U.S. patent application number 09/300595 was filed with the patent office on 2002-01-31 for optical transmission path having sections which overcompensate for dispersion occurring in the sections.
Invention is credited to NAITO, TAKAO, TANAKA, TOSHIKI.
Application Number | 20020012162 09/300595 |
Document ID | / |
Family ID | 14930992 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020012162 |
Kind Code |
A1 |
TANAKA, TOSHIKI ; et
al. |
January 31, 2002 |
OPTICAL TRANSMISSION PATH HAVING SECTIONS WHICH OVERCOMPENSATE FOR
DISPERSION OCCURRING IN THE SECTIONS
Abstract
An optical communication system which includes a transmission
path through which a light is transmitted to a specific point, such
as to a receiver. The transmission path includes a plurality of
sections so that the light travels through the sections to the
specific point. Each section overcompensates for dispersion
produced in the respective section for the light so that an amount
of dispersion for the light at the specific point is substantially
zero.
Inventors: |
TANAKA, TOSHIKI; (TOKYO,
JP) ; NAITO, TAKAO; (KAWASAKI-SHI, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
700 11TH STREET, NW
SUITE 500
WASHINGTON
DC
20001
US
|
Family ID: |
14930992 |
Appl. No.: |
09/300595 |
Filed: |
April 28, 1999 |
Current U.S.
Class: |
359/341.1 |
Current CPC
Class: |
H04B 10/25253
20130101 |
Class at
Publication: |
359/341.1 |
International
Class: |
H01S 003/00; H04B
010/12 |
Foreign Application Data
Date |
Code |
Application Number |
May 8, 1998 |
JP |
10-126268 |
Claims
What is claimed is:
1. An optical communication system comprising: a transmission path
through which a light is transmitted to a specific point, the
transmission path including a plurality of sections so that the
light travels through the sections to the specific point, each
section overcompensating for dispersion produced in the respective
section for the light so that an amount of dispersion for the light
at the specific point is substantially zero.
2. An optical communication system as in claim 1, further
comprising: a transmitter; and a receiver at the specific point,
the transmitter transmitting the light to the receiver through the
transmission path.
3. An optical communication system as in claim 1, wherein the light
includes at least one wavelength.
4. An optical communication system as in claim 1, wherein the light
is a wavelength division multiplexed signal.
5. An optical communication system as in claim 1, wherein the light
includes a plurality of different wavelengths including a center
wavelength, and each section overcompensates for the dispersion
produced in the respective section for light at the center
wavelength so that the amount of dispersion for the light at the
center wavelength at the specific point is substantially zero.
6. An optical communication system as in claim 1, further
comprising: a plurality of optical amplifiers arranged along the
transmission path in equal intervals.
7. An optical communication system as in claim 1, wherein each
section comprises: a plurality of optical amplifiers arranged along
the transmission path; and a dispersion compensator
overcompensating for dispersion produced in the section.
8. An optical communication system as in claim 7, wherein the
optical amplifiers in each section are arranged along the
transmission path in equal intervals.
9. An optical communication system as in claim 1, wherein each
section comprises: a plurality of optical amplifiers arranged along
the transmission path in equal intervals from a first optical
amplifier to a last optical amplifier; and a dispersion compensator
overcompensating for dispersion produced in the section.
10. An optical communication system as in claim 6, wherein the
amount of overcompensation of each section is changeable without
changing the positions of the optical amplifiers.
11. An optical communication system as in claim 7, wherein, for
each section, the amount of overcompensation is changeable without
changing the positions of the optical amplifiers in the
section.
12. An optical communication system as in claim 1, wherein each
section includes a cable providing dispersion to the light as the
light travels through the cable, to thereby cause the section to
overcompensate for dispersion produced in the section.
13. An optical communication system as in claim 1, wherein each
section includes a cable selectable from the group consisting of a
cable having a dispersion generating fiber through which the light
travels, a cable having a dispersion compensating fiber through
which the light travels, and a cable having a dispersion generating
fiber and a dispersion compensating fiber connected together so
that the light travels through both the dispersion generating fiber
and the dispersion compensating fiber, to vary the amount of
dispersion compensation provided by the section.
14. An optical communication system as in claim 12, wherein the
cable includes a plurality of fibers providing different amounts of
dispersion and which are individually selectable so that the light
travels through the selected fiber, the cable thereby allowing the
amount of overcompensation provided by the section to be
selected.
15. An optical communication system as in claim 14, wherein a fiber
of the cable is selectable during installation of the system.
16. An optical communication system as in claim 12, wherein the
cable is a multi-core cable having individually selectable cores so
that the light travels through the selected core, the cable thereby
allowing the amount of overcompensation provided by the section to
be selected.
17. An optical communication system as in claim 1, wherein when the
transmission path provides a positive dispersion to the light, each
section overcompensates for the dispersion produced in the
respective section by adding a negative dispersion to the light,
and when the transmission path provides a negative dispersion to
the light, each section overcompensates for the dispersion produced
in the respective section by adding a positive dispersion to the
light.
18. An optical communication system as in claim 1, wherein, in
addition to travelling through the plurality of sections, the light
travels through an additional portion of the transmission path to
reach the specific point.
19. An optical communication system as in claim 1, wherein, in
addition to travelling through the plurality of sections, the light
travels through an additional portion of the transmission path to
reach the specific point, the total amount of overcompensation
provided by the plurality of sections being substantially equal to
the amount of dispersion produced in said additional portion.
20. An optical communication system as in claim 7, wherein the
dispersion compensator is a variable dispersion compensator which
is controllable to vary the amount of overcompensation.
21. An optical communication system comprising: a transmission path
through which a light is transmitted to a specific point, the
transmission path including a plurality of sections so that the
light travels through the sections to the specific point, each
section overcompensating for dispersion produced in the respective
section for the light to control the amount of dispersion for the
light at the specific point.
22. An optical communication system as in claim 21, further
comprising: a transmitter; and a receiver at the specific point,
the transmitter transmitting the light to the receiver through the
transmission path.
23. An optical communication system as in claim 21, wherein the
light includes a plurality of different wavelengths including a
center wavelength, and each section overcompensates for the
dispersion produced in the respective section for light at the
center wavelength so that the amount of dispersion for the light at
the center wavelength at the specific point is substantially
zero.
24. An optical communication system as in claim 21, wherein each
section comprises: a plurality of optical amplifiers arranged along
the transmission path; and a dispersion compensator
overcompensating for dispersion produced in the section.
25. An optical communication system as in claim 21, wherein each
section comprises: a plurality of optical amplifiers arranged along
the transmission path in equal intervals from a first optical
amplifier to a last optical amplifier; and a dispersion compensator
overcompensating for dispersion produced in the section.
26. An optical communication system as in claim 25, wherein the
dispersion compensator is a variable dispersion compensator which
is controllable to vary the amount of overcompensation.
27. An optical communication system as in claim 21, wherein when
the transmission path provides a positive dispersion to the light,
each section overcompensates for the dispersion produced in the
respective section by adding a negative dispersion to the light,
and when the transmission path provides a negative dispersion to
the light, each section overcompensates for the dispersion produced
in the respective section by adding a positive dispersion to the
light.
