Fault location in optical systems

Details Fault location in optical systems Fault location in optical systems

1994-02-28

1996-06-18

Boudreau, Leo

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

359177, 359179, 356 731, H04B 1008

Patent

active

055284043

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to the location of faults in optical telecommunications systems, especially to submarine systems.


BACKGROUND OF THE INVENTION

Optical submarine systems usually comprise cables which contain repeaters spaced at suitable intervals, e.g. about 50 km. Each cable contains several, e.g. 6 or 8 optical fibres which transmit the telecommunications traffic. In addition to the fibres, the cable usually contains a conductor to provide electrical power to the repeaters and strength elements, e.g. tensile wires, to increase its mechanical strength and to protect the fibres. Usually the tensile wires are in contact with the conductor so that the wires can assist in carrying the electrical current. The whole of the structure is enclosed in a waterproof sheath, usually polyethylene, which also provides electrical insulation. The cable usually has an annular structure with the fibres at the centre surrounded by the electrical conductor and the tensile elements and with the sheath on the outside. To give some idea of the dimensions, a typical cable has an overall diameter of 25 mm, the sheath is 5 mm thick, and the centre core, which contains all of the fibres, is usually about 2 mm thick. In shallow water, where cables are liable to be damaged by maritime operations such as fishing and dropping anchors, the structure described above may be contained inside armour.
The repeaters are needed because fibres attenuate signals whereby amplification is required at suitable distances. This invention is particularly concerned with repeaters in which the amplification is provided by a fibre amplifier. A fibre amplifier usually comprises a suitable length of fibre, e.g. 1-20 m, which contains a lasing additive such as a rare earth element. The fibre amplifier comprises a pump, e.g. a laser operating at 1480 nm, which produces a population inversion in the energy states of the lasing additive, whereby optical signals are amplified by laser action. The fibre amplifier usually includes an automatic gain control device (AGC) which monitors the strength of the amplified signals. The amplifier includes control means which adjust the power in the pump laser to maintain a constant signal level at the output. One method of improving the performance of the AGC comprises providing a control tone on the optical signals. The AGC detects the control tone and maintains its amplitude at a constant value. This technique guards against optical noise, e.g. from pumps, affecting the performance of the AGC.
It is possible that the optical cables described above may get damaged and, therefore, it is desirable to provide the system with a default mode which is adopted when the fibre is damaged. Clearly the breakage of a fibre means that no signals are transmitted through the break in the fibre and the amplifiers after the break receive no input. This implies that there is no amplified output or that the amplified output falls below a threshold level. Where a control tone is used the control tone falls below a threshold level. When low or no output is detected the system adopts a default condition. It should be noted that, because the conductor and the fibres represent a small filament in the centre of the cable, if any element is damaged it is usual that all the elements are damaged. Thus, although breakages are themselves unusual, when a breakage does occur, it usually affects all the systems of the cable. EP-0,331,304, (British Telecommunications) and its corresponding counterpart U.S. Pat. No. 4,995,100 describe a means of using an AGC (Automatic Gain Control) to amplify signals and also to detect the existence of breaks in cables. In that specification the AGC circuit responds to a control tone transmitted at a different frequency to the data signals. If the control tone drops below a predetermined threshold level the repeater then switches into distress mode. This will indicate either a break in the optical fibre or a failure of an amplifier.
JP60-177238 (Mitsubishi Electric Corp) also describes a meth

REFERENCES:
patent: 5013907 (1991-05-01), Bateman
patent: 5083874 (1992-01-01), Aida et al.
patent: 5093568 (1992-05-01), Maycock
patent: 5214728 (1993-05-01), Shigematsu et al.
patent: 5298965 (1994-05-01), Spirit et al.

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