Optical power monitor device, optical amplifier, and optical transmitter

An optical power monitor device capable of monitoring optical power with the influence of optical crosstalk eliminated, and an optical amplifier and an optical transmitter each having the monitor device. The monitor device is applied to a light source for outputting first and second beams, for example. The first beam is divided into first and second branch beams by a beam splitter. The first branch beam is supplied to a first photodetector. The first photodetector outputs a first signal having a level corresponding to the power of the first branch beam. The second beam from the light source is supplied to a second photodetector. The second photodetector outputs a second signal having a level corresponding to the power of the second beam. The first and second signals are supplied to a first subtracter. The first subtracter outputs a first error signal corresponding to the difference between the first and second signals. The first signal and the first error signal are supplied to a second subtracter. The second subtracter outputs a second error signal corresponding to the difference between the first signal and the first error signal. Thus, the second error signal is obtained by using the first and second subtracters to thereby cancel a noise component caused by optical crosstalk and therefore eliminate the influence of optical crosstalk from the second error signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to optical power monitoring which 
can cancel optical crosstalk, and more particularly to an optical power 
monitor device, and an optical amplifier and an optical transmitter each 
having the optical power monitor device. 
2. Description of the Related Art 
In recent years, an optical amplifier such as typically, an erbium doped 
fiber amplifier (EDFA) has been put to practical use, and the power of 
light to be handled has been increased. Accordingly, it is required to 
perform monitoring of optical power with the influence of optical 
crosstalk eliminated. 
Conventionally known is an optical amplifier including an optical 
amplifying medium and means for pumping the optical amplifying medium so 
that the optical amplifying medium has a gain band. In an optical pumping 
type of optical amplifier, pump light having a wavelength properly set 
according to the wavelength of signal light to be amplified is supplied to 
an erbium doped fiber (EDF), for example. When signal light is input into 
the EDF being pumped, the signal light is amplified in the EDF in 
accordance with the principle of stimulated emission. 
Also known is a semiconductor laser type of optical amplifier. In this 
type, a bias current is supplied to an optical amplifying medium provided 
as a semiconductor chip, thereby pumping the optical amplifying medium. 
In a practical optical amplifier, optical power is monitored for various 
purposes. For example, the power of signal light to be supplied to an 
optical amplifying medium is monitored, so as to stop pumping of the 
optical amplifying medium when the signal light input into the optical 
amplifier is cut off. Since the power of the signal light input into the 
optical amplifier is small, it is desirable to eliminate the influence of 
optical crosstalk. The optical crosstalk is caused by, for example, ASE 
(amplified spontaneous emission) generated in the optical amplifying 
medium. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an optical 
power monitor device which can eliminate the influence of optical 
crosstalk. 
It is another object of the present invention to provide an optical 
amplifier having an optical power monitor device which can eliminate the 
influence of optical crosstalk. 
It is still another object of the present invention to provide an optical 
transmitter having an optical power monitor device which can eliminate the 
influence of optical crosstalk. 
In accordance with a first aspect of the present invention, there is 
provided an optical power monitor device for a light source outputting 
first and second beams. The first beam is divided into first and second 
branch beams by a beam splitter. The first branch beam is supplied to a 
first photodetector. The first photodetector outputs a first signal having 
a level corresponding to the power of the first branch beam. The second 
beam from the light source is supplied to a second photodetector. The 
second photodetector outputs a second signal having a level corresponding 
to the power of the second beam. The first and second signals are supplied 
to a first subtracter. The first subtracter outputs a first error signal 
corresponding to the difference between the first and second signals. The 
first signal and the first error signal are supplied to a second 
subtracter. The second subtracter outputs a second error signal 
corresponding to the difference between the first signal and the first 
error signal. 
In this manner, the second error signal is obtained by using the first and 
second subtracters to thereby cancel a noise component caused by optical 
crosstalk and therefore eliminate the influence of optical crosstalk from 
the second error signal. 
