U.S. patent number RE38,289 [Application Number 09/834,558] was granted by the patent office on 2003-10-28 for chromatic dispersion compensation in wavelength division multiplexed optical transmission systems.
This patent grant is currently assigned to Tyco Telecommunications (US) Inc.. Invention is credited to Neal S. Bergano.
United States Patent |
RE38,289 |
Bergano |
October 28, 2003 |
Chromatic dispersion compensation in wavelength division
multiplexed optical transmission systems
Abstract
A method and apparatus is provided for managing dispersion in a
WDM optical transmission system so that transmission performance is
improved. The usable optical bandwidth of the transmission system
is divided into sub-bands that individually undergo dispersion
compensation before being re-combined. Accordingly, in comparison
to known dispersion mapping techniques, more WDM data channels
reside near a wavelength corresponding to the average zero
dispersion wavelength.
Inventors: |
Bergano; Neal S. (Lincroft,
NJ) |
Assignee: |
Tyco Telecommunications (US)
Inc. (Morristown, NJ)
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Family
ID: |
25055855 |
Appl.
No.: |
09/834,558 |
Filed: |
April 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
759493 |
Dec 4, 1996 |
06137604 |
Oct 24, 2000 |
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Current U.S.
Class: |
398/75; 385/122;
385/24 |
Current CPC
Class: |
H04B
10/2519 (20130101); H04B 10/2525 (20130101); G02B
6/2932 (20130101); G02B 6/29394 (20130101); H04B
2210/258 (20130101); H04J 14/02 (20130101) |
Current International
Class: |
H04B
10/18 (20060101); H04J 014/02 () |
Field of
Search: |
;359/124,130,134,161,173,179,188 ;385/122,24 |
References Cited
[Referenced By]
U.S. Patent Documents
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5467213 |
November 1995 |
Kaede et al. |
5696614 |
December 1997 |
Ishikawa et al. |
6181449 |
January 2001 |
Taga et al. |
|
Foreign Patent Documents
Other References
RW. Tkach et al., "Four-Photon Mixing and High-Speed WDM Systems",
Journal of Lightwave Technology, vol. 13, No. 5, May 1995, pp.
841-849.* .
21.sup.st European Conference on Optical Communications, Brussels,
Belgium, paper Th. A.3.1, Sep. 1995..
|
Primary Examiner: Negash; Kinfe-Michael
Claims
What is claimed is:
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 optical amplifier; a dispersion compensator disposed at
an intermediate point along said transmission path, said
compensator including: an optical splitter for dividing a signal
introduced therein onto a plurality of optical paths, said signal
having a prescribed bandwidth; a bandpass filter disposed along
each of said optical paths, said filters dividing the prescribed
bandwidth of the signal into a plurality of distinct sub-bands; a
dispersion compensating element coupled to each of the bandpass
filters, said dispersion compensating optical elements each
substantially compensating for dispersion at a prescribed
wavelength within the bandpass of its respective bandpass filter; a
coupler for recombining said distinct sub-bands and coupling said
recombined distinct sub-bands onto said optical fiber transmission
path.
2. The communication system of claim 1 wherein said plurality of
sub-bands are substantially non-overlapping in wavelength.
3. The system of claim 1 wherein said dispersion compensating
elements are single-mode optical fibers.
4. The system of claim 1 wherein said dispersion compensating
elements are fiber diffraction gratings.
5. The system of claim 4 wherein said fiber diffraction gratings
are chirped gratings.
6. The system of claim 5 wherein said gratings are linearly
chirped.
7. The system of claim 5 wherein said gratings are quadratically
chirped.
8. The system of claim 1 wherein said signal is a soliton
signal.
9. The system of claim 1 further comprising an additional
dispersion compensating element preceding said optical splitter for
providing a common amount of dispersion to all of the
sub-bands.
10. The system of claim 9 wherein said additional dispersion
compensating element is an extension of the fiber transmission
path.
