U.S. patent application number 09/779201 was filed with the patent office on 2001-10-11 for method and apparatus for providing chromatic dispersion compensation in a wavelength division multiplexed optical transmission system.
Invention is credited to Abbott, Stuart M., Bergano, Neal, Evangelides, Stephen G., Golovchenko, Ekaterina, Harvey, George, Kerfoot, Franklin W. III, Lin, Chinlon, Pedersen, Bo.
Application Number | 20010028758 09/779201 |
Document ID | / |
Family ID | 22352320 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010028758 |
Kind Code |
A1 |
Abbott, Stuart M. ; et
al. |
October 11, 2001 |
Method and apparatus for providing chromatic dispersion
compensation in a wavelength division multiplexed optical
transmission system
Abstract
A WDM optical communication system is provided that includes a
transmitter and a receiver. An optical fiber transmission path
couples the transmitter to the receiver. The transmission path
includes at least one repeater having an optical amplifier located
therein. A dispersion compensator is disposed at an intermediate
point along the transmission path. The intermediate point is
located outside of the repeater. The compensator includes a
wavelength routing device for dividing a signal having a prescribed
bandwidth into a plurality of distinct sub-bands. A plurality of
output paths is provided for respectively receiving the plurality
of distinct sub-bands. The dispersion compensator also includes a
dispersion compensating optical element coupled to each of the
output paths. Each dispersion compensating optical element
substantially compensates for dispersion at a prescribed wavelength
within the bandpass of its respective sub-band. A coupler
recombining the distinct sub-bands and couples them back onto the
optical fiber transmission path.
Inventors: |
Abbott, Stuart M.;
(Marlboro, NJ) ; Bergano, Neal; (Lincroft, NJ)
; Evangelides, Stephen G.; (Red Bank, NJ) ;
Golovchenko, Ekaterina; (Colts Neck, NJ) ; Harvey,
George; (Princeton, NJ) ; Kerfoot, Franklin W.
III; (Red Bank, NJ) ; Lin, Chinlon; (Holmdel,
NJ) ; Pedersen, Bo; (Rumson, NJ) |
Correspondence
Address: |
TyCom (US) Inc.
John P. Maldjian
Senior Patent and Trademark Counsel
250 Industrial Way West, Room 2B-106
Eatontown
NJ
07724
US
|
Family ID: |
22352320 |
Appl. No.: |
09/779201 |
Filed: |
February 8, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09779201 |
Feb 8, 2001 |
|
|
|
09113923 |
Jul 14, 1998 |
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Current U.S.
Class: |
385/24 ;
398/147 |
Current CPC
Class: |
H04B 10/25253 20130101;
H04B 2210/258 20130101; G02B 6/29376 20130101 |
Class at
Publication: |
385/24 ; 359/161;
359/173 |
International
Class: |
G02B 006/28 |
Claims
1. A WDM optical communication system, comprising: a transmitter
and a receiver; an optical fiber transmission path coupling said
transmitter to said receiver, said transmission path including at
least one repeater having an optical amplifier located therein; a
dispersion compensator disposed at an intermediate point along said
transmission path outside of the repeater, said compensator
including: a wavelength routing device for dividing a signal having
a prescribed bandwidth into a plurality of distinct sub-bands; a
plurality of output paths for respectively receiving said plurality
of distinct sub-bands; a dispersion compensating optical element
coupled to each of the output paths, said dispersion compensating
optical elements each substantially compensating for dispersion at
a prescribed wavelength within the bandpass of its respective
sub-band; a coupler for recombining said distinct sub-bands and
coupling said recombined distinct sub-bands onto said optical fiber
transmission path.
2. The system of claim 2 wherein said transmission path comprises
at least one optical fiber and a cable supporting said optical
fiber, said cable having a prescribed portion in which said
dispersion compensator is located.
3. The system of 1 wherein said plurality of distinct sub-bands
comprises two distinct sub-bands corresponding to upper and lower
bands.
4. The system of claim 3 wherein said lower band includes a first
dispersion equalizing fiber having a positive dispersion.
5. The system of claim 4 wherein the dispersion of said first
dispersion equalizing fiber is approximately equal to 19
ps/km-nm.