In accordance with a second aspect of the present invention, there is 
provided an optical transmitter having the optical power monitor device 
according to the first aspect of the present invention. The optical 
transmitter further has the above-mentioned light source, an optical 
amplifying medium, and means for pumping the optical amplifying medium so 
that the optical amplifying medium has a gain band. The second branch beam 
from the beam splitter is supplied to the optical amplifying medium. The 
wavelength of the second branch beam is included in the gain band. 
In this optical transmitter, the influence of optical crosstalk due to ASE 
generated in the optical amplifying medium can be effectively eliminated. 
In accordance with a third aspect of the present invention, there is 
provided an optical power monitor device for an optical amplifier having 
an optical path including an optical amplifying medium. A photodetector is 
operatively connected to the optical path. The photodetector outputs an 
electrical signal having a level corresponding to an optical power in the 
optical path. The electrical signal is supplied to a peak detecting 
circuit. The peak detecting circuit outputs a first signal providing a 
first peak level corresponding to a maximum level of the electrical signal 
and a second signal providing a second peak level lower than the first 
peak level. The first and second signals are supplied to a subtracter. The 
subtracter outputs an error signal corresponding to the difference between 
the first and second signals. 
The optical power in the optical path is reflected in the error signal. 
Furthermore, a component of optical crosstalk is canceled by the operation 
of the peak detecting circuit. Accordingly, it is possible to perform 
monitoring of the optical power with the influence of optical crosstalk 
eliminated. 
In accordance with a fourth aspect of the present invention, there is 
provided an optical amplifier having the optical power monitor device 
according to the third aspect of the present invention. The optical 
amplifier further has the above-mentioned optical amplifying medium and 
pumping means, and a beam splitter. The beam splitter divides an optical 
beam propagating along the optical path into first and second branch 
beams. The first branch beam is supplied to the photodetector. The optical 
path including the optical amplifying medium is set between an input port 
and an output port. The beam splitter is provided between the input port 
and the optical amplifying medium or between the optical amplifying medium 
and the output port, for example. 
Also in this optical amplifier, the influence of optical crosstalk can be 
effectively eliminated as similarly to the device according to the third 
aspect of the present invention. 
In this specification, the wording that an element and another element are 
operatively connected includes the case that these elements are directly 
connected, and also includes the case that these elements are so provided 
as to be related with each other to such an extent that an electrical 
signal or an optical signal can be mutually transferred between these 
elements. 
The above and other objects, features and advantages of the present 
invention and the manner of realizing them will become more apparent, and 
the invention itself will best be understood from a study of the following 
description and appended claims with reference to the attached drawings 
showing some preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Some preferred embodiments of the present invention will now be described 
in detail with reference to the attached drawings. 
Referring to FIG. 1, there is shown a first basic configuration of the 
optical power monitor device according to the present invention. This 
device is usable for a light source 2 for outputting a first beam B1 and a 
second beam B2. The light source 2 is, for example, a laser diode for 
outputting forward light and backward light as will be described later. 
The power of the backward light has a linear relation to the power of the 
forward light. 
The first beam B1 is divided into a first branch beam BB1 and a second 
branch light BB2 by a beam splitter 4. The first branch beam BB1 is 
supplied to a first photodetector 6. The photodetector 6 outputs a first 
signal S1 having a level corresponding to the power of the branch beam 
BB1. The second beam B2 from the light source 2 is supplied to a second 
photodetector 8. The photodetector 8 outputs a second signal S2 having a 
level corresponding to the power of the beam B2. The signals S1 and S2 are 
supplied to a first subtracter 10. The subtracter 10 outputs a first error 
signal ES1 corresponding to the difference between the signals S1 and S2. 
The first signal Si and the first error signal ES1 are supplied to a 
second subtracter 12. The subtracter 12 outputs a second error signal ES2 
corresponding to the difference between the signals S1 and ES1. 
