WO2020044041A1

WO2020044041A1 - Fibre optic communication apparatus

Info

Publication number: WO2020044041A1
Application number: PCT/GB2019/052405
Authority: WO
Inventors: Alastair Godfrey; Christopher MINTO
Original assignee: Optasense Holdings Limited
Priority date: 2018-08-31
Filing date: 2019-08-28
Publication date: 2020-03-05
Also published as: GB201814159D0

Abstract

This application relates to methods and apparatus for communication using fibre optic sensing techniques. A communication apparatus (210) has a first optical fibre (101) coupled to a first material (212) configured to vary in dimension in response to a variation in applied magnetic field (221), such that, a strain is induced in the first optical fibre (101) due to the variation in applied magnetic field. A fibre optic interrogator (102) is configured to interrogate the first optical fibre (101) with optical radiation to provide sensing and provide at least one first measurement signal indicative of variations in applied magnetic field. A processor (103) is configured to analyse said at least one first measurement signal to detect a characteristic signature indicative of a data transmission. In use a transmitter (220) can transmit data by deliberately modulating a generated magnetic field (221) in the vicinity of the optical fibre (101).

Description

FIBRE OPTIC COMMUNICATION APPARATUS

This application relates to methods and apparatus for fibre optic sensing, and in particular, to a fibre optic sensing apparatus and method of operation suitable for communication.

Various types of fibre optic sensing are known, where an optical fibre is deployed as a sensing fibre and interrogated with electromagnetic radiation to provide sensing of environmental stimuli along its length, e.g. dynamic strains acting on the sensing fibre.

One known type of fibre optic sensing is distributed fibre optic sensing, which involves interrogating the sensing fibre with optical radiation and analysing the backscatter from inherent scattering sites within the sensing optical fibre to detect environmental disturbances on the sensing fibre. By analysing this radiation backscattered from within the fibre, the fibre can effectively be divided into a plurality of discrete sensing portions which may be (but do not have to be) contiguous. Within each discrete sensing portion disturbances of the fibre, for instance, dynamic strains due to incident acoustic waves, cause a variation in the properties of the radiation which is

backscattered from that portion. This variation can be detected and analysed and used to give an indication of the disturbance acting on the fibre at that sensing portion. Thus the distributed fibre optic sensor effectively acts as a linear sensing array of sensing portions of the optical fibre. Distributed acoustic sensing (DAS) is one particular form of distributed fibre optic sensing used to sense mechanical disturbances or strains acting on the sensing fibre.

Various types of DAS sensor have been demonstrated including sensors based on Rayleigh scattering of coherent light from the sensing fibre. Light transmitted into an optical fibre will be Rayleigh scattered from various inherent, i.e. intrinsic, scattering sites within an optical fibre. A mechanical vibration or dynamic strain acting on the fibre, such as caused by an incident acoustic wave, will effectively alter the distribution of scattering sites resulting in a detectable change in the properties of the Rayleigh backscattered light. Analysing such changes allows vibrations/acoustic stimuli acting on sensing portions of the optical fibre to be detected. Fibre optic distributed sensing systems have been employed in a variety of

applications. Distributed sensing systems can provide accurate localised sensing along an optical fibre up to 40 km in length. As such, DAS-based systems have been used for the monitoring of large-scale linear assets such as borders, oil and gas pipelines and infrastructure monitoring such as roads and rail tracks. The robust nature of fibre optic cables has also meant DAS-based systems perform well over other sensing systems in harsh environments. DAS systems have therefore also been proposed in environments such as borehole or down-well monitoring and in sub-sea environments as well.

In some applications of DAS it has been proposed that a controlled acoustic source at a first location could be used to generate specific acoustic disturbances that act upon part of the sensing fibre as a way of conveying some information from the first location via the DAS sensor. This may advantageously allow the sensing fibre to be used for both distributed sensing and communication of some data.

Embodiments relate to a distributed fibre optic sensing system suitable for

communication with advantages for particular applications.

Thus according to an embodiment of the present disclosure there is provided a communication apparatus, comprising:

a first optical fibre coupled to a first material configured to vary in dimension in response to a variation in applied magnetic field, such that, a strain is induced in the first optical fibre due to the variation in applied magnetic field;

a fibre optic interrogator configured to interrogate the first optical fibre with optical radiation to provide sensing on the first optical fibre and provide at least one first measurement signal indicative of variations in applied magnetic field; and

a processor configured to analyse said at least one first measurement signal to detect a characteristic signature indicative of a data transmission.

In some examples the characteristic signature may be a signature indicative of data transmission according to a defined data transmission protocol. In some examples the characteristic signature may correspond to a predetermined sequence that, in use, is transmitted prior to data transmission according to a defined data transmission protocol. Upon detection of the characteristic signature the processor may be configured to process the first measurement signal so as to extract a data signal according to the defined data transmission protocol.

The fibre optic interrogator may be configured to provide a measurement signal from each of a plurality of sensing portions of the first optical fibre. The fibre optic interrogator may be configured to interrogate the first optical fibre with optical radiation to provide distributed fibre optic sensing.

In some embodiments a second optical fibre may be deployed alongside the first optical fibre. The second optical fibre may be configured to be substantially insensitive to any variation in applied magnetic field. The fibre optic interrogator may be further configured to interrogate the second optical fibre to provide sensing and wherein. The processor may be further configured to use second measurement signals from interrogating the second optical fibre to compensate for any non-magnetically induced disturbances affecting the measurement signals from interrogating the first optical fibre. In some implementation interrogation of the second optical fibre may provide distributed acoustic sensing.

The first material may comprise a magnetostrictive material.

In some examples a magnetic field generating element may be deployed along at least part of the length of the first optical fibre. The magnetic field generating element may be configured to generate a locating magnetic field. The magnetic field producing element may, in some examples, be deployed substantially along an entire length of the first optical fibre. The magnetic field generating element may be configured to generate the locating magnetic field periodically.

