Abstract:
An apparatus for detecting and/or locating a fibre-optic cable ( 170 ) by applying a magnetic field with a component parallel to the cable ( 170 ) and detecting the cumulative rotation of polarisation of a polarised beam passing through the magnetic field multiple times. The beam is preferably input into the cable ( 170 ) multiple times, with the same polarisation and amplitude each cycle. In this case, a regeneration stage ( 800 ) is provided to recycle the beam with the correct amplitude and polarisation. A method of detecting a fibre-optic cable ( 170 ) is also disclosed.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an apparatus for detecting the position of fibre-optic cables and a method for carrying out the same.  
         BACKGROUND OF THE INVENTION  
         [0002]    There are many known methods of identification of underground objects. Many of these make use of a magnetic field generated by passage of an alternating current through the object, or the current induced by a magnetic field applied to the object.  
           [0003]    Fibre-optic cables may be protected by a metallic sheath. However, use of such a metallic sheath can cause damage to the fibres it is intended to protect, for example where a lightening strike occurs causing very high currents to pass along the cable.  
           [0004]    Therefore, it is desirable to protect the cables by using a non-metallic sheath, such as Kevlar™, in order to avoid such damage. Additionally, non-metallic sheaths are cheaper and the cost of ownership is lower. However, use of such non-metallic materials means that it is not possible by conventional electromagnectic techniques.  
           [0005]    Because many known detectors make use of metallic elements in the cable to be identified and located, they are not suitable to detect non-metallic cables. U.S. Pat. No. 6,480,635 (the entire contents of which are incorporated herein by reference) discloses the use of the so-called Faraday effect to detect fibre-optic cables underground. The Faraday effect causes a rotation in the polarisation of linearly polarised light when a magnetic field is applied in the direction of propagation of the light. If a linearly polarised beam of light is applied to one end of a fibre-optic cable to be located and an antenna is placed in proximity to the fibre-optic cable, inducing a rotation of the polarisation of the light in the cable, this rotation can be determined at the other end of the cable by measuring the polarisation state of the light as it exits the fibre.  
           [0006]    However, the Faraday effect is very weak, and the magnetic field required to be applied to obtain a noticeable rotation of polarisation state is high. The application of the Faraday effect has thus far been of limited use and success.  
           [0007]    It is an object of the invention to provide an improved optical cable detection system.  
         SUMMARY OF THE INVENTION  
         [0008]    An aspect of the invention provides a method of detecting or locating a fibre-optic cable by applying a magnetic field to the cable and detecting the cumulative rotation of polarization of a beam of polarized light passed through the cable multiple times.  
           [0009]    In an embodiment, the beam passes along the cable in both a first or outward and second or return direction. It may only pass along the cable in the outward direction with the beam returning, to pass in the outward direction along the fibre again, via a different route not within the cable. In an alternative embodiment the beam only passes through the cable in the return direction, the outward path being via a different route, not within the cable. In an embodiment where the beam travels along the cable in both directions, it may pass along the same fibre in both directions, or a different fibre in each direction.  
           [0010]    Where the beam travels along the same fibre in both directions, a polarization conjugate mirror may be provided to reflect the beam, such that, at each point on the fibre, the beam on its return path is the polarization conjugate of the beam on its outward path.  
           [0011]    Any arrangement where the beam passes through a magnetic field to have multiple rotations of the polarization of the beam, which are then detected may be used in the present invention.  
           [0012]    An embodiment of the invention provides an apparatus, for use in the detection and/or location of a fibre-optic cable, comprising a generator and a detector. The generator generates light, which is polarised and can be linearly polarised or can have any other predetermined polarisation. The generated light may be a pulse, or may be a continuous wave. A method is provided as a further embodiment, of detecting cables where a rotation of the polarization has been imparted by a magnetic field as the beam passes though the field multiple times.  
           [0013]    In another embodiment, the apparatus sends a polarized beam out along a first cable fibre and receives the beam back from a second cable fibre, which may or may not be in the same cable as the first fibre. Once again, however, the polarization of the received beam is detected for cumulative rotation of the polarization whilst in the fibre on outward and return parts of the beam&#39;s path.  
           [0014]    In one embodiment, the frequency of the magnetic field applied to the fibre is such that it generates a standing wave, as observed by a photon traversing the cable to be detected/located.  
           [0015]    More than one polarisation state may be generated, either concurrently in a single beam or successively in successive beams, and these polarisation states may be orthogonal. More than one frequency of light may be generated, again either concurrently or successively to be input into a cable to be detected or located.  
           [0016]    For the avoidance of doubt, the cable or cables do not comprise part of embodiments of the invention, but are used with the invention, and located/detected by embodiments of the invention.  
           [0017]    In an embodiment of the invention, the detector detects the polarisation state of the light from the generator. The detector may detect the absolute polarisation of the light and the difference between two orthogonal polarisations. The detector may detect the intensity of two orthogonal linear states. The detector may therefore detect the polarisation state of a beam, by detecting the projection of the polarisation onto two orthogonal axes.  