6. The system of claim 4 wherein said upper band includes a second
dispersion equalizing fiber having a negative dispersion.
7. The system of claim 6 wherein the dispersion of the second
dispersion equalizing fiber is within a range of about -80 to -100
ps/km-nm.
8. The system of claim 7 wherein said optical fiber transmission
path has a path average dispersion of -2.5 ps/km-nm.
9. The system of claim 8 wherein said first dispersion equalizing
fiber has a length providing a total dispersion of approximately
817 ps/nm.
10. The system of claim 9 wherein said second dispersion equalizing
fiber has a length providing a total dispersion of approximately
-1387 ps/nm.
11. The system of claim 6 wherein said first and second dispersion
equalizing fibers have first and second lengths, respectively, that
provide gain equalization between said upper and lower bands.
12. The system of claim 6 wherein said lower band further comprises
a loss element to provide gain equalization between said upper and
lower bands.
13. The system of claim 6 wherein said upper band further comprises
a third dispersion equalizing fiber having a negative dispersion
with a magnitude less than the magnitude of the second dispersion
equalizing fiber.
14. The system of claim 13 wherein the dispersion of the third
dispersion equalizing fiber is approximately equal to -2 ps/km-nm.
Description
RELATED APPLICATIONS
[0001] This application is related to U.S. Appl. Ser. No. [Bergano
18] entitled "Chromatic Dispersion Compensation in Wavelength
Division Multiplexed Optical Transmission Systems."
FIELD OF THE INVENTION
[0002] The invention relates to the optical transmission of
information and, more particularly, to a method and apparatus for
compensating for chromatic dispersion that accrues over optical
fiber transmission systems.
BACKGROUND OF THE INVENTION
[0003] The availability of high performance optical amplifiers such
as the Erbium-Doped Fiber-Amplifier (EDFA) has renewed interest in
the use of wavelength division multiplexing (WDM) for optical
transmission systems. In a WDM transmission system, two or more
optical data carrying channels are combined onto a common path for
transmission to a remote receiver. Typically, in a long-haul
optical fiber system, the set of wavelength channels would be
amplified simultaneously in an optical amplifier based repeater.
The Erbium-Doped Fiber-Amplifier is particularly useful for this
purpose because of its ability to amplify multiple wavelength
channels without crosstalk penalty.
[0004] Typically, it is advantageous to operate long-haul
transmission systems at high data rates per channel. For example,
useful data rates include multiples of the Synchronous Digital
Hierarchy (SDH) standard, i.e., 2.5 and 10 Gb/s. While both 2.5 and
10 Gb/s are available, in many applications it is desirable to
operate at the highest standard bit rate possible. This lowers the
cost and physical dimensions of the terminal equipment. For
example, a transmission system with a capacity of 160 Gb/s would
require 64 2.5 Gb/s transmitters/receiver pairs, whereas a system
based on 10 Gb/s would only require 16 10 Gb/s pairs of terminal
equipment. Unfortunately, as the transmission bit rates are
increased, so do the transmission penalties associated with the
transmission fiber's chromatic dispersion and nonlinear index of
refraction. It has been found both experimentally and by
simulations that by using a combination of dispersion management
techniques and distortion tolerant transmission formats, 10 Gb/s
channels can be transmitted successfully over transoceanic
distances over a limited range of wavelengths.
[0005] One of the more important parameters that limit the usable
wavelength space is the so-called "dispersion slope", or "higher
order" dispersion of the transmission fiber. The dispersion slope
causes each WDM channel to experience a slightly different amount
of dispersion. The efficacy of the dispersion mapping technique,
reviewed by Forghieri et al., (Chapter 8 in Optical Fiber
Telecommunications IIIA, Academic Press 1997) is limited because
the amount of dispersion that occurs in a typical optical fiber
depends on the operating wavelength that is employed. That is, only
one given wavelength can operate at average zero dispersion.
