Key Distribution Based on Phase Fluctuation Between Polarization Modes in Optical Channel

This letter proposes and experimentally demonstrates an approach for generating and sharing a secret key based on the random phase fluctuation between orthogonal polarization modes (OPMs). A delay interferometer (DI) is placed to extract the phase differences inside the optical channel, where reconfigurable lengths of polarization-maintaining fiber are applied as the main source of fluctuation. It is proven that the output signals from the DI can generate random and highly correlated waveforms that can be extracted as the key and shared between the two communication parties. Due to the environment-related randomness and channel-dependent uniqueness of phase fluctuation in OPMs, the security enhancement of the proposed scheme is evaluated with the capability against various tapping attacks. As a proof of concept, the key generation rate of 220 bits/s with bit-error rate of 5% is successfully demonstrated over 25-km standard single-mode fiber. Moreover, the proposed scheme has the advantage of low cost, compatible with current optical fiber networks, and suitable for long-distance transmission, since optical amplifiers can be applied.