_2+ASE_R_2), (1)
[0038] in which the expression "0.5 * (ASE_L_2+ASE_R_2)" represents
the extrapolated in-band ASE noise level of channel 2. The OSNR
values of channels 1, 3, and 4 may be estimated in a similar
manner.
[0039] It is noted that the single pre-emphasis technique performed
by the conventional DWDM optical transmission system 100 is
typically only employed to compensate for unequal channel
performance over a limited fiber transmission distance.
[0040] FIG. 3 depicts a block diagram of a DWDM optical
transmission system 300 capable of performing a multiple
pre-emphases technique, in accordance with the present invention.
The multiple pre-emphases technique performed by the DWDM optical
transmission system 300 may be employed to compensate for unequal
channel performance over an extended fiber transmission
distance.
[0041] In the illustrated embodiment, the DWDM optical transmission
system 300 includes a first plurality of optical amplifiers
304.1-304.3 serially coupled along a transmission path between a
transmitter end 302 and a pre-emphasis device 314, and a second
plurality of optical amplifiers 304.4-304.6 serially coupled along
the path between the pre-emphasis device 314 and a receiver end
306. It is noted that the DWDM optical transmission system 300
performs the multiple pre-emphases technique by performing a first
pre-emphasis technique at the transmitter end 302 and a second
pre-emphasis technique at the pre-emphasis device 314. Further,
although the DWDM optical transmission system 300 includes the
single pre-emphasis device 314, it should be understood that the
system 300 may alternatively include a plurality of such
pre-emphasis devices to compensate for unequal channel performance
over a desired fiber transmission distance.
[0042] Specifically, the transmitter end 302 includes a plurality
of optical transmitters Tx1-TxN configured to transmit respective
channels 1-N of information at different wavelengths
.lambda..sub.1-.lambda..sub.N- , and a plurality of VOAs
308.1-308.N configured to receive the respective channels 1-N and
reduce non-uniform optical power gains across the channels 1-N to a
uniform level. The transmitter end 302 further includes an optical
multiplexor 310 configured to receive the respective channels 1-N
and combine the channels 1-N into a multi-wavelength optical signal
for transmission on a single transmission fiber. The optical
amplifiers 304.1-304.6 are configured as repeaters to amplify the
multi-wavelength optical signal at intervals along the transmission
path. For example, the optical amplifiers 304.1-304.6 may comprise
respective Erbium Doped Fiber Amplifiers (EDFAs), and the single
transmission fiber may comprise a single mode optical transmission
fiber.
[0043] The receiver end 306 includes an optical demultiplexor 312
configured to separate the multi-wavelength optical signal into its
component channels 1N, and a plurality of optical receivers Rx1-RxN
configured to receive and detect the information carried by the
channels 1-N.
[0044] As described above, the DWDM optical transmission system 300
includes the pre-emphasis device 314. In the illustrated
embodiment, the pre-emphasis device comprises an Optical
Equalization Node (OEQN) such as the OEQN apparatus described in
the above-referenced U.S. Provisional patent application Ser. No.
60/261,564 filed Jan. 12, 2001. However, it should be understood
that the pre-emphasis device 314 may alternatively comprise a
Dynamical Gain Flatness Filter (DGFF) or any other suitable device
capable of performing the multiple pre-emphases technique disclosed
herein.
[0045] Specifically, the pre-emphasis device 314 includes an
optical de-multiplexor 316 configured to separate the
multi-wavelength optical signal provided by the optical amplifier
304.3 into its component channels 1-N, a plurality of VOAs
configured to reduce non-uniform optical power gains across the
channels 1-N, and an optical multiplexor 320 configured to combine
the channels 1-N back into a multi-wavelength optical signal for
further transmission on the single transmission fiber.
[0046] It is noted that the optical amplifiers 304.1-304.6 may have
wavelength dependent gain and noise profiles that can cause unequal
performance across the channels 1-N. The above-mentioned multiple
pre-emphases technique comprising the first and second pre-emphasis
techniques performed at the transmitter end 302 and by the
pre-emphasis device 314, respectively, may be employed to
compensate for such unequal channel performance over an extended
fiber transmission distance.
