Optical waveguides – Optical waveguide sensor – Including physical deformation or movement of waveguide
Patent
1989-11-13
1992-04-28
Lee, John D.
Optical waveguides
Optical waveguide sensor
Including physical deformation or movement of waveguide
359116, 359124, 359900, 25022714, G02B 616
Patent
active
051094436
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a method and apparatus for detecting changes in stress applied to an optical fibre, and particularly but not exclusively to a method and apparatus for detecting stresses applied to such fibres by movements caused for example by attempts to tap into such fibres in optical communication systems; and to systems incorporating such apparatus or employing such a method.
BACKGROUND OF THE INVENTION
The unauthorised tapping of communication systems is a problem of concern to commercial and financial interests. When the first optical fibre systems were deployed it was throught that clandestine access to the optical signal in the fibre would be virtually impossible. More recent developments in fibre handling and techniques for injecting and extracting optical signals from the fibre have lead to concern that optical tapping could be possible without the system operator readily becoming aware of it.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method and apparatus for detecting such attempts. Accordingly there is provided a method of detecting changes in stress applied to an optical fibre, comprising the steps of: a data-carrying light beam in a single mode an electromagnetic signal having a wavelength at which the fibre supports propagation in a plurality of modes; and fibre by the interaction of said modes.
According to a second aspect of the present invention there is provided monitoring apparatus for detecting movement of an optical fibre, comprising means adapted for coupling an electromagnetic signal into an optical fibre communication link adapted to convey a data-carrying light beam in a single mode an electromagnetic signal having a wavelength at which the fibre supports propagation in a plurality of modes and means for detecting changes in the interference pattern produced at an output of the fibre by the interaction of said modes.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described with reference to the drawings, in which;
FIG. 1 is a schematic diagram of an experimental arrangement illustrating the general principle of operation of the invention; and
FIG. 2 is a schematic diagram of a data transmission system according to the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
Multimode fibres are so called because the optical energy passing along the fibre is distributed between a large number of optical paths or `modes`. The energy distribution between the modes depends on the distance travelled in the fibre because transfer from one mode to another can arise at joints, connectors, bends and from stresses imposed by the surroundings of the fibre. The method of the present invention is based on the monitoring of part or all of the emergent interference pattern from a fibre which allows the transmission of more than one optical mode. A fibre intended for single mode operation in a particular wavelength band is monitored at a shorter wavelength where its waveguiding properties allow more than one mode to propagate. At a multimode wavelength the fibre may pass from two to many hundreds of individual modes.
On emerging from the fibre, the relative phases of the various modes constructively or destructively interfere in a complex way to produce an interference pattern that can be observed, when using visible light, by imaging it onto a screen with a lens. The interference pattern so produced is sometimes called a speckle pattern. A speckle pattern is also produced by non-visible electromagnetic radiation.
The speckle pattern consists of random, alternating high and low intensities whose distribution on the screen changes in response to disturbances in the fibre. The overall energy remains constant but in a selected part of the field of view (10% for example although other portions may be chosen) the energy will, it has been found by the applicant, increase or decrease as the stresses imposed on the fibre vary. A photodetector, for example a silicon or III-V compound PIN diode with suitable amplification, regis
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Hill Godfrey R.
Stanley Ian W.
White Kenneth I.
British Telecommunication public limited company
Lee John D.
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