It is now assumed that the first photodetector 6 is under the influence of 
optical crosstalk. In this case, the influence of optical crosstalk is 
exhibited by the signal S1 output from the photodetector 6. However, since 
the signal S1 is input to the two subtracters 10 and 12 cascaded together, 
a component due to the optical crosstalk is canceled, so that the error 
signal ES2 output from the subtracter 12 is not influenced by the optical 
crosstalk. 
The level of the error signal ES2 corresponds to the powers of the two 
beams B1 and B2 output from the light source 2, so that this preferred 
embodiment allows monitoring of optical power with the influence of 
optical crosstalk eliminated. 
In the case that the branch beam BB2 from the beam splitter 4 is supplied 
to an optical waveguide structure 14 as shown in FIG. 1, returned light 
from the optical waveguide structure 14 (e.g., reflected light from a 
fiber end) becomes optical crosstalk. However, the influence of this 
optical crosstalk is eliminated in accordance with the above-mentioned 
principle in this optical power monitor device. Particularly in the case 
that the optical waveguide structure 14 includes an optical amplifying 
medium generating ASE as in an optical amplifier to be hereinafter 
described, the influence of optical crosstalk due to the ASE can be 
effectively eliminated. 
Referring to FIG. 2, there is shown a basic configuration of the optical 
transmitter according to the present invention. This optical transmitter 
includes the light source 2, the optical power monitor device shown in 
FIG. 1, an optical amplifying medium 16, and pumping means 18. 
The optical amplifying medium 16 is operatively connected to the beam 
splitter 4 to receive the second branch beam BB2 from the beam splitter 4. 
The pumping means 18 pumps the optical amplifying medium 16 so that the 
optical amplifying medium 16 has a gain band. In this configuration, the 
gain band includes the wavelength of the branch beam BB2. Modulating means 
20 is operatively connected to the light source 2, so as to modulate at 
least the beam B1 of the beams B1 and B2 output from the light source 2 
according to a main signal MS. In the case of carrying out direct 
modulation of a laser diode, the modulating means 20 includes means for 
superimposing a modulating current corresponding to the main signal MS on 
a DC or controlled bias current to be supplied to the laser diode. In the 
case that the light source 2 outputs carrier light having a constant 
power, the modulating means 20 includes an optical modulator for 
modulating the carrier light. 
In this optical transmitter, the beam B1 becomes signal light. A branching 
ratio between the branch beams BB1 and BB2 in the beam splitter 4 is 1:10, 
for example. The branch beam BB2 is amplified by the optical amplifying 
medium 16 and next transmitted to an optical transmission line not shown. 
Thus, the optical transmitter has a post-amplifier comprising the optical 
amplifying medium 16 and the pumping means 18, so that a large output 
power can be obtained. 
Further, the power of signal light to be supplied to the optical amplifying 
medium 16 can be accurately monitored without being influenced by optical 
crosstalk due to ASE or the like generated from the optical amplifying 
medium 16. 
Referring to FIG. 3, there is shown a second basic configuration of the 
optical power monitor device according to the present invention. This 
optical power monitor device is usable for an optical amplifier having an 
optical path OP including the optical amplifying medium 16. 
A photodetector 22 is operatively connected to the optical path OP. The 
photodetector 22 outputs an electrical signal S10 having a level 
corresponding to the optical power P in the optical path OP. The signal 
S10 is supplied to a peak detecting circuit 24. The circuit 24 outputs a 
first signal S11 providing a first peak level corresponding to a maximum 
level of the signal S10 and a second signal S12 providing a second peak 
level lower than the first peak level. The first and second signals S11 
and S12 are supplied to a subtracter 26. The subtracter 26 outputs an 
error signal ES11 corresponding to the difference between the signals S11 
and S12. 
In an optical amplifier used for optical communication, the optical power P 
in the optical path OP is modulated by a main signal and/or a supervisory 
signal. Accordingly, by using such a peak detecting circuit as mentioned 
above, monitoring of the optical power P can be carried out without the 
influence of optical crosstalk. 