Aspects also relate to a communication system comprising the communication apparatus of any of the variants discussed above and a transmitter for generating a controllably varying magnetic field and operable to control the magnetic field for data transmission according to a defined data protocol.

In some implementations the transmitted may be configured, prior to transmitting data according to the defined data protocol, to control the magnetic field in accordance with a predetermined coded sequence. In use the transmitter may be physically uncoupled from the first optical fibre. In some examples the transmitter may be located as part of a vehicle.

In cases where the first optical fibre is located with a magnetic field generating element for generating a locating magnetic field, the transmitter may comprise a magnetic field detection capability for detecting the locating magnetic field. A transmitter apparatus, including the transmitter, may further comprise a processor for locating the first optical fibre based on a magnetic field strength of the locating magnetic field.

In another aspect there is provided a method of communication, comprising:

interrogating an optical fibre with optical radiation to provide sensing so as to generate measurement signals indicative of variations in applied magnetic field; and processing said measurement signals to detect a characteristic signature indicative of a data transmission;

wherein, the first optical fibre is coupled to a material configured to vary in dimension in response to the variations in applied magnetic field, such that, a strain is induced in the first optical fibre.

The method of communication of this aspect may be implemented using any of the apparatus variants described above. The method may, in particular, further comprises transmitting a characteristic varying magnetic field corresponding to the characteristic signature.

Unless expressly indicated or otherwise clearly incompatible, any of the features described herein may be implemented in combination with any one or more of the other features. The invention will now be described by way of example only with respect to the accompanying drawings, of which:

Figure 1 illustrates a conventional DAS sensor apparatus;

Figure 2 illustrates a communication system comprising a communication apparatus and a transmitter according to an embodiment;

Figure 3 illustrates one example of a characteristic signal indicative of data

transmission;

Figure 4 illustrates a communication apparatus for communication and for sensing acoustic disturbances and variations in magnetic field according to an embodiment; and

Figure 5 illustrates an example implementation of a communication system according to an embodiment.

Embodiments of the present disclosure relate to a fibre optic system suitable for use for communication, and to methods of communication using such a system, using the principles of fibre optic sensing, in particular distributed fibre optic sensing. Fibre optic sensing is a sensing technique where an optical fibre, which is deployed in an area of interest as a sensing fibre, is interrogated with optical radiation so as to detect external environmental disturbances acting on the sensing fibre. In particular, embodiments relate to distributed fibre optic based sensing based on analysing radiation which is Rayleigh backscattered from within the sensing optical fibre. The distributed fibre optic sensing system of embodiments of the disclosure employs a fibre optical cable structure that is sensitive to variations in magnetic field, such that a variation in magnetic field incident on at least part of the cable results in a detectable strain on a sensing optical fibre within that part of the cable.

In use a suitable transmitter is configured to generate a varying magnetic field in the vicinity of the magnetically sensitised fibre optic cable, wherein the magnetic field varies according to a defined protocol for data transmission. Data encoded into the varying magnetic field will be manifest as a corresponding varying strain in the sensing optical fibre of the magnetically sensitised cable, which can be detected by the fibre optic distributed sensor. A processor of the distributed sensing fibre optic system can analyse measurement signals generated by interrogation of the optical fibre to detect signals indicative of the data transmission. If data transmission is detected, the processor can extract a data signal according to the defined protocol. The distributed fibre optic sensing system can thus be used as part of a system of communication.

Figure 1 shows a schematic of a fibre optic sensing arrangement 100, which in this example is a distributed fibre optic sensing arrangement. A length of sensing optical fibre 101 is removably connected at one end to an interrogator 102. The output from interrogator 102 is, in some implementations, passed to a signal processor 103, which may be co-located with or integrated into the interrogator or may be remote therefrom. Optionally there may also be a user interface/graphical display 104, which may be co located with the signal processor or may be remote therefrom and in practice may be realised by an appropriately specified PC.

As discussed, the sensing fibre 101 can be many kilometres in length and can, in some applications be tens of kilometres in length, say up to 40 km or more. For distributed fibre optic sensing the sensing fibre may be a standard, unmodified single mode optic fibre such as is routinely used in telecommunications applications without the need for deliberately introduced reflection sites such a fibre Bragg grating or the like. The fibre will be protected by containing it with a cable structure which may contain more than one optical fibre.

In use the fibre optic cable comprising the sensing fibre 101 is deployed in an area of interest to be monitored. Depending on the particular use case, the sensing fibre may be deployed in a relatively permanent manner, e.g. being buried or otherwise secured in place. If continuous monitoring is not required the interrogator 102 may be coupled to the sensing fibre when required and removed when sensing is not required.

In operation, the interrogator 102 launches coherent electromagnetic radiation into the sensing fibre, which will be referred to as interrogating radiation. The sensing fibre may, for instance, be repeatedly interrogated with pulses of optical radiation. In some embodiments a single pulse of optical radiation may be used for each interrogation, although in some embodiments multiple pulses may be used, in which case the optical pulses may have a frequency pattern as described in GB patent publication

GB2,442,745 or optical characteristics such as described in WO2012/137022, the contents of which are hereby incorporated by reference thereto. Note that as used herein the term“optical” is not restricted to the visible spectrum and optical radiation includes infrared radiation and ultraviolet radiation. Any reference to“light” should also be construed accordingly.

As described in GB2,442,745 and WO2012/137022 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 105 and at least one optical modulator 106 for producing interrogating radiation, for example pairs of interrogating pulses separated by a known optical frequency difference. The interrogator also comprises at least one photodetector 107 arranged to detect radiation which is Rayleigh backscattered from the intrinsic scattering sites within the fibre 101. A Rayleigh backscatter DAS sensor is very useful but systems based on Brillouin or Raman scattering are also known.

