U.S. patent application number 11/979672 was filed with the patent office on 2008-03-27 for raman amplifier and optical communication system including the same.
This patent application is currently assigned to SUMITOMO ELECTRIC INDUSTRIES, LTD.. Invention is credited to Motoki Kakui, Junichi Kumasako, Takafumi Terahara, Tetsufumi Tsuzaki.
Application Number | 20080074733 11/979672 |
Document ID | / |
Family ID | 19064224 |
Filed Date | 2008-03-27 |
United States Patent
Application |
20080074733 |
Kind Code |
A1 |
Tsuzaki; Tetsufumi ; et
al. |
March 27, 2008 |
Raman amplifier and optical communication system including the
same
Abstract
The present invention relates to a Raman amplifier where
flexibility in device design considering both of Raman
amplification and dispersion compensation is high. In the Raman
amplifier, the Raman amplification optical fiber included in the
optical amplification section and the dispersion compensating
optical fiber included in the dispersion compensation section are
arranged while being optically connected to each other. Since the
optical amplification section and the dispersion compensation
section are provided as independent optical devices, one device can
be designed without being restricted to the design conditions of
the other device.
Inventors: |
Tsuzaki; Tetsufumi;
(Yokohama-shi, JP) ; Kakui; Motoki; (Yokohama-shi,
JP) ; Terahara; Takafumi; (Kawasaki-shi, JP) ;
Kumasako; Junichi; (Kawasaki-shi, JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Assignee: |
SUMITOMO ELECTRIC INDUSTRIES,
LTD.
OSAKA
JP
|
Family ID: |
19064224 |
Appl. No.: |
11/979672 |
Filed: |
November 7, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10208198 |
Jul 31, 2002 |
7307782 |
|
|
11979672 |
Nov 7, 2007 |
|
|
|
Current U.S.
Class: |
359/334 |
Current CPC
Class: |
H01S 3/06754 20130101;
H01S 3/094096 20130101; H01S 3/09408 20130101; H01S 3/06725
20130101; H01S 3/094003 20130101; H01S 3/302 20130101 |
Class at
Publication: |
359/334 |
International
Class: |
H01S 3/00 20060101
H01S003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 31, 2001 |
JP |
P2001-232282 |
Claims
1-49. (canceled)
50-52. (canceled)
53. A Raman amplifier provided at a predetermined position of an
optical fiber transmission line and having an input end for
entering signal light which propagates through said optical fiber
transmission line and an output end for outputting said
Raman-amplified signal light, comprising: an optical amplification
section provided between said input end and said output end, and
including a Raman amplification optical fiber for Raman-amplifying
signal light which enters through said input end by supplying
pumping light into said Raman amplification optical fiber; wherein
said Raman amplifier further comprises a dispersion compensation
section provided between said input end and said output end while
being optically connected to said Raman amplification optical
fiber, said dispersion compensation section compensating for a
total chromatic dispersion of said optical fiber transmission line
positioned outside of said Raman amplifier and said Raman
amplification optical fiber positioned inside of said Raman
amplifier, in a signal light wavelength band, and wherein said
Raman amplification optical fiber has an effective area of 30
.mu.m.sup.2 or less in the pumping light wavelength, and a
parameter g.sub.R of 2.3.times.10.sup.-14 m/W or more.
54. A Raman amplifier provided at a predetermined position on an
optical fiber transmission line and has an input end for entering
signal light which propagates through said optical fiber
transmission line and an output end for outputting said
Raman-amplified signal light, comprising: an optical amplification
section which is installed between said input end and said output
end, and includes a Raman amplification optical fiber for
Raman-amplifying signal light which enters through said input end
by the supply of pumping light into said Raman amplification
optical fiber; a dispersion compensation section provided between
said input end and said output end while being optically connected
to said Raman amplification optical fiber, and compensates for a
total chromatic dispersion in the signal light wavelength of said
optical fiber transmission line positioned outside of said Raman
amplifier and said Raman amplification optical fiber positioned
inside of said Raman amplifier; and a pumping light source supply
system, comprising a first pumping light source, and supplies said
pumping light to said Raman amplification optical fiber, and a
first optical multiplexing structure for guiding the pumping light
from said first pumping light source to said Raman amplification
optical fiber without passing through said dispersion compensating
optical fiber, and wherein said Raman amplification optical fiber
has an effective area of 30 .mu.m.sup.2 or less in the pumping
light wavelength, and a parameter g.sub.R of 2.3.times.10.sup.-14
m/W or more.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a Raman amplifier
Raman-amplifying signal light of a plurality of channels having
wavelengths different from each other, and an optical communication
system including the same.
[0003] 2. Related Background Art
[0004] A rare earth element-doped optical fiber amplifier which
uses a rare earth element-doped optical fiber as an optical
amplification medium is an optical device having a structure for
supplying pumping light having a wavelength to pump the rare earth
element into the optical fiber, and amplifying signal light by
using the transition between the energy levels of the rare earth
element. Therefore, in a rare earth element-doped optical fiber
amplifier, the wavelength band range of signal light which can be
amplified is limited. Whereas the Raman amplifier is an optical
amplifier using the Raman scattering phenomena in an optical fiber
where signal light propagates, and if the transmission medium of
the signal light is a silica-based optical fiber, then the signal
light can be Raman-amplified by supplying pumping light, having a
wavelength about 100 nm shorter than the signal light wavelength,
to the optical fiber. Therefore, with the Raman amplifier, the
wavelength band range of signal light which can be amplified is
arbitrary, and the pumping light wavelength can be appropriately
set according to the signal light wavelength.
[0005] As the Raman amplifier, not only a structure for
Raman-amplifying signal light in an optical fiber transmission line
laid in the relay section, but also a structure, as a module
provided in a repeater, for Raman-amplifying signal light in the
repeater is known. The Raman amplifier is an optical device using
Raman scattering, which is one type of non-linear optical phenomena
in a Raman amplification optical fiber. Since a dispersion
compensating optical fiber compensates for a chromatic dispersion
of the optical fiber transmission line is, in general, an optical
fiber having a small effective area and a high non-linearity, a
structure for Raman-amplifying signal light by using this
dispersion compensating optical fiber as a Raman amplification
optical fiber is also known.
SUMMARY OF THE INVENTION
[0006] The present inventors studied conventional Raman amplifiers,
and discovered the following problems. In the case of a Raman
amplifier where the dispersion compensating optical fiber is
applied as a Raman amplification optical fiber, it is necessary
that one optical fiber realizes both of the Raman amplification
function and the dispersion compensation function, so one function
is restricted by the other function. For example, in order to
compensate for a chromatic dispersion of the optical fiber
transmission line, the length of a dispersion compensating optical
fiber is set according to not only the chromatic dispersion and the
length of the optical fiber transmission line, but also according
to the chromatic dispersion of the dispersion compensating optical
fiber itself. But, if the dispersion compensating optical fiber for
which the length is set like this is applied to the Raman amplifier
as a Raman amplification optical fiber, a sufficient Raman
amplification gain may not be obtained. Therefore, in a
conventional Raman amplifier, the design flexibility thereof is low
for both of the device design considering Raman amplification and
the device design considering dispersion compensation.