[0047] Specifically, the first pre-emphasis technique is employed
to compensate for unequal channel performance occurring between the
transmitter end 302 and the pre-emphasis device 314. As a result,
each of the component channels 1-N at an output of the pre-emphasis
device 314 has a designated OSNR value. Further, the second
pre-emphasis technique is employed to compensate for unequal
channel performance occurring between the pre-emphasis device 314
and the receiver end 306. As a result, each of the component
channels 1-N at outputs of the receiver end 306 has a designated
OSNR value.
[0048] In the illustrated embodiment, both designated and measured
output power levels of the optical amplifiers 304.1-304.6 are
employed to estimate the designated OSNR values for the optical
transmission system. Further, during the estimation of the
designated OSNR values, the optical amplifiers 304.1-304.6 are
preferably operated under a constant gain control mode to simplify
system provisioning and channel turn-up procedures.
[0049] The designated OSNR value "OSNR_D" of each of the channels
1-N is generally pre-determined in the design and is dependent on
the channel power and the total power requirements. In the event
the OSNR_D is not predetermined, it can be estimated using the
following equation:
OSNR_D=OSNR_M+min[Power_D_OA(I)--Power_M_OA(I)], (2)
[0050] in which "OSNR_M" is the measured OSNR value of the channel
in a particular pre-emphasis section of the transmission path,
"Power D_OA(I)" is the designated output power level of the
I.sup.th optical amplifier in that pre-emphasis section, and
"Power_M_OA(I)" is the measured output power level of the I.sup.th
optical amplifier in that pre-emphasis section. It is noted that in
the illustrated embodiment, the index "I" ranges from 1 to 3.
Accordingly, the expression "min[Power_D_OA(I)--Power- _M_OA(I)]"
represents the minimum of the differences between the designated
output power level and the measured output power level of the
respective optical amplifiers in that pre-emphasis section.
[0051] FIG. 4 depicts a diagram including exemplary waveforms at
outputs of the optical transmitters Tx1-Tx4, at inputs and outputs
of the pre-emphasis device 314, and at inputs of the optical
receivers Rx1-Rx4 (see FIG. 3). The exemplary waveforms illustrate
an OSNR estimation technique employing out-of-band ASE noise level
measurements, which may be used in conjunction with the
above-mentioned first and second pre-emphasis techniques.
[0052] Specifically, the diagram of FIG. 4 shows the exemplary
waveforms of component channels 1-4 at the Tx1-Tx4 outputs having
non-uniform optical power gains, and at the device 314 input.
Because the optical amplifiers 304.1-304.3 may have wavelength
dependent gain and noise profiles, the diagram of FIG. 4 shows a
non-flat ASE noise floor across the component channels 1-4 at the
device 314 input.
[0053] The first pre-emphasis technique is performed at the
transmitter end 302 to estimate designated OSNR values at the
pre-emphasis device 314. As described above, the designated OSNR
value "OSNR1_D" of each of the channels 1-4 is estimated using
equation (2). Accordingly, the output power levels "Power1_M_OA(I)"
of the respective optical amplifiers 304.1-304.3 are measured by a
measurement device such as an optical spectrum analyzer, while the
output power levels "Power1_D_OA(I)" of the respective optical
amplifiers 304.1-304.3 are designated to reduce the total power
requirements of this first pre-emphasis section.
[0054] Further, the measured OSNR value "OSNR1_M" of each of the
channels 1-4 is estimated by way of extrapolation. For example, to
estimate the OSNR value of channel 2, the optical signal power
level of channel 2 "Power1_2" for this first pre-emphasis section
is measured. Next, out-of-band ASE noise levels at a left shoulder
portion "ASE1_L" and at a right shoulder portion "ASE1_R" of the
optical signal of channel 2 are measured. The measured OSNR value
of channel 2 "OSNR1_M_2" for this first pre-emphasis section is
then estimated using equation (1), in which the expression "0.5 *
(ASE1_L+ASE1_R)" represents the extrapolated in-band ASE1 noise
level of channel 2. It is noted that the measured OSNR values of
channels 1, 3, and 4 at the pre-emphasis device 314 may be
estimated in a similar manner.