For a distributed fibre optic sensor, the backscatter from the sensing optical fibre will depend on the distribution of the inherent scattering sites within the optical fibre, which will vary effectively randomly along the length of the fibre. Thus the backscatter intensity from any given interrogating pulse will exhibit a random variation from one sensing portion to the next but, in the absence of any environmental stimulus, the backscatter intensity from any given sensing portion should remain the same for each repeated interrogation (provided the characteristics of the interrogating pulse remains the same). However an environmental stimulus acting on the relevant sensing portion of the fibre will result in an optical path length change for that section of fibre, e.g. through stretching/compression of the relevant section of fibre and/or a refractive index modulation. As the backscatter from the various scattering sites within the sensing portion of fibre will interfere to produce the resulting intensity, a change in optical path length will vary the degree of interference. The variation in distribution of the scattering sites will result in a variation in intensity of backscattered from an affected sensing portion, which can be detected and used as an indication of a disturbance acting on the fibre, such as an incident acoustic wave. Additionally or alternatively, if each interrogation comprises spatially separated pulses at different frequencies to one another, or the backscatter is mixed with a local oscillator signal at a different frequency, the change in optical path length for a sensing portion with result in a change in phase of a carrier signal at the difference frequency, which can be detected and used as an indication of the disturbance.

The signal from the photodetector may thus be processed by processing module 108 of the interrogator 102 to provide a measurement signal which is representative of disturbances acting on the sensing portions or channels of the fibre. Some processing may additionally or alternatively be done by signal processor 103. As noted, in some implementations, the processing may demodulate the returned signal based on a frequency difference between optical pulses of the interrogating radiation. The processing module may, in some implementations, process the detected backscatter for example as described in any of GB2,442,745, W02012/137021 or WO2012/137022 and may also apply a phase unwrap algorithm. The phase of the backscattered light from various sections of the optical fibre can therefore be monitored. As described previously any changes in the effective optical path length within a given section of fibre, such as would be due to incident pressure waves causing strain on the fibre, can therefore be detected.

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, a measurement signal from a disturbance, for instance an acoustic signal, sensed at one sensing portion can be provided substantially independently of the sensed signal at an adjacent portion. Note that the term acoustic shall be taken to mean any type of pressure wave or mechanical disturbance or varying strain generated on the optical fibre and for the avoidance of doubt the term acoustic will be used in the specification to include other types of vibration. As used in this specification the terms“distributed fibre optic sensing” and“distributed acoustic sensing” will be taken to mean sensing by optically interrogating an optical fibre to provide a plurality of discrete sensing portions distributed longitudinally along the fibre and the terms“distributed fibre optic sensor” and“distributed acoustic sensor” shall be interpreted accordingly.

Such a sensor may be seen as a fully distributed or intrinsic sensor, as it uses the intrinsic scattering process inherent in an optical fibre and thus distributes the sensing function throughout the whole of the optical fibre.

In embodiments of the present disclosure the sensing optical fibre is formed as part of a fibre optic cable structure that is sensitised to magnetic fields, along at least part of the length of the cable structure. In particular the optical fibre 101 may be

mechanically coupled to a first material with magnetostrictive properties.

Magnetostrictive materials, as will be understood by one skilled in the art, undergo a physical dimension change in response to a change in magnetic field. As such, a magnetostrictive material formed as part of a fibre optic cable structure and

mechanically coupled to sensing optical fibre 101 can be configured to induce a strain in the optical fibre in response to a change in magnetic field. This varying strain will result in a change in the backscatter properties from the relevant sensing portion which may be detected by the interrogator 102 in the same manner as discussed above.

Thus using such a magnetically sensitised fibre optic cable, a measurement signal produced by the interrogator 102 can be indicative of magnetic field variations at a given sensing portion of the optical fibre 101.

In embodiments of the present disclosure a fibre optic sensing system which is operable to detect variations in magnetic field can be used as part of a communication system. In use a transmitter may generate a magnetic field which is controllably modulated for data transmission according to a defined protocol. In use the transmitter can thus controllably vary a generated magnetic field in the vicinity of the magnetically sensitised cable. The varying magnetic field will result in a corresponding varying induced strain in the optical fibre, due to the response of the magnetostrictive material. The strain change can be detected by the interrogator from variations in backscatter radiation as discussed above. A processor may analyse the measurement signals generated by the interrogator so as to detect signals which are characteristic of data transmission and, on detection of such a characteristic, may process the measurement signal(s) to extract the corresponding data signal. Figure 2 illustrates a communication system 200 according to an embodiment.

Communication system 200 comprises a distributed fibre optic sensor arranged as a communication apparatus 210 to detect and receive data which is transmitted from transmitter 220. The communication apparatus 210 comprises a fibre optic sensor arrangement, in this example a distributed fibre optic sensor, and thus comprises an optical fibre 101 deployed as a sensing optical fibre and an interrogator 102 configured to interrogate the optical fibre 101 to provide distributed fibre optic sensing as discussed above with reference to figure 1. Communication apparatus 210 further comprises processor 103 configured to analyse measurement signals generated by the interrogator 102 from the interrogation of optical fibre 101. In some examples, interrogator unit 102 and processor 103 may be co-located and formed as part of the same module. In other examples processor 103 may be separate to the interrogator 102, and in some instances located remotely from the interrogator 102, and arranged to receive measurement signals from the interrogator 102.

Optical fibre 201 is formed as part of a fibre optic cable structure 211. The fibre optic cable structure 211 comprises a first material 212, which is configured to vary in at least one dimension in response to a variation in applied magnetic field, i.e. the first material 212 exhibits magnetostrictive properties. The first material 212 is configured within the fibre optic cable 211 such that a change in dimension of the first material 212 resulting from a variation in magnetic field at a first part of the fibre optic cable will translate to a variation in strain on the optical fibre 101 at that part of the cable, such that the effective optical path length of that part of the optical fibre 101 will vary. The first material 212 may be directly coupled to the optical fibre 101 or may be

mechanically coupled via some intermediate material or structure, for instance via some compliant material. The fibre optic cable structure may also comprise at least one jacket layer 213 and may also comprise some other filler material 214, such as a compliant buffer material. Figure 1 illustrates a single optical fibre 101 for ease but in some embodiments the fibre optic cable 211 could comprise a plurality of optical fibres, at least one of which is mechanically coupled to the first material 212 as described.

