GB2186683A - Optical fibre intrusion detector - Google Patents

Abstract

An optical fibre intrusion detector for detecting the general position of an intruder on a perimeter to be protected includes first and second optical transmission highways 13,14 for alternately receiving respective portions of optical fibres 5,6,7 and 8 in coded sequence along respective sectors of the perimeter A,B,C, etc, the first transmission highway 13 being responsive to disturbances and the second highway 14 being substantially isolated from both the perimeter and the first highway. Means such as a Gray code are used for identifying which fibre or fibres are disturbed to thereby indicate at 15 which sector of the perimeter is being disturbed by an intruder. In the first highway 13 the fibres are located in co- operating channels formed in top and bottom layers of resilient plastics material. Undulations 24 are provided along the channels to dynamically deform the fibres in response to a disturbance by an intruder at any point along the perimeter. <IMAGE>

Description

SPECIFICATION Optical fibre seismometer This invention relates to an optical fibre seismometer of the type which can be used in an intruder alarm system.

It has been known for many years that optical fibres can be used in e.g. acoustic monitoring arrangements which are particularly suitable for use as perimeter intrusion warning systems. Such known arrangements usually take the form of one or more fibres laid around a perimeter to be protected, usually below ground level, and light from a laser or other coherent light source is directed along the fibre. Disturbances caused by e.g. an intruder walking over the area to be protected change the light transmission properties of the fibre in that area and this change in light transmission is detected to give an alarm condition. An example of this type of arrangement is shown in UK patent 1497995, the disclosure of which is incorporated herein by reference, in which a pair of optical fibres are closely spaced together and laid around an area to be protected.Light from a light source is transmitted simultaneously down both fibres in opposite directions and separate detectors are used to detect the optical signals transmitted through each fibre. In this way, it is possible to determine the relative time delay between acoustic modulations in the optical fibres caused by the presence of e.g. an intruder and thereby calculate the position of the intruder in the perimeter.

In another arrangement as shown in UK patent application 2046437A, now withdrawn, the optical seismometer takes the form of an intruder alarm system comprising a length of multimode optical fibre supplied with light from a coherent light source such as a laser, which fibre produces a speckled pattern when displayed upon a detector, the speckled pattern being the result of modal interference within the multimode fibre. Whenever deformation of the optical fibre occurs by even a small amount the optical path length within the fibre changes and this in turn correspondingly changes the speckled pattern falling onto the detector. This change in pattern is used to indicate the presence of e.g. an intruder, although no method is disclosed as to how it is possible to detect the position of the intruder as well as his presence.

It is an object of one aspect of the present invention to provide an optical seismometer suitable for use in intrusion detection systems in which the position of an intruder can be readily and relatively accurately ascertained. It is an object of a second aspect of the present invention to provide an optical seismometer which is relatively false alarm free and yet which is also sensitive enough to detect the presence of even light-weight intruders.

According to a first aspect of the invention there is provided an optical fibre seismometer comprising a coherent light source, optical fibres for positioning around a perimeter to be protected, detector means for detecting light from the light source transmitted by the optical fibres, monitoring means for monitoring changes in said detected light as a consequence of a disturbance indicative of the presence of an intruder on the perimeter to thereby generate an alarm condition, and means for detecting the general position of such an intruder on the perimeter, which means comprises first and second optical fibre transmission highways for alternately receiving respective portions of the optical fibres in coded sequence along respective sectors of the perimeter, the first said transmission highway being responsive to such a said disturbance, the second highway being substantially acoustically isolated from the perimeter and first highway, and means for identifying which fibre or fibres are disturbed, thereby to indicate which sector of the perimeter is being disturbed and generating the alarm condition.

The invention according to the first aspect therefore provides means whereby only a portion or portions of each length of optical fibre is exposed to vibration and/or pressure disturbance around the perimeter, the remaining part of each fibre being substantially isolated by the second transmission highway. By alternating the positions in which adjacent fibres are exposed or isolated in coded sequence along the perimeter, it is possible to define sectors of the perimeter and hence identify the general position of an intruder.

Conveniently, the first and second optical fibre transmission highways carry four optical fibres which are alternately and sequentially received by each in turn. The location of each fibre, being known, is thus used to codify the perimeter into sectors, each having a unique combination of fibres. Thus, for example, if four fibres are used, the first transmission highway could carry over a first sector of the perimeter only the first optical fibre, the second, third and fourth fibres being isolated within the second transmission highway. A second sector of the first highway could carry the first and second optical fibres, with the third and fourth fibres being carried by the second transmission highway etc. and so on.

Preferably, a Gray code is used in which case up to fifteen sectors could be uniquely identified within a given perimeter having only four optical fibres for light transmission therearound.