11. The system of claim 1 further comprising a plurality of loss
elements disposed in said optical paths to provide gain
equalization to the sub-bands.
12. The system of claim 1 wherein said prescribed wavelengths are
substantially equal to center wavelengths of the respective
bandpass filters.
13. The system of claim 12 wherein said prescribed wavelengths are
offset from the respective center wavelengths by a predetermined
amount.
14. The system of claim 4 wherein said plurality of optical paths
are defined by reflections from different fiber gratings.
15. A dispersion compensator for use in a WDM optical communication
system that includes a transmitter, receiver, and an optical fiber
transmission path coupling said transmitter to said receiver, said
transmission path including at least one optical amplifier, said
dispersion compensator comprising: an optical splitter for dividing
a signal introduced therein onto a plurality of optical paths, said
signal having a prescribed bandwidth, said optical splitter being
adapted to receive said signal from an intermediate point along
said optical fiber transmission path; a bandpass filter disposed
along each of said optical paths, said filters dividing the
prescribed bandwidth of the signal into a plurality of distinct
sub-bands; a dispersion compensating optical element coupled to
each of the bandpass filters, said dispersion compensating optical
elements each substantially compensating for dispersion at a
prescribed wavelength within the bandpass of its respective
bandpass filter; a coupler for recombining said distinct sub-bands,
said coupler being adapted to couple said recombined distinct
sub-bands onto said optical fiber transmission path.
16. The dispersion compensator of claim 15 wherein said plurality
of sub-bands are substantially non-overlapping in wavelength.
17. The dispersion compensator of claim 15 wherein said dispersion
compensating elements are single-mode optical fibers.
18. The dispersion compensator of claim 15 wherein said dispersion
compensating elements are fiber diffraction gratings.
19. The dispersion compensator of claim 18 wherein said fiber
diffraction gratings are chirped gratings.
20. The dispersion compensator of claim 19 wherein said gratings
are linearly chirped.
21. The dispersion compensator of claim 19 wherein said gratings
are quadratically chirped.
22. The dispersion compensator of claim 15 wherein said signal is a
soliton signal.
23. The dispersion compensator of claim 15 further comprising an
additional dispersion compensating element preceding said optical
splitter for providing a common amount of dispersion to all of the
sub-bands.
24. The system of claim 23 wherein said additional dispersion
compensating element is an extension of the fiber transmission
path.
25. The dispersion compensator of claim 15 further comprising a
plurality of loss elements disposed in said optical paths to
provide gain equalization to the sub-bands.
26. The dispersion compensator of claim 15 wherein said prescribed
wavelengths are substantially equal to center wavelengths of the
respective bandpass filters.
27. The dispersion compensator of claim 26 wherein said prescribed
wavelengths are offset from the respective center wavelengths by a
predetermined amount.
28. The dispersion compensator of claim 18 wherein said plurality
of optical paths are defined by reflections from different fiber
gratings.
29. A method for compensating for dispersion in a WDM optical
communication system that includes a transmitter, receiver, and an
optical fiber transmission path coupling said transmitter to said
receiver, said method comprising the steps of: splitting a signal
at an intermediate point along the transmission path to be directed
onto a plurality of optical paths, said signal having a prescribed
bandwidth; filtering the signals along each of said optical paths
to divide the prescribed bandwidth of the signal into a plurality
of distinct sub-bands; compensating for dispersion at a prescribed
wavelength within the distinct sub-bands; recombining said distinct
sub-bands and directing said recombined distinct sub-bands onto
said optical fiber transmission path.
30. The method of claim 29 wherein said plurality of sub-bands are
substantially non-overlapping in wavelength.
31. The method of claim 29 wherein the step dispersion compensation
is accomplished with single-mode optical fibers.
32. The method of claim 29 wherein the step of dispersion
compensation is accomplished with fiber diffraction gratings.
33. The method of claim 32 wherein said fiber diffraction gratings
are chirped gratings.