[0007] It is an object of the present invention to provide a Raman
amplifier having high design flexibility for both of the device
design considering Raman amplification and the device design
considering dispersion compensation, and an optical communication
system including the same.
[0008] The Raman amplifier according to the present invention is an
optical device for Raman-amplifying signal light (WDM signal light)
of a plurality of channels having wavelengths different from each
other, which is provided at a predetermined position on an optical
fiber transmission line for capturing signal light propagating the
optical fiber transmission line and has an input end, and an output
end for outputting Raman-amplified signal light. Particularly, the
Raman amplifier according to the present invention comprises a
light amplification section and a dispersion compensation section,
which are provided between the input end and the output end
respectively while being optically connected to each other. The
optical amplification section includes a Raman amplification
optical fiber for Raman-amplifying the signal light by supplied
pumping light. The dispersion compensation section includes a
dispersion compensating optical fiber, for example, and compensates
for a chromatic dispersion of the optical fiber transmission line
and the Raman amplification optical fiber in a signal light
wavelength band.
[0009] The pumping light may be the pumping light of a plurality of
channels having wavelengths different from each other, so as to
enable Raman amplification with a wider signal light wavelength
band.
[0010] In the Raman amplifier according to the present invention,
it is preferable that the signal light propagation path from the
input end to the output end, excluding the dispersion compensation
section, has a cumulative chromatic dispersion whose absolute value
is 5 ps/nm or less in the signal light wavelength band. In this
case, the dispersion compensation section is designed such that the
optical fiber transmission line, positioned outside the Raman
amplification section, becomes the dispersion compensation
target.
[0011] The Raman amplifier according to the present invention
further comprises a pumping light supply system for supplying
pumping light having at least a sufficient power to cause induced
Raman scattering to the Raman amplification optical fiber. This
pumping light supply system constitutes a part of the light
amplification section of the Raman amplifier, and includes a
pumping light source (first pumping light source) for supplying
pumping light to the Raman amplification optical fiber, and a first
optical multiplexing structure for guiding the pumping light from
the first pumping light source to the Raman amplification optical
fiber without passing through the dispersion compensation section,
such as a dispersion compensating optical fiber.
[0012] As described above, the Raman amplifier according to the
present invention has a dispersion compensation section for
implementing the dispersion compensation function and a light
amplification section for implementing the Raman amplification
function, which are provided as optical devices independent from
each other, so high flexibility is obtained for both of the device
design considering Raman amplification and the device design
considering dispersion compensation. Specifically, when the signal
light propagation path from the input end to the output end,
excluding the dispersion compensation section in the Raman
amplifier, has a cumulative chromatic dispersion whose absolute
value is 5 ps/nm or less in the signal light wavelength band, it is
substantially unnecessary for the dispersion compensation section
to compensate for the chromatic dispersion in the Raman
amplification optical fiber in the Raman amplifier, so an even
higher design flexibility is obtained.
[0013] In the Raman amplifier according to the present invention,
Raman amplification can be performed in the dispersion compensation
section if the dispersion compensation section has a configuration
which includes a dispersion compensating optical fiber. In this
case, it is preferable that the pumping light supply system
includes a pumping light source (second pumping light source) for
supplying pumping light having a sufficient power to cause induced
Raman scattering to the dispersion compensating optical fiber, and
a second optical multiplexing structure for guiding the pumping
light from the pumping light source to the dispersion compensating
optical fiber without passing through the Raman amplification
optical fiber. These first and second pumping light sources may be
a common pumping light source. Raman amplification is also possible
in the dispersion compensating optical fiber by installing the
dispersion compensating optical fiber at a position where the
pumping light which propagated at least a part of the Raman
amplification optical fiber reaches.
[0014] In particular, Raman amplification in the dispersion
compensating optical fiber can make the dispersion compensating
optical fiber to be substantially no loss in the signal light
wavelength band. In other words, it is preferable that the pumping
light to be supplied to the dispersion compensating optical has a
sufficient power or more to obtain Raman gain to cancel
transmission loss in the dispersion compensating optical fiber. In
other words, the signal light is Raman-amplified also in an part
excluding the light amplification section (dispersion compensation
section), so an effective loss of the dispersion compensation
section in the signal light wavelength band decreases, and the loss
becomes substantially none. In this case, the effective gain of
Raman amplification in the Raman amplifier is roughly the same as
the Raman amplification gain in the light amplification section, so
the flexibility of the device design considering both Raman
amplification and dispersion compensation further increases, and a
deterioration of noise figure is effectively controlled.
[0015] In the Raman amplifier according to the present invention,
the Raman amplification optical fiber may include a forward stage
Raman amplification optical fiber provided at the upstream side and
a backward stage Raman amplification optical fiber provided at the
downstream side, in view from the signal light propagation
direction. In this case, the Raman amplifier Raman-amplifies the
signal light in both of the forward stage Raman amplification
optical fiber and the backward stage Raman amplification optical
fiber, so the signal light can be Raman-amplified at low noise and
high gain. In particular, it is preferable that the dispersion
compensation section is arranged between the forward stage Raman
amplification optical fiber and the backward stage Raman
amplification optical fiber to effectively control the
deterioration of noise figure characteristic.
[0016] In the Raman amplifier according to the present invention,
the Raman amplification optical fiber may have a chromatic
dispersion whose absolute value is 5 ps/nm/km or more in the signal
light wavelength band, or may have a zero dispersion wavelength of
shorter than the shortest wavelength of the pumping light to be
supplied. In this case, the generation of four wave mixing
(including remote four wave mixing) is effectively controlled, and
an excellent Raman amplification characteristic is obtained.
Particularly, when such Raman amplification optical fiber comprises
the forward stage and backward stage optical fibers, the signal
light propagation path from the input end to the output end in the
Raman amplifier, excluding the dispersion compensation section, can
have a cumulative chromatic dispersion whose absolute value of
which is 5 ps/nm or less in the signal light wavelength band, if
the polarity of the chromatic dispersion of the forward stage
optical fiber and the polarity of the chromatic dispersion of the
backward stage optical fiber are set to be opposite.
[0017] Also in the Raman amplifier according to the present
invention, it is preferable that the Raman amplification optical
fiber has an effective area of 30 .mu.m.sup.2 or less at a pumping
light wavelength. This is because the Raman gain coefficient
increases and high efficiency Raman amplification becomes possible.
