The FIBER FENCE Security System utilizes bend-sensitive fiber optic cable MAKING THE FIBER A SENSOR! Duplex fiber (2 fibers in one jacket) is used for a single strand FIBER FENCE. Using a LoopBack Module, installed by FIS, a single cable can be mounted to a fence. It can also be used to monitor an Opti-Mag COVER COP on a gate, door, manhole cover, metal valve, or lever.