           [0018]    In embodiments of the invention, the “same” beam of light, i.e. a beam with the same physical attributes as a previous beam, is input into the cable more than once. A loop or reflector may be provided to return the beam into the cable. The polarisation state of this re-entered light in each cycle is preferably the same as the polarisation state when the beam first entered the cable. A regeneration stage or beam amplifier may be provided in order to ensure that successive beams input into the cable are of approximately equal intensity.  
           [0019]    In an embodiment of the present invention, the pulse is input into a first fibre in the cable a plurality of times and on each pass along the first fibre, a Faraday rotation is imparted by a magnetic field. In a further embodiment a Faraday rotation is also imparted as each pulse returns along a second fibre. In an embodiment, the outward and return paths are through the same fibre.  
           [0020]    The detected cumulative rotation of polarisation may be read at every cycle and processed, or may only be processed after a predetermined number of cycles of the beam.  
           [0021]    The invention may be used in a system comprising a source, a detector and an antenna, which is located remote from the source and detector and can irradiate the cable to be detected with a magnetic field with a component substantially parallel to the cable to be detected.  
           [0022]    The detector and beam input may both be at one end of a fibre cable to be located/detected. Alternatively, the detector and beam input may be at opposite ends of the cable. The light generator or source is conveniently placed at the same end of the cable as the beam input.  
           [0023]    The beam may be a pulse, and may be of varying a duration equal to the time taken for a photon to pass from the first end of the cable to the second end. Alternatively, the duration may be shorter or longer than this.  
           [0024]    Embodiments of the invention can be used for locating an underground, or otherwise inaccessible fibre optic cable.  
           [0025]    Using embodiments of the invention, the detector can indicate when an antenna irradiating the cable is in proximity to the cable. The intensity of the detected polarisation change increases as the distance between the antenna and cable reduces.  
           [0026]    Although the terms fibre optic cable and cable fibre are used in the description and claims, it will be appreciated that other optical communication or transmission media may also be used.  
           [0027]    There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto.  
           [0028]    In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.  
           [0029]    As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    Embodiments of the invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:  
         [0031]    [0031]FIG. 1 is a diagram of an apparatus according to a first embodiment of the invention;  
         [0032]    [0032]FIG. 2 is a flow chart showing the method of operation of the apparatus according to the first embodiment of the invention;  
         [0033]    [0033]FIG. 3 is a flow chart showing the operation of a generation stage according to the first embodiment of the present invention;  
         [0034]    [0034]FIG. 4 is a flow chart showing the operation of a detection stage according to the first embodiment of the present invention;  
         [0035]    [0035]FIG. 5 is a graph showing the detection response of a detector of the first embodiment of the invention;  
         [0036]    [0036]FIG. 6 shows an antenna, which can be used with the first embodiment of the invention;  
         [0037]    [0037]FIG. 7 is a flow chart showing the steps occurring in the fibre optic cable according to the first embodiment of the present invention;  
         [0038]    [0038]FIG. 8 is a diagram of an apparatus according to a second embodiment of the present invention;  
         [0039]    [0039]FIG. 9 is a flow chart showing the operation of the apparatus according to the second embodiment;  
         [0040]    [0040]FIG. 10 is a flow chart showing the operation of a regeneration stage according to the second embodiment of the present invention;  
         [0041]    [0041]FIG. 11 is a flow diagram showing the processing steps for determining the state of polarisation to be applied to pulses in the second embodiment;  
         [0042]    [0042]FIG. 12 is a Poincaré Sphere showing polarisation states in the second embodiment of the invention;  
         [0043]    [0043]FIG. 13 is a diagram showing various parameters of the second embodiment of the invention;  
         [0044]    [0044]FIG. 14 is a diagram showing a third embodiment of the present invention;  
         [0045]    [0045]FIG. 5 is a diagram showing a regeneration stage of a fourth embodiment of the present invention;  
         [0046]    [0046]FIG. 16 is a diagram showing a fifth embodiment of the present invention;  
         [0047]    [0047]FIG. 17 is a diagram showing a sixth embodiment of the present invention; and  
         [0048]    [0048]FIG. 18 is a flow diagram showing the operation of the sixth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0049]    An apparatus according to a first embodiment of the present invention is shown in FIG. 1 and comprises a source  110 , a first optical coupler  120  and a detection stage  130 .  
         [0050]    The source  110  is optically coupled to the optical coupler  120 , and generates laser light, which is input into the first optical coupler  120 . The detection stage  130  is also optically coupled to the first optical coupler  120  and receives light output from the first optical coupler  120 .  
         [0051]    The source  110  comprises a fibre Distributed Feedback (DFB) Laser  112 , a pump  114 , a Wavelength Division Multiplexer (WDM)  116  and an isolator  118 . The pump  114  generates a beam which is then passed through the WDM  116  and into the DFB laser  112  before being output from the source  110 . The isolator  118  prevents light from re-entering the source  110  which would cause lasing.  
         [0052]    Any coherent source may alternatively be utilised, but a fibre DFB laser is preferred because a narrow line width is obtained. A narrow line width (30 kHz) reduces noise signals over large lengths of fibre (for example over many hundreds of kilometres).  