Accordingly, because of this characteristic of the dispersion
slope, the various channels employed in a WDM system cannot all
operate at the wavelength of average zero dispersion. This
limitation can be overcome to a limited degree by using individual
channel dispersion compensation at the receiver. However, since
these systems are subject to nonlinear penalty, the ability to
correct for the non-zero dispersion at the receiver terminal is
limited. Thus, channels located far from the average zero
dispersion wavelength can experience large amounts of
distortion.
SUMMARY OF THE INVENTION
[0006] In accordance with the present invention, a method and
apparatus is provided for managing dispersion in a WDM optical
transmission systems so that transmission performance is improved
using a so-called "split-band" dispersion management scheme. The
split-band scheme is accomplished by providing select repeaters
that can divide the WDM channels into two (or more) bands. All the
WDM channels enter the first select repeater on a common fiber. The
select repeaters are followed by sections of transmission cable
having therein twice (or more) the usual number of optical fibers.
These cable sections allow the bands to experience differing
amounts of chromatic dispersion. The WDM channels are then
re-combined onto a common fiber. Since in this arrangement
differential dispersion is provided in an actual section of
transmission cable rather than in a repeater, it is particularly
efficient in terms of excess loss.
[0007] In one particular embodiment of the invention, a WDM optical
communication system is provided that includes a transmitter and a
receiver. An optical fiber transmission path couples the
transmitter to the receiver. The transmission path includes at
least one repeater having an optical amplifier located therein. A
dispersion compensator is disposed at an intermediate point along
the transmission path. The intermediate point is located outside of
the repeater. The compensator includes a wavelength routing device
for dividing a signal having a prescribed bandwidth into a
plurality of distinct sub-bands. A plurality of output paths is
provided for respectively receiving the plurality of distinct
sub-bands. The dispersion compensator also includes a dispersion
compensating optical element coupled to each of the output paths.
Each dispersion compensating optical element substantially
compensates for dispersion at a prescribed wavelength within the
bandpass of its respective sub-band. A coupler recombining the
distinct sub-bands and couples them back onto the optical fiber
transmission path.
BRIEF DESCRIPTION OF THE DRAWING
[0008] FIG. 1 shows a simplified block diagram of an optical fiber
transmission system in accordance with the present invention.
[0009] FIG. 2 shows a simplified block diagram of one embodiment of
the dispersion compensator shown in FIG. 1.
[0010] FIG. 3 shows a simplified block diagram of another
embodiment of the invention employing wavelength routing devices
FIG. 4 shows the wavelength allocations for a dispersion
compensator that divides the signal into two wavebands.
[0011] FIG. 5 shows a block diagram of the dispersion compensator
employed in FIG. 4.
[0012] FIG. 6 shows a particular embodiment of the dispersion
compensator of FIG. 5.
[0013] FIG. 7 shows the differential dispersion between the upper
and lower bands for the dispersion compensator shown in FIG. 6 as a
function of the length of true-wave fiber.
[0014] FIG. 8 shows an example of the dispersion map that may be
used for a transmission system employing the dispersion compensator
shown in FIG. 5.
DETAILED DESCRIPTION
[0015] FIG. 1 shows a simplified block diagram of an exemplary
optical fiber transmission system in accordance with the present
invention. The system includes an optical transmission path 100, a
transmitting terminal 101, and a receiving terminal 102. While only
a single transmission path is shown, one of ordinary skill in the
art will recognize that a second path may be employed to support
bi-directional communication. The transmitting terminal 101
provides an optical data signal that is to be transmitted to the
remote receiving terminal via the optical fiber transmission path
100. The optical signal presented by the terminal 101 to the
transmission path 100 may comprise a plurality of WDM optical
carriers each carrying an SDH signal. FIG. 1 shows a section of the
amplified transmission path consisting of optical amplifiers 103,
spans of transmission fiber 104, and dispersion compensator 105. In
a typical long-haul system, this series of components constituting
the dispersion map period might be repeated a number of times over
the length of the system. The optical amplifiers 103 maybe EDFAs,
for example, which amplify optical signals in the 1550 nm
wavelength band. In undersea communication systems a pair of such
optical amplifiers supporting opposite-traveling signals is housed
in a single unit known as a repeater. In one embodiment of the
invention the transmission fibers 104 may be dispersion-shifted
single-mode fibers with an average zero dispersion wavelength
higher than the operating wavelengths of the system. For example,
the transmission fiber in 104 may be similar to those used in
Bergano et al. (OFC'98 Post deadline paper PD12, San Jose Calif.,
Feb. 1998.), in which the transmission fiber had an average
dispersion of -2.5 ps/km-nm and a dispersion slope of about 0.09
ps/km-nm.sup.2. The fiber spans disclosed therein are a hybrid
combination of large mode fiber and True-Wave minus.TM.0 fiber
available from Lucent Technologies. This combination achieves an
adequate trade-off between a decrease in the nonlinear effects
resulting from using a large mode fiber and a reduction in
dispersion slope found in the large mode fiber.
