Optical fiber amplifier

An optical fiber amplifier is disclosed including: an optical line through which an optical signal is transmitted; a rare-earth-doped fiber, doped with a predetermined rare earth ion, being set on the optical line; a pumping light beam source for generating a pumping light beam having a predetermined wavelength; a first multiplexer for coupling the pumping light beam to the optical line; a second multiplexer for dividing the pumping light beam outputted from the rare-earth-doped fiber; and pumping light beam looping means for looping the pumping light beam divided by the second multiplexer back to the rare-earth-doped fiber. The residual pumping light beam outputted from the rare-earth-doped pumping light beam is looped back by the feedback loop or reflection loop, to be reprovided to the rare-earth-doped fiber through the multiplexer. Accordingly, it is possible to reduce the length of the rare-earth-doped fiber remarkable, compared to the conventional case, and prevent the residual pumping light from being transmitted through the optical line.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical fiber amplifier for amplifying 
optical signals, specifically, to an optical fiber amplifier which can 
reduce the amount of active fiber which constructs the optical fiber 
amplifier, and improve the amplification efficiency as well, by looping a 
pumping light beam used as an exciting signal in a rare-earth-doped fiber 
back to the rare-earth-doped fiber through a feedback loop or reflection 
loop. 
2. Discussion of Related Art 
Optical communication techniques transmitting information through optical 
fibers have been developed and are widely being used. The optical 
communication techniques, which can transmit large amount of information 
at high speed, are applied for information communications between 
countries through submarine cables because they do not suffer from signal 
disturbance or crosstalk due to electromagnetic induction. As multiplex 
and network techniques for the optical communications have been developed 
recently, the optical communication techniques gradually enlarge the range 
of their use to the key communication networks for high-speed broadband 
multimedia communications including voice and data communications between 
switches, cable TV or video on demand (VOD). 
The optical communication techniques have been improved according to the 
development of optical signal amplifiers which provide high-speed optical 
signal transmission and superlong-distance transmission. Recently, there 
have been actively carried out researches about amplifiers having flat 
gain wavelength, which is used in wavelength multiplex, and high-gain 
amplifiers for image distribution techniques. 
An early optical signal amplifier converts an optical signal into an 
electric signal through an avalanche-type photodiode to amplify, and 
reconverts the amplified electric signal into the optical signal using a 
laser diode. Present optical signal amplifiers employ rare-earth-doped 
fibers so that the signal conversion process for optical signal 
amplification can be omitted. The aforementioned rare-earth-doped fiber is 
formed in a manner that an active optical fiber is doped with a rare earth 
ion such as Er, Pr and Nd. When a pumping light beam having a 
predetermined wavelength is supplied to the rare-earth-doped fiber, 
stimulated photon having a predetermined wavelength is emitted due to 
excitation of the rare earth ion, which amplifies the optical signal 
propagated through a corresponding optical fiber ultimately. 
FIG. 1 shows a configuration of a conventional optical fiber amplifier 
using the rare-earth-doped fiber. Referring to FIG. 1, an optical signal S 
is coupled to a first optical line 1, and a pumping light beam P is 
coupled to a second optical line 2, first and second optical lines 1 and 2 
being coupled to a multiplexer 3 as its inputs. A third optical line 4 
corresponding to the output of multiplexer 3 is connected to a fourth 
optical line 7 which is the output line, through a rare-earth-doped fiber 
5 and isolator 6. In this configuration, optical signal S and pumping 
light beam P applied through first and second optical lines 1 and 2 
respectively are coupled with each other by multiplexer 3 so that they are 
included together in third optical line 4 corresponding to the output of 
multiplexer 3. 
Optical signal S and pumping light beam P are applied to the 
rare-earth-doped fiber 5 where pumping light beam P excites rare earth 
ions doped thereinto, to generate stimulated photon having a predetermined 
wavelength. This light is introduced into optical signal S and effects 
optical amplification. Isolator 6 prevents opposite optical signals from 
being introduced into rare-earth-doped fiber 5, which proceed in a 
direction opposite to optical signal S and include, for example, pumping 
light beam from another rare-earth-doped fiber located in the following 
stage or reflection signal of optical signal S. 
The maximum output power of the optical fiber amplifier is determined, 
depending on the dopant doped into the optical fiber, concentration of the 
dopant, the length of the doped optical fiber, the wavelength of pumping 
light, and the output of pumping light. As the optical fiber doped with 
the rare earth ion is very expensive, it requires to be shortened. 
However, when the rare-earth-doped fiber becomes shorter, amplification of 
the optical signal is not sufficiently carried out, and thus an optimum 
optical signal cannot be obtained. 
Furthermore, the pumping light beam as an exciting light in the 
rare-earth-doped fiber corresponds to a noise signal in terms of the 
optical signal transmitted through the fiber. Accordingly, to prevent a 
residual pumping light, which is not consumed but left in the 
rare-earth-doped fiber, from being transmitted through the optical fiber, 
the conventional optical fiber amplifier includes a reflection mirror at 
its output terminal to reflect the pumping light beam. However, the 
reflection mirror reflects not only the pumping light beam outputted from 
the rare-earth-doped fiber but also a portion of the optical signal 
transmitted through the optical fiber. Thus, it may deteriorate the output 
level of the optical signal. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention is directed to an optical fiber 
amplifier which substantially obviates one or more of the problems due to 
limitations and disadvantages of the related art. 
An object of the present invention is to provide an optical fiber amplifier 
which remarkably reduces the length of an rare-earth-doped fiber used 
therein. 
A further object of the present invention is to provide an optical fiber 
amplifier which prevents a residual pumping light beam from being 
outputted from the optical fiber amplifier, without using a reflection 
mirror. 