In the Raman amplifier according to the present invention, it is
preferable that the Raman amplification optical fiber has a cut-off
wavelength of shorter than the shortest wavelength of the pumping
light to be supplied. This is because the pumping light to be
supplied to the Raman amplification optical fiber propagates in a
single mode, so stable gain can be obtained. Also in the Raman
amplifier according to the present invention, it is preferable that
the signal light propagation path from the input end to the output
end is 1 ps or less in the signal light wavelength band. In this
case, deterioration of the transmission characteristic is
controlled for the Raman amplifier.
[0018] The pumping light supply system in the Raman amplifier
according to the present invention may include a pumping light
source for outputting pumping light and a drive circuit for driving
the pumping light source. The pumping light source and drive
circuit may be provided separately from the optical amplification
section, so that installation is possible after Raman amplification
optical fibers are installed.
[0019] The optical communication system according to the present
invention comprises an optical fiber transmission line where signal
light of a plurality of channels propagate, and a Raman amplifier
having the above mentioned structure. Particularly to enable a long
haul transmission, the optical communication system according to
the present invention may comprises a plurality of Raman amplifiers
each having a structure similar to the Raman amplifier.
[0020] In the optical communication system according to the present
invention, various modifications to improve the SN ratio is
possible to further improve system performance. In other words, the
optical communication system according to the present invention may
have a configuration to further improve the noise characteristic by
causing induced Raman scattering in the optical fiber transmission
line at the input end side of the Raman amplifier. Specifically,
the optical communication system may comprise a pumping light
source (third pumping light source) for supplying new pumping light
to the optical fiber transmission line at the input end side, and a
third optical multiplexing structure for guiding the pumping light
from the pumping light source to the optical fiber transmission
line. In the case of a configuration where a plurality of Raman
amplifiers are provided on an optical fiber transmission line, it
is efficient to supply the pumping light to the optical fiber
transmission line at the input end side of the Raman amplifier
which locates at the most upstream side among the plurality of
Raman amplifiers. Also this optical communication system may
comprise a bypass transmission line for supplying pumping light,
which propagates through at least a part of the Raman amplification
optical fiber of the Raman amplifier, to the optical fiber
transmission line at the input end side of the Raman amplifier, and
a fourth optical multiplexing structure for guiding the pumping
light, which propagates through the bypass transmission line, to
the optical fiber transmission line. In this case, it is preferable
that the Raman amplifier includes an optical demultiplexer for
guiding the light propagated through the Raman amplification
optical fiber to the bypass transmission line, and an optical
filter for transmitting the pumping light out of the demultiplexed
lights by the optical demultiplexer.
[0021] The present invention will become more fully understood from
the detailed description given hereinbelow and the accompanying
drawings which are given by way of illustration only, and thus are
not to be considered as limiting the present invention.
[0022] Further scope of applicability of the present invention will
become apparent from the detailed description given hereinafter.
However, it should be understood that the detailed description and
specific examples, while indicating preferred embodiments of the
invention, are given by way of illustration only, since various
changes and modifications within the spirit and scope of the
invention will become apparent to those skilled in the art from
this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a diagram depicting the configuration of the first
embodiment of the Raman amplifier according to the present
invention;
[0024] FIG. 2 is a diagram depicting the configuration of the
second embodiment of the Raman amplifier according to the present
invention;
[0025] FIG. 3 is a diagram depicting the configuration of the Raman
amplifier of the first comparison example;
[0026] FIG. 4 is a diagram depicting the configuration of the Raman
amplifier of the second comparison example;
[0027] FIG. 5A and FIG. 5B are graphs depicting the gain
characteristic and the noise figure characteristic of the first
embodiment, second embodiment, first comparison example, and second
comparison example respectively;
[0028] FIG. 6 is a table showing the output power of each
semiconductor laser light source (pumping light source) of the
Raman amplifiers of the first embodiment, second embodiment, first
comparison example, and second comparison example respectively;
[0029] FIG. 7 is a graph depicting the relationship between the
relative refractive index difference and g.sub.R in the core region
of the optical fiber;
[0030] FIG. 8 is a graph depicting the relationship between the
relative refractive index difference and transmission loss .alpha.
in the core region of the optical fiber;
[0031] FIG. 9 is a diagram depicting a general configuration of the
Er element added optical fiber amplifier;
[0032] FIG. 10 is a graph depicting the relationship between the
signal light output power and the power penalty per channel of the
Raman amplifier;
[0033] FIG. 11 is a diagram depicting the configuration of the
first embodiment of the optical communication system according to
the present invention; and
[0034] FIG. 12A and FIG. 12B are diagrams depicting the
configuration of the second embodiment of the optical communication
system according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] Embodiments of the Raman amplifier and the optical
communication system according to the present invention will now be
described with reference to the FIGS. 1-4, 5A-5B, 6-11, and 12A and
12B. In these drawings, the same elements are denoted with the same
symbols, where redundant descriptions are omitted.
First Embodiment of Raman Amplifier
[0036] FIG. 1 is a diagram depicting the configuration of the first
embodiment of the Raman amplifier according to the present
invention. The Raman amplifier 1 according to the first embodiment
comprises an optical isolator 111, a Raman amplification optical
fiber 121, an optical coupler 141 (included in the first optical
multiplexing structure), a dispersion compensating optical fiber
132, an optical coupler 142 (included in the second optical
multiplexing structure), and an optical isolator 112, which are
sequentially provided from the input end 101 to the output end 102.
The semiconductor laser light sources 161a to 161c (first pumping
light source) are connected to the optical coupler 141 through the
optical multiplexer 151. The semiconductor laser light sources 162a
to 162c (second pumping light source) are connected to the optical
coupler 142 through the optical multiplexer 152. The optical
couplers 141 and 142 (first and second optical multiplexing
structures), the optical multiplexers 151 and 152, and the
semiconductors laser light sources 161a to 161c and 162a to 162c
(first and second pumping light sources) constitute the pumping
light supply system 100.
[0037] In the Raman amplifier 1 according to the first embodiment,
the optical amplification section includes the Raman amplification
optical fiber 121, the optical coupler 141, the optical multiplexer
151, and the semiconductor laser light sources 161a to 161c. The
dispersion compensation section includes the dispersion
compensating an optical fiber 132, and pumping light is supplied
from the semiconductor laser light sources 162a to 162c is supplied
to the dispersion compensating optical fiber 132 through the
optical multiplexer 152 and the optical coupler 142.
[0038] The optical isolator 111 transmits the light reaching from
the input end 101 to the Raman amplification optical fiber 121, and
blocks the light which propagates in a direction opposite from this
transmission light. The optical isolator 112 transmits the light
reaching from the optical coupler 142 to the output end 102, and
blocks the light which propagates in a direction opposite from this
transmission light. The Raman amplification optical fiber 121
Raman-amplifies the signal light guided from the optical isolator
111 by pumping light supplied from the optical coupler 141.
