Intruder alarm arrangement for an optical communication system

摘要

An intelligence modulated optical beam is injected into an optical waveguide through a beam splitter. At the output end of the optical waveguide the optical beam has a portion thereof transmitter through a partial reflector to a detector or receiver with a portion of the optical beam at the output of the waveguide being reflected back into the waveguide with the reflected beam being reflected by the beam splitter to a correlator. A replica of the intelligence of the injected optical beam delayed an amount equal to the round trip transit time of the optical beam is supplied as the other input of the correlator. The delayed replica of the intelligence of the optical beam and reflected optical beam are correlated to produce an output from the correlator proportional to the attenuation or intensity of the reflected beam. When the output of the correlator falls below a given value, an alarm is actuated indicating that an intruder has intercepted intelligence of the optical beam transmitted through the waveguide.