U.S. patent application number 09/769532 was filed with the patent office on 2001-07-19 for optical amplifier for use in optical communications equipment.
Invention is credited to Chikama, Terumi, Kinoshita, Susumu, Ohshima, Chihiro.
Application Number | 20010008459 09/769532 |
Document ID | / |
Family ID | 26556076 |
Filed Date | 2001-07-19 |
United States Patent
Application |
20010008459 |
Kind Code |
A1 |
Ohshima, Chihiro ; et
al. |
July 19, 2001 |
Optical amplifier for use in optical communications equipment
Abstract
The output level of pumping light supplied from a pumping light
source is varied using a variable attenuator, which is controlled
by a variable attenuator driver circuit. By separating a portion
containing the pumping light source from a portion containing an
amplification medium, it becomes possible to prevent heat emitted
by the pumping light source from adversely affecting the
amplification medium. By arranging the portion containing the
pumping light source such that a pumping light source or sources
can be added when necessary, optical transmission system
requirements of having more pumping light sources for system
upgrade can be accommodated readily. By packaging portions
containing amplification media, an optical amplifier can be made
small.
Inventors: |
Ohshima, Chihiro;
(Kawasaki-shi, JP) ; Kinoshita, Susumu;
(Kawasaki-shi, JP) ; Chikama, Terumi;
(Kawasaki-shi, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
700 11TH STREET, NW
SUITE 500
WASHINGTON
DC
20001
US
|
Family ID: |
26556076 |
Appl. No.: |
09/769532 |
Filed: |
January 26, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09769532 |
Jan 26, 2001 |
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09677759 |
Oct 3, 2000 |
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09677759 |
Oct 3, 2000 |
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09042790 |
Mar 17, 1998 |
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Current U.S.
Class: |
359/341.44 ;
359/341.32; 359/341.33; 359/345 |
Current CPC
Class: |
H01S 3/094061 20130101;
H01S 3/13013 20190801; H01S 3/09408 20130101; H01S 3/094003
20130101; H01S 3/1001 20190801; H01S 3/06754 20130101; H01S 3/06704
20130101 |
Class at
Publication: |
359/341.44 ;
359/341.32; 359/341.33; 359/345 |
International
Class: |
H01S 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 17, 1997 |
JP |
09-285905 |
Claims
What is claimed is:
1. An optical amplifier for amplifying incoming signal light in
response to pumping light applied thereto, comprising: a variable
attenuator for varying the input level of the pumping light applied
to the optical amplifier to thereby tune the amplification
characteristic of the optical amplifier.
2. The optical amplifier according to claim 1, wherein optical
coupler means for supplying the pumping light by coupling pumping
light beams generated by multiple pumping light sources into one is
provided.
3. The optical amplifier according to claim 1, wherein optical
splitter means for supplying the pumping light by splitting a
pumping light beam by one pumping light source into a plurality of
pumping lights, is provided.
4. The optical amplifier according to claim 1, wherein optical
coupler/splitter means for supplying the pumping light by coupling
pumping light beams generated by multiple pumping light sources
into one beam and then splitting the one beam into a plurality of
beams, is provided.
5. The optical amplifier according to claim 2, wherein the multiple
pumping light sources and the optical coupler means are assembled
into a unit, and the unit is arranged such that pumping light from
a separate pumping light source or a separate unit can be
additionally coupled to the optical coupler means.
6. The optical amplifier according to claim 4, wherein the multiple
pumping light sources and the optical coupler means are assembled
into a unit, and the unit is arranged such that pumping light from
a separate pumping light source or a separate unit can be
additionally coupled to the optical coupler/splitter means.
7. The optical amplifier according to claim 2, wherein the multiple
pumping light sources, the optical coupler means and the variable
attenuator are assembled into a unit.
8. The optical amplifier according to claim 3, wherein the pumping
light source, the optical splitter means and the variable
attenuator are assembled into a unit.
9. The optical amplifier according to claim 4, wherein the multiple
pumping light sources, the optical coupler/splitter means and the
variable attenuator are assembled into a unit.
10. The optical amplifier according to claim 7, wherein the optical
coupler means is arranged such that pumping light from a separate
pumping light source or a separate unit can be additionally coupled
thereto.
11. The optical amplifier according to claim 9, wherein the optical
coupler/splitter means is arranged such that pumping light from a
separate pumping light source or a separate unit can be
additionally coupled thereto.
12. A package incorporating multiple optical amplification units
each of which comprises an amplification medium for amplifying
incoming signal light in response to application thereto of pumping
light supplied from an optical splitter means for splitting a
pumping light beam generated by one pumping light source into
multiple light beams or an optical coupler/splitter means for
coupling pumping light beams generated by multiple pumping light
sources into one beam and splitting the one beam into a plurality
of beams.
13. An optical communications device comprising: a pumping light
source unit and an optical amplification unit for amplifying
incoming signal light in response to application thereto of pumping
light from the pumping light source unit, wherein the pumping light
source unit is placed in a location in the optical communications
device where heat radiation conditions are good.
14. An optical amplifier for amplifying incoming signal light in
response to application thereto of pumping light from a pumping
light source unit having a pumping light source for generating a
pumping light beam and optical coupler means for coupling multiple
pumping light beams, the pumping light source unit comprising: a
polarization plane rotating unit to rotate the plane of
polarization of output pumping light from the optical coupler means
through a first angle of rotation for transmission and rotating the
plane of polarization of return light, resulting from the output
pumping light being reflected from a connector connecting the
pumping light source unit and other components of the optical
amplifier back to the pumping light source unit, through a second
angle of rotation, thereby inputting to the pumping light source
return light different in wavelength from the pumping light source
generated by the pumping light source.
15. An optical amplifier for amplifying incoming signal light in
response to application thereto of pumping light from a pumping
light source unit having multiple pumping light sources each
generating a pumping light beam and optical coupler/splitter means
for coupling multiple pumping light beams and splitting into a
plurality of beams, the pumping light source unit comprising: a
polarization plane rotating unit to rotate the plane of
polarization of output pumping light from the optical coupler means
through a first angle of rotation for transmission and to rotate
the plane of polarization of return light, resulting from the
output pumping light being reflected from a connector connecting
the pumping light source unit and other components of the optical
amplifier together back to the pumping light source unit, through a
second angle of rotation, thereby inputting to the pumping light
source return light different in wavelength from the pumping light
source generated by the pumping light source.
16. An optical amplifier in which a pumping light source unit
having a pumping light source for generating pumping light and an
optical amplification unit having an amplification medium for
amplifying incoming signal light in response to application of the
pumping light thereto are connected together by means of a
connector that allows the pumping light to be transmitted to the
optical amplification unit, the optical amplification unit
comprising: a unit to determine whether or not the connection
between the pumping light source unit and the optical amplification
unit is established by means of the connector on the basis of the
output level of the pumping light from the pumping light source
unit.
17. An optical amplifier in which a pumping light source unit
having a pumping light source for generating pumping light and an
optical amplification unit having an amplification medium for
amplifying incoming signal light in response to application of the
pumping light thereto are connected together by means of a
connector that allows the pumping light to be transmitted to the
optical amplification unit, the pumping light source unit
comprising: a unit to determine whether or not the connection
between the pumping light source unit and the optical amplification
unit is established by means of the connector on the basis of the
level of return light reflected from the connector.
