Apparatus for monitoring the operation of an optical repeater

Apparatus for monitoring a plurality of optical repeaters of an optical communication system in which each said repeater directly amplifies an optical signal transmitted in an optical fiber and passing through the respective optical repeater, including an optical division circuit for dividing an outgoing main line signal and a monitoring signal, a format conversion circuit by which monitoring information is converted into binary information, a previous modulation circuit by which a monitoring signal is modulated with a sine wave, a comparison circuit which compares the monitoring signal with a reference voltage, a voltage controlled amplifier which amplifies the modulated monitoring signal, an automatic gain control circuit which regulates the output from the outgoing optical repeater, and an outgoing optical repeater which includes a stimulating light source, a wave synthesizer and a doped optical fiber.

BACKGROUND OF THE INVENTION 
The present invention relates to apparatus for monitoring the operation of 
the optical repeater of an optical communication system employing the 
repeater, such that the optical signal transmitted in an optical fiber can 
be directly amplified when the signal is in the form of light. 
The function of monitoring from a terminal station, the operation of 
optical repeaters made of semiconductor laser amplifiers or optical fiber 
amplifiers, while continuing the service of an optical communication 
system, so that an optical signal transmitted in an optical fiber is 
directly amplified by each repeater at a plurality of stages when the 
signal is in the form of light, is very important for the stable operation 
of the system. 
FIG. 3 is a block diagram for describing a conventional method of 
monitoring the operation of an optical repeater. As shown in FIG. 3, the 
predetermined incoming optical repeater 32 to be monitored is controlled 
by a prescribed control signal C through an incoming optical fiber 31 from 
a sending terminal station 30. For the incoming optical repeater 32 
appointed by the control signal C, appointed monitoring information is 
created in a prescribed format by a monitoring circuit 33. An outgoing 
main line signal S is subjected to prescribed modulation by a created 
monitoring signal M through an outgoing optical repeater 34 in an outgoing 
line and then sent to a receiving terminal station 35. For control and 
modulation of the signals, a method in which an incoming main line signal 
and the outgoing main line signal are subjected to a low degree of 
amplitude modulation at a frequency sufficiently lower than the speed of 
the signals was already proposed by the present applicant in Japanese 
Patent Application No. 63-270729 entitled "Monitoring and controlling 
method for optical repeater". When the outgoing main line signal S in 
subjected to amplitude modulation, the degree of the modulation needs to 
be kept sufficiently constant so as not to affect the transmission 
characteristics of the signal. The monitoring information is usually 
converted into a binary signal which has previously been subjected to 
modulation so as to become a low-frequency signal by which the outgoing 
main line signal S is modulated. The signal S is thus subjected to 
substantially constant amplitude modulation so that the monitoring signal 
is superposed on the outgoing main line signal which is sent to the 
receiving terminal station 35. Although conventional amplitude modulation 
is performed by directly increasing and decreasing the gain of the optical 
repeater which is comprised of an optical amplifier, the amplification 
characteristics of the repeater are greatly affected by environmental 
conditions such as temperature so that it is difficult to keep the degree 
of modulation constant. For that reason, the degree of modulation of the 
main line signal is not constant. As a result, the transmission 
characteristics of the main line signal will be affected. 
SUMMARY OF THE INVENTION 
The present invention provides a monitoring apparatus for an optical 
repeater that solves the above-mentioned problems. 
Accordingly, it is an object of the present invention to provide apparatus 
in which a main line signal is modulated without affecting the 
transmission characteristics thereof. 
The method provides for monitoring of the optical repeaters of an optical 
communication system in which the repeaters, in each of which an optical 
signal transmitted in an optical fiber is directly amplified when the 
signal is in the form of light, are provided at a large number of stages. 
