摘要
随着混沌同步的实现,利用混沌信号作为载波进行保密通信受到了广泛关注。通过引入反馈或外部注入,半导体激光器可以产生较大带宽的高维混沌载波,从而使激光混沌通信成为目前的研究热点。垂直腔面发射激光器(VCSELs)作为一种新型的微腔激光器,具有圆形出光、与光纤耦合效率高、易于二维阵列集成、易实现单纵模等优点,将它应用于混沌通信将大大改善现有系统的性能。基于以上考虑,本文对基于VCSELs的级联及多信道系统混沌同步与通信进行研究。
首先,研究了开环全光级联混沌通信系统的中继性能。对系统同步条件,相关度,同步健壮性,输出功率谱进行了分析;分别就混沌掩藏(CMS)、混沌调制(CM)和混沌键控(CSK)调制方式的编解码过程及性能进行了仿真研究;通过正弦信号传输,观察了混沌滤波(CPF)效应;分析了信息的多级解码能力以及再生信号的传递。结果表明,上级激光器足够强的光注入可以保证相邻激光器足够高的同步质量,并且对参数失配具有较强的鲁棒性。发送端传送的数字信号在三种调制方式下均可很好的隐藏于混沌载波中,并在接收端得到正确恢复。相同条件下,CMS和CSK的解码性能相对好于CM。CPF效应是实现混沌解码的物理基础,跟调制频率有关。由于CPF效应,有用信号每次传递都受到衰减,经过两次传递,有用信号几乎被滤除。并且前两级的CPF效应明显,能够较好的实现信息的解码,而第三级以后无法解调出正确的信息;通过再生传输中衰变的信号,实现了多级联混沌通信。
在级联系统的基础上,构建了多信道全光混沌通信系统模型,研究了信道串扰对系统混沌同步及解码性能的影响。利用描述其特性的数率方程模型,采用相似指数作为参考标准来评价同步性能,并进行了2.5Gb/s混沌掩藏方式的编解码数值模拟。结果表明,相对注入强度越小,信道间隔越大,系统同步的性能越好;双信道系统的同步质量相比主从式单信道系统会有明显下降,而进一步增加信道数量,同步品质并不发生显著改变。在同步质量足够高时,数字和模拟信号都可以得到较好的恢复。
With the achievement of the chaos synchronization, secure communications with messages encoded in chaotic waveforms have attracted considerable interest. Semiconductor lasers are able to generate hyper-dimension chaotic carriers with large bandwidth subject to optical feedback or injection. Optical chaotic communication has been a subject of intensive research. Meanwhile, vertical cavity surface emitting lasers (VCSELs), as the new type micro-cavity lasers, have many excellent features, such as circular beams, high efficiency for coupling with fibers, easy to be integrated as high density 2D arrays, single longitude mode operation. Therefor, it can largely promote the performance of chaotic optical communication system. Based on above considerations, synchronization and communication in cascade and multi-channel systems with chaotic VCSELs are theoretically investigated in this thesis.
Firstly, the relaying performance in open-loop cascade chaotic optical communication system is investigated. The synchronization condition, correlation degree, synchronization robustness, and spectra of chaotic outputs are analyzed thoroughly. The encoding/decoding processes and performance in chaotic masking (CMS), chaotic modulation (CM), and chaos-shift keying (CSK) are simulated. The chaos-pass filtering (CPF) effects are observed by transmission of sinusoid signals. Multilevel decoding ability for the messages, and the relay of regenerated signals are the issues we focus on. The results indicate that a high synchronization performance between the neighboring two lasers can be guaranteed by sufficient optical injection from the former laser, and is very robust for variations in the mismatch of parameters. With the three encoding methods, the digital signals sent in transmitting part can be well hidden by chaotic carrier and recovered correctly in the receiver. At the same conditions, the decoding performances in CMS and CSK are better than CM. CPF effect is the physical basis for chaotic decoding, and it is relative to the modulation frequency. The signal is attenuated due to the CPF effects, and nearly filtered out after transferring twice. The original message can be extracted correctly in the first two grades because of the notable CPF effects, and can hardly be recovered in the third or later. The multilevel cascade chaotic communication can be realized by regenerating the depleted signal in relaying.
A multi-channel chaotic optical communication system is established on the basis of cascade system, in order to investigate the effects of the cross talk on synchronization and decoding. Based on the corresponding rate equations, similarity index is used to evaluate the synchronization performance, and 2.5Gb/s message coding/decoding of chaotic masking method is numerically analyzed. The results show that the system can get better synchronization quality with less relative injection strength and more channel interval. The synchronization performance of dual-channel system decreases obviously compared with master-slave single channel system, and changes little when the channel number increases. The digital and analog signals can be correctly recovered when the synchronization is good enough.
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