18. An optical amplifier in which a pumping light source unit
having a pumping light source for generating pumping light and an
optical amplification unit having an amplification medium for
amplifying incoming signal light in response to application of the
pumping light thereto are connected together by means of a
connector that allows the pumping light to be transmitted to the
optical amplification unit, comprising: a unit to determine whether
or not the connection between the pumping light source unit and the
optical amplification unit is established by means of the
connector.
19. The optical amplifier according to any one of claims 16, 17 and
18, wherein, when it is determined that the connection between the
pumping light source unit and the optical amplification unit is not
established by means of the connector, the pumping light source
unit attenuates the output level of the pumping light to a safety
level or stops the output of the pumping light.
20. An optical amplification unit having an amplification medium
for amplifying incoming signal light in response to application
thereto of pumping light from a separate pumping light source unit,
the optical amplification unit and the pumping light source unit
being connected by a connector to form an optical amplifier,
wherein a variable attenuator for adjusting the level of pumping
light input to the amplification medium is provided.
21. A pumping light source unit having a pumping light source for
generating pumping light to be output to a separate optical
amplification unit, the pumping light source unit and the optical
amplification unit being connected by a connector to form an
optical amplifier, wherein a variable attenuator for adjusting the
level of pumping light to be output to the optical amplification
unit is provided.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an optical amplifier for
use in optical communications equipment.
[0003] 2. Description of the Related Art
[0004] In recent years, wavelength division multiplexing (WDM)
transmission systems have been introduced. In transmitting
stations, increasing the number of signal channels results in an
increase in the number of post amplifiers used. An optical
amplifier takes up a lot of space in optical transmitting
equipment. It is therefore desired that the optical amplifier be
decreased in size and produced in integrated form.
[0005] In the optical amplifier, a laser diode (LD) is used as a
pumping light source and a component that gives off heat generated
by the LD is associated with the light source. Of components that
construct the optical amplifier, the light source and its
associated radiator are very large, preventing the optical
amplifier from becoming reduced in size. Thus, separating the
pumping light source from the optical amplifier and grouping
components necessary for use as a pumping light source, such as a
light source and its associated driving circuit, as a pumping light
source unit were thought of.
[0006] FIGS. 1A, 1B and 1C show arrangements of conventional
pumping light source units.
[0007] As shown in FIGS. 1A, 1B and 1C, three types of pumping
light source units may be considered (note that the example of FIG.
1A uses four pumping LDs). The example of FIG. 1C is used in the
WDM system developed by CIENA (see catalog "CIENA Multiwave Line
Amplifier Block Diagram").
[0008] In FIGS. 1A to 1C, 1 denotes a circuit for driving a pumping
light source, 2, 3, 4, 5 and 8; pumping light sources, 6; a
polarization beam splitter (PBS), 7; a wavelength division
multiplexing (WDM) coupler, 9; an optical splitter for separating
pumping light, and 10; an optical coupler/splitter for coupling and
separating pumping light.
[0009] FIG. 1A, each of the four pumping light sources 2, 3, 4 and
5 is driven (for example, supplied with current) by a respective
one of the four pumping light driving circuits 1 to produce light.
The light from each of the pumping light sources 2 to 5 is output
as linearly polarized light. The beams of light output from the
pumping light sources 2 and 3 can be set substantially equal to
each other in wavelength but will be differently polarized, The
beams of light which are substantially equal to each other in
wavelength but differently polarized are polarization-coupled by
the polarization beam splitter 6. The pumping light sources 4 and 5
are of the same type as the pumping light sources 2 and 3 but will
differ from the pumping light sources 2 and 3 in output wavelength.
This indicates that, even if the pumping light sources 4 and 5 are
of the same type as the pumping light sources 2 and 3, their
wavelengths do not necessarily match because of variations in
manufacturing process. The polarization-coupled pumping light from
the pumping light sources 2 and 3 and the polarization-coupled
pumping light from the pumping light sources 4 and 5 are coupled by
the WDM coupler 7. The light output from the WDM coupler is sent to
an optical amplification medium, for example, an erbium doped fiber
(EDF), for use as pumping light for amplifying light signals. The
WDM coupler is so named because it couples wavelength-multiplexed
signal light and pumping light of a specific wavelength, but in
practice it can be an ordinary optical coupler.
[0010] This arrangement is used when only one pumping light source
cannot provide a sufficient amplification action to the optical
amplification medium and intended to obtain pumping light with a
larger power through the use of two or more pumping light
sources.
[0011] The pumping light source unit of FIG. 1B comprises one
pumping light source driver 1, one pumping light source 8, and an
optical splitter 9 for separating pumping light from the pumping
light source 8. This arrangement is used when the pumping light
source has a power large enough to supply two or more optical
amplification media (not shown). This arrangement allows the two or
more optical amplification media to be operated equally with one
pumping light source having a single wavelength and a single
polarized wave.
[0012] The pumping light source unit of FIG. 1C comprises two or
more pumping light source drivers 1, an equal number of pumping
light sources 8, and an optical coupler/splitter 10 that couples
and separates pumping beams of light from the pumping light
sources. This arrangement is intended to supply two or more optical
amplification media with pumping light from a single pumping light
source unit, but it has two or more pumping light sources 8 to
provide pumping light with higher power because a single pumping
light source alone cannot give all of the optical amplifiers a
sufficient amplifying action. As described above, however, there
are variations in wavelength between two or more pumping light
sources 8. Thus, the use of each of the pumping light sources 8 for
a respective one of the optical amplification media will cause
their respective amplification actions to vary. For this reason,
this pumping light source unit is arranged such that beams of
pumping light from the pumping light sources 8 are first coupled to
produce a single beam of light and then the single beam of light is
separated to thereby supply each of the optical amplification media
with pumping light of the same property.
[0013] Hereinafter, what includes a pumping light source unit, an
optical amplification medium or media, and other circuits including
a control circuit shall be called an optical amplifier.
[0014] In an optical amplifier, automatic gain control (AGC) or
automatic level control (ALC) is sometimes performed to control the
supply amount of pumping light to an optical amplification medium.
In conventional optical amplifiers, the amount of output light of a
pumping light source is varied to vary the supply amount of pumping
light to the optical amplification medium by changing a drive
current to the pumping light source.
[0015] In the WDM transmission system, signal light beams of
different wavelengths are collectively amplified by an optical
amplifier. After the start of system implementation, the WDM
transmission system is sometimes modified to increase the signal
multiplexing degree (i.e., the system is upgraded). When the
multiplexing degree is increased, the optical amplifier requires
more pumping power in order to increase the supply amount of
pumping light to the optical amplification media.
[0016] In the pumping light source units shown in FIGS. 1B and 1C,
the ratio of the output power of each pumping light is fixed to the
ratio of the splitting by the splitter 9 or the optical
coupler/splitter 10, and the amount of pumping light at each output
port cannot be varied arbitrarily.
[0017] The optical amplifier contains components that are
associated with a pumping light source to radiate heat generated by
it. These components are relatively large among components
composing the optical amplifier, preventing the optical amplifier
from becoming reduced in size. When LDs as pumping light sources
that generate heat and driver circuits therefor are placed close
together within the optical amplifier, it will create an excessive
rise in temperature, reducing the performance and reliability of
the optical amplifier.