Another object of the present invention is to provide an optical fiber 
amplifier which enhances the efficiency of the electric power used by 
optimizing its amplification efficiency. 
Additional features and advantages of the invention will be set forth in 
the description which follows, and in part will be apparent from the 
description, or may be learned by practice of the invention. The 
objectives and other advantages of the invention will be realized and 
attained by the structure particularly pointed out in the written 
description and claims hereof as well as the appended drawings. 
To accomplish the objects of the invention, an optical fiber amplifier 
according to a first aspect of the present invention includes: an optical 
line through which an optical signal is transmitted; a rare-earth-doped 
fiber, doped with a predetermined rare earth ion, being set on the optical 
line; a pumping light beam source for generating a pumping light beam 
having a predetermined wavelength; a first multiplexer for coupling the 
pumping light beam to the optical line; a second multiplexer for dividing 
the pumping light beam outputted through the rare-earth-doped fiber; and 
pumping light beam feedback means for looping a portion of the divided 
pumping light beam back to the rare-earth-doped fiber. 
A further optical fiber amplifier according to the first aspect of the 
present invention includes: a first optical line through which an optical 
signal is transmitted; a rare-earth-doped fiber, doped with a 
predetermined rare earth ion, being set on the optical line; a first 
pumping light beam source for generating a first pumping light beam having 
a predetermined wavelength; a second pumping light beam source for 
generating a second pumping light beam having a predetermined wavelength; 
a first multiplexer for coupling the first pumping light beam to the 
optical signal transmitted through the optical line, the first pumping 
light beam and optical signal being transmitted in the same direction; a 
second multiplexer for coupling the second pumping light beam to the 
optical signal transmitted through the optical line, the second pumping 
light beam being transmitted in a direction opposite to the optical 
signal; a third multiplexer for dividing the pumping light beam which is 
outputted through the rare-earth-doped fiber and transmitted in the same 
direction as the optical signal; a fourth multiplexer for dividing the 
pumping light beam which is outputted through the rare-earth-doped fiber 
and transmitted in a direction opposite to the optical signal; and a 
second optical line optically coupled to the third and fourth 
multiplexers, in which the third and fourth multiplexers output the 
divided pumping light beam through the second optical line, and couple the 
pumping light beam received through an optical line to the first optical 
line. 
Another optical fiber amplifier according to the first aspect of the 
invention further includes feedback pumping light beam detection means for 
detecting the amount of the pumping light beam looped back by the feedback 
loop, and control means for controlling the output level of the first and 
second pumping light beam sources on the basis of the amount of the 
pumping light beam detected by the feedback pumping light beam detection 
means. 
To accomplish the objects of the invention, an optical fiber amplifier in 
accordance with a second aspect of the invention includes: an optical line 
through which an optical signal is transmitted; a rare-earth-doped fiber, 
doped with a predetermined rare earth ion, being set on the optical line; 
a pumping light beam source for generating a pumping light beam having a 
predetermined wavelength; a first multiplexer for coupling the pumping 
light beam to the optical line; and pumping light beam looping means for 
dividing the pumping light beam outputted from the rare-earth-doped fiber, 
and providing it to the rare-earth-doped fiber again. 
A further optical fiber amplifier in accordance with the second aspect of 
the invention includes: an optical line through which an optical signal is 
transmitted; a rare-earth-doped fiber, doped with a predetermined rare 
earth ion, being set on the optical line; a first pumping light beam 
source for generating a first pumping light beam having a predetermined 
wavelength; a second pumping light beam source for generating a second 
pumping light beam having a predetermined wavelength; a first multiplexer 
for coupling the first pumping light beam to the optical signal 
transmitted through the optical line, the first pumping light beam and 
optical signal being transmitted in the same direction; a second 
multiplexer for coupling the second pumping light beam to the optical 
signal transmitted through the optical line, the second pumping light beam 
being transmitted in a direction opposite to the optical signal; first 
pumping light beam looping means for dividing the pumping light beam which 
is outputted through the rare-earth-doped fiber and transmitted in the 
same direction as the optical signal, and providing it to the 
rare-earth-doped fiber again; and second pumping light beam looping means 
for dividing the pumping light beam which is outputted through the 
rare-earth-doped fiber and transmitted in a direction opposite to the 
optical signal, and providing it to the rare-earth-doped fiber again. 
Another optical fiber amplifier of the second aspect of the invention 
further includes at least one pumping light beam detection means for 
detecting the amount of the pumping light beam reflected by the reflection 
loop, and control means for controlling the output level of the first and 
second pumping light beams sources on the basis of the amount of the 
pumping light beam detected by the pumping light beam detection means. 
According to the present invention as constructed as above, the residual 
pumping light beam outputted from the rare-earth-doped fiber is coupled to 
the feedback loop or reflection loop through the multiplexer, and the 
residual pumping light beam looped back by the feedback loop or reflection 
loop is reprovided to the rare-earth-doped fiber by the multiplexer. 
Accordingly, it is possible to reduce the length of the rare-earth-doped 
fiber remarkably compared to the conventional case, and prevent the 
pumping light beam from being transmitted through the optical fiber. 
Furthermore, as the control means controls the output of the pumping light 
beam source on the basis of a monitor signal supplied from the feedback 
pumping light beam feedback means, it is also possible to control the 
amplification efficiency of the optical fiber amplifier in an optimum 
state. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory and are 
intended to provide further explanation of the invention as claimed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
Reference will now be made in detail to the preferred embodiments of the 
present invention, examples of which are illustrated in the accompanying 
drawings. 
