Optical fiber transducer

Details Optical fiber transducer Optical fiber transducer

1999-09-29

2001-04-24

Spyrou, Cassandra (Department: 2872)

Optical waveguides

With optical coupler

C385S016000, C385S004000, C385S013000

Reexamination Certificate

active

06222952

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to detection of a measurand utilising a transducer unit in association with an optical fibre or optical fibre cable.
According to a first aspect the present invention provides a transducer unit adapted to be fitted to an optical fibre, the transducer unit comprising a support structure carrying an assembly of clamp members mutually spaced from one another in a generally linear direction, the clamp members being adapted and arranged to be secured individually to an optical fibre extending in said direction, obstructive means disposed in the path of the optical fibres between first and second clamp members of the assembly for pre-bending the fibre from its lenear path when the fibre is clamped, a sensor element which is sensitive to a particular measurand disposed between at least one of the first and second clamp members and another clamp member of the assembly, the sensor element being such that in operation exposure to the particular measurand causes a dimensional change of the sensor element in said linear direction, the arrangement of the transducer unit being such that the magnitude of the bend in the fibre is enlarged to an abnormal level which introduces a measurable loss in the transmission characteristics of the fibre when the sensor element is exposed to the particular measurand.
SUMMARY OF THE INVENTION
The sensor element may be of the type which dimensionally enlarges or dimensionally contracts in said linear direction. Such sensor elements may be made of hydrogel material or shape memory alloy material. Dimensionally contracting sensor elements are disposed between the first and second clamp members.
The sensor element is preferably made of a material where the dimensional change is reversible on removal from exposure to the particular measurand. Such sensor elements are preferably secured at each end to at least one of the first and second clamp members and to the said another clamp member of the assembly.
The sensor element may be in the form of a hydrogel plug for sensing aqueous environments such as the presence of water or water vapour or pH value. It may be a rubber plug for sensing hydrocarbon presence. It may be a swellable wax plug for sensing heat or it may be a shape-memory-alloy element such as a spring for sensing heat. It will of course be recognised that the foregoing materials are given only by way of example of the many materials which are suitable to form the sensor elements.
In one embodiment the assembly of clamp members are three in number and have limited movement with respect to the support structure and are arranged so as to be mutually separated by respective distances each of which is less than the length of the corresponding fibre portion when the clamp members are secured to be fibre. The sensor element is disposed between the second and third clamp members. In this way when the sensor element is of the type which becomes enlarged on exposure to the particular measurand it causes movement of the second or central clamp member without being restrained by the presence of a comparatively rigid portion of fibre between the first and second clamp members.
It is preferred that a resilient biasing element is located between the first and second clamp members for restoring the position of the central or second clamp member after activation of the sensor element and in the absence of the particular measurand.
The optical fibre is preferably a single mode fibre or a bundle of single mode fibres forming a fibre cable. In this connection it will be understood that the integrity of the optical fibre does not require to be compromised by use of the transducer unit in that there is no stripping of fibre cladding or overcovering or the like.
The condition of the optical fibre and the transducer unit associated therewith may be interrogated by transmission of a signal along the length of the fibre and which is detected at the fibre output and by a power threshold detection unit. Alternatively a detection system which uses optical time domain reflectometry (OTDR) may be used to locate the position of a transducer unit incorporating an abnormally bent fibre. This is of particular value where several transducer units are fitted along the length of the optical fibre. Several different types of sensor elements can be used in transducer units along the length of the same cable in order to detect different measurands. The loss characteristics can be tuned so that different sensor types have different loss signatures on activation thereby enabling detection of different measurands to be prioritised.


REFERENCES:
patent: 4725124 (1988-02-01), Taylor
patent: 4729630 (1988-03-01), Martinez
patent: 4789241 (1988-12-01), Michal et al.
patent: 5093569 (1992-03-01), Krumboltz et al.

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