[0018] Heretofore, even if the WDM transmission system is upgraded,
pumping light can only be output up to the allowable maximum output
of a pumping light source installed in an optical amplifier at the
beginning of system implementation. As an example, assume that, in
a 16-channel WDM transmission system, only four channels are
employed at the beginning of system implementation. In this case,
the optical amplifier used is naturally equipped with pumping light
sources necessary to accommodate 16 channels, which increases the
initial investment at the time of system installation.
[0019] In an optical communications device equipped with an optical
amplifier of the built-in pumping light source type, heat generated
in the device is difficult to radiate and it is therefore necessary
to cool the device with a fan, dissipating extra power.
[0020] When pumping light of a narrow spectrum width emitted by an
LD as a pumping light source reflects from optical parts composing
an optical amplifier or a fiber junction back to the pumping LD,
the operation of the LD becomes unstable, which makes the operation
of the optical amplifier unstable. To avoid this problem,
conventionally the optical amplifier has an optoisolater built in
on the output side of the pumping LD. This arrangement requires
more optical parts.
[0021] In the most used type of an optical amplifier, the
amplification characteristic of an amplification medium is
wavelength-dependent, and pumping LDs have variations in wavelength
due to variations in the manufacturing process. For this reason,
the optical amplifier has variations in amplification
characteristic due to variations in wavelength.
[0022] In an optical amplifier in which a pumping light source unit
and an optical amplification medium are coupled together by means
of a connector, it is necessary to sound an alarm in the case that
the connector has come off, because pumping light leaking out
through the connector is very dangerous for persons at work.
SUMMARY OF THE INVENTION
[0023] It is an object of the present invention to provide an
optical amplifier which has a function of changing the supply
amount of pumping light to an amplification medium, and is small in
size and little affected by heat generated by a pumping light
source.
[0024] According to a first aspect of the present invention, there
is provided an optical amplifier for amplifying incoming signal
light in response to pumping light applied thereto, characterized
by including a variable attenuator for varying the input level of
the pumping light applied to the optical amplifier to thereby tune
the amplification characteristic of the optical amplifier.
[0025] In the optical amplifier of the present invention, two or
more optical amplification units that contain amplification media
are assembled into one package.
[0026] The optical communications device of the present invention
comprises a pumping light source unit having at least one pumping
light source for generating pumping light and a unit for separating
or coupling pumping light from the at least one pumping light
source, and an optical amplification unit for amplifying incoming
signal light in response to application thereto of pumping light
from the pumping light source unit and is characterized in that the
pumping light source unit is placed in a location in the optical
communications device where heat radiation conditions are good.
[0027] According to a second aspect of the present invention, there
is provided an optical amplifier for amplifying incoming signal
light in response to application thereto of pumping light from a
pumping light source unit having a pumping light source for
generating a pumping light beam and an optical coupler unit for
coupling multiple pumping light beams, characterized in that the
pumping light source unit includes a polarization plane rotating
unit for rotating the plane of polarization of output pumping light
from the optical coupler unit through a first angle of rotation for
transmission and rotating the plane of polarization of return
light, resulting from the output pumping light being reflected from
a connector connecting the pumping light source unit and other
components of the optical amplifier back to the pumping light
source unit, through a second angle of rotation, thereby inputting
to the pumping light source return light, different in wavelength
from the pumping light source generated by the pumping light
source.
[0028] According to a third aspect of the present invention, there
is provided an optical amplifier for amplifying incoming signal
light in response to application thereto of pumping light from a
pumping light source unit having multiple pumping light sources
each generating a pumping light beam and an optical
coupler/splitter unit for coupling multiple pumping light beams and
splitting into individual light beams, characterized in that the
pumping light source unit includes a polarization plane rotating
unit for rotating the plane of polarization of output pumping light
from the optical coupler unit through a first angle of rotation for
transmission and rotating the plane of polarization of return
light, resulting from the output pumping light being reflected from
a connector connecting the pumping light source unit and other
components of the optical amplifier together back to the pumping
light source unit, through a second angle of rotation, thereby
inputting to the pumping light source return light different in
wavelength from the pumping light source generated by the pumping
light source.
[0029] According to a fourth aspect of the present invention, there
is provided an optical amplifier in which a pumping light source
unit having a pumping light source for generating pumping light and
an optical amplification unit having an amplification medium for
amplifying incoming signal light in response to application of the
pumping light thereto are connected together by means of a
connector that allows the pumping light to be transmitted to the
optical amplification unit, characterized in that the optical
amplification unit includes a unit for determining whether or not
the connection between the pumping light source unit and the
optical amplification unit is established by means of the connector
on the basis of the output level of the pumping light from the
pumping light source unit.
[0030] According to a fifth aspect of the present invention, there
is provided an optical amplifier in which a pumping light source
unit having a pumping light source for generating pumping light and
an optical amplification unit having an amplification medium for
amplifying incoming signal light in response to application of the
pumping light thereto are connected together by means of a
connector that allows the pumping light to be transmitted to the
optical amplification unit, characterized in that the pumping light
source unit includes a unit for determining whether or not the
connection between the pumping light source unit and the optical
amplification unit is established by means of the connector on the
basis of the level of return light reflected from the
connector.
[0031] According to a sixth aspect of the present invention, there
is provided an optical amplifier in which a pumping light source
unit having a pumping light source for generating pumping light and
an optical amplification unit having an amplification medium for
amplifying incoming signal light in response to application of the
pumping light thereto are connected together by means of a
connector that allows the pumping light to be transmitted to the
optical amplification unit, characterized by comprising a unit for
determining whether or not the connection between the pumping light
source unit and the optical amplification unit is established by
means of the connector.
[0032] An optical amplification unit of the present invention has
an amplification medium for amplifying incoming signal light in
response to application thereto of pumping light from a separate
pumping light source unit, the optical amplification unit and the
pumping light source unit being connected by a connector to form an
optical amplifier, and is characterized by the provision of a
variable attenuator for adjusting the level of pumping light input
to the amplification medium.
[0033] A pumping light source unit of the present invention has a
pumping light source for generating pumping light to be output to a
separate optical amplification unit, the pumping light source unit
and the optical amplification unit being connected by a connector
to form an optical amplifier, and is characterized by the provision
of a variable attenuator for adjusting the level of pumping light
to be output to the optical amplification unit.
[0034] According to the present invention, the pumping light,
which, in the conventional system, has its output level maintained
constant or adjusted by controlling the pumping light source
itself, can be level-adjusted easily by the use of the variable
attenuator, which allows pumping light with a suitable intensity to
be supplied to the amplification medium.
[0035] In addition, since the pumping light source is separated
from the optical amplification unit, two or more optical
amplification units can be grouped into one package, ensuring
compactness of the optical amplifier.
[0036] Being separated from the optical amplification unit, the
pumping light source can be placed in a location where heat
radiating conditions are good and can suppress the effect of heat
on the light amplification unit.
[0037] By the provision of the unit for rotating the plane of
polarization of pumping light output from the pumping light source
through a predetermined angle in the pumping light source unit, the
operational instability of the light source due to return light can
be eliminated.