         [0053]    A DFB laser is also advantageous because this means that the cable location system can be used on a live fibre, carrying communications traffic, as well as location information. In order to do this, the laser has a channel spacing of 25 GHz, as this is the current specification of a DWDM system.  
         [0054]    The pump  114  is set at a wavelength of 975 nm.  
         [0055]    Between the source  110  and the first optical coupler  120  are a polarisation controller  150 , and a first Acousto-Optic Modulator (AOM)  160  arranged in series.  
         [0056]    In the first embodiment, instead of using an output from the source  110 , which is pulsed by the first AOM  160 , the polarisation controller  150  and first AOM  160  may be omitted. If these are omitted, a continuous wave is obtained, which alternatively may be used in this embodiment.  
         [0057]    The source  110 , together with polarisation controller  150  and first AOM  160 , comprise a generation stage  180 .  
         [0058]    The first optical coupler  120  comprises four ports  121 ,  122 ,  123 ,  124  arranged in two sets of two ports  121 ,  122  and  123 ,  124 . Each port from the first set  121 ,  122  is coupled without bias to both ports from the second set  123 ,  124  and vice versa. The source  110  is connected to the first port  121 .  
         [0059]    Therefore, any light entering the first optical coupler  120  by the first port  121  is output to the third and fourth ports  123  and  124  equally, i.e. 50% exits via the third port  123  and 50% exits via the fourth port  124 .  
         [0060]    One of the second set ports  123 ,  124  of the optical coupler  120  is connected to a fibre to be located  170 . The other port of the second set of ports  124  is blanked by index matching in order to reduce reflection. The second port of the first set  122  of the first optical coupler  120  is connected to the detection stage  130 .  
         [0061]    The detection stage  130  comprises a second optic coupler  132 , which is the same as the first optical coupler  120  except that a bias is applied between each of the ports of the first set, and between each of the ports of the second set. The bias is 90/10, meaning that 90% of the light entering in one of the first set of ports of the second optical coupler  132  exits via the first port of the second set and the remaining 10% does so via the second port of the second set.  
         [0062]    The second port  122  of the first optical coupler  120  is connected to one of the first set of ports of the second optical coupler  132 .  
         [0063]    A polarisation controller  134  and a polarimeter  136  are connected to respective ports of the second set of the second optical coupler  132 . The polarisation controller  134  is connected to a Polarisation Division Multiplexer (PDM)  139 , and a differential detector  138  is attached to both outputs of the PDM  139 . The output from the differential detector  138  is sent to a processor, which is a computer, running processing software. Alternatively, the processor may be purely hardware implemented or purely software implemented or any combination thereof.  
         [0064]    The light path of a pulse generated by the apparatus of the first embodiment of the invention will now be described with reference to FIGS.  2  to  6  of the drawings.  
         [0065]    [0065]FIG. 2 shows a flow diagram of steps involved in the first embodiment of the invention.  
         [0066]    At S 210 , the generation stage  180  generates a linearly polarised laser pulse. The Pulse enters fibre to be located  170  via the First Optical Coupler  120  at S 220 . The pulse exits the fibre to be located  170  at the same end that it entered at S 230 . The pulse then enters the detection stage  130  via the First Optical Coupler  120  at S 240 . The polarisation state of the pulse is then detected by the detection stage  130  at S 250 . Alternatively, a CW laser beam may be used.  
         [0067]    [0067]FIG. 3 shows the process of the generation stage  180  in detail. This is a narrow line-width source. The pump  114  is set at a wavelength of 975 nm and feeds the fibre DFB laser  112  via the WDM  116  at S 310 . The fibre DFB laser  112  then outputs the laser light from the source  110  at S 320 .  
         [0068]    The light exiting the source  110  passes through a polarisation controller  150  at S 360 , and a first Acousto-Optic Modulator (AOM)  160  at S 380 , which opens and closes to allow pulses out of the generation stage  180 .  
         [0069]    The polarisation controller  150  and AOM  160  are arranged in series between the source  110  and the optical coupler  120 .  
         [0070]    The polarisation controller ensures that a predetermined state of polarisation is fed to the first optical coupler  120 . The first AOM  160  acts as a high-speed switch to turn on and off the light from the source  110 . The AOM  160  allows a pulse of a predetermined duration to be fed into the first optical coupler  120 .  
         [0071]    The pulse then exits the generation stage  180 , and enters the first optical coupler  120  at the first port  121 . The optical coupler splits the pulse equally, half leaving via the third port  123  and the other half leaving through the fourth port  124 . The half leaving through the fourth port  124  is discarded.  
         [0072]    The pulse exits the third port  123  and enters the fibre to be located  170 . The pulse is reflected at the far end of the fibre to be located  170  by a Faraday mirror  190 , having properties as described in more detail below.  
         [0073]    The pulse returning from the fibre to be located  170  re-enters the third port  123  of the first optical coupler  120 . The pulse exits the second port  122  of the first optical coupler, and enters the detection stage  130 .  