FIG. 2 shows a configuration of an optical fiber amplifier including a 
pumping light beam feedback loop according to a first embodiment of a 
first aspect of the present invention, which employs a feedback loop as 
looping means of the pumping light beam. Referring to FIG. 2, an input 
optical signal S is coupled to a first optical line 21, and a pumping 
light beam P generated by a laser diode 22 is coupled to a second optical 
line 23. First and second optical lines 21 and 23 are provided to a first 
wavelength division multiplexer 24 as its inputs. Here, optical signal S 
has a wavelength of 1520 to 1570 nm, for example, and pumping light beam P 
has a wavelength of 980 nm or 1480 nm. The output power of laser diode 22 
is determined by operation current supplied from a control circuit 42. 
First multiplexer 24 couples optical signal S with pumping light beam P and 
sends them to a second wavelength division multiplexer 26 through a third 
optical line 25 coupled to one input of second multiplexer 26. Second 
wavelength division multiplexer 26 couples optical signal S and pumping 
light beam P with feedback pumping light beam P received from an eighth 
optical line 34 which will be explained below, and sends them to a 
rare-earth-doped fiber 28 through a fourth optical line 27 which is 
connected to rare-earth-doped fiber 28 as its input. Fiber 28 is doped 
with a rare earth ion, for example, Er, and its length is set to, for 
example, half the rare-earth-doped fiber in the conventional optical fiber 
amplifier. The length of fiber 28 is not fixed to a specific one but 
depends on the wavelength or the power of pumping light beam P generated 
by laser diode 22. 
The output light beam from rare-earth-doped fiber 28 is coupled to a fifth 
optical line 29 which is connected to a third wavelength division 
multiplexer 30. Here, since rare-earth-doped fiber 28 is remarkably 
shortened, compared to the conventional fiber, if the power of pumping 
light beam P generated by laser diode 22 is similar to that in the 
conventional optical fiber amplifier, pumping light beam P will remain on 
fifth optical line 29 together with optical signal S. Third multiplexer 30 
divides optical signal S and pumping light beam on the basis of their 
wavelengths and couples them to sixth and seventh optical lines 31 and 32, 
respectively. 
Pumping light beam P propagated through seventh optical line 32 is coupled 
to eighth optical line 34 through a first tap coupler 33, eighth optical 
line 34 being connected to the other input of second multiplexer 26. Here, 
seventh and eighth optical lines 32 and 34 construct a feedback loop for 
the residual pumping light beam outputted from rare-earth-doped fiber 28. 
First tap coupler 33 divides pumping light beam P applied from seventh 
optical line 32 in a predetermined ratio, for example, 99:1, and couples 
each of divided portions of the pumping light beam to eighth optical line 
34 and a ninth optical line 35, respectively. First photodiode 36 
photoelectric-converts the pumping light beam applied through ninth 
optical line 35 to supply to control circuit 42 as a monitor signal. 
Optical signal S coupled to sixth optical line 31 by third multiplexer 30 
is coupled to a second tap coupler 38 through an isolator 37 which blocks 
a reflective optical signal. Second tap coupler 38 divides optical signal 
S in a predetermined ratio, for example, 99:1, and couples each of divided 
portions to tenth and eleventh optical lines 39 and 40, respectively. The 
portion of optical signal S coupled to tenth optical line 39 is 
transmitted as an output signal, and portion of optical signal S coupled 
to eleventh optical line 40 is photoelectric-converted by a second 
photodiode 41 and applied to control circuit 42 as a monitor signal. 
Control circuit 42 controls the amount of operation current supplied to 
laser diode 22 on the basis of the monitor signals applied through first 
and second photodiodes 36 and 41. That is, control circuit 42 can control 
the optical fiber amplifier in an optimum state, by reducing the amount of 
the operation current supplied to laser diode 22 when the level of the 
monitor signal from first photodiode 36 is high, and increasing the amount 
of the operation current when the level of the monitor signal from second 
photodiode 41 is low. 
In the optical fiber amplifier as constructed as above, optical signal S 
inputted through first optical line 21 and pumping light beam P generated 
by laser diode 22 are coupled with each other by first multiplexer 24, to 
be outputted through third line 25. Optical signal S and pumping light 
beam P are coupled with the pumping light beam looped back by feedback 
loop 34 through second multiplexer 26, to be applied to rare-earth-doped 
fiber 28. Pumping light beam P excites the rare earth ion doped into the 
fiber 28 to generate stimulated photons having a predetermined wavelength 
which are introduced to optical signal S to be amplified. Optical signal S 
amplified in rare-earth-doped fiber 28 is coupled to sixth optical line 31 
through third multiplexer 30, with hardly having loss, to be transmitted 
through isolator 37. 
In this case, as the length of fiber 28 is set shorter, compared to the 
conventional case, pumping light beam P remains in the output light beam 
from rare-earth-doped fiber 28. The residual pumping light beam is 
wavelength-divided by third multiplexer 30 and coupled to second 
multiplexer 26 through the feedback loop constituted of seventh and eighth 
optical lines 32 and 34. Second multiplexer 26 couples the residual 
pumping light beam P looped back by the feedback loop to fourth optical 
line 27, to reprovide the pumping light beam P to rare-earth-doped fiber 
28. 
That is, in the above-described configuration of the optical fiber 
amplifier, pumping light beam P generated by laser diode 22 passes through 
rare-earth-doped fiber 28 at least twice through feedback loops 32 and 34. 
Accordingly, the length of rare-earth-doped fiber 28 can be remarkably 
shortened, compared to the conventional case. Furthermore, since the 
residual pumping light beam P outputted from rare-earth-doped fiber 28 is 
coupled to feedback loops 32 and 34 through third multiplexer 30, it is 
possible to prevent pumping light beam P from being propagated through the 
optical lines without using a separate reflection mirror. Moreover, 
control circuit 42 controls the output of laser diode 22 on the basis of 
the monitor signals applied from first and second photodiodes 36 and 41. 
