Abstract: An advance in the art is made according to aspects of the present disclosure directed to methods for earthquake sensing that employ a supervisory system of undersea fiber optic cables. Earthquakes and other environmental disturbances are detected by monitoring the polarization of interrogation light instead of its phase. More specifically, our methods monitor the transfer matrix rather than just polarization and isolate disturbance location by monitoring eigenvalues of the polarization transfer matrix. From results obtained we have demonstrated experimentally that we can monitor disturbances that affect signal polarization on a span-by-span basis using High Loss Loop Back (HLLB) paths. It is shown that by measuring the polarization rotation matrix and determining the polarization rotation angle we can identify the span where the disturbance occurred with 35 dB extinction with no limitation on the magnitude of the disturbance and the number of affected spans.

Abstract: A hybrid Quantum Key Distribution (QKD) protocol and method for secure transmission in optical communications systems and in particular long distance optical communications systems such as those in undersea environments. Our inventive method advantageously exploits fundamental security features of quantum-base encryption with the added intrinsic security of encapsulated and sealed equipment of undersea optical networks. Our method employs an intermediate node that generates an optical signal, and a pair of quantum state generators that respectively generate quantum states of the optical signal and transmit the generated quantum states to secure nodes, respectively. The secure nodes then communicate securely using a quantum key distribution (QKD) protocol.