         [0074]    As shown in FIG. 4, the pulse entering the detection stage  130  is input into the second optical coupler  132  at S 400 . The second optical coupler  132  splits the pulse at S 402 . From here 90% of the pulse intensity is coupled to the second polarisation controller  134  at S 404  and 10% is coupled to the polarimeter  136  at S 406 .  
         [0075]    The polarimeter  136  measures the polarisation state of the pulse as it enters the detection stage  130  at S 408 . This information is used to calibrate the first polarisation controller  150  for subsequent pulses sent through the apparatus, and to control the second polarisation controller  136  at S 410 .  
         [0076]    The pulse travels through the polarisation controller  134  and the polarisation state of the pulse is adjusted (described below) at S 410 . The PDM  139  then splits the pulse into two orthogonal linear polarisations at S 412 . One half of the differential detector  138  receives and detects each of the linear polarisations at S 414  and S 416 .  
         [0077]    The polarisation of the pulse detected by the differential detector  138  can be altered by adjusting the polarisation controller  134 . FIG. 5 shows the effect of adjusting the linear component of the polarisation of the light on the signal detected by the differential detector  138 .  
         [0078]    The smallest changes in the polarisation of the pulse can be detected when the gradient of the line charting polarisation angle against voltage in the detector  138  is highest. This occurs when the angle of polarisation of the pulse coming into the differential detector  138  is set at 45° to the differential detector  138 . The polarisation controller  134  is therefore adjusted such that this condition is fulfilled using suitable software, the operation of which is described with reference to FIG. 11, below.  
         [0079]    The first embodiment of the invention can detect the location of the fibre-optic cable by the variation in polarisation of the pulse as it is detected. The variation in the pulse is induced, as shown in FIG. 6, by an antenna  600 , remote from the detection stage, which applies a modulated field to the fibre to be located  170 .  
         [0080]    The antenna  600  comprises electric dipole oscillator  610 , which produces a magnetic field that is perpendicular to the axis of the dipole along the line of the cable. The dipole  610  is aligned along the axis perpendicular to the fibre to be located  170 . A control unit  640  powered by a power supply  650  drives the dipole  610 .  
         [0081]    Further antennae  620 ,  630  may also be used; these further antennae are not essential. Where more than one antenna is used, spaced laterally to the cable, this can give more accurate positional results of the location of the cable.  
         [0082]    The field produced by the antenna  600  causes a rotation in the linear component of state of polarisation of light in the fibre to be located  170  under the antenna  600  due to the Faraday effect.  
         [0083]    The dipole(s) produce(s) a magnetic field, the line integral of which is non-zero along the axis of the cable. This ensures that the overall Faraday effect on light travelling along the cable is non-zero.  
         [0084]    However, birefringence within the fibre to be located  170  causes polarisation mode dispersion, where a first state of polarisation propagates along the fibre to be located  170  faster than a second state of polarisation, and circular birefringence, which rotates the polarisation state as the light propagates.  
         [0085]    Because of this birefringence, the state of polarisation of the light underneath the antenna will not generally be the same as the state of polarisation entered into the fibre to be located  170 . Therefore, successive pulses are input into the fibre to be located  170  from the generation stage  180 . Three different orthogonal polarisation states are input into the fibre to be located  170  from the generation stage  180 . These states are generated using the polarisation controller  150 .  
         [0086]    If these three orthogonal states are entered into the fibre to be located  170  sequentially, then one of the states will be at least 67% linearly polarised directly under the antenna  600 . This is because if the three states are orthogonal, the worst match with the polarisation under the antenna  600  is that all three states are 45° relative to the magnetic field under the antenna  600 , giving a projection of (sqrt.2)/2 onto the axis of the fibre to be located  170 .  
         [0087]    Assuming that one of the three pulses input into the fibre to be located  170  by the generation stage  180  is suitably linearly polarised under the antenna  600 , the angle of polarisation of the pulse travelling along the fibre to be located  170  is rotated by the modulated magnetic field. The extent of the rotation is proportional to the magnetic field applied to the fibre to be located  170 .  
         [0088]    The closer the antenna is to the fibre to be located  170 , the larger the angle of rotation of the polarisation state. If the antenna  600  is off, or sufficiently far away, no rotation occurs due to the antenna  600  and a control reading can be taken. In this way, the location of the fibre to be located  170  can be established by observing the rotation of the polarisation of the light while the antenna is moved. The detector of the embodiment may further provide feedback to the antenna in order to enable a user of the antenna to quickly locate the fibre to be located  170 . The feedback may be by radio carrier.  
         [0089]    It has already been stated that the Faraday effect is very weak, and it is made weaker by the fact that the magnetic field attenuates between the antenna  600  and the fibre to be located  170 .  
         [0090]    The pulse of light in the fibre to be located  170  is therefore passed back along the fibre to be located  170 , where it is rotated by the magnetic field a second time. It is important that the rotation of the state of polarisation on the return path of the pulse is constructive with the rotation given on the outward path, rather than destructive. The state of polarisation of the pulse changes along the fibre to be located  170 . Therefore, a means of ensuring the polarisation state of the pulse as it passes under the antenna  600  on the return path must be used.  
