U.S. patent application number 09/768689 was filed with the patent office on 2002-07-25 for wdm optical communication system using co-propagating raman amplification.
Invention is credited to Du, Mei, Nielsen, Torben N., Rottwitt, Karsten, Stentz, Andrew John.
Application Number | 20020097481 09/768689 |
Document ID | / |
Family ID | 25083219 |
Filed Date | 2002-07-25 |
United States Patent
Application |
20020097481 |
Kind Code |
A1 |
Du, Mei ; et al. |
July 25, 2002 |
WDM OPTICAL COMMUNICATION SYSTEM USING CO-PROPAGATING RAMAN
AMPLIFICATION
Abstract
The use of a co-propagating fiber Raman amplifier in an optical
WDM transmission system has been found to be practical in the
situation where the fiber amplifier is operated into depletion and
the characteristics of the input signals are controlled to exhibit
a reduced integrated relative intensity noise (RIN) over the fiber
crosstalk bandwidth. In particular, the reduction in the integrated
RIN can be achieved by increasing the number of input channels (by
adding more messages or simply using dummy channels), encoding the
data in a particular fashion to reduce the integrated RIN, or
decorrelating the plurality of N input signals below a
predetermined, relatively low frequency (for example, 2 MHz).
Inventors: |
Du, Mei; (Scotch Plains,
NJ) ; Nielsen, Torben N.; (Monmouth Beach, NJ)
; Rottwitt, Karsten; (Basking Ridge, NJ) ; Stentz,
Andrew John; (Clinton, NJ) |
Correspondence
Address: |
Wendy W. Koba, Esq.
P O Box 556
Springtown
PA
18081
US
|
Family ID: |
25083219 |
Appl. No.: |
09/768689 |
Filed: |
January 24, 2001 |
Current U.S.
Class: |
359/334 |
Current CPC
Class: |
H01S 3/302 20130101;
H04B 10/2916 20130101; H04J 14/02 20130101; H01S 3/094003 20130101;
H01S 3/06754 20130101 |
Class at
Publication: |
359/334 |
International
Class: |
H01S 003/00 |
Claims
What is claimed is:
1. In an optical WDM transmission system, a fiber Raman amplifier
for generating optical amplification in a plurality of N input
signals, said fiber Raman amplifier comprising at least one pump
source disposed to provide a co-propagating pump in said fiber
Raman amplifier with said plurality of N input signals so as to
provide optical amplification into the pump depletion region,
wherein said plurality of N input signals are decorrelated in the
signal-pump-signal (SPS) crosstalk bandwidth.
2. An optical WDM transmission system as defined in claim 1 wherein
the plurality of N input signals are statistically independent to
exhibit a reduced value for the integrated relative intensity noise
of said plurality of N input signals over the fiber crosstalk
bandwidth, when compared with a plurality of N correlated
signals.
3. An optical WDM transmission system as defined in claim 2 wherein
the integrated relative intensity noise is defined as the integral
over all frequencies of the relative intensity noise value of the
plurality of N input signals interacting with a Raman pump, and the
associated crosstalk transfer function.
4. An optical WDM transmission system as defined in claim 2 wherein
the fiber crosstalk bandwidth is defined as all frequencies below a
predefined null in a transfer function curve associated with the
crosstalk efficiency.
5. An optical WDM transmission system as defined in claim 4 wherein
the fiber crosstalk bandwidth is defined as all frequencies below
the second null in the transfer function curve.
6. An optical WDM transmission system as defined in claim 2 wherein
the fiber crosstalk bandwidth is defined by the relation:
.omega.=3.pi./[L.sub.e(1/.nu..sub.p-1/.nu..sub.S)], where
.nu..sub.p is defined as the group velocity at the pump wavelength,
.nu..sub.S is the group velocity at the signal wavelength, and
L.sub.e is the "effective length" of the amplification region.
7. An optical WDM transmission system as defined in claim 2 wherein
the fiber crosstalk bandwidth is approximately 750 MHz.
8. An optical WDM transmission system as defined in claim 2 wherein
the reduced value of the integrated relative intensity noise of
said plurality of N input signals is achieved by using a relatively
large number of channels to reduce the integrated relative
intensity noise.
9. An optical WDM transmission system as defined in claim 8 wherein
a plurality of dummy channels are used in association with a
plurality of message channels to provide a relatively large
plurality of input channels.
10. An optical WDM transmission system as defined in claim 8
wherein a plurality of 5 dummy channels are used with a plurality
of 5 message channels to reduce the SPS crosstalk.
11. An optical WDM transmission system as defined in claim 10
wherein a plurality of 5 dummy channels is used with the plurality
of 5 message channels, at 5 dB pump depletion with 10 dB Raman
gain, to reduce the SPS crosstalk to a value less than 1 dB.
12. An optical WDM transmission system as defined in claim 10
wherein a plurality of 15 dummy channels is used with the plurality
of 5 message channels, at 5 dB pump depletion with 10 dB of Raman
gain, to reduce the SPS crosstalk to a value less than 0.5 dB.
13. An optical WDM transmission system as defined in claim 10
wherein a plurality of 35 dummy channels is used with the plurality
of 5 message channels, at 5 dB pump depletion with 10 dB of Raman
gain, to reduce the SPS crosstalk to a value less than 0.2 dB.
14. An optical WDM transmission system as defined in claim 2
wherein the plurality of N input signals are particularly encoded
to reduce the integrated relative intensity noise value over the
fiber crosstalk bandwidth.
15. An optical WDM transmission system as defined in claim 14
wherein the plurality of N input signals are particularly encoded
such that for any signal channel, the ratio of the integrated
relative intensity noise within the fiber crosstalk bandwidth
divided by the crosstalk bandwidth to the integrated relative
intensity noise over the entire electrical bandwidth divided by the
entire electrical bandwidth of the data is relatively small.
