Technical Field

The present invention relates to optical fiber sensing systems, such as optical fiber distributed acoustic sensors (DASs), and optical fiber distributed temperature sensors (DTSs). Specifically, embodiments of the present invention provide for improved sensitivity of such sensing systems by providing multiple parallel sensing fibers to increase the amount of reflections or backscatter from any particular sensing point.

Background to the Invention and Prior Art Optical fiber based sensing systems are known already in the art. OTDRs are used to determine fiber condition and properties, such as splice or connector losses and attenuation, whereas DAS and DTS systems use backscatter from along the fiber to sense acoustic energy incident on the fiber, or ambient temperature around the fiber, as appropriate. An example prior art DAS system is the Silixa® iDAS™ system, available from Silixa Ltd, of Elstree, UK, the details of operation of which are available at the URL htf|t: /ww .^ and which is also described in our earlier patent application WO2010/0136809, any details of which that are necessary for understanding the present invention being incorporated herein by reference. An example DTS system is the Silixa® Ultima™ system, described at http ://w w w . sili xa.com/technoIogy/dts/. At a high level, DAS and DTS systems operate by sending sensing pulses down an optical fiber deployed in the environment which is to be monitored. For a DAS system the vibrations of an incident acoustic wave on the fiber cause modulations in the backscatter or reflections from the fiber as the pulse travels along the fiber. By measuring the backscatter or reflections and detecting such modulation then the incident acoustic wave can be determined. For a DTS system, ambient temperature affects the amount of backscatter from different parts of the fiber at different ambient temperatures, so that again temperature along the fiber can be inferred by monitoring the backscatter.

The performance of the distributed sensor systems, for most applications, is limited by the system's signal to noise ratio (SNR). Improving the SNR can lead to, for example, quantification of flow, seismic and leak signals which are otherwise unmeasurable. The SNR of the sensor is mostly dependent on the optical SNR, which is the relationship of the magnitude of the optical signal and the associated detection noise. The optical SNR, and so the acoustic SNR in the case of a DAS system, is optimised by maximising the amount of optical signal returning to the sensing unit from the optical fibre. The returning optical signal can be maximised using a number of techniques, including using a shorter wavelength source light (shorter wavelengths scatter more), using a fibre with a large scattering coefficient or a large capture angle, or by introducing reflectors into the optical path. However, such solutions are not usually optimal taking into account other requirements of the sensor systems, such as maximising the spatial resolution and/or range performance.

Another technique to increase the SNR is to increase the amount of light that is launched into the sensing optical fiber in the first place. In this respect, the amount of backscatter light that can be generated and then captured by a distributed sensor is governed by how much light can be usefully transmitted through the sensing fibre. The amount of light which can be usefully transmitted is usually limited by the onset of optical non-linear effects, which destroy the integrity of the backscatter signal. In order to maximise the distributed sensor performance, the instrument is usually configured such that the launch light power is set to just below the non-linear threshold for the target measurement length.

Special fibres may be used to increase the amount of light which can be launched before non-linear effects take hold. For example, since the non-linear threshold is dependent on the optical intensity (power/area), fibres with larger cores can be used to launch more power. However, such fibres do not usually have the optimum performance for other aspects of the distributed measurement requirements (which are typically the same requirements as are important for communications fibres). For example, a larger core fibre (in the case of multimode fibre) will lead to modal dispersion which spreads out the light and so worsens the spatial resolution. Special fibres may also have poorer optical loss characteristics, and so will limit the sensing range. Significantly, there is also the major consideration of the loss at the connection between the sensing fibre and the sensing unit, which will normally be composed of components made of standard single mode (SM) or graded index multimode (MM) fibres. For example, if a 50 micron multimode fibre is connected to a sensing unit built with single mode components (for example the Silixa® iDAS™ described above) although it is possible to launch significantly more light into the MM fibre before non-linear effects take hold, this advantage will be negated due to the losses incurred by the backscatter signal at the SM/MM connector (see Fig 1). Similarly, there is no advantage to be gained by connecting a 62.5 micron core MM fibre to a DTS, which is built using 50 micron components. In this respect, Figure 1 illustrates that there are no problems launching light from single mode fiber 1 provided with a narrow core 5 to multimode fibre 3 having a wider core 7 (i.e. from left to right) but that only a proportion of the backscattered light is transmitted to the core of the single mode fibre 1 from the multimode fiber 7 (i.e. from right to left).

