GB2190187A - Optical fibre sensors - Google Patents

Abstract

An array of optical fibre sensors comprises an optical fibre structure having a series of reflective discontinuities formed by spaced fusion splices between fibre elements of different refractive indices. The structure may comprise a number of equal long lengths of fibre 4 of one refractive index joined by short lengths of fibre 5 of a different refractive index. One end of each short length is fully fusion spliced to one long fibre length (no reflective interface) whilst either the other end is partially fusion spliced or an index matching medium is disposed between the other end and the other long length in order to obtain a single reflective interface. Alternatively, alternate long lengths of fibre (6,7) with different refractive index may be partially fused together at their ends to form reflective interfaces (8). Pairs of laser pulses of slightly different frequency passing along a fibre having equally spaced reflective interfaces are partly reflected to a detector where they are heterodyned to detect phase shifts due to deformation forces (eg acoustic waves) acting on the fibre. <IMAGE>

Description

SPECIFICATION Optical fibre sensors This invention relates to optical fibre sensors and in particular to fibre sensor arrays, especially acoustic fibre sensor arrays, and to methods of fabricating the same.

U.K. Patent application 8220793 (Serial No.

2126820 A) discloses an optical sensing system in which an optical fibre arranged to be subjected along its length to fibre deforming forces, e.g. acoustic waves, is provided with a plurality of equally spaced discontinuities. A light signal is transmitted along the fibre and a small proportion of the light signal is reflected from each discontinuity. Suitable processing of the reflected signals indicates changes in the signals caused by deformation of the respective length of the fibre due to the forces acting thereon. The sole disclosure in the patent specification as to the nature of these discontinuities is that they may, for example, be formed by suitable joints in the optical fibre.

Various methods of producing suitable joints in optical fibres have been disclosed in 'Progress with multiplexed sensor arrays based on reflection at spliced joints between sensors' by J.P. Dakin and C.A. Wade, AGARD Conference Pre-print No. 383, p.5.1-5.5, Istanbul, 23/27 September 1985.

It was there stated that a convenient way of producing a small reflection with low excess loss is to introduce a refractive index mismatch into the optical fibre. This can be by way of a small air gap between fibre ends, or by setting the fibre ends in a transparent medium having a different refractive index to that of the fibre. Both these techniques have the disadvantage however that the two fibre ends constitute a pair of refractive interfaces giving rise to double reflections.

According to one aspect of the present invention there is provided a method of manufacturing an optical fibre reflective element including the step of forming a fusion splice between two optical fibre elements of different refractive indices.

According to another aspect of the present invention there is provided an optical fibre reflective element comprising two lengths of optical fibre interposed between whose ends is an element of different refractive index which is fully fusion spliced to one said length, a reflective interface being provided between the other said length and the element.

According to a further aspect of the present invention there is provided an optical fibre sensor array comprising an optical fibre structure provided with a series of spaced optical discontinuities along its length, characterised in that each optical discontinuity includes a fusion splice between optical fibre elements of different refractive indices.

Preferably each optical discontinuity includes a partially fused splice between the optical fibre elements, whereby to provide a reflective interface thereat, and the discontinuities are equally spaced.

In a preferred embodiment of the invention the fibre structure consists of lengths of optical fibre of one refractive index, each length being substantially equal to the distance between successive discontinuities, alternating with short pieces of fibre of a different refractive index, the short pieces being fully fusion spliced to one adjoining length of fibre and partially fusion spliced to the other adjoining length of fibre, whereby to form said partially fused splices.

In and alternative embodiment the fibre structure consists of alternating equal lengths of optical fibres of different refractive indices, each length being partially fusion spliced to an adjoining length whereby to form said partially fused splices.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 illustrates schematically a reflective element between two optical fibres; Fig. 2 illustrates schematically a sensor array according to the embodiment of the present invention; Fig. 3 illustrates schematically a sensor array according to another embodiment of the invention; Fig. 4 illustrates an alternative reflector configuration and Fig. 5 illustrates a sensor array employed in a sensing system.

As mentioned above, to cause reflection a discontinuity in the refractive index of the transmission medium is required, that is a reflective element. Systems have been described in the aforementioned paper using an optical cement between two adjoining fibres.

This method is inherently weak, generally of high loss and of dubious stability. An all-glass system, however, should be of higher strength and reliability and involve low signal attenuation.

