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We propose a novel method to simultaneously improve the physical layer security and the transmission performance of the orthogonal frequency division multiplexing (OFDM) passive optical network system by using chaos and fractional Fourier transform (FrFT) techniques. The designed 3-D chaotic sequences are used to form the training sequence for time synchronization, to perform the OFDM subcarriers masking, and to control the fractional order of the FrFT operation. The analyses show that the whole key space size of the proposed scheme could be beyond 10 ⁵⁰ , and the peak-to-average-power-ratio of the transmitted OFDM signal can be decreased by about 0.5 dB. Furthermore, we successfully demonstrate an 8.18 Gbps 16-quadrature-amplitude- modulation (QAM)-OFDM data transmission experiment with chaotic and FrFT operations over 25 km single mode fiber. The results show that the proposed …