Coherent alarm for a secure communication system

摘要

A high speed secure fiber optic communication system that includes a coherent alarm system utilizes a pair of single mode fiber optic cables in combination with one or more light sources, phase modulators, detectors and polarization scrambling elements to form a Sagnac interferometer. The phase modulator is driven so that counter propagating light beams in the Sagnac loop experience a different optical path as they pass through the loop. When the two beams are recombined on the central beamsplitter of the Sagnac loop, the two beams interfere with each other and the data impressed as phase modulation on the light beams by the phase modulator is recovered as amplitude modulation on the output detector of the Sagnac interferometer. The coherent alarm system applies a relatively low frequency background signal to the Sagnac interferometer and watches for changes occurring to the background signal that indicate the presence of an intruder. Although the system is very secure, such coherent alarm systems with or without other alarms, intrusion control, random pathlength changes and the like can make undetected, unauthorized access to the system impossible with available interception techniques.