28. An optical communication system as in claim 21, wherein, in
addition to travelling through the plurality of sections, the light
travels through an additional portion of the transmission path to
reach the specific point, the total amount of overcompensation
provided by the plurality of sections being substantially equal to
the amount of dispersion produced in said additional portion.
29. An optical communication system comprising: a transmission path
through which a light is transmitted to a specific point, the
transmission path including a plurality of sections so that the
light travels through the sections to the specific point, the
plurality of sections together overcompensating for dispersion
produced in the sections for the light to reduce the total amount
of dispersion for the light at the specific point.
30. An optical communication system as in claim 29, further
comprising: a transmitter; and a receiver at the specific point,
the transmitter transmitting the light to the receiver through the
transmission path.
31. An optical communication system as in claim 29, wherein the
light includes a plurality of different wavelengths including a
center wavelength, and each section overcompensates for the
dispersion produced in the respective section for light at the
center wavelength so that the total amount of dispersion for the
light at the center wavelength at the specific point is
substantially zero.
32. An optical communication system as in claim 29, wherein each
section comprises: a plurality of optical amplifiers arranged along
the transmission path; and a dispersion compensator
overcompensating for dispersion produced in the section.
33. An optical communication system as in claim 29, wherein each
section comprises: a plurality of optical amplifiers arranged along
the transmission path in equal intervals from a first optical
amplifier to a last optical amplifier; and a dispersion compensator
overcompensating for dispersion produced in the section.
34. An optical communication system as in claim 33, wherein the
dispersion compensator is a variable dispersion compensator which
is controllable to vary the amount of overcompensation.
35. An optical communication system as in claim 29, wherein when
the transmission path provides a positive dispersion to the light,
each section overcompensates for the dispersion produced in the
respective section by adding a negative dispersion to the light,
and when the transmission path provides a negative dispersion to
the light, each section overcompensates for the dispersion produced
in the respective section by adding a positive dispersion to the
light.
36. An optical communication system as in claim 29, wherein, in
addition to travelling through the plurality of sections, the light
travels through an additional portion of the transmission path to
reach the specific point, the total amount of overcompensation
provided by the plurality of sections being substantially equal to
the amount of dispersion produced in said additional portion.
37. An optical communication system comprising: a transmission path
through which a light is transmitted to a specific point, the
transmission path including a plurality of sections through which
the light travels to the specific point, the plurality of sections
together overcompensating for dispersion produced in the sections
for the light, a total amount of overcompensation in the sections
taken together being substantially equal to a residual dispersion
in the light at the specific point which would occur if the
dispersion for the light in each section was approximately
zero.
38. An optical communication system as in claim 37, further
comprising: a transmitter; and a receiver at the specific point,
the transmitter transmitting the light to the receiver through the
transmission path.
39. An optical communication system as in claim 37, wherein the
light includes a plurality of different wavelengths including a
center wavelength, and the plurality of sections together
overcompensate for the dispersion produced in the sections for
light at the center wavelength so that the amount of dispersion for
the light at the center wavelength at the specific point is
substantially zero.
40. An optical communication system as in claim 37, wherein each
section comprises: a plurality of optical amplifiers arranged along
the transmission path; and a dispersion compensator
overcompensating for dispersion produced in the section.
41. An optical communication system as in claim 37, wherein each
section comprises: a plurality of optical amplifiers arranged along
the transmission path in equal intervals from a first optical
amplifier to a last optical amplifier; and a dispersion compensator
overcompensating for dispersion produced in the section.
42. An optical communication system as in claim 40, wherein the
dispersion compensator is a variable dispersion compensator which
is controllable to vary the amount of overcompensation.
43. An optical communication system as in claim 37, wherein when
the transmission path provides a positive dispersion to the light,
the plurality of sections together overcompensate for dispersion by
adding a negative dispersion to the light, and when the
transmission path provides a negative dispersion to the light, the
plurality of sections together overcompensate for dispersion by
adding a positive dispersion to the light.
44. An optical communication system as in claim 37, wherein, in
addition to travelling through the plurality of sections, the light
travels through an additional portion of the transmission path to
reach the specific point, the total amount of overcompensation
provided by the plurality of sections being substantially equal to
the amount of dispersion produced in said additional portion.
45. An optical communication system comprising: a transmission path
through which a light is transmitted to a specific point; m
dispersion compensators positioned along the transmission path to
divide the transmission path into (m+1) blocks, each dispersion
compensator overcompensating for dispersion produced in the
preceding block so that the amount of dispersion for the light at
the specific point is substantially zero.
46. An optical communication system as in claim 45, wherein the
dispersion compensators are equally spaced along the transmission
path, and each dispersion compensator provides a dispersion amount
of approximately ((m+1)/m).multidot.100%.
47. An optical communication system as in claim 45, wherein each
dispersion compensator provides a dispersion amount of
approximately ((m+1)/m).multidot.100%.
48. An optical communication system as in claim 45, further
comprising: a transmitter; and a receiver at the specific point,
the transmitter transmitting the light to the receiver through the
transmission path.
49. An optical communication system as in claim 45, wherein the
light includes a plurality of different wavelengths including a
center wavelength, and each dispersion compensator overcompensates
for the dispersion produced in the proceeding block for light at
the center wavelength so that the amount of dispersion for the
light at the center wavelength at the specific point is
substantially zero.
50. An optical communication system as in claim 45, wherein each
block comprises: a plurality of optical amplifiers arranged along
the transmission path.
51. An optical communication system as in claim 45, wherein each
block comprises: a plurality of optical amplifiers arranged along
the transmission path in equal intervals.
52. An optical communication system as in claim 45, wherein at
least one of the dispersion compensators is a variable dispersion
compensator which is controllable to vary the amount of
overcompensation provided by the dispersion compensation.
53. An optical communication system as in claim 45, wherein when
the transmission path provides a positive dispersion to the light,
the dispersion compensators overcompensate for dispersion by adding
a negative dispersion to the light, and when the transmission path
provides a negative dispersion to the light, the dispersion
compensators overcompensate for dispersion by adding a positive
dispersion to the light.
54. An optical communication system as in claim 45, wherein, in
addition to travelling through the blocks, the light travels
through an additional portion of the transmission path to reach the
specific point, the total amount of overcompensation provided by
the dispersion compensators being substantially equal to the amount
of dispersion produced in said additional portion.
55. An optical communication system comprising: a transmission path
through which a light is transmitted to a specific point; a
dispersion compensator positioned along the transmission path
before the specific point and overcompensating for dispersion
provided by the transmission path to the light up to a point along
the transmission path before the specific point, so that the amount
of dispersion for the light at the specific point is substantially
zero.
56. An optical communication system as in claim 55, further
comprising: a transmitter; and a receiver at the specific point,
the transmitter transmitting the light to the receiver through the
transmission path.
57. An optical communication system as in claim 55, wherein the
light includes a plurality of different wavelengths including a
center wavelength, and the dispersion compensator overcompensates
for dispersion so that the amount of dispersion for the light at
the center wavelength at the specific point is substantially
zero.