[0055] Accordingly, this first pre-emphasis technique is completed
by adjusting the pre-emphasis attenuation or gain of the respective
channels 1-4 by way of the VOAs 308.1-308.4 based on the measured
OSNR1_M values of channels 1-4 to achieve the designated OSNR1_D
values of channels 1-4 at the pre-emphasis device 314.
[0056] Next, the second pre-emphasis technique is performed at the
pre-emphasis device 314 to estimate designated OSNR values at the
receiver end 306. Specifically, the diagram of FIG. 4 shows the
exemplary waveforms of component channels 1-4 at the device 314
output, and at the Rx1-Rx4 inputs. Because the out-of-band ASE
noise "ASE1_L" and "ASE1_R" may be modified by dispersion
components such as the optical de-multiplexor 316 and the optical
multiplexor 320 included in the pre-emphasis device 314,
conventional techniques generally cannot be employed to measure the
OSNR values of the channels 1-4 at the receiver end 306.
[0057] Again, the designated OSNR value "OSNR2_D" of each of the
channels 1-4 is estimated using equation (2). Accordingly, output
power levels "Power2_N_OA(I)" of the respective optical amplifiers
304.4-304.6 are measured by an optical spectrum analyzer, while
output power levels "Power2_D_OA(I)" of the respective optical
amplifiers 304.4-304.6 are designated to reduce the total power
requirements of this second pre-emphasis section.
[0058] Next, the measured OSNR value "OSNR2_M" of each of the
channels 1-4 at the pre-emphasis device 314 is estimated. For
example, to estimate the OSNR value of channel 2, out-of-band ASE
reference noise levels at a left shoulder portion "ASE2_REF_L" and
at a right shoulder portion "ASE2_REF_R" of the optical signal of
channel 2 at the device 314 output are measured.
[0059] Specifically, the ASE2_REF_L and ASE2_REF_R noise levels may
be located by searching for minimum ASE noise levels within
predetermined wavelength ranges of the optical signal. For example,
for 100 GHz channel spacing and a central channel wavelength of
.lambda..sub.c, a suitable predetermined wavelength range for
ASE2_REF_L is
[(.lambda..sub.c-0.5)nm, (.lambda..sub.c-0.3)nm], (3)
[0060] and a suitable predetermined wavelength range for ASE2_REF_R
is
[(.lambda..sub.c+0.3)nm, (.lambda..sub.c+0.5)nm]. (4)
[0061] Next, out-of-band ASE noise levels at a left shoulder
portion "ASE2_MEAS_L" and at a right shoulder portion "ASE2_MEAS_R"
of the optical signal of channel 2 at the Rx1-Rx4 inputs are
measured. Corresponding actual out-of-band ASE noise levels
"ASE2_ACT_L" and "ASE2_ACT_R" for this second pre-emphasis section
are then estimated using the following equations:
ASE2_ACT_L_LINEAR=ASE2_MEAS_L_LINEAR-ASE2_REF_L_LINEAR (5)
ASE2_ACT_R_LINEAR=ASE2_MEAS_R_LINEAR-ASE2_REF_R_LINEAR. (6)
[0062] It is noted that names of parameters in linear format
include the "LINEAR" suffix, as shown in equations (5) and (6) (see
also equations (7)-(11) below). Parameters that are not in linear
format are in logarithmic format.