[0016] A simple linearized chromatic dispersion relationship
between the signal wavelength .lambda..sub.sig and the dispersion D
is given in equation 1
D=SL(.lambda..sub.sig-.lambda..sub.0) (1)
[0017] where the dispersion D is measured in units of ps/nm, the
dispersion slope S is measured in units of ps/km-nm.sup.2, and the
average zero dispersion wavelength .lambda..sub.0 of the
transmission fiber is measured in units of nm. As equation 1
clearly indicates, the point of minimum dispersion only occurs at
one particular wavelength .lambda..sub.0. Accordingly, as disclosed
in U.S. Pat. No. 5,559,920, if a set of WDM channels were
transmitted along the transmission path 100, a dispersion
compensating fiber could only translate one channel back to the
zero dispersion wavelength. The remaining channels would accumulate
dispersion. As previously noted, this problem can be alleviated
with individual channel dispersion compensation at the receiver;
however, since these systems are subject to nonlinear penalty, the
ability to correct for the non-zero dispersion at the receiver
terminal is limited. Thus, to transmit the channels with low
dispersion penalty, there is an upper bound on the maximum amount
of accumulated dispersion that each channel can tolerate, which is
bit rate dependent. As the bit rate of each channel is increased,
the allowable amount of accumulated dispersion per channel is
reduced. This problem is overcome by the dispersion compensator
shown in FIG. 2.
[0018] FIG. 2 shows one embodiment of the chromatic dispersion
compensator 105 constructed in accordance with the present
invention. In operation, the dispersion compensator first splits
the bandwidth of the optical signals traversing the optical
amplifiers 103 into a series of bands, equalizes the dispersion of
each band individually, and finally recombines the signals onto a
common path for continued transmission. In FIG. 2, the signals
reach the compensator on fiber path 201 and enter an optional first
dispersion compensating fiber 202. The signals next enter a
1.times.N optical splitter 203, which divides the power of the
optical signal onto output paths 209.sub.1, 209.sub.2, 209.sub.3, .
. . 209.sub.N. The signals propagating along the N output paths
respectively enter optical band-pass filters 204.sub.1, 204.sub.2,
204.sub.3, . . . 204.sub.N with a center wavelength of
.lambda..sub.1, .lambda..sub.2, .lambda..sub.3, . . .
.lambda..sub.N, respectively. The optical bandpass filters 204
separate the usable bandwidth into N distinct bands. In a preferred
embodiment of the invention the wavelengths transmitted by the
bandpass filters 204 do not overlap one other and have sufficient
extinction in their stop-bands so that when the bands are
recombined in coupler 206, any interference effects will be
sufficiently small to avoid adversely impacting the system's
performance. The signals emerging from bandpass filters 204.sub.1,
204.sub.2, 204.sub.3, . . . 204.sub.N each enter a respective
dispersion equalizing fiber 205.sub.1, 205.sub.2, 205.sub.3, . . .
205.sub.N and possibly loss elements 208.sub.1, 208.sub.2,
208.sub.3, . . . 208.sub.N. The signals are subsequently recombined
in coupler 206 before exiting the dispersion compensator on fiber
207. The dispersion in each of the plurality of compensating fibers
205.sub.1, 205.sub.2, 205.sub.3, . . . 205.sub.N is selected so
that the average chromatic dispersion of the concatenated
transmission spans 104 upstream from the dispersion compensator 105
and the equalizing sections 202 and 205 are substantially returned
to zero at each of the center wavelengths .lambda..sub.N.