16. An optical WDM transmission system as defined in claim 15
wherein the ratio is less than -3 dB.
17. An optical WDM transmission system as defined in claim 2
wherein the plurality of N input signals are decorrelated to reduce
the integrated relative intensity noise over the fiber crosstalk
bandwidth.
18. An optical WDM transmission system as defined in claim 17
wherein the plurality of N input signals are decorrelated down to a
frequency of at least 2 MHz.
19. An optical WDM transmission system as defined in claim 18
wherein the plurality of N input signals comprises a plurality of
10 signal channels, up to 5 dB pump depletion with 10 dB Raman
gain, exhibiting a SPS crosstalk penalty of less than 1.0 dB.
20. An optical WDM transmission system as defined in claim 18
wherein the plurality of N input signals comprises a plurality of
20 signal channels, up to 5 dB pump depletion with 10 dB Raman
gain, exhibiting a SPS crosstalk penalty of less than 0.5 dB.
21. An optical WDM transmission system as defined in claim 18
wherein the plurality of N input signals comprises a plurality of
40 signal channels, up to 5 dB pump depletion with 10 dB Raman
gain, exhibiting a SPS crosstalk penalty of less than 0.1 dB.
22. In a bi-directional optical WDM transmission system, a fiber
Raman amplifier for generating optical amplification in a first
plurality of N input signals propagating in a first direction
through the fiber Raman amplifier and in a second plurality of N
input signals propagating in a second, opposite direction through
said fiber Raman amplifier, so as to provide optical amplification
into the pump depletion region, wherein said first and second
pluralities of N input signals are decorrelated in the
signal-pump-signal (SPS) crosstalk bandwidth.
23. A bi-directional optical WDM transmission system as defined in
claim 22 wherein the at least one pump source comprises a pump
source disposed to be co-propagating with the first plurality of N
input signals and counter-propagating with the second plurality of
N input signals.
24. A bi-directional optical WDM transmission system as defined in
claim 22 wherein the at least one pump source comprises a pump
source disposed to be co-propagating with the second plurality of N
input signals and counter-propagating with the first plurality of N
input signals.
25. A bi-directional optical WDM transmission system as defined in
claim 22 wherein the at least one pump source comprises a first
pump source disposed to be co-propagating with the first plurality
of N input signals and a second pump source disposed to be
co-propagating with the second plurality of N input signals.
26. A bi-directional optical WDM transmission system as defined in
claim 22 wherein the first and second pluralities of N input
signals are statistically independent to exhibit a reduced value
for the integrated relative intensity noise of said first and
second pluralities of N input signals over the fiber crosstalk
bandwidth, when compared with a plurality of N correlated
signals.
27. A bi-directional optical WDM transmission system as defined in
claim 26 wherein the integrated relative intensity noise is defined
as the collective modulation of the first plurality of N input
signals as they interact with the depleted pump in the fiber Raman
amplifier.
28. A bi-directional optical WDM transmission system as defined in
claim 22 wherein the reduced value of the integrated relative
intensity noise of said first and second pluralities of N input
signals is achieved by using a relatively large number of channels
to reduce the integrated relative intensity noise.
29. A bi-directional optical WDM transmission system as defined in
claim 28 wherein a plurality of dummy channels are used in
association with the plurality of message channels to provide a
relatively large plurality of input channels.
30. A bi-directional optical WDM transmission system as defined in
claim 28 wherein the first and second pluralities of N input
signals are particularly encoded to reduce the integrated relative
intensity noise value over the fiber crosstalk bandwidth.
31. An optical WDM transmission system as defined in claim 28
wherein the first and second pluralities of N input signals are
particularly encoded such that for any signal channel, the ratio of
the integrated relative intensity noise within the fiber crosstalk
bandwidth divided by the crosstalk bandwidth to the integrated
relative intensity noise over the entire electrical bandwidth
divided by the entire electrical bandwidth of the data is
relatively small.
Description
TECHNICAL FIELD
[0001] The present invention relates to the utilization of a
co-propagating Raman amplifier in an optical communication system
and, more particularly, to the utilization of a co-propagating
Raman amplifier in a wavelength-division-multiplexed (WDM) optical
communication system.
BACKGROUND OF THE INVENTION
[0002] Raman amplifiers have played an important role in advancing
optical communication systems, primarily as a result of being
capable of both increasing the capacity of such systems (in terms
of higher data rate and more channels) and the transmission
distance of the systems. Currently, most Raman amplifiers use a
counter-pumped configuration in which the pump and message signals
propagate in opposite directions through the fiber amplifier. To
date, there have been limited applications of a "co-pumped" Raman
amplifier where the pump and message signals propagate in the same
direction through the fiber amplifier. The co-pumped architecture
has been avoided due to the (presumed) increased presence of noise
in the co-pumped amplifier, where noise is defined as "pump-signal
crosstalk" (i.e., the noise originating from the pump being coupled
to the message signals through Raman gain) and "signal-pump-signal
crosstalk" (i.e., the encoded signal(s) impressing information to
the same or different signal wavelength via the Raman process). The
issue of pump-signal crosstalk has been addressed by the applicants
in their co-pending application No. 60/186,797, filed Mar. 3, 2000.
The ability to reduce pump-signal crosstalk using the methods
disclosed in the co-pending application has lead to the ability to
analyze and overcome the problems associated with
signal-pump-signal crosstalk.
[0003] In general, Raman amplification is an extremely fast
process, where the amplitude modulation of the encoded signal
channels over a limited bandwidth is impressed upon the Raman pump.