58. An optical communication system as in claim 55, further
comprising: a plurality of optical amplifiers equally spaced along
the transmission path.
59. An optical communication system as in claim 55, wherein the
dispersion compensator is a variable dispersion compensator which
is controllable to vary the amount of overcompensation.
60. An optical communication system as in claim 55, wherein when
the transmission path provides a positive dispersion to the light,
and the dispersion compensator overcompensates for dispersion by
adding a negative dispersion to the light, and when the
transmission path provides a negative dispersion to the light, the
dispersion compensator overcompensates for dispersion by adding a
positive dispersion to the light.
61. An optical communication system comprising: a transmission path
through which a light is transmitted to a specific point; a
dispersion compensator positioned along the transmission path
before the specific point and overcompensating for dispersion
provided by the transmission path to the light up to a point along
the transmission path before the specific point, to control the
amount of dispersion for the light at the specific point.
62. An optical communication system as in claim 61, further
comprising: a transmitter; and a receiver at the specific point,
the transmitter transmitting the light to the receiver through the
transmission path.
63. An optical communication system as in claim 61, wherein the
light includes a plurality of different wavelengths including a
center wavelength, and the dispersion compensator overcompensates
for dispersion so that the amount of dispersion for the light at
the center wavelength at the specific point is substantially
zero.
64. An optical communication system as in claim 61, further
comprising: a plurality of optical amplifiers equally spaced along
the transmission path.
65. An optical communication system as in claim 61, wherein the
dispersion compensator is a variable dispersion compensator which
is controllable to vary the amount of overcompensation.
66. An optical communication system as in claim 61, wherein when
the transmission path provides a positive dispersion to the light,
and the dispersion compensator overcompensates for dispersion by
adding a negative dispersion to the light, and when the
transmission path provides a negative dispersion to the light, the
dispersion compensator overcompensates for dispersion by adding a
positive dispersion to the light.
67. A method comprising: providing a transmission path through
which a light is transmitted to a specific point; and
overcompensating for dispersion provided by the transmission path
to the light up to a point along the transmission path before the
specific point, to control the amount of dispersion for the light
at the specific point.
68. A method as in claim 67, wherein said overcompensating causes
the amount of dispersion for the light at the specific point to be
substantially zero.
69. A method as in claim 67, wherein the light includes a plurality
of different wavelengths including a center wavelength, and said
overcompensating overcompensates for dispersion so that the amount
of dispersion for the light at the center wavelength at the
specific point is substantially zero.
70. A method as in claim 67, further comprising: providing a
plurality of optical amplifiers equally spaced along the
transmission path.
71. A method as in claim 67, wherein when the transmission path
provides a positive dispersion to the light, said overcompensating
overcompensates for dispersion by adding a negative dispersion to
the light, and when the transmission path provides a negative
dispersion to the light, said overcompensating overcompensates for
dispersion by adding a positive dispersion to the light.
72. A method comprising: providing a transmission path through
which a light is transmitted to a specific point, the transmission
path including a plurality of sections so that the light travels
through the sections to the specific point; and in each section,
overcompensating for dispersion produced in the respective section
for the light so that an amount of dispersion for the light at the
specific point is substantially zero.
73. A method as in claim 72, wherein the light includes a plurality
of different wavelengths including a center wavelength, and in each
section, said overcompensating overcompensates for the dispersion
produced in the respective section for light at the center
wavelength so that the amount of dispersion for the light at the
center wavelength at the specific point is substantially zero.
74. An apparatus comprising: a transmission path through which a
light is transmitted to a specific point; and means for
overcompensating for dispersion provided by the transmission path
to the light up to a point along the transmission path before the
specific point, to control the amount of dispersion for the light
at the specific point.
75. An optical communication system comprising: a transmission path
including a section which overcompensates for dispersion occurring
in the section so that total dispersion for light travelling
through the section is approximately zero at a point downstream of
the section.
76. An optical communication system as in claim 75, wherein the
transmission path includes a plurality of sections which together
overcompensate for dispersion occurring in the plurality of section
so that total dispersion for light travelling through the plurality
of sections is approximately zero at a point downstream of the
plurality of sections.
77. An optical communication system comprising: a transmission path
through which a light is transmitted to a specific point, the
transmission path including a plurality of sections so that the
light travels through the sections to the specific point, at least
one section overcompensating for dispersion produced in the
respective section for the light so that an amount of dispersion
for the light at the specific point is substantially zero.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is based on, and claims priority to,
Japanese application number Heisei 10-26268, filed on May 8, 1998,
in Japan, and which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an optical communication
system which compensates for dispersion. More specifically, the
present invention relates to an optical communication system having
a transmission path with sections which overcompensate for
dispersion occurring in the sections, so that the total dispersion
at a point downstream of the sections is approximately zero.
[0004] 2. Description of the Related Art
[0005] In ocean transversal long-haul optical communication systems
covering distances of several thousands kilometers, signal
transmission is conducted using optical regenerating repeaters
which convert an optical signal to an electrical signal to perform
retiming, reshaping and regenerating.
[0006] However, optical amplifiers which can directly amplify
light, without converting the light into an electrical signal, are
being investigated for use in optical communication systems. The
use of such optical amplifiers can greatly reduce the number of
parts in a repeater, improve reliability, and drastically reduce
cost, as compared to the use of conventional optical regenerating
repeaters.
[0007] Moreover, wavelength division multiplexing (WDM) is being
used in optical communication systems in increase transmission
capacity. With WDM, two or more optical signals at different
wavelengths are multiplexed together into a WDM signal. The WDM
signal is then transmitted through a single optical fiber as a
transmission line. WDM can be compared to a conventional optical
communication system where only one optical signal is transmitted
through the optical fiber.
[0008] An optical amplifier, which directly amplifies light without
converting the light into an electrical signal, can be used to
amplify a WDM signal. In this case, the optical amplifier will
simultaneously amplify each optical signal in the WDM signal.
[0009] Therefore, an optical communication system which uses WDM in
combination with optical amplifiers can provide high capacity,
long-haul optical transmission with a relatively simple, economical
structure. Unfortunately, an optical signal transmitted through
such an optical communication system can experience a large amount
of distortion.
SUMMARY OF THE INVENTION
[0010] Accordingly, it is an object of the present invention to
provide an optical communication which appropriately compensates
for dispersion for optical signals transmitted through the
system.
[0011] Additional objects and advantages of the invention will be
set forth in part in the description which follows, and, in part,
will be obvious from the description, or may be learned by practice
of the invention.
[0012] The foregoing objects of the present invention are achieved
by providing an optical communication system which includes a
transmission path through which a light is transmitted to a
specific point, such as to a receiver. The transmission path
includes a plurality of sections so that the light travels through
the sections to the specific point. Each section overcompensates
for dispersion produced in the respective section for the light so
that an amount of dispersion for the light at the specific point is
substantially zero.
[0013] Objects of the present invention are also achieved by
providing an optical communication system which includes a
transmission path having a plurality of sections so that the light
travels through the sections to a specific point. Each section
overcompensates for dispersion produced in the respective section
for the light to control the amount of dispersion for the light at
the specific point.
