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Secure optical-fiber communication system

  • US 4,965,856 A
  • Filed: 05/23/1989
  • Issued: 10/23/1990
  • Est. Priority Date: 05/23/1989
  • Status: Expired due to Fees
First Claim
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1. A system for alerting a user to the fact of an unauthorized tapping of an optical fiber communication system, comprising:

  • a means for generating data signals;

    means for modulating a first optical carrier signal with said data signals;

    means for providing a reference signal;

    means for modulating a second optical carrier signal by said reference signal;

    means for multiplexing the first and second modulated optical carrier signals into a fiber optic communication link;

    means, coupled to said fiber optic communications link, for receiving, demultiplexing and demodulating the first and second modulated optical carrier signals to recover the data and reference signals;

    means for providing a replica of said reference signal;

    means for measuring the phase difference between the recovered reference signal and the replica of said reference signal as received over a communications link that is physically separate from said fiber optic communication link, there being an effective path-length difference therebetween; and

    means for creating an alert signal when the measured phase difference between the recovered reference signal and the received replica of the reference signal becomes non-zero.

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