[0038] According to the present invention, the optical amplifier is
separated into the pumping light source unit containing the pumping
light source and the optical amplification unit containing the
amplification medium and a connector is therefore required to
couple these units together. The connector may come off while a
person is working. Not only has the pumping light high power, but
it is converged by an optical fiber. In the event that the
connector has come off, therefore, the person at work may be
exposed to danger. To avoid such danger, the optical amplifier of
the present invention is provided with means for detecting whether
the connector is off or not.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIGS. 1A, 1B and 1C show arrangements of conventional
pumping light source units;
[0040] FIG. 2 illustrates the principle of the present
invention;
[0041] FIG. 3A shows an arrangement of a variable attenuator;
[0042] FIG. 3B shows an arrangement of the Faraday rotator of FIG.
3A;
[0043] FIG. 4 shows a first embodiment of an optical amplifier of
the present invention;
[0044] FIG. 5 shows a second embodiment of the optical amplifier of
the present invention;
[0045] FIG. 6 shows a third embodiment of the optical amplifier of
the present invention;
[0046] FIG. 7 shows a first arrangement of the pumping light source
unit;
[0047] FIG. 8 shows a second arrangement of the pumping light
source unit;
[0048] FIG. 9 shows a third arrangement of the pumping light source
unit;
[0049] FIG. 10 shows a fourth arrangement of the pumping light
source unit;
[0050] FIG. 11 shows a fifth arrangement of the pumping light
source unit;
[0051] FIG. 12 shows a sixth arrangement of the pumping light
source unit;
[0052] FIG. 13 shows a seventh arrangement of the pumping light
source unit;
[0053] FIG. 14 is a schematic representation of the optical
amplifier;
[0054] FIG. 15 shows a first arrangement of the optical
amplification unit;
[0055] FIG. 16 shows a second arrangement of the optical
amplification unit;
[0056] FIG. 17 shows a third arrangement of the optical
amplification unit;
[0057] FIG. 18 shows a fourth arrangement of the optical
amplification unit;
[0058] FIG. 19 shows a fifth arrangement of the optical
amplification unit;
[0059] FIG. 20 shows an arrangement of the pumping light source
unit adapted to operate the pumping light source stably;
[0060] FIG. 21 shows a fourth embodiment of the optical amplifier
of the present invention;
[0061] FIG. 22 shows an arrangement adapted to monitor connection
between the optical amplification unit and the pumping light source
unit; and
[0062] FIGS. 23A and 23B are diagrams for use in explanation of how
to connect a pumping light source or sources in the pumping light
source unit.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0063] Referring now to FIG. 2, there is illustrated the principle
of the present invention.
[0064] An arrangement of an optical amplifier, with a pumping light
source excluded, is illustrated here. Hereinafter, the other
portion of an optical amplifier than a pumping light source is
referred to as an optical amplification unit.
[0065] Reference numeral 11 denotes an amplification medium that
amplifies signal light by being injected with pumping light, 12; a
coupler that couples the signal light and the pumping light, 13; a
variable attenuator that varies the supply amount of the pumping
light to the amplification medium, and 14; a driver that drives the
variable attenuator.
[0066] In the present invention, signal light and pumping light
having its output level adjusted by the variable attenuator 13
driven by the driver 14 are input to the coupler 12, then coupled
and transmitted to the amplification medium 11. The amplification
medium consists of, for example, an erbium doped fiber. By the
pumping light, the amplification medium (fiber amplifier) 11 is
excited to initiate stimulated emission, thereby amplifying the
signal light.
[0067] With the arrangement of FIG. 2, even if the light
multiplexing degree increases, sufficient light amplification can
be achieved by merely adjusting the output level of the pumping
light with the variable attenuator 13.
[0068] In addition, control of the output power of the pumping
light through the use of the variable attenuator 13 eliminates the
need of controlling a pumping light source itself and moreover
allows the pumping light source and the optical amplification unit
to be arranged separately. Separating the heat emitting pumping
light source from the optical amplification unit susceptible to
heat ensures that the optical amplifier operates stably.
[0069] FIGS. 3A and 3B illustrate an example of the variable
attenuator.
[0070] FIG. 3A illustrates the entire arrangement of the variable
attenuator.
[0071] The variable attenuator comprises a lens 20 which collimates
incoming light from an optical fiber 25, a birefringent wedge 21
which splits an incoming light beam into two components, depending
on a difference in polarization, a variable Faraday rotator 22
which can vary the angle of Faraday rotation, a birefringent wedge
23 which splits an incoming light beam into two components, and a
lens 24 which focuses light emerging from the birefringent wedge
23. The light transmitted through the lens 24 is directed to an
optical fiber 26, in which case, depending on the Faraday rotation
angle in the Faraday rotator 22, there are two cases; the case
where 100% of light is directed to the optical fiber, and the case
where only a part of light is directed. When only a part of light
is directed to the optical fiber, the intensity of light directed
to the fiber is attenuated. On the other hand, when 100% of light
is directed to the fiber, the intensity of light reaches the
maximum level.
[0072] Thus, by controlling the Faraday rotation angle of the
Faraday rotator, the degree to which a light path is bent in the
birefringent wedge 23 is varied, allowing the intensity of light
directed to the optical fiber 26 to be controlled.
[0073] FIG. 3B illustrates an example of the variable Faraday
rotator.
[0074] This Faraday rotator comprises a permanent magnet 27, an
electromagnet 28, a magneto-optical crystal 29, and a variable
current source for supplying current to the electromagnet. A light
beam 31 incident on the magneto-optical crystal 29 has its plane of
polarization rotated by a magnetic field produced within the
magneto-optical crystal by the permanent magnet 27 and the
electromagnet 28. The direction of the magnetic field produced
within the magneto-optical crystal changes with the magnitude of a
magnetic field produced by the electromagnet. The angle of rotation
of the plane of polarization of the light beam 31 is determined by
that component of the magnetic field which is parallel to the
direction in which the light beam travels. Thus, the angle of
rotation of the plane of polarization of the light beam 31 can be
changed by changing the direction of the magnetic field within the
magneto-optical crystal 29 with its magnitude unchanged. Here, the
permanent magnet 27 is used to saturate the magnetic field within
the magneto-optical crystal 29. By saturating the magnetic field
within the magneto-optical crystal, the magnitude of the internal
magnetic field can be kept unchanged even if any magnetic field is
produced by the electromagnet 28. By changing the direction of the
internal magnetic field, the magnitude of the magnetic field
component parallel to the direction in which the light beam 31
travels is increased or decreased. Thereby, the angle of rotation
of the plane of polarization of the light beam 31 is
controlled.
[0075] FIG. 4 illustrates a first embodiment of the optical
amplifier of the present invention.
[0076] This optical amplifier comprises a pumping light source unit
40 and an optical amplification unit 41. The pumping light source
unit couples pumping light emitted from two or more pumping light
sources 43 in a coupler 42 for transmission to the optical
amplification unit 41. The provision of two or more light sources
allows a pumping light beam of large power to be transmitted to the
optical amplification unit 41. If the pumping light source unit 40
is arranged such that a pumping light source or sources can be
added, the optical amplifier requirements of having higher-power
pumping light at a later time will be accommodated.
[0077] The optical amplification unit 41 is equipped with a WDM
coupler 44 that couples incoming signal light and pumping light for
transmission to an amplification medium 46 via an optical isolator
45. The amplification medium 46 is excited by the pumping light to
amplify the light signal, which in turn is directed to an optical
isolator 47. The amplified signal is then input to a wavelength
selective filter 48 where only the signal is derived and then input
to an optical splitter 49. When the signal light is amplified in
the amplification medium 46 by the use of the pumping light, which
emit signal light in the opposite direction to the transmission
direction. The optical isolators 45 and 47 are provided to prevent
the opposite direction signal light.