Accordingly, the amplification efficiency of the optical fiber amplifier 
can be controlled in an optimum state. 
FIG. 3 shows a configuration of an optical fiber amplifier according to a 
second embodiment of the first aspect of the present invention, which 
employs a reverse-direction excitation mode in which pumping light beam P 
is provided in a opposite direction to optical signal S, in contrast to 
the forward-direction excitation mode of the first embodiment in which 
pumping light beam P and optical signal S are provided in the same 
direction. Referring to FIG. 3, input optical signal S is coupled to a 
first optical line 51 which is coupled to a first wavelength division 
multiplexer 52 as its one input. First multiplexer 52 couples optical 
signal S to a second optical line 53, and also couples a residual pumping 
light beam P applied from a rare-earth-doped fiber 54 through second 
optical line 53, to a third optical line 55. 
Pumping light beam P coupled to third optical line 55 is coupled to a first 
tap coupler 56 which divides pumping light beam P in a predetermined 
ratio, for example, 99:1, and couples each of divided portions to a fourth 
and fifth optical lines 57 and 58, respectively. Fourth optical line 57 is 
connected to a second multiplexer 61 as its input, and constructs a 
feedback loop for the residual pumping light beam P outputted from fiber 
54, together with third optical line 55. The portion of pumping light beam 
P coupled to fifth line 58 by first tap coupler 56 is applied to a first 
photodiode 59 to be photoelectric-converted, and provided to a control 
circuit 72 as a monitor signal. 
Optical signal S coupled to second line 53 is sent to rare-earth-doped 
fiber 54 whose output light beam is applied to second multiplexer 61 
through a sixth optical line 60. Second multiplexer 61 couples optical 
signal S received through sixth line 60 to a seventh optical line 62, and 
also couples pumping light beams P applied through seventh optical line 62 
and from feedback loop 55 and 57 to the input of rare-earth-doped fiber 54 
through sixth optical line 60. 
In this second embodiment of the first aspect of the invention, for 
example, a light beam with a wavelength of 1520 to 1570 nm is used as 
optical signal S, and light beam with a wavelength of 980 or 1480 nm is 
used as pumping light beam P. The fiber 54 is doped with a rare earth ion, 
for example, Er ion, and its length is set to half the fiber used in the 
conventional optical fiber amplifier. In FIG. 3, reference numeral 63 
denotes a laser diode which generates a pumping light beam P with a 
predetermined wavelength under the control of control circuit 72. Pumping 
light beam P generated from laser diode 63 is coupled to an eighth optical 
line 64 connected to a third multiplexer 65 as its input. Third 
multiplexer 65 couples optical signal S received through seventh optical 
line 62 to a ninth optical line 66, and also couples pumping light beam P 
applied from eighth optical line 64 to seventh optical line 62. 
Optical signal S propagated through ninth optical line 66 is coupled to a 
second tap coupler 68 through an isolator 67 which blocks a reflective 
optical signal. Second tap coupler 68 divides optical signal S in a 
predetermined ratio, for example, 99:1, and couples each of the divided 
portions to tenth and eleventh optical lines 69 and 70, respectively. The 
portion of optical signal S coupled to tenth optical line 69 is 
transmitted as an output signal, and the portion of optical signal coupled 
to eleventh optical line 70 is photoelectric-converted through a second 
photodiode 71 and provided to control circuit 72 as a monitor signal. 
Control circuit 72, similar to the first embodiment, controls the amount of 
operation current supplied to laser diode 63 on the basis of the monitor 
signals applied through first and second photodiodes 59 and 71, in such a 
manner that the amount of the operation current is reduced when the level 
of the monitor signal applied from first photodiode 59 is high, and the 
operation current amount is increased when the level of the monitor signal 
applied from second photodiode 71 is low, thereby controlling the optical 
fiber amplifier in an optimum state. 
In the optical fiber amplifier according to the second embodiment of the 
first aspect of the invention, pumping light beam P generated by laser 
diode 63 is sent to rare-earth-doped fiber 54 through third and second 
multiplexers 65 and 61, and excites the rare earth ion doped into fiber 
54, to generate stimulated photons with a predetermined wavelength which 
are introduced to optical signal S inputted through first multiplexer 52 
and being transmitted through fiber 54, to amplify optical signal S. 
Furthermore, since the length of rare-earth-doped fiber 54 is remarkably 
reduced compared to the conventional case, a predetermined amount of the 
pumping light beam is outputted from fiber 54, and the residual pumping 
light beam is applied to feedback loop 55 and 57 through first multiplexer 
52, looped back by the feedback loop, and coupled to sixth optical line 60 
through second multiplexer 61, to be reprovided to rare-earth-doped fiber 
54. Control circuit 72 controls the output of laser diode 63 on the basis 
of the monitor signals from first and second photodiodes 59 and 71. 
Accordingly, the electric power efficiency of the optical fiber amplifier 
can be optimized as well as rare-earth-doped fiber 54 can be shortened. 
Moreover, pumping light beam P outputted from rare-earth-doped fiber 54 is 
continuously reprovided thereto through feedback loop 55 and 57, it is 
possible to prevent the residual pumping light beam from being transmitted 
through the optical lines. 
FIG. 4 shows a configuration of an optical fiber amplifier including a 
reflection loop according to a third embodiment of the first aspect of the 
present invention, which employs bidirectional excitation mode. Referring 
to FIG. 4, input optical signal S and first pumping light beam P1 
generated by a laser diode 81 are coupled with each other by a first 
wavelength division multiplexer 82, pass through a second wavelength 
division multiplexer 83, with hardly having loss, and then applied to a 
rare-earth-doped fiber 84. The length of fiber 84 corresponds to half the 
rare-earth-doped fiber of the conventional optical fiber amplifier. 
