The present invention relates to optical fibres and fibre optic cables suitable for use in distributed fibre optic sensing, especially for use in distributed acoustic fibre optic sensing and to enhancements in the design, application and manufacture of optical fibre and/or fibre optic cable for distributed fibre optic sensors.

Various sensors utilizing optical fibres are known. Many such sensors rely on fibre optic point sensors or discrete reflection sites such as fibre Bragg gratings or the like being arranged along the length of an optical fibre. The returns from the discrete point sensors or reflection sites can be analysed to provide an indication of the temperature, strain and/or vibration in the vicinity of the discrete sensors or reflection sites.

Fully distributed fibre optic sensors are also known in which the intrinsic scattering from a continuous length of optical fibre is used. Such sensors allow use of standard fibre optic cable without deliberately introduced reflection sites such fibre Bragg gratings or the like. The entire optical fibre from which a backscatter signal can be detected can be used as part of the sensor. Time division techniques are typically used to divide the signal returns into a number of time bins, with the returns in each time bin

corresponding to a different portion of the optical fibre. Such fibre optic sensors are referred to as distributed fibre optic sensors as the sensor options are fully distributed throughout the entire optical fibre. As used in this specification the term distributed fibre optic sensor will be taken to mean a sensor in which the optical fibre itself constitutes the sensor and which does not rely on the presence of specific point sensors or deliberately introduced reflection or interference sites, that is an intrinsic fibre optic sensor.

For example US patent No. 5,194,847 describes a distributed acoustic fibre optic sensor for intrusion sensing. A continuous optical fibre without any point sensors or specific reflection sites is used. Coherent light is launched into the optical fibre and any light which is Rayleigh backscattered within the optical fibre is detected and analysed. A change in the backscattered light in a time bin is indicative of an acoustic or pressure wave incident on the relevant portion of optical fibre. In this way acoustic disturbances any portion of the fibre can be detected. GB patent application publication No. 2,442,745 describes a distributed acoustic fibre optic sensor system wherein acoustic vibrations are sensed by launching a plurality of groups of pulse modulated electromagnetic waves into a standard optical fibre. The frequency of one pulse within a group differs from the frequency of another pulse in the group. The Rayleigh backscattering of light from intrinsic reflection sites within the fibre is sampled and demodulated at the frequency difference between the pulses in a group.

Distributed fibre optic sensing therefore provides useful and convenient sensing solutions that can monitor long lengths of optical fibre. Standard telecommunications optical fibre, e.g. single mode 125pm optical fibre, can be used which means that the sensing fibre is relatively cheap and readily available and, in some instances, it may be possible to use existing optical fibres for acoustic monitoring say.

A standard optical fibre comprises a core, which is optically transmissive at the wavelength of operation, surrounded by cladding material which has a different refractive index to the core, the core and cladding together co-operating to guide optical radiation within the core of the fibre. The cladding is generally surrounded by a jacket material to protect the optical fibre and a buffer material may be disposed between the jacket and the core.

A fibre optic cable may comprise a single optical fibre, in which the jacket of the optical fibre may also serve as the jacket of the cable, although typically there would be more than one jacket layer. In many applications however many optical fibres are included in a single cable.

One form of fibre optic cable, which is especially useful for harsh environments or otherwise where a robust cable is desired, is a gel filled cable. In such an arrangement the or each optical fibre is located within a cable jacket that it filled with a gel material. The gel material helps protect the optical fibre from environmental contamination and also provides protection against mechanical shock whilst maintaining flexibility of the resultant cable. It is known that the gel material may be a thixotropic material, i.e. a non-Newtonian shear thinning material. Thixotropic gels are used as, during the manufacturing process the shear thinning nature makes the cable easier to fill. In some instances though the use of standard telecommunications fibre optic cable may not provide optimal sensing. It is therefore an aim of the present invention to provide improved optical fibre and fibre optic cables for use in distributed fibre optic sensing.