Where it is convenient to be able to utilise spare combinations of fibres, such as where all of the fibres occupy a single sector in the first or second transmission highways, this permutation can be used to indicate a disturbance or instability in the coherent light source, which would be transmitted simultane ously along the four fibres. Thus, when four fibres are used, although it is theoretically possible to have fifteen sectors, using the Gray code referred to above, in practice only thirteen sectors would preferably be used. The sector in which all four fibres are present in the first data highway being used to monitor the stability of the light source. The remaining sector is preferably not used in order to maintain a Gray code sequence.

According to a second aspect of the invention, there is provided an optical fibre seismometer comprising a coherent light source, an optical fibre or fibres for positioning around a perimeter to be protected, detector means for detecting light from the source transmitted by the optical fibre or fibres, and monitoring means for monitoring changes in said detected light as a consequence of a disturbance indicative of the presence of an intruder on the perimeter, the fibres being at least partially housed within a cable, disposed around the perimeter, incorporating undulations adapted to dynamically deform the or each fibre in response to a disturbance from an intruder, at any point along the perimeter.

Conveniently, according to the second aspect of the invention, the seismometer is provided with at least one transmission highway made up of a substantially flat cable, within which is sandwiched the optical fibre. The undulations inside the cable may be in the form of corrugations adapted to compress or kink the fibre or fibres in response to vibrations and/or pressure. An alternative to a flat cable construction is a tubular construction having undulations on the inner side against which the optical fibres can be sandwiched.

Conveniently, the cable construction of the optical seismometer according to the second aspect of the invention may incorporate spacers which prevent the undulations from contacting the or each fibre when the cable is unstressed or unpressurised externally. This arrangement ensures that when, for example, the cable or cables are buried below ground the pressure of soil above the cable does not unduly deform the or each optical fibre but is still sufficient to couple satisfactorily with the or each fibre. This feature is useful, when, for example, the perimeter extends over a large distance where it is important for the or each optical fibre to be largely unstressed so as to minimize light attenuation and ensure that a good signal is received by the detecting means.

The invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic view of a seismometer installed underground, Figure 2 is a table showing the code used in the seismometer of Figure 1.

Figure 3 is an enlarged view of the cable construction used in part of the seismometer of Figure 1.

Referring firstly to Figure 1, a seismometer according to one embodiment of a first aspect of the invention comprises a coherent light source in the form of a solid state laser 1 which is buried underground and powered by an external electrical supply via a surface mounted control box 2, to which it is connected by wires 3,4. The control box 2 includes a filter circuit which is used to suppress electrical power surges from the external electrical supply which could otherwise damage the laser 1.

Optically connected to laser 1 are four optical fibres 5,6,7 and 8 which each extend along a perimeter to be protected, each fibre terminating at a respective photo-detector 9, 10, 11 and 12 at the other end of the perimeter which is also buried underground. The perimeter itself is divided into a total of 13 sectors (only 7 of which are shown) marked A, B, C, D, E, F, G, and so on.

Each of the fibres 5,6,7 and 8 are carried along the perimeter underground by respective first and second transmission highways 13, 14. Transmission highway 13 is a flexible elongate cable having undulations which serve to acoustically couple the fibres contained therein to the ground within which the first highway 13 is buried.

The second highway 14 is buried below the first highway 13 and is acoustically isolated from it and the surrounding ground in which it is buried. The second highway 14 is a tube which has strengthening members in the walls thereof, the tube loosely receiving the fibres, 5,6,7 and 8 at various positions along the perimeter.

The fibres 5,6,7 and 8 are distributed between the first and second highways along the perimeter in a specified manner as shown in Figure 2. In that part of the perimeter shown, defined by sectors A, B, C, D, E and F, fibre 5 is received in the first transmission highway only at sectors A, B, E and F, it being received in the second transmission highway 14 at sectors C and D. Fibre 6 is received in the first transmission highway 13 at sectors B, C, D and E and in the second transmission highway 14 at sectors A and F.

Fibre 7 is received in the first transmission highway 13 at sectors D, E and F and is received in the second transmission highway 14 at sectors A, B and C. Fibre 8, in this part of the perimeter, is received in sectors, A, B, C, D, E and F wholly in the second transmission highway 14. The code used to define each sector of the perimeter is known as a Gray code. In this manner each sector is uni quely identified by a combination of cables being received in the first and second transmission highways.

Surface mounted above each of the photodetectors 9, 10, 11 and 12 is a display box 15 which in this simplified embodiment of the invention has 4 illuminable panels 16, 17, 18 and 19 each corresponding, respectively, to photo-detectors 9, 10, 11 and 12.

In operation, laser 1 provides a light source suitable for transmission along each of the fibres 5, 6, 7 and 8, which light is detected respectively by photo-detectors 9, 10, 11 and 12. The laser 1 provides a steady state level of light for transmission along the optical fibres and this steady-state level is monitored by the photo-detectors 9, 10, 11 and 12 to thereby indicate that the perimeter is clear.

