Optical fiber light amplifier

An erbium-doped optical fiber for amplifying a light signal 1 is wound to form a plurality of nearly-closed rings 1a that are arranged in a flat expanded configuration. A light-receiving device 4 receives a spontaneous emission light emitted by the entirety of erbium-doped optical fiber 1 and supplies an electric signal corresponding to the received spontaneous emission light. A laser diode 2 supplies a pumping light to the erbium-doped optical fiber 1 to create a population inversion in the erbium-doped optical fiber. An output-control device 3 controls an output of laser diode 2 in response to the electric signal supplied from the light-receiving device 4. The light-receiving device 4 is located so that the light-receiving device can receive the spontaneous emission light emitted from the entirety of the rings 1a.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a light amplifier, particularly to an 
optical fiber amplifier of controllable gain doped with a rare earth 
element such as erbium . The erbium-doped fiber will be hereinafter 
referred to as EDF. 
2. Description of the related art 
A wide variety of devices has been proposed in the field of EDF light 
amplifiers. For example, Japanese Patent Laid-open No. 22687/95 
(hereinafter, referred to as reference 1) describes an EDF light amplifier 
capable of performing a constant-gain control even when an input of a 
light signal is absent. 
In reference 1, a light signal and a reference signal that has a wavelength 
slightly different from that of the light signal are entered into the EDF 
which has been placed in a state of population inversion by a pumping 
light supplied from a laser diode. Stimulated emission is caused in the 
EDF by both the light signal and the reference signal. A part of the 
output from the EDF is optically filtered to extract the reference signal 
component, which is in turn converted to an electric signal. A laser 
driving circuit controls the output of the laser diode in response to the 
electric signal. Controlling the output of the laser diode so that the 
ratio of the filtered light signal to the reference signal is constant 
yields a constant-gain control. In this way, a constant-gain control 
become feasible even when an input of a light signal is absent. 
Japanese Patent Laid-open No. 97941/97 describes an EDF light amplifier in 
which a light signal component of a desired wavelength included in a 
wavelength-multiplex light signal is controlled to have a constant output 
level. Hereinafter, this light amplifier is referred to as reference 2. 
In the light amplifier of reference 2, the light signal component of the 
desired wavelength is modulated at a prescribed frequency beforehand. The 
wavelength-multiplex light signal including the light signal of concern is 
entered into the EDF that has been placed in a state of population 
inversion, thereby causing a stimulated emission of radiation. 
A part of the output of the EDF is converted to an electric signal, from 
which an electric signal of the prescribed frequency is filtered. The 
detected level of the filtered electric signal is supplied to a control 
circuit, which controls the intensity of the pumping light corresponding 
to the light signal of the prescribed frequency . In this way, the light 
signal of a prescribed wavelength is controlled so as to have a desired 
signal level. 
Japanese Patent Laid-open No. 327104/93 (hereinafter, referred to as 
Reference 3) describes a light amplifier of a low noise figure. 
In this light amplifier, a pumping light creates population inversion in 
the EDF; a light signal fed to the EDF gives rise to stimulated emission 
of the EDF, causing amplification of the light signal; the amplified light 
signal is attenuated by a high-loss optical fiber; and a part of the 
output of the high-loss optical fiber is fed back to control the intensity 
of the pumping light so as to adjust the output of the light amplifier. 
Due to the highloss optical fiber, the level of the pumping light becomes 
high compared to a light amplifier of the same output level having no 
high-loss optical fiber. The high intensity of the pumping light allows a 
low noise figure. 
A light amplifier has been known from Japanese Patent Laid-open No. 
364790/92 (hereinafter, referred to as Reference 4), in which the output 
of the light amplifier is adjusted to be constant by a plurality of 
pumping light sources. 
In this amplifier, an input light signal is amplified by an EDF that is 
activated (placed in a state of a population inversion) by a pumping light 
fed from the plurality of pumping light sources. A part of the output 
light signal is branched to be converted to a first electric signal. The 
first electric signal is compared with a reference signal to generate an 
error signal. The error signal is branched into the plurality of error 
signal components corresponding to the number of the pumping light 
sources. Each error signal component is supplied to each of output control 
circuits. Each output control circuit controls the corresponding pumping 
light source. Each of output control circuits receives, as a feed-back 
signal, a second electric signal that is produced by photoelectric 
conversion of a branched part of the pumping light. 
Each output control circuit controls the pumping light in two modes, a 
short-time response mode and a longtime response mode. In the long-time 
response mode, the output control circuit controls the pumping light 
source so as to minimize the error signal component. In the shorttime 
response mode, the output control circuit controls the pumping light 
source so as to minimize the deviation of the second electric signal from 
the error signal component. In this way, constant-output control is 
attained. 
Another light amplifier has been proposed in which a depolarized light 
source is used to check polarization dependence of the gain associated 
with the pumping light.(Japanese Patent Laid-open No. 308547/94: This 
reference is referred to as Reference 5). In one specified embodiment, a 
passive polarization scrambler is used to depolarize the pumping light. 
