Fig. 1. Experimental setup of the secure communication system. LD, tunable laser diode; PC, polarization controller; RF, radio-frequency amplifier; IM, intensity modulator; EDFA, erbium-doped fiber amplifier; PD, photodetector; PM, phase modulator; FBG, fiber Bragg grating; DL, tunable delay line; OC, optical coupler; SSMF, standard single-mode fiber; SPMF, self-phase-modulated optical feedback.

Fig. 2. (a) Time series of the original data signal at the output of intensity modulator; (b) time series of the encrypted signal at the output of FBG2; (c) time series of the decrypted signal at the output of FBG4; (d) eye diagram of the original data signal at the output of intensity modulator; (e) eye diagram of the encrypted signal at the output of FBG2; (f) eye diagram of the decrypted signal at the output of FBG4.

Fig. 3. (a) RF spectrum of the data signal; (b) RF spectrum of the encrypted signal; (c) RF spectrum of the decrypted signal.

Fig. 4. (a) BER variation of the decrypted signal with time-delay mismatch between DL1 and DL2; (b) BER variation of the decrypted signal with dispersion mismatch between FBG2 and FBG3; (c) BER variation of the decrypted signal with dispersion mismatch between FBG1 and FBG4.

Fig. 5. (a) ACF of time series with original parameter settings; (b) ACF of time series with time-delay concealment.

Fig. 6. (a) Peak size of ACF values under different dispersions of FBG1; (b) peak size of ACF values under different dispersions of FBG2.

Fig. 7. System structures for complexity measurement. (a) Phase chaos structure; (b) phase chaos with an external phase noise source; (c) SPMF loop with an external phase noise source.

Fig. 8. Experiment structure of the proposed encryption scheme for WDM.

Fig. 9. Spectra of DWDM channels.