[0039] The dispersion compensating optical fiber 132 compensates
for the chromatic dispersion in the signal light wavelength in the
optical fiber transmission line where this Raman amplifier 1 is
provided, and in the Raman amplification optical fiber 121. When
the absolute value of the cumulative chromatic dispersion in the
signal light propagation path from the input end 101 to the output
end 102, excluding the dispersion compensating optical fiber 132,
is 5 ps/nm or less in the signal light wavelength band, the
compensation target of the dispersion compensating optical fiber
132 becomes the chromatic dispersion in the signal light wavelength
of the optical fiber transmission line where the Raman amplifier 1
is provided.
[0040] The semiconductor laser light sources 161a to 161c output
laser beams with different wavelengths respectively. The optical
multiplexer 151 multiplexes the laser beams which were output from
the semiconductor laser light sources 161a to 161c respectively,
and outputs the multiplexed light to the optical coupler 141 as the
pumping light of a plurality of channels. The optical coupler 141
directly guides the pumping light reaching from the optical
multiplexer 151 to the Raman amplification optical fiber 121, and
also guides the signal light of the plurality of channels reaching
from the Raman amplification optical fiber 121 to the dispersion
compensating optical fiber 132.
[0041] The semiconductor laser light sources 162a to 162c output
laser beams with different wavelengths respectively. The optical
multiplexer 152 multiplexes the laser beams which were output from
the semiconductor laser light sources 162a to 162c respectively,
and outputs the multiplexed light to the optical coupler 142 as the
pumping light of the multiple channels. The optical coupler 142
directly guides the pumping light reaching from the optical
multiplexer 152 to the dispersion compensating optical fiber 132,
and outputs the signal light reaching from the dispersion
compensating optical fiber 132 to the optical isolator 112.
[0042] For example, signal light to be Raman-amplified is a WDM
signal in the S band (wavelength band range of 1460 nm to 1530 nm),
the output wavelength (pumping channel wavelength) of the
semiconductor laser light sources 161a and 162a respectively is
1390 nm, the output wavelength (pumping channel wavelength) of the
semiconductor laser light sources 161b and 162b is 1405 nm
respectively, and the output wavelength (pumping channel
wavelength) of the semiconductor laser light sources 161c and 162c
respectively is 1430 nm.
[0043] The optical transmission line is, for example, a single mode
optical fiber which has a zero dispersion wavelength near the
wavelength of 1.3 .mu.m, and has a positive chromatic dispersion in
the signal light wavelength band. For the Raman amplification
optical fiber 121, an optical fiber having a small effective area
and high non-linearity is suitable, and specifically, high Raman
amplification efficiency can be obtained if the effective area
thereof is 30 .mu.m.sup.2 or less at the pumping light
wavelength.
[0044] The Raman amplification optical fiber 121 may have a
chromatic dispersion whose absolute value is 5 ps/nm/km or more in
the signal light wavelength band, and have a zero dispersion
wavelength of shorter than the shortest wavelength of the pumping
light. In this case, the generation of four wave mixing (including
remote four wave mixing) is effectively controlled, and an
excellent Raman amplification characteristic can be obtained. It is
preferable that the Raman amplification optical fiber 121 has a
cut-off wavelength of shorter than the shortest wavelength of the
pumping light, and in this case, stable gain can be obtained since
the pumping light propagates in the Raman amplification optical
fiber 121 in single mode.
[0045] It is preferable that the signal light propagation path from
the input end 101 to the output end 102 has a 1 ps or less
polarization mode dispersion in the signal light wavelength band.
In this case, deterioration of the Raman amplification
characteristic can be effectively controlled.
[0046] The Raman amplifier 1 according to the first embodiment
operates as follows. The laser beams which were output from the
semiconductor laser light sources 161a to 161c respectively are
multiplexed by the optical multiplexer 151, and the pumping light
of the plurality of channels, which is the multiplexed laser beam,
is supplied to the Raman amplification optical fiber 121 through
the optical coupler 141. The laser beams which were output from the
semiconductor laser light sources 162a to 162c respectively are
multiplexed by the optical multiplexer 152, and the pumping light
of the plurality of channels, which is the multiplexed laser beam,
is supplied to the dispersion compensating optical fiber 132
through the optical coupler 142. The signal light of the plurality
of channels entered from the input end 101 reach the Raman
amplification optical fiber 121 through the optical isolator 111,
and is Raman-amplified in the Raman amplification optical fiber
121.
[0047] The signal light which was Raman-amplified in the Raman
amplification optical fiber 121 passes through the optical coupler
141 and reaches the dispersion compensating optical fiber 132, and
is further Raman-amplified in the dispersion compensating optical
fiber 132. The signal light which was Raman-amplified in the
dispersion compensating optical fiber 132 then passes through the
optical coupler 142 and the optical isolator 112 sequentially, and
is output from the output end 102 to the optical fiber transmission
line outside. The dispersion compensating optical fiber 132 not
only Raman-amplifies the signal light, but also functions so as to
compensate for the chromatic dispersion in the signal light
wavelength of the optical fiber transmission line and Raman
amplification optical fiber 121.
[0048] Therefore the Raman amplifier 1 according to the first
embodiment provides high flexibility to the device design
considering both Raman amplification and dispersion compensation.
In other words, the loss of signal light which propagates through
the optical fiber transmission line is compensated by the Raman
amplification in the Raman amplification optical fiber 121 of this
Raman amplifier 1. The chromatic dispersion in the signal light
wavelength of the optical fiber transmission line and the Raman
amplification optical fiber 121, on the other hand, is compensated
by the dispersion compensating optical fiber 132 in the Raman
amplifier 1. Since the dispersion compensating optical fiber 132
which implements the dispersion compensation function, and the
Raman amplification optical fiber 121 which implements the Raman
amplification function, are optically connected in this way, the
Raman amplifier 1 can provide high design flexibility for both
Raman amplification and dispersion compensation.
[0049] In the Raman amplifier 1 according to the first embodiment,
signal light is Raman-amplified not only in the Raman amplification
optical fiber 121, but also in the dispersion compensating optical
fiber 132, so the dispersion compensating optical fiber 132 has
less effective loss in the signal light wavelength, and becomes a
transmission medium with substantially loss-less. In this case, the
effective gain of Raman amplification of the signal light in the
Raman amplifier 1 is roughly the same as the Raman amplification
gain of the signal light in the Raman amplification optical fiber
121, so the design flexibility for both Raman amplification and
dispersion compensation further increases, and deterioration of the
noise figure can also be effectively controlled.
Second Embodiment of Raman Amplifier
[0050] FIG. 2 is a diagram depicting the configuration of the
second embodiment of the Raman amplifier according to the present
invention. The Raman amplifier 2 according to the second embodiment
is different from the Raman amplifier 1 according to the first
embodiment in that a new Raman amplification optical fiber 122 is
provided between the dispersion compensating optical fiber 132 and
the optical coupler 142. The dispersion compensating optical fiber
132 is arranged between the forward stage Raman amplification
optical fiber 121 and the backward stage Raman amplification
optical fiber 122.