The scrambler is connected with an output of the single-frequency laser 
that generates the pumping light. 
The above-described references are mainly directed to the methods of 
performing a constant-output control, in which the intensity of the 
pumping light is controlled by an output of the fiber light amplifier. 
However, there has been developed a fiber light amplifier based on other 
concept. In this fiber light amplifier, spontaneous emission (hereinafter, 
referred to as SE) emitted from an optical fiber doped with rare earth 
element, such as erbium, is detected when the optical fiber is in the 
state of population inversion, and the pumping light source is controlled 
by the detected value of the SE. 
As is known well, the probability per photon that an input light signal 
induces stimulated emission, i.e., macroscopically, the ratio of the 
intensity of the output light signal with respect to the intensity of the 
input light signal, or the gain of the fiber light amplifier, is 
proportional to the number of the rare earth atoms in a state of 
population inversion. 
Since the intensity of the SE as well is proportional to the number of the 
rare earth atoms in a state of population inversion, the intensity of the 
SE is proportional to the gain of the fiber light amplifier. 
For this reason, control of the SE by controlling the pumping light implies 
a control of the gain by controlling the pumping light. 
The fiber light amplifier based on the SE concept above is shown in FIG. 1. 
Referring now to FIG. 1, a light signal applied to input terminal A is 
transmitted to EDF 5 through optical branching circuit 6 and isolator 7. 
Monitor (photo-detector) 8 monitors the part of the light signal that has 
branched at optical branching circuit 6. 
A pumping light emitted by pumping light source 2 is entered into EDF 5 
through optical multiplexer 9. The population inversion produced in EDF 5 
by the pumping light causes to amplify the input light signal. The 
amplified light signal is taken out from output terminal B through optical 
multiplexer 9 and optical isolator 10. 
EDF 5 is wound in a shape of a coil and is fixed. Photo-detector 4 for 
detecting the SE emitted from EDF 5 is arranged radially above the side 
surface of the EDF coil, as is shown in FIG. 2. 
Photo-detector 4 detects the SE emitted from EDF 5. The output of 
photo-detector 4 is supplied to output-control section 3, which in turn 
controls the bias current of pumping light source (laser diode) 2 in 
response to the intensity of the SE, thereby adjusting the gain of the 
light amplifier to be constant. 
A problem in a light amplifier of the earlier development described above 
has been in the structure of the fiber amplifier that the EDF is wound to 
shape a coil, above the side surface of which photo-detector 4 is settled, 
as is depicted in FIG. 2. 
In this structure, photo-detector 4 detects an SE emitted only from the 
outermost wound EDF. Since the intensity of the SE emitted from the EDF 
depends on the longitudinal position of the fiber, monitoring of the SE 
emitted from only a certain special position does not provide information 
about the SE sufficient to carry out a precise constant-gain control. For 
this reason, it is necessary to conduct monitoring of the SE over the 
entire length of the EDF. 
SUMMARY OF THE INVENTION 
The present invention is directed to solving the above-described problem. 
It is an object of the present invention to provide a light amplifier 
capable of performing detection of the spontaneous emission precise enough 
to carry out an accurate constant-gain adjustment and also capable of 
promoting feasibility of reducing the size of the light amplifier. 
In order to attain the object above, a light amplifier according to the 
present invention comprises: an optical fiber doped with an rare earth 
element for amplifying a light signal wound to form a plurality of 
nearly-closed rings, the plurality of nearly-closed rings being arranged 
in a flat expanded configuration; a light-receiving device for receiving a 
spontaneous emission light emitted from the optical fiber and supplying an 
electric signal corresponding to the received spontaneous emission light; 
pumping light source for supplying a pumping light to the optical fiber to 
create a population inversion in the optical fiber; and an output-control 
circuit for controlling an output of the pumping light source in response 
to the electric signal supplied from the light-receiving device. 
Since the plurality of nearly-closed rings are arranged in a flat expanded 
configuration, the spontaneous emission emitted from the entirety of the 
optical fiber can be detected. 
The above and other objects, features, and advantages of the present 
invention will become apparent from the following description referring to 
the accompanying drawings which illustrate examples of a preferred 
embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 is a block diagram of a first embodiment of the light amplifier 
according to the present invention. In the Figure, the same parts as those 
in FIG. 1 are labeled with the same reference numbers. 
In this embodiment, erbium is doped to the optical fiber as a typical 
example of a rare earth element. 
The light amplifier comprises optical branching circuit 6, optical isolator 
7, EDF 1, monitor 8, optical multiplexer 9, optical isolator 10, pumping 
light source (laser diode LD) 2, output control section 3 and SE receiver 
(photo-detector PD) 4. 
A light signal to be amplified is applied to input terminal A. Optical 
branching circuit 6 branches a part of the input light signal from a main 
part to be amplified. Monitor 8 is a light-receiving element such as a 
photodiode and receives the part of the branched light signal, thereby 
monitoring the level of an incoming light signal. Optical isolators 7 and 
10 block a spurious oscillation caused by multiple reflections of the 
light beam within the light amplifier. 