Examples of magnetostrictive materials include iron, nickel, cobalt (the

magnetostrictive elements) or more sensitive alloys designed for their magnetostrictive properties such as Terfenol-D. Experiments have shown that magnetostrictive materials can be applied as a coating to the fibre in the form of a powder set in a flexible binder matrix. Methods of coupling a magnetostrictive material to an optical fibre are discussed in WO2012/028846, the contents of which are hereby incorporated by reference thereto.

In some embodiments the fibre optic cable 211 is configured so as to be magnetically sensitive over substantially its entire length, and thus the first material 212 may be deployed throughout the whole length of the cable 211. In some examples however there may be one or more parts of the fibre optic cable which are configured to be magnetically sensitive and one or more parts which are configured to be magnetically insensitive. The magnetically insensitive parts may be substantially free of any magnetostrictive material. Within a magnetically sensitive part of the cable 211 the magnetostrictive material may be deployed substantially continuously over the length of that part of the cable 211 , or there may be discrete sections of the first material disposed at intervals along the length of the cable.

In use, to transmit data, the transmitter 220 is configured to generate a controllably varying magnetic field 221 and is operable to modulate the magnetic field 221 for data transmission according to a defined data protocol. The defined protocol is a

predetermined protocol for encoding data, e.g. to provide a particular encoded sequences that can be decoded to recover the data. There are a number of different data protocols that could be used to encode data using a varying magnetic field as would be understood by one skilled in the art. For instance the magnetic field could be generated as an oscillating magnetic field and frequency modulation techniques could be used to encode data. In some embodiments the magnetic field could be pulsed high or low to provide digital data encoding.

In use the varying magnetic field is generated so as to be incident on the fibre optic cable structure 211. Due to the presence of the magnetostrictive material 212 the varying magnetic field can induce a corresponding varying strain on at least some sensing portions of the optical fibre 101 , resulting in a corresponding modulation in the measurement signal from the relevant sensing portion(s). To extract the data the processor 103 may decode the measurement signal from a given sensing portion according to the defined data protocol.

In use, however, a given sensing portion of the optical fibre may also respond to other strains induced on the optical fibre, and/or in use there may be variations in magnetic field in the environment that arise from other sources. Therefore there may be variations in the measurement signals from some sensing portions that do not correspond to transmitted data. It is thus necessary to identify signals that do correspond to transmitted data.

The processor 203 is thus configured to analyse the measurement signals from the interrogator 102 so as to detect a characteristic signal indicative of data transmission.

In some embodiments the characteristic signature detected by the processor 103 may be the characteristics of data transmission itself. For example figure 3 illustrates one possible example of data transmission protocol. The top plot of figure 3 illustrates the magnetic field B generated by the transmitter 220 as a function of time. In this example the transmitter 220 is initially not transmitting data and then commences data transmission at a time TD. In this example the data transmission includes a series of periods of oscillating magnetic field, where a value may be encoded by the frequency of oscillation. Thus in this example the transmitter generates a first varying field at first frequency f1 for a first period T_B and then varies the frequency to a second frequency f2 for a second period TB. The frequencies used to encode the data may be selected from a predefined range. This is just one example of a possible data protocol however. The lower plot shows an example of a possible measurement signal M from a sensing portion on which the varying magnetic field is incident. It can be seen that prior to the start of data transmission at time TD, the measurement signal may vary due to background noise, or environmental changes in magnetic field, but these may, on the whole, be relatively low intensity and/or may be outside the defined range of frequencies. In particular it may expected that any variations which are not due to the data transmission may not exhibit the property of having a defined frequency for the defined period TB. The processor may therefore be configured to detect a pre-determined characteristic corresponding to a defined data transmission protocol for a given communication system 200 that is used to actually encode and transmit the data. This may, for instance, be the case where the data encoding protocol used is quite distinctive and unlikely to arise other than as a deliberate transmission.

In some embodiments however, the transmitter may be configured to modulate the magnetic field according to a predetermined coded sequence that, in use, is transmitted prior to transmission of data encoded according to a defined data transmission protocol. In other words there may be some pre-transmission sequence that is not, itself, used to encode any data to be transmitted but which is used to signal that data transmission will follow. This predetermined sequence may be designed so as to be relatively distinct and easily identifiable in the measurement signal from a distributed fibre optic sensor. The processor may be configured to detect such characteristic signature, which is indicative that a data transmission will follow, and thus may subsequently process the relevant measurement signal to extract a data signal. Data transmission may commence immediately after the characteristic signature is transmitted. Alternatively, there may be some defined period of latency between transmission of the characteristic signature and the data transmission.

The use of a predetermined sequence to signal that data transmission is to follow, can, as mentioned, allow the use of a distinctive sequence to signal data transmission is expected. The predetermined sequence may have characteristics that are different to the characteristics of the data encoding protocol. Depending on the data encoding protocol used, it may be possible for some naturally occurring disturbances on the optical fibre to appear to correspond to characteristics of the pre-defined data transmission protocol. Were the processor 103 to respond purely to characteristics of the data transmission protocol, in such a situation the processor may falsely detect, and try to decode, a data transmission. Alternatively, the data protocol may not be clearly recognised as being distinct from the background signals and the existence of the data transmission may be missed. Using a distinctive characteristic signature that indicates that a data transmission is about to be sent may avoid such a problem and clearly indicate that data is to follow. In some embodiments, on detection of the characteristic signature, the processor 103 may be reconfigured so as to operate in a different mode of operation, which may improve the ability to detect the data

transmission.