[0078] In the optical splitter 49, most of the signal light
propagates straight, but part of the signal light branches off and
its total output is detected by a photodiode 50. The output of the
photodiode is input to a monitor circuit 51 to make a decision of
whether or not the amplified signal light has reached a
predetermined output level. If the decision is that the amplified
signal light has not reached the predetermined output level, then
the monitor circuit sends a signal to an attenuator driver circuit
52. In response to this signal, the driver circuit controls the
variable attenuator 53 to thereby adjust the pumping light output
to the fiber amplifier 46. By using such a feedback loop, the
output level of the amplified signal light can be maintained
constant. The monitor circuit 51 may be formed of a differential
amplifier.
[0079] In this arrangement, being separated from the pumping light
sources 43 grouped as a pumping light source unit, the optical
amplification unit 41 will not suffer from heat emitted by the
pumping light sources. In addition, unlike the conventional system,
the pumping light output level is adjusted by the variable
attenuator 53. Thus, even if the signal multiplexing degree in
signal light is changed, a required output level of pumping light
can be obtained readily by controlling the variable attenuator
53.
[0080] FIG. 5 illustrates a second embodiment of the optical
amplifier of the present invention.
[0081] In this figure, the same reference numerals are used to
denote parts corresponding to those in FIG. 4 and their
descriptions are omitted.
[0082] The second embodiment of FIG. 5 is arranged such that a
single light source 62 can provide sufficient pumping light to each
of two or more amplification media 70 and 46. A pumping light
source unit 60 comprises the pumping light source 62 and an optical
splitter 63 that splits pumping light from the pumping light source
for transmission to the amplification media 70 and 46. An optical
amplification unit 61 comprises two or more amplifiers (first and
second amplification units 74 and 75). The arrangement of the
second optical amplification unit 75 is the same as that described
in conjunction with FIG. 3 and hence its description is omitted
here.
[0083] The first optical amplification unit 74 takes a
configuration called bidirectional excitation in which pumping
light is input to the amplification medium 70 from its both ends.
Two pumping light beams from the pumping light source unit 60 are
directed to the first optical amplification unit 74 where their
output levels are respectively adjusted by variable attenuators 64
and 67 driven by driver circuits 65 and 66 and then coupled with
signal light by WDM couplers 68 and 71. The two pumping light beams
are input to the amplification medium 70 from its both ends to
thereby amplifying the signal light. Optical isolators 69 and 72
are provided, as described previously, so as to remove spurious
light that propagates in the opposite direction to the direction in
which the signal light propagates. The light transmitted through
the optical isolator 72 is input to a wavelength selective optical
filter 73 where only the main signal light is extracted for
transmission over a transmission path (not shown).
[0084] In the first optical amplification unit 74, unlike the
second optical amplification unit 75, no feedback is used to
maintain the output level of amplified signal light constant. Of
course, a feedback arrangement may be provided in the first optical
amplification unit 74 as well. In this case as well, the feedback
arrangement is such that the output level of a fraction of main
signal light emerging from the wavelength selective optical filter
73 is detected through the use of a photodiode and a monitor
circuit and the variable attenuators 64 and 67 are controlled by
the driver circuits 65 and 66 supplied with an output signal of the
monitor circuit.
[0085] The first optical amplification unit 74 uses bidirectional
excitation, which is effective in using a very long erbium-doped
fiber for the amplification medium 70. That is, in such a case,
pumping light input from the WDM coupler 68 to an end of the
amplification medium 70 will be dissipated before it reaches the
other end of the medium. By inputting pumping light from the WDM
coupler 71 to the other end of the amplification medium as well,
therefore, the pumping light is allowed to spread over the entire
medium. According to this scheme, even if the amplification medium
70 consists of a very long erbium-doped fiber, the entire medium
can be used to amplify signal light.
[0086] FIG. 6 shows a third embodiment of the optical amplifier of
the present invention.
[0087] In FIG. 6, as in FIG. 5, the optical amplification unit 61
comprises the first and second optical amplification units 74 and
75 and like reference numerals are used to denote corresponding
parts to those in FIG. 5.
[0088] Using bidirectional excitation, the first optical
amplification unit 74 in the optical amplification unit 61 is
arranged such that pumping light is directed to both ends of the
amplification medium 70. The advantage of this scheme has been
described previously. The second optical amplification unit 75,
arranged identically to that of FIG. 4, uses only one pumping light
beam. As with the second optical amplification unit 75, in the
first optical amplification unit 74 feedback control may be
performed on the variable attenuators 64 and 65.
[0089] In FIG. 6, a pumping light source unit 80 containing pumping
light sources and the optical amplification unit 61 are provided
separately and, in the optical amplification unit 61, pumping light
from the pumping light source 80 to the amplification media 70 and
46 is controlled by the variable attenuators 64, 67 and 53. Such an
arrangement prevents heat emitted by the pumping light sources 81
from adversely affecting the stable operation of the optical
amplification unit 61 and allows the required pumping light to be
obtained only by controlling the variable attenuators 64, 67 and
53. In addition, by arranging the pumping light source unit so as
to allow a pumping light source or sources to be added to its
optical coupler/splitter 82, a requirement of providing such
high-power pumping light as is not obtainable from the previously
installed pumping light sources 81 can be accommodated simply by
adding a pumping light source or sources to the pumping light
source unit 80.
[0090] The pumping light source unit 80 of FIG. 6 is equipped with
two or more pumping light sources 81 to thereby provide an intense
pumping light that is not obtainable from a single pumping light
source. That is, lights output from the pumping light sources 81
are coupled and then split by an optical coupler/splitter 82 for
transmission to the amplification media 70 and 46. As described
previously, by coupling lights from two or more pumping light
sources and then splitting them for transmission to two or more
amplification media, there is provided an advantage that all the
amplification media can operate with substantially the same
characteristics. That is, variations in operating characteristics
between amplification media due to variations in a manufacturing
process among pumping light sources, which would occur if one
pumping light source were allocated for one amplification medium,
can be eliminated.
[0091] FIG. 7 illustrates a first arrangement of the excitation
light source unit.
[0092] The pumping light source unit of FIG. 7 is provided with two
or more pumping light sources 90 to supply one amplification medium
with a high-power pumping light which is not obtainable from a
single pumping light source. Unlike the previous arrangement, in
this arrangement a variable attenuator 93 and an attenuator driver
circuit 92 are installed in the pumping light source unit as
opposed to the optical amplification unit. In this case, in order
to adjust the output level of the pumping light in response to the
output level of signal light amplified by the amplification medium,
it is required to input a feedback control signal to the driver
circuit 92 from the optical amplification unit (not shown). As
described previously, since the pumping light source unit and the
optical amplification unit are assembled as separate units,
electrical wiring is needed to input the feedback control signal to
the attenuator driver circuit 92. Although the feedback control
arrangement will not be particularly mentioned below, it will be
apparent to those skilled in the art.
[0093] FIG. 8 shows a second arrangement of the pumping light
source unit.
