Optical waveguides – Optical waveguide sensor – Including physical deformation or movement of waveguide
Patent
1992-05-05
1993-10-05
Gonzalez, Frank
Optical waveguides
Optical waveguide sensor
Including physical deformation or movement of waveguide
385128, 385140, 25022716, 374 17, G02B 602
Patent
active
052512746
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an optical fibre cable for detecting a change in temperature, which fibre cable comprises an optical fibre with a core and a cladding having separate refractive indexes, and a temperature-responsive device which surrounds the optical fibre along at least a part of its length, wherein in a non-influenced state of the fibre cable, a light pulse having been sent into the fibre from its one end will be attenuated substantially uniformly along the length of the fibre, and upon microbending of the optical fibre caused by the temperature-responsive device, the light pulse will be further attenuated.
BACKGROUND ART
Microbending will occur in an optical fibre which is subjected to an external force, with subsequent light attenuation. Although this is a disadvantage in signal transmission, microbending can be utilized in a manner which will enable the fibre to be used as a detector or sensor element. An example of one such force detecting element is found described in European Patent Specification Number 0188512. An optical fibre having a core and cladding is embraced by a primary cover, and a twisted tape is disposed between the cladding and the primary cover. The twisted tape is responsible for the microbending when a force is exerted on the fibre.
A fibre cable for detecting temperature is illustrated in Japanese Patent Application No. 59-50676. The optical-fibre cladding has wound therearound a wire consisting of memory-metal, which contracts when exposed to a given detection temperature and therewith exerts pressure on the cladding and causes microbending of the fibre. Each molecule in the fibre core reflects light by Rayleigh-scattering and the intensity of the light reflected is directly proportional to the intensity of the passing light pulse. The intensity of the reflected light pulse decreases as a result of microbending of the fibre. The reflected light pulse can be detected and the position at which microbending occurs along the fibre can be established by so-called OTDR, Optical Time Domain Reflectometry. The drawback with this device is that in many applications it is difficult to obtain a memory metal which will produce microbends of desired sizes and at desired temperatures. The device is also relatively complicated to manufacture.
DESCRIPTION OF THE INVENTION
According to the invention, an optical fibre has a casing made of polymer material, the coefficient of heat expansion of which has a desired value in a limited temperature range. The value of the coefficient of heat expansion can be chosen by appropriate choice of the polymer material. Either a positive or a negative coefficient can be chosen and the limits of the temperature range can be chosen through selection of the polymer material. This enables the optical fibre with its casing, the fibre cable, to be given the desired attenuation properties.
BRIEF DESCRIPTION OF THE DRAWINGS
An exemplifying embodiment of the inventive fibre cable will now be described in more detail with reference to the accompanying drawings, in which
FIG. 1 illustrates schematically a building with the fibre cable and a monitoring system;
FIG. 2 illustrates the fibre cable in cross-section;
FIG. 3 is a schematic illustration of the structure of a polymer material;
FIG. 4 is a diagram of the coefficient of heat expansion of the polymer material;
FIG. 5 is a diagram of the force-effect achieved at different temperatures;
FIG. 6 illustrates the attenuation of a light pulse along the fibre cable;
FIG. 7 illustrates the coefficient of heat expansion of an alternative polymer material;
FIG. 8 is a cross-sectional view of an alternative embodiment of the fibre cable; and
FIG. 9 is a diagram showing the coefficient of heat expansion of the materials in the alternative fibre cable of FIG. 8.
BEST MODE OF CARRYING OUT THE INVENTION
FIG. 1 illustrates schematically a building 1, for instance a greenhouse, whose temperature is to be monitored. The greenhouse is divided into several rooms 2, 3, 4 and it is of interest to
REFERENCES:
patent: 4417782 (1983-11-01), Clarke et al.
patent: 4463254 (1984-07-01), Asawa et al.
patent: 4713538 (1987-12-01), Theocharous
patent: 4729627 (1988-03-01), Saito et al.
patent: 4767219 (1988-08-01), Bibby
Carlstrom Bengt J. A.
Forsberg Gunnar S.
Gonzalez Frank
Telefonaktiebolaget LM Ericsson
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