[0014] Objects of the present invention are further achieved by
providing an optical communication system which includes a
transmission path. Light is transmitted through the transmission
path to a specific point. The transmission path includes a
plurality of sections so that the light travels through the
sections to the specific point. Each section overcompensates for
dispersion produced in the respective section for the light to
reduce the total amount of dispersion for the light at the specific
point.
[0015] Objects of the present invention are also achieved by
providing a transmission path including a plurality of sections
through which light travels to a specific point, wherein the
plurality of sections together overcompensate for dispersion
produced in the sections for the light. The total amount of
overcompensation in the sections taken together is substantially
equal to a residual dispersion in the light at the specific point
which would occur if the dispersion for the light in each section
was approximately zero.
[0016] Further, objects of the present invention are achieved by
providing an optical communication system including a transmission
path through which a light is transmitted to a specific point,
where m dispersion compensators are positioned along the
transmission path to divide the transmission path into (m+1)
blocks. Each dispersion compensator overcompensates for dispersion
produced in the preceding block so that the amount of dispersion
for the light at the specific point is substantially zero.
[0017] In addition, objects of the present invention are achieved
by providing an optical communication system which includes a
transmission path through which a light is transmitted to a
specific point. A dispersion compensator is positioned along the
transmission path before the specific point and overcompensates for
dispersion provided by the transmission path to the light up to a
point along the transmission path before the specific point, so
that the amount of dispersion for the light at the specific point
is controlled, reduced, or is substantially zero.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] These and other objects and advantages of the invention will
become apparent and more readily appreciated from the following
description of the preferred embodiments, taken in conjunction with
the accompanying drawings of which:
[0019] FIG. 1 is a diagram showing a difference in residual
accumulated wavelength dispersion amount in different wavelengths
by influence of dispersion slope.
[0020] FIG. 2 is a diagram showing an average wavelength of a
wavelength distribution.
[0021] FIG. 3 is a diagram illustrating a wavelength dispersion map
when a wavelength dispersion compensation is applied in an optical
communication system.
[0022] FIG. 4 is a diagram showing a section of an optical
communication system for compensating for dispersion.
[0023] FIG. 5 is a diagram showing a residual wavelength dispersion
amount due to manufacturing error in a transmission path.
[0024] FIG. 6 is a diagram illustrating an optical communication
system, according to an embodiment of the present invention.
[0025] FIG. 7 is a diagram illustrating an example of a wavelength
dispersion map for the optical communication system in FIG. 6,
according to an embodiment of the present invention.
[0026] FIG. 8 is a diagram illustrating a section of a transmission
path of an optical communication system having a wavelength
dispersion map as in, for example, FIG. 7, according to an
embodiment of the present invention.
[0027] FIG. 9 is a diagram illustrating a section of a transmission
path of an optical communication system, according to an additional
embodiment of the present invention.
[0028] FIGS. 10(A), 10(B) and 10(C), and 11(A), 11(B) and 11(C) are
diagrams illustrating the use of versatile cables for varying the
dispersion compensating amount of an optical communication system,
according to an embodiment of the present invention.
[0029] FIG. 12 is a diagram showing multi-core structure of a
cable, according to an embodiment of the present invention.
[0030] FIG. 13 is a diagram showing change of fiber length of a
dispersion compensator, according to an embodiment of the present
invention.
[0031] FIG. 14 is a diagram showing change of cable length of a
dispersion compensator, according to an embodiment of the present
invention.
[0032] FIGS. 15-24 are diagrams showing wavelength dispersion maps,
according to embodiments of the present invention.
[0033] FIG. 25 is a diagram showing an optical communication
system, according to an embodiment of the present invention.
[0034] FIG. 26 is a diagram illustrating an example of a dispersion
map for an optical communication system, according to an embodiment
of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] Reference will now be made in detail to the present
preferred embodiments of the present invention, examples of which
are illustrated in the accompanying drawings, wherein like
reference numerals refer to like elements throughout.
[0036] As indicated above, an optical communication system which
uses WDM in combination with optical amplifiers can provide high
capacity, long-haul optical transmission with a relatively simple,
economical structure.
[0037] As an example, a nine-wave multiplex amplifying and
repeating optical communication system with a transmission rate of
2.5 Gb/s per wave will be explained. In this example, nine signal
waves are set in the range from 1551.0 nm to 1559.0 nm with an
interval of 1.0 nm with assignment of channel numbers from the
short wavelength side.
[0038] For the transmission path, a 1.5 .mu.m zero dispersion fiber
and a 1.3 .mu.m zero dispersion fiber are used. The former one is
called a dispersion shifted fiber (DSF), and the latter one is
called a dispersion compensating fiber (DCF) because it compensates
the dispersion accumulated in the DSF. Dispersion of the DSF should
be -2 ps/nm/km in average for the wavelength of 1558 nm and that of
the DCF should be +18 ps/nm/km. The repeating interval is set to 70
km.
[0039] FIG. 1 is a diagram showing difference in the residual
accumulated wavelength dispersion amount in each wavelength due to
the influence of dispersion slope. FIG. 1 indicates the condition
of the wavelength dispersion map of three channels, namely, the
center wavelength (channel 5, in this case), the channel near the
shortest wavelength side (channel 1, in this case) and the channel
near the longest wavelength side (channel 9, in this case) of the
signal beam wavelengths.
[0040] When a receiver is positioned at a transmission distance of
3000 km, a compensation amount of dispersion to be compensated in
the receiver is different for each wavelength. This difference is
due to the difference in residual dispersion at the receiver in
each wavelength caused by the dispersion slopes.
[0041] FIG. 2 is a diagram showing that the center wavelength of
distributed signal beam wavelengths is the average wavelength of
the wavelength distribution of the signal beam. In the case of
designing wavelength dispersion of the system as a whole, the
optimum wavelength dispersion design of the center wavelength of
the distributed signal beam wavelengths is very important to assure
good balance of the transmission characteristics in all
channels.
[0042] FIG. 3 is a diagram showing the wavelength dispersion map in
the center wavelength (channel 5) of the distributed signal beam
wavelengths in above example. As shown in FIG. 3, as a result of
optimum wavelength dispersion design, compensation of 100% is
executed for the accumulated wavelength dispersion for the
transmission path distance of 700 km for each insertion of DCF
having a length of 70 km. Thus, the accumulated wavelength
dispersion becomes zero for each insertion of the DCF. Thereafter,
above structure is repeated along the transmission path.
[0043] In FIG. 3, after a transmission of 3000 km, the signal beam
is dispersion-compensated (post-compensation) using a DCF in the
receiver. More specifically, a dispersion amount indicated by a
dotted line in FIG. 3 is compensated by a dispersion compensator,
such as a DCF, provided in the receiver. The channels other than
the center wavelength channel 5 are also dispersion-compensated by
adjusting the length of DCF for each channel (signal beam
wavelength) at the receiver because the residual dispersion
accumulated in the transmission path by the influence of the
dispersion slope is different depending on the characteristics in
FIG. 1.