Referring to FIGS. 4A and 4B, there are shown two basic configurations of 
the optical amplifier according to the present invention. In each 
configuration, the optical path OP including the optical amplifying medium 
16 is set between an input port 28 and an output port 30. The optical 
amplifying medium 16 is pumped by the pumping means 18, thereby obtaining 
a gain band. 
In the configuration shown in FIG. 4A, a beam splitter 32 operatively 
connected to the photodetector 22 is provided between the input port 28 
and the optical amplifying medium 16. In accordance with the principle of 
operation of the optical power monitor device shown in FIG. 3, the optical 
power of an input beam to be supplied to the optical amplifying medium 16 
is provided according to the error signal ES11. 
In contrast with FIG. 4A, the configuration shown in FIG. 4B is 
characterized in that a beam splitter 32' operatively connected to the 
photodetector 22 is provided between the optical amplifying medium 16 and 
the output port 30. With this configuration, the power of a beam directed 
from the optical amplifying medium 16 to the output port 30 or the power 
of a beam directed from the output port 30 to the optical amplifying 
medium 16 is provided according to the error signal ES11. The beam 
directed from the optical amplifying medium 16 to the output port 30 is 
amplified signal light, for example, whereas the beam directed from the 
output port 30 to the optical amplifying medium 16 is returned light from 
an end face of an optical fiber not shown, for example. 
The operation of the peak detecting circuit in each of the optical power 
monitor devices shown in FIGS. 3, 4A, and 4B will now be described with 
reference to FIGS. 5A and 5B. 
Referring to FIG. 5A, there is shown a case where the optical power P is 
modulated by a main signal having a high level (H) and a low level (L). 
The vertical axis represents optical power P, and the horizontal axis 
represents time t. In this case, the first peak level in the peak 
detecting circuit 24 corresponds to an optical power P.sub.1 providing the 
high level, and the second peak level corresponds to an optical power 
P.sub.2 providing the low level. That is, the signals S11 and S12 to be 
supplied from the peak detecting circuit 24 to the subtracter 26 
respectively correspond to the optical powers P.sub.1 and P.sub.2. An 
average optical power of signal light in the optical path OP. is reflected 
by (P.sub.1 -P.sub.2), and a noise component due to optical crosstalk is 
superimposed on both the high level (H) and the low level (L). 
Accordingly, by obtaining the error signal ES11 corresponding to the 
difference between the signals S11 and S12 by means of the subtracter 26, 
the average optical power in the optical path OP can be accurately 
monitored. 
Referring to FIG. 5B, there is shown another case where the optical power P 
in the optical path OP is modulated by a main signal 34 and a 
superimposing signal 36 slower than the main signal 34. The vertical axis 
represents optical power P, and the horizontal axis represents time t. The 
frequency (bit rate or speed) of the main signal 34 is about several GHz, 
for example, and the frequency (bit rate or speed) of the superimposing 
signal 36 is about several KHz to several MHz, for example. The 
superimposing signal 36 has an amplitude A1 smaller than the amplitude of 
the main signal 34. That is, the high level of the main signal 34 changes 
between an optical power P.sub.3 and an optical power P.sub.4 (P.sub.4 
&lt;P.sub.3) according to the superimposing signal 36. In this case, the 
second peak level is provided as the difference between the first peak 
level and the amplitude of the superimposing signal in the peak detecting 
circuit 24. More specifically, the signals S11 and S12 to be supplied from 
the peak detecting circuit 24 to the subtracter 26 respectively correspond 
to the optical powers P.sub.3 and P.sub.4. An average optical power in the 
optical path OP is reflected by (P.sub.3 -P.sub.4), and a noise due to 
optical crosstalk is superimposed on the optical power uniformly with 
time. Accordingly, the average optical power in the optical path OP can be 
accurately monitored by the error signal ES11 output from the subtracter 
26. 
