A new fiber-optic system with an active unbalanced Michelson interferometer (AUMI) was designed for multi-zone perimeter intrusion detection by employing only two single-mode fibers for each sensing zone. A pump laser at the monitor site was used to remotely pump the erbiumdoped fibers located, respectively, at the two arms of the AUMI for each sensing zone. Configured into a fiber laser cavity, the AUMI served as an optical switch that would modulate the optical power when one arm of the AUMI was perturbed by intrusion to induce a phase offset variation. In the study, an armored cable with a length of 200 meters in total was divided into four sections each guarding a perimeter zone. The experimental results showed that the AUMI system correctly responded to perturbations induced by footstepping on a fiber cable buried underground, vibrating a netted fence, or knocking a window. A high alarm-upon-intrusion rate and zero false alarm rate could be reached by using the presented detection algorithm. Index Terms-Pump laser, Michelson interferometer, multi-zone perimeter intrusion detection, fiber laser cavity. I. INTRODUCTION F IBER-OPTIC perimeter intrusion detection (PID) systems offer some advantages over counterparts using wired or wireless hardwares [1]-[3]. They can find applications in detecting and locating unauthorized excavation of buried pipelines and trespassing intruders in order to protect premises for personal, commercial, and utility purposes. Many types of fiber-optic PID systems have been recently studied for not only detecting intrusion but providing locating ability for multiple events [4]-[6]. Some of these systems are Manuscript

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