         [0091]    The Faraday mirror  190  is used in order to obtain the orthogonal polarisation state at the point the pulse passes the antenna  600  when it is travelling in the opposite direction. Alternatively, any other suitable polarisation conjugate return device may be used instead of a Faraday mirror. The mirror  190  inputs back into the fibre to be located  170  light that is the conjugate of the light it receives. At every point along the fibre to be located  170 , the light travelling in one direction is the conjugate of the light travelling in the other. Because of the opposing direction of the outward and return paths, a constructive Faraday effect is achieved as the pulse returns along the return path in the fibre to be located  170 .  
         [0092]    In order to ensure that the Faraday effect is additive on the second pass, the excitation frequency should be chosen such that the wavefront must be seen by the light propagating in the fibre to have the same phase when passing in both directions.  
         [0093]    The detection stage  130  receives a pulse that has twice the rotation applied to it that would have been applied if the detection stage  130  were at the second end of the fibre to be located  170 .  
         [0094]    In one round trip, the light in the pulse travels twice the optical length of the fibre to be located  170  together with any additional optical length introduced by the Faraday mirror  190  and the polarisation controller and detector. This overall length is called the cavity length.  
         [0095]    The allowed magnetic field excitation frequencies that can be used are determined by the cavity length. This is because a standing wave must be set up to ensure that the same phase occurs when travelling in each direction. The cavity length can be determined by simply timing how long it takes for a pulse of light to travel through the optical cavity.  
         [0096]    However, the situation is further complicated in that a standing wave solution generates a series of peaks and nulls. In order to be able to detect the cable at every point along its length, there must be frequency diversity in the excitation. The antenna control unit  640  is then tuned to suitable frequencies, which generate standing waves in the optical cavity without substantial nulls.  
         [0097]    [0097]FIG. 7 shows a flow diagram of the pulse travelling through the fibre to be located  170 . The pulse enters the fibre to be located  170  at S 700 , and travels along the fibre to be located at S 702 . The pulse travels through the magnetic field applied by the antenna  600  at S 704  and is rotated at S 706 . The pulse then continues along the fibre to be located  170  at S 708  and is reflected at the far end of the fibre to be located  170  at S 710 .  
         [0098]    The conjugate of the pulse then re-enters the fibre to be located  170  at S 712  and travels through the magnetic field for a second time at S 714 . The Faraday rotation of polarisation is doubled by this at S 716  and the pulse continues along the fibre to be located  170  at S 718  until it exits the fibre to be located  170  at S 720 .  
         [0099]    A second embodiment of the invention will now be described. The second embodiment is a variation on the first embodiment described above, and like parts will retain the same numbering as in the first embodiment.  
         [0100]    [0100]FIG. 8 shows the apparatus of the second embodiment of the invention together with the fibre to be located  170 . The generation stage  180  and detection stage  130  of the second embodiment correspond to those of the first embodiment and no further explanation of these will be given.  
         [0101]    The detection stage  130  is arranged differently to the first embodiment, in that it is connected to the fourth port  124  of the first optical coupler  120 , rather than the second port  122 .  
         [0102]    A regeneration stage  800  is connected to the first port  121  of the first optical coupler  120  in the second embodiment. The regeneration stage  800  comprises a first circulator  810 , a Fibre amplifier  820 , a second circulator  830 , a tuned grating  840 , a second AOM  850  and a third polarisation controller  860 .  
         [0103]    The first circulator  810  has three ports. The second port  814  connects to the second port  122  of the first optical coupler  120 . The third port  816  connects to the fibre amplifier  820 , and the first port  812  connects to the third polarisation controller  860 .  
         [0104]    The fibre amplifier  820  comprises a pump  822  and WDM  824 , as in the source  110 . Instead of a laser, however, an erbium-doped fibre section  826  is provided. The output of the fibre amplifier is connected to the second circulator  830 .  
         [0105]    The second circulator  830  has three ports. The first port  832  is connected to the output of the fibre amplifier  820 . The second port  834  is connected to the tuned grating  840 , and the third port is connected to the second AOM  850 .  
         [0106]    The AOM  850  is connected in series between the second circulator  830  and the third polarisation controller  860 .  
         [0107]    The method of regeneration in the regeneration stage  800  will now be described with reference to FIG. 9.  
         [0108]    At S 900  the pulse enters the regeneration stage from the fibre to be located  170  via the first optical coupler  120 . The pulse travels through the first circulator  810  from the second port  814  to the third port  816  at S 902  and enters the fibre amplifier  820 . The fibre amplifier amplifies the pulse as it travels through it at S 904 .  
         [0109]    However, the fibre amplifier  820  will also emit undesired broadband spontaneous emission in addition to the amplified signal, which itself is amplified. Therefore, the pulse then enters the first port  832  of the second circulator  830  and exits the second port  834  of the second circulator  830  to the tuned grating  840 . The grating  840  is a narrow band fibre grating corresponding to the channel spacing. This grating  840  filters out any undesired amplified spontaneous emission at S 906  and reflects the desired signal.  
