Fig. 1. Experimental setup. (a) Chaotic transmitter and (b) the offline pilot-based chaotic coherent fiber transmission system. CW, continuous wave laser; MZM, Mach–Zehnder modulator; OC, optical coupler; DL, optical delay line; VOA, variable optical attenuator; PD, photodiode; EA, electrical amplifier. PM-IQM, polarization multiplexing-in-phase/quadrature modulator; SMF, single-mode fiber; EDFA, erbium-doped fiber amplifier; OSC, oscilloscope; DSP, digital signal processing; and NN, neural network.

Fig. 2. Pilot structure at the transmitter and the DSP module at the receiver. (a) Frame structure of pilot symbols; (b) pilot tone structure of chaotic encrypted carriers; (c) principle of FO compensation by pilot tone; and (d) receiver pilot-based DSP module. CD, chromatic dispersion; CPR, carrier phase recovery; RPE, residual phase estimation; and NN, neural network.

Fig. 3. Equalization algorithm process and constellations of the encrypted and decrypted QPSK signal. (a) Jointly LMS equalization and phase noise compensation by pilot symbols; (b) constellation of decrypted QPSK signals in the back-to-back case; (c) constellation of chaotic encrypted carrier; (d) constellation of decrypted QPSK signal by NN after 800 km fiber transmission.

Fig. 4. Experimental setup and results of real-time pilot-based coherent chaotic fiber transmission system. (a) Experimental setup; (b) pilot tone structure of chaotic encrypted carriers; (c) plot of chaotic time series collected from PD2 and NN synchronization; (d) constellation of decrypted QPSK signals in the back-to-back case; (e) constellation of chaotic encrypted carrier; and (f) constellation of decrypted QPSK signal by NN after 800 km fiber transmission.

Fig. 5. BER performance. BER performance of decrypted QPSK signal after 800-km fiber transmission (a) with the length of Np and (b) with the pilot rate of NS.