This invention relates to optical fibre sensors for use in optical sensing systems such as for the detection of acoustic pressure waves (e.g. hydrophone applications) or temperature, for example.

More specifically, the invention relates to optical fibre sensors comprising an optical fibre effectively divided into a plurality of sensor elements by partially reflective splices or discontinuities. An optical sensing system embodying optical fibre sensors of this construction form the subject of our British Patent No. 2126820B to which attention is hereby directed.

One of the problems experienced with such optical fibre sensors is the tendency for multiple reflections to occur within the optical fibre sensor causing cross-talk between the sensing elements of the sensor.

The present invention is directed to minimising cross-talk within an optical fibre sensor of the form described.

According to the present invention there is provided an optical fibre sensor for use in optical fibre sensing arrangements (eg. pressure or temperature sensing arrangements), in which the optical fibre sensor has optical discontinuities spaced along its length to serve as partial reflectors to light propagating along the sensor, in which each of the discontinuities comprises light reflecting means which extends transversely into the path of light propagating in at least one direction along the optical fibre whereby some of the propagating light travels along the optical fibre unimpeded by the reflecting means whereas the remainder of the propagating light is

reflected by impingement on said light reflecting means and in which the angle of reflection of the light propagating in one direction is such that the reflected light travels back along the optical fibre whilst the angle of reflection of light, if any, propagating from the other direction is such that the reflected light is lost by transmission through the wall of the optical fibre.

It will be appreciated that optical fibre sensors constructed in accordance with the present invention prevent the occurrence of the previously mentioned multiple reflections since that part of light travelling in one direction along the sensor which is reflected will be reflected in its entirety out through the wall of the optical fibre.

The reflection means may comprise separate reflectors (e.g. plates or suitably shaped blocks) which extend radially into the optical fibre path at respective splices therealong. In the case where reflector plates are used the spliced ends of the optical fibre may be slightly displaced transversely relative to one another to provide the requisite directional reflective characteristics of the discontinuity.

Alternatively, the reflection means may comprise suitably profiled sections of the optical fibre which may remain unsevered along its length or a suitably reflective medium may be introduced into the optical fibre core by diffusion at suitable points along the optical fibre sensor and shaped to provide the requisite reflective characteristics of the discontinuity.

By way of example the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 illustrates the occurrence of multiple reflections in a conventionally spliced optical fibre sensor; and.

Figures 2 to 6 are diagrams of parts of an optical fibre sensor having a partial reflective discontinuity in accordance with the present invention.

Referring to Figure 1 of the drawings an optical fibre sensor 1 is provided along its length with a plurality of partially . reflective splices two of which are shown at 2 and 3.

As depicted in the drawing some of the light propagating along the fibre in the direction A will be directed back along the fibre from the splice 2 whereas some light will pass through the splice 2 and will be reflected by the splice 3 but instead of all passing back through the splice 2 some of it will be reflected back again to splice 3 where it is reflected so that it finally passes through the splice 2. Such multiple reflections cause unwanted cross-talk or cross - coupling between optical fibre sensor elements 4, 5, and 6 defined by the sections of optical fibre between discontinuities.

Referring now to Figures 2(a) and 2(b) these show a splice from a discontinuity for providing partial reflection of light in both directions along a monomode or multimode optical fibre 7. As can be seen, a tapered reflective block 8 is interposed between the spliced ends of the optical fibre so that the block extends transversely by a short distance (set by a microadjuster) into the path of light travelling along the core of the optical fibre. The remaining space between the ends of the fibre is filled with material 9 which is index matched to the optical fibre 7.

In operation of the optical fibre sensor some of the light propagating along the sensor 7 in the direction B will be reflected back along the fibre as shown by the vertical surface 10 of the block

8. The remainder of the light will pass through the index matched splice to the next discontinuity (not shown) along the optical fibre sensor. Most of the light reflected back along the optical fibre in direction C from the next and any succeeding discontinuities will pass straight through the index matched splice material 9 but some will impinge on the inclined reflective surface 11 of the block 8 and will be reflected through the wall of the optical fibre to prevent the occurrence of unwanted multiple reflections which would give rise to cross-coupling between sensor elements.

In the Figure 3(a) and 3(b) arrangement a plate reflector 12 is introduced into a spliced connection between the ends 13 and 14 of an optical fibre with an index - matched filling -material 15 being provided. As can best be seen in Figure 3(b) the fibre ends 13 and 14 are slightly displaced relative to one another in the transverse direction so that a segmental section 16 of the reflector 12 is exposed to light propagating along the fibre in direction D to enable a proportion of light to be reflected back along the fibre but any light propagating in the direction E due to reflection from the next or any succeeding discontinuties will pass straight through the index- matched material without being reflected and causing multiple reflections. Only a small proportion of the reflected light will be lost due to the transverse displacement of the fibre ends indicated by the segment 17 in Figure 3(b).

Referring to Figures 4, 5, and 6 in these constructions the respective discontinuities 18, 19 and 20 are provided in the optical fibre 21 without the need to sever the fibre.

In Figure 4 the core 22 of the optical fibre is etched away as shown to provide a vertical reflection surface 23 and a tapered reflection surface 24. A small proportion of light propagating in the direction F will be reflected back along the optical fibre 21 whereas a small proportion of any light refleced from the next or any succeeding discontinuities along the fibre will be reflected out of the fibre by the tapered reflecting surface 24.

In the Figure 5 construction the refractive index of a region 25 of the fibre core is changed by diffusion of material into the core so that propagating light impinging on the region from direction G will be reflected back along the fibre as shown whereas light reflected from the next or succeeding discontinuities along the fibre and impinging on the region 25 will be reflected out of the fibre as indicated thereby avoiding the problem of multiple reflections.

Finally, in the Figure 6 construction the optical fibre core and cladding is profiled as shown whereby a small proportion of light propagating in directon H is reflected by the vertical reflecting surface 26 whereas some of the light reflected by the next or any succeeding discontinuities is reflected and transmitted through the tapered surfaces 27 and 28 of the core and cladding to prevent multiple reflections occurring.

As will be observed from the foregoing description of various embodiments of the invention the reflecting means utilised at each of the discontinuities have different reflectivities in different directions and although this would appear to suggest a breach of reciprocity or energy conservation this is not the case if the reflection is either diffuse, energy is absorbed or the unwanted signal is simply side-

tracked onto an irrelevant path. In an optical fibre this is particularly easy to accomplish and can provide the basis for reflectors which suppress reflections in one direction within the fibre.