[0019] Compensating fiber 202 is optionally provided as a potential
cost saving step to perform any dispersion compensation that is
required by all of the N wavebands. For example, if the required
amount of dispersion compensation ranged from -1000 ps/nm for band
1 to -500 ps/nm for band N, equalizing fiber 202 advantageously may
provide -500 ps/nm of dispersion, which is required by each of the
N wavebands. Accordingly, the amount of equalizing dispersion
needed in the plurality of fibers 205.sub.1, 205.sub.2, 205.sub.3,
. . . 205.sub.N would range from -500 ps/nm to 0 ps/nm. The
equalizing fiber 202 may be directly incorporated into the
transmission path itself, thus yielding a significant cost savings.
For example, the equalizing fiber 202 may be an extension of the
cable defining the transmission path.
[0020] The dispersion compensation scheme shown in FIG. 2 is used
to equalize the differential dispersion over a plurality of N
wavebands, which results from the dispersion slope of the
transmission fiber. The differential dispersion needed to
accomplish this task is provided by the plurality of fibers
205.sub.i.
[0021] When provided, the loss elements 208.sub.1, 208.sub.2,
208.sub.3, . . . 208.sub.N facilitate the equalization of the gain
for the respective N wavebands. For example, an EDFA-based
transmission system may require some degree of gain equalization
when employed in wide-band applications. The loss elements
208.sub.1, 208.sub.2, 208.sub.3, . . . 208.sub.N may be selected to
equalize the received signal-to-noise ratio of the transmitted WDM
channels in the N wavebands.
[0022] In the embodiment of the invention shown in FIG. 2 the
equalizing elements 202 and 205 are single-mode fibers. Of course,
those of ordinary skill in the art will recognize that many other
optical devices may be employed to provide the necessary dispersion
compensation. For example, fiber diffraction gratings may be used
instead of single-mode fibers. One advantage accruing from the use
of a fiber diffraction grating is that the slope of the dispersion
characteristic as well as the dispersion itself may be
appropriately adjusted. If compensating elements 205 comprise
single-mode fibers having equal dispersions per unit length, the
resulting propagation delay for the different bands would be
different. If this posed any system problems, the propagation
delays could be equalized by constructing equalizing fibers having
differing dispersions per unit length. In this manner the required
dispersion compensation is provided while equalizing the
propagation length for all the bands. In addition, while it might
be advantageous to completely equalize the differential dispersion
caused by the dispersion slope of the transmission fiber, in some
circumstances it may be sufficient to only partially compensate for
this dispersion.
[0023] FIG. 3 shows an alternative embodiment of the invention in
which the functions performed by the optical splitter 203 and
bandpass filters 204 in the FIG. 2 embodiment are performed by a
wavelength routing device 303 such as disclosed in U.S. Pat. Nos.
5,002,350 and 5,412,744 to Dragone, for example. Similarly, the
optical coupler 206 also may be replaced with a wavelength routing
device 305. In FIG. 3, signals are directed to optional first
dispersion compensating fiber 302 on fiber 301 before entering
wavelength routing device 303. The wavelength routing device 303
divides the incoming signals into N output bands, which are each
directed to a respective dispersion equalizing fiber 304.sub.1,
304.sub.2, 304.sub.3, . . . 304.sub.N. The dispersion compensated
signals enter respective loss element 307.sub.1, 307.sub.2,
307.sub.3, . . . 307.sub.N (if employed) before being recombined in
wavelength routing device 305 and emerging on fiber 306.
[0024] FIG. 4 shows the allocation of wavelengths for an embodiment
of the invention in which a signal comprising sixteen WDM channels
is divided into two wavebands. FIG. 4 shows the lower and upper
wavebands 403 and 404 into which the signal is divided. Sixteen 10
Gb/s channels are divided into an "upper" band and a "lower" band.
The eight channels within each band are separated by 0.7 nm. A 2 nm
guard band is provided between the upper and lower bands, which
allows for a transition region in the multiplexing and
de-multiplexing devices.
