Abstract: A diffraction grating portion (12) is formed in an optical fiber (10), having a diameter of 125 .mu.m and serving to transmit light, along its optical axis. The optical fiber is concentrically surrounded by a lower coating portion (14) having an outer diameter of 300 .mu.m and consisting of a silicone resin. The lower coating portion is concentrically surrounded by a coating portion (16) having an outer diameter of 900 .mu.m and consisting of a liquid crystal polymer, e.g., polyester amide. The coating portion is further surrounded by an outermost coating portion (18) having an outer diameter of 1 mm and consisting of a UV curing resin colored for identification. Both the optical fiber (10) and the lower coating portion (14) have positive thermal expansion coefficients. In contrast to this, the coating portion (16) consisting of the liquid crystal polymer has a negative thermal expansion coefficient.

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.