Thus, in this environment, even a perfectly "quiet" pump (i.e., a
pump without noise) will become noisy during Raman amplification.
This noise on the pump may then be impressed upon other message
signals through the process of Raman amplification. For the
purposes of understanding the teaching of the present invention,
this effect will be defined as "signal-pump-signal" crosstalk
(hereinafter referred to as "SPS crosstalk), since the crosstalk
between the signal channels is mediated by the pump. See, for
example, the article entitled "Cross talk in Fiber Raman
Amplification for WDM Systems", by W. Jiang et al., appearing in
the Journal of Lightwave Technology, Vol. 7, No. 9, 1989 at app.
1407-1411. In this theoretical paper, the crosstalk between two
channels in a Raman amplifier was calculated. The crosstalk, even
in the linear amplifier range (i.e., pump non-depletion), was shown
to be severe for the co-propagating configuration. In its
conclusion, the Jiang et al. reference stated that a
counter-propagating arrangement would be preferred, since as the
pump travels against the message signal, a stronger averaging
effect exists, reducing the crosstalk.
[0004] It is also known in the art that SPS crosstalk depends on
the modulation frequency of the channels, due to the relative
propagation speed difference (i.e., group velocities) between the
pump and message signals, as well as between the signals
themselves. Such relative propagation speed difference introduces
the walk-off of information in time, therefore effectively
averaging the SPS crosstalk for higher frequencies. Such an effect
results in a limited crosstalk bandwidth over which the SPS
crosstalk may occur, which is much smaller as compared to the
electrical bandwidth of the data. The crosstalk bandwidth in the
counter-propagating configuration is known to be relatively small
when compared to that encountered in the co-propagating
configuration. See, for example, an article entitled "Bandwidth of
cross talk in Raman amplifiers" by F. Forghieri et al. appearing in
the OFC '94 Technical Digest at page 294. In this paper, the
crosstalk bandwidth was defined as the frequency bandwidth in which
the modulation depth onto the second continuous-wave signal channel
is more than -20 dB. In this definition, the crosstalk bandwidth is
determined by the fiber dispersion parameters at the wavelengths of
the pump and signals, as well as the amount of Raman gain and pump
depletion. In their study, the crosstalk bandwidth was determined
to be approximately 100 MHz for a co-pumped Raman amplifier, as
compared to only a 10 kHz bandwidth for a counter-pumped
arrangement. Therefore, the Forghieri et al. paper concludes that
the performance of an intensity modulated WDM system using a
co-pumped Raman amplifier was severely limited by SPS crosstalk,
noting as preferable, then, the counter-propagating configuration.
A similar conclusion was made in the article "Crosstalk due to
stimulated Raman scattering in single-mode fibers for optical
communication in wavelength division multiplex systems" by H. F.
Mahlein appearing in Optical and Quantum Electronics 16, (1984), p.
409 et seq.
[0005] It is well-known by those skilled in the art that the term
"relative intensity noise" (RIN) is often used to characterize
fluctuations in photocurrents. RIN is defined in terms of detected
electrical power as the power-spectral density of the photocurrent
in a 1-Hz bandwidth at a specified frequency divided by the average
power of the photocurrent. Although the term "RIN" indicates that
this quantity is usually used to characterize fluctuations arising
from noise, the same quantity can be used to characterize
fluctuations due to signal modulation, as is the case here.
[0006] As mentioned above, SPS crosstalk depends on both pump
depletion and Raman gain. It is known by those skilled in the art
that the amount of Raman gain is known as the "on/off" gain, and
defined as ratio of output signal power in the presence of the
Raman pump to the output signal power in the absence of the Raman
pump. If a Raman pump amplifies an optical signal to an optical
power comparable to the power of the Raman pump, the Raman pump
will experience "pump depletion". This means that the power of the
pump, at some position within the amplifier, will become
significantly less than it would be in the absence of the signal,
and that the "net" Raman amplification will be reduced. In
practice, pump depletion is measured at the output of the amplifier
and is defined as the intensity difference of the pump output
"with" and "without" signal channels. In general, the smaller the
pump depletion and Raman gain, the smaller the SPS crosstalk.
Therefore, it is expected from the prior art studies that co-pumped
Raman amplifiers are limited to applications with small levels of
Raman gain and pump depletion. See, for example, the article
entitled "Wide-Bandwidth and Long-Distance WDM Transmission using
Highly Gain-Flattened Hybrid Amplifier" by S. Kawai et al.,
appearing in IEEE Photonics Technology Letters, Vol. 11, No. 7,
July 1999. In this article, 4 dB of Raman gain in a
co-propagating-pump geometry was used in a discrete Raman amplifier
as a part of the gain-flattened hybrid amplifier.
[0007] The prior art is replete with references describing the SPS
crosstalk problem present in fiber Raman amplifiers and the
utilization of a counter-propagating amplifier configuration to
overcome this problem. The Jiang et al. article referenced above
describes a solution involving limiting the signal gain and the
injected pump power to values well below the threshold for Raman
amplification. The Forghieri et al. and Mahlein articles suggest a
solution in terms of eliminating all components within the
crosstalk bandwidth (for example, 100 MHz), which is not practical
in WDM applications. Several papers have stated that a
counter-propagating configuration eliminates SPS crosstalk through
averaging. For example, see an article by S. A. E. Lewis et al.
appearing in Electronic Letters, Vol. 35, No. 11, 1999, at page
923. To date, therefore, most Raman amplifiers have been limited to
the counter-propagating configuration.
[0008] However, as WDM optical communication systems continue to
grow in capacity and reach longer distances, more system benefits
could be realized from co-propagating Raman amplification. For
example, a co-propagating Raman amplifier would allow for
bi-directional pumping of a Raman amplifier, as well as
bi-directional transmission of the message signals. Also, the
aggregate signal powers have reached magnitudes that can easily
deplete a Raman pump.