EDF 1, activated or pumped by an input pumping light beam, produces 
population inversion and performs stimulated emission in response to an 
input light signal. The erbium atoms doped to the optical fiber make 
spontaneous emission. 
While the stimulated emission is confined within the optical fiber due to 
the refractive index of the optical fiber wall, the spontaneous emission 
(SE) propagates toward the exterior as well as the interior of the optical 
fiber. 
PD 4 (SE receiver 4) receives the SE emitted by EDF 1 and converts the 
received SE to an electric signal. Output control section 3 provides an 
output of an output control signal in response to the electric signal 
supplied from PD 4. Pumping light source (LD ) 2 generates a pumping light 
for pumping EDF 1 in response to the output control signal. 
Optical multiplexer 9 serves to transmit the output of EDF 1 to optical 
isolator 10 as well as to transmit the pumping light supplied from LD 2 to 
EDF 1. The amplified light signal is transmitted outside from output 
terminal B through optical isolator 10. 
FIG. 4 represents EDF 1 of the present embodiment. 
EDF 1 of the present embodiment is wound to form a plurality of 
substantially circular rings, each ring la being shifted laterally with 
respect to the adjoining ring so that the plurality of substantially 
circular rings have a flat expanded configuration. The circular rings are 
sealed and fixed on a resin sheet 11 or the like, as is shown in FIG. 4. 
PD 4 is placed to face the resin sheet 11 so that PD 4 can receive the SE 
emitted from the entirety of EDF 1. 
This structure of the light amplifier according to the present invention 
differs from that shown in FIG. 2 in that the entire EDF rings are exposed 
to PD 4, thereby enabling correct information to be acquired about the SE. 
In operation, a light signal received at terminal A is guided to EDF 1 
through optical branching circuit 6 and optical isolator 7. A part of the 
input light signal is branched at optical branching circuit 6 and 
monitored by monitor 8. 
A pumping light beam is guided to EDF 1 from pumping light source 2 through 
optical multiplexer 9 to produce a population inversion in the electronic 
states of erbium atoms. The input light signal induces transitions of the 
electronic states to cause the stimulated emission. 
The light generated by the stimulated emission is guided, as an amplified 
light signal, to output terminal B through optical multiplexer 9 and 
optical isolator 10. Since EDF 1 is arranged between optical isolators 7 
and 10, a spurious oscillation due to multiple light reflections is 
suppressed. 
EDF 1 emits an SE as well as the stimulated emission. The SE as well varies 
with a change in an output of LD 2. Forming EDF 1 in a flat configuration 
and locating PD 4 above EDF 1 with its entire rings exposed to PD 4 allow 
PD 4 to detect the SE emitted from entire EDF 1. The output of PD 4 is 
supplied to output-control section 3 to create an output-control signal. 
The output-control signal is supplied to LD (pumping light source) 2 to 
control the bias current thereof, thereby enabling to adjust the intensity 
of the pumping light and further enabling to control the gain of the light 
amplifier as well as the level of the SE to be constant. 
It is to be noted that the gain of the light amplifier linearly depends on 
the intensity of the SE emitted from the entire length of EDF. For this 
reason, the detection of the SE emitted from the entire length of EDF 1 is 
indispensable to perform a precise adjustment of the gain of the light 
amplifier. 
Next, a second embodiment of the present invention will be explained below. 
FIG. 6 is a block diagram of the second embodiment. As is known by 
comparing FIG. 6 with FIG. 3, the constitution of the light amplifier 
shown in FIG. 6 is similar to that shown in FIG. 3 except EDF 1. For this 
reason, the same reference numbers are labeled to the corresponding parts 
in the light amplifiers shown by FIGS. 6 and 3, and the explanation about 
FIG. 6 will be omitted for simplicity. 
Unlike the first embodiment, EDF 1 of the second embodiment is spirally 
wound radially inwardly with circular rings having decreasing radii. The 
EDF 1 is sealed and fixed on a resin sheet 11 or the like, as is shown in 
FIG. 7. 
In the second embodiment, both ends of EDF 1 are spaced apart from each 
other by 30 mm and led out of the resin sheet 11 . The light amplifier of 
the second embodiment operates in a similar manner to that of the first 
embodiment. 
In summary, since the EDF of the present invention has an expanded 
sheet-like structure, it is enabled to detect the spontaneous emission 
emitted from the entirety of the doped optical fiber. As a result, it 
become enabled to carry out a precise constant-gain control and also to 
reduce the volume for mounting the doped optical fiber, thereby reducing 
the size of the light amplifier. 
It is to be understood, however, that although the characteristics and 
advantages of the present invention have been set forth in the foregoing 
description, the disclosure is illustrative only, and changes may be made 
in the shape, size, and arrangement of the parts within the scope of the 
appended claims.