The apparatus includes a conversion means by which monitoring information 
is converted into binary information; a previous modulation means by which 
a monitoring signal which contains the binary information is subjected to 
previous modulation with a prescribed sine wave; a modulation means by 
which the optical signal passing through the repeater is subjected to 
amplitude modulation with a portion of the monitoring signal subjected to 
the previous modulation; a filter means by which a component corresponding 
to the optical signal subjected to the amplitude modulation is extracted 
through tuning of the sine wave for the previous modulation; and an 
amplitude control means by which the amplitude of the monitoring signal 
subjected to the previous modulation is controlled so that the amplitude 
of the extracted component is nearly equal to a prescribed reference 
value. This results in solving of the above-mentioned problems. 
With the above-mentioned apparatus, the degree of the amplitude modulation 
of the main line signal does not depend on the optical repeater whose 
amplification characteristics are much affected by environmental 
conditions such as temperature. This differs from a conventional amplitude 
modulation apparatus in which the gain of an optical repeater is directly 
increased and decreased. The main line signal passing through the repeater 
is subjected to the amplitude modulation with the monitoring signal 
subjected to the amplitude control after the previous modulation. The 
degree of the amplitude modulation of the main line signal is kept 
constant so that an amplitude value obtained by extracting a portion of 
the modulated main line signal through the filter means tuned to a 
frequency for the previous modulation is equal to the prescribed reference 
value, thus preventing the transmission characteristics of the main line 
signal from being adversely affected.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT 
FIG. 1 is a block diagram of an optical repeater monitoring apparatus 
according to an embodiment of the present invention. FIG. 1 shows an 
optical fiber 1 for transmitting an optical signal, an incoming optical 
repeater 2, an optical division circuit 3 for dividing an incoming main 
line signal A, optical detectors 4 and 4' which are light receivers and 
which convert optical signals into electric signals, and filters 5 and 5' 
for extracting control signals C which are control instruction signals. A 
format conversion circuit 6 decodes the control signal C and converts 
monitoring information to a prescribed format of a monitoring signal M1. 
An oscillator 7 generates a low-frequency signal for performing the 
previous modulation of the monitoring signal M1, and a previous modulation 
circuit 8 modulates the monitoring signal M1 with low-frequency signal. A 
voltage-controlled amplifier 9 whose gain is changed under external 
control amplifies the modulated signal M2. The signal is then supplied to 
an outgoing optical repeater 10 comprised of an optical fiber amplifier 
and including a stimulating light source 10a, a wave synthesizer 10b and a 
doped optical fiber 10c. Optical fibers 11 and 12 are connected as 
outgoing transmission media to the input and output ends of the doped 
optical fiber 10c. Further, an optical division circuit 13 is provided for 
dividing a signal into an outgoing line and a monitoring line. A band-pass 
filter 14 for extracting the low-frequency signal component of a 
monitoring signal and an automatic gain control (AGC) circuit 17 are 
connected to the output of optical division circuit 13 through optical 
detector 4'. AGC circuit 17 regulates the output from the outgoing optical 
repeater 10 to a constant level. A reference voltage generator 15 
generates a reference voltage, and a comparison circuit 16 compares the 
monitoring signal from band-pass filter 14 with the reference voltage in 
order to control voltage-controlled amplifier 9. Stimulating light source 
10a is a pumping light source made of a semiconductor laser unit and 
generates stimulating light which functions as pumping light for 
stimulating the doped optical fiber 10c. Wave synthesizer 10b synthesizes 
the stimulating light with an outgoing main line signal B. The doped 
optical fiber 10c is manufactured by doping a quartz fiber with a rare 
earth element such as erbium. FIG. shows a means for monitoring optical 
repeater 2 provided in an incoming line, but does not show any means for 
monitoring optical repeater 10 provided in the outgoing line. However, 
such a monitoring means will be disposed at the position of filter 5' in 
the same manner as the monitoring means for the incoming line. FIG. 1 also 
shows an incoming main line signal A containing the control signal C, an 
outgoing main line signal B, and portions M1, M2 and M3 of monitoring 
information in optical repeater 2. 