[0044] FIG. 4 is a diagram showing a section of an optical
communication system providing the wavelength dispersion map of
FIG. 3. Referring now to FIG. 4, a transmission distance of 700 km
is realized by utilizing ten optical amplifiers/repeaters OA1
through OA10 spaced apart from each other in intervals of 70 km.
DSF is used as the transmission fiber between the optical
amplifiers/repeaters. The transmission path between optical
amplifier/repeater OA10 and an optical amplifier/repeater OA11 is
formed of a DCF. In this case, as illustrated in FIG. 4, dispersion
compensation is conducted so that dispersion becomes zero at the
770 km point.
[0045] When a dispersion compensation technique as in FIGS. 3 and 4
is used, and when the transmission path becomes longer, the amount
of dispersion compensating fiber at the receiver increases. As a
result, the receiver will likely increase in size.
[0046] In addition, the dispersion amount to be compensated in the
receiver must be adjusted to compensate for wavelength dispersion
generated by manufacturing error of wavelength dispersion value in
the DSF and DCF used in the transmission path.
[0047] Moreover, the residual dispersion is different in each
wavelength due to the influence of the dispersion slope. Therefore,
the dispersion compensation amount compensated in the receiver is
different for each wavelength. If residual wavelength dispersion at
the center wavelength is large, a large difference is generated
between the channels having good and bad transmission
characteristics. As a result, balance of the total system is lost
and the size of the receiver becomes large.
[0048] For example, transmission for the distance of 10,000 km
(ocean transversal distance) will be explained. Since the center
wavelength of the distributed signal beam wavelengths is thought to
be important, attention will be paid to this wavelength. Here, for
example, it is assumed that wavelength dispersion in the DSF is set
to -1.8 ps/nm/km, wavelength dispersion in the DCF is set to +18
ps/nm/km, the DCF is inserted in every other ten repeating points
for every repeating distance of 70 km, and the accumulated
wavelength dispersion of every DCF is compensated by 100%. As a
calculation result using these typical values without any
consideration of manufacturing error, a DCF of about 1260 ps/nm is
required at the receiver. This is a relatively large DCF.
[0049] Moreover, manufacturing error generated when the fiber is
actually manufactured must also be considered.
[0050] FIG. 5 is a diagram showing the residual wavelength
dispersion amount of a transmission path due to manufacturing error
of a DSF and a DCF forming the transmission path. More
specifically, FIG. 5 shows the residual wavelength dispersion
amount when the manufacturing error of wavelength dispersion of a
1.5 .mu.m zero dispersion DSF is .+-.0.2 ps/nm/km and the
manufacturing error of wavelength dispersion of a DCF is .+-.0.5
ps/nm/km.
[0051] In accordance with the above, the rate of use of DSF and DCF
in the transmission path is almost identical to an inverse number
ratio of the wavelength dispersion and is about 10:1. In this case,
the residual dispersion reaches .+-.2275 ps/nm, where manufacturing
error of the DSF is .+-.1820 ps/nm and that of the DCF is .+-.455
ps/nm. In order to compensate for this residual dispersion, a DCF
about .+-.120 km is necessary as the extra fiber. This distance
corresponds to four sections.
[0052] As explained above, the DCF of 3535 ps/nm in maximum
(1260+2275 ps/nm) is required to compensate for the residual
wavelength dispersion at the receiver, by the technique described
in FIGS. 1-4. In order to realize this dispersion compensation
amount, a DCF of about 200 km is necessary. When loss of the fiber
is assumed as 0.2 dB/km, the loss generated in this DCF is about 40
dB.
[0053] Here, two units of optical amplifiers having a gain of about
20 dB are required to compensate for this loss. Therefore, the
structure and installation size of the receiver become large.
Moreover, if residual dispersion is large, deterioration of
transmission characteristic may be generated.
[0054] Therefore, as can be seen from above, when a signal is
transmitted by WDM as shown in FIG. 1, dispersion compensation must
be conducted for all channels in the receiver. Additional
compensation for error of dispersion compensation in the receiver
is also necessary to extend further the transmission distance.
Accordingly, wavelength dispersion compensation at the receiver is
necessary to prevent deterioration of the transmission
characteristics.
[0055] Moreover, as can be seen from above, the technique disclosed
in FIGS. 3-4 typically requires a large amount of dispersion
compensation at the receiver. It would be desirable to reduce or
eliminate this amount of dispersion compensation, to thereby reduce
the size, cost and complexity of the receiver.
[0056] In contrast to the technique disclosed in FIGS. 3-4, and as
will be described in more detail below, according to embodiments of
the present invention, a DCF having a dispersion compensating
amount larger than the accumulated wavelength dispersion amount is
periodically inserted to approximate the residual dispersion to be
compensated at the receiver to zero. As a result, in a single wave
communication system, dispersion compensation in the receiver may
be reduced or eliminated.
[0057] In addition, as will be discussed in more detail below,
according to embodiments of the present invention, with WDM
communication, the residual dispersion to be compensated by the
center wavelength at the receiver is approximated to zero. The
other wavelengths may be distributed around zero to reduce the
dispersion compensation amount in the receiver. As a result, the
size of the receiver can be reduced without deterioration of the
transmission characteristics.
[0058] Moreover, as will be discussed in more detail below,
according to embodiments of the present invention, since the
dispersion compensating amount of the DCF inserted to the
transmission path may be varied and fluctuation of residual
dispersion amount by the manufacturing error can be reduced, the
dispersion compensating amount in the receiver can be reduced.
[0059] FIG. 6 is a diagram illustrating an optical communication
system, according to an embodiment of the present invention. In
FIG. 6, DCFs are inserted into the transmission path to divide the
transmission path into blocks. Residual dispersion is not set to
zero in blocks for each insertion of the DCF. Instead, the final
residual dispersion becomes substantially zero for the center
wavelength in the distribution of a signal beam wavelength in a WDM
communication system, or for the communication wavelength in a
single wavelength communication system.
[0060] More specifically, referring now to FIG. 6, an n-wave signal
beam is multiplexed in a transmitter 100 and is then demultiplexed
in a receiver 110. The signal beam travels from transmitter 100 to
receiver 110 through a transmission path 120. In transmission path
120, a dispersion compensating fiber DCF for dispersion
compensation is introduced, for example, in every other several
sections of DSF.
[0061] In a WDM communication system, for the center wavelength of
distribution in the signal beam wavelength, the dispersion
compensation amount of the DCF used for dispersion compensation is
not set to zero at the output of the DCF. Instead, the use of DSFs
and DCFs in transmission path 120 causes the final residual
dispersion amount at receiver 110 to be approximately zero.
[0062] Particularly, when the dispersion compensating interval is
equal, compensation amount of the DCF inserted into transmission
path 120 exceeds the dispersion amount accumulated during
transmission by DSFs up that point along transmission path 120.
When the number of sections of DCF is defined as (m) and the number
of blocks of DSF required for compensation (corresponding to
several sections of DSF) as (m+1), compensation of about
((m+1)/m).times.100% is required.