A further solution that improves sensitivity by increasing the SNR of the system, while not being detrimental to other performance criteria such as spatial resolution and range is therefore desirable.

Summary of Invention In order to address the above, some embodiments of the invention provide optical fiber sensing systems wherein sensitivity is increased by the provision of multiple parallel- connected sensing fibers. The fibers are in some embodiments substantially contiguous to each other along the sensing parts of their lengths, for example by being incorporated into a common cable or the like. That is, the fibers in some embodiments run substantially parallel to each other and/or are further arranged such that incident energy, or temperature, at a particular location is incident upon known corresponding sections of the respective fibers substantially simultaneously. The provision of multiple fibers allows for multiple backscatter or reflection signals to be generated in response to a particular event to be sensed, the multiple backscatter or reflection signals pertaining to the same event to be sensed (e.g. temperature change, or incident acoustic energy) being combined together to provide for a greater overall signal for inputting into an optical fiber sensing system for processing. The multiple sensing fibers also permit the optical fiber sensing system to output optical signals for inputting into the fibers, typically as optical pulses, of a greater power than would otherwise be possible without entering the domain of non- linear fiber responses. This is because the output signal is divided between the plurality of sensing fibers, and hence the individual optical signal into each sensing fiber remains within the linear response region. However, the overall backscatter and/or reflections that is/are generated and upon which sensing is based is increased in line with the overall increase in signal power, and hence overall the signal to noise ratio of the system is increased. Importantly, embodiments of the invention typically may make use of telecoms- standard optical fibers, and as such, make use of their optimum transmission characteristics of low loss and low dispersion, while simultaneously allowing more useful light to be launched than would be received from a single fiber. These could be fibers in existing installations, for example multiple spare fibers in a telecoms cable.

In view of the above, from one aspect there is provided an optical fiber sensing system, comprising: an optical sensing system and a plurality of optical fiber cores, the arrangement being such that in use the optical sensing system outputs optical signals into the plurality of optical fiber cores such that the optical signals propagate therealong substantially simultaneously, the optical sensing system determining one or more properties of external conditions along the optical fiber cores in dependence on backscattered and/or reflected signals received from along the plurality of optical fiber cores.

In one embodiment the plurality of optical fiber cores are contained within a plurality of respective optical fibers. Alternatively, in another embodiment a multiple core optical fiber may be provided. Moreover, in a further embodiment some of the plurality of optical fiber cores may be contained within a first multi-core fiber, and others of the plurality of optical fiber cores may be contained within either a second multi core fiber, or respective single core fibers. Hence, multi-core fibers may be mixed with single core fibers, as deployment needs dictate.

Preferably at least two or more of the optical fiber cores extend substantially in parallel with each other. With such an arrangement, typically energy relating to an external condition is incident on spatially corresponding sections of the parallel fiber cores substantially simultaneously.

In one embodiment, the optical sensing system outputs optical signals of an amplitude in a range greater than a first level that if input into a single optical fiber the signals would be in the non-linear region of operation of the single fiber, but less than a second level that if input into the plurality of fiber cores and shared therebetween would be in the non- linear region of any of the plurality of fiber cores. In this way, greater overall optical power can be output, thus increasing the signal to noise ratio, and the optimum transmission properties of the individual fibres are used..

In one embodiment the fiber cores are coupled to the optical sensing system by a coupler. The coupler may be a 1 x n coupler, wherein there are up to n optical fiber cores.

In one embodiment a respective optical amplifier is provided for at least a subset of the optical fiber cores, and in other embodiments a optical amplifier is provided for each of the optical fiber cores. Usually the amplifiers are coupled to the respective optical fiber cores in series with the respective connections to the coupler. The amplifiers may in some embodiments be bi-directional, whereas in other embodiments they may amplify only in the fiber return direction i.e. amplify the reflections and/or backscatter returning from along the fiber.

The amplifiers typically comprise one or more optical amplifiers in series with the respective sensing fibers and arranged to increase the power of sensing pulses travelling along the fiber in the fiber forward path and/or backscatter and/or reflections returning back along the fiber in the fiber return path.

Some embodiments of the invention typically further comprise optical amplifier bypass optical componentry arranged to permit the forward direction optical pulses travelling along the sensing fiber to bypass the one or more optical amplifiers. In some embodiments the optical componentry includes one or more optical amplifiers arranged to amplify the optical backscatter so as to maintain the backscatter above the noise floor. This allows the range of the sensor to be increased.

