Abstract:
A security system, including a cable having a groove formed in the inside thereof to accommodate therein an optical fiber with an excessive length, a plurality of supports having mandrels to support the optical cable so as to provide a fence, a light transmitter to inject light into one end of the optical cable and a light receiver to detect the light projecting out of the other end of the optical cable so that any transmission loss through the optical cable due to an intrusion of the fence may be measured. The cable is formed so that when tension is applied to the optical cable, a transmission loss is detected. The resulting system has a threshold capability which distinguishes valid from false intrusions.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a security system using an optical fiber barrier for detecting an invader breaking into an area surrounded by the barrier. 
     2. Prior Art 
     Conventionally, as a security system, employed are an infrared camera, a system using infrared rays, or a method of transmitting an optical signal through an optical fiber. 
     An area which can be watched by one unit of such an infrared camera, a system using infrared rays, or the like, has a limit, resulting in difficulty to watch a wide area. On the other hand, a conventional system using an optical fiber is used to detect whether the optical fiber is cut or not by a person. However, in these systems, repair has usually been required to recover the system, and a large amount of optical fiber is required so that the person cannot invade an area surrounded by the optical fiber without cutting the optical fiber. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to solve the above problems accompanying the conventional systems, and to provide a security system in which a wide area can be watched without optical fiber being damaged. 
     In order to solve the above problems, the present invention provides a security system which comprises an optical cable having at least one gap formed in the inside thereof so as to accommodate therein an optical fiber with a length in excess of the minimum lineal length of fiber which could be used; a fence on which one or more lines of the optical cable are provided between supports containing mandrels, which provides support for the optical cable by winding around the mandrels; and a light transmitter for injecting light into one end of the optical fiber and a light receiver for receiving the light projecting out of the other end thereof to detect the transmission loss of an optical signal, wherein the optical cable is formed so that when predetermined tension is exerted to the optical cable, a predetermined transmission loss of the optical signal is generated in the optical cable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an explanatory view of the whole configuration of the present invention; 
     FIG. 2 shows a sectional view of an optical cable applied to the present invention; 
     FIG. 3 shows a sectional view of another optical cable applied to the present invention; 
     FIG. 3a shows a cut away side view of a length of optical cable with a spiral groove. 
     FIGS. 4a and 4b show sectional views of optical fibers applied to the present invention; 
     FIG. 5 shows a view for explaining the principle of the present invention; 
     FIGS. 6 and 7 show the measured values of the results obtained by the examples of the present invention; and 
     FIG. 8 shows a sectional view of an optical fiber applied to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described with reference to the accompanying drawings. 
     In FIGS. 1 and 2, an optical cable 3 is provided with gaps 8 therein, which accommodates optical fibers 6, 6-2, 6-3, and 6-4 with an excessive length. A plurality of supports 1 are provided with a plurality of mandrels 2 in such a manner that one or more lines of the optical cable 3 are arranged between the supports 1 while winding around the mandrels 2 to form a fence. A light transmitter 4, injects light into one end of the optical cable 3, and a receiver 5 receives the light from the other end thereof. 
     The optical cable 3 is formed in such a manner that the optical fiber 6 is received in the gap 8 which is in the form of a linear or spiral groove provided in a surface of a cylindrical body 7, and a cover layer 9 is provided over the cable 3. 
     The cylindrical body 7 is made of reinforced plastics including glass fibers, but a copper wire 10 may be used at a center of the cylindrical body 7 for increasing tensile strength. 
     The cover layer 9 is made of resin such as polyethylene, nylon, or the like. As shown in FIG. 3, a rough surface 11 is provided on an inner wall of the groove 8 so that microbend is easily generated in the optical fiber 6 when tension is exerted on the optical cable 3 which is bent by the mandrels 2. 
     It is preferable to provide three or more grooves 8 in one cylindrical body, so that any one of the optical fibers will be maintained on the outside of the bending radius of the optical cable 3 when the optical cable 3 is bent. 
     In an alternative method, as shown in FIG. 3, a marking or a structural groove location mark 15 is provided on a cover layer 9-1 at the position of a groove 8-1, whereby a cable is positioned so that the groove 8-1 is allowed to be maintained on the outside of the bending radius around each mandrel 2. 
     Further, as shown in FIG. 4a, the optical fiber 6 is made such that solid particles 12 are mixed in a coating 13 of a glass fiber 14 so that microbend is generated easily. The solid particles 12 are made of spherical glass, fluorine resin, alumina, or the like. The size of the solid particles 12 is approximately 30 to 50 μm, and the number thereof included in the coating 13 is approximately 50/mm. The same effect can be obtained if the solid particles 12 are attached to the outer surface of the coating 13 by the use of an adhesive 21 used to maintain the particles in place as shown in FIG. 8, or if unevenness 22 is provided on the outer surface of the coating 13 as shown in FIG. 4b. 