In telecommunications using optical fibres techniques have been developed for making low-loss joints between successive lengths of like fibre. One such technique being widely adopted is that of fusion splicing. This involves creating a localised arc and introducing the fibre ends into the arc until they abut. The arc is then maintained for a predetermined time before being removed. As the fibre ends enter the arc they begin to soften and fusion takes place after they come into contact with each other. Typically a single mode silica fibre having an outside diameter of 125,am is provided with a silicone primary coat and a nylon secondary coat. To form a low loss fusion splice the primary and secondary coats are first removed from the ends to be joined and the two ends are brought together in abutting relationship in an arc.In the apparatus currently used by the applicants in the telecommunications field the arc typically draws approximately 15.2 mA and is maintained for a period of 5 seconds after the fibre ends abut. The resulting splice is extremely strong and creates negligible reflection for the optical signal owing to the complete fusion of the silica fibres. Whereas the reflection produced by an interface depends on the refractive index difference, we have found that it is also dependent to some extent on the thermal history. Indeed we have found that interfaces produced using arc parameters as for normal fusion splices result in much lower reflection than would be expected from the refractive index difference. Applying shorter fusion times and lower temperatures, however, can produce reflectors with the expected reflectivities.The standard fusion time and temperature are thought to allow diffusion of the dopants resulting in a blurring of the index boundary between the two surfaces.

One embodiment of a reflective element is illustrated in Fig. 1. Between two standard single mode silica fibres 1 and 2 is inserted a thin slice of glass 3 possessing a refractive index higher or lower than the core refractive index of the fibres. The level of reflection can be tailored by and is partially dependent on the refractive index of the inserted slice.

As mentioned above, a fully fused splice between two similar fibres results in negligible reflection for an optical signal. Between two fibres of different refractive indices there can be sufficient diffusion to prevent reflection if the fusion temperature is high enough. To prevent such diffusion, therefore, and to provide a sufficiently sharp interface, the fusion temperature should be kept as low as possible consistent with achieving a fusion splice, this may be referred to as partial fusion as opposed to the fully fused splice required between two similar fibres for low loss joints in telecommunications systems.

To produce the structure of Fig. 1 a length of reflector fibre from which glass slice 3 is to be formed is fusion spliced to a single mode fibre 1. The reflector fibre is then cleaved close to this splice leaving a thin slice 3 of the reflector fibre attached to the single mode fibre 1. A further fusion splice is then made to attach a single mode fibre such as 2 to the other end of the slice 3 of reflector fibre. In our co-pending Application Serial No.

2158603 (A.J. Robertson 5) there is disclosed an attenuator structure incorporating an intermediate fibre member (silica rod) between two fibres. The length of the intermediate element (slice 3) is very much shorter for the present application since it is required to reflect rather than throw away optical power as in the attenuator application.

If both fusion splices are partially fused two reflecting interfaces will be formed as in the prior art, and thus involve the Fabry-Perot effect due to having two parallel interfaces in close proximity. Two reflections of equal amplitude but unknown relative phase will be obtained so that the overall reflection from the discontinuity will be wavelength dependent and for some lengths of spacer fibre (slice 3)/wavelengths the second reflection will cancel the first. The slice has to be short in order to avoid loss of light in the unguided region comprised by it (lowvsignal attenuation). Alternatively a short length of a guiding fibre could be used as described hereinafter.

If, however, one fusion splice is fully fused so that no reflective interface is formed between fibre 1 and slice 3, whereas the other fusion splice is only partially fused so that a reflective interface is formed between slice 3 and fibre 2, then the insertion of a silica slice between two similar optical fibres can be performed in such a manner as to obtain a single reflective interface, and the Fabry-Perot effect is thus eliminated. The thin slice 3 may, for example, be 15-20 um thick and the actual splicing process based on that described above with respect to, making low loss joints, but modified as necessary to obtain the reflective interface. Typically this is achieved with an arc current of 12mA and a fusion time of 0.5 seconds, the splice being tacked rather than fully fused as in the case with the 15mA and 5 second process described above.

Using such an embodiment with one reflective interface, reflector elements may be made using a thin slice from a silica reflector fibre having a reflective index lower than that of the single mode fibres it joins. These are expected to give a reflection of the order of -50dB to -55dB down on the transmitted signal. Another reflector element may employ a thin slice from a reflector fibre having a refractive index higher than that of the core of the single mode fibre it joins. In this case reflection of the order of 30 dB to 35 dB down on the transmitted signal was achieved.