[0051] In the Raman amplifier 2 according to the second embodiment,
the optical amplification section comprises the forward stage Raman
amplification optical fiber 121, the optical coupler 141 (first
optical multiplexing structure), an optical multiplexer 151, the
semiconductor laser light sources 161a to 161c (first pumping light
source), the backward stage Raman amplification optical fiber 122,
the optical coupler 142 (second optical multiplexing structure),
the optical multiplexer 152, and the semiconductor laser light
sources 162a to 162c (second pumping light source). The optical
couplers 141 and 142 (first and second optical multiplexing
structures), the optical multiplexers 151 and 152, and the
semiconductor laser light sources 161a to 161c and 162a to 162c
(first and second pumping light sources) constitute the pumping
light supply system 100. The dispersion compensation section
includes the dispersion compensating optical fiber 132, and pumping
light, which was supplied from the semiconductor laser light
sources 162a to 162c to the backward stage Raman amplification
optical fiber 122 through the optical multiplexer 152 and optical
coupler 142, and which propagated through the backward stage Raman
amplification optical fiber 122, is supplied to this dispersion
compensating optical fiber 132.
[0052] Pumping light, which was output from the semiconductor laser
light sources 161a to 161c (multiplexed light multiplexed by the
optical multiplexer 151), is supplied to the forward stage Raman
amplification optical fiber 121 through the optical fiber 141. This
Raman amplification optical fiber 121 Raman-amplifies the signal
light reaching from the optical isolator 111, and the
Raman-amplified signal light is output to the optical coupler
141.
[0053] Pumping light, which was output from the semiconductor laser
light sources 162a to 162c (multiplexed light multiplexed by the
optical multiplexer 152), is supplied to the backward stage Raman
amplification optical fiber 122 through the optical coupler 142.
This Raman amplification optical fiber 122 Raman-amplifies the
signal light reaching from the dispersion compensating optical
fiber 132, and the Raman-amplified signal light is output to the
optical coupler 142.
[0054] It is preferable that the Raman amplification optical fibers
121 and 122 are transmission medium having a small effective area
and a high non-linearity respectively, and specifically, the
optical fibers for amplification 121 and 122 have an effective area
of 30 .mu.m.sup.2 or less at the pumping light wavelengths
respectively, so as to obtain high Raman amplification efficiency.
It is also preferable that each one of the Raman amplification
optical fibers 121 and 122 has a chromatic dispersion whose
absolute value is 5 ps/nm/km or more in the signal light wavelength
band, and a zero dispersion wavelength of shorter than the shortest
wavelength of the pumping light, and in this case, the generation
of four wave mixing (including remote four wave mixing) is
effectively controlled, and an excellent Raman amplification
characteristic is obtained. Also it is preferable that the Raman
amplification optical fibers 121 and 122 have a cut-off wavelength
of shorter than the shortest wavelength of the pumping light
respectively, and in this case, stable gain can be obtained since
the pumping light propagates in the Raman amplification optical
fibers 121 and 122 in single mode. It is preferable that the signal
light propagation path from the input end 201 to the output end 202
has a 1 ps or less polarization mode dispersion in the signal light
wavelength band, and in this case, deterioration of the Raman
amplification characteristic is effectively controlled.
[0055] Particularly, it is preferable that the signal light
propagation path from the input end 201 to the output end 202,
excluding the dispersion compensating optical fiber 132, has a
cumulative chromatic dispersion whose absolute value is 5 ps/nm or
less in the signal light wavelength band. Therefore it is
preferable that the Raman amplification optical fibers 121 and 122
have a chromatic dispersion with different signs in the signal
light wavelength band respectively. In this case, the compensation
target of the dispersion compensating optical fiber 132 is the
optical fiber transmission line where this Raman amplifier 2 is
inserted, and the chromatic dispersion in the signal light
wavelength of the optical fiber transmission line is compensated
for.
[0056] The Raman amplifier 2 according to the second embodiment
operates as follows. The laser beams which were output from the
semiconductor laser sources 161a to 161c respectively are
multiplexed by the optical multiplexer 151, and the pumping light
of a plurality of channels, which is the multiplexed laser beam, is
supplied to the Raman amplification optical fiber 121 through the
optical coupler 141. The laser beams which were output from the
semiconductor laser light sources 162a to 162c respectively are
multiplexed by the optical multiplexer 152, and the pumping light
of the plurality of channels, which is the multiplexed laser beam,
is sequentially supplied to the Raman amplification optical fiber
122 and the dispersion compensating optical fiber 132 through the
optical coupler 142.
[0057] The signal light of the plurality of channels entering from
the input end 201 passes through the optical isolator 111, reaches
the Raman amplification optical fiber 121, and is Raman-amplified
in the Raman amplification optical fiber 121. The signal light
which was Raman-amplified in the Raman amplification optical fiber
121 passes through the optical coupler 141 and reaches the
dispersion compensating optical fiber 132 and the Raman
amplification optical fiber 122 sequentially, and is
Raman-amplified also in both the optical fibers 132 and 122. The
signal light which was Raman-amplified in the Raman amplification
optical fiber 122 passes through the optical coupler 142 and the
optical isolator 112 sequentially, and is output from the output
end 202 to the optical fiber transmission line outside. Also the
dispersion compensating optical fiber 132 not only Raman-amplifies
the signal light, but also functions so as to compensate for the
chromatic dispersion in the signal light wavelength of the optical
fiber transmission line and the Raman amplification optical fibers
121 and 122.
[0058] Therefore the Raman amplifier 2 according to the second
embodiment provides high design flexibility for both Raman
amplification and dispersion compensation, just like the case of
the first embodiment. In other words, the loss of the signal light,
which propagates through the optical fiber transmission line, is
compensated by the Raman amplification in the Raman amplification
optical fibers 121 and 122 of this Raman amplifier 2. The chromatic
dispersion in the signal light wavelength of the optical fiber
transmission line and the Raman amplification optical fibers 121
and 122, on the other hand, is compensated for by the dispersion
compensating optical fiber 132 in the Raman amplifier 2. Since the
dispersion compensating optical fiber 132, which implements the
dispersion compensation function, and the Raman amplification
optical fibers 121 and 122, which implement the Raman amplification
functions, are provided while being optically connected to each
other, the Raman amplifier 2 can provide high design flexibility
for both Raman amplification and dispersion compensation.