Once data transmission is detected, the processor 103 may be configured to extract a data signal according to the defined data protocol.

In some embodiments the data transmission protocol may include some encoding that signals the end of transmission and/or the transmitter 220 may be configured to generate a predetermined sequence to indicate that the data transmission has ended. Alternatively the transmitter may transmit the data it has to transmit and may then stop generating the varying magnetic field, in which case the processor 103 may detect that encoding according to the defined protocol has ceased and determine that the particular instance of data transmission has ended.

Transmitter 220 may comprise any suitable magnetic field generating device capable of generating a controllably varying magnetic field. For example the transmitter 220 may have at least one conductive loop 222 which may be driven with varying current of different strength and/or polarity so as to generate the magnetic field 221.

As the transmitter 220 encodes the data as a varying magnetic field it is not necessary for the transmitter 220 to be in physical contact with the optical fibre 211 nor is there any need for good acoustic coupling between the transmitter 220 and the optical fibre. Transmitter 220 may thus be physically uncoupled from optical fibre 211.

Such an arrangement is advantageous as it means that the transmitter 220 may be mobile, and may for instance be deployed in, or on, a mobile apparatus such as vehicle for instance. Additionally the fibre optic cable 211 does not itself need to be accessible to the transmitter and thus the fibre optic cable may be buried, at least in part, or embedded within a structure. Provided the transmitter 220 is located, in use, relatively close to the fibre optical cable structure, a relatively modest magnetic field strength may generate a sufficient dimension change in the magnetostrictive material 212 so as to induce a strain of the optical fibre 101 which is readily detectable by the interrogator 102. For example in some embodiments, where the distributed fibre optic sensor determines a phase change, a modulation of the order of about 1 radian may provide a clear signal so as to allow relatively high data rates. Such a modulation can be provided by the strain induced by a magnetic field strength of about 50 mT, which can be readily generated in use using a transmitter loop 222 and a relatively small variation in current.

The communication system 200 can thus be advantageously used for receiving communication from a mobile apparatus. In particular the communication system may be advantageously used for receiving communications from submersible apparatus or vehicles.

Communication with underwater apparatus and vehicles can be problematic, especially beyond certain depths. Wired communication to the surface may not be practical for underwater vehicles that move over significant distances and wired communication to communication buoys or the like may run the risk or fouling or snagging. All but extremely long wave radio signals may be significantly attenuated at depths beyond a few metres. It thus can be problematic to receive communications from undersea vehicles and equipment.

A communication system according to embodiments of the present disclosure may be advantageously used to receive communications from underwater equipment, as illustrated with respect to figure 4. Figure 4 illustrates that a magnetically sensitised fibre optic cable 211 may run into a body of water from the surface 401. In the example illustrated in figure 4 the fibre optic cable may run from the shore where an optical fibre 101 is coupled to interrogator 201. In other examples however the interrogator could be located on some fixed or floating platform in the body of water. In the example of figure 4 the cable 211 runs down to the seabed 402 and at least part of the cable 211 may run along or be buried in the seafloor.

Some underwater equipment, in this example a submersible vehicle 403, is configured with a transmitter 220 as discussed with respect to figure 2. To transmit data the vehicle 403 may descend to the seabed in the vicinity of the fibre optic cable and generate a varying magnetic field to transmit data as discussed previously. The varying magnetic field will be incident on the magnetically sensitised cable 211 and lead to a strain on the optical fibre 101 , which can lead to a detectable modulation of the measurement signal generated by interrogator 102. The characteristics of data transmission may be identified by signal processor 103 and the relevant data signal extracted, possibly for onward transmission. In this way data may be communicated from the underwater equipment 403 to the surface via the magnetically sensitised fibre optic cable 211 in real time, but without requiring the underwater equipment to be in contact with the fibre optic cable 211.

It will be noted that instead of using a magnetically sensitised fibre optic cable it would be possible to use a standard fibre optic cable as a sensing fibre and generate a coded acoustic sequence to induce strain on the sensing optical fibre. However, to generate a sufficient strain on the optical fibre for reliable data transmission, the acoustic source may need to be physically coupled to the fibre optic cable or, especially in submarine environments, generate a large amplitude acoustic signal so as to ensure that the acoustic signals pass via the water and through the seabed to the fibre optic cable. Such large amplitude acoustic signals may be undesirable in terms of disturbing the local environment. Such large amplitude acoustic signals may also travel for a relatively long distance, which could allow the location of the equipment to be identified and/or the data transmission to be intercepted, which may be undesirable in such instances. Using a varying magnetic field avoids the need to generate large acoustic pulses and the field strength of the varying magnetic field 221 dissipates rapidly with increasing distance from the source of the varying magnetic field i.e. from transmitter 220. This may be advantageous as the data transmission encoded into the varying magnetic field is unlikely to be intercepted by a third party.

As the fibre optic sensor can be implemented with a sensing fibre that is tens of kilometres in length, the magnetically sensitised fibre optic cable 211 may be deployed to run over a large area of the area of interest, in this example the seabed, allowing mobile equipment the ability to roam over a relatively large area and still be relatively near at least part of the cable 211. Depending on the application the fibre optic cable could deployed in the area of interest in any desired pattern, whether generally straight in parts, curved in parts or in some parts arranged in a back-and-forth pattern to provide 2D coverage. In some embodiments, in use, a mobile equipment or vehicle 403 may move to part of the area of interest which is too far away from the fibre optic cable 211 for reliable data transmission. In which case, when the mobile equipment or vehicle 403 has data to transmit, it may move to close to the fibre optic cable 211 to transmit data and, after transmitting data, move on the next desired location. The location of the fibre optic cable 211 in at least part of the area of interest may thus be pre-determined and known to the mobile equipment or vehicle 403. In some implementations however it may be useful for the mobile equipment or vehicle 403 to be able to identify the present position of the fibre optic cable 211 so it can position itself optimally for data transfer. For instance it may be difficult to determine that the vehicle is correctly located with respect to the cable 211 if the cable is buried. In addition, in the underwater scenario described with reference to figure 4, in practice the position of the fibre optic cable loosely buried in silt may move over time due to the action of waves, tides etc.