However, when an intruder is present on the perimeter in one sector, the disturbance caused by pressure and/or vibrations due to his presence is transmitted only to the or each optical fibre present in the first transmission highway in the same sector. Thus, in the drawing it will be seen that the illuminable panel 16 on the display box 15 has been activated and this is indicative of a disturbance being applied only to optical fibre 5 which disturbance is detected by photo-detector 9. Since, in the arrangement shown, there is only one sector along the perimeter in which this could happen i.e. sector A, the display indicates that an intruder is present on that part of the sector. If, instead, the intruder were present on sector B, illuminable panels 16 and 17 would become activated. If the intruder were present on Sector C, only panel 17 would become activated and so on.

A refinement (not shown) to this simplified arrangement is to pass the signals received from the four fibres through a decoder first, and then to a display which shows directly the sector at which the fibre or fibres, as the case may be, have been disturbed.

Referring now to Figure 2 of the drawings, there is shown a part-sectional perspective view of the first transmission highway 13 which comprises two facing layers 20, 21 of a resilient plastics material on the inner face of each of which are longitudinally extending ridges 22, the space between adjacent pairs of ridges 22 defining a channel 23. Within each channel 23 of each layer 20, 21 are undulations in the form of corrugations 24 set transversely to the longitudinal ridges 22. An optical fibre 5 is shown disposed within one of the channels 23 and is attached to the top layer 20 but not the bottom layer 21. Because the height of each ridge 22 within each channel 23 is greater than the corresponding height of each corrugation 24, a gap exists between the fibre-optic 5 and the corrugations in the bottom layer 21.Ridges 22 act as spacers which ensure that when the cable 13 is in an unstressed condition, i.e. not pressurised externally, the respective corrugations 24 do not act to compress the optical fibre 5.

In operation, the cable 13 is laid flat around a perimeter to be protected within a trench, typically around 6" below surface level. The trench is then filled in with in-fill such as soil or aggregate and it is the weight of this in-fill that compresses the ridges 22 in the cable to such an extent that corrugations 24 begin to compress the fibre-optic cable. The cable is constructed so that the weight of in-filling is just sufficient to permit acoustic coupling of the or each optical fibre but is not sufficient to severely distort the or each fibre, which, over a perimeter extending several kilometers, would severely attenuate the light from the light source. This could result in little or no light being transmitted to the detectors at the other end of the perimeter.

Claims (10)

1. An optical fibre seismometer comprising a coherent light source, optical fibres for positioning around a perimeter to be protected, detector means for detecting light from the light source transmitted by the optical fibres, monitoring means for monitoring changes in said detected light as a consequence of a disturbance indicative of the presence of an intruder on the perimeter to thereby generate an alarm condition, and means for detecting the generate position of such an intruder on the perimeter, which means comprises first and second optical fibre transmission highways for alternately receiving respective portions of the optical fibres in coded -sequence along respective sectors of the perimeter, the first said transmission highway being responsive to such a said disturbance, the second highway being substantially acoustically isolated from the perimeter and first highway, and means for identifying which fibre or fibres are disturbed, thereby to Indicate which sector of the perimeter is being disturbed and generating the alarm condition.

2. A seismometer according to claim 1 in which the first and second optical fibre transmission highways carry four optical fibres which are alternately and sequentially received by each in turn.

3. A seismometer according to claim 1 or claim 2 in which a Gray code is used to uniquely identify sectors of the perimeter.

4. A seismometer according to any preceding claim in which a spare combination of fibres is used, such as where all of the fibres occupy a single sector in the first or second transmission highways, the arrangement being such that this permutation is used to indicate a disturbance or instability in the coherent light source itself, which would be transmitting simultaneously along the fibres.

5. An optical fibre seismometer comprising a coherent light source, an optical fibre or fibres for positioning around a perimeter to be protected, detector means for detecting light from the light source transmitted by the optical fibre or fibres, and monitoring means for monitoring changes in said detected light as a consequence of a disturbance indicative of the presence of an intruder on the perimeter, the fibres being at least partially housed within a cable, disposed around the perimeter, incorporating undulations adapted to dynamically deform the or each fibre in response to a disturbance from an intruder at any point along the perimeter.

6. A seismometer according to claim 5 having at least one transmission highway made up of a substantially flat cable, within which is sandwiched the or each optical fibre.

7. A seismometer according to claim 6 in which the undulations inside the cable are in the form of corrugations adapted to compress or kink the fibre or fibres in response to vibrations and/or pressure.

8. A seismometer according to claim 5 having at least one transmission highway made up of a substantially tubular construction having undulations on the inner side against which the optical fibre or fibres can be sandwiched.

9. A seismometer according to any one of claims 5 to 8 in which the cable construction incorporates spacers which prevent the undulations from contacting the or each fibre when the cable is unstressed or unpressurised externally.

10. A seismometer substantially as hereinbefore described with reference to and as shown in the accompanying drawings.