[0059] In the Raman amplifier 2 according to the second embodiment,
the signal light is Raman-amplified not only in the Raman
amplification optical fibers 121 and 122, but also in the
dispersion compensating optical fiber 132, so the dispersion
compensating optical fiber 132 has less effective loss in the
signal light wavelength, and becomes a transmission medium with
substantially no loss. In this case, the effective gain of Raman
amplification of the signal light in the Raman amplifier 2 is
roughly the same as the Raman amplification gain of the signal
light in the Raman amplification optical fibers 121 and 122, so the
design flexibility for both Raman amplification and dispersion
compensation further increases, and deterioration of the noise
figure can also be effectively controlled.
[0060] Also if the signal light propagation path from the input end
201 to the output end 202, excluding the dispersion compensating
optical fiber 132, is designed so as to have a cumulative chromatic
dispersion whose absolute value is 5 ps/nm or less at the signal
light wavelength band in the Raman amplifier 2 according to the
second embodiment, then the dispersion compensating optical fiber
132 compensates for the chromatic dispersion of the optical fiber
transmission line at the signal light wavelength, with the optical
fiber transmission line where this Raman amplifier 2 is provided as
a target to be compensated for. Therefore this Raman amplifier 2
has high design flexibility for both Raman amplification and
dispersion compensation, and effectively controls the generation of
a non-linear optical phenomena, such as self phase modulation. Also
in the Raman amplifier 2 according to the second embodiment, the
dispersion compensating optical fiber 132 is arranged between the
forward stage Raman amplification optical fiber 121 and the
backward stage Raman amplification optical fiber 122, so Raman
amplification with low noise and high gain becomes possible.
Comparison Example
[0061] Two comparison examples of the Raman amplifier according to
the present invention will now be described.
[0062] FIG. 3 is a diagram depicting the configuration of the Raman
amplifier 3 according to the first comparison example. The
difference between the Raman amplifier 3 according to the first
comparison example and the Raman amplifier 1 according to the first
embodiment is that the dispersion compensating optical fiber 131 is
provided instead of the Raman amplification optical fiber 121. The
Raman amplifier 3 according to this first embodiment has only the
dispersion compensating optical fibers 131 and 132, which implement
the dispersion compensation function, and does not have the Raman
amplification optical fiber for implementing the Raman
amplification function. In this first comparison example, both of
the dispersion compensating optical fibers 131 and 132 function as
the Raman amplification optical fibers, and also function as the
dispersion compensation section for compensating for the chromatic
dispersion of the optical fiber transmission line.
[0063] The Raman amplifier 3 according to this first comparison
example operates as follows. The signal light of a plurality of
channels entering from the input end 301 passes through the optical
isolator 111, reaches the dispersion compensating optical fiber
131, and is Raman-amplified in the dispersion compensation optical
fiber 131. The signal light, which was Raman-amplified in the
dispersion compensating optical fiber 131, passes through the
optical coupler 141, reaches the dispersion compensation optical
fiber 132, and is also Raman-amplified in this dispersion
compensating optical fiber 132. And the signal light, which was
Raman-amplified in the dispersion compensating optical fiber 132,
passes through the optical coupler 142 and the optical isolator 112
sequentially, and is output from the output end 302 to the optical
fiber transmission line outside. The dispersion compensating
optical fibers 131 and 132 not only Raman-amplify the signal light,
but also function so as to compensate for the chromatic dispersion
of the optical fiber transmission line at the signal light
wavelength.
[0064] FIG. 4 is a diagram depicting the configuration of the Raman
amplifier 4 according to the second comparison example. The
difference between the Raman amplifier 4 according to the second
comparison example and the Raman amplifier 1 according to the first
embodiment is that the Raman amplification optical fiber 121, the
optical the coupler 141, the optical multiplexer 151, and the
semiconductor laser light sources 161a to 161c, are not provided.
In the Raman amplifier 4 according to the second comparison
example, the dispersion compensating optical fiber 132 implements
the dispersion compensation function and the Raman amplification
function, and the Raman amplification optical fiber is not
provided.
[0065] The Raman amplifier 4 according to this second comparison
example operates as follows. The signal light of a plurality of
channels entering from the input end 401 passes through the optical
isolator 111, reaches the dispersion compensating optical fiber
132, and is Raman-amplified in this dispersion compensating optical
fiber 132. The signal light, which was Raman-amplified in the
dispersion compensating optical fiber 132, passes through the
optical coupler 142 and the optical isolator 112 sequentially, and
is output from the output end 402 to the optical fiber transmission
line outside. The dispersion compensating optical fiber 132 not
only Raman-amplifies the signal light, but also functions so as to
compensate for the chromatic dispersion of the optical fiber
transmission line at the signal light wavelength.
Comparison of First and Second Embodiments, and First and Second
Comparison Examples
[0066] Now the Raman amplifier 1 according to the first embodiment
(FIG. 1), Raman amplifier 2 according to the second embodiment
(FIG. 2), Raman amplifier 3 according to the first comparison
example (FIG. 3), and the Raman amplifier 4 according to the second
comparison example will be compared with each other.
[0067] In the Raman amplifiers 1 and 2, the length of the Raman
amplification optical fiber 121 is 3 km, the length of the Raman
amplification optical fiber 122 is 3 km, and the length of the
dispersion compensating optical fiber 132 is 15 km respectively. In
the Raman amplifier 3, the length of the dispersion compensating
optical fiber 131 is 3 km, and the length of the dispersion
compensating optical fiber 132 is 12 km. And in the Raman amplifier
4, the length of the dispersion compensating optical fiber 132 is
15 km.
[0068] In this way, in each one of the Raman amplifiers 1 to 4, the
total length of the dispersion compensating optical fiber is set so
as to match 15 km.
[0069] In each one of the Raman amplifiers 1 to 4, the insertion
loss of the optical isolator 111 is 1 dB, the insertion loss of the
optical coupler 141 is 0.6 dB, and the insertion loss of both the
optical coupler 142 and optical isolator 112 is 1.2 dB. In the
Raman amplifier 2, the connection loss of the dispersion
compensating optical fiber 132 and Raman amplification optical
fiber 122 is 0.3 dB. And in each one of the Raman amplifiers 1 to
4, the output power of each semiconductor laser light source is set
such that the average gain in the S band becomes 20 dB.
[0070] FIGS. 5A and 5B are graphs depicting the gain characteristic
and noise figure characteristic of the Raman amplifiers 1 to 4
respectively. In FIG. 5A, the graph Gain 1 indicates the gain
characteristic of the Raman amplifier 1, graph Gain 2 indicates the
gain characteristic of the Raman amplifier 2, graph Gain 3
indicates the gain characteristic of the Raman amplifier 3, and
graph Gain 4 indicates the gain characteristic of the Raman
amplifier 4 respectively. In FIG. 5B, graph NF1 indicates the noise
figure characteristic of the Raman amplifier 1, graph NF2 indicates
the noise figure characteristic of the Raman amplifier 2, graph NF3
indicates the noise figure characteristic of the Raman amplifier 3,
and graph NF4 indicates the noise figure characteristic of the
Raman amplifier 4 respectively. As FIGS. 5A and 5B show, the noise
figures of the Raman amplifiers 1 and 2 according to the first and
second embodiments are lower than the noise figures of the Raman
amplifiers 3 and 4 according to the first and second comparison
examples respectively.