A supervisory signal having a constant amplitude to be superimposed on a 
main signal for the purpose of supervision of an optical repeater or the 
like may be used as the superimposing signal 36. In the case of performing 
stabilization of an operation point of an optical modulator by using a 
low-frequency signal as disclosed in U.S. Pat. No. 5,170,274, the 
low-frequency signal may be used as the superimposing signal 36. In the 
case that such a supervisory signal or a low-frequency signal as mentioned 
above is not used, the superimposing signal 36 may be superimposed on the 
main signal 34 with a small modulation factor by using a dedicated optical 
modulator. 
Referring to FIG. 6, there is shown a preferred embodiment of the optical 
amplifier to which the present invention is applicable. This optical 
amplifier includes a front-stage optical module 38, an erbium doped fiber 
(EDF) 40 as the optical amplifying medium, and a rear-stage optical module 
42, which are arranged in this order along an optical path extending from 
the input port 28 to the output port 30. 
The front-stage optical module 38 includes an optical coupler 46 for 
separating a monitor beam 44 from signal light supplied to the input port 
28, an optical isolator 48 for passing signal light from the optical 
coupler 46, and a photodiode 50 for receiving the monitor beam 44. 
The EDF 40 has a first end 40A and a second end 40B operatively connected 
to the input port 28 and the output port 30, respectively. The signal 
light passed through the optical isolator 48 is supplied from the first 
end 40A into the EDF 40. 
Two pump LD modules 52 and 54 are used to pump the EDF 40 so that the EDF 
40 has a gain band including the wavelength of the signal light. Each of 
the modules 52 and 54 has an LD (laser diode) as a pump light source. The 
wavelength of pump light to be output from each LD falls within a band of 
0.98 .mu.m or 1.48 .mu.m, for example. 
The rear-stage optical module 42 has a polarization beam splitter 56 for 
combining the pump light from the pump LD modules 52 and 54 to output a 
pump beam. The pump beam is supplied through an optical coupler 58 of a 
wavelength-division multiplexing (WDM) type to the EDF 40 from its second 
end 40B. The signal light amplified in the EDF 40 is passed through the 
optical coupler 58 and an optical isolator 60 in this order. 
The rear-stage optical module 42 further includes an optical coupler 66 for 
separating monitor beams 62 and 64 between the optical isolator 60 and the 
output port 30, and photodiodes 68 and 70 for respectively receiving the 
monitor beams 62 and 64. The monitor beam 62 corresponds to amplified 
signal light directed from the optical isolator 60 to the output port 30 
(this beam will be hereinafter referred to as "forward beam"), and the 
monitor beam 64 corresponds to a beam directed from the output beam 30 to 
the optical isolator 60 (this beam will be hereinafter referred to as 
"backward beam"). In this preferred embodiment, the backward beam is 
monitored for the reason of preventing laser hazard in case of 
disconnection of an optical connector (not shown) operatively connected to 
the output port 30. That is, if the optical connector is disconnected, 
Fresnel reflection on a fiber end face becomes large to increase the power 
of the backward beam. Therefore, such an increase in power is intended to 
be monitored. 
A back power monitor (BPM) 72 for the LD included in the pump LD module 52 
is provided to monitor the power of the pump light output from the module 
52. A signal obtained in the back power monitor 72 is output from a port 
74. A temperature controller 76 is provided to maintain the temperature of 
the LD in the pump LD module 52 constant. The temperature controller 76 
functions also to monitor the temperature of the LD and output the result 
of the temperature monitoring from a port 78. There are also provided a 
back power monitor 80, a port 82, a temperature is controller 84, and a 
port 86 for the pump LD module 54, which respectively correspond to the 
back power monitor 72, the port 74, the temperature controller 76, and the 
port 78. 
A photocurrent generated in the photodiode 50 according to the power of the 
monitor beam 44 is converted into a voltage signal in an I/V converter 88. 
This voltage signal is supplied to a monitor circuit 90 to which the 
present invention is applicable. The monitor circuit 90 outputs from a 
port 92 an input level monitor signal corresponding to the power of the 
beam supplied to the input port 28, and also outputs an input reduction 
alarm signal from a port 94 when the input level monitor signal becomes 
smaller than a predetermined value. 