According to a first aspect of the present invention there is provided a fibre optic cable comprising at least one optical fibre and comprising a non-Newtonian shear thickening fluid disposed within the cable.

The non-Newtonian fluid is a shear thickening fluid, i.e. a fluid whose apparent viscosity increases as a function of shear stress. The non-Newtonian fluid may be a dilatant material, i.e. a material whose apparent viscosity increases with the rate of shear, and/or it may be rheopectic and thus have an apparent viscosity which increases with increasing duration of applied shear force.

As the skilled person will appreciate a non-Newtonian fluid is one in which there is not a linear relationship between shear stress and strain rate. In a Newtonian fluid the relationship between shear stress and strain rate is linear and determined by the coefficient of viscosity of the fluid, thus the viscosity of the fluid remains the same irrespective of the amount of force applied.

By contrast, a non-Newtonian fluid the viscosity is not constant and depends on the shear rate or kinematic history of the fluid. The skilled person will appreciate that non- Newtonian fluids may be categorised as those in which the shear stresses, and apparent viscosity, depend on strain rate and those in which the shear/stress rate is time dependent.

Materials where the apparent viscosity is based on the rate of strain include

pseudoplastic materials (strain thinning) or dilatent materials (strain thickening).

In a shear thickening dilatant non-Newtonian fluid the apparent viscosity of the fluid increases with applied shear. Thus, if subjected to a low rate of shear the material may exhibit a relatively low viscosity and the fluid will flow. However if subject to a rapid rate of shear the viscosity may increase significantly to the extent that the material exhibits solid like properties and does not flow at all. Thus the flow resistance of the material increases when subject to rapid movement. There may also be a yield stress or strain rate. For instance a pseudoplastic (strain thinning material) may have a yield stress that is required prior to deformation such materials are known as Bingham plastics). There may also be a yield dilatent material.

Materials in which the apparent viscosity is time dependent may be divided into thixotropic (thinning) or rheopectic (thickening) materials.

In a rheopectic material the apparent viscosity increases with the duration of the applied shear. Thus application of a constant rate of shear may result in the apparent viscosity increasing with time.

It will be appreciated that a material may exhibit a dependence on both the rate and duration of shear, i.e. it may dilatant and rheopectic. It will further be appreciated that a material may exhibit shear thickening behaviour associated with the rate of shear (dilatant) but may also have a time-dependent thinning (thixotropic behaviour). Thus a material subject to a high rate of shear may solidify whereas if subjected to a constant, relatively low rate of shear, it may actually thin.

As used in this specification the term shear-thickening material shall be taken to mean a material that exhibits an apparent increase in viscosity, i.e. a resistance to flow, when subject to stress, whether dependent on the rate of shear and/or the duration, i.e. it applies to a material which exhibit dilatant behaviour and/or rheopectic behaviour.

The present invention uses a shear thickening non-Newtonian fluid disposed within a fibre optic cable to provide a fibre optic cable with advantageous properties for distributed sensing, especially for distributed acoustic sensing. This is the opposite to the conventional use of non-Newtonian fluids in fibre optic cables wherein thixotropic, i.e. shear thinning gels, have previously been used.

The shear thickening fluid may flow at low shear rates and thus a fibre optic cable comprising such a fluid may be nearly as flexible as a standard gel filled fibre optic cable provided that no sudden bending forces are applied. Thus the cable of the present invention has the same advantageous in terms of easy of installation and flexibility in use as standard fibre optic cable. However, when subject to sudden forces the shear thickening material will not flow as readily and adopt more solid like properties. This will have the effect of making the relevant section of fibre relatively rigid.

