Image Number 14 for United States Patent #5311592.
A secure fiber optic communication system capable of tens of gigabits/second data transfer rates that 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 system can be configured to operate full duplex on two optical fibers by using light at different wavelengths or time division multiplexing data. The system can also be configured as a multi-node network. Alarms, intrusion control, random pathlength changes and the like can be included to make undetected, unauthorized access to the system even more difficult.