[0063] FIG. 7 is a diagram illustrating an example of a wavelength
dispersion map for the optical communication system in FIG. 6,
according to an embodiment of the present invention. In this
example, wavelength dispersion in the DSF is set to -1.8 ps/nm/km,
wavelength dispersion in the DCF is set to +18 ps/nm/km for the
center wavelength of distribution in the signal beam wavelength,
the transmission distance is 10,000 km, the transmission distance
of one block of DSF (typically corresponding to several sections of
DSF) is set to 700 km, and the transmission distance of one section
of DCF is set to 76 km. One block of DSF plus one section of DCF
can be considered to be one section of transmission path 120.
[0064] As shown in FIG. 7, in each section of transmission path 120
which includes a DCF, residual wavelength dispersion is not set to
zero, but it finally becomes zero at the receiver. Namely, when the
dispersion compensating interval is equal, if the number of
sections of DCF is set to (m) and the number of blocks of DSF
required for compensation is set to (m+1), compensation amount of
the DCF may be set to about ((m+1)/m).times.100%.
[0065] FIG. 8 is a diagram illustrating a section of a transmission
path having a wavelength dispersion map as in, for example, FIG. 7,
according to an embodiment of the present invention. Here, it is
assumed that the DCFs divide the transmission path into blocks, and
the section of the transmission path illustrated in FIG. 8 includes
one block of DSF and an associated section of DCF.
[0066] Referring now to FIG. 8, the dispersion value of the section
of the transmission path exceeds 0. A positive dispersion value can
be obtained by setting each transmission path DSF from the
transmitter to optical amplifier/repeater OA1, and between optical
amplifier/repeaters OA1 to OA10, to, for example, 70 km,
respectively, and by inserting a DCF of, for example, 76 km,
between optical amplifier/repeaters OA10 and OA11.
[0067] In FIGS. 7 and 8, the transmission path DSF has minus
dispersion value, while the DCF has plus dispersion value.
[0068] FIG. 9 is a diagram illustrating a section of a transmission
path, where the signal is transmitted for 10,000 km, the
transmission path DSF has plus dispersion value and the DCF has
minus dispersion value, according to an embodiment of the present
invention. As illustrated in FIG. 9, the dispersion value does not
become zero in each section of the transmission path, but the
residual dispersion amount is finally set to zero at the position
where the receiver is installed.
[0069] In FIGS. 8 and 9, for a WDM optical communication system,
the dispersion value is not set to zero in each section of the
transmission path where the DCF is inserted for the center
wavelength of distribution in the signal beam wavelength. Instead,
the final residual dispersion is set to substantially zero.
However, for WDM, when dispersion compensation is conducted for the
particular wavelength for communication, it is also possible to
conduct dispersion compensation so that the dispersion value does
not become zero for the particular wavelength in the section of the
transmission path having the particular wavelength. As an example
of the particular wavelength, the wavelength of a channel to
transmit high bit rate signal and the wavelength of channel
required to have a low error rate can be considered. The
transmission quality can be improved by arranging the wavelengths
of these channels as the center wavelengths of total
wavelengths.
[0070] FIGS. 8 and 9 illustrate all the DSFs as providing positive
dispersion, or all the DSFs as providing negative dispersion.
However, the present invention is not intended to be limited to
this. Instead, for example, the transmission path can include a
mixture of DSFs providing positive and negative dispersion.
Similarly, the DCFs are not limited to all providing positive
dispersion, or all providing negative dispersion. Instead, the
transmission path can have a mixture of DCFs providing positive or
negative dispersion.
[0071] According to embodiments of the present invention, it may be
possible to vary the dispersion compensating amount of DCFs
inserted to the transmission path, adjust the dispersion amount and
reduce fluctuation of the residual dispersion amount.
[0072] For example, the manufacturing error of wavelength
dispersion of a DSF might vary by several percent. As a result, for
example, if the manufacturing error of the DSF deviates to a
negative accumulated value, the manufacturing error can be
compensated by increasing the dispersion compensating value of a
DCF inserted to the transmission path. If the manufacturing error
of the DSF deviates to a positive accumulated value, the
manufacturing error can be compensated by reducing dispersion
amount of a DCF inserted to the transmission path.
[0073] To compensate for manufacturing error in the dispersion
amount of the DSF, the dispersion compensating amount of a DCF
inserted to the transmission path can be varied by, for example,
(a) using versatile cables; (b) using cables having an increased
number of fiber cores which can be selected; (c) changing the fiber
length of the DCF; and (d) using DCFs having versatile wavelength
dispersion values.
[0074] FIGS. 10(A), 10(B), 10(C) and 11(A), 11(B) and 11(C) are
diagrams illustrating the use of versatile cables for varying the
dispersion compensating amount, according to an embodiment of the
present invention. A cable corresponding to the required dispersion
compensating value can be selected from cables providing different
amounts of dispersion compensation.
[0075] FIG. 12 is a diagram illustrating a multi-core cable for
varying the dispersion compensating amount, according to an
embodiment of the present invention. As illustrated in FIG. 12, the
multi-core cable includes versatile fibers in different dispersion
compensating amounts within a single cable. The fiber of the
optimum dispersion compensating amount is selected at the time of
cable connection. The structure shown in FIG. 12 is economically
preferable in comparison with a case where a single fiber is
prepared for a single cable, and a case where a plurality of cables
having different dispersion values are prepared, because cable is
generally more expensive than fiber.
[0076] The dispersion compensation amount can also be varied by
changing fiber length or cable length without changing the
installation position of optical amplifiers/repeaters within a
certain section.
[0077] For example, FIGS. 13 and 14 are diagrams showing a DCF of a
dispersion compensating section of a transmission path being
physically formed in a longer length than the cable, so that the
dispersion compensation amount can be varied, according to an
embodiment of the present invention.
[0078] In FIG. 13, the DCF length can be set longer than the cable
length by changing the radius of curvature when the DCF is wound to
the tensile force line within the cable. The DCF length can be
adjusted by changing a radius of curvature.
[0079] In FIG. 14, the cable laying section, namely optical
amplifier/repeater interval, is set in the same distance as that of
the other transmission section. The radius of curvature for
meandering the cable when it is laid is selected to a value
different from that of the DSF. Thereby, distance can substantially
be extended or reduced to change the dispersion compensating
amount.
[0080] Versatile wavelength dispersion compensating values of
fibers can also be used to vary the wavelength dispersion
compensating amount. In this case, versatile dispersion
compensating fibers are prepared in different wavelength dispersion
values in a certain section. Manufacture can be done easily by
changing a ratio of the lengths of DSF and DCF forming the
dispersion compensating section.
[0081] To realize a compact structure and installation at the
receiver without deterioration of transmission characteristic, it
may be effective to use a combination of the above-described
techniques for varying the dispersion compensating amount based on
the dispersion error for each dispersion compensating section of
the transmission path.
[0082] In an example as explained above, the dispersion
compensating amount can be adjusted and fluctuation of the residual
wavelength dispersion due to the manufacturing error of fiber can
be compensated by preparing versatile DCF in different dispersion
amounts.