In a distributed acoustic fibre optic sensor such as described in GB2,442,745 optical radiation is transmitted into the optical fibre and any optical radiation which is Rayleigh back-scattered within the optical fibre is detected. Any incident acoustic signal causes mechanical vibration of the fibre which changes the amount of Rayleigh back-scattering at that part of the fibre. The variation in back-scatter is related to the strain on the optical fibre, in other words the amount of bending, experienced by the optical fibre. Other distributed fibre optic vibration sensors also rely on bending of the optical fibre changing the amount of back-scatter from that portion of the optical fibre.

With a standard gel filled cable, if an acoustic wave, is incident on the cable, the outside of the cable will be subject to vibration. (For the purposes of the this specification the term acoustic wave shall be taken to mean any type of mechanical vibration wave and in particular shall include seismic waves). At least some of this vibration will be transferred to the interior of the cable and will cause the optical fibre(s) disposed inside to vibrate. However the effect of the vibration on the gel will be to cause the gel to flow, at least partly, around the optical fibres and thus only some of the acoustic wave energy will be coupled to the optical fibre. This will especially be the case if the are any spaces into which the gel can flow, such as spaces created by trapped air during the filling process.

With a shear thickening fluid, such as a dilatant fluid however a relatively high frequency mechanical wave incident on the cable will cause the viscosity of the fluid to increase. In effect the relevant section of fibre will be become more rigid. If the stimulus were sufficiently intense the relevant section of cable may effectively become solid. In any case the increased viscosity and resultant resistance to flow means that more of the excitation of the outside of the cable will be transferred to the optical fibres within the cable, thus increasing the strain on the cable and hence the detected signal when used for distributed sensing. With a strain-thickening non-Newtonian fluid, even if there are spaces into which the fluid could flow, the increase in viscosity in response to an applied vibration means that that fluid will not flow during the application of the vibration. For a distributed fibre optic sensor this means that there is no loss of signal and the stress of the vibration will be transmitted to the optical fibre. The skilled person will be aware of a variety of shear thickening non-Newtonian fluids that would be suitable. Clearly the fluid should ideally be relatively inert and harmless. In some applications the fluid should have predictable behaviour at elevated temperature. The shear thickening non-Newtonian fluid should be stable, in the sense that the fluid maintains a non-Newtonian behaviour for a significant period. In some applications the fibre optic cable may be buried or embedded in a structure and may not be easily retrieval. In such cases the fibre may be used for monitoring for years. Preferably the fluid maintains its shear thickening non-Newtonian behaviour for such timescales. The shear thickening non-Newtonian fluid may therefore be resistant to biodegradation or breakdown and may comprise one or more additives providing stability and/or resistance to biodegradation. In some applications however the fluid may be biodegradable such that, provided that the fluid is within the protective cover of the fibre it is stable, but in the event of a breakage and the fluid escapes to the environment, it harmlessly degraded. Ideally the fluid should be relatively non- hazardous.

The shear thickening non-Newtonian fluid may comprise dispersed particles in a fluid wherein the particles are ultrafine particles, for instance nanoparticles with an average size of 100nm or less or 10 nm or less. By average size is meant the average size of the maximum dimension of the particles.

A strain-thickening non-Newtonian fluid may be formed from a plurality of closely packed particles within a liquid. At low strain rates the liquid acts as lubricant and the particles can relatively easily move past each other so that the whole fluid can flow. At higher strain rates however the liquid is less able to fill the gaps between particles and friction, and hence viscosity, increases. At high enough strain rates the particles can, in effect lock up and cease to flow.

In the application to a distributed fibre optic sensor however, where the shear thickening non-Newtonian fluid is contained within a fibre optic cable, there may be a lower limit to the amplitude of vibration to which the non-Newtonian fluid will exhibit a significant change in viscosity. Ideally the non-Newtonian fluid would exhibit an increase in viscosity even at low amplitude vibrations. The use of ultrafine particles maximises the ability of the fluid to flow at low strain rates but to interact so as to increase the viscosity, and ideally effectively solidify, at relatively low amplitude vibrations. Ultrafine particles tend to have a relatively high surface energy which may limit the amount of particles by weight that can be included in the fluid. The skilled person will appreciate that the loading level by weight of particles needs to be sufficient to ensure the desired strain thickening behaviour.