[0009] Thus, a need remains in the art for an arrangement capable
of providing significant co-propagating Raman amplification in a
WDM optical transmission system.
SUMMARY OF THE INVENTION
[0010] The need remaining in the prior art is addressed by the
present invention, which relates to the utilization of a
co-propagating Raman amplifier in optical communication system and,
more particularly, to the utilization of a co-propagating Raman
amplifier exhibiting reduced signal-pump-signal (SPS) crosstalk in
a wavelength-division-multiplexed (WDM) optical communication
system.
[0011] In accordance with the present invention, it has been
discovered that co-propagating Raman amplification, when utilized
into the pump depletion region, can be used in virtually any WDM
optical transmission system, as long as the signal channels
interacting with any given Raman pump exhibit small integrated RIN
values over the fiber crosstalk bandwidth. In particular, it can be
shown that SPS crosstalk in co-propagating fiber Raman amplifiers
can be significantly reduced by altering the properties of the
input signals so as to minimize the integrated RIN values of the
signal channels over the fiber crosstalk bandwidth.
[0012] In accordance with the present invention, one or more of the
following methods is used to provide the desired reduction of the
integrated RIN values of the signal channels interacting with any
given co-propagating Raman pump in the fiber crosstalk bandwidth:
(1) transmission of a sufficient number of independent channels
(using "dummy" channels, if necessary); (2) encoding the data such
that any given signal channel has a small ratio of integrated RIN
values over the fiber crosstalk bandwidth to that over the entire
signal electrical bandwidth; and/or (3) decorrelating a plurality
of input signals to reduce the integrated RIN values over the fiber
crosstalk bandwidth. In general, the purpose of the present
invention is to reduce the integrated RIN values over the fiber
crosstalk bandwidth so as to essentially eliminate problems
associated with SPS crosstalk in a co-propagating Raman
amplifier.
[0013] In a preferred embodiment of the present invention, all
three of these concepts would be used to provide for the greatest
reduction in SPS crosstalk. However, it has been found that the
implementation of even one of these features is sufficient to
provide for a viable commercial WDM system using a co-propagating
fiber Raman amplifier.
[0014] Various and further features and aspects of the present
invention will become apparent during the course of the following
discussion and by reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Referring now to the drawings,
[0016] FIG. 1 contains a block diagram of an exemplary (simplified)
prior art WDM system in a typical testbed setting;
[0017] FIG. 2 illustrates a simplified model for understanding the
generation of SPS crosstalk in a WDM system using Raman
amplification;
[0018] FIG. 3 is a plot that illustrates fiber crosstalk bandwidth
for three different exemplary amounts of pump depletion;
[0019] FIG. 4 is a diagram of an exemplary experimental setup used
to evaluate SPS crosstalk in a fiber Raman amplifier using a
co-propagating configuration;
[0020] FIG. 5 is a plot of the Q factor as a function of Raman pump
depletion for three different levels of data correlation;
[0021] FIG. 6 is a graph of RF power density as a function of
frequency for the same three levels of data correlation as used
with the plot of FIG. 5;
[0022] FIG. 7 is a graph of the relative intensity noise (RIN)
value as a function of the number of signal channels in a fiber
Raman amplifier;
[0023] FIG. 8 contains a graph of the probe Q factor as a function
of pump depletion for systems utilizing different numbers of
information channels;
[0024] FIG. 9 is a graph of RF power density as a function of
different data pattern lengths;
[0025] FIG. 10 plots the Q factor as a function of pump depletion
for different data pattern lengths;
[0026] FIG. 11 illustrates an exemplary co-propagating system
exhibiting reduced input signal correlation in accordance with the
present invention;
[0027] FIG. 12 illustrates an exemplary bi-directional
co-propagating system with reduced signal-pump-signal crosstalk in
accordance with the present invention; and
[0028] FIG. 13 illustrates an exemplary WDM optical transmission
system configured to reduce SPS crosstalk in the fiber Raman
amplifier, using a bi-directional pumping scheme.
DETAILED DESCRIPTION
[0029] FIG. 1 contains a simplified block diagram of a typical WDM
transmission arrangement 10 that is used to analyze Raman amplified
systems in a testbed environment. In particular, a number of
separate channels, denoted S.sub.1-S.sub.N are applied as separate
inputs to a wavelength division multiplexer (WDM) 12 and combined
onto a single transmission path 14. The combined output then passes
through a modulator 16 and a decorrelator 18, where decorrelator 18
is used to "walk-off" the data bits in time between channels. In
most cases, decorrelator 18 comprises a section of dispersion
compensated fiber (DCF), or fiber delay lines. The output of
decorrelator 18 then enters a transmission fiber 20, as shown in
FIG. 1.
[0030] FIG. 2 is a simplified illustration that is useful in
understanding the parameters determining SPS crosstalk in a WDM
optical transmission system. A plurality of message signals
M.sub.1-M.sub.N are shown in FIG. 2, where exemplary data patterns
are depicted for channels 1 and N. As shown, the modulation of the
input signals is impressed upon a continuous-wave pump through pump
depletion. This noise is then impressed upon all of the various
signals that are likewise passing through the same Raman amplifier.
The presence of this noise is illustrated in FIG. 2, which shows a
noise from channel i (shown with the dashed line) as impressed on
the signal in channel j. As will be discussed below, an aspect of
the present invention is to consider that as the number of signal
channels increases, the probability that a symbol "1" is
simultaneously transmitted on all N channels decreases. Indeed, for
the purposes of the present invention, it is important to consider
the statistics as based on N channels, not limited to experimental
set-ups involving only two channels. Indeed, it is the collective
modulation of the N signal channels that serves as the noise source
for SPS crosstalk. This noise source, as discussed above, can be
represented by the RIN value of the N signal channels.
