Optical waveguides – Optical transmission cable
Patent
1995-11-15
1997-09-30
Healy, Brian
Optical waveguides
Optical transmission cable
385 12, 385123, 385147, 385140, 174 681, 356 731, G02B 644, H01B 700, G01N 2100
Patent
active
056733518
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a method and an apparatus for inducing a temporary localised transmission loss (point loss) in a telecommunications cable. One particular form of the invention relates to inducing a temporary localised optical transmission loss in an optical fiber cable, which loss can be detected from one end thereof.
BACKGROUND OF THE INVENTION
In a communications system which utilizes optical fibres for the mission of signals over long distances, faults which may occur from time to time along the optical fibre mission path may be difficult to accurately locate. The optical fibres are commonly grouped together into an optical fibre cable in which a number of fibres are encased in various protective materials to enable the cable to be buried along the transmission path between telecommunications exchanges. A fault may occur at a point along the optical fibre cable by virtue of, for example, attack by termites or pressure from a rock where the cable is buried, which introduces a transmission loss for signals transmitted along the optical fibre affected by the fault. Along a long distance optical fibre telecommunications path there may be up to 50 km of optical fibre cable between telecommunications exchanges, and perhaps 2 to 20 km between splices in the optical fibre cable.
Optical time domain reflectometry (OTDR) is a technique which may be used to determine the approximate position of a point loss in an optical fibre from a remote end of the fibre. The use of OTDR for fault location is quite well known, and examples of its application may be found in: Localization of Optical Amplifier Submarine Cable Systems Yukio Horiuchi, Shiro Ryu, Kiyofumi Mochizuki and Hiroharu Wakabayashi IEEE Photonics Technology Letters, Vol. 2, No. 4, Apr. 1990 Containing Optical Circulators Yoshiaki Sato and Koh-ichi Aoyama IEEE Transactions Photonics Technology Letters, Vol. 3, No. 11 , Nov. 1991
Briefly, the OTDR apparatus generates a pulse of light from a laser diode and launches it through a directional coupler into the optical fibre to be measured. Reflected light returning to the OTDR receiver is directed by the directional coupler to a detector and converted into an electrical signal. The timing of pulse reflections detected at the OTDR receiver is indicative of the distance along the optical fibre of the cause of such reflections, which may be determined from a trace of the OTDR receiver output. An intrinsic mechanism of losses in an optical fibre results from Rayleigh scattering, which is caused by continuous microscopic refractive index variations "frozen" into the fibre during the production process. This is a property of the material and cannot be eliminated. As a result, small mounts of light are scattered in all directions throughout the fibre and some of this scattered light is captured by the fibre and guided back to the OTDR receiver. This light is referred to as back scattered light, and is observable on an OTDR trace as a continuous slope indicating continuous loss along the length of the fibre. Point losses are also observable by use of OTDR, and these may result, for example, from fibre splices or increased micro bending losses at a point of the fibre. A fault along an optical fibre cable, such as a kink in the cable or a damaged or broken optical fibre therein results in a point loss causing reflected or back scattered light which may be observed on an OTDR trace.
The accuracy with which a fault can be located along an optical fibre cable depends on a number of factors, many outside that of the resolution of the OTDR instrument itself. In addition to the resolution limits of the OTDR instrument, one factor which may affect the accuracy with which a fault can be located is the discrepancy which can occur between the optical path length and the physical length of the optical fibre cable exterior. Since OTDR relies upon signals transmitted along the fibres themselves, the distances determined thereby relate to the distance along the actual optical fibre within the optical
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Clarke Kenneth Paul
Coulson David
Gibbs Ernest Edward
Keon Barry John
Kruijshoop Alfred Willem
Healy Brian
Telstra Corporation Limited
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