The shear thickening non-Newtonian fluid may comprise particles that are relatively uniform in size or it may comprise a range of different sized particles. There may be two or more different types of particles disposed within the fluid, the different types of particle having different sizes and/or being formed from or treated with different materials. At least some of the particles may be provided to ensure that the fluid has desired properties.

The shear thickening non-Newtonian fluid may be disposed in the cable in various ways. For example the cable may comprise one or more optical fibres disposed within the non-Newtonian fluid within a cable jacket in a similar fashion to conventional gel filled cables. Alternatively the cable may comprise one or more channels, for instance a central channel, containing the non-Newtonian fluid, for instance one or more optical fibres could be held together, possibly with a buffer material surround which has an outer layer of non-Newtonian fluid and/or a central channel containing non-Newtonian fluid. The buffer may be a solid buffer or a standard gel buffer.

The fibre optic cable may comprise various outer protective layers and may comprise various strengthening fibres. As used herein the term fibre optical cable shall be taken to mean an apparatus comprising one or more optical fibres, each having a core and cladding material.

As mentioned the fibre optic cable of the present invention is particularly suited for distributed sensing, especially distributed acoustic sensing (DAS). Therefore in another aspect of the invention there is provided a distributed fibre optic sensor comprising an optical source coupled to a fibre optic cable to transit optical radiation into said fibre optic cable, a detector arranged to detect radiation back-scattered from said fibre and a processor to process the back-scattered radiation to provide a plurality of discrete longitudinal sensing portions of said fibre wherein fibre optic cable is a cable as described above. In general the present invention relates to the use of a shear thickening non-Newtonian fluid as a filler material in a fibre optic cable. Preferably the non-Newtonian fluid is a dilatant material.

The invention will now be described by way of example only with respect to the following drawings of which:

Figure 1 illustrates the basic components of a distributed fibre optic sensor;

Figures 2 shows an embodiment of the present invention; and

Figure 3 illustrates how the embodiment of the present invention operates.

Figure 1 shows a schematic of a distributed fibre optic sensing arrangement. A length of sensing fibre 104 is connected at one end to an interrogator 106. The output from interrogator 106 is passed to a signal processor 108, which may be co-located with the interrogator or may be remote therefrom, and optionally a user interface/graphical display 110, which in practice may be realised by an appropriately specified PC. The user interface may be co-located with the signal processor or may be remote therefrom.

The sensing fibre 104 can be many kilometres in length, and in this example is approximately 40km long. In conventional applications of optical fibre distributed sensors the sensing fibre is at least partly contained within a medium which it is wished to monitor. For example, the fibre 104 may be buried in the ground to provide monitoring of a perimeter or monitoring of a buried asset such as a pipeline or the like.

The invention will be described in relation to a distributed acoustic sensor, although the skilled person will appreciate that the teaching may be generally applicable to any type of distributed fibre optic sensor.

In operation the interrogator 106 launches interrogating electromagnetic radiation, which may for example comprise a series of optical pulses having a selected frequency pattern, into the sensing fibre. The optical pulses may have a frequency pattern as described in GB patent publication GB2.442.745 the contents of which are hereby incorporated by reference thereto. As described in GB2,442,745 the phenomenon of Rayleigh backscattering results in some fraction of the light input into the fibre being reflected back to the interrogator, where it is detected to provide an output signal which is representative of acoustic disturbances in the vicinity of the fibre. The interrogator therefore conveniently comprises at least one laser 112 and at least one optical modulator 114 for producing a plurality of optical pulse separated by a known optical frequency difference. The interrogator also comprises at least one photodetector 116 arranged to detect radiation which is backscattered from the intrinsic scattering sites within the fibre 104.

