Abstract: A distributed fiber optic sensing system for detecting mechanical, acoustic, or ultrasonic disturbances is disclosed. This system is optimized for use as an intrusion detection system in security applications. Light is launched into opposite ends of a sensing fiber loop such that two beams circulate through the loop in opposite directions and then recombine to produce an interference pattern on a photoelectric detector. Disturbances of the sensing fiber loop produce corresponding changes in the interference pattern and thereby the signal level at the detecter. The changes in the electrical signal from the detector are interpreted to provide alarms when the sensing fiber loop is impinged upon by intrusive disturbances, and to provide location of disturbances along the loop. Means are incorporated to decode uniquely coded sonic or ultrasonic signals from personal transmitters borne by authorized personnel approaching the sensing fiber loop.