         [0110]    The filtered pulse then re-enters the second port  834  of the second circulator  830  and exits from the third port  836  of the second circulator  830 . The pulse can then be stopped at any time by closing the second AOM  850 . The polarisation of the pulse can be adjusted by the third polarisation controller  860 , before the pulse enters the first port  812  of the first circulator  810 , where it is output from the second port  814  of the first circulator  810  back into the first optical coupler  120 .  
         [0111]    The overall path taken by a pulse of light in the second embodiment will now be described. FIG. 10 shows a flow diagram giving the overall process of the light path.  
         [0112]    The pulse is generated at S 1000  by the generation stage  180 . The pulse is then split by the first optical coupler  120 . Half of the pulse enters the fibre to be located  170  at S 1010 . The other half of the pulse enters the detection stage  130  at S 1020 .  
         [0113]    The pulse that entered the fibre to be located  170  is reflected by the faraday mirror  190  at the far end and exits the fibre to be located  170 . This pulse re-enters the first optical coupler  120 .  
         [0114]    The pulse from the fibre to be located  170  then enters the regeneration stage  800  at S 1030 . The amplified pulse is returned from the regeneration stage  800  at S 1040  and is split by the first optical coupler  120 , with half of the amplified pulse re-entering the fibre to be located  170  and the other half entering the detection stage  130  at S 1050 .  
         [0115]    Multiple passes are sent down the fibre to be located  170  in the same way as in the first embodiment, in order to provide different orthogonal polarisation states. However, because of the regeneration stage, the same pulse, with a Faraday effect rotation already introduced, can be re-inputted into the fibre to be located  170 . It is therefore possible to obtain much increased rotation of the polarisation state.  
         [0116]    However, in order to achieve increased rotation of the polarisation state, the Faraday effect introduced on each pass must be constructive, rather than destructive. Assuming that the fibre to be located  170  has constant properties, this can be done by ensuring that the polarisation state of the pulse is the same every time it enters the fibre to be located  170  as it was the first time it entered.  
         [0117]    Therefore, on the first pulse, the polarisation state of the pulse is measured by the polarimeter  136  of the detection stage  130  before it has passed through the fibre to be located  170 .  
         [0118]    Subsequent passes can then compared with the first pass, and the third polarisation controller  860  can be adjusted, by use of suitable feedback software, so that the regeneration stage  800  acts as a mirror for the pulse such that each time it enters the fibre to be located  170  it does so with the same initial polarisation state and same initial power.  
         [0119]    In order to determine a suitable state of polarisation to be applied to pulses input into the cavity, a calibration process is carried out. FIG. 11 shows a flow diagram of the calibration process. A pulse is launched into the optical cavity at S 1100 . The regeneration stage is configured to allow N passes through the optic system at S 1102  in one series.  
         [0120]    On each pass through the system, the state of polarisation of the pulse is measured and stored at S 1104  before being regenerated and relaunched into the cavity at S 1106 . S  1104  and S 1106  are repeated N times. The first pulse in the series is ignored at S 1108 .  
         [0121]    The processor then calculates the axial centre generated by the remaining N-1 pulses, as shown in FIG. 12, at S 1110 . The processor calculates the spin of the pulses at S 1112 , and the sequence is then repeated M times, with the spin of the axis recalculated and averaged on each repeat with the previous spins.  
         [0122]    The processor then calculates the opposite axis to the averaged axis at S 1114 , and the opposite axis of polarisation is applied to the pulses re-launched into the cavity at S 1116 .  
         [0123]    [0123]FIG. 12 shows a Poincaré Sphere showing the typical polarisations of a successive set of regenerated pulses. The Poincaré Sphere shows all linear polarisations around the equator of the sphere with right handed polarisations in the upper hemisphere and left handed polarisations in the lower hemisphere with full circular polarisation at the poles. The actual polarisation is generally a combination of such orthogonal polarisation states and is at a point on the surface of the sphere. Parts lying within the sphere denote partially polarised light.  
         [0124]    Point 1 shows the initial polarisation i.e. the first captured pulse. This polarisation state is determined simply by the first polarisation controller  150  and the birefringence between it and the polarimeter  136 .  
         [0125]    This state evolves as it travels through the optical cavity and regeneration stage to pulse 2 shown at point 2. Successive regeneration pulses each experience the same evolution and describe a circle on the surface of the Poincaré Sphere (points 3, 4, 5 and 6). The centre of this circle corresponds to the eigen-axis of the fibre to be located  170  and regeneration stage  800 .  
         [0126]    By measuring the polarisation of this centre and the amount of rotation about it in each successive regeneration pulse it is possible to adjust the third polarisation controller  860 , in the regeneration stage  800 , to compensate for this birefringence, by adjusting the polarisation controller  860  such that it has the same eigan-axis but with the opposite spin. When this is done, each regeneration pulse entering the fibre to be located  170  will do so with the same polarisation state, so enabling constructive Faraday modulation on each pass.  