[0025] FIG. 5 shows the structure of the two band dispersion
compensator that is used in connection with FIG. 4. This dispersion
compensator employs readily available optical fibers with different
dispersions. The different dispersion shifted optical fibers that
are currently commercially available include the so-called DCF
fiber, Z-fiber (i.e., pure silica core fiber), and True-Wave
minus.TM. fiber. The DCF fiber has a dispersion per unit length of
about -80 to -100 ps/km-nm, the Z-fiber has a dispersion per unit
length of about +19 ps/km-nm, and the true-wave fiber has a
dispersion per unit length of -2 ps/km-nm. Another parameter
characterizing the dispersion shifted fibers is a figure of merit
denoting the dispersion per unit of optical loss. For example, the
DCF fiber has a figure of merit greater than the figure of merit of
the Z-fiber (about -160 ps/nm/dB versus about 100 ps/nm/dB).
[0026] Referring again to FIG. 5, signals enter a common fiber 501.
The signals are then directed to wavelength routing device 503,
which divides the two wavebands onto fibers 504 and 505. The lower
band channels 1-8 are directed to equalizing fiber 507, which
comprises 43 km of Z-fiber and thus has a total dispersion of about
817 ps/nm (43 km.times.+19 ps/km-nm). The higher band channels 9-16
are directed on path 504 to equalizing fiber 506, which comprises
14.6 km of DCF fiber having a dispersion per unit length of -95
ps/km-nm. The total dispersion of equalizing fiber 506 is thus
-1387 ps/nm (14.6 km.times.-95 ps/km-nm). The high and low bands
are then recombined in wavelength dependent coupler 508 onto a
common path 509. It is anticipated that the extinction ratio of
wavelength dependent couplers 503 and 508 will be sufficiently
large such that signals traveling through unwanted paths would be
at a low enough level to minimize the impact of any interference
effects on the system's end-to-end performance.
[0027] Since as noted above the two types of dispersion equalizing
fiber have different figures of merit, the loss in equalizing fiber
506 may be different than the loss in equalizing fiber 507. As
previously mentioned, the loss between the upper and lower
wavebands may be equalized by inserting a separate loss element 510
in fiber 505. Alternatively, as in FIG. 5, the loss may be
equalized by increasing the length of equalizing fiber 507,
eliminating the need for a distinct loss element. This extra length
may, for example, be located in the repeater housing. This approach
is particularly advantageous because by increasing the length of
equalizing fiber 507, the differential dispersion between the lower
and upper wavebands is increased. Of course, the present invention
also contemplates the use of both a distinct loss element and an
increased length of the equalizing fiber having the smaller figure
of merit (e.g., equalizing fiber 507 in the FIG. 5 embodiment).
[0028] In some embodiments of the invention it may be advantageous
to incorporate the dispersion equalizer 105 shown in FIG. 1 into
the repeater which houses the adjacent optical amplifier 103 (or in
a bi-directional system an amplifier pair). In an undersea
communication system, this arrangement minimizes the number of
hermetically-sealed housings that must be deployed. In some
alternative embodiments of the invention, however, it may be
advantageous to deploy the dispersion equalizer 105 outside of the
repeater. When disposed outside the repeater, the dispersion
compensating fibers may function as a span of the transmission path
that contributes to traversing the distance between the
transmitting and receiving terminals. That is, the dispersion
compensating fibers provide additional path length. In contrast,
when the dispersion equalizer 105 is located within a repeater, it
simply adds excess length without covering any distance.
[0029] In some embodiments of the invention it may be advantageous
if the dispersion equalizing fiber located within a repeater
housing is arranged to traverse the housing at least twice, thus,
reducing the required fiber length. If such an arrangement were
employed, the dispersion compensating fiber would be surrounded by
a circulator and a Faraday rotator mirror. This would also have the
added benefit of reducing any residual polarization dependence in
the fiber, such as polarization mode dispersion.
[0030] Spans of the transmission path between repeaters typically
comprise optical fiber located in a cable sheath that serves to
protect the optical fibers and any electrical power lines that also
may be supported therein. When the dispersion equalizer is located
outside the repeaters, special sections of cable must be provided
that will accommodate the dispersion compensating fibers. The
special cable sections must be capable of housing N dispersion
compensating fibers corresponding to the N bands into which the
optical signal is divided by the dispersion equalizer.