The signal from the photodetector is processed by signal processor 108. The signal processor conveniently demodulates the returned signal based on the frequency difference between the optical pulses such as described in GB2,442,745. The signal processor may also apply a phase unwrap algorithm as described in GB2,442,745.

The form of the optical input and the method of detection allow a single continuous fibre to be spatially resolved into discrete longitudinal sensing portions. That is, the acoustic signal sensed at one sensing portion can be provided substantially

independently of the sensed signal at an adjacent portion. The spatial resolution of the sensing portions of optical fibre may, for example, be approximately 10m, which for a 40km length of fibre results in the output of the interrogator taking the form of 4000 independent data channels.

In this way, the single sensing fibre can provide sensed data which is analogous to a multiplexed array of adjacent independent sensors, arranged in a linear path.

Conventional distributed fibre optic sensors use standard telecommunications optical fibre. The present invention provides enhancements to fibre optic cable design that improve the sensitivity or functionality of distributed fibre optic sensors.

Figure 2 shows a cross section of a fibre optic cable 201 according to an embodiment of the invention.

The fibre optic cable illustrated comprises three optical fibres each having an optical core 208 surrounded by a cladding material 206 as is usual in the field of optical fibres. It will be appreciated however that a fibre optical cable may comprise a single optical fibre or may comprise a bundle of optical fibres with many tens or hundreds of optical fibres. The cladding of each optical fibre may also be surrounded in an individual jacket layer (not shown) The core 208 and cladding 206 may be produced by standard optical fibre production techniques and may for instance comprise pulled silica glass.

Surrounding the optical fibres is a non-Newtonian fluid 202 contained within a jacket material 204. The non-Newtonian fluid is a shear thickening fluid. Conveniently the non-Newtonian fluid is a dilatant fluid selected such that the fluid has a liquid or gel like consistency at a low shear rate (at the expected operating temperature and pressure of the fibre optic cable) but when subject to a mechanical stress of sufficient intensity at acoustic frequencies exhibits a markedly increased viscosity and exhibits solid like properties.

This behaviour means that, whilst the cable is flexible for normal handling and the fluid flow at low strain rates, in response to an acoustic impetus of sufficient stimulus the response of the fibre is increased. This is illustrated with respect to Figure 3. Figure 3a shows a fibre optic cable 301 having an optical fibre disposed therein. In response to an incident acoustic wave the cable will vibrate, with the intensity of the vibrations being linked to intensity of the acoustic wave. Figure 3b illustrates how a standard gel filled cable may respond to such a stimulus. Figure 3b shows the cable 301 at a particular time during the vibration. It can be seen that the cable has been bent upwards slightly. However as the cable is gel filled and the gel can flow within the cable casing the optical fibre 302 has been displaced by a lesser amount. In essence the gel has propagated the acoustic wave past the optical fibre and has not transferred all of the strain to the optical fibre.

It will also be appreciated that the cable may not be completely filled with gel. During the manufacturing process air bubbles or the like may mean that there are spaces within the cable which are void of fluid. In response to a force on the outside of the cable the gel may simply flow into the void, displacing and/or compressing the air bubble. This effectively means that the energy of incident vibration is used to move the gel and it is not transferred to the optical fibre. In a standard telecoms application of an optical fibre this response would be beneficial. However for a distributed optical fibre sensor it effectively means that some of the incident signal is wasted.

Figure 3c shows the response of a cable according to an embodiment of the present invention. As the incident acoustic wave acts on the fibre casing, causing it to move, the non-Newtonian within the fibre is forced to flow. This creates shear within the non- Newtonian fluid with a resultant increase in the apparent viscosity of the fluid. Thus the non-Newtonian fluid does not flow around the optical fibre and instead passes more of the incident energy to the fibre, thus increasing the amount of strain on the fibre. As the response of the fibre in a DAS system is dependent on the amount of strain on the fibre the result is that the response from a fibre optic cable according to the present invention is improved as compared to a standard gel filled cable.