[0031] FIG. 3 depicts the results of an exemplary measurement of
the fiber crosstalk bandwidth as discussed above. Although
"crosstalk bandwidth" has been numerically simulated (as, for
example, by F. Forghieri et al.), there are no experimental
measurements in the literature to date. The difficulty in measuring
crosstalk bandwidths arises from the inability to separate out
other noise sources during this measurement, in particular, the
noise created by pump-signal crosstalk in co-propagating Raman
amplifiers. Our co-pending application Serial No. 60/186,777
describes an arrangement where the pump is optimized to
significantly reduce the pump-signal crosstalk, thereby permitting
for the purposes of the present invention, the ability to measure
the fiber crosstalk bandwidth. In this example, a "noisy" signal
and "quiet" pump were used at the input of a co-propagating Raman
amplifier. The pump output was characterized with an RF spectrum
analyzer to determine the frequency-dependence of the noise
transferred from the signal through the pump depletion. This
measurement yields the signal-to-pump crosstalk bandwidth, which is
similar to the pump-to-signal crosstalk bandwidth. As an
approximation, therefore, the measurement can be used to ascertain
the frequency range that is critical for SPS crosstalk.
[0032] A transfer function can be used to characterize the
frequency dependence of the SPS crosstalk. In particular, FIG. 3
illustrates the transfer function, which represents the ratio of
the noise added to the pump by Raman amplification normalized to
the signal noise at the input. The plots illustrate the transfer
function for the noise from the signal to the pump, where a
transfer function value of "1" represents a 100% transfer from
signal noise to the pump at frequency .omega.. In particular, FIG.
3 illustrates the plot of the transfer function as a function of
frequency for three different amounts of pump depletion through the
Raman process in a particular transmission fiber exhibiting a group
delay of approximately 0.2 ns/km between the pump and signal. As
shown, the transfer efficiency of the noise from the signal to the
pump decreases as the frequency increases. Therefore, at the low
frequency, more efficient noise transfer occurs. Also, at the same
frequency, larger depletion results in more efficient noise
transfer. The envelope of the transfer function decreases as 1/f
and the nulls appear at the frequency for which the propagation
time difference between the pump and signal is an odd integer of
the signal modulation frequency.
[0033] The "noise" source for SPS crosstalk originates from the
collective modulation of the N channels that interact with the
depleted Raman pump. This collective modulation can be
characterized by the frequency-dependent RIN value of the N
channels. An important quantity that affects the level of SPS
crosstalk in co-propagating Raman amplifiers, as discussed above,
is the integral of the product of this RIN value produced by the N
signal channels interacting with a given Raman pump and the
crosstalk transfer function over all the frequencies. For the
purposes of understanding the teaching of the present invention,
this quantity will hereinafter be referred to as the "integrated
RIN", or IRIN.
[0034] Inspect of the transfer function in FIG. 3 reveals that the
greatest contribution to the IRIN comes from the integration of the
signal RIN values at frequencies less than the second null in the
transfer function. Therefore, the IRIN value can be accurately
approximated by simply integrating the signal RIN values over
frequencies less than the second null in the transfer function.
Thus, for the purposes of the present invention, the "crosstalk
bandwidth" will be defined as the frequency range below the second
null. For the example depicted in FIG. 3, therefore, the crosstalk
bandwidth is approximately 750 MHz.
[0035] As previously discussed, the crosstalk bandwidth also
depends on the fiber dispersion parameters at the pump and signal
wavelengths. The crosstalk bandwidth can therefore be estimated
with the following simplified equation:
.omega.=3.pi./[L.sub.e(1/.nu..sub.p-1/.nu..sub.s)],
[0036] where .nu..sub.p is defined as the group velocity at the
pump wavelength, .nu..sub.s is the group velocity at the signal
wavelength, and L.sub.e is the "effective length" of the
amplifier.
[0037] Based on the above, therefore, it is concluded in accordance
with the present invention that SPS crosstalk is determined by the
following parameters: (1) the statistics of the N signal channels;
(2) pump depletion (which determines the magnitude of modulation
transfer from the message signals to the pump); (3) the Raman gain
(which determines the magnitude of the modulation transfer from the
pump back to the signals; and (4) the fiber crosstalk bandwidth in
which most of the SPS crosstalk occurs. More particularly, the
integrated RIN over the crosstalk bandwidth is a critical factor in
overcoming SPS crosstalk problems in a co-propagating Raman
amplifier.
[0038] In many applications, the signal channels are completely
independent. Under these circumstances, the IRIN decreases as the
number of channels increases. More particularly, as N approaches
infinity, the IRIN will approach zero. When N=1, the IRIN is at its
maximum. Therefore, one means of reducing the effects of SPS
crosstalk in accordance with the present invention is to transmit a
sufficient number of statistically independent channels through the
Raman amplifier. If a "sufficient" number of data-bearing channels
are not present, independent "dummy" channels may be added to
reduce the SPS crosstalk. In either of these circumstances, the
IRIN of the collection of channels should be substantially less
than the IRIN of one channel.
[0039] In accordance with the present invention, another method of
reducing the IRIN is to encode each channel such that it has
reduced RIN values at frequencies less than the crosstalk
bandwidth. In other words, each signal is encoded so as to
substantially reduce the magnitude of its frequency components at
frequencies less than the crosstalk bandwidth. Under these
circumstances, the RIN values of a given channel, integrated over
frequencies less than the crosstalk bandwidth and normalized by the
crosstalk bandwidth, should be substantially less than the RIN
values of the same channel integrated over frequencies less than
the data bandwidth and normalized by the data bandwidth.