In some embodiments therefore the fibre optic cable 211 may be provided with a means of identifying its location to the mobile equipment or vehicle 403. Conveniently the means of identifying the location of the fibre optic cable 211 may involve generating a magnetic field, which may be referred to as a locating magnetic field. Therefore, fibre optic cable structure 211 may be configured with at least one magnetic field generating element deployed at least partly along the length of the cable. The magnetic field generating element(s), in use, generates the locating magnetic field so as to allow the location of the cable to be determined, at least to within a suitable range for data transmission as discussed above.

One or more magnetic field generating elements may, in some embodiments, be deployed substantially along the entire length of the fibre optic cable 211. A magnetic field generating element may be deployed alongside or may be formed as part of the fibre optic cable 211. In any event, the magnetic field generating element is located with the fibre optic cable 211.

Referring back to figure 2, the fibre optic cable structure 211 may thus comprise at least one magnetic field generating element 215a, 215b that may be deployed to run along at least part of the length of the fibre optic cable structure 211. In some embodiments at least magnetic field generating element may comprise one or more permanent magnetic materials, however in some embodiments the magnetic field generating element may comprise at least one conductor 215a connected in use to an electrical source 216. Figure 2 illustrates that there may be a first conductor 215a and a second conductor 215b running along the length of the cable 211. In use the first and second conductors 215a and 216b may be connected to different terminals of the electrical source 216 at or near the end of the cable 211 coupled to the interrogator 102 and may be conductively coupled to one another at the opposite end of the cable so as to complete a circuit. In some instances the electrical source 216 could be part of the interrogator 102. In some embodiments however there may be only a single conductor, say 215a which runs the length of the cable and which, in use, is electrically connected to a defined voltage or grounded at the distal end. The first conductor 215a and/or second conductor 215b (if present) may be formed by any suitable conductive material and could for example be a wire running through the cable or could be a conductive layer, which may, in any case, be provided for some other reason. For example, some fibre optic cable designs may include an annular metallic sleeve for protection and/or some metallic braiding, that could be used as a conductor. In use the electric source 216 generates a potential across the conductor(s) so as to cause a current to flow and thus generate the locating magnetic field.

The mobile equipment or vehicle 403 may thus be provided with a magnetic field detector 404 in order to detect the locating magnetic field. It will be understood by one skilled in the art that a transmitter 220 such as illustrated in figure 2 may also be operable to detect magnetic fields via loop 222 and thus at least some of the same components used for the transmitter 220 may also be operable as part of a magnetic field detector 404.

In use the magnetic field generating element 215a will generate a locating magnetic field around the fibre optic cable 211. As the mobile equipment or vehicle 403 approaches the fibre optic cable 211 , its magnetic field detector 404 will begin to detect the effect of the locating magnetic field generated around the fibre optic cable 211.

The mobile equipment or vehicle 403 may then manoeuvre until the detected field strength is at its strongest and/or, in some embodiments, above a certain threshold - which may be set with respect to a known field strength of the magnetic field generating element. If the detector 404 of the mobile equipment or vehicle 403 can detect a magnetic field strength above a threshold from the magnetic field generating element 215a this may indicate that the varying magnetic field generated by the transmitter 220 in use will induce a readily detectable strain in the optical fibre 101 of the cable 211.

In some instances the magnetic field generating element 215a may generate the locating magnetic field as a non-varying magnetic field. In which case the magnetic field generating element could comprise a permanent magnet material, or a current carrying element may be driven with a DC current. The field strength of the non varying field could be chosen to be greater than any other magnetic field which may be expected in the area of interest in use arising from other sources.

In some instances however, in order that the magnetic field detector 404 of the mobile equipment/vehicle 403 does not mistakenly identify a magnetic field arising from other sources as the locating magnetic field, the locating magnetic field may be generated as a varying magnetic field, having a predetermined variation which is unlikely to occur form another source. The magnetic field detector 404 of the mobile equipment or vehicle 403 may thus comprise a processor which is configured to detect the locating magnetic field by detecting the predetermined variation.

In one embodiment, the magnetic field generating element 215a may generate the locating magnetic when the processor 103 does not detect any characteristics of data transmission. In other words, if the processor 103 does not detect any characteristics in the measurement signals that indicate that data transmission is underway or about to start, the processor 103 may control the electronic source 216 to generate the locating magnetic field, whether continuously or at various intervals, so as to allow a mobile equipment or vehicle to locate the fibre optic cable. If the processor 103 then detects the characteristics of data transmission, for instance a predetermined sequence indicative that data transmission is about to begin, the processor 103 may be configured to stop generating the locating magnetic field, or, in embodiments where the locating magnetic field is generated to have a predefined sequence, to swap to a non varying field.

It will be appreciated that the magnetic field generated by the magnetic field generating element 215a will also have an effect on the magnetostrictive material of fibre optic cable structure 211. It may therefore be desirable in some instances to stop generating the magnetic field from magnetic field generation element 215a once data transmission is detected to avoid any interference with the varying magnetic field generated by the transmitter 220. In some embodiments however applying a non-varying field may apply a DC bias to the magnetostrictive material that may, in some instances, improve sensitivity. This may also allow the magnetic field detector 404 of the mobile equipment or vehicle 403 to continue to determine the location of the cable 211 and/or check that the detected field strength remains above a threshold, which may be useful if the equipment or vehicle 403 is moving. In some instances the locating magnetic field may be generated, at least periodically, whilst data transmission is occurring, for instance to allow the equipment or vehicle 403 to stay close to the cable 211 when moving, in which case the processor 103 may be configured to compensate the measurement signals from the interrogator 102 based on the known variation of the locating magnetic field.

