Fig. 1. Schematic diagram of the proposed classical secure key distribution scheme. DL, driving laser; PRCM, private response chaotic module; TRKG, true random key generator; CW, continuous-wave laser; OPS, optical phase scrambler; OPD, optical phase descrambler; TRKR, true random key receiver; PD, photo-detector.

Fig. 2. Experimental setup of the proposed high-speed secure key distribution scheme. SLD, super-luminescent diode; TOF, tunable optical filter; DFB, distributed feedback laser; CW, continuous-wave laser; OC, optical coupler; VOA, variable optical attenuator; PC, polarization controller; CIR, circulator; EDFA, erbium-doped-fiber-amplifier; PD, photo-detector; EA, electrical amplifier; MZI, Mach–Zehnder interferometer; DCF, dispersion compensation fiber; TDL, tunable delay line; PM, phase modulator; DSO, digital sampling oscilloscope.

Fig. 3. Temporal waveforms (left column) and corresponding correlation plots (right column) of the output chaos measured at points (a), (b) S and A, (c), (d) A and B, (e), (f) S and C, and (g), (h) C and D, respectively.

Fig. 4. (a) Optical spectra for the driving signal, free-running response signals, and synchronized signals from DFB lasers and (b) the synchronized chaotic scrambling signals after feedback loops.

Fig. 5. (a) Synchronization coefficient and (b) residual driving-response cross-correlation versus PM modulation depth and dispersion.

Fig. 6. (a) Correlation coefficients for different points versus the PM modulation depth and (b) DCF dispersion.

Fig. 7. Effects of DFB laser inner parameters mismatch on chaos synchronization.

Fig. 8. Correlation coefficient at C and D versus (a) dispersion mismatch; (b) feedback delay time mismatch; (c) PM modulation depth mismatch; (d) MZI interference delay time mismatch.

Fig. 9. Waveforms and eye diagrams for (a), (b) the demodulated key without scrambling, (c), (d) the scrambled signal without demodulation, (e), (f) the illegible eavesdropped signal using only an MZI, (g), (h) the properly descrambled key in BtB case, and (i), (j) the descrambled signal after transmission.

Fig. 10. Optical spectra for the original signal, chaotic phase scrambled signal, and descrambled signal, respectively.

Fig. 11. BER performance versus mask coefficient for the legitimate user and an eavesdropper in the BtB and transmission scenarios.

Fig. 12. BER performances versus (a) the PM modulation depth mismatch and (b) the channel synchronization delay time mismatch.