     Similar to the cover layer 9, resin is used for the coating 13. In order to allow the optical fiber 6 to have excessive length, it is necessary to make the groove 8 larger than the outer diameter of the optical fiber 6. Alternatively, the groove 8 may be shaped so as to be elongated so that the excessive length of the optical fiber 6 may be accommodated by the depth. 
     If a person intends to enter an area encircled by optical cables 3 arranged like a fence through a gap between optical cables 3, it is necessary for that person to enlarge the distance between the optical cables 3 as shown in FIG. 5. Letting the interval between supports 1-1 and 1-2 be L and letting the enlargement of the distance between the optical cables 3-1 and 3-2 be Δa per optical cable, then the percent of extension ΔL is expressed in the following Equation (1). ##EQU1## 
     On the other hand, the minimum length L MIN  of the optical fiber 6 in the groove 8 of the cable 3 is determined by the structure of the cable 3. For example, in a case of an optical fiber 6 in a groove 8 of a cylindrical body 7 longitudinally having a spiral groove as shown in FIG. 3a, the minimum length L MIN  of optical fiber 6 which can span the length of the groove 8 can be expressed by the following Equation (2) when the length of the cable 3, the pitch of the spiral groove, and the distance from the cable center to the groove bottom are represented by Lca, P and b, respectively. ##EQU2## 
     In the case of a linear groove, L MIN  becomes equal to L ca . 
     When an excessive length of optical fiber is received by the groove 8 so that the optical fiber is longer than L MIN  by only ΔK. the optical fiber 6 exists freely within the groove 8 when not subjected to lateral pressure. Lateral pressure F is finally applied to the optical fiber 6 from the groove bottom when ΔL&gt;ΔK. This lateral pressure F is expressed by the following Equation (3). ##EQU3## where, E, S and R represent the Young&#39;s modulus of the optical fiber 6, the sectional area of the optical fiber 6, and the radius of curvature of the optical fiber 6, respectively, the radius of curvature of the optical fiber 6 being determined by the spiral groove and satisfying the Equation of R=b+(P/2π) 2  /b. Accordingly, the lateral pressure according to the present invention occurs with a threshold. 
     In the case where the radius of curvature R of the optical fiber 6, determined by the spiral groove, is large, or in the case of a linear groove, the lateral pressure threshold may not be sufficiently obtained. In this case, if mandrels 2 each having a small radius of curvature are attached to the supports 1 and the optical cable 3 winds around the mandrels 2 so that the groove 8 is directed toward the outer circumference of each of the mandrels 2, the lateral pressure threshold is lowered such that when the optical fiber 6 is subjected to a large lateral pressure it generates microbend, which increases the transmission loss of the optical signal. 
     On the other hand, in the case where the radius of curvature of each mandrel 2 is larger or in the case of no mandrel 2 at all, the lateral force exerted on the cable 3 in a certain section may cause sufficient enough pressure on the optical fiber 6 in the section in question. In this case, it is effective to provide a structure where the optical fiber 6 is fixed with resin or the like in the groove 8 of the cable 3 at intervals substantially equal to those of the supports (preferably, at positions a little before or a little after the respective supports 1). 
     The cables 3 shown in FIGS. 2 and 3 are of a concentric type that has the copper wire 10 positioned in the center of the cable, however, the cable may be an eccentric type where the copper wire is not in the center of the cable, as shown in FIG. 8. 
     EXAMPLE 1 
     In the cable structure shown in FIG. 3, an optical fiber 6-1 with an excessive length of 0.05% of the minimum length thereof was received in a spiral groove 8-1 to thereby form a cable 3. The cable 3 was attached on supports 1 provided at intervals of 2 m through mandrels 2 of 35 mm in diameter attached to the supports 1 in such a manner that the groove 8 was directed toward the outer circumference of each mandrel 2. Light of 1.55 μm in wavelength was transmitted through the optical fiber 6, and the cable 3 at the intermediate portion between the supports 1 was displaced from 0 to 50 mm while the quantity of light reception was being detected at the other end of the optical fiber 6. As shown in FIG. 6, the loss caused by the displacement of the cable 3 and the microbend in the optical fiber due to the displacement of the cable 3 did not increase the light loss through the range of 0 to 35 mm, while the light loss showed a tendency to increase relative to the displacement beyond the above range. 
     EXAMPLE 2 
     Four optical fibers 6, 6-2, 6-3 and 6-4 were received in respective spiral grooves under the conditions similar to those in the Example 1 to form a cable 3 as shown in FIG. 2, and similar measurement was performed. As a result, as shown in FIG. 7, the light loss through the optical fiber 6 had a tendency to increase as in Example 1, while at the same time no increase of loss occurred in the other three optical fibers 6-2, 6-3 and 6-4. This was considered to be because only the optical fiber 6 was attached in the radially outer side of the mandrel 2. 
     In the security system using the present invention, the change of loss shows a threshold behavior which depends upon the force exerted onto the cable or the quantity of expansion of the distance between the cable portions. Accordingly, in only the case where sufficiently large force is exerted will the system detect the optical loss, thereby avoiding false intrusions cause by a fine displacement of the cable due to wind pressure, temperature change, or the like.