A photocurrent generated in the photodiode 68 according to the power of the 
monitor beam 62 is converted into a voltage signal in an I/V converter 96. 
This voltage signal is supplied to a monitor circuit 98 to which the 
present invention is applicable. The monitor circuit 98 outputs from a 
port 100 an output level monitor signal corresponding to the power of the 
beam to be output from the output port 30, and also outputs this monitor 
signal to an output alarm circuit (OAC) 102. The output alarm circuit 102 
outputs an output alarm signal from a port 104 when the output level 
monitor signal deviates from a predetermined range. 
A photocurrent generated in the photodiode 70 according to the power of the 
monitor beam 64 is converted into a voltage signal in an I/V converter 
106. This voltage signal is supplied to a-monitor circuit 108 to which the 
present invention is applicable. The monitor circuit 108 outputs from a 
port 110 a reflection level monitor signal corresponding to the power of 
the backward beam. 
In this preferred embodiment, generation or increase of returned light is 
detected according to a ratio in power between the backward beam and the 
forward beam. To this end, the output level monitor signal and the 
reflection level monitor signal are supplied to logarithmic converters 112 
and 114, respectively. A subtracter 116 is provided to obtain a difference 
between the output levels of the logarithmic converters 112 and 114. The 
ratio in power between the backward beam and the forward beam is reflected 
in an output signal from the subtracter 116. The output signal from the 
subtracter 116 is supplied to a returned light alarm circuit (RAC) 118 
when the difference in output level between the logarithmic converters 112 
and 114 becomes larger than a predetermined value, the alarm circuit 118 
determines that the power of the backward beam has been increased, and 
then outputs a returned light alarm signal from a port 120. 
Bias current circuits (BCC) 122 and 124 are provided to apply bias currents 
to the LDs in the pump LD modules 52 and 54, respectively. The bias 
current to be output from each of the bias current circuits 122 and 124 is 
controlled by an APC (automatic power control) circuit 126 so that the 
output level monitor signal from the monitor circuit 98 is maintained 
constant. The bias current circuit 122 monitors the bias current to be 
supplied to the module 52 and outputs the result of this monitoring from a 
port 128. The bias current circuit 124 monitors the bias current to be 
supplied to the module 54 and outputs the result of this monitoring from a 
port 130. 
A pump alarm circuit () 132 is connected to the bias current circuits 
122 and 124, so as to monitor abnormality of pumping. The alarm circuit 
132 outputs a pump alarm signal from a port 134 when at least one of the 
bias currents deviates from a predetermined range. 
A temperature alarm circuit (TAC) 136 is connected to the temperature 
controllers 76 and 84, so as to monitor abnormality of temperatures of the 
two pump LDs. The alarm circuit 136 outputs a temperature alarm signal 
from a port 138 when the temperature of at least one of the pump LDs 
deviates from a predetermined range. 
Shutdown circuits (SDC) 140 and 142 for the bias currents to the pump LDs 
are connected to the APC circuit 126, so as to stop pumping in case of 
abnormality. The shutdown circuit 140 stops the pumping of the EDF 40 when 
receiving the input reduction alarm signal from the monitor circuit 90. 
The reason for this operation of the shutdown circuit 140 is to determine 
that the supply of signal light to the input port 28 has become off when 
the input reduction alarm signal is generated, thereby preventing abnormal 
increase in the power of the pump beam. The shutdown circuit 142 stops the 
pumping of the EDF 40 when receiving the returned light alarm signal. The 
reason for this operation of the shutdown circuit 142 is to determine that 
the above-mentioned optical connector has been disconnected when the 
returned light alarm signal is generated, thereby avoiding laser hazard. 
Referring to FIG. 7, there is shown a first preferred embodiment of the 
optical power monitor device. 
This device is applicable to the monitor circuit 90 shown in FIG. 6. 