34. The method of claim 33 wherein said gratings are linearly
chirped.
35. The method of claim 33 wherein said gratings are quadratically
chirped.
36. The method of claim 29 wherein said signal is a soliton
signal.
37. The method of claim 29 further comprising the step of
compensating for a select amount of dispersion prior to the step of
filtering the signals.
38. The method of claim 37 wherein said select amount of dispersion
compensation is provided along a portion of said fiber transmission
path.
39. The method of claim 29 further comprising the step of
introducing loss along the optical paths to provide gain
equalization to the sub-bands.
40. The method of claim 29 wherein said prescribed wavelengths are
substantially equal to center wavelengths of respective bandpass
filters employed to perform the step of filtering the signals.
41. The method of claim 29 wherein said prescribed wavelengths are
offset from the respective center wavelengths by a predetermined
amount.
42. The method of claim 32 wherein said plurality of optical paths
are defined by reflections from different fiber gratings.
43. A WDM optical communications 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 optical amplifier; a dispersion compensator disposed at
an intermediate point along said transmission path, 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.
44. The communication system of claim 43 wherein said plurality of
sub-bands are substantially non-overlapping in wavelength.
45. The system of claim 43 wherein said dispersion compensating
elements are single-mode optical fibers.
46. The system of claim 43 wherein said dispersion compensating
elements are fiber diffraction gratings.
47. The system of claim 46 wherein said fiber diffraction gratings
are chirped gratings.
48. The system of claim 47 wherein said gratings are linearly
chirped.
49. The system of claim 47 wherein said gratings are quadratically
chirped.
50. The system of claim 43 wherein said signal is a soliton
signal.
51. The system of claim 43 further comprising an additional
dispersion compensating element preceding said wavelength routing
device for providing a common amount of dispersion to all of the
sub-bands.
52. The system of claim 51 wherein said additional dispersion
compensating element is an extension of the fiber transmission
path.
53. The system of claim 43 further comprising a plurality of loss
elements disposed in said optical paths to provide gain
equalization to the sub-bands.
54. The system of claim 43 wherein said prescribed wavelengths are
substantially equal to center wavelengths of the respective
bandpass filters.
55. The system of claim 54 wherein said prescribed wavelengths are
offset from the respective center wavelengths by a predetermined
amount.
56. The system of claim 46 wherein said plurality of optical paths
are defined by reflections from different fiber gratings.
57. The system of claim 43 wherein said coupler comprises a second
wavelength routing device.
58. 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 optical amplifier; a dispersion compensator disposed at
an intermediate point along said transmission path, said
compensator including: a circulator having at least an input port
for receiving optical signals from the transmission path, an output
port for transmitting optical signals onto the transmission path,
and a third port; a wavelength routing device coupled to said third
port of the circulator 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 Faraday rotator mirror coupled to each of the
dispersion compensating optical elements.
59. The communication system of claim 58 wherein said plurality of
sub-bands are substantially non-overlapping in wavelength.
60. The system of claim 58 wherein said dispersion compensating
elements are single-mode optical fibers.
61. The system of claim 58 wherein said signal is a soliton
signal.
62. The system of claim 58 further comprising an additional
dispersion compensating element preceding said wavelength routing
device for providing a common amount of dispersion to all of the
sub-bands.
63. The system of claim 62 wherein said additional dispersion
compensating element is an extension of the fiber transmission
path.
64. The system of claim 58 further comprising a plurality of loss
elements disposed in said optical paths to provide gain
equalization to the sub-bands.
65. The system of claim 58 wherein said prescribed wavelengths are
substantially equal to center wavelengths of the respective
bandpass filters.
66. The system of claim 65 wherein said prescribed wavelengths are
offset from the respective center wavelengths by a predetermined
amount..Iadd.