Optical signal S outputted from rare-earth-doped fiber 84 passes through 
third and fourth wavelength multiplexers 85 and 86, with hardly having 
loss. 
The residual first pumping light beam P1 outputted from rare-earth-doped 
fiber 84 is wavelength-divided by third multiplexer 85 and coupled to a 
feedback loop 87. The first pumping light beam P1 loops back to second 
multiplexer 83 through feedback loop 87, to be reprovided to 
rare-earth-doped fiber 84. A second pumping light beam P2 generated by a 
second laser diode 88 is coupled to the input of fourth multiplexer 86, to 
be supplied to rare-earth-doped fiber 84 through third multiplexer 85. The 
residual second pumping light beam P2 outputted from fiber 84 is 
wavelength-divided by second multiplexer 83, to be coupled to feedback 
loop 87. Then, the residual second pumping light beam P2 looped back to 
third multiplexer 85 by feedback loop 87 is reprovided to fiber 84. 
Feedback loop 87 includes a first tap coupler 89 which divides first or 
second pumping light beam P1 or P2 supplied from the feedback loop 87 in a 
predetermined ratio, for example, 99:1, and applies a portion of the 
divided pumping light beam to a first photodiode 90 which 
photoelectric-converts the received pumping light beam and provides it to 
a control circuit 94 as a monitor signal. Optical signal S outputted from 
fourth multiplexer 86 is coupled to a second tap coupler 92 through an 
isolator 91 which blocks a reflective optical signal. Second tap coupler 
92 divides optical signal S outputted from isolator 91 in a predetermined 
ratio, for example, 99:1, and applies a portion of the divided optical 
signal to a second photodiode 93 which photoelectric-converts the received 
optical signal and provides it to control circuit 94 as a monitor signal. 
Control circuit 94 controls the amount of operation current supplied to 
first and second laser diodes 81 and 88 on the basis of the monitor 
signals provided through first and second photodiodes 90 and 93. That is, 
control circuit 94 controls the optical fiber amplifier in an optimum 
state, by reducing the amount of operation current supplied to first and 
second laser diodes 81 and 88 when the level of the monitor signal 
provided from first photodiode 90 is high, and increasing the operation 
current amount when the level of the monitor signal provided from second 
photodiode 93 is low. 
As described above, optical signal S is applied to rare-earth-doped fiber 
84 through first and second multiplexers 82 and 83. Here, as an exciting 
signal for rare-earth-doped fiber 84, first pumping light beam P1 
generated by first laser diode 81 is provided thereto through first and 
second multiplexers 82 and 83, and second pumping light beam P2 generated 
by second laser diode 88 is also provided thereto through fourth and third 
multiplexers 86 and 85. First and second pumping light beams P1 and P2 
have the same wavelength. 
The length of rare-earth-doped fiber 84 is set shorter than that of the 
conventional one. Accordingly, when the outputs of first and second laser 
diodes 81 and 88 are as much as those of the conventional optical fiber 
amplifier, first and second pumping light beams P1 and P2 are not all 
consumed but remain in rare-earth-doped fiber 84. The residual light beams 
of first and second pumping light beams P1 and P2 are respectively 
wavelength-divided by third and second multiplexers 85 and 83 to be 
coupled to feedback loop 87. Then, they are looped back to third and 
second multiplexers 85 and 83 by feedback loop 87 to be applied to 
rare-earth-doped fiber 84. This feedback operation for first and second 
pumping light beams P1 and P2 is continuously performed when the pumping 
light beam is not all consumed but remains in rare-earth-doped fiber 84. 
When first and second pumping light beams P1 and P2 are provided to 
rare-earth-doped fiber 84, stimulated photons are emitted from fiber 94 to 
be introduced to optical signal S being transmitted fiber 94, to amplify 
the optical signal. Control circuit 94 controls the amplification 
efficiency of the optical fiber amplifier in an optimum state, controlling 
the output of first and second laser diodes 81 and 90, based on the 
monitor signals supplied from first and second photodiodes 90 and 93. 
In the third embodiment of the first aspect of the invention, the pumping 
light beam outputted from rare-earth-doped fiber 84 is looped back thereto 
by feedback loop 87. Accordingly, if the outputs of first and second laser 
diodes 81 and 88 are as much as those in the conventional optical fiber 
amplifier, the length of rare-earth-doped fiber 84 can be reduced to below 
half the fiber length in the conventional optical fiber amplifier. 
Furthermore, the residual pumping light beams P1 and P2 outputted from 
rare-earth-doped fiber 84 are continuously reprovided thereto, for 
complete consumption of the pumping light beam. Here, it is possible to 
set the electric power efficiency of optical fiber amplifier in an optimum 
state since the amount of the pumping light beam reprovided to fiber 84 is 
detected by first photodiode 90, and the output powers of first and second 
laser diodes are controlled on the basis of the detected signal. 
While the present invention is applied to forward-direction excitation mode 
and reverse-direction excitation mode using a single laser diode (pumping 
light source) in the embodiments shown in FIGS. 2 and 3, the invention can 
be also applied in the same manner to an optical fiber amplifier having a 
plurality of pumping light sources, as shown in FIGS. 5 and 6. 
FIG. 7 shows a configuration of an optical fiber amplifier according to a 
first embodiment of a second aspect of the present invention, which 
employs a reflection loop as looping means for the pumping light beam. 