The monitor circuit 90 has a peak detecting circuit 144 and an operational 
amplifier 146 respectively corresponding to the peak detecting circuit 24 
and the subtracter 26 shown in FIG. 3. A voltage signal from the I/V 
converter 88 is supplied to the peak detecting circuit 144, and two 
voltage signals from the peak detecting circuit 144 are supplied to a 
positive input port and a negative input port of the operational amplifier 
146, respectively. Accordingly, an error signal output from the 
operational amplifier 146 is a voltage signal (input level monitor 
signal), and this error signal is supplied to the port 92 and a comparator 
148. The comparator 148 compares a voltage level of the supplied error 
signal with a reference voltage Vref1 (voltage source 150), and outputs an 
input reduction alarm signal when the voltage level of the error signal 
becomes lower than the reference voltage Vref1. 
When the input reduction alarm signal is generated, the shutdown circuit 
140 shown in FIG. 6 is operated to stop the supply of the bias currents to 
the pump LD modules 52 and 54, thus stopping the pumping of the EDF 40. 
Generally, in the optical amplifier as shown in FIG. 6, ASE generated in 
the EDF 40 is supplied from the first end 40A to the front-stage optical 
module 38, and a residual component of the pump light having not 
contributed to the pumping of the EDF 40 is also supplied to the 
front-stage optical module 38. Such undesired light causes optical 
crosstalk. According to the preferred embodiment shown in FIG. 7, the 
second basic configuration shown in FIG. 3 is applied to the monitor 
circuit 90, the optical power at the input port 28 can be accurately 
monitored without the influence of the optical crosstalk mentioned above. 
Particularly in the case that the optical amplifier is provided in an 
optical repeater or an optical receiver, the optical power at the input 
port 28 is considerably small such as about -30 dBm and it is accordingly 
susceptible to optical crosstalk. Therefore, this preferred embodiment is 
effective in such a case. 
Further, according to this preferred embodiment, the pumping of the EDF 40 
is stopped according to the input reduction alarm signal, so that abnormal 
increase in power of the pump beam upon cutting off the supply of signal 
light to be amplified can be prevented. 
Referring to FIG. 8, there are shown second and third preferred embodiments 
of the optical power monitor device. The second and third preferred 
embodiments are applicable to the monitor circuits 98 and 108 shown in 
FIG. 6, respectively. 
The monitor circuit 98 has a peak detecting circuit 152 and an operational 
amplifier 154 respectively corresponding to the peak detecting circuit 24 
and the subtracter 26 shown in FIG. 3. A voltage signal from the I/V 
converter 96 is supplied to the peak detecting circuit 152, and two 
voltage signals output from the peak detecting circuit 152 are supplied to 
a positive input port and a negative input port of the operational 
amplifier 154, respectively. An error signal (output level monitor signal) 
output from the operational amplifier 154 is supplied to the port 100, the 
output alarm circuit 102, the logarithmic converter 112, and the APC 
circuit 126 shown in FIG. 6. 
The monitor circuit 108 has a peak detecting circuit 156 and an operational 
amplifier 158 respectively corresponding to the peak detecting circuit 24 
and the subtracter 26 shown in FIG. 3. A voltage signal from the I/V 
converter 106 is supplied to the peak detecting circuit 156, and two 
voltage signals output from the peak detecting circuit 156 are supplied to 
a positive input port and a negative input port of the operational 
amplifier 158. An error signal (reflection level monitor signal) output 
from the operational amplifier 158 is supplied to the port 110 and the 
logarithmic converter 114 shown in FIG. 6. 
According to the second and third preferred embodiments, the second basic 
configuration shown in FIG. 3 is applied to each of the monitor circuits 
98 and 108, thereby allowing accurate monitoring of optical power without 
the influence of optical crosstalk. 
Further, the bias current to be supplied to each of the pump LD modules 52 
and 54 shown in FIG. 6 is controlled by the APC circuit 126 so that the 
error signal output from the monitor circuit 98 becomes constant, thereby 
maintaining the output level of the optical amplifier constant. 