67. A dispersion compensator comprising: a plurality of optical
paths configured to carry a wavelength division multiplexed optical
signals having a prescribed bandwidth; a bandpass filter disposed
along each of said optical paths, said filters dividing the
prescribed bandwidth of the signal into a plurality of distinct
sub-bands; and a dispersion compensating element coupled to each of
said bandpass filters, said dispersion compensating optical
elements each substantially compensating for dispersion at a
prescribed wavelength within the bandpass of its respective
bandpass filter..Iaddend..Iadd.
68. The dispersion compensator in accordance with claim 67 wherein
said prescribed wavelengths are substantially equal to center
wavelengths of the respective bandpass filters..Iaddend..Iadd.
69. The dispersion compensator in accordance with claim 67 wherein
said dispersion compensating elements are single-mode optical
fibers..Iaddend..Iadd.
70. The dispersion compensator in accordance with claim 67 wherein
said plurality of sub-bands are substantially non-overlapping in
wavelength..Iaddend..Iadd.
71. A method for compensating for dispersion in a wavelength
division multiplexed optical communication system that includes
optical signal having a prescribed bandwidth and a plurality of
optical paths configured to carry said optical signal, said method
comprising the steps of: filtering the signals along each of said
optical paths to divide the prescribed bandwidth of the signal into
a plurality of distinct sub-bands; and compensating for dispersion
at a prescribed wavelength within the distinct
sub-bands..Iaddend..Iadd.
72. The method for compensating for dispersion in accordance with
claim 71 wherein said prescribed wavelengths are substantially
equal to center wavelengths of respective bandpass filters employed
to perform the step of filtering the signals..Iaddend..Iadd.
73. The method for compensating for dispersion in accordance with
claim 71 wherein said prescribed wavelengths are offset from
respective center wavelengths with the distinct sub-bands by a
predetermined amount..Iaddend..Iadd.
74. The method for compensating for dispersion in accordance with
claim 71 wherein said plurality of sub-bands are substantially
non-overlapping in wavelength..Iaddend.
Description
FIELD OF THE INVENTION
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
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
system. 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.
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. As the bit rates increase through
the gigabit per second range, the optical powers launched into the
transmission fiber need to approach 1 mW per channel. As was
demonstrated by Bergano et al. (European Conference on Optical
Communications, Brussels, Belgium, paper Th.A. 3.1 Sept. (1995) the
Non-Return-to-Zero (NRZ) transmission format is over optically
amplified fiber paths. However, NRZ channels operating over long
distances require sufficient control over the total amount of
chromatic dispersion to ensure low dispersion penalties.
Accordingly, the preferred transmission medium for such a system is
dispersion shifted optical fibers.
Crosstalk, or the mixing of channels through the slight
nonlinearity in the transmission fiber, may arise from the
combination of long distance, low dispersion and high channel
power. The transmission of many WDM channels over transoceanic
distances may be limited by nonlinear interactions between
channels, which in turn is affected by the amount of dispersion.
This subject was reviewed by Tkach et al. (Journal of Lightwave
Technology in Vol. 13, No. 5, May 1995 pp. 841-849). As discussed
in Tkach et al., this problem may be overcome by a technique known
as dispersion mapping, in which the generation of mixing products
is reduced by offsetting the zero dispersion wavelengths of the
transmitter. This technique employs a series of amplifier sections
having dispersion shifted fiber spans with either positive or
negative dispersion. The dispersion accumulates over multiple fiber
spans of approximately 500 to 1000 km. The fiber spans of either
positive or negative sign are followed by a dispersion-compensating
fiber having dispersion of the opposite sign. This subsequent
section of fiber is sufficient to reduce the average dispersion
(averaged over the total length of the transmission system)
substantially to zero. That is, a fiber of high negative (positive)
dispersion permits compensation by a length of positive (negative)
transmission fiber.
The efficacy of the dispersion mapping technique 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. The
wavelength dependence of the dispersion coefficient is sometimes
referred to as the dispersion slope of the fiber. 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.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method and apparatus is
provided for managing dispersion in a WDM optical transmission
system so that transmission performance is improved. In accordance
with the inventive method, the usable optical bandwidth of the
transmission system is divided into sub-bands that individually
undergo dispersion compensation before being re-combined.