It will be appreciated that a distributed fibre optic sensor such as provided by the interrogator 102 and sensing fibre 101 is operable to provide an independent measurement signals from each of a plurality of sensing portions of the optical fibre. The communication apparatus 210 may, in some embodiments, be configured to provide localised sensing and thus the interrogator 102 may be operable to provide a measurement signal from each of a plurality of sensing portions of the optical fibre 101. In such an implementation, the sensing portion or channel for which the measurement signal corresponds to data transmission may be identified. This may be useful to allow the location of a vehicle or mobile equipment to be identified and in some instances different transmitters could transmit signals to different parts of the sensing fibre at the same time.

However, in some embodiments, in particular if the communication apparatus 210 is used purely for communication i.e. for only the detection of the data transmission and not for any other acoustic or magnetic sensing, and parallel data transmission is not expected, it may be sufficient to detect an incident varying magnetic field

corresponding to data transmission acting on any part of the optical fibre 101. As such the specific location along the length of the cable 211 may not of interest to the system 200. In such the interrogator may be configured to provide a single measurement signal from the entire length of optical fibre 101 , i.e. the entire length of the optical fibre may act as a single sensing portion, which may allow for some reduced processing overhead.

In some embodiments however it may be desirable to use the distributed fibre optic sensor not only to be able to receive data transmission, via a deliberately modulated magnetic field, but also provide sensing of at least one environmental parameter. In other words the interrogator 102 may interrogate the sensing optical fibre 101 to provide distributed fibre optic sensing for monitoring of the area of interest in addition to monitoring for data transmission.

The optical fibre 101 is coupled to the first material 212 of the magnetically sensitised cable 211 and thus will be sensitive to any magnetic field variation acting on the cable 211. A varying magnetic field 221 generated by transmitter 220 will be of interest for detecting and receiving a data transmission. However, magnetic field variations arising from other sources may be of interest and provide some useful information about the environment in which the cable 211 is deployed. Interrogation of the optical fibre 101 of the fibre optic cable 211 by interrogator 102 may thus provide a measurement signal from each of a plurality of sensing portions of the first optical fibre so as to allow localised sensing of variations in magnetic field.

Additionally or alternatively the sensing optical fibre 101 of the fibre optic cable 211 will be sensitive to strains induced by other external disturbances such as pressure waves, e.g. acoustic waves, and temperature variation, which may cause additional detectable signals in optical fibre 211. In may be desirable to provide sensing for such other disturbances acting on the fibre optic cable 211.

In some embodiments it may be desirable to discriminate any signals due to variation in magnetic fields from signals due to other disturbances, whether for sensing environmental parameters or to reduce the amount of non-magnetically induced signals so as to avoid unwanted signal components (i.e. noise) in a measurement signal prior to trying to extract any data signal component. Figure 5 illustrates fibre optic apparatus 500 that can be used as a communication apparatus 210 as discussed above and which may optionally be used for magnetic and/or acoustic sensing. The apparatus 500 includes a first optical fibre 101a which is mechanically coupled to a first material 212 with magnetostrictive properties in the same way as discussed previously. In addition a second optical fibre 101 b is deployed to run generally along at least part of the same path as the first optical fibre 101a. The second optical fibre 101b is substantially mechanically uncoupled to any

magnetostrictive material, such as the first material 212. The second optical fibre may for instance be coupled to a second material, which may be a compliant material 214 but configured such that any changes in dimension of the first material 212 do not induce any significant strain on the second optical fibre 101 b.

Both the first optical fibre 101a and the second optical fibre 101b are, in use, interrogated with optical radiation by interrogator 102 so as to provide distributed fibre optic sensing. The first optical fibre 101a will experience optical path length changes induced by incident pressure waves or vibration, temperature changes and, via the magnetostrictive material 212, variations in magnetic field. The second optical fibre 101 b will experience optical path length changes induced by incident pressure waves or vibration or temperature changes, but will be insensitive to magnetic field variations.

Thus, second optical fibre 101 b effectively acts as a control fibre. The second material 214 may, in some instances, be selected to provide similar acoustic and thermal properties as the first material 212, for instance having a similar matrix and metal loading, except second material 334 is insensitive to variations in magnetic field and comprises non-magnetostrictive material. The second optical fibre 101b may be deployed close to the magnetically sensitised optical fibre 101b so that any acoustic or thermal variations would be expected to be the same for both optical fibres. In some instances the first and second optical fibres may be deployed in first and second fibre optic cable structures 211a and 211 b but in some implementation the first and second optical fibres could be arranged as part of the same fibre optic cable structure, with suitable isolation of the second optical fibre 101b from the magnetostrictive material 212. This could be achieved by installing the fibres in a standard gel filled outer casing and/or using a specially manufactured casing which isolates the fibres from each other. In use measurement signals acquired from interrogation of the second optical fibre 101 b can thus be used as an indication of acoustic/thermal variations. These measurement signals can be used to compensate the measurement signals acquired from interrogation of the first optical fibre 101a, for instance by subtraction.

The signal subtraction could be done in various ways. The interrogator 102 could comprise two separate interrogator sub-systems 102a and 102b, one for each optical fibre. Alternatively, a single interrogation unit could be used and multiplexed between the two optical fibres 101a, 102b in a time multiplexed manner for example, i.e.

arranged to send interrogating radiation alternately down the magnetically sensitised first optical fibre 101a and subsequently down the non-magnetically sensitised second optical fibre 101b. In another arrangement, a single interrogator could be used with the sensitised fibre 101a coupled in series with the non-sensitised fibre 101b and with the fibre doubled back on itself. In this method, the doubling back of the fibre may mean some loss at the connection (if separate fibres are used and spliced together), and the range will be halved. In some embodiments, instead of two different fibres being coupled together, a single fibre could be manufactured with different coatings applied on at different points on the same fibre, so as to provide a first length of fibre which is magnetically sensitised and a second length of the same fibre which is not.