Furthermore, if the signal channels are less than completely
independent, a decorrelator may be used to substantially
decorrelate the channels at frequencies less than the crosstalk
bandwidth such that the IRIN of the channels is reduced before the
signals are injected into the Raman amplifier.
[0040] FIG. 4 illustrates an experimental setup 30 that can be used
to evaluate SPS crosstalk in accordance with the principles of the
present invention. As will be discussed in detail below, problems
with this setup in the past have resulted in incorrect conclusions
regarding the workability of co-propagating fiber Raman amplifiers.
Referring to FIG. 4, a total of N channels (for example, 100 GHz
spaced WDM channels) are used to deplete a co-propagating Raman
pump. The depleting tones are modulated in a Mach-Zehnder modulator
32 at a frequency of 9.95 Gb/s, using a 2.sup.x-1 pseudo-random bit
stream (PRBS) sequence, with x=7, 9 or 31. The bit patterns of the
N depleting tones are made statistically independent (i.e.,
decorrelated) by time-shifting their bit patterns relative to each
other, for example by using a pair of AWGR's connected with fiber
delay lines having different lengths for the N individual
wavelengths. A probe signal P is also modulated at 9.95 Gb/s, in a
second Mach-Zehnder modulator 34, using a 2.sup.23-1 PRBS sequence.
The two PRBS sequences are generated from individual pattern
generators having independent clocks. The modulated probe signal is
then combined with the statistically independent depleting tones,
passed through an erbium-doped preamplifier 35 and launched into a
fiber span 36 consisting of 80 km of fiber. Fiber span 36 is
co-pumped with light from a semiconductor Raman pump source 38. In
an exemplary experimental setup, the Raman pump power launched into
fiber span 36 may be 270 mW, with the pump wavelength centered at
1435 nm, providing a peak Raman gain of approximately 13 dB in the
1525-1545 nm window. By adjusting the launch power of the N
depleting tones (along with the probe signal) with an attenuator
40, the amount of Raman pump depletion can be varied between zero
(with low launch power) and 15 dB. The pump depletion is measured
by monitoring the pump throughput at the end of fiber span 36 with
a power meter 37. Careful choice of the probe signal wavelength and
power will minimize the amount of cross-phase modulation between
the depleting tones and the probe channel. The large number of
depleting tones ensures that the launch power per tone is
sufficiently low so as to avoid SBS in the transmission fiber. At
the receiver, probe signal P is selected with a bandpass filter 42
and a section of dispersion compensating fiber (DCF) 44 is used to
compensate for the dispersion accumulated through the 80 km of
transmission fiber. By adjusting the input level to an erbium-doped
preamplifier 46, it can be ensured that the OSNR at the output of
preamplifier 46 is fixed at 21.4 dB, with 0.1 nm resolution
bandwidth, for any level of Raman pump depletion. The output is
then passed through an optical receiving element 48 (such as a PIN
photodiode) to convert the received optical signal into an
electrical equivalent. The penalties caused by SPS crosstalk
between the depleting tones and the probe are then quantified by
the "Q" of the probe channel by measuring the bit error rate (BER)
versus receiver decision level threshold, in an error measurement
arrangement 50.
[0041] It is to be noted that experiments involving transmission
systems are often conducted using multiple channels that are
modulated by a common modulator, followed by a decorrelator, simply
because this equipment set is substantially less expensive than a
set of completely independent channels, each with its own
modulator. However, the level of decorrelation at frequencies less
than the crosstalk bandwidth is typically not sufficient to
accurately characterize SPS crosstalk. With this system as it is
now understood in accordance with the present invention, there are
at least three cases where the SPS crosstalk measurements using the
arrangement of FIG. 4 may lead to incorrect conclusions on the
usefulness of co-propagating Raman amplifier configurations. Each
of these three cases will be discussed in detail below.
[0042] FIG. 5 is a plot of the Q factor, as defined above, as a
function of Raman pump depletion for the arrangement of FIG. 4,
where 16 channels between 1542 nm and 1555 nm were chosen for use
as depletion tones with a 2.sup.31-1 PRBS pattern and a 1534 nm
signal (with a 2.sup.23-1 PRBS sequence used as a probe). Peak gain
for the undepleted Raman amplifier was determined to be 12.5 dB.
Curve A in FIG. 5 is associated with the channels when a
decorrelator 52 is not present in the system. As shown, a sever Q
factor penalty will result without decorrelation. For example, at a
pump depletion level of 1.5 dB, a Q factor penalty of 1.6 dB was
observed. Curve B illustrates an application where the decorrelator
only effectively decorrelates the bits above 100 MHz in frequency.
At the same 1.5 dB Q factor penalty, the amplifier can tolerate
over 6 dB of pump depletion. In both situations, when measured in a
counter-pumped Raman amplifier configuration at the same signal
average power, no Q penalty was observed up to 8 dB pump depletion.
Thus, it is concluded that the observed Q penalty can be attributed
to SPS crosstalk. Based on these measurements, therefore, the prior
art studies determined that the SPS crosstalk was severe for
co-pumped amplification.
[0043] As mentioned above, SPS crosstalk is a "pump mediated"
crosstalk. The origin of this crosstalk is the collective signal
modulation, which is coupled to the Raman pump through the pump
depletion process. The integrated RIN of the combined signals can
be used as a measure of such collective modulation. More
particularly, the greater the integrated RIN, the greater the noise
sources for SPS crosstalk. Using the same average power of the 16
channels, the RF power densities can be used to represent RIN. FIG.
