Line monitoring of an optical transmission system using pilot signals

An optical transmission system allows for the remote determination of the total output power of each repeater. The optical transmission system includes two terminals, an optical path that transmits a plurality of optical signals between the two terminals, and a plurality of repeaters spaced along the optical path. In each of the repeaters, pilot signals are inserted by impressing them upon the amplified optical signals output from the repeater. The pilot signals are derived from an oscillator whose frequency is unique to the repeater in which the pilot signals are inserted. The magnitude of each pilot signal is proportional to the output power of its associated repeater. Each terminal provides a measurement of the total power output from each repeater using a receiving device. The receiving device selects the unique oscillator frequency for each of the repeaters desired to be measured, and measures the pilot signals at the selected frequency.

FIELD OF THE INVENTION 
The present invention is directed to line monitoring of optical 
transmission systems. More particularly, the present invention is directed 
to line monitoring of optical transmission systems using pilot signals to 
determine the total output power of each repeater in the system. 
BACKGROUND OF THE INVENTION 
Long distance optical transmission systems generally require a plurality of 
amplifiers located along the length of the optical fibers to periodically 
amplify the optical signals. It is essential in these systems to provide 
the ability to remotely monitor the performance of any amplifier, and to 
locate the source of system degradation or fault to a particular amplifier 
or cable section. 
Most known methods for remotely monitoring the performance of optical 
amplifiers in an optical transmission system require an optical loopback 
path between adjacent amplifiers on the forward and return optical paths, 
and the generation of a test signal on at least one end of the system. For 
example, U.S. Pat. No. 5,436,746 discloses an optical transmission system 
that includes multiple loopbacks. A test signal is generated at the local 
station, or terminal, and transmitted on a forward path. The test signal 
is returned to the local station via the optical loopbacks and a return 
path. Measurement of the test signal provides information that is related 
to the performance of the amplifiers within the optical transmission 
system. 
The method of using loopback paths to remotely measure the performance of 
amplifiers has several disadvantages. Specifically, the loopback method 
requires test signals to be both transmitted and received on associated 
fiber pairs at a terminal of the transmission system. The test signals 
must travel over an optical fiber pair (i.e., the forward path and the 
return path). Therefore, the loop loss information provided by the 
loopback method is ambiguous because there is no way to tell how the loop 
losses are distributed between the forward and return path. 
Further, the loop loss information provided by the loopback method is 
redundant because the same information is measured at both terminals of 
the transmission system. In addition, the optical loopback paths between 
adjacent amplifiers cause a significant transmission impairment in the 
form of crosstalk or added noise. Finally, the loopback method, when used 
to provide information in-service (i.e., while the optical transmission 
system is transmitting signals) requires a long time (approximately 2-8 
hours) to obtain a measurement due to the typical poor signal-to-noise 
(S/N) ratio of the monitoring signal. Transmission systems that utilize 
multiple carrier wavelengths, and their corresponding monitoring signal, 
have lower S/N ratios than single wavelength systems, and therefore 
obtaining measurements using the loopback method in these systems impose 
an even greater time delay. 
Based on the foregoing, there is a need for a method and apparatus for 
remotely measuring amplifier performance that does not require a fiber 
pair to obtain measurements, and provides measurement information quicker 
and more accurately than known methods, especially when multiple carrier 
wavelengths are used. 
SUMMARY OF THE INVENTION 
The above-described needs are met by the present invention which remotely 
determines the total output power of each repeater in an optical 
transmission system. In one embodiment, the optical transmission system 
includes two terminals, an optical path that transmits a plurality of 
optical signals between the two terminals, and a plurality of repeaters 
spaced along the optical path. 
In each of the repeaters, pilot signals are inserted by impressing them 
upon the amplified optical signals output from the repeater. The magnitude 
of each pilot signal is proportional to the output power of its associated 
repeater. The pilot signals are derived from an oscillator whose frequency 
is unique to the repeater in which the pilot signals are inserted. Each 
terminal provides a measurement of the total power output from each 
repeater using a receiving device. The receiving device selects the unique 
oscillator frequency for each of the repeaters desired to be measured, and 
measures the pilot signals at the selected frequency.

