Patent ID: 7519242

Claim:
A perimeter security system including; at least a first waveguide and at least a second waveguide buried below ground level and extending along a perimeter which defines an area to be monitored; means for launching light into the first and second waveguides; a detector for detecting light which has propagated through the waveguides so as to detect a change in a parameter of the light propagating through the waveguides due to an intrusion across the ground beneath which the waveguides are buried and for providing an indication of that intrusion; the first and second waveguides being provided in separate cables and the separate cables being buried beneath ground level in a zig-zag spaced apart relationship with respect to one another to define a perimeter region having a substantial width which will be traversed by a person intruding into the area; a first of the said cables contains said at least one waveguide and a second said cable contains said second waveguide; a further waveguide being contained within the first cable; first coupling means at one end of the said first, second and further waveguides for coupling the waveguides so that light launched into the said further waveguide is able to propagate through the further waveguide and then into the said first and said second waveguides to propagate in a first direction through the said first and second waveguides; second coupling means at the other end of said first and said second waveguides so that the light propagating in the said first direction through said first and second waveguides is able to coherently recombine and interfere at the second coupling means; and light also being able to be launched through said second coupling means and into said first and second waveguides to travel in a direction opposite said first direction and coherently recombine at the first coupling means so the light travelling in the opposite direction is able to interfere and then propagate through the said further waveguide.