In some embodiments the optical componentry has backscatter conditioning componentry arranged to maintain or improve the spectral form of the backscatter. Again, such measures helps to improve the range of the sensor. The backscatter conditioning componentry may include one or more bandpass filters.

In some embodiments pulse conditioning componentry is further provided, arranged to condition sensing pulses travelling along the fiber. Preferably the pulse conditioning componentry includes a bandpass filter to help to maintain the spectral form of the pulses. In preferred embodiments the optical amplifiers are optical fiber amplifiers, and more preferably erbium doped fiber amplifiers or ytterbium doped fiber amplifiers. In some embodiments the optical amplifiers are fed via optical fiber from a laser pump located remotely therefrom. Such arrangements are particularly advantageous for measuring following a long offset - for example for measuring in subsea wells using optoelectronics housed remotely.

The sensing system may be any optical fiber sensing system. In some embodiments the sensing system is an optical fiber distributed acoustic sensor (DAS), whereas in other embodiments the sensing system is an optical fiber distributed temperature sensor (DTS). In addition, in further embodiments the sensing system is a Brillouin optical time domain reflectometry (BOTDR) system.

Further features and advantages of embodiments of the invention will be apparent from the appended claims.

Brief Description of the Drawings Further features and advantages of the present invention will become apparent from the following description of an embodiment thereof, presented by way of example only, and by reference to the drawings, wherein like reference numerals refer to like parts, and wherein:

Figure 1 is a diagram illustrating the loss of backscatter signal at a connection between single mode and multimode fibers;

Figure 2 is a diagram of a range extender module used in embodiments of the present invention;

Figure 3 is a diagram of a further version of a range extender module used in embodiments of the present invention; Figure 4 is a block diagram of a first variant of a third embodiment of the invention;

Figure 5 is a block diagram of a first embodiment of the invention;

Figures 6 and 7 are block diagrams of different variants of the first embodiment of the invention; Figures 8 and 11 are block diagrams of the arrangements of pertinent parts of a prior art DTS system;

Figures 9 and 12 are block diagrams of the arrangement of pertinent parts of a second embodiment of the invention; and Figure 13 is a block diagram of a second variant of a third embodiment of the invention

Description of the Embodiments

Embodiments of the invention will now be described. Generally, embodiments of the invention address the issue of the limitation on the amount of power that can be input into an optical fiber that forms part of a sensing system through the concept of simultaneous measurement on multiple, standard optical fibres. In this case, the launch power in each fibre can be independently optimised to be at the non-linear threshold, meaning the total amount of optical power launched, and so signal generated, increases proportionally with the number of fibres connected. Significantly, the sensing fibres may be of any standard type (e.g. standard SM or MM multimode fibres). This provides the advantages of:

1) Utilising the optimised optical properties of standard fibres, which have undergone extensive development for telecoms applications;

2) The fibres are cheap and readily available; and

3) There is the opportunity to use the arrangements described herein with existing cables which typically hold multiple standard fibres.

Using multiple fibres allows us to optimally launch more power but the technical challenge remains how to minimise losses when coupling these signals into the sensor unit. This is resolved in embodiments of the invention by the provision of two possible configurations - one for sensors using Rayleigh and/or Brillouin signals (for example a DAS or a BOTDR sensor) and the other for Raman sensors (for example a DTS). Two embodiments are described as the Rayleigh/Brillouin embodiments (i.e. that used for DAS or BOTDR systems) use optical amplifiers, which are not suitable for use with the weak Raman light that is used in DTS systems. The embodiments to be described are extremely cost-effective ways to increase the sensing SNR in that they replicate the performance of running multiple complete sensing units in parallel, through the addition of only a few extra components.

Note, the use of multiple fibres in this way requires matching of the fibre optical length with the physical cable length in order to ensure each equivalent position in each fibre senses the same measurand. This can be readily achieved, within the tolerances required, through appropriate cable design or through the use of ribbon fibres, multicore fibres etc.