Accordingly, in comparison to known dispersion mapping techniques,
more WDM data channels reside near a wavelength corresponding to
the average zero dispersion wavelength.
In one embodiment of the invention, a WDM optical communication
system is provided that includes a transmitter, receiver, an
optical fiber transmission path coupling the transmitter to the
receiver, and at least one optical amplifier. A dispersion
compensator, which is disposed at an intermediate point along the
transmission path, includes an optical splitter for dividing a
signal introduced therein onto a plurality of optical paths. The
signal has a prescribed bandwidth. A bandpass filter is disposed
along each of the optical paths and divides the prescribed
bandwidth of the signal into a plurality of distinct sub-bands. A
dispersion compensating element is coupled to each of the bandpass
filters. The dispersion compensating optical elements each
substantially compensate for dispersion at a prescribed wavelength
within the bandpass of its respective bandpass filter. A coupler is
employed to recombine the distinct sub-bands and couple the
recombined distinct sub-bands onto the optical fiber transmission
path.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows a simplified block diagram of an optical fiber
transmission system in accordance with the present invention.
FIG. 2 shows a simplified block diagram of one embodiment of the
dispersion compensator shown in FIG. 1.
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.
FIG. 5 shows a block diagram of the dispersion compensator employed
in FIG. 4.
FIG. 6 shows the accumulated dispersion versus wavelength for a
system utilizing the dispersion compensator shown in FIG. 4.
FIG. 7 shows an alternative embodiment of the dispersion
compensator which employs chirped fiber gratings.
FIG. 8 shows the propagation delay of the signal reflected off of
the two fiber grating filters shown in FIG. 7.
FIG. 9 shows an alternative embodiment of the invention that
employs Faraday rotator mirror reflectors.
DETAILED DESCRIPTION
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. 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 carries each carrying an SDH signal. FIG. 1 shows a
single period of the dispersion map 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 may be EDFAs, for example, which amplify optical
signals in the 1550 nm wavelength band. 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 fibers 104 may be similar to those used in Bergano
et al. (European Conference on Optical Communications, Brussels,
Belgium, paper Th.A.3.1, Sept. 1995), in which the transmission
fiber had an average zero dispersion wavelength of 1580 nm and a
dispersion slope of about 0.073 ps/km-nm.sup.2.
A simple linearized chromatic dispersion relationship between the
signal wavelength .lambda..sub.sig and the dispersion D is given in
equation 1:
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.
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.sup.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.sup.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.
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.
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 required 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.
In the embodiment of the invention shown in FIG. 2 the equalizing
elements 202 and 205 are signal-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.
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.
FIG. 4 shows the allocation of wavelengths for an embodiment of the
invention in which a signal comprising ten WDM channels is divided
into two wavebands. FIG. 4 shows the lower and upper wavebands 403
and 404 into which the signal is divided. This example assumes that
the transmission fibers 104 have a nominal zero dispersion
wavelength of 1580 nm and a dispersion slope of 0.073
ps/km-nm.sup.2. The dispersion compensators are assumed to be
spaced at 500 km intervals along the transmission path of the
transmission system. The lower and upper wavebands are centered at
1552 nm and 1559 nm, respectively. Within each waveband the channel
spacing is 1 nm and the wavelength spacing between the bands (i.e.
between channels 5 and 6) is 3 nm.
FIG. 5 shows the structure of the two band dispersion compensator
that is used in connection with FIG. 4. Signals enter a common
compensating fiber 502, which is 45 km in length, on optical fiber
501. The average chromatic dispersion of fiber 502 is about 17
ps/km-nm, which is typical of a conventional step-index single-mode
fiber with a center wavelength .lambda..sub.o of 1310 nm.