6 illustrates the RF power densities of the pump throughput at 5 dB
depletion for the three different data correlations discussed above
in association with FIG. 5, where the same curve letterings are
used for each. In FIG. 6, curve A represents the greatest RF power
within the crosstalk bandwidth. In this case, 16 channels have the
greatest correlation, where the only decorrelation of the channels
comes from the dispersion walk-off within the transmission fiber.
Curve B has less RF power within the fiber crosstalk bandwidth than
curve A, where in this case the 16 channels pass through a
decorrelator, but only the bits above 100 MHz are decorrelated.
[0044] In accordance with the present invention, it has been
determined that the penalties shown in curves A and B are
overestimated, as a result of testbed limitations. Curve C
illustrates the situation where a special decorrelator capable of
randomizing the message signal inputs above 2 MHz is used. As
shown, curve C has the least RF power within the fiber crosstalk
bandwidth. Referring back to FIG. 5, this same configuration yields
a small Q penalty for up to 8 dB of pump depletion. Therefore, it
is shown that a co-propagating fiber Raman amplifier is practical
for use in a WDM transmission system operated into the pump
depletion region (i.e., operated into the saturation region of the
Raman amplifier), as long as the signal channels exhibit
significant decorrelation within the fiber crosstalk bandwidth.
[0045] As mentioned above, many prior art studies focused on the
situation where two signal channels were used to determine the
effects of crosstalk in Raman amplifiers. It has now been
determined that this is a "worst case" scenario for studying SPS
crosstalk. FIG. 7 is a graph of the RIN value of signal channels at
50 MHz as a function of the number, N, of signal channels that are
included in the system (in an environment where the channels are
decorrelated down to the 2 MHz level discussed above). As shown,
and as further determined in accordance with the present invention,
as the number of channels increases, the RIN value decreases. As
the RIN value decreases, the integrated RIN value over the
crosstalk bandwidth decreases. Therefore, the SPS crosstalk will
decrease as well.
[0046] FIG. 8 contains a graph illustrating the Q factor of the
probe as a function of pump depletion for a set of WDM optical
transmission systems, each utilizing a different number of input
channels (e.g., 5 channels, 10 channels, 20 channels and 40
channels). Peak gain for the undepleted Raman amplifier is about
12.5 dB. At the same total input signal power (that is, same pump
depletion), a significant improvement in system performance is
observed as the number of channels increases. These results again
indicate that the reduction of RIN can reduce the SPS crosstalk
penalty. In accordance with the present invention, therefore, the
number of channels serves to average out the collective modulation
of the signals and thus reduces the SPS crosstalk present in any
one channel. In systems which by their nature have a limited number
of channels, a set of "dummy channels" may be added, in accordance
with the teachings of the present invention, to reduce the
integrated RIN value.
[0047] In the conventional test bed as illustrated in FIG. 4, the
data stream is often a pseudo-random bit sequence. At a certain
data rate (for example, 10 Gb/s), the frequency components in the
crosstalk bandwidth will be different for different data lengths.
FIG. 9 illustrates the relative RF power density as a function of
the data pattern length, where the 16 signal channels were
decorrelated above 100 MHz. When the depletion tones carry a
2.sup.7-1 PRBS pattern, the lowest frequency component is
approximately 78 MHz. Therefore, the discrete components in a PRBS
2.sup.7-1 pattern has the least integrated RIN within the fiber
crosstalk bandwidth. Even for the case where a 2.sup.9-1 PRBS
pattern is used (where the lowest frequency component is
approximately 19.6 MHz), the limited RF components within the fiber
crosstalk bandwidth will substantially impact the integrated RIN
value. However, for the 2.sup.31-1 PRBS case, the discrete RF
components will have a spacing of only approximately 4.6 Hz, and
the integrated RIN value will be a significant source of noise.
FIG. 10 is a graph of the Q factor (i.e., SPS crosstalk penalty) as
a function of pump depletion at different pattern lengths (i.e., 7,
9, 31) for 16 channels decorrelated above 100 MHz. It is shown that
if a 2.sup.7-1 PRBS pattern is used (such as usually the case with
"test" systems and computer simulations), the presence of SPS
crosstalk will be completely missed. Therefore, when a real
application is analyzed (which is similar as using a 2.sup.31-1
PRBS pattern), the problem of SPS crosstalk will appear. The
recognition of the importance of the word length of the data
pattern was heretofore unknown. This result, in accordance with the
present invention, leads to three important conclusions: (1) the Q
factor difference at 2.sup.7-1 and 2.sup.31-1 PRBS can be used in a
system testbed to separate the SPS crosstalk penalty from other
sources of noise penalty; (2) computer simulations using a
2.sup.7-1 PRBS may completely miss the SPS crosstalk penalty; and
(3) if the data is encoded such that for any signal channel the
ratio of the integrated RIN value within the fiber crosstalk
bandwidth to the integrated RIN in the electrical bandwidth is
small, the SPS crosstalk will be reduced.
[0048] Based on the recognition of these aspects of SPS crosstalk
in a co-propagating fiber Raman amplifier, it is possible, in
accordance with the teachings of the present invention, to use
co-propagating fiber Raman amplifiers in an optical communication
system, as long as the operating parameters are well understood and
controlled.
[0049] In particular, FIG. 11 illustrates, in simplified block
diagram form, an exemplary WDM optical transmission system 100 of
the present invention that is configured to reduce
signal-pump-signal (SPS) crosstalk in the amplifier region. As
shown, a plurality of N separate input signals S.sub.1-S.sub.N are
applied as separate inputs along associated communication paths
120.sub.1-120.sub.N, and then to a wavelength division multiplexer
(WDM) 140. WDM 140 is a component well-known in the art and
functions to combine the separate signals S.sub.1-S.sub.N and
coupled the signals onto a single output transmission fiber 160, as
shown in FIG. 11. In accordance with one particular embodiment of
the present invention, a decorrelator 180 is disposed along
transmission fiber 160 and is used to introduce a predetermined
randomization function between input signals S.sub.1-S.sub.N.