Figure 5 is a block diagram of a first embodiment of the invention. This embodiment is intended for use with DAS and other OTDR or BOTDR sensor systems, where the returning light along the fiber is at a suitable level to be subject to amplification. As shown in Figure 5, the basic arrangement is to provide a fiber multiplexing module 72, referred to sometimes herein and in the Figures as an "Optopus"™ module, that attaches via an optical fiber 74 to a conventional optical fiber sensing system processing module 70, such as for example, the Silixa® iDAS™, referenced previously. The multiplexing module 76 multiplexes a plurality of optical fibers 76 together, the optical fibers being substantially co-located such that they run in parallel next to each other, such that, for example, an incident acoustic signal is incident on a corresponding length of each fiber at substantially the same position along each fiber substantially simultaneously. No modification of the DAS system 70 is required (although in some embodiments a minor modification such as the removal of one or two of the optical amplifiers in the iDAS may be beneficial). Note, the DAS architecture and processing is unaffected - the DAS will see the multiplexing module 72 merely as if it were a single optical fibre with a high backscatter coefficient. This is important as it means it is possible to use a conventional DAS system such as the Silixa® iDAS™ and run it with or without the multiplexing module 72 and also match the number of outputs from the module 72 to the number of sensing fibres 76 available.

The multiplexing module 72 comprises two building blocks, as shown in Figures 6 and 7 - one optical combiner 78 and a number of directional amplifiers 14, 16, 18, and 19, connected together by suitable optical waveguides such as optical fibers. The amplifiers 14, 16, 18, etc in most embodiments will be bi-directional amplifiers, but in some embodiments may act to operate in the return direction only, to amplify backscatter from along the fiber. This would be possible if the DAS system 70 has sufficient output power to illuminate the each of the fibres optimally (accounting for the splitting in the circulator). The optical combiner 78 can most simply be a 1 x n coupler (for example a 1 x 8 coupler for an 8 -fibre multiplexing module). One bi-directional amplifier (14, 16, 18, and 19) is needed for each output fibre. Where bi-directional amplifiers are used, such amplifiers may be identical to the range extending units described in our earlier copending patent application GB 1421470.4, referred to in the Figures as "eLITE" modules. Note the architecture is very flexible - for example, a 1 x 8 coupler could be used with any number of amplifier modules and output fibres from 1 to 8 without harm.

In the example shown in Figure 6, the launch light from the DAS 70 is shared between the four outputs of the 1 x 4 coupler 78. Depending on the launch light power from the DAS, this light may then be amplified by each amplifier module 14, 16, 18, and 19 and launched into the respective sensing fibres 76. However, if the launch light power from the DAS is sufficient such that when shared between the plural fibers and accounting for coupling losses it is still powerful enough such that backscatter or reflections from the end of the sensing fiber can still be detected at the DAS, then no forward amplification may be required, in which case amplifier modules 14, 16, 18, and 19 may act to amplify backscattered or reflected light on the return path only.

As described previously, the sensing fibers 76 are arranged to run in parallel next to each other, and may conveniently be combined together into a single cable. Note, the amplification provided by each amplifier 14, 16, 18, and 19 may be optimised for each unit, independently, in turn, before the start of the measurement. The reflected and/or backscattered light, which is dependent upon incident vibrations along the sensing fiber, returning along the sensing fibers is amplified by each amplifier and then combined at the coupler 78 and returned into the DAS 70 for measurement. The operation of the DAS 70 is conventional, as described, for example, in WO2010/0136809.

In this embodiment the backscattered or reflected light is amplified before the coupler. This is advantageous because the SNR is determined by the backscatter or reflected light level at the point of amplification. If the amplification stage were not performed before the coupler, then the backscatter or reflected light would suffer the losses at the coupler, before amplification within the DAS; this would cancel any benefit of using multiple fibres. Note, the coupler losses are also significant - with a 1 x n coupler, only 1/n of the backscattered or reflected light incident on the coupler would pass into the DAS (i.e. ¾ of the backscatter or reflected light incident on the coupler as shown in Figure 6 would be lost). Although the light in the multiplexing module 72 still suffers this coupler loss, this is after the amplification stage and so does not impact upon the SNR performance of the system. Figure 7 shows a configuration where only three amplifiers 14, 16, and 18 are used, feeding into three parallel sensing fibers 76. In this case, there is no harm in still using a 1 x 4 coupler 78 in this system. In this case, the DAS 70 would receive ¾ of the light compared to the system in Figure 6 but would suffer no other ill effects. This illustrates that the system is highly modular - a single DAS in combination with a 1 x n (say n = 8) coupler can operate with a pre-installed cable with as many fibers as are already present in the cable (up to 8). All that is needed to adapt the arrangement to the cable is to provide as many amplifier units 14, 16, 18, ... as there are available fibres in the cable.

