GB2204679A

GB2204679A - Fibre optic sensor

Info

Publication number: GB2204679A
Application number: GB8710274A
Authority: GB
Inventors: Dr Frank Kvasnik; Anthony Michael Campbell
Original assignee: UNIVERSITY OF MANCHESTER INSTITUTE OF SCIENCE AND TECHNOLOGY
Current assignee: UNIVERSITY OF MANCHESTER INSTITUTE OF SCIENCE AND TECHNOLOGY
Priority date: 1987-04-30
Filing date: 1987-04-30
Publication date: 1988-11-16
Also published as: GB8710274D0

Abstract

An optical fibre sensor comprises first and second microbending elements (1, 2) between which is sandwiched an optical fibre coil (3). <IMAGE>

Description

FIBRE OPTIC SENSOR The present invention relates to a fibre optic sensor.

So-called microbend sensors have been described, for example, by J.N. Fields et al (J. Acoust Soc. Am 67(3) 816-818). A practical application of such sensors is illustrated in U.K.-A-2131171. Briefly these prior sensors comprise a straight length of optical fibre placed between two plates with ridges running transversely to the fibre. At least one of the plates is subject to an applied force (e.g. as a result of a displacement being measured) and light is passed along the fibre. Variations in the applied force applied to the plate cause the fibre to be locally 'distorted' by the ridges resulting in modulation of the light. This modulation is dependent on the applied force and may be detected to provide a measurement of this force.

However, the above device suffers from a number of disadvantages. In particular, a straight optical fibre may not give the sensitivity required without increasing the length of the fibre beyond that which is practical (e.g. because the sensor must be of a particular maximum size). One solution which has been proposed is to bend the fibre into a "zig-zag" configuration so that the fibre makes several passes between the plates. This solution suffers from the disadvantage that the sensor becomes bulky and, in any event, cannot be minituarised because of the lower limit imposed by the minimum radius of bend of the fibre. Additionally, the insertion loss of the device also increases as the bend radius is decreased.

Alternatively, special fibres with small numerical apertures (NAs) can be used to improve the sensitivity. The optimum ridge spacing, required to achieve specific sensitivity, in this case is critically dependent on the NA of the fibre used (L.T. Wood et al (1985), Opt. Lett., 10, 632-634).

It is an object of the present invention to obviate or mitigate the abovementioned disadvantages.

According to the present invention there is provided an optical fibre sensor comprising first and second microbending elements in face-to-face relationship and relatively displaceable towards and away from each other, and a coil of an optical fibre sandwiched between said elements.

Preferably, the coil is a flat coil.

The sensor of the invention may of course be used in a manner entirely analogous to the prior art microbending sensors, i.e. by using the same light sources, detectors etc.

The use of a coil enables a large fibre length to be confined into a relatively small space and this increased length improves the sensitivity of the device. In a preferred embodiment of the invention the microbending elements each have a plurality of ridges running transversely to the coil (i.e, radially thereof) with the ridges of the two elements being identically spaced whereby the ridges may be superimposed on each other with the fibre in between. The use of such ridges further increases the sensitivity of the sensor. However, the use of such ridges is not essential since the general surface roughness of the bending elements will, when the elements are displaced towards each other, be sufficient to produce a degree of modulation of the light which may be sufficient for certain measurements.

Where ridges are used, the ridge spacing need not be optimised hence the same microbending elements can be used with a range of fibres. Extra high sensitivity can however be obtained by the use of long lengths of special fibres and appropriate choice of ridge spacing.

Performance of the device can be further improved by the use of cladding modes strippers or by surrounding the coil with a material with appropriate refractive index to remove cladding modes.

Further advantages of the invention are as follows: (a) the device exhibits good sensitivity to applied force even when manufactured from large NA fibre with thick acrylic coating; (b) different coils of optical fibre may be used to provide sensors of different sensitivity; (c) the device has low insertion loss; (d) there is a smaller distorting force applied to unit length of fibre which should increas reliability; (e) the device is easy to construct; and (f) the device is of small size, e.g. a flat coil with an outside diameter of 75mm may contain 7m of 400um diameter fibre.

The invention will be further described by way of example only with reference to the accompanying drawings, in which: Fig. 1 is an exploded view of one embodiment of sensor in accordance with the invention; and Fig. 2 illustrates the assembled sensor.

As shown in Fig. 1, the sensor comprises first and second generally circular microbending elements 1 and 2 between which locates a flat coil 3 of optical fibre. Element 1 is generally circular and has an upstanding central projection 4 over which the element 2 (of generally annular configuration) will locate (see also Fig. 2). Elements 1 and 2 are thus adapted to locate in generally face-to-face relationship (with coil 3 therebetween) and each of these faces has a plurality of radial ridges 5 as shown.

In use of the sensor, the end 6 of coil 3 is connected to a light source whereas the end 7 isconnected to a detector. Operation of the sensor is as previously described.

Claims

1. An optical fibre sensor comprising first and second microbending elements in face-to-face relationship and relatively displacable towards and away from each other, and a coil of an optical fibre sandwiched between said elements.

2. A sensor as claimed in claim 1 wherein the coil is a flat coil.

3. A sensor as claimed in claim 1 or 2 wherein the microbending elements each have a plurality of ridges running transversley to the coil.

4. A sensor as claimed in claim 3 wherein the ridges extend radially of the coil.

5. A sensor as claimed in any one of claims 1-4 wherein one of the microbending elements has a central upstanding projection and the other microbending element has a central aperture through which the projection locates.

6. An optical fibre sensor substantially as hereinbefore described with reference to the accompanying drawings.