[0063] Next, an in-band ASE2 noise level of channel 2 for this
second pre-emphasis section is calculated using the following
equation:
ASE2_ACT_LINEAR=10[0.5 * (ASE2_L_2+ASE2_R_2)/10]. (7)
[0064] An actual in-band ASE1 noise of channel 2 for the first
pre-emphasis section is then estimated based on the measured
optical signal power level (Power1_2) and the measured OSNR value
(OSNR1_M_2) using the following equation:
ASE1_ACT=Power1_2-OSNR1_M_2. (8)
[0065] Next, equation (8) is converted into linear form as
follows,
ASE1_ACT_LINEAR=10[0.5 * (ASE1_L_2+ASE1_R_2)/10]. (9)
[0066] The total ASE noise level at the receiver end 306 of the
DWDM optical transmission system 300 (see FIG. 3) for channel 2 is
then calculated using the following equation:
ASE_LINEAR=ASE1_LINEAR+ASE2_LINEAR, (10)
[0067] in which "ASE1" and "ASE2" are determined using equations
(7) and (9), as indicated above.
[0068] Next, the optical signal power level of channel 2 "Power2_2"
for this second pre-emphasis section is measured. The total
measured OSNR value at the receiver end 306 for channel 2 is then
estimated using the following equation:
OSNR(TOTAL)_M_2=Power2_2-10*LOG(ASE_LINEAR). (11a)
[0069] Alternatively, this value can be estimated through OSNR
measurement using the following equation:
OSNR(TOTAL)_M_2=-10*LOG(1/OSNR1_ACT_2_LINEAR+1/OSNR2_ACT_2_LINEAR),
(11b)
[0070] in which OSNR1_ACT_2 and OSNR2_ACT_2 are the OSNR values at
channel 2 from the first pre-emphasis section and from the second
pre-emphasis section, respectively. It is noted that the total
measured OSNR values of channels 1, 3, and 4 at the receiver end
306 may be estimated in a similar manner.
[0071] Accordingly, this second pre-emphasis technique is completed
by adjusting the pre-emphasis attenuation or gain of the respective
channels 1-4 by way of the VOAs 318.1-318.4 based on the measured
OSNR(TOTAL)_M values of channels 1-4 to achieve the designated
OSNR2_D values of channels 1-4 at the receiver end 306.
[0072] FIG. 5 depicts a diagram including exemplary waveforms at
the inputs of the optical receivers Rx1-Rx4 (see FIG. 3). The
exemplary waveforms illustrate an OSNR measurement technique
including alternately turning channel power "on" and "off", which
may be used in conjunction with the above-described multiple
pre-emphases technique comprising the first and second pre-emphasis
techniques. Specifically, the diagram of FIG. 5 shows the exemplary
waveforms of component channels 1-4 at the Rx1-Rx4 inputs with
optical power for all channels turned-on, and with the optical
power for channel 2 turned-off.
[0073] This OSNR measurement technique is based on the concept that
once the total input power provided to each of the optical
amplifiers 304.1-304.6 is set within a suitable range to maintain a
desired gain, the ASE noise levels of the respective optical
amplifiers are dependent only on those desired gains and are
independent of both the individual channel power and the total
channel power.
[0074] Accordingly, in order to measure the total OSNR value at the
receiver end 306, the optical signal power levels of the channels
1-4 at the receiver end 306 are measured, and then the power levels
of corresponding ASE noise of the channels 1-4 are measured after
turning the optical signal power for the respective channels "off".
For example, the diagram of FIG. 5 shows the optical spectrum of
channel 2 with the optical signal power "Power_2" turned-on, and
with Power_2 turned-off to reveal the corresponding ASE noise
"Power_ASE_2". The OSNR(TOTAL)_M value for channel 2 can then be
calculated using the following equation:
OSNR(TOTAL)_M_2=Power_2-Power_ASE_2. (12)
[0075] It is noted that this OSNR measurement can be performed
using an optical spectrum analyzer. Further, this OSNR measurement
technique may be employed with optical transmission systems that
comprise Optical Add-Drop Nodes (OADNs), which generally complicate
out-of-band ASE noise level measurements.