Alternatively, system 100 may utilize a plurality of "dummy"
channels with input signals S.sub.1-S.sub.N, or include a specific
encoder to reduce IRIN to a predetermined amount within the fiber
crosstalk bandwidth. Referring back to FIG. 11, the decorrelated
signals, denoted DS.sub.1-DS.sub.N, thereafter propagate along
fiber 200 and enter fiber Raman amplifier region 220. A pump source
240 is disposed as shown to couple a pump input (operating at a
wavelength of, for example, 1435 nm) into fiber 260 with the
decorrelated signals DS.sub.1-DS.sub.N to provide a plurality of
amplified signals A.sub.1-A.sub.N, with minimal SPS crosstalk at
the output of co-pumped Raman amplifier 220.
[0050] A bi-directional WDM communication system 300 of the present
invention is illustrated in FIG. 12. A number of the elements in
system 300 are identical to those described above in association
with system 100 of FIG. 11 and carry the same reference numeral.
System 300 is considered "bi-directional" in the sense that a
second plurality of input signals, denoted S'.sub.1-S'.sub.N
propagate along a second set of input signal paths 320 and are
applied as inputs to a second WDM 340. This second plurality of
input signals is then provided as an output along a single
transmission path 360 and passes through a second decorrelator 380
(or otherwise modified using a plurality of dummy channels and/or
specific encoding) to produce a set of decorrelated signals
DS'.sub.1-DS'.sub.N. A second pump source 400 is included in system
300 of FIG. 12 and, as shown, produces a second pump signal
P.sub.2, where P.sub.2 co-propagates with decorrelated signals
DS'.sub.1- DS'.sub.N. Indeed, for bi-directional system 300 of FIG.
12, the existence of two separate pump signals (from sources 240
and 400), results in providing both co- and counter-propagating
pumps for the information signals amplified within fiber Raman
amplifier 220. Thus, it is important that the problems associated
with SPS crosstalk be addressed in a bi-directional communication
arrangement, since such a system will inevitably employ a
co-propagating pump.
[0051] Referring to FIG. 12, a first pair of wavelength division
multiplexers (WDM) 460, 440 are used to provide for the
bi-directional transmission of information signals. In particular,
the first plurality of input signals S.sub.1-S.sub.N will exit WDM
140 and be directed as an input to first decorrelator 180. The
plurality of decorrelated information signals DS.sub.1-DS.sub.N
subsequently pass through a WDM 440 and into fiber Raman amplifier
220. At the output of fiber Raman amplifier 220, amplified signals
A.sub.1-A.sub.N enter WDM2 460 and are separated from the second
plurality of signals. In a similar fashion, the second plurality of
input signals S'.sub.1-S'.sub.N will exit second WDM 340 and enter
second decorrelator 380. The decorrelated plurality of signals
DS'.sub.1-DS'.sub.N will then pass through WDM2 460 and enter fiber
Raman amplifier 220. At the output of fiber Raman amplifier 220,
the second plurality of amplified signals, denoted
A'.sub.1-A'.sub.N, will enter WDM 440. Amplified signals
A'.sub.1-A'.sub.N will then be separated from first plurality of
signals.
[0052] FIG. 13 illustrates, in simplified block diagram form, an
exemplary WDM optical transmission system 1000 of the present
invention that is configured to reduce SPS crosstalk in the
amplifier region using a bidirectional pumping scheme, where a pump
signal 1080 is co-propagating with the input message signals and
pump 1090 is counter-propagating with the input message signals. As
shown, a plurality of N separate input signals S.sub.1-S.sub.N are
applied as separate inputs along associated communication paths
1020.sub.1-1020.sub.N and thereafter coupled as inputs into a WDM
1030. WDM 1030 is a component well-known in the art and functions
to combine the separate signals S.sub.1-S.sub.N and couple the
combined signals onto a single output fiber 1040, as shown.
[0053] In accordance with the present invention, a decorrelator
1050 (or other arrangement, as discussed above, for adding "dummy
channels" or signal encoding) is disposed along fiber 1040 and is
used to reduce IRIN to a predetermined value. The decorrelated
signals, denoted DS.sub.1-DS.sub.N, thereafter propagate along
fiber 1060 and enter fiber Raman amplifier region 1070. A pump
source 1080 is also disposed as shown to couple a pump (operating
at a wavelength of, for example, 1435 nm) into fiber 1260 with
decorrelated signals DS.sub.1-DS.sub.N to provide a plurality of
amplified signals A.sub.1-A.sub.N with minimal SPS crosstalk at the
output of Raman amplifier 1070.
[0054] Therefore, in accordance with the present invention, the
capability of decorrelating the input signals within the fiber
crosstalk bandwidth will greatly reduce the presence of SPS
crosstalk. By also encoding the data to reduce the ratio of RIN
within the fiber crosstalk bandwidth to that within the data
electrical bandwidth, the SPS crosstalk will be further reduced.
Lastly, if necessary, "dummy" channels can be added to increase the
number of data channels present in any given system. The present
invention is also applied to the discrete Raman amplifiers.
[0055] It is to be understood that various other modifications will
be readily apparent to those skilled in the art without departing
from the scope and spirit of the present invention as described
above. Accordingly, it is not intended that the scope of the
present invention be limited to the description set forth herein,
but rather that the claims be construed as encompassing all of the
features of patentable novelty that reside in the claims appended
hereto, including all features that would be treated as equivalents
thereof by those skilled in the art to which this invention
pertains.
* * * * *