Referring to FIG. 7, input optical signal S is coupled to a first optical 
line 121, and pumping light beam P generated by a laser diode 122 is 
coupled to a second optical line 123. First and second optical lines 121 
and 122 are provided to a first wavelength division multiplexer 124 as its 
inputs. Here, optical signal S has a wavelength of 1520 to 1570 nm, for 
example, and pumping light beam P has a wavelength of 980 nm or 1480 nm. 
The output power of laser diode 122 is determined by operation current 
supplied from a control circuit 140. 
First multiplexer 124 couples optical signal S with pumping light beam P 
and sends them to a third optical line 125 which is coupled to a 
rare-earth-doped fiber 126 as its input. The rare-earth-doped fiber 126 is 
doped with a rare earth ion, for example, Er, and its length is set to 
half the fiber length in the conventional optical fiber amplifier. Here, 
the length of fiber 126 is not a specific one but depends on the 
wavelength or power of pumping light beam P generated by laser diode 122. 
The output light beam from rare-earth-doped fiber 126 is coupled to a 
fourth optical line 127 which is connected to a second multiplexer 128. 
Here, as described above, since rare-earth-doped fiber 126 is shorter 
compared to the conventional case, if the output of pumping light beam p 
generated by laser diode 122 is as much as that in the conventional 
optical fiber amplifier, pumping light beam P will remain on fourth 
optical line 127 together with optical signal S. 
Second multiplexer 128 divides optical signal S and pumping light beam P on 
the basis of their wavelengths, and couples them to fifth and sixth 
optical lines 129 and 130, respectively. Pumping light beam P propagated 
through sixth optical line 130 is coupled to a reflection loop 132 through 
a first tap coupler 131, with hardly having loss. Then, pumping light beam 
P is looped back by reflection loop 132 to be coupled to first tap coupler 
131 again. First tap coupler 129 divides pumping light beam P looped back 
by reflection loop 132 in a predetermined ratio, for example, 99:1, and 
couples each of the divided portions to sixth and seventh optical lines 
130 and 133, respectively. The portion of pumping light beam P coupled to 
sixth optical line 130 is coupled to second multiplexer 128 as its input. 
Then, second multiplexer 128 sends pumping light beam P to 
rare-earth-doped fiber 126 through fourth optical line 127. The portion of 
pumping light beam P coupled to seventh optical line 133 by first tap 
coupler 131 is applied to a first photodiode 134, photoelectric-converted, 
and then applied to control circuit 140 as a monitor signal. 
Optical signal S coupled to fifth optical line 129 by second multiplexer 
128 is coupled to a second tap coupler 136 through an isolator 136 which 
blocks a reflective optical signal. Second tap coupler 136 divides optical 
signal S in a predetermined ratio, for example, 99:1, and couples each of 
the divided portions to eighth and ninth optical lines 137 and 138, 
respectively. The portion of optical signal S coupled to eighth optical 
line 137 is transmitted as an output signal, and the portion of optical 
signal S coupled to ninth optical line 138 is photoelectric-converted by a 
second photodiode 139, to be applied to control circuit 140 as a monitor 
signal. 
Control circuit 140 controls the amount of operation current provided to 
laser diode 122 on the basis of the monitor signals applied from first and 
second photodiodes 134 and 139. That is, control circuit 140 controls the 
optical fiber amplifier in an optimum state, by reducing the amount of 
operation current supplied to laser diode 122 when the level of the 
monitor signal applied from first photodiode 134 is high, and increasing 
the operation current amount when the level of the monitor signal applied 
from second photodiode 139 is low. 
In the optical fiber amplifier as constructed as above, input optical 
signal S and pumping light beam P generated by laser diode 122 are coupled 
with each other by first multiplexer 124 and applied to rare-earth-doped 
fiber 126 where pumping light beam P excites the rare earth ion doped 
thereto to generate stimulated photons with a predetermined wavelength. 
The stimulated photons are introduced to optical signal S and amplify it. 
Here, since the length of rare-earth-doped fiber 126 is set shorter 
compared to the conventional case, pumping light beam P remains in the 
output light beam from fiber 126. The residual pumping light beam is 
coupled to reflection loop 132 through second multiplexer 128, and looped 
back by the reflection loop to be recoupled to second multiplexer 128 
through tap coupler 131. Second multiplexer 128 sends pumping light beam P 
to rare-earth-doped fiber through fourth optical line 127. 
In the above-described optical fiber amplifier, pumping light beam P 
generated by laser diode 122 passes through rare-earth-doped fiber 126 at 
least twice. Accordingly, the length of fiber 126 can be remarkably 
reduced, compared to the fiber in the conventional optical fiber 
amplifier. Furthermore, the residual pumping light beam outputted from 
rare-earth-doped fiber 126 is coupled to reflection loop 132 through 
second multiplexer 128, looped back by the reflection loop, and reapplied 
to rare-earth-doped fiber 126 through second multiplexer 128. Accordingly, 
it is possible to prevent pumping light beam P from being transmitted 
through the optical fiber without establishing a separate mirror. 
Moreover, control circuit 140 controls the amplification efficiency of the 
optical fiber amplifier in an optimum state by controlling the output of 
laser diode 122 which generates pumping light beam P, based on the monitor 
signals applied from first and second photodiodes 134 and 130. 
FIG. 8 shows a configuration of an optical fiber amplifier having a pumping 
light beam reflecting loop according to a second embodiment of the second 
aspect of the present invention, which employs reverse-direction 
excitation mode in which pumping light beam P is provided in a direction 
opposite to optical signal S, in contrast to the forward-direction 
excitation mode of the first embodiment in which pumping light beam P and 
optical signal S are provided in the same direction. Referring to FIG. 8, 
input optical signal S is coupled to a first optical line 141 which is 
connected to a first wavelength division multiplexer 142 as its input. 
