Fig. 1. Schematic diagram for security-enhanced long-distance bidirectional chaos communication system

Fig. 2. Evolution maps of (a) σD and (b) WD of the chaos outputs from DL in the parameter space of frequency detuning ΔfID and injection strength kin1

Fig. 3. Evolution maps of (a) σR1 and (b) WR1 of the chaos outputs from RL1 in the parameter space of frequency detuning ΔfDR and injection strength kin, where the lengths of fiber links F1 and F2 are also 10 km

Fig. 4. Evolution maps of maximal cross-correlation coefficients between two lasers for (a) RL1 and RL2 and (b) DL and RL1 in the parameter space of frequency detuning ΔfDR and injection strength kin, where the lengths of fiber links F1 and F2 are also 10 km

Fig. 5. Evolution maps of maximal cross-correlation coefficient between two lasers for (a) RL1 and RL2 and (b) DL and RL1 in the parameter space of frequency detuningΔfDR and injection strength kin, where the lengths of fiber links F1 and F2 are also 50 km

Fig. 6. (a1)-(a3) Time series, (b1)-(b3) power spectra and (c1)-(c3) corresponding cross-correlation coefficients for arbitrary two chaotic outputs from DL, RL1 and RL2, where the lengths of fiber links F1 and F2 are also 10 km

Fig. 7. Maximal cross-correlation coefficients Cmax_R1,R2 between RL1 and RL2, Cmax_D,R1 between DL and RL1 versus internal parameters of lasers and parameter mismatches between F1 and F2

Fig. 8. Q factors of 20 Gbit/s decoded messages for different bidirectional transmission distances in dispersion-shifted fibers. (a) Q factors of decoded messages versus the transmission distance; (b)-(d) eye diagrams of decoded messages for transmission distances of 20, 80, 120 km along 2 opposite directions

Fig. 9. Q factors of 1 Gbit/s decoded messages for different bidirectional transmission distances in dispersion-shifted fiber. (a) Q factors of decoded messages versus the transmission distance, (b)-(d) eye diagrams of decoded message for transmission distances of 20 , 80, 120 km along 2 opposite directions