[0094] In this arrangement as well, the pumping light source is
equipped with variable attenuators 97 and an attenuator driver
circuit 96. This arrangement is suitable for a case where a single
pumping light source 94 has an output high enough to provide
pumping light to each of two or more amplification media (not
shown). An optical splitter 95 splits light from the pumping light
source 94. The variable attenuators 97 each adjust the output level
of corresponding pumping light from the optical splitter. This is
intended to tune each of the amplification media individually.
Although, in FIG. 8, the variable attenuators 97 are controlled in
common by the one driver circuit 96, one driver circuit may be
provided for each variable attenuator. For pumping light output
control feedback, a monitor circuit is provided to detect the
output level of signal light amplified by an amplification medium
to produce a control signal and the control signal is then applied
to the attenuator driver circuit 96 connected to the monitor
circuit.
[0095] FIG. 9 shows a third arrangement of the pumping light source
unit.
[0096] In this arrangement, lights from two or more pumping light
sources 98 are first coupled and then split by an optical
coupler/splitter 99. This is intended to obtain pumping light of a
required output level by coupling the two or more pumping light
sources 98 and to prevent variations in wavelength among lights to
the amplification media by first coupling all lights from the
pumping light sources into one light beam and then dividing it. The
division of one light eliminates variations in wavelength among
light beams sent to the amplification media, allowing each
amplification medium to operate uniformly. In the arrangement of
FIG. 9 as well, variable attenuators 101 and an attenuator driver
circuit 100 are provided in the pumping light source unit. The
variable attenuators 101 are each provided for a light output of
the optical splitter and controlled by the driver circuit 100. As
described previously in conjunction with FIG. 8, an attenuator
driver circuit may be provided for each of the variable
attenuators. For pumping light output feedback control, a control
signal is applied from a monitor circuit not shown to the
attenuator driver circuit 100.
[0097] In the arrangements of FIGS. 8 and 9, pumping light divided
by the optical splitter 95 or optical coupler/splitter 99 is
power-controlled by each variable attenuator 97 or 101. This is
equivalent to varying the dividing ratio in the splitter or
coupler/splitter.
[0098] FIG. 10 shows a fourth arrangement of the pumping light
source unit.
[0099] This pumping light source unit is arranged such that lights
from two or more pumping light sources 110 are coupled by an
optical coupler 111 into one for application to a single
amplification medium and moreover the number of light sources
connectable to the coupler can be increased or decreased. For
example, the optical coupler 111 has light source connectors and
optically coupling components built in which are larger in number
than pumping light sources required at optical transmission system
startup, and a minimum required number of pumping light sources is
connected at the time of system installation. When there arises a
need to increase the optical multiplexing degree in the optical
transmission system (for system upgrade), an additional pumping
light source or sources (including a separate pumping light source
unit or units) are coupled to the connectors of the optical coupler
111 which have been provided in advance. That is, when the optical
multiplexing degree of the optical transmission system increases,
the power of signal light increases accordingly. In this case, when
the power of pumping light remains unchanged, the gain of the fiber
amplifier drops. It is thus required to elevate the output level of
the pumping light. For this reason, the pumping light source unit
is arranged to allow a pumping light source or sources to be added.
This avoids the need to install more pumping light sources than is
necessary at the stage of initial investment. In addition,
higher-power pumping light requirements at the time of system
upgrade can be accommodated readily.
[0100] FIG. 11 shows a fifth arrangement of the pumping light
source unit.
[0101] In the arrangement of FIG. 11, lights from two or more
pumping light sources 110 are first coupled and then separated for
transmission to each amplification medium. In this case as well, an
optical coupler/splitter 113 is provided beforehand with connectors
and optical elements which allow an additional pumping light source
or sources to be added. When higher-power pumping light becomes
necessary for system upgrade, an upgrading pumping light source or
a separate pumping light source unit can be connected to the
optical coupler/splitter 113.
[0102] As shown in FIG. 10 or FIG. 11, since the optical coupler
111 or optical coupler/splitter 113 is formed to allow for
connection of additional pumping light source or sources, a minimum
required number of pumping light sources suffices at system
startup. For system upgrade, a pumping light source or sources have
only to be added to obtain pumping light at a required level. Thus,
the initial investment in the optical transmission system can be
controlled to a minimum and the system can be upgraded readily.
[0103] FIG. 12 shows a sixth arrangement of the pumping light
source.
[0104] This arrangement contains a variable attenuator 115 and an
attenuator driver circuit 114 in addition to the arrangement of
FIG. 10. Although, in the arrangement of FIG. 10, the variable
attenuator 115 and the attenuator driver circuit 114 are provided
in the optical amplification unit not shown, they may be provided
in the pumping light source unit as shown in FIG. 12. This
arrangement corresponds to the arrangement of FIG. 7 but differs in
that the optical coupler 111 is formed to allow for addition of a
pumping light source or sources. As described in conjunction with
FIG. 7, to feedback control the pumping light output from the
optical coupler 111, wiring is needed to input a control signal to
the attenuator driver circuit 114.
[0105] FIG. 13 shows a seventh arrangement of the pumping light
source unit. This arrangement, corresponding to the arrangement of
FIG. 11, has variable attenuators 116 each corresponding to a
respective one of pumping light outputs of an optical
coupler/splitter 113 and an attenuator driver circuit 117 for
driving the attenuators. This arrangement allows pumping light
output to each amplification medium to be controlled individually.
This is substantially equivalent to varying the light dividing
ratio in the optical coupler/splitter 113.
[0106] The arrangement of FIG. 13, which is adapted to supply
pumping light to multiple amplification media, can meet situations
where the optical multiplexing degree is increased only for some of
transmission paths and the pumping light output level need not be
elevated for all the amplification media that the pumping light
source accommodates. That is, with system upgrade, pumping light
from a pumping light source 112 or separate pumping light source
unit is input to the optical coupler/splitter 113 and the light
attenuation level in each variable attenuator 116 is set
individually. More specifically, pumping light of the same output
level as prior to the upgrade is applied to amplification media for
which the pumping light output level does not need to be elevated,
while pumping light of a higher output level is applied to
amplification media for which the pumping light output level needs
to be elevated. Thus, the arrangement of FIG. 13 can accommodate
various system upgrades.
[0107] As described previously, in the arrangement of FIG. 13, one
attenuator driver circuit may be provided for each variable
attenuator. For feedback control of the output level of each
pumping light, a control signal is applied from a monitor circuit,
not shown, to the attenuator driver circuit.
[0108] FIG. 14 is a schematic of an optical amplifier.
[0109] The optical amplifier of FIG. 14 is arranged such that a
pumping light source unit 120 is independent of an optical
amplification unit and many optical amplification units are
incorporated into a package 121. By incorporating optical
amplification units 122-1 to 122-n into one package in this manner,
optical amplifiers on multiple light transmission paths can be
integrated, which will not take up too much space in the optical
communications device. The pumping light source unit 120 is
installed independently of the optical amplifier unit integrating
package 121, which provides greater freedom in placement of the
pumping light source unit in the optical communications device.
Thus, the pumping light source unit 120, which emits a large amount
of heat and may adversely affect the amplification media in the
optical amplification units, can be placed in a cooling-efficient
location within the optical communication device e.g., in the upper
portion of the device or near to a fan.