         [0127]    Although the pulse length is set to be substantially equal to the cavity length, Rayleigh backscattering will give rise to noise. As the pulse propagates along the fibre to be located  170 , a small proportion is backscattered. This backscattering is at the same wavelength as the source, and so is amplified in the regeneration stage  800 . This, over multiple passes is a substantial source of noise.  
         [0128]    This noise can be reduced by reducing the pulse length to half of the cavity length. In this way, some backscattered light can be removed from the cycle by closing the second AOM  850  when the pulse is travelling away from the apparatus, as only backscattered light will be received in this time, and reopening the second AOM  850  in the second half of the cycle, when the pulse is returning from the fibre to be located  170 .  
         [0129]    The recycling of the pulse can be continued for any number of cycles until the noise becomes too great, or a result is obtained. The second AOM  850  is then closed and the pulse is dumped. A fresh pulse can then be generated by the generation stage and the process started again.  
         [0130]    The amplification of the second embodiment will now be described. FIG. 13 shows various timing parameters during successive cycles of pulses through the apparatus and fibre to be located  170 .  
         [0131]    In order to use the second embodiment of the invention, the pulse is sent through the fibre to be located  170  a predetermined number of times. On the final cycle, the processor processes the rotation of polarisation detected by differential detector  138 . The pulse is then removed from the system by closing the second AOM  850  as described above.  
         [0132]    A new pulse is then generated by opening the generation stage  180  by opening the first AOM  160  and allowing a pulse with length equal to the cavity length out of the generation stage  180 , before re-closing the first AOM  160 .  
         [0133]    This process is then repeated in order to provide successive results.  
         [0134]    In order for the system to operate correctly, the net gain for each cycle of a pulse as it is regenerated and between successive pulses must be set to approximately 1. If a gap is left between pulse regeneration or between successive pulse trains, for example, if the first and second AOMs  160 ,  850  are both shut for a period, then there will be an impulse in the amplifier gain, due to the time that the pulse was allowed to “charge” in the amplifier during this period. If both AOMs  160 ,  850  are left open at the same time then lasing will occur.  
         [0135]    The timing should therefore be set such that no gaps are left, and no overlap occurs, between regeneration cycles and between successive pulse trains. The amplifier gain can then be controlled by a computer such that the amplitude of the pulses remains constant.  
         [0136]    [0136]FIG. 13 shows the opening and closing of the first and second AOMs  160 ,  850 .  
         [0137]    The first pulse carries no information regarding the fibre to be located  170 , as it has not yet travelled down it. However, it contains initial polarisation data. The first regeneration of the pulse carries twice the Faraday modulation, the second regeneration pulse carries four times the Faraday modulation and so on. The pulse is detected by the polarimeter  136  in the first half of every cycle of the pulse along the fibre to be located  170  and back to allow for feedback into the regeneration stage  800 .  
         [0138]    The pulses have progressively higher initial intensities (which have been exaggerated in the Figure) due to noise increases in successive regeneration pulses.  
         [0139]    The detected results from all of the regeneration pulses can be recorded. However, in this embodiment, only the results from the sixth pulse are taken from the differential detector and processed by the processing software. The sixth pulse will carry 10 times the modulation of a single pass. If data from the 100 th  pass is processed, the modulation carried will be 198 times the modulation from a single pass.  
         [0140]    [0140]FIG. 14 shows a regeneration stage  1400  according to a third embodiment of the present invention. The other elements of the embodiment are the same as those of the second embodiment and so are not shown. Additionally, a third AOM is placed at the far end of the fibre to be located before the Faraday mirror.  
         [0141]    The regeneration stage  1400  of the third embodiment is similar to that of the second embodiment. First circulator  1410 , amplifier  1420  (shown only schematically), second circulator  1430 , tuned grating  1440  and polarisation controller  1460  all operate in the same way as the corresponding units of the second embodiment.  
         [0142]    However, the second AOM  1450  of the regeneration stage is placed between the first optical coupler  120  and the first circulator  1410 . A delay loop  1470  is placed between the polarisation controller  1460  and the first circulator  1410 . The delay loop  1470  has an optical delay corresponding in length to the optical length of the fibre to be located  170 .  
         [0143]    The second AOM  1450  selectively allows pulses returning from the fibre to be located into the regeneration stage. The second and third AOMs  1450 ,  1480  are controlled such that Rayleigh backscatter is reduced by not allowing it to enter the regeneration stage  1400 . By selectively opening the second and third AOMs  1450 ,  1480 , Rayleigh backscatter from the pulse as it travels in both directions can be reduced.  
         [0144]    [0144]FIG. 15 shows an alternative regeneration stage  1500  of a fourth embodiment of the invention. As in the third embodiment, a third AOM is placed at the far end of the fibre to be located  170  before the Faraday mirror  180 .  
         [0145]    In this embodiment, the delay loop  1570  is placed between the second circulator  1530  and the tuned grating  1540 . This configuration provides an advantage that the regeneration stage  1500  is acoustically insensitive.  