[0071] FIG. 6 is a table showing the output power of each
semiconductor laser light source of the Raman amplifiers 1 to 4
respectively. As this table shows, the required pumping light power
of the Raman amplifiers 1 and 2 according to the first and second
embodiments are lower than the required pumping light power of the
Raman amplifiers 3 and 4 according to the first and second
comparison examples respectively.
Raman Amplifiers According to the First and Second Embodiments
[0072] The Raman amplifiers 1 and 2 (particularly the Raman
amplification optical fibers 121 and 122) according to the first
and second embodiments will now be described.
[0073] Generally, compared with a rare earth element-doped optical
fiber amplifier, the Raman amplifier has an advantage in that there
is no limit in the wavelength band that has gain, but there is a
disadvantage in that the pumping efficiency is low. However the
Raman gain coefficient (g.sub.R/A.sub.eff) of the Raman
amplification optical fibers 121 and 122 can be increased by
decreasing the effective area A.sub.eff of the Raman amplification
optical fibers 121 and 122.
[0074] FIG. 7 is a graph depicting the relationship between the
relative refractive index difference of the core region and
g.sub.R. FIG. 7 shows the relationship between the relative
refractive index difference of the core region and g.sub.R for
various optical fibers, such as a standard single mode optical
fiber where GeO.sub.2 is added to the core region, a single mode
optical fiber where the core region is pure silica glass and an F
element is added to the cladding region, a dispersion-shifted
optical fiber where the zero dispersion wavelength is shifted to
the longer wavelength side at wavelength 1.3 .mu.m, a dispersion
compensating optical fiber where the chromatic dispersion is
negative at wavelength 1.55 .mu.m, and an optical fiber which
effective area is small, and non-linearity is high. As FIG. 7
shows, g.sub.R is substantially in a linear relationship with the
relative refractive index difference, and is 2.3.times.10.sup.-14
m/W or more. Therefore in the following description, it is assumed
that g.sub.R=2.3.times.10.sup.-14 m/W.
[0075] FIG. 8 is a graph depicting the relationship between the
relative refractive index difference of the core region and
transmission loss .alpha.. In FIG. 8, graph L1 shows the
relationship between the relative refractive index difference of
the core region in a typical optical fiber and transmission loss
.alpha. at wavelength 1.45 .mu.m, and graph L2 shows the
relationship between the relative refractive index difference of
the core region in a typical optical fiber and transmission loss
.alpha. at the wavelength 1.55 .mu.m. According to FIG. 8, the
transmission loss .alpha. at the pumping light wavelength is
assumed to be 0.55 dB/km, and the actual length L of the Raman
amplification optical fiber, where the effective length L.sub.eff
of the Raman amplification optical fiber does not become 1/2 or
less of the actual length, is assumed to be 14.5 km.
[0076] The power pump P.sub.pump of the pumping light to be
supplied to the optical fiber for Ramon amplification is assumed to
be 500 mW, which is equivalent to the maximum input pumping light
power to the optical fiber for amplification with a general
configuration of the Er-doped optical fiber amplifier, which is
commercialized as a centralized optical amplifier.
[0077] The Er-doped optical fiber amplifier 9 with a general
configuration comprises, for example, an optical isolator 911,
optical coupler 941, Er-doped optical fiber 931, optical isolator
912, optical coupler 942a, Er-doped optical fiber 932, optical
coupler 942b, dispersion compensator 971, optical isolator 913,
optical coupler 943a, Er-doped optical fiber 933, and optical
coupler 943b, which are arranged sequentially from input end 901
towards output end 902, as shown in FIG. 9. The pumping light,
which is output from the semiconductor laser light source 961, is
supplied to the Er-doped optical fiber 931 through the optical
coupler 941 in the forward direction with respect to the signal
light. The pumping light, which is output from the semiconductor
laser light source 962, is branched into two by the optical
branching unit 952. One of the branched lights is supplied to the
Er-doped optical fiber 932 through the optical coupler 942a in the
forward direction with respect to the signal light. The other
branched light is supplied to the Er-doped optical fiber 932
through the optical coupler 942b in the back direction with respect
to the signal light. The pumping light, which is output from the
semiconductor laser light source 963, is supplied to the Er-doped
optical fiber 933 through the optical coupler 943a in the forward
direction with respect to the signal light. The pumping light,
which is output from the semiconductor laser light sources 964a and
964b respectively, is multiplexed by the optical multiplexer 954.
And this multiplexed light is supplied to the Er-doped optical
fiber 933 through the optical coupler 943b in the backward
direction with respect to the signal light.
[0078] If attenuation of the pumping light is ignored, then the
Raman amplifier gain G.sub.Raman (dB) is given by the following
formula. G Raman = 10 log .times. { exp ( g R A eff .times. L eff
.times. P pump ) } ##EQU1##
[0079] As a consequence, if the effective area A.sub.eff of the
Raman amplification optical fiber is 30 .mu.m.sup.2 or less at the
pumping light wavelength, then the absolute value of the Raman
amplification gain G.sub.Raman becomes a loss of 25 dB or more per
span (one relay section) in a typical land optical communication
system, which is desirable.
[0080] Since the Raman amplification optical fiber is long, the
wavelength deterioration of signal light tends to occur in Raman
amplification optical fiber due to a non-linear optical phenomena,
such as self phase modulation and four wave mixing, if the
effective area A.sub.eff is small. However, in the case of the
Raman amplifier according to the present invention, each of the
chromatic dispersion and polarization mode dispersion of the Raman
amplification optical fiber is appropriately set, so wave form
deterioration of the signal light is effectively controlled.
[0081] If the Raman amplifier is applied to the optical
communication system as a preamplifier, the loss of the optical
demultiplexer, arranged between the Raman amplifier as a
pre-amplifier and the light receiving section, is generally about
10 dB, and the light receiving dynamic range of the light receiving
section per channel is generally -16 dBm/ch to -10 dBm/ch.
Therefore the signal light output power per channel of the Raman
amplifier requires -6 dBm/ch or more.