Further, generation of returned light can be accurately detected according 
to the ratio in power between the backward beam and the forward beam in 
the rear-stage optical module 42. That is, when the difference in output 
level between the logarithmic converters 112 and 114 becomes larger than a 
predetermined value, the returned light alarm signal is generated. 
Accordingly, an increase in returned light due to disconnection of the 
optical connector or the like can be easily detected. Further, the 
shutdown circuit 142 is operated in response to the generation of the 
returned light alarm signal, thereby quickly stopping the pumping of the 
EDF 40 and therefore avoiding laser hazard. 
Referring to FIG. 9, there is shown a fourth preferred embodiment of the 
optical power monitor device. In this preferred embodiment, a light source 
module 160 is added to the optical amplifier shown in FIG. 6. That is, 
this preferred embodiment provides an optical transmitter having this 
optical amplifier as a post-amplifier. 
The light source module 160 has a laser diode (LD) 162 for outputting 
forward light and backward light, and a photodiode 164 for receiving the 
backward light from the LD 162. The forward light from the LD 162 is 
supplied to the input port 28. A modulating circuit 166 for changing a 
drive current for the LD 162 according to a main signal MS is provided to 
modulate the LD 162. A low-speed photodiode not responsive to the main 
signal MS is used as the photodiode 164, so that a photocurrent 
corresponding to an average output power of the LD 162 is generated in the 
photodiode 164. This photocurrent is converted into a voltage signal by an 
I/V converter 168. 
A monitor circuit 90' corresponding to the monitor circuit 90 shown in FIG. 
6 is used. The monitor circuit 90' has operational amplifiers 170 and 172 
respectively corresponding to the first subtracter 10 and the second 
subtracter 12 shown in FIG. 1. Voltage signals from the I/V converters 88 
and 168 are supplied to a positive input port and a negative input port of 
the operational amplifier 170, respectively. The output voltage signal 
from the I/V converter 88 is supplied also to a positive input port of the 
operational amplifier 172, and an error signal provided as an output 
voltage signal from the operational amplifier 170 is supplied to a 
negative input port of the operational amplifier 172. An error signal 
(input level monitor signal) provided as an output voltage signal from the 
operational amplifier 172 is supplied to the port 92 and a comparator 174. 
The comparator 174 compares a voltage level of the error signal supplied 
from the operational amplifier 172 with a reference voltage Vref2 (voltage 
source 176), and supplies an input reduction alarm signal to the port 94 
when the voltage level of the error signal becomes lower than the 
reference voltage Vref2. 
According to this preferred embodiment, when the input reduction alarm 
signal is generated, it can be determined that the LD 162 has become off. 
Further, when the LD 162 becomes off, the shutdown circuit 140 shown in 
FIG. 6 is operated according to the input reduction alarm signal, thereby 
stopping the pumping of the EDF 40. Accordingly, abnormal increase in the 
bias currents to be supplied to the pump LD modules 52 and 54 can be 
prevented. 
Further, the influence of optical crosstalk is eliminated in accordance 
with the principle described with reference to FIG. 1, thereby allowing 
accurate monitoring of the output power of the LD 162. 
It is to be noted that the present invention is not limited by the 
preferred embodiments mentioned above. For example, while the optical 
transmitter having the post-amplifier (the optical amplifying medium 16 
and the pumping means 18) has been described with reference to FIG. 2, an 
optical transmitter having no post-amplifier is also included in the scope 
of the present invention. For example, in the case that the optical power 
monitor device shown in FIG. 1 is applied to an optical transmitter having 
no optical post-amplifier, the output power of the light source 2 can be 
accurately monitored according to the error signal ES2 output from the 
second subtracter 12. Accordingly, by providing a feedback loop such that 
the monitored optical power becomes constant, automatic power control 
(APC) of the optical transmitter is allowed. Thus, the scope of the 
invention is defined by the appended claims and all changes and 
modifications as fall within the equivalence of the scope of the claims 
are therefore to be embraced by the invention.