         [0146]    In all of the above embodiments, the fibre to be located  170  may actually comprise two fibres optically linked at the end distal to the apparatus. The pulses may enter the first fibre and travel to the distal end before being routed into the second fibre to follow a return path back towards the apparatus. The Faraday mirror can then be placed at the second end of the second fibre to reflect the light back to the apparatus through both fibres. An advantage of this is that the pulse travels underneath the antenna a total of four times for each round trip, so doubling the Faraday rotation of the pulse compared with the rotation when the Faraday mirror is simply placed at the far end of a single fibre.  
         [0147]    In a fifth embodiment, the above two fibre system is altered so that, instead of using a Faraday mirror at the second end of the second fibre, the second fibre is directly connected to the first optical coupler  1620 . Such an embodiment is shown in FIG. 16. The generation stage  180  and the regeneration stage  800  are the same as the second embodiment, and are shown only schematically.  
         [0148]    The first fibre  1670  is connected to the first optical coupler  1620  as in previous embodiments. The distal ends of the first and second fibres are optically coupled with an optical link  1640 . The detection stage of the second embodiment is moved, and the second fibre  1675  is connected to the first optical coupler  1620  in its place. The detection stage is connected via a third optical coupler  1650 . The first optical coupler of the fifth embodiment differs from that of the previous embodiments in that the fourth port  1624 , to which is connected the second fibre  1675 , does not receive pulses generated by the generation stage  180 , but only inputs pulses which have travelled around the fibre loop. This is achieved by modifying the coupler  1620 . Alternatively, an optical isolator may be placed between the third optical coupler  1650  and the fourth port  1624  of the first optical coupler  1620 .  
         [0149]    The detection stage  1630  is the same as the first and second embodiments other than its location, and has been shown only schematically.  
         [0150]    [0150]FIG. 17 shows a sixth embodiment of the present invention. The sixth embodiment is similar to the fifth embodiment in that two fibres  1770 ,  1775  are used, and both the first and second fibres are connected to the first optical coupler  120  in order to form a closed loop configuration.  
         [0151]    However, in the sixth embodiment, the arrangement of the detection and regeneration stages is different from that of the fifth embodiment. The generation stage  180  is connected to the first port  121  of the first optical coupler. The first fibre  1770  is connected to the third port  1723  of the first optical coupler  1720  and the second fibre is connected to the fourth port  1724 , as in the fifth embodiment. A further optical coupler  1710  is connected to the second port of the first optical coupler  1720 .  
         [0152]    The further optical coupler  1710  has a detection stage  1730  as described in the first and second embodiments connected to it, and is therefore shown only schematically, and also a tuned grating  1750 .  
         [0153]    The second fibre  1775  is connected to a regeneration stage. The regeneration stage comprises an AOM  1760 , an amplifier  1780  and a polarisation controller  1790 . The AOM switches to reduce Rayleigh backscattering, as described above. The amplifier is of the same form as the amplifier in the detection stage of the second embodiment, and is therefore shown only schematically. The polarisation controller has the same function as in the second embodiment and will not be described further.  
         [0154]    [0154]FIG. 18 shows a flow diagram according to the sixth embodiment of the invention.  
         [0155]    A pulse is generated in the generation stage  180  at S 1800 . The pulse travels through the first coupler  1720  and into the first fibre  1770  at S 1802 . The pulse travels along the first fibre, and has a rotation of its polarisation imparted on it by a magnetic field at S 1804 . The pulse is routed into the second fibre  1775  and returns along the second fibre  1775  where a further constructive rotation of its polarisation is imparted on it by the magnetic field on its return path at S 1806 .  
         [0156]    The pulse is switched by AOM  1760  at S 1808 , and amplified by the amplifier  1780  at S 1810 . The polarisation of the pulse is adjusted by the polarisation controller  1790  at S 1812 .  
         [0157]    The pulse returns from the second fibre  1775  into the first optical coupler  1720  at S 1814  and is routed into the further optical coupler  1710  at S 1816 . The amplified spontaneous emission is removed by the tuned grating  1750  at S 1818 . The pulse then re-enters the further optical coupler  1710  where it is routed both into the detection stage  1730  to be detected and to re-enter the first fibre  1770  at S 1820 .  
         [0158]    Alternatively, the fifth and sixth embodiments, the two fibres may be in separate cables, which follow differing physical paths. The Faraday rotation is only imparted onto the pulse in the first fibre  1670 ,  1770  to which is applied the magnetic field and no Faraday rotation is imparted in the second fibre  1675 ,  1775  to which no magnetic field is applied. In this case a single rotation is imparted in each cycle of the pulse. These embodiments provide multiple Faraday effect rotation.  
         [0159]    The present invention has been described purely by way of example and modifications can be made within the spirit of the invention. The invention also consists in any individual features described or implicit herein or shown or implicit in the drawings, or any combination of any such features, or any generalisation of any such features or in combination. Each feature disclosed in the specification, including the abstract, claims, and drawings, may be replaced with an alternative feature or features serving the same, equivalent or similar purpose, unless expressly stated otherwise. For example, differential detector  138  need not be a differential detector. Also, AOMs need not be used; other types of switches may also be used. Those skilled in the art will realize the different possible individual features possible in carrying out the invention and will not be limited to the embodiments described herein.  
         [0160]    The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.