The arrangement of the amplifier modules 14, 16, 18, 19 will next be described. As mentioned previously, these modules are similar to those described previously in our prior co-pending patent application GB 1421470.4. Figures 2 and 3illustrate different variants for the amplifier modules 14, 16, and 18 of Figures 6 and 7, and will be described further next.

Figure 2 illustrates a first variant of the optical amplifier modules 14, 16, or 18. In the amplifier forward path the amplifier module 14, 16, 18 comprises a first circulator 22, which is a three port device where a signal input at a first port is output at a second port, whereas a signal input at the second port is output at the third port. Correspondingly, a signal input at the third port is output at the first port. Optical circulators are known in the art, and no further description of the internal operation thereof will be undertaken. It should be noted that the amplifier forward path is connected in the return path of the fiber so as to amplify the backscattered or reflected light, and as such the first port of the optical circulator 22 is connected to one of the respective optical fiber lengths 76, whereas the second port of the optical circulator is connected via an optical fiber to the input of an optical amplifier 24, such as an optical fiber amplifier like an erbium doped fiber amplifier (EDFA) or ytterbium doped fiber amplifier. The optical amplifier 24 acts to amplify the power of the backscatter or reflections received from the optical circulator, and then feeds them along a length of fiber via an optional band pass filter 26, to a first port of a second optical circulator 28. The second optical circulator 28 then outputs the amplified and filtered backscatter or reflections on its second port, to be fed to coupler 78, as appropriate. With this amplifier forward path, therefore, (which corresponds to the backscatter or reflections return path back along the fiber) backscatter and/or reflections from along the fiber are amplified and band pass filtered before being fed back to the optical coupler.

With respect to signal pulses from the DAS 70 to be sent into the fibers 76, these pulses are received from the DAS 70 via the optical coupler 78 at the second port of the optical circulator 28, and output on the third port thereof via fiber path 29 to the third port of the first optical circulator 22, which then routes the pulses on to the respective fiber 76, for propagation therealong. With such operation, therefore, forward pulses from the DAS (via the coupler) are routed around the amplification circuitry for subsequent injection on to the fiber 76, whereas backscatter from all the way along the length of the respective fiber 76 is routed through the forward optical amplifiers and band pass filters, so that it is then amplified prior to being input into the coupler 78 and then being received back at the DAS 70 for measurement. As explained previously, amplification of the backscatter/reflections at this point is advantageous because the SNR is determined by the backscatter or reflected light level at the point of amplification. If the amplification stage were not performed before the coupler, then the backscatter or reflected light would suffer the losses at the coupler, before amplification within the DAS; this would cancel any benefit of using multiple fibres. In addition, amplification of the backscatter signal ensures that it can be kept above the noise floor so that it can be detected by the DAS 70.

Figure 3 illustrates a further variant, which is a variant of the arrangement of Figure 2. Here, the amplifier forward path (i.e. the return path of the fiber) is identical to the arrangement of Figure 2, and will not therefore be described further. However the amplifier return path (i.e. the forward path of the fiber) is also provided with an optical amplifier, such as an optical fiber amplifier like an erbium doped fiber amplifier 32, which receives the forward signal pulses from the DAS 70 via the coupler 78from the third port of the optical circulator 28, and amplifies them. The amplified forward signal pulses are then input into a band pass filter 34 where they are band pass filtered, before then being fed to the third port of the optical circulator 22, which then routes the amplified and filtered signal pulses back onto the optical fiber 76 to carry on in the forward direction along the fiber. In this way, the forward signal pulses can also be amplified (and filtered if required), so as to compensate for losses in the coupler 78 and ensure that the optimum signal pulse is injected onto the fiber 76.

Various modifications may be made to the above amplifier arrangements to provide further arrangements. For example, the laser pumps required for the various erbium doped fiber amplifiers may be located away from the fiber amplifiers themselves. For example, where the optical fiber sensor is being used in a downhole application, it may be possible to have another fiber run from a laser pump provided at the surface downhole parallel with the sensing fiber, in order to feed the one or more erbium doped fiber amplifiers from the surface laser pump. Additionally or alternatively, the multiplexing module 72 may be sited remotely from the DAS 70, connected thereto by a length of single fiber extending over a region where increased sensing sensitivity is not required. Such an arrangement would have the advantage that as only the single fibre is needed over the section which is not of interest then we may keep the DAS launch power relatively low (so as not to induce non-linear effects) with this then being boosted to a useful level by the amplifier modules for sensing in the fibers 76 connected at the multiplexing module.

