Enhanced physical-layer security in WDM optical communications based on chaotic optical encryption with frequency-hopping filtered feedback.

We propose and numerically demonstrate a physical-layer-security-enhanced 4 × 50 Gb/s 16 QAM WDM optical communication system using optical phase encryption with frequency-hopping filtered feedback. A broadband chaotic signal generated by a filtered opto-electronic feedback loop is employed as the phase key. With a strong modulation index and an 80 GHz optical filter spanning nearly two adjacent channels (50-GHz channel spacing), the beating of two selected channels at the photo-detector generates a broadband chaotic signal (over 36 GHz), significantly broader than that obtained with a single-channel feedback. The broadband phase encryption makes the proposed scheme highly sensitive to dispersion mismatch, filter center-frequency detuning, and filter bandwidth mismatch, thereby significantly enhancing transmission security. Despite the strong encryption, long-haul transmission remains feasible. For example, with a modulation index of 1.2, the proposed system achieves transmission beyond 400 km. Moreover, by applying frequency hopping filtering to the WDM signal, the center frequencies of the filters at the transmitter and receiver are randomly varied in each time slot, providing an additional layer of security.