[0110] To provide an optical amplifier with uniform
characteristics, optical amplification units of the same type are
connected to the pumping light source unit, For this reason,
pumping lights having the same wavelength or the same wavelength
component are input to the amplification media in the optical
amplification units, which allows the optical amplifier to have
uniform characteristics regardless of the wavelength dependence of
the amplification media.
[0111] FIGS. 15 to 18 show various arrangements of the optical
amplification unit described in conjunction with FIGS. 4 and 5.
[0112] FIG. 15 shows a first arrangement of the optical
amplification unit.
[0113] In FIG. 15, 131 denotes an amplification medium (fiber
amplifier) consisting of an erbium-doped fiber, 132; a WDM coupler
for coupling pumping light and signal light, 133; an optical
isolator for preventing oscillation of light propagating in the
reverse direction, 134; a wavelength selective optical filter which
allows signal light wavelength components to pass through, 135; a
variable attenuator which varies the supply amount of pumping light
to the amplification medium, 136; an attenuator driver circuit for
supplying a drive current to the optical attenuator, 137; an
optical splitter for branching part of signal light emerging from
the filter for the purpose of controlling the output level of the
pumping light, 138; a photodiode for converting signal light
branched by the optical splitter into an electrical signal, and
139; a control circuit which receives the electrical signal and
performs necessary calculations to send a control signal to the
attenuator driver circuit.
[0114] The optical amplification unit of FIG. 15 performs automatic
level control (ALC) to keep the level of output light constant and
has the same arrangement as that described in conjunction with FIG.
4. Part of signal light is branched by the optical splitter 137 and
then converted into an electrical signal, which, in turn, is
compared in magnitude with a preset value in the control circuit
139. The ALC is performed by controlling the drive current to the
variable attenuator 135 via the driver circuit 136 so that a
difference between the electrical signal and the preset value will
reach zero.
[0115] Separating the optical amplifier into the optical
amplification unit containing the amplification medium susceptible
to heat and the pumping light source unit containing a pumping
light source or sources allows the amplification medium to operate
stably and the pumping light source unit to have facilities for
system upgrades as described previously. Further, unlike the
conventional amplifier in which the output level of pumping light
is controlled by varying current to a pumping light source, the
present invention performs the pumping light level control by
varying the power of pumping light to the amplification medium
using the variable attenuator while keeping the output level of the
pumping light source constant. This makes it possible to form an
optical amplifier from separate units: an optical amplification
unit and a pumping light source unit.
[0116] FIG. 16 shows a second arrangement of the optical
amplification unit.
[0117] In this figure, like reference numerals are used to denote
corresponding parts to those in FIG. 15 and their descriptions are
omitted.
[0118] In FIG. 16, 140 denotes an optical splitter which branches
part of incoming signal light to control the output level of
pumping light, 141; a photodiode which converts the signal light
branched by the optical splitter into an electrical signal, and
142; a control circuit which receives the electrical signals
produced by the photodiodes 138 and 141 and performs necessary
calculations to produce a control signal for the attenuator driver
circuit 136.
[0119] This amplification unit is arranged to perform automatic
gain control (AGC) to keep the signal light gain constant. Part of
output signal light branched by the splitter 137 is converted into
an electrical signal by the photodiode 138, while part of incoming
signal light branched by the optical splitter 140 is converted into
an electrical signal by the photodiode 141. These electrical
signals are applied to the control circuit 142 where the ratio of
output signal light to input signal light, i.e., the gain, is
obtained. This gain is compared with a preset value to produce a
difference therebetween. The AGC control is performed by
controlling current to the variable attenuator 135 via the driver
circuit 136 so that the difference will reach zero.
[0120] The WDM coupler 132, the optical isolator 133, the
wavelength selective filter 134 and the amplification medium 131
operate as described previously.
[0121] FIG. 17 shows a third arrangement of the optical
amplification unit.
[0122] In this figure, like reference numerals are used to denote
corresponding parts to those in FIGS. 15 and 16 and their
descriptions are omitted.
[0123] In FIG. 17, 143 denotes an optical splitter which branches
part of pumping light for control, 144; a photodiode which converts
the pumping light branched by the optical splitter into an
electrical signal, and 145; a control circuit which receives the
electrical signal produced by the photodiode and performs necessary
calculations to produce a control signal for the attenuator driver
circuit 136.
[0124] This amplification unit is arranged to perform automatic
power control (APC) to keep the amount of pumping light to the
amplification medium constant. Part of pumping light branched by
the optical splitter 143 is converted into an electrical signal by
the photodiode 144. The electrical signal is applied to the control
circuit 145 where it is compared with a preset value to produce a
difference therebetween. The APC control is performed by
controlling current to the variable attenuator 135 via the driver
circuit 136 so that the difference will reach zero.
[0125] The pumping light having a constant level is coupled with
incoming signal light in the WDM coupler 132 to amplify the signal
light in the amplification medium 131. The amplified signal light
is applied to the wavelength selective filter 134 from which only
the main signal light is derived for transmission over a light
transmission path (not shown). As described previously, the optical
isolator 133 is provided to avoid oscillation of light propagating
in the reverse direction.
[0126] In this arrangement in which the power of pumping light is
constant, when higher-power pumping light becomes necessary for
system upgrade, the preset value in the control circuit 145 must be
changed to a suitable value. However, if there is no need of
increasing the power of pumping light, the arrangement will
function identically to the ALC control and the AGC control.
[0127] FIG. 18 shows a fourth arrangement of the optical
amplification unit. This arrangement is adapted for bidirectional
pumping light amplification. By varying current to each of variable
attenuators 135 through a corresponding one of driver circuits 136,
the supply amount of pumping light to the amplification medium from
each of its input and output ends can be controlled individually.
In the arrangement of FIG. 18, two pumping light sources are used:
one for pumping light supply to the amplification medium from its
input end, and one for pumping light supply from the output
end.
[0128] It is desirable that a common pumping light source be used
to supply pumping light to the amplification medium from its both
ends. An arrangement that uses such a common pumping light source
is shown in FIG. 19.
[0129] The operation of each component for bidirectional excitation
has been described in conjunction with FIG. 5 and the description
is thus omitted here.
[0130] FIG. 19 shows a fifth arrangement of the optical
amplification unit.
[0131] In FIG. 19, like reference numerals are used to denote
corresponding parts to those in FIGS. 15 to 18 and their
descriptions are omitted. In this figure, 146 denotes an optical
splitter which splits pumping light.
[0132] The optical amplification unit of FIG. 19 has the same
bidirectional excitation arrangement as the unit of FIG. 18 but
differs in that pumping light from a pumping light source is split
by the optical splitter 146 and then input to the variable
attenuators 135. Instead of placing the variable attenuators 135 as
shown in FIG. 19, a single variable attenuator may be placed ahead
of the optical splitter 146 to vary the amount of pumping light
before it is split by the splitter. In this case, the ratio between
the supply amount of pumping light to the amplification medium from
its input end and the supply amount of pumping light from the
output end depends on the dividing ratio in the optical
splitter.
[0133] FIG. 20 shows an arrangement for operating pumping light
sources stably in the pumping light source unit of the optical
amplifier.
[0134] In general, the connectors adapted for optical components,
though having varying degrees of reflectiveness, reflect no little
incoming light to produce return light. When the return light falls
on a pumping light source, its operation becomes unstable, causing
the wavelength of oscillating light to drift. This is because, when
a pumping light source consists of a laser which uses a resonator
structure, the return light upon incidence disturbs the resonant
state of the resonator.
