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Optical cable fault locating using Brillouin optical time domain reflectometer and cable localized heating method






Optical cable fault locating using Brillouin optical time
domain reflectometer and cable localized heating method
Y G Lu, X P Zhang, Y M Dong, F Wang and Y H Liu
Institute of Optical Communication Engineering, School of Engineering &
Management, Nanjing University, Nanjing 210093

E-mail: luyg@nju.edu.cn
Abstract. A novel optical cable fault location method, which is based on Brillouin optical time
domain reflectometer(BOTDR) and cable localized heating, is proposed and demonstrated. In
the method, a BOTDR apparatus is used to measure the optical loss and strain distribution
along the fiber in an optical cable, and a heating device is used to heat the cable at its certain
local site. Actual experimental results make it clear that the proposed method works effectively
without complicated calculation. By means of the new method, we have successfully located
the optical cable fault in the 60 km optical fiber composite power cable from Shanghai to
Shengshi, Zhejiang. A fault location accuracy of 1 meter was achieved. The fault location
uncertainty of the new optical cable fault location method is at least one order of magnitude
smaller than that of the traditional OTDR method.
42.81.Cn
Keywords
: fault location, Brillouin optical time domain reflectometer, optical time domain
reflectometer, localized heating, optical cable, optical fiber

1. Introduction
Optical cable, which is the channel to be used to transmit information in the form of optical pulses of a
specific wavelength, is the basic component of the optical communication networks. Unfortunately,
optical cables laid on the sea floor and under the earth are vulnerable to all kinds of threats. For
IOP Publishing
Journal of Physics: Conference Series 48 (2006) 138749
doi:10.1088/1742-6596/48/1/257
International Symposium on Instrumentation Science and Technology
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© 2006 IOP Publishing Ltd






instance, under-earth optical cables are often damaged because of natural disasters and human
interventions. Similarly, man-made attacks, such as ships anchoring and fishing activities, are the
major cause of submarine cable faults. If a cable line fault occurs, the huge amount of information will
be lost, which makes optical cable fault location as well as optical cable maintenance a crucial
problem to solve.
Generally, Optical Time Domain Reflectometer (OTDR) is used to identify optical cable fault[1-3].
By means of OTDR, one can get the distance from the fault site to the measurement site along the
optical fiber housed in the optical cable. In most kind of optical cables, optical fibers are usually
housed in a tube, and are laid with an excess length of fiber compared to the length of the tube in order
to protect the sensitive optical fibers against external mechanical and chemical factors. Therefore, in
such optical cables the length of the optical fiber is not equal to that of the cable. Thus one cannot
directly obtain the spatial location of the optical cable fault by using the OTDR equipment. In order to
locate the spatial site of optical cable fault, related project documentation, such as the cable lines
property sheet and the map of cable connection, and complicated calculating based on the scale on
each map of cable routes are need. In general, in this method the fault location accuracy of under-earth
optical cable and that of submarine optical cable is respectively about 40~50 meters and hundreds of
meters, though the OTDR can provide sub-meter or even mm-order spatial resolution[4]. In order to
improve the accuracy and efficiency of the fault location, a simple and direct optical cable fault
location method is proposed in this paper. The new optical cable fault location method is based on
BOTDR and cable localized heating. Experimental results verify the effectiveness of the new method.

2. Principle
The Brillouin optical time domain reflectometer(BOTDR) is a fiber optic strain and temperature
distributed sensing system, which can measure strain and temperature along arbitrary regions of an
optical fiber. If the optical cable is heated in certain part of the cable, and the temperature distribution
curves along the fiber before and after the heating process are measured by the BOTDR, the
corresponding heated point in the temperature distribution curves along the fiber can be identified by
comparing the difference between the two curves. Since the location of the optical fiber fault also can
be obtained by using the BOTDR, the distance between the faulty site and the heated site along the
cable can be determined by reading the distance between the faulty site and heated site along the fiber.
Therefore, the optical cable fault location can be determined without related project documentation
and complicated calculating. The accuracy of the fault location depends on the spatial resolution of the
BOTDR and the distance between the heated site and the faulty site. The less distance there is, the
more accurate the fault locating result is.

We can increase the times of the heating process to decrease
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the distance between the heated site and the faulty one, and thereby obtain higher fault location
accuracy. The schematic of the proposed optical cable fault location method is shown in figure 1.


Figure 1. Schematic of the proposed optical cable fault location method.

The process of optical cable fault location is as following:
1.
Firstly a BOTDR is connected to certain optical fiber in the optical cable and the optical loss
distribution or distributed strain along the optical fiber is measured. The faulty site in the fiber is
determined by observing the distribution curve of the optical loss or strain along the fiber.
2.
Estimate the approximate spatial site of optical cable fault according to the faulty site in the fiber
and then heat 1 meter length of optical cable at the estimated cable fault site with an automatic
temperature-control heating device. The distance between the estimated faulty site and the
measurement site along the cable depends on the excess length of the fiber in the optical cable.

In
general, the distance is chosen as about 0.85
0.95 of the distance between the faulty site and the
measurement site along the fiber.
3.
After the temperature of the local optical cable increase to a certain value, the BOTDR is used to
measure the distribution of the strain along the fiber once more. Because the variation of
temperature and strain both can cause a frequency shift of spontaneous Brillouin scattered light,
there will be a sharp increase corresponding to the heated location in the strain curve map,
provided that there is no other frequency shift factor except the temperature increase in the heated
site. Thus we can obtain the heated site in the strain curve map and accurately determine the
distance between the heated site and the faulty one along the fiber.
4.
Move forward the above determinate distance along the cable route and observe the optical cable
at the new location. If there is evident cable damage at this location, then the position is just the
location of the optical cable fault. Otherwise, heat the optical cable at the new location.
5.
Repeat step 3 and 4 until finding the evident damage existing on the cable or until the heated site
and the faulty site in the fiber are the same point in the strain curve map.
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Generally two or three repeated processes are enough to determine the location of the optical cable
fault. Depending on the spatial resolution of the BOTDR and the repeated times of heating process, the
optical cable fault location accuracy is good to 1 meter. In next section, the actual application results
will make it clear that the proposed method works effectively. The fault location result by use of the
new method also will be compared with that by use of the traditional OTDR method.
3. Experimental results
In the actual experiment, a 60 km optical fiber composite power cable that contains 5 optical fibers,
which was laid on the shallow sea floor from Shanghai to Shengshi island, Zhejiang, China, was
damaged by ship anchor and needed to fault location before repairing. The BOTDR system
commercialized by Ando Electric Corporation, AQ8603, was used in this experiment. The BOTDR
was placed at the end of the cable on the shore and connected to certain optical fiber in the cable. A
special cable localized heating box manufactured for the experiment also used in the actual optical
cable fault location process. In the fault location process, the heating box was placed in the sea to heat
certain part of the cable. In order to compare the performance of the proposed method and the
traditional OT