[0083] As an example, assume that wavelength dispersion in the DSF
is set to -1.8 ps/nm/km, wavelength dispersion in the DCF is set to
+18 ps/nm/km, and DCF is inserted in every other ten repeaters of
the repeating distance of 70 km for the transmission distance of
10,000 km. Moreover, assume that the manufacturing error of
wavelength dispersion of the DSF is set to .+-.0.2 ps/nm/km, and
the manufacturing error of wavelength dispersion of the DCF is set
to .+-.0.5 ps/nm/km. In this case, since fluctuation of .+-.2275
ps/nm is generated in the residual dispersion due to the
manufacturing error, fluctuation of about .+-.120 km is generated
as the dispersion compensating fiber length. To control such
fluctuation, for example, as described above, cables having
different rates of dispersion compensating fiber can be used.
[0084] First, three kinds of fiber in the rate of dispersion
compensating fiber of 0%, 50% and 100% are assumed, considering the
merits in the manufacturing process and economical aspect. If
residual wavelength dispersion amount of the system as a whole is
deviated to negative, the rate of the DCF is changed to 100% from
50% and if it is deviated to positive, the rate of the DCF is
changed to 0% from 50%, by arranging the section where the rate of
DCF is 50% when the manufacturing error is not included.
[0085] Residual wavelength dispersion can be adjusted in the
wavelength dispersion value step of a half section of the repeating
section. Namely, when the repeating section is 70 km, the residual
wavelength dispersion can be adjusted in the step of about 700
ps/nm.
[0086] Moreover, since fluctuation of about .+-.120 km exists as
the fiber length, about three sections (3.times.35 km) are required
in this case to compensate for such fluctuation.
[0087] An example where the section having the rate of the
dispersion compensating fiber of 50% is introduced up to three
sections will be explained below.
[0088] FIG. 15 is a diagram showing the wavelength dispersion map
not including manufacturing error, according to an embodiment of
the present invention. Here, it is assumed that the optimum
wavelength dispersion design is conducted when manufacturing error
is not included. In FIG. 15, the repeating section length is 70
km.
[0089] FIG. 16 is a diagram showing the wavelength dispersion map
including the manufacturing error of wavelength dispersion of DSF
of +0.2 ps/nm/km and manufacturing error of wavelength dispersion
of dispersion compensating fiber +0.5 ps/nm/km, according to an
embodiment of the present invention. The repeating section length
is 70 km.
[0090] On the other hand, FIG. 17 is a diagram showing the
wavelength dispersion map when the wavelength dispersion of the
system as a whole is adjusted, because dispersion is deviated to
positive, by changing the rate of dispersion compensating fiber to
0% from 50%, according to an embodiment of the present invention.
In FIG. 17, the wavelength dispersion map includes a manufacturing
error of wavelength dispersion of DSF of +0.2 ps/nm/km and a
manufacturing error of wavelength dispersion of dispersion
compensating fiber of +0.5 ps/nm/km. In FIG. 17, the repeating
section length is 70 km.
[0091] Moreover, FIG. 18 is a diagram showing the wavelength
dispersion map including a manufacturing error of wavelength
dispersion of DSF of -0.2 ps/nm/km and a manufacturing error of
wavelength dispersion of dispersion compensating fiber of -0.5
ps/nm/km, according to an embodiment of the present invention. In
FIG. 18, the repeating section length is 70 km.
[0092] Meanwhile, FIG. 19 is a diagram showing the wavelength
dispersion map when the wavelength dispersion of the system as a
whole is adjusted, because dispersion is deviated to negative, by
changing the rate of dispersion compensating fiber to 100% from
50%, according to an embodiment of the present invention. In FIG.
19, the wavelength dispersion map includes a manufacturing error of
wavelength dispersion of DSF of -0.2 ps/nm/km and a manufacturing
error of wavelength dispersion of dispersion compensating fiber of
-0.5 ps/nm/km. The repeating section length is 70 km.
[0093] Therefore, as explained above, fluctuation by the
manufacturing error can be controlled.
[0094] Next, the case where the repeating interval is set to 50 km
will be explained. Since the fluctuation of about .+-.120 km exists
within the fiber length, about four repeating sections (4.times.25
km) will be required for compensating such fluctuation.
[0095] The case where the section having the rate of DCF of 50% is
used, up to four sections will be explained. Since the repeating
section is 50 km, residual wavelength dispersion amount of the
system can be adjusted in the step of about 500 ps/nm.
[0096] FIG. 20 is a diagram showing the wavelength dispersion map
not including manufacturing error, according to an embodiment of
the present invention. Here it is assumed that optimum wavelength
dispersion design is conducted when the manufacturing error is not
included. The repeating section length is 50 km.
[0097] FIG. 23 is a diagram showing the wavelength dispersion map
including a manufacturing error of wavelength dispersion of DSF of
+0.2 ps/nm/km and a manufacturing error of wavelength dispersion of
dispersion compensating fiber of +0.5 ps/nm/km, according to an
embodiment of the present invention. The repeating section length
is 50 km.
[0098] On the other hand, FIG. 24 is a diagram showing the
wavelength dispersion map when wavelength dispersion of system as a
whole is adjusted by changing the rate of dispersion compensating
fiber to 0% from 50% because dispersion is deviated to positive,
according to an embodiment of the present invention. In FIG. 24,
the wavelength dispersion map includes a manufacturing error of
wavelength dispersion of DSF of +0.2 ps/nm/km and a manufacturing
error of wavelength dispersion of dispersion compensating fiber of
+0.5 ps/nm/km. The repeating section length is 50 km.
[0099] Moreover, FIG. 21 is a diagram showing the wavelength
dispersion map including a manufacturing error of wavelength
dispersion of DSF of -0.2 ps/nm/km and a manufacturing error of
wavelength dispersion of dispersion compensating fiber of -0.5
ps/nm/km, according to an embodiment of the present invention. The
repeating section length is 50 km.
[0100] FIG. 22 is a diagram showing the wavelength dispersion map
when the wavelength dispersion of system as a whole is adjusted by
changing the rate of dispersion compensating fiber to 100% from 0%
because dispersion is deviated to positive, according to an
embodiment of the present invention. In FIG. 22, the wavelength
dispersion map includes a manufacturing error of wavelength
dispersion of DSF of -0.2 ps/nm/km and a manufacturing error of
wavelength dispersion of dispersion compensating fiber of -0.5
ps/nm/km. The repeating section length is 50 km.
[0101] As explained above, fluctuation by the manufacturing error
can be controlled. Moreover, when it is required to set the
adjusting step of residual wavelength dispersion smaller value, it
can be realized by further increasing kinds of the rates of the
dispersion compensating fiber.
[0102] FIG. 25 is a diagram showing an optical communication system
having a wavelength dispersion adjusting section for compensating
the manufacturing error of wavelength dispersion of the system as a
whole, according to an embodiment of the present invention.
Referring now to FIG. 25, a wavelength dispersion adjusting section
200 is provided along transmission path 120.
