Chaotic optical communications provide high security for physical layer encryption. We propose a chaotic laser communication system based on a balanced detection optoelectronic delayed feedback system, which has a higher performance compared with one based on single-photodiode detection. Our numerical analysis proves that dynamic behaviors are achieved at only half feedback gain, and the safety hazard chaotic dead zone is eliminated. The generated chaos features high pseudo-randomness and a good capacity to conceal time-delay signatures, which makes it hard for an unauthorized party to remove the chaotic carriers. 10 Gbps message hidden in wideband chaotic carries is transmitted back-to-back, and the BER 10−1 of the eavesdropper achieved at a low feedback gain β=3.2 proves its high confidentiality. Our proposed system has the potential to promote chaotic laser systems to practical appliances with simple structure and high performance.
Quadrature Phase Shift Keying (QPSK) is an important digital signal modulation method, which has the advantages of high spectrum efficiency and strong anti-interference. Coherent laser communication has the advantages of high speed, large capacity, light and small equipment as well as high sensitivity. In order to better introduce the QPSK modulation method into the space downlink coherent laser communication system, we give a bit error rate (BER) of QPSK modulation model with laser linewidth suitable for space downlink coherent laser communication system with QPSK modulation under atmospheric turbulence. Based on the model, we simulate and analyze the effects of atmospheric wind speed, zenith angle, gain of Erbium doped optical fiber amplifier (EDFA) and communication rate on the linewidth tolerance of space downlink coherent laser communication system with QPSK modulation. The results show that the linewidth tolerance of the system increases when the zenith angle decreases or the gain of EDFA increases or the wind speed decreases. And the effects of zenith angle, gain of EDFA and communication rate on the linewidth tolerance of the space downlink coherent laser communication system with QPSK modulation are large. The effect of wind speed on the linewidth tolerance of space downlink coherent laser communication system with QPSK modulation is smaller. And the larger the wind speed is, the smaller the effect of increasing wind speed on the linewidth tolerance. For the communication rate, the linewidth tolerance increases and then decreases with the increase of the communication rate. This work can be a reference for the design of space downlink coherent laser communication system with QPSK modulation.
Atmospheric intensity scintillation effect deteriorates the communication quality of space chaos laser communication. To analyze the effect of internal mismatch and internal mismatch induced by intensity scintillation on bit error rate (BER) for space downlink chaos laser communication system, numerical simulation results based on BER formula are conducted and analyzed. Numerical results demonstrate that BER worsens with the effect of intensity scintillation. Although the probability that received optical power causes larger mismatch noises is small, its corresponding larger instantaneous BER deteriorates the ensemble average BER significantly. Results also indicate that internal mismatch deteriorates BER more than external mismatch. These results are beneficial for designing practical space chaos laser communication system with optimal performance.
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