As a further modification to the amplifier arrangements, the amplifier signal paths may also include respective attenuators. Inclusion of attenuators allows an optimum configuration where the optical amplifiers are operated at their respective optimum gain settings for noise performance (at which settings the amplifiers normally emit too much light), but then the power is adjusted down to the appropriate level using respective variable attenuators placed in the respective signal paths after the optical amplifiers. Attenuators may be included in all of the signal paths, or only some of them, as required.

A second embodiment will now be described, relating to an optical fiber distributed temperature sensor (DTS) system, which again makes use of plural parallel sensing optical fibers, to increase the SNR of the system, and hence the sensitivity.

A DTS system uses the amplitude of backscattered Raman light as the sensing mechanism. This Raman light is unsuitable for optical amplification; for this reason, the multiplexing module described above in respect of the first embodiment cannot be used with a DTS. In order to use plural sensing fibers with a DTS a different approach is required that works through the use of a new wavelength division multiplexing (WDM) component, built into the DTS. This new component combines the backscattered light from the multiple sensing fibres into a larger core fibre prior to detection or, alternatively, combines the light directly onto a large area detector. This approach of coupling light from multiple standard sensing fibres into a large core receiver fibre allows efficient transmission and detection of light while still using optimum sensing fibres. Although the receiver fibre is not optimum for optical transmission, its length is short (<lm) and so will have minimal effect on the system's performance.

Figure 8 shows the existing DTS architecture, where all optical fibres are of the same type (standard single mode or standard multimode). Here, light from the laser 82 is guided via standard fiber 83 to the wavelength division multiplexer 88 (WDM), and from there is directed towards the sensing fibre 89. The backscattered Stokes and anti-Stokes signals are directed by the WDM 88 towards their respective receivers 84 and 86 via respective fibers 85 and 87. An example DTS system operating in accordance with the above is the Silixa® Ultima™ DTS, available from Silixa Limited, of Elstree, United Kingdom.

Figure 9 shows the concept of the DTS multiplexer according to the second embodiment. Here the WDM component 90 is modified to enable the multiplexing functionality, to allow multiple sensing fibers 96 to be connected thereto. In this respect, the WDM is modified to have multiple outputs, with the laser light shared equally between the sensing fibres. The fibres 92 and 94 connecting the Stokes and Anti-Stokes receivers 84 and 86 to the WDM 90 are also made to be of a larger core size, as depicted by the thicker lines. Alternatively, in another embodiment the receivers could be incorporated into the body of the WDM 90, in order to save these connections.

A conceptual design for a standard WDM, as is conventionally used, is shown in Figure 11 : In the standard WDM, all fibres are of the same type (SM or MM). At one side of the WDM, the laser fiber feeds in light from laser 82, which is collimated into a beam via lens 112, and directed towards the anti-Stokes filter and the Stokes filter in turn, before then being focussed by lens 114 into the sensing fiber 89, connected to an opposite port of the WDM than the laser fiber. With respect to backscattered light from the fiber, lens 114 also acts to collimate the backscattered light into a beam that is directed towards the Stokes filter 120, and anti-Stokes filter 122 i.e. in the opposite direction to the forward light from the laser fiber 83. The anti-Stokes filter 122 and Stokes filter 120 direct respective anti-Stokes and Stokes beams substantially orthogonally to respective lenses 118 and 116, which focus the beams into respective anti-Stokes and Stokes fibers 87 and 85, for guidance to the anti-Stokes and Stokes receivers 86 and 84 respectively.

Figure 12 shows the equivalent schematic for the multiplexing WDM 90 of the second embodiment. Here, the arrangement is similar to the arrangement of Figure 11, with the differences that a bundle of substantially parallel fibers 96 are used as the sensing fiber, and the anti-Stokes and Stokes fibers 92 and 94 connecting to the receivers 84 and 86 are of larger core size. The lenses' 112 and 114 focal lengths are also optimised such that the sensing fibre bundle 96 is uniformly illuminated by the laser light from laser fiber 83. It is acceptable for there to be significant losses going into the fibre bundle 96 as this can be compensated by increasing the laser power. Due to the use of a fibre bundle 96 rather than a single fibre, the spot size incident on the receiver fibres 116 and 118 will be larger than in the simple WDM case of Figure 11. This light is collected through the use of larger fibre cores. Alternatively, large area receivers can be packaged directly in the WDM 90.

