Fig. 1. Optical covert communication system based on DPSK-QNSC

Fig. 2. Structure of DPSK balanced receiver

Fig. 3. Error rate curves. (a) Bob; (b) Eve

Fig. 4. Mask number under different key bases. (a) With ASE noise; (b) without ASE noise

Fig. 5. Time domain waveform of pulse after broadening with different dispersions. (a) 80 ps/nm; (b) 800 ps/nm

Fig. 6. Spectra with different key bases. (a) 7 bit; (b) 11 bit

Fig. 7. SNR of transmitter in covert channel with different key bases

Fig. 8. Eavesdropper recovery signal. (a) Time domain waveform; (b) eye diagram

Fig. 9. Bit error rate at the receiving end under different key bases. (a) Covert channel; (b) public channel

Fig. 10. Eye diagrams under different key bases. (a) 7 bit; (b) 11 bit