Accordingly, the total dispersion in this fiber is about 765 ps/nm
(or 45 km.times.17 ps/km-nm). The signals are then directed to
wavelength routing device 503 and the two wavebands are separated
onto fibers 504 and 505. The low band channels 1-5 are directed to
equalizing fiber 506, which is 15 km long with a dispersion of 255
ps/nm (15 km.times.17 ps/km-nm). The high band channels 6-10 are
directed on path 505 to attenuator 507, which has approximately the
same attenuation as fiber 506, which would be about 3 dB for a
typical fiber. However, it is anticipated that attenuator 507 could
alternatively have a different value that would aid in equalizing
the average gains for the low and high bands, considering the
non-equal gains in the EDFA repeaters. 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 made 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.
FIG. 6 shows the accumulated chromatic dispersion versus wavelength
over a 9000 km system that employs the dispersion compensator shown
in FIG. 5. The maximum accumulated dispersion for all 10 channels
is 1310 ps/nm, as indicated by the vertical lines crossing
dispersion characteristic 601 for the low band and dispersion
characteristic 602 for the high band. Therefore, the two band
dispersion compensator reduces by over 50% (relative to known
dispersion compensators) the maximum amount of dispersion
experienced by the outermost WDM channels.
FIG. 7 shows another alternative embodiment of the two band
dispersion compensator shown in FIG. 5. In this case chirped fiber
gratings are provided, which perform both band selection, and at
least part of the requisite dispersion compensation. In FIG. 7, the
signal, which comprises a plurality of WDM channels, is directed to
a common compensating fiber 702 on fiber 701. In one particular
embodiment of the invention compensating fiber 702 is about 52.6 km
in length. Fiber 702 has an average chromatic dispersion of about
17 ps/km-nm, which is typical of conventional step-index
single-mode fiber with a center wavelength .lambda..sub.o of 1310
nm. Accordingly, the total dispersion in this fiber is about 894
ps/nm (or 52.6 km.times.17 ps/km-nm). Fiber 702 shifts the mean
zero dispersion wavelength from its value of 1580 nm arising in the
fiber preceding fiber 701 to a wavelength of 1555.5 nm. This
wavelength denotes the cross-over point indicated in FIG. 4, which
separates the lower from the upper waveband. The WDM channels enter
input port 710 of a three port circulator 703, which, for example,
may be similar to the device provided by JDS Fitel Inc. (570 West
Hunt Club Road, Nepean, Ontario, Canada K2G 5W8) under model number
CR2500. The WDM channels exit the circulator 703 on output port 720
of circulator 703 and enter the first chirped fiber grating filter
704. Fiber grating filters 704 and 705 are linearly chirped
gratings that reflect signals over different wavelength bands.
Fibers of this type have been described by Cole et al. in
"Continuously chirped, broadband dispersion-compensating fiber
gratings in a 10 Gb/s 110 km standard fiber link," presented at the
22.sup.nd European Conference on Optical Communication, paper
ThB.3.5. The reflection from fiber grating 704 provides a constant
amplitude response for channels 1-5 (as denoted in FIG. 4) and a
constant dispersion of +128 ps/nm. The reflection from fiber
grating 705 provides a constant amplitude response for channels
6-10 (as denoted in FIG. 4) and a constant dispersion of -128
ps/nm. Since fiber grating filter 704 is reflective only for
channels 1-5, channels 6-10 will be transmitted therethrough with
minimal attenuation. The reflected channels 1-10 enter circulator
703 on port 720 and exit port 730 onto fiber 706. The dispersion
characteristic that results over a system length of 9,000 km is the
same as shown in FIG. 6. The common compensating fiber 702 is used
to reduce the stringency of the design requirements for fiber
gratings 704 and 705. However, the dispersion characteristics of
these gratings alternatively may include the dispersion
compensation imparted by fiber 702. For example, a comparable
amount of dispersion compensation can be achieved by removing fiber
702 and specifying the dispersion in fiber gratings 704 and 705 to
be .sup.+ 1022 ps/nm and .sup.+ 767 ps/nm, respectively. Of course,
one of ordinary skill will recognize that the embodiment of the
invention shown in FIG. 7 can be extended to accommodate a
multiplicity of wavebands by adding additional fiber gratings.