In some embodiments the sensing fibre bundle 96 shown in Figure 12 may be required to fan out the fibres from the WDM component 90 to individual connectors on the DTS unit (for connection to the fibres in the sensing cable). A possible arrangement of the fibre cores in the WDM 90, for a 7-channel WDM, is shown in Figure 10. Here, the fiber cores 102 are arranged substantially contiguously, and are illuminated by laser illumination 100 i.e. the illumination 102 extends beyond the extent of the fibers, to ensure uniform illumination of all of the fibers.

Note, although the DTS of the second embodiment would be capable of the simultaneous measurement on n fibres, it would be equally capable of measuring on any number of fibres up to n. For example, a 7-fibre DTS (e.g. with 7 fibers in the bundle 96) would also work correctly if only one fibre was connected.

A third embodiment of the invention will now be described with respect to Figures 4 and 13. The third embodiment is based on the embodiments described above, but instead of the plural optical fibers being co-located and running parallel to each other, in this embodiment the fibers diverge, so as to run in different directions. In this way the fibers can be dispersed from each other so as to extend over a wide area. The result of the above is to provide a completely different effect from the above described embodiments, and in particular because the plural fibers do not run parallel to each other there is no increase in sensitivity. In fact, because the fibres 76 run in different directions there would be crosstalk between fibres, in that it would not be possible to tell immediately at the DAS or DTS from which particular fiber backscatter or reflections caused by an external sensing event (i.e. incident acoustic energy on the particular fiber or a local temperature change at a point along the fiber) originate. This is because in the case of the DAS the backscatter and/or reflections are coupled from the plural fibers on to a single fiber at the optical coupler 78, and at the point the information relating to which fiber the backscatter came from is lost. Likewise lens 144 in the DTS system acts to combine the light from the plural fibers together in a similar manner. Moreover, because the fibers are not co-located with each other the same incident external energy will not be incident on the plural fibers at the same time, and hence the effects of the above described embodiments of increased sensitivity will also not be obtained. However, where such an embodiment is useful is that it allows a single optical sensing system (DAS or DTS) to provide simultaneous and parallel coverage over a wider area than a single fiber (or plural co-located fibers). The plural fibers can effectively be fanned out as shown so as to be spatially distributed from each other to provide respective sensing areas around each fiber. Those sensing areas may overlap partially (or in some cases even wholly, in which case the processing gain effects of the above described embodiments may be obtained for the overlapping areas), or may not overlap at all, but the main effect obtained is that where the sensing areas do not overlap an increase in total sensing area from the system is obtained. For example, each fiber may be deployed such that the respective sensing areas of each do not overlap at all, in which case the total sensing area of the whole system is increased so as to be a multiple (equal to the number of fibers) of the sensing area of a single fiber (assuming equal fiber lengths). Generally, where the sensing zones of each fiber do not overlap, then the total sensing area is the sum of the sensing zones. Where there is some overlap then the total sensing area would be reduced by the overlap i.e. the overlapping zones should only be counted once in the calculation of total sensing area.

Operationally, the present embodiment therefore allows area coverage using a signal optical fiber sensing system over a larger area. This is useful in some cases (for example in an alarm system). In such a case, the DAS or DTS system may monitor a large number of fibres that are deployed distributed away from each other over a larger area simultaneously, and then if an external event is detected (for example unusual frequency content or signal amplitude on a DAS or a temperature rise for a DTS), then the system may turn off each optical path in turn to identify the exact location, and along which fibre, of the event. In this way, area coverage is obtained from a single system, and then when an event is detected, by then operating only a single fiber in turn in a conventional manner, the location of the event may then be found. In this respect, controllable optical switches 132 may be provided in the fiber paths 76, to allow the fibers to be switched in and out in turn.

Note that the above concept of the third embodiment may be applied to both DAS systems (as shown in Figure 4, and which would make use of an optical coupler) and DTS systems (as shown in Figure 13, which do not include a coupler), and the operational concept of each in terms of having fanned out fibers distributed over a wider area so as to provide area coverage may be provided by both types of system, as described above. Various further modifications to the above described embodiments, whether by way of addition, deletion or substitution, will be apparent to the skilled person to provide additional embodiments, any and all of which are intended to be encompassed by the appended claims.