[0082] FIG. 10 is a graph depicting the relationship between the
signal light output power per channel of the Raman amplifier and
power penalty. Here 8 channels of multiplexed signal light are
input to the Raman amplifier. The Raman amplification optical fiber
has high non-linearity. In FIG. 10, graph P1 shows the relationship
in the Raman amplification optical fiber with the chromatic
dispersion of +2 ps/nm/km, graph P2 shows the relationship in the
Raman amplification optical fiber with the chromatic dispersion of
-2 ps/nm/km, graph P3 shows the relationship in the Raman
amplification optical fiber with the chromatic dispersion of -5
ps/nm/km, graph P4 shows the relationship in the Raman
amplification optical fiber with the chromatic dispersion of -7
ps/nm/km, graph P5 shows the relationship in the Raman
amplification optical fiber with the chromatic dispersion of -20
ps/nm/km, and graph P6 shows the relationship in the Raman
amplification optical fiber with the chromatic dispersion of -40
ps/nm/km. As FIG. 10 shows, if the Raman amplification optical
fiber has high non-linearity and has a chromatic dispersion whose
absolute value is 5 ps/nm/km or less, then the power penalty caused
by the four wave mixing does not become 1 dB or less unless the
signal light output power per channel is 2 dBm or less. When the
result is applied to the case of 64 channel signal light
transmission, the signal light output power per channel is -7 dBm,
which is outside the light receiving dynamic range of the light
receiving section in the optical communication system where the
Raman amplifier is applied to the preamplifier. However, as
mentioned above, this problem can be avoided if each one of the
Raman amplification optical fibers 121 and 122 has a chromatic
dispersion whose absolute value is 5 ps/nm/km or more in the signal
light wavelength band.
[0083] For example, it is preferable that the Raman amplification
optical fiber 121 is mainly made from silica glass, and comprises a
GeO.sub.2-doped core region having an outer diameter of 4.0 .mu.m,
and an F-doped cladding region surrounding the core region, where
the relative refractive index difference of the core region is
+2.5%, and the relative refractive index difference of the cladding
region is -0.7% with respect to the pure silica glass. If the
refractive index of the pure silica glass is n.sub.0, the
refractive index of the core region is n.sub.1, the refractive
index of the cladding region is n.sub.2, then the relative
refractive index difference .DELTA..sub.1 of the core region and
the relative refractive index difference .DELTA..sub.2 of the
cladding region with respect to the pure silica glass are given by
the following formulas respectively.
.DELTA..sub.1=(n.sub.1.sup.2-n.sub.0.sup.2)/2n.sub.0.sup.2
.DELTA..sub.2=(n.sub.2.sup.2-n.sub.0.sup.2)/2n.sub.0.sup.2 In this
case, the Raman amplification optical fiber 121 has a chromatic
dispersion of -9.0 ps/nm/km at a wavelength of 1.55 .mu.m, an
effective area of 9.9 .mu.m.sup.2, and a Raman gain coefficient of
5.8.times.10.sup.-3/Wm at the wavelength of 1.55 .mu.m.
[0084] Also, if the land main optical communication system with a
relay section of 100 km.times.6 span at bit rate 10 Gb/s is
assumed, for example, the polarization mode dispersion which is
allowed in this optical communication system is 10 ps or less.
Therefore as mentioned above, if the polarization mode dispersion
in the signal light propagation path from the input end to the
output end of the Raman amplifier is 1 ps or less in the signal
light wavelength band, then the transmission quality in this
optical communication system is excellent.
(Optical Communication System)
[0085] The optical communication system according to the present
invention includes an optical fiber transmission line where the
signal light of a plurality of channels propagates, and a Raman
amplifier having the above mentioned structure. Particularly to
enable a long haul transmission, the optical communication system
according to the present invention may have a plurality of Raman
amplifiers having a structure similar to the Raman amplifier. The
optical communication system according to the present invention can
be modified in various ways to further improve system performance
by improving the optical SN ratio.
[0086] As FIG. 11 shows, the optical communication system according
to the first embodiment has a configuration to further improve the
noise characteristic by causing induced Raman scattering in the
optical fiber transmission line 10 at the input end side of the
Raman amplifier, which locates at the most upstream side in the
Raman amplifiers 1(2) (Raman amplifier according to the present
invention), which is provided at a predetermined position of the
optical fiber transmission line 10 laid between the transmitting
station 11 and the receiving station 12. FIG. 11 is a diagram
depicting the configuration of the first embodiment of the optical
communication system according to the present invention.
[0087] Specifically, the optical communication system according to
the first embodiment comprises a pumping light source 13 (third
pumping light source) for supplying new pumping light to the
optical fiber transmission line 10 at the input end side, and
optical coupler 14 (third optical multiplexing structure) for
guiding the pumping light from the pumping light source 13 to the
optical fiber transmission line 10. In this way, by
Raman-amplifying the signal light in advance, before the signal
light is input to the Raman amplifiers 1(2) arranged on the optical
fiber transmission line 10, the noise characteristic is
dramatically improved.
[0088] FIGS. 12A and 12B are diagrams depicting the configuration
of the second embodiment of the optical communication system
according to the present invention. In the optical communication
system according to the second embodiment as well, a plurality of
Raman amplifiers 1(2) (Raman amplifiers according to the present
invention) are arranged on the optical fiber transmission line 10
laid between the transmitting station 11 and the receiving station
12.
[0089] In particular, the optical communication system according to
the second embodiment comprises bypass transmission lines 15a, 15b
and 15c for supplying pumping light which propagated at least a
part of the Raman amplification optical fiber 121 of the Raman
amplifier, and optical couplers 14a, 14b and 14c for guiding the
pumping light which propagated the bypass transmission lines 15a,
15b and 15c to the optical fiber transmission line (fourth optical
multiplexing structure) on the optical fiber transmission line at
the input end side in each Raman amplifier 1(2) to improve the
noise characteristic of the entire optical communication
system.
[0090] Also to guide the pumping light from each Raman amplifier
1(2) to the bypass transmission lines 15a, 15b and 15c, in this
optical communication system, an optical demultiplexer 16 for
guiding the light which propagated through the Raman amplification
optical fiber 121 to the bypass transmission lines 15a, 15b and 15c
respectively, and an optical filter 17 for transmitting the pumping
light out of the light demultiplexed by the optical demultiplexer
16, are provided in each Raman amplifier 1(2), as shown in FIG.
12B.
[0091] As a consequence, according to the present invention, the
dispersion compensation section which implements the dispersion
compensation function, and the optical amplification section which
implements the Raman amplification function, are provided as
independent device composing elements. Therefore high design
flexibility is obtained for both the device design considering
Raman amplification and the device design considering dispersion
compensation, without being restricted by the respective design
conditions. In particular, when the signal light propagation path
in the Raman amplifier, excluding the dispersion compensation
section, has a cumulative chromatic dispersion whose absolute value
is 5 ps/nm or less in the signal light wavelength band, then
flexibility of device design considering both Raman amplification
and dispersion compensation further increases.
[0092] From the invention thus described, it will be obvious that
the embodiments of the invention may be varied in many ways. Such
variations are not to be regarded as a departure from the spirit
and scope of the invention, and all such modifications as would be
obvious to one skilled in the art are intended for inclusion within
the scope of the following claims.
* * * * *