First multiplexer 142 couples pumping light beam P applied thereto through 
a second optical line 143 to a third optical line 144, and also couples 
pumping light beam P applied through third optical line 144 to optical 
signal S applied through first optical line 141, to send them to second 
optical line 143. 
Pumping light beam P coupled to third optical line 144 by first multiplexer 
142 is coupled to a reflection loop 146 through a first tap coupler 145, 
with hardly having loss, looped back by the reflection loop, to be coupled 
to first tap coupler 145 again. First tap coupler 145 divides pumping 
light beam P looped back by reflection loop 146 in a predetermined ratio, 
for example, 99:1, and couples each of divided portions to third and 
fourth optical lines 144 and 147, respectively. The portion of pumping 
light beam P coupled to third optical line 146 is reapplied to first 
multiplexer 142 as its input. Accordingly, the light beam coupled to 
second optical line 143 by first multiplexer 142 includes optical signal S 
inputted through first optical line 141 and pumping light beam looped back 
by reflection loop 146. The portion of pumping light beam P coupled to 
fourth optical line 147 by first tap coupler 145 is sent to a first 
photodiode 148, photoelectric-converted, and then applied to a control 
circuit 160 as a monitor signal. Optical signal S and reflected pumping 
light beam P which are coupled to second optical line 143 are applied to a 
rare-earth-doped fiber 149 whose output light beam is coupled to second 
wavelength multiplexer 151 as its input, through a fifth optical line 150. 
In this second embodiment of the second aspect of the invention, as optical 
signal S, for example, a light beam with a wavelength of 1520 to 1570 nm 
is used, and light beam with a wavelength of 980 or 1480 nm is used as 
pumping light beam P. The fiber 149 is doped with a rare earth ion, for 
example, Er, and its length is set to half the fiber length in the 
conventional optical fiber amplifier. In FIG. 8, reference numeral 152 
denotes a laser diode which generates a pumping light beam P with a 
predetermined wavelength under the control of control circuit 160. Pumping 
light beam P generated by laser diode 152 is coupled to a sixth optical 
line 153 connected to second multiplexer 151 as its input. Second 
multiplexer 151 couples pumping light beam P received through sixth 
optical line 153 to fifth optical line 150 to provides it to 
rare-earth-doped fiber 149 as an exciting light beam. 
Optical signal S from second multiplexer 151 is coupled to a seventh line 
154 which is connected to a second tap coupler 156 through an isolator 155 
which blocks a reflective optical signal. Second coupler 156 divides 
optical signal S in a predetermined ratio, for example, 99:1, and couples 
each of the divided portions to eighth and ninth optical lines 157 and 
158, respectively. The portion of optical signal coupled to eighth optical 
line 157 is transmitted as an output signal, and the portion of the 
optical signal coupled to ninth optical line 158 is 
photoelectric-converted through a second photodiode 159, to be provided to 
control circuit 160 as a monitor signal. 
Control circuit 160, similar to the first embodiment, controls the amount 
of operation current supplied to laser diode 152 on the basis of the 
monitor signals applied through first and second photodiodes 148 and 159, 
in such a manner that the amount of the operation current is reduced when 
the level of the monitor signal applied from first photodiode 148 is high, 
and the operation current amount is increased when the level of the 
monitor signal applied from second photodiode 159 is low, thereby 
controlling the optical fiber amplifier in an optimum state. 
In the optical fiber amplifier according to the second embodiment of the 
second aspect of the invention, pumping light beam P generated from laser 
diode 152 is sent to rare-earth-doped fiber 149 through second 
multiplexers 151, and excites the rare earth ion doped thereinto to 
generate stimulated photons with a predetermined wavelength which are 
introduced to optical signal S inputted through first multiplexer 142 and 
being transmitted through fiber 54, thus amplifying optical signal S. 
Furthermore, since the length of rare-earth-doped fiber 149 is remarkably 
reduced compared to the conventional case, a portion of pumping light beam 
P remains in the output light beam from rare-earth-doped fiber 149. And 
the residual pumping light beam is sent to reflection loop 146 through 
first multiplexer 142. The residual pumping light beam P looped back to 
reflection loop 146 is coupled to first multiplexer 142 through tap 
coupler 145, and first multiplexer 142 provides the looped pumping light 
beam to rare-earth-doped fiber 149, recoupling it to second optical line 
143. Control circuit 160 controls the output of laser diode 152 on the 
basis of the monitor signals from first and second photodiodes 148 and 
159. Accordingly, the electric power efficiency of the optical fiber 
amplifier can be optimized as well as rare-earth-doped fiber 149 can be 
shortened. 
FIG. 9 shows a configuration of an optical fiber amplifier including a 
pumping light beam reflecting loop according to a third embodiment of the 
second aspect of the present invention, which employs bidirectional 
excitation mode. Referring to FIG. 9, input optical signal S and first 
pumping light beam P1 generated by a first laser diode 171 are coupled 
with each other by a first wavelength division multiplexer 172, pass 
through a second wavelength division multiplexer 173, with hardly having 
loss, and then applied to a rare-earth-doped fiber 174. The length of 
fiber 174 is set to half the fiber length in the conventional optical 
fiber amplifier. Optical signal S outputted from rare-earth-doped fiber 
174 passed through third and fourth wavelength multiplexers 175 and 176, 
with hardly having loss. 