The embodiments of the invention described prior to FIG. 7 do not
include any provision for correcting the dispersion slope. However,
the embodiment shown in FIG. 7 can correct for both the dispersion
and the slope of the dispersion. That is, the total accumulated
dispersion for all channels can be reduced essentially to zero.
This is achieved by quadratically chirping rather than linearly
chirping the fiber gratings 704 and 705 so that the delay in the
dispersion compensator is approximately equal in magnitude but
opposite in sign to the delay in the fiber span, as shown in FIG.
8. FIG. 8 shows the relative propagation delay of the reflected
signal from two fiber grating filters designed to flatten the
dispersion over a wide waveband when used in a dispersion
compensator similar to that shown in FIG. 7. In FIG. 8 the delay
characteristic is parabolic to correct for the fiber's chromatic
dispersion over a substantial part of the required waveband.
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.
FIG. 9 shows an alternative embodiment of the dispersion
compensator shown in FIG. 3, in which the functions of splitting
and recombining the signal are performed by a wavelength routing
device. In this embodiment, Faraday rotator mirror reflectors are
employed so that the optical signals traverses the wavelength
routing device, dispersion compensating elements, and the
attenuators on two occasions. This arrangement may be advantageous
because it requires only a single routing device and because the
deleterious effects of polarization dependence in the optical
components can be reduced.
In operation, the optical signal is received by the compensator on
input fiber 901 and enters the common compensating fiber 902, if
provided. The signal next enters port 910 of a three port
circulator 903, exits on port 920 of circulator 903, and is
received by wavelength routing device 904. The wavelength routing
device 904 divides the signal into a plurality of output bands
which are directed along distinct optical paths to a respective one
of the dispersion compensating fibers 905.sub.1, 905.sub.2,
905.sub.3, . . . 905.sub.N and loss bands traverse the dispersion
compensating fibers 905.sub.1, 905.sub.2, 905.sub.3, . . .
905.sub.N and loss elements 906.sub.1, 906.sub.2, 906.sub.3, . . .
906.sub.N, respectively, can be half of that imparted by the
corresponding components in the embodiment of FIG. 3. The optical
bands each enter a Faraday rotator mirror 907, which reflects the
bands back on themselves with a state-of-polarization that is
orthogonal to its respective input state. The Faraday rotator
mirror may be, for example, of the type supplied by E-TEK Dynamics,
Inc. (1885 Lundy Ave., San Jose, Calif. 95131) as model HSFM.
One of ordinary skill in the art will recognize that when an
optical element such as the wavelength routing device is placed
between a circulator and a Faraday rotator mirror, the polarization
dependence of the optical element is effectively removed (or at
least substantially reduced when observed from the input and output
of the circulator). Accordingly, the embodiment of the invention
shown in FIG. 9 advantageously allows the use of a wavelength
router having less stringent polarization dependent loss
specifications.
The optical bands reflected by mirrors 907 once again traverse loss
elements 906.sub.1, 906.sub.2, 906.sub.3, . . . 906.sub.N and
dispersion compensating elements 905.sub.1, 905.sub.2, 905.sub.3, .
. . 905.sub.N and are recombined onto a single fiber in the routing
device 904. The recombined signal enters port 920 of circulator 903
and is returned to the transmission path of the system on fiber 908
by port 930 of circulator 903.
It should be appreciated by those skilled in the art that the
common compensating fiber 902 may be alternatively located between
port 920 of circulator 903 and the wavelength routing device 904.
In this case the signal would traverse the compensating fiber
twice, thus only requiring it to impart half the dispersion that
would be required by the configuration shown in FIG. 9.
* * * * *