LAUSR

We propose and experimentally demonstrate a novel real-valued chaotic orthogonal matrix transform-based encryption scheme for improving the physical-layer security in fast orthogonal frequency-division multiplexing (fast-OFDM) passive optical network. Chaotic orthogonal matrices generated by a four-dimensional hyperchaotic system are employed for encryption, which shows more complex randomness and higher unpredictability compared to the primitive permutations used in conventional physical security systems. Besides, we also extend the encryption to the time domain without causing the issues of synchronization, delay, or additional noise. A total key space of $\sim 3.4\times 10^{274}$ against the exhaustive trial is realized. Encryption of 18.82-Gb/s PAM4 fast-OFDM signals has been successfully experimentally demonstrated over 20-km standard single-mode fiber, with about 1.5-dB additional improvement in the receiver sensitivity at the bit error ratio of $1\times 10^{-3}$ .