The residual first pumping light beam P1 outputted from rare-earth-doped 
fiber 174 is wavelength-divided by third multiplexer 175 and coupled to a 
first reflection loop 178 through a first tap coupler 177. The residual 
first pumping light beam P1 looped back by first reflection loop 178 is 
recoupled to third multiplexer 175 through first tap coupler 177 to be 
provided to rare-earth-doped fiber 174. First tap coupler 177 divides 
first pumping light beam P1 looped back by first reflection loop 178 in a 
predetermined ratio, for example, 99:1, and applies a portion of the 
divided first pumping light beam to a first photodiode 179. The portion of 
first pumping light beam P1 is photoelectric-converted by first photodiode 
179 to be applied to a control circuit 187 as a monitor signal. 
A second pumping light beam P2 generated by a second laser diode 180 is 
coupled to the input of fourth multiplexer 176, to be supplied to 
rare-earth-doped fiber 174 through third multiplexer 175. The residual 
second pumping light beam P2 outputted from the rare-earth-doped fiber 174 
is wavelength-divided by second multiplexer 173 and coupled to a second 
reflection loop 182 through a second tap coupler 181. Then, the residual 
second pumping light beam P2 is looped back by second reflection loop 182, 
and coupled to the input of second multiplexer 173 as its input through 
second tap coupler 181, to be provided to rare-earth-doped fiber 174. 
Second tap coupler 181 divides second pumping light beam P2 in a 
predetermined ratio, for example, 99:1, and applies a portion of the 
divided second pumping light beam to a second photodiode 183 which 
photoelectric-converts the received pumping light beam and provides it to 
control circuit 187 as a monitor signal. Optical signal S outputted from 
fourth multiplexer 176 is coupled to a third tap coupler 185 through an 
isolator 184 which blocks a reflective optical signal. Third tap coupler 
185 divides optical signal S outputted from isolator 84 in a predetermined 
ratio, for example, 99:1, and applies a portion of the divided optical 
signal to a third photodiode 186 which photoelectric-converts the received 
optical signal and provides it to control circuit 187 as a monitor signal. 
Control circuit 187 controls the amount of operation current supplied to 
first and second laser diodes 171 and 183 on the basis of the monitor 
signals provided through first, second and third photodiodes 179, 183 and 
186. That is, control circuit 187 controls the optical fiber amplifier in 
an optimum state, by reducing the amount of operation current supplied to 
first or second laser diode 171 and 180 when the level of the monitor 
signal provided from corresponding first or second photodiode 179 or 180 
is high, and increasing the operation current amount when the level of the 
monitor signal provided from third photodiode 186 is low. 
As described above, optical signal S is applied to rare-earth-doped fiber 
174 through first and second multiplexers 172 and 173. Here, as an 
exciting signal for rare-earth-doped fiber 174, first pumping light beam 
P1 generated by first laser diode 171 is provided to the rare-earth-doped 
fiber 174 through first and second multiplexers 172 and 173, and second 
pumping light beam P2 generated by second laser diode 180 is also provided 
to the rare-earth-doped fiber 174 through fourth and third multiplexers 
176 and 175. Here, first and second pumping light beams P1 and P2 have the 
same wavelength. 
The length of rare-earth-doped fiber 174 is set shorter than that of the 
conventional optical fiber amplifier. Accordingly, when the outputs of 
first and second laser diodes 171 and 180 are as much as those of the 
conventional optical fiber amplifier, first and second pumping light beams 
P1 and P2 are not all consumed but remain in rare-earth-doped fiber 174. 
The residual light beams of first and second pumping light beams P1 and P2 
are respectively wavelength-divided by third and second multiplexers 175 
and 173, to be coupled to first and second reflection loop 178 and 182. 
Then, the residual pumping light beams P1 and P2 looped back by reflection 
loops 178 and 182 are reapplied to rare-earth-doped fiber 174 through 
third and second multiplexers 175 and 173. This reflection operation for 
first and second pumping light beams P1 and P2 is continuously performed 
when the pumping light beam is not all consumed but remains in 
rare-earth-doped fiber 174. 
When first and second pumping light beams P1 and P2 are provided to 
rare-earth-doped fiber 174, stimulated photons are emitted from 
rare-earth-doped fiber 174 to be introduced to optical signal S being 
transmitted therethrough to be amplified. Control circuit 187 controls the 
amplification efficiency of the optical fiber amplifier in an optimum 
state, controlling the output of first and second laser diodes 171 an 180, 
based on the monitor signals supplied from first, second and third 
photodiodes 179, 183 and 186. 
In the third embodiment, the pumping light beam outputted from 
rare-earth-doped fiber 174 is reflected thereto through the reflection 
loop, and this reflection is repeatedly carried out through first and 
second reflection loops 178 and 182. Accordingly, the length of fiber 174 
can be reduced to below half the fiber length in the conventional optical 
fiber amplifier. Furthermore, the residual pumping light beam outputted 
from rare-earth-doped fiber 174 is continuously reprovided thereto through 
first and second reflection loops 178 and 182, for complete consumption of 
the pumping light beam. Here, the amount of the pumping light beam 
reprovided to fiber 174 is detected by first and second photodiodes 179 
and 183, and the output powers of first and second laser diodes 171 and 
180 are controlled on the basis of the detected signal, thereby setting 
the power efficiency of the optical fiber amplifier in an optimum state. 
As described above, according to the present invention, the length of the 
rare-earth-doped fiber used in the optical fiber amplifier can be 
remarkably reduced. Furthermore, the pumping light beam is completely 
consumed in the rare-earth-doped fiber, to improve the amplification 
efficiency of the optical fiber amplifier, and to prevent the residual 
pumping light beam from being transmitted through the optical lines. 
It will be apparent to those skilled in the art that various modifications 
and variations can be made in the optical fiber amplifier of the present 
invention without departing from the spirit or scope of the invention. 
Thus, it is intended that the present invention cover the modifications 
and variations of this invention provided they come within the scope of 
the appended claims and their equivalents.