Shown at (a), (b) and (c) in FIG. 2 are the waveforms of the monitoring 
signals M1, M2 and M3 in optical repeater 2. Monitoring signal M1 is a 
binary signal generated by the conversion in format conversion circuit 6. 
Monitoring signal M2 is a signal subjected to the previous modulation with 
the low-frequency signal generated by oscillator 7 and having a frequency 
Fr. Monitoring signal M3 is an optical signal generated at the output side 
of outgoing repeater 10 through the synthesis and amplification of 
outgoing main line signal B and monitoring signal M2. Although the level 
of optical signal M3 is kept constant by automatic gain control circuit 17 
shown in FIG. 1, there is a minute amplitude fluctuation of the optical 
signal at the place of the superposition of monitoring signal M3 on 
outgoing main line signal B, as shown in FIG. 2(c). Optical signal M3 is 
then sent to a receiving terminal station not shown in the drawings. 
Stimulating light source 10a is driven by both the output from previous 
modulation circuit 8 and a laser driving direct current supplied from 
automatic gain control circuit 17 via voltage-controlled amplifier 9. 
Since the output from doped optical fiber 10c is proportional to electric 
power for stimulating light source 10a, optical signal M3 as the output 
from outgoing optical repeater 10 has the waveform shown in FIG. 2(c). 
Optical signal M3 is converted into an electric signal by light receiver 
4. Only a frequency (Fr) component is extracted from the electric signal 
by filter 14 tuned to the oscillation frequency Fr of oscillator 7, and is 
then compared with the reference voltage generated by reference voltage 
generator 15. The difference between the frequency component and the 
reference voltage is fed back to voltage-controlled amplifier 9 so that 
the amplitude of the frequency component is controlled to be constant. 
Because of this control, a monitoring amplitude shown at M3 in FIG. 2, 
which is equal to the degree of the modulation with regard to the ratio of 
the amplitude to that of the optical output from repeater 10, is always 
kept constant. This makes it possible to send the stable outgoing main 
line signal B and the monitoring optical signal M3 superposed thereon 
toward the receiving terminal station. 
In the method, the oscillation frequency Fr of oscillator 7 and the 
frequency of filter 14 need to be stably tuned to each other for a long 
time. For that purpose, it is effective to make oscillator 7 a crystal 
oscillator and/or filter 14 a crystal filter. 
Although oscillator 7 is disposed in the optical repeater in the embodiment 
so as to feed the low-frequency signal, the oscillator may be disposed in 
a terminal station so as to feed the low-frequency signal together with 
the control instruction signal from the station to produce the same effect 
as the above-described embodiment. Further, although the optical fiber 
amplifier employing the doped optical fiber is provided as an optical 
amplifier in the embodiment, a different optical amplifier may be provided 
instead. If a semiconductor laser amplifier is provided as the optical 
amplifier in the embodiment, the monitoring signal M2 shown in FIG. 2 may 
be superposed on an injected current for the semiconductor laser 
amplifier. 
As described above, the low-frequency signal component contained in the 
modulation signal superposed on the main line signal is extracted by the 
filter tuned to the component. The amplitude of the modulation signal 
injected into the stimulating light source is controlled so that the level 
of the component is constant. For that reason, the main line signal can be 
modulated without affecting the transmission characteristics thereof. 
Oscillator 7, which generates a sine-wave signal for the previous 
modulation may be made of a crystal oscillator in order to stably tune the 
frequency of the oscillator and that of filter 14 to each other for a long 
time. Filter 14 may be made of a crystal filter to stably tune the 
frequency of oscillator 7 and that of the filter to each other for a long 
time. 
The monitoring information on the optical repeater can thus be stably 
transmitted while a optical communication system employing the repeater is 
in service. For that reason, the optical repeater monitoring method can be 
used for a long-distance optical communication system, whether the system 
is in service or out of service. This produces a highly desirable effect.