In this case an optical fibre sensor array will be comprised by substantially equal lengths 4 (Fig. 2) of an optical fibre with a core of one refractive index, which lengths 4 are joined by short pieces of fibre 5 of a different absolute refractive index, the short pieces being fully fusion spliced to one adjoining length of fibre and partially fusion spliced to the other adjoining length of fibre, to provide a number of reflective elements along the overall length of the optical fibre structure.

Another technique of providing an optical fibre structure with discontinuities spaced along it and employing fusion splices comprises constructing the fibre of alternate lengths of single mode fibre of different core refractive indices (and possibly cladding), each length being partially fusion spliced to an adjoining length, so that a series of reflections is generated, one reflection at each interface between adjacent fibres. The distance between the reflection sites is determined by the lengths of the fibre sections. Fig. 3 illustrates such a structure. The sensor array comprises, for example, eight lengths of fibre, four lengths 6 being of one core refractive index and four lengths 7 being of a different core refractive index. The lengths are partially fused together producing reflection sites at seven points 8.To ensure similar transmission characteristics in the two fibre types 6 and 7 they need to be designed to give closely matching mode field shapes and dimensions. The difference in the refractive indices of the cores must be selected to provide the required level of reflection. It may also be necessary to ensure that the An's of the two fibre types (An=n1-n2 where n,=core refractive index and n2=cladding refractive index) are very similar.

The ideal form of this other technique only requires that the two fibre types have the same spot size (and shape) and that the absolute refractive indices are different. The first condition ensures that all the power not transmitted is reflected in the fundamental mode, that is no power is lost from the fibre.

For small An/n, that is weakly guiding fibres, this means that the two fibres should have almost identical An but different refractive index. This might be achieved, for example, where fibre type 6 has an increased index core in a silica cladding and fibre type 7 has a silica core with a depressed cladding.

An alternative reflector configuration is illustrated in Fig. 4. To a single mode fibre 41 is fully fusion spliced a fibre 42 with a different refractive index (high index core) which is then cleaved to leave a short stub. The single mode fibre 41 and fibre stub 42 are then butted to another single mode fibre 43 using an index matching medium 44 and, for example, as in a conventional commercialiy available splicing system. In this arrangement reflections are suppressed at the fibre 41/stub 42 fusion splices by using normal splice conditions and at the single mode fibre 43 interface with the index matching medium. This leaves the interface between the high index stub 42 and the index matching medium 44 as the sole source of reflection. The level of reflection can be varied by changing the stub material and/or the index matching medium.

The intermediate element between the fibres (1,2 or 44,43) to be joined may thus comprise an element which provides no guiding (as in Fig. 1), a fibre which is not necessarily single mode, or a single mode fibre which enables losses associated with the length thereof to be minimised.

The fibre structure of Figs. 2 and 3 may be used for various sensor applications, in particular but not exclusively, hydrophone sensor applications since they are sensitive to acoustic energy. This is achieved by virtue of the reflective elements provided along their lengths which are such that a small amount of coherent light pulses transmitted down the fibre are reflected from each of these elements. Processing of these coherent reflected impulses provides a measure of the deforming forces of incident acoustic energy acting to change the length of the optical fibre.

The fibre structures may be used in optical sensing systems as shown in Fig. 5. The fibre structure 10 is arranged to be subjected along its length to fibre deforming forces during operation of the system. Equally spaced reflective elements are indicated by 11 to 17.

Means 18 including a pulsed laser produces pairs of output pulses of coherent light, each pulse of a pair having slightly different frequencies. This two-pulse light signal passes through a beam splitter 19 and launched into fibre 10.

As each two-pulse light signal reaches the first reflective element 11 a small proportion of the signal will be fed back along fibre 10 to the beam splitter 19 which directs it to a detector 20. The remaining part of the twopulse light signal travels on to reflective element 12 at which a further small proportion thereof will be reflected back to detector 20.

This continues until the last reflective element 17 has been employed, when a fresh twopulse optical signal is transmitted and the cycle repeated.