[0135] To solve such a problem, there are provided two pumping
light sources 157 and 158 of different wavelengths. The pumping
light sources 157 and 158 are respectively set to output light
beams of .lambda.1 and .lambda.2 in wavelength which are linearly
polarized perpendicular to each other. The output beams of the
light sources 157 and 158 are polarization-coupled by a
polarization beam splitter (PBS) 159. A Faraday rotator 160 is set
to rotate the plane of polarization of incoming light through 22.5
degrees. After having been polarization-coupled, the pumping light
has its plane of polarization rotated through 22.5 degrees by the
Faraday rotator 160 and then output from the pumping light source
unit. Return light, resulting from the output pumping light being
reflected from optical components within the optical amplifier, has
its plane of polarization rotated through 22.5 degrees again in the
Faraday rotator. It thus follows that the return light has its
plane of polarization rotated through 45 degrees with respect to
the original light (the output light of the PBS). Thus, the return
light will have wavelength components of .lambda.1 and
.lambda.2.
[0136] Thus, if the return light has a component different in
wavelength from the output light of an pumping light source, its
action of disturbing the resonant state within the resonator of a
laser as the pumping light source will be suppressed. The
operational instability of the pumping light source due to return
light can therefore be eliminated.
[0137] That is, output light of pumping light sources (LDs) having
different output wavelengths is used as pumping light so that part
of the pumping light that is reflected by optical components and
falls on the pumping light source unit will not be uniform in
wavelength. This allows return light to an LD to contain a
wavelength component other than the output wavelength of the LD,
thus eliminating the operational instability of the optical
amplifier.
[0138] FIG. 21 shows a fourth arrangement of the optical
amplifier.
[0139] In this arrangement, optical amplification unit 170 is
provided with a pumping light input monitor 177 which comprises an
optical splitter 174 for branching part of pumping light, a
photodiode 175 for receiving the part of pumping light branched by
the optical splitter, and a monitor circuit 176 for monitoring the
output level of the received pumping light. The other components of
the optical amplification unit remain unchanged from those of FIG.
4 and their descriptions are thus omitted. In this arrangement, the
output level of pumping light is monitored by the pumping light
input monitor 177 to activate an alarm, such as a buzzer or lamp,
when the output level of pumping light has dropped abnormally.
[0140] In addition, when the input amount of pumping light to the
optical amplification unit 170 has dropped, the monitor circuit
sends an electrical signal to the pumping light source unit 171 to
shut off the pumping light source unit. Alternatively, when the
pumping light source unit has an output adjusting optical
attenuator (not provided in the arrangement of FIG. 21), the
attenuator is controlled to increase its light attenuation amount
to lower the output amount of pumping light to zero or a safety
level.
[0141] Such control is performed by sending a control signal from
the monitor circuit 176 to a control circuit 173 in the pumping
light source unit 171, thereby causing light source drivers 172 to
shut off pumping light sources 43 or causing a variable attenuator,
if provided, to increase the light attenuation amount.
[0142] Such control is intended to detect a state where a connector
178 that connects the pumping light source unit 171 and the optical
amplification unit 170 together has become disconnected. Not only
has the pumping light output from the pumping light source unit 171
very high power, but it is also converged by an optical fiber. If,
when the connector has come off, a person at work should be exposed
to pumping light, this would entail great dangers to his or her
skin and eyes. For this reason, the pumping light input monitor 177
is provided in the optical amplification unit to detect the output
level of the pumping light, thereby monitoring the state of the
connector 178. When the output level of the pumping light has
dropped below a predetermined level, the connector is considered to
have become disconnected, whereupon the alarm is sounded or the
pumping light source is shut down to keep the person at work from
dangers.
[0143] FIG. 22 shows another arrangement for monitoring the
connection between the optical amplification unit and the pumping
light source unit.
[0144] In this arrangement, light beams from multiple pumping light
sources 183 are first coupled and then split by an optical
coupler/splitter 184. The pumping light source unit is also
connected with the optical amplification unit by means of
connectors 180. In this case as well, the state of each connector
is monitored.
[0145] That is, there is provided a reflection monitor 182, in
which reflected light from the connector 180 is branched by an
optical splitter 185 and then detected by a photodiode 190 which
sends the detected output to a monitor circuit 186. When the
connector is off, the power of the reflected light from the
connector becomes higher than when the connector is connected. When
the power of the reflected light has become higher than a preset
level, the monitor circuit 186 produces an alarm signal. Otherwise,
when a variable attenuator is provided in the pumping light source
unit, the monitor circuit controls the attenuator to reduce the
output amount of pumping light to zero or a safety level.
Alternatively, the monitor circuit 186 sends a control signal to a
control circuit 187 to cause driver circuits 188 to shut off the
pumping light sources 183.
[0146] FIGS. 23A and 23B illustrate how pumping light sources are
connected in the pumping light source unit.
[0147] In FIG. 23A, pumping light sources (laser diodes; LDs) are
connected in series with a transistor 200. This arrangement
requires less transistors and less power dissipation than when each
of the LDs is driven individually.
[0148] As an example, consider a case where, as shown in FIG. 23B,
one LD is attached to a transistor 201. Assuming that the voltage
across the LD is V and the current flowing through the transistor
is I, the power dissipation in the transistor becomes P=(Vcc-V) I.
On the other hand, when multiple LDs are connected in series as
shown in FIG. 23A, the power dissipation in the transistor 200
becomes P'=(V'cc-4V) I where Vcc=(1+.alpha.)V, V'cc=(4+.alpha.') V,
and .alpha. and .alpha.' are assumed to be nearly equal to each
other.
[0149] A comparison in transistor power dissipation between the
arrangements of FIGS. 23A and 23B indicates that
4P-P'=(4.alpha.-.alpha.'- ) and hence 4P is larger than P'. Thus, a
combination of series-connected LDs and one transistor requires
less power dissipation than with multiple combinations of each of
an LD and a transistor. Further, less transistors are required.
[0150] However, the pumping LDs need not be connected in series.
For example, if pumping LDs are connected in parallel within the
light source driver circuits, a failing LD will be replaced with a
good one more easily than with the series connection, providing
redundancy.
[0151] The embodiments have been described in terms of an optical
fiber amplifier. An optical semiconductor amplifier can also be
used which uses a semiconductor as the amplification medium.
[0152] The output power of pumping light to the amplification
medium is adjusted by a variable attenuator, making it easy to
adjust the output power of pumping light.
[0153] The optical amplifier is separated into the pumping light
source unit containing a pumping light source or sources and the
optical amplification unit susceptible to heat, which allows the
amplification medium in the optical amplification unit to be free
from the influence of heat emitted by the pumping light source and
operate stably.
[0154] System upgrades, such as are intended to increase the signal
multiplexing degree, can be accommodated by adjusting the
attenuation amount of the variable attenuator. By arranging the
pumping light source unit so that an additional pumping light
source or sources can be attached thereto, it becomes unnecessary
to install redundant pumping light sources at the time of system
installation, allowing the initial investment to be cut.
[0155] When pumping light is supplied to multiple amplification
media from the pumping light source unit, the power of pumping
light can be adjusted for each amplification medium.
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