[0103] In wavelength dispersion adjusting section 200, various of
the above-described techniques can be used to adjust wavelength
dispersion. For example, in wavelength dispersion adjusting section
200, an optimum fiber can be selected at both terminal stations.
Therefore, the final wavelength dispersion can be equalized, for
example, by selecting an adequate fiber from those having versatile
dispersion values prepared from the fiber and cable formed as
described above.
[0104] In FIG. 25, characteristics of the fibers laid in the course
of laying the fibers respectively from the transmitter and receiver
are investigated. Wavelength dispersion error of the system as a
whole is adjusted using any one or combination of the techniques
disclosed herein considering the section for connecting the cable
laid from the transmitter and the cable laid from the receiver as
wavelength dispersion adjusting section 200. The dispersion amount
is adjusted so that the transmission characteristics do not
deteriorate, by adjusting the dispersion amount to make
substantially zero the wavelength dispersion at the receiver
position.
[0105] According to embodiments of the present invention, when a
signal is transmitted in WDM mode, it is no longer required to
conduct dispersion compensation for an average wavelength of a
wavelength distribution in the receiver and accumulated dispersion
for all channels can be reduced. Moreover, since adjustment is
conducted using the center wavelength in WDM mode, fluctuation of
dispersion of the other wavelengths becomes almost zero, and total
amount of dispersion in the dispersion compensator required in the
receiver can be reduced. Accordingly, an optical amplifier for
compensating loss by the dispersion compensator can be eliminated
in the receiver.
[0106] In addition, conventionally, it was required to compensate
for the error of dispersion compensation in the receiver to expand
the transmission distance. However, according to embodiments of the
present invention, the dispersion compensating amount in the
receiver can be reduced or eliminated because an error is also
compensated by the dispersion compensator provided in the
transmission path.
[0107] According to the above embodiments of the present invention,
an optical amplifier provided for compensating loss by the
dispersion compensator can be eliminated. Moreover, in various
embodiments of the present invention, a dispersion compensator in a
receiver can be eliminated or reduced in size.
[0108] According to the above embodiments of the present invention,
an optical communication system includes a transmission path
through which a light is transmitted to a specific point, such as
to a receiver. The transmission path includes a plurality of
sections so that the light travels through the sections to the
specific point. For example, in FIGS. 8 and 9, each block of
optical amplifiers optically connected together, along with the
DCF, forms a respective section of the transmission path. As
illustrated in FIGS. 8 and 9, each section overcompensates for
dispersion produced in the respective section for the light so that
an amount of dispersion for the light at the specific point is
controlled, reduced, or made to be substantially zero.
[0109] Moreover, according to the above embodiments of the present
invention, an optical communication system includes a transmission
path having a plurality of sections through which light travels to
a specific point. The plurality of sections together overcompensate
for dispersion produced in the sections for the light. The total
amount of overcompensation in the sections taken together is
substantially equal to a residual dispersion in the light at the
specific point which would occur if the dispersion for the light in
each section was approximately zero. Therefore, generally, a
specific amount of dispersion compensation is required to
compensate for dispersion occurring in portions of the transmission
path other than the section providing dispersion compensation. This
specific amount of dispersion compensation is essentially
"distributed" to the sections. For example, in FIGS. 8 and 9,
dispersion compensation is required to compensate for dispersion
occurring between optical amplifier/repeaters OA10 and OA11.
According to embodiments of the present invention, this amount of
dispersion is essentially distributed to the various sections, so
that the total amount of overcompensation provided by the sections
substantially equals that required to compensate for dispersion
occurring between optical amplifiers OA10 and OA11.
[0110] According to embodiments of the present invention, m
dispersion compensators are positioned along the transmission path
to divide the transmission path into (m+1) blocks. Each dispersion
compensator overcompensates for dispersion produced in the
preceding block so that the amount of dispersion for the light at
the specific point is substantially zero. For example, see FIGS. 8
and 9.
[0111] Moreover, according to embodiments of the present invention,
an optical communication system includes a transmission path
through which a light is transmitted to a specific point. A
dispersion compensator is positioned along the transmission path
before the specific point. The dispersion compensator
overcompensates for dispersion provided by the transmission path to
the light up to a point along the transmission path before the
specific point, so that the amount of dispersion for the light at
the specific point is controlled, reduced, or is substantially
zero. According to the above embodiments of the present invention,
various blocks or sections of a transmission path include optical
amplifiers/repeaters. For example, FIGS. 8 and 9 illustrate one
block as including ten optical amplifiers/repeaters. However, a
block or section is not intended to be limited to having any
specific number of optical amplifiers/repeaters. Instead, the
number of optical amplifiers/repeaters used in a specific
configuration will depend, for example, on the design
specifications of the system.
[0112] According to the above embodiments of the present invention,
a DCF is typically positioned in a section of a transmission line
after the last optical amplifier/repeater in the section. For
example, in FIGS. 8 and 9, a DCF is positioned after the last
optical amplifier/repeater OA 10. However, the present invention is
not intended to be limited to this positioning of the DCF. Instead,
a DCF can be positioned anywhere along the section. Moreover, a DCF
is not intended to be limited to being a single DCF positioned in
the section. Instead, many different DCFs positioned at different
locations in a section of the transmission path can together be
considered as being a DCF or dispersion compensator. Thus, for
example, in FIGS. 8 and 9, each section can have, for example, two
DCFs positioned somewhere along the section, where the two DCFs,
taken together, provide the required amount of dispersion
compensation.
[0113] The present invention will also be operable as far as the
total dispersion at a specific point is controlled to become
substantially zero. This is shown in FIG. 26 as another embodiment
of the present invention. In FIG. 26, the dispersion in at least
one section is overcompensated, and the dispersions in other
sections are undercompensated.
[0114] According to various embodiments of the present invention,
sections of a transmission path overcompensate for dispersion
produced in the respective sections so that an amount of dispersion
for light travelling through the sections to a the specific point
(such as the location of a receiver) is substantially zero.
However, the present invention is not intended to be limited to
controlling the dispersion at the specific point to be
"substantially zero". Instead, the sections can be seen as simply
controlling or reducing the amount of dispersion at the specific
point. Thus, the dispersion is not limited to being substantially
zero at the specific point.
[0115] According to the above embodiments of the present invention,
a dispersion compensator is used to provide dispersion
overcompensation. The dispersion compensator can be, for example, a
variable dispersion compensator which is controllable to vary the
amount of overcompensation. For example, the various configurations
in FIGS. 10-14 can be considered to be variable dispersion
compensators, since the amount of dispersion provided by these
configuration can be changed. Typically, with these configurations,
the amount of dispersion provided by the dispersion compensator is
set during installation. However, a variable dispersion compensator
can be used in which the amount of dispersion compensation is
changeable after installation and/or after operation of the system.
Moreover, the present invention is not intended to be limited to
dispersion compensators formed of dispersion compensating fibers,
and other types of dispersion compensators can be used.
[0116] Although a few preferred embodiments of the present
invention have been shown and described, it would be appreciated by
those skilled in the art that changes may be made in these
embodiments without departing from the principles and spirit of the
invention, the scope of which is defined in the claims and their
equivalents.
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