The delay between the two transmitted pulses of a pair is chosen to be equal to the two-way propagation time through each section of fibre between two reflective elements, so that the reflection of the first pulse from a particular reflective element is received by detector 20 simultaneously with the reflection of the second pulse from the preceding reflective eiement. The two therefore mix on the detector and generates a heterodyne signal, whose phase depends on the difference in optical paths. However their paths only differ by twice the length of the fibre between the two reflective elements concerned and therefore changes in the length of this fibre, caused for example by acoustic signals, modulate the phase of the heterodyne signal. The detector output thus consists of a sequence of short bursts of phase-modulated heterodyne signal, each corresponding to a particular length of fibre. If the whole cycle is repeated continuously the photodetector output consists of a set of phase-modulated carriers time-divisionmultiplexed together. The acoustic signal on a particular length of the fibre can be recovered by demultiplexing and phase-modulating the detector output.

Alternatively the fibre structure can be employed in the sensor arrangement of our copending application No.

Claims (20)

1. A method of manufacturing an optical fibre reflective element including the step of forming a fusion splice between two optical fibre elements of different refractive indices.

2. A method as claimed in claim 1 wherein the fusion splice is a partially fused splice.

3. A method as claimed in claim 2 including the step of fully fusion splicing one end of one of two optical fibre elements to a length of optical fibre and partially fusion splicing the other end of said one optical fibre element to the other of said two optical fibre elements.

4. A method as claimed in claim 3 wherein said one optical fibre element is comprised by a length of reflector fibre whose one end is fully fusion spliced to said length of optical fibre, which reflector fibre is then cleaved close to said splice to leave a thin slice thereof whose free end is then partially fusion spliced to the other said optical fibre element.

5. A method as claimed in claim 2 wherein the optical fibre elements comprise equal lengths of optical fibres of different refractive indices.

6. A method as claimed in claim 2 including the step of fully fusion splicing one end of one of the two fibre elements to a length of optical fibre, cleaving the length of fibre close to the splice to leave a short length thereof which comprises the other fibre elements, and splicing the short length to a further fibre by an index matching medium method, a reflective interface thus being formed between the short length and the index matching medium.

7. An optical fibre reflective element manufactured by a method as claimed in any one of claims 1 to 6.

8. An optical fibre reflective element comprising two lengths of optical fibre interposed between whose ends is an element of different refractive index which is fully fusion spliced to one said length, a reflective interface being provided between the other said length and the element.

9. An element as claimed in claim 8, wherein the other said length is partially fusion spliced to said element whereby to provide said reflective interface.

10. An element as claimed in claim 8, wherein an index matching medium is disposed between said other length and the element, the reflective index being between the element and the index matching medium.

11. An optical fibre sensor array comprising an optical fibre structure provided with a series of spaced optical discontinuities along its length, characterised in that each optical discontinuity includes a fusion splice between optical fibre elements of different refractive indices.

12. An array as claimed in claim 11 wherein each optical discontinuity includes a partially fused splice between said optical fibre elements.

13. An array as claimed in claim 12, wherein the optical discontinuities are equally spaced along the length of the fibre structure.

14. An array as claimed in claim 13, wherein the fibre structure consists of lengths of optical fibre of one refractive index, each length being substantially equal to the distance between successive discontinuities, alternating with short pieces of fibre of a different refractive index, the short pieces being fully fusion spliced to one adjoining length of fibre and partially fusion spliced to the other adjoining length of fibre, whereby to form said partially fused splice.

15. An array as claimed in claim 13, wherein the fused structure consists of alternating equal lengths of optical fibre of different refractive indices, each length being partially fusion spliced to an adjoining length whereby to form said partially fused splice.

16. An array as claimed in claim 11 wherein each optical discontinuity includes a fully fused splice between said optical fibre elements and a further optical fibre element together with an index matching medium disposed between one of said first mentioned optical fibre elements and said further optical fibre element.

17. An optical fibre sensor array substantially as herein described with reference to Fig.

2 or Fig. 3 of the accompanying drawings.

18. An optical sensing system including an optical fibre sensor array as claimed in any one of claims 1 to 7.

19. An optical sensing system including an optical fibre sensor array as claimed in any one of claims 1 to 7 and substantially as herein described with reference to Fig. 5 of the accompanying drawings.

20. An optical fibre reflective element substantially as herein described with reference to and as illustrated in Fig. 1 or Fig. 4 of the accompanying drawings.