DETAILED DESCRIPTION 
The present invention provides a measurement of the output power of each 
repeater in an optical transmission system. FIG. 1 illustrates an optical 
transmission system in accordance with one embodiment of the present 
invention. The transmission system of FIG. 1 is a long distance underwater 
system that transmits optical communication signals between a terminal 2 
and a terminal 3. The communication signals are sent from terminal 2 to 
terminal 3 via optical path 4 and are sent from terminal 3 to terminal 2 
via optical path 5. Each optical path 4, 5 includes up to four optical 
fibers. 
A plurality of repeaters 10-13 are located in a series of predetermined 
intervals along optical paths 4 and 5. Repeaters 10-13 include an 
amplifier for each optical fiber passing through them. The amplifiers 
amplify the optical signals as they travel between terminals 2 and 3. 
Terminal 2 includes a constant DC current source 6 which produces a 
constant electrical current of predetermined magnitude. The current 
provides power to repeaters 10-13 via center conductor 30. Center 
conductor 30 is connected to voltage source 7 in terminal 3. Both current 
source 6 and voltage source 7 are connected to ground. 
FIG. 2 is a detailed illustration of repeater 10 in accordance with one 
embodiment of the present invention. Optical fibers 32 and 33, which form 
optical path 5 shown in FIG. 1, transmit optical signals from right to 
left in FIG. 2. Optical fibers 34 and 35, which form optical path 4, 
transmit optical signals in a direction opposite to that of optical fibers 
32 and 33. 
A pump manifold 50 includes a plurality of pump lasers. The pump lasers are 
powered by current received from a pump bias control circuit 68 on path 
100. The pump lasers produce pumping power proportional to the input 
current in a known manner. The pumping power is output from pump manifold 
50 on paths 52. 
Optical fibers 32-35 each include identical amplifier components. Referring 
to optical fiber 32, path 52 is coupled to a directional wavelength 
selective coupler 40. Coupler 40 causes the optical energy output by pump 
manifold 50 on path 52 to be directed into an erbium doped fiber 42 which 
amplifies optical signals on optical fiber 32. Optical fiber 32 also 
includes an optical isolator 44 which prevents power from flowing 
backwards. 
Repeater 10 further includes an oscillator circuit that inserts pilot 
signals by impressing the pilot signals onto the amplified optical signals 
output from repeater 10. The oscillator circuit includes a crystal 
oscillator 60 that generates a signal at a frequency unique to repeater 
10. Each repeater 10-13 in the optical transmission system is identical 
except that its corresponding oscillator generates a signal at a different 
frequency. In one embodiment, the frequency separation between repeaters 
10-13 is approximately 200-400 Hz, and all of the oscillator frequencies 
in the optical transmission system is within a band of approximately 
0.5-0.6 MHz. 
The AC current signal from crystal oscillator 60 is input to a multiplier 
74 via path 90. Also input to multiplier 74 is a DC current signal via 
path 94 that is proportional to the total output power of the amplifiers 
in repeater 10. The DC current signal is generated by photo detectors 
80-83 which are coupled to optical fibers 32, 34, 35 and 33, respectively 
via couplers, and measure the power of the amplified optical signals. 
Approximately ten percent of the amplified power on fibers 32-35 is 
directed to photo detectors 80-83 by the couplers. Photo detectors 80-83 
output a current that is proportional to their input optical power. 
The outputs of photo detectors 80-83 are input to a summation device 78. 
The output of summation device 78 is input to a DC amplifier 72 which 
outputs an amplified DC signal component. The output of amplifier 72 is 
input to multiplier 74 as the total power signal via path 94. The output 
of multiplier 74 is an AC current with a frequency equal to the frequency 
of crystal oscillator 60 and with a magnitude proportional to the total 
output power of repeater 10. 
The AC current output from multiplier 74 is input to summing device 66 via 
path 75. Also input to summing device 66 via path 64 is the DC current 
from center conductor 30. The level of the AC current is very small in 
comparison to the level of the DC current. In one embodiment, the level of 
the AC current is one percent of the level of the DC current. 
The output of summing device 66 is input to pump bias control circuit 68. 
Pump bias control circuit 68 outputs a DC bias current and the AC current. 
The output from pump bias control circuit 68 is coupled to the pump lasers 
in pump manifold 50 via path 100. Because amplifier output power is 
proportional to pump current, the AC current that is coupled to the pump 
lasers via path 100 causes the amplifier output power on optical fibers 
32-35 to change very slightly at the rate of oscillator 60. This causes 
amplitude modulated sidebands, referred to as pilot signals, to be 
impressed on all amplified optical signals output from repeater 10. The 
magnitude of the pilot signals corresponds to the total power output of 
repeater 10. 
FIG. 3 is a detailed illustration of a receiving device 102 within terminal 
2. Terminal 3 also includes a similar receiving device. The receiving 
device 102 shown in FIG. 3 is used to recover and measure the line 
monitoring pilot signals that are inserted in each repeater 10-13. 
As shown in FIG. 3, receiving device 102 is coupled to optical fiber 32 
through an optical coupler 104. However, receiving device 102 can be 
coupled to any fiber transmitting incoming optical signals to terminal 2 
(i.e., fibers 32 or 33). 
A portion of the optical signals transmitted on optical fiber 32 is coupled 
to fiber 105 by an optical coupler 104. The optical signals are then 
amplified by an optical amplifier 110 and converted to electrical signals 
by a photo detector 112. 
A control & data storage unit 120 selects a repeater for measuring the 
total output power. A repeater is selected by specifying a frequency that 
coincides with the frequency of an oscillator included in a repeater 
10-13. The frequency is input to a phase-locked homodyne or heterodyne 
selective level detector 118 via path 121. The signal output from photo 
detector 112 is also input to selective level detector 118. Selective 
level detector 118 locks on to the portion of the input signal that 
coincides with the input frequency. Therefore, selective level detector 
118 isolates and selects the pilot signal inserted by the repeater that 
includes the oscillator of the selected frequency. 
Selective level detector 118 includes a selective level measuring set that 
measures the selected pilot signal power. This power is proportional to 
the total power output of the repeater that inserted the selected pilot 
signals. The power level is stored in control and data storage device 120 
via path 122 for later reference. 
The selection process by level detector 118 and control and data storage 
device 120 is repeated until the total output power level is measured for 
each repeater 10-13. Therefore, the combination of the repeaters and the 
receiving device in accordance with the present invention provides a 
measurement of total output power of each of the repeaters in the optical 
transmission system. 
The present invention provides many advantages over the prior art loopback 
method. Measurements of total power output of each repeater using the 
present invention can be performed in approximately one second. Further, 
the present invention requires only receiving equipment at the terminal. 
In addition, the present invention does not provide ambiguous information 
because each terminal only provides measurements for the received signals. 
One embodiment of the present invention is specifically illustrated and/or 
described herein. However, it will be appreciated that modifications and 
variations of the present invention are covered by the above teachings and 
within the purview of the appended claims without departing from the 
spirit and intended scope of the invention. 
For example, although an underwater long distance optical transmission 
system is described, the present invention can be implemented on any 
optical transmission system that includes repeaters. Further, other 
alternative methods of impressing amplitude modulation on the amplified 
signals other than the method of pump current modulation that is used in 
the described embodiment can also be implemented.