[0031] One problem associated with the use of high negative
dispersion fiber is that such fiber inherently has a smaller
effective cross-sectional area than a negative dispersion fiber
having a smaller relative magnitude. Thus, DCF fiber (with a
dispersion of about -100 ps/km-nm) has a smaller cross-sectional
area than true-wave fiber (with a dispersion of about -2 ps/km-nm).
As a result, the power intensity will be greater in the DCF fiber
than in the true-wave fiber, thus increasing nonlinearities in the
DCF fiber.
[0032] In the embodiment of the invention shown in FIG. 5,
nonlinearities may be reduced by replacing a portion of DCF fiber
506 with true-wave fiber. FIG. 6 shows such an arrangement in which
the upper waveband traverses true-wave fiber 612 of length L1 and
DCF fiber 606 of length (L-L1) and the lower waveband traverses
Z-fiber 607. FIG. 7 shows the differential dispersion between the
upper and low wavebands as a function of the length L1 of true-wave
fiber 612. Clearly, the differential dispersion is maximized when
the length of true-wave fiber is zero and is minimized when the
length of the true-wave fiber is L (i.e., when no DCF fiber 606 is
employed).
[0033] Thus, true-wave fiber 612 may be used to reduce
nonlinearities, but at the expense of a decrease in the
differential dispersion.
[0034] The dispersion equalizer shown in FIG. 5 may be employed in
a transmission system that has 16 channels transmitting data at a
rate of 10 Gb/s. The channel spacing is 0.7 nm with a guard-band of
2 nm. Hybrid spans of fiber are used with an average dispersion
slope of 0.09 ps/km-nm-nm. The center of the upper and lower
wavebands are separated by 6.9 nm. Accordingly, the differential
dispersion between the two bands is (0.09 ps/km-nm-nm).times.6.9
nm=0.621 ps/km-nm. FIG. 7 shows that the maximum amount of
differential dispersion is 1980 ps/nm, and thus a single dispersion
equalizer can correct for dispersion in (1980 ps/nm)/(0.621
ps/km-nm)=3,188 km of the transmission path. Thus, in a system
about 9,600 km in length, three such dispersion equalizers may be
used, which could be positioned 1/4, 1/2, and 3/4 of the way along
the system. This would result in the center channels of the upper
and lower bands having zero end-to-end accumulated dispersion.
[0035] FIG. 8 shows an example of the dispersion map for the
previously mentioned transmission system. The figure shows the
accumulated dispersion for the center wavelength of the upper and
lower wavebands. Because of the fiber's non-zero dispersion slope,
the two wavelengths accumulate dispersion at different rates, thus
causing the spread in the accumulated dispersion seen in FIG. 8.
The period of the dispersion map is 500 km. The difference in
dispersion between the two wavelengths after a single period of the
dispersion map is 310 ps/nm (500 km.times.0.621 ps/km-nm).
Therefore, after each subsequent period of the dispersion map the
difference in dispersion between the two center wavelengths will
increase by 310 ps/mn.
[0036] In the previously described embodiments of the invention it
was assumed that the transmission fiber had a negative dispersion
and that the dispersion compensator had a positive dispersion. Of
course, those skilled in the art will recognize that the invention
alternatively could operate in connection with a transmission fiber
having a positive dispersion and a dispersion compensator that has
negative dispersion. Moreover, the invention is not limited to
signals arranged in an NRZ transmission format. For example, the
invention is also applicable to soliton transmission systems,
particularly those systems that use sliding frequency-guiding
jitter control. In such systems the optical powers should be
directly tied to the average chromatic dispersion. The dispersion
slope causes the different soliton WDM channels to operate at
different optical powers. Accordingly, the present invention may
advantageously equalize the optical powers by allowing the channels
to operate at similar values of chromatic dispersion. Other soliton
systems in which the invention may be advantageously employed
include a dispersion managed soliton system in which accumulated
jitter is minimized by periodically reducing dispersion with a
dispersion equalizer.
* * * * *