First optical fibre 101a, sensitive to variations in magnetic field will still be able to provide a communication function by responding to a varying magnetic field generated by a transmitter. The control fibre, second optical fibre 101b will not respond to variations in magnetic field and thus second fibre 101 b will not detect any data transmission, however subtracting the measurement signals generated from

interrogating the second optical fibre 101b from those generated from interrogating the first optical fibre 101a may allow a clearer detection of a signal component

characteristic of data transmission.

The measurement signals generated by interrogation of the second optical fibre 101 b can provide an indication of acoustic signals incident on the cable, free of any magnetic variations. Both fibre may be subject to thermal variations, but the system may be deployed with the optical fibres in an environment where rapid variations in

temperature are unlikely. For example in the underwater implementation discussed with respect to figure 4, rapid variations in temperature effecting the fibre optic cable 211 may be unlikely and thus thermal variations may be removed by filtering out any low frequency component.

Embodiments thus relate to a fibre optic sensor apparatus that can be used as a communication apparatus to receive data. In embodiments of the present invention a sensing optical fibre, which, in use, is interrogated with optical radiation, is coupled to a material or structure that varies in dimension or configuration with varying magnetic fields in order to induce a strain on the optical fibre. Strain induced by a varying magnetic field thus modulates the properties of the interrogating optical radiation in a way that can be detected. Embodiments thus make use of such a magnetically sensitised optical fibre as a part of a fibre optic sensing apparatus to detect deliberately generated variations in magnetic field as part of a communication system.

Embodiments may particularly be implemented using distributed fibre optic sensing techniques, and in particular Rayleigh based sensing, where backscatter from inherent scattering sites within the optical fibre is of interest. However the principles are also applicable to sensing techniques that rely on the interrogating radiation interacting with deliberately introduced features such as Fibre Bragg gratings (FBGs). FBGs act as point sensors within the optical fibre. In use a strain acting on the fibre in the vicinity of a FBG changes the grating spacing and thus varies the modulation properties of the grating in a way that can be detected. A sensing fibre provided with FBGs which is coupled to a magnetostrictive material may also be sensitive to detect varying magnetic fields.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim,“a” or“an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.

Claims

1. A communication apparatus, comprising:

a fibre optic interrogator configured to interrogate the first optical fibre with

optical radiation to provide sensing on the first optical fibre and provide at least one first measurement signal indicative of variations in applied magnetic field; and

2. The communication apparatus of claim 1 , wherein the characteristic signature is a signature indicative of data transmission according to a defined data transmission protocol.

3. The communication apparatus of claim 1 , wherein the characteristic signature corresponds to a predetermined sequence that, in use, is transmitted prior to data transmission according to a defined data transmission protocol.

4. The communication apparatus of claim 2 or claim 3, wherein upon detection of the characteristic signature the processor is configured to process the first measurement signal so as to extract a data signal according to said defined data transmission protocol.

5. The communication apparatus of any preceding claim, wherein the fibre optic interrogator is configured to provide a measurement signal from each of a plurality of sensing portions of the first optical fibre.

6. The communication apparatus of any preceding claim further comprising a second optical fibre deployed alongside the first optical fibre wherein the fibre optic interrogator is further configured to interrogate the second optical fibre to provide sensing and wherein the second optical fibre is configured to be substantially insensitive to any variation in applied magnetic field.

7. The communication apparatus of claim 6 wherein the processor is further

configured to use second measurement signals from interrogating the second optical fibre to compensate for any non-magnetically induced disturbances affecting the measurement signals from interrogating the first optical fibre.

8. The communication apparatus of claim 6 or claim 7, wherein interrogation of the second optical fibre provides distributed acoustic sensing.

9. The communication apparatus of any preceding claim wherein the first material comprises a magnetostrictive material.

10. The communication apparatus of any preceding claim further comprising a magnetic field generating element deployed along at least part of the length of the first optical fibre and is configured to generate a locating magnetic field.

1 1. The communication apparatus of claim 10, wherein the magnetic field

producing element is deployed substantially along an entire length of the first optical fibre.

12. The communication apparatus of claims 10 or 11 , wherein the magnetic field generating element is configured to generate the locating magnetic field periodically.

13. A communication system comprising the communication apparatus according to any preceding claim and a transmitter for generating a controllably varying magnetic field and operable to control the magnetic field for data transmission according to a defined data protocol.

14. A communication system of claim 13, wherein prior to transmitting data

according to the defined data protocol, the transmitter is configured to control the magnetic field in accordance with a predetermined coded sequence.

15. The communication system of claims 13 or 14 wherein the transmitter is physically uncoupled from the first optical fibre.

16. The communication system of any of claims 13 to 15, wherein the transmitter is located as part of a vehicle.

17. The communication system of any of claims 13 to 16, when dependent on any of claims 10 to 12, wherein the transmitter comprises a magnetic field detection capability for detecting the locating magnetic field.

18. The communication system of claim 17, wherein the transmitter is comprised in a transmitter apparatus, the transmitter apparatus further comprising a processor for locating the first optical fibre based on a magnetic field strength of the locating magnetic field.

19. A method of communication, comprising:

interrogating an optical fibre with optical radiation to provide sensing so as to generate measurement signals indicative of variations in applied magnetic field; and

processing said measurement signals to detect a characteristic signature

indicative of a data transmission;

wherein, the first optical fibre is coupled to a material configured to vary in

dimension in response to the variations in applied magnetic field, such that, a strain is induced in the first optical fibre.

20. The method of communication of claim 19 further comprising transmitting a characteristic varying magnetic field corresponding to the characteristic signature.