[0076] It is also noted that such OSNR measurements can be
performed by monitoring channel powers at each amplifier input,
such as by dithering the optical transmitters Tx1-TxN (see FIG. 3).
For example, the optical transmitters Tx1-TxN may be dithered using
dither signals with distinct frequencies or the same frequencies.
Further, the dither frequencies may be high enough to avoid
interference with the modulated optical signals and low enough to
avoid cross-modulation of the optical amplifiers 304.1-304.6, which
typically have low response bandwidths.
[0077] Moreover, at each of the optical amplifiers 304.1-304.6, the
dither signal may be detected using a photo-detector and suitable
digital signal processing. The detected dither signals may then be
used to estimate the optical power of the channels 1-N. Next, the
OSNR value for each channel "K" can be calculated using the
following equation:
OSNR_K=58+10*LOG[1/(.sub.INF_I/P_in_I_K)], (13)
[0078] in which "NF_I" is the Noise Figure (NF) at optical
amplifier "I", and "P_in_I_K" is the optical input power of channel
"K" at optical amplifier "I".
[0079] As described above, the multiple pre-emphases technique is
completed by adjusting the pre-emphasis attenuation or gain of the
respective channels 1-N based on the measured OSNR values (OSNR_M)
to achieve the designated OSNR values (OSNR_D) of the channels 1-N
at the end of each pre-emphasis section. Specifically, the OSNR
values of the channels 1-N at each pre-emphasis section can be
balanced by measuring the OSNR_M value at each channel to be
pre-emphasized. Next, in the event the OSNR_M value is not within
the range "OSNR_D+TOL", e.g., TOL=0.5 dB, the optical output power
of the channel is adjusted according to the following equation:
ChannelPower_Adj (dB)=OSNR_M-OSNR_D. (14)
[0080] The channel optical output power is then adjusted until the
OSNR_M is within the range "OSNR_D+TOL".
[0081] FIG. 6 depicts a block diagram of an alternative embodiment
600 of the DWDM optical transmission system 300 (see FIG. 3). The
DWDM optical transmission system 600 is capable of automatically
performing the above-described multiple pre-emphases technique. In
the illustrated embodiment, the DWDM optical transmission system
600 includes a transmitter end 602, a receiver end 606, and a
plurality of pre-emphasis devices 614.1-614.2 serially coupled
between the transmitter and receiver ends 602 and 606. Further, at
least one optical amplifier 604.1 is coupled between the
transmitter end 602 and the pre-emphasis device 614.1 to form a
pre-emphasis section I, at least one optical amplifier 604.2 is
coupled between the pre-emphasis device 614.1 and the pre-emphasis
device 614.2 to form a pre-emphasis section II, and at least one
optical amplifier 604.3 is coupled between the pre-emphasis device
614.2 and the receiver end 606 to form a pre-emphasis section
III.
[0082] It is noted that the pre-emphasis devices 614.1-614.2 are
depicted as respective OEQNs. However, it should be understood that
the pre-emphasis devices 614.1-614.2 may comprise any other
suitable device capable of performing the multiple pre-emphases
technique disclosed herein.
[0083] The DWDM optical transmission system 600 further includes a
Network Management System (NMS) 622, which is configured to control
the performance of the above-described multiple pre-emphases
technique. Specifically, the NMS 622 includes at least one memory
such as ROM and/or RAM for storing operating systems and
application software modules, and at least one processor for
executing applications for controlling the performance of three (3)
pre-emphasis techniques. For example, the NMS 622 may control the
three (3) pre-emphasis techniques for sequentially achieving
designated OSNR values at each of the pre-emphasis sections I-III
by suitably adjusting optical channel power levels at the
pre-emphasis device 614.1, at the pre-emphasis device 614.2, or at
the transmitter end 602.
[0084] It will further be appreciated by those of ordinary skill in
the art that modifications to and variations of the above-described
system and method may be made without departing from the inventive
concepts disclosed herein. Accordingly, the invention should not be
viewed as limited except as by the scope and spirit of the appended
claims.
* * * * *