| With the development of nonlinear science,a secure communication based on chaos has become a hotspot in the field of information security owing to chaotic signal has complex trajectory and unpredictability.Due to the large bandwidth of optical fiber,chaos communication based on semiconductor laser has attracted more and more attention in recent years.The physical basis of optical chaotic communication is chaos synchronization,as well as the robustness against the external perturbation,namely,once receiver is synchronized to the transmitter,its chaotic dynamics is not affected by perturbation of transmitter,so that one can resume the sending message by detecting the synchronization error at both sides.The security of chaotic communication mainly relies on the match of the internal parameters of laser(such as nonlinear gain,linewidth enhancement factor and the photon lifetime,etc.)and the external parameters(feedback delay,etc.),such parameters' match is equivalent to a key provided for both sides of communication in order to realize secure communication.Therefore,except system synchronization,it is also necessary to consider the robustness of synchronization.A synchronization with good robustness can not only reduce the bit error rate,but also is the key to realize chaotic secure communication.Based on the principle of chaos synchronization,three chaotic secure communication schemes are designed according to different ways generating chaos.Then,the nonlinear chaotic dynamics effect of the system is studied in the chaotic system above-mentioned,and the robustness of the system synchronization is analyzed.The main work and research results of this paper are as follows:1)Two schemes about point-to-multipoint and ring network communication based on chaotic semiconductor lasers with optical feedback are designed;2)A scheme about bidirectional message transmission in a chaos-based semiconductor laser system with positive optoelectronic feedback and coupling is designed;3)A multiple-access bidirectional communication system using chaotic semiconductor lasers subject to optical-electric feedback and coupling is analyzed.Firstly,the theoretical schematic and rate equations of these systems are given.Then,the chaotic dynamics characteristics of the system are analyzed respectively.After that,the synchronization characteristics and robustness of chaotic synchronization against the external perturbation are discussed.Furthermore,the chaotic information transmission processes of systems are simulated numerically,and the information transmission quality is measured by eye diagram.The results show that:1)By solving the rate equation,the bifurcation diagram,the maximum Lyapunov exponential diagram and Lempel-Ziv complexity diagram of the system are obtained,and the chaotic dynamics characteristics of the system are studied.These results indicate that these systems could generate chaotic signals with high complexity.2)By numerically simulating,one draws the synchronization diagram,the cross-correlation coefficient diagram and the attractor diagram to study the synchronization characteristics of these systems;the robustness of the system is studied under non-ideal conditions.A square wave signal modulating the bias is acted as the external disturbances.When the disturbance appears,it will be asynchronous between the receiver side and the transmitter side of these systems.Once the perturbation disappears,the synchronization between the receiver and transmitter can instantly restore,the fact indicates that the synchronization of the system owns excellent robustness.3)Then,the secure communication simulation has carried out,by monitoring the synchronization error(e.g.P_k-P_n at laser k)at the receiving end,and then performing an operation for the error with the local message(e.g.mk-[P_k-P_n],mk is local message at laser k),the sending information(e.g.m_n,which is the local message at laser n)on the other end can be decrypt.The qualities of the above communication schemes are verified by eye-diagrams,which show that the above systems own good ability of information transmission.It is difficult for eavesdropper to obtain the initial sending message,because eavesdropper only monitors the message difference,but has no clue to obtain the any party's message bit,so they own excellent security performance.4)After that,in the multi-access bidirectional communication system,the wavelength conversion has been carried out according to the needs of the route,and the wavelength division multiplexer is used for multiplexing and demultiplexing the multiplex signals.To sum up,starting from the rate equations of semiconductor laser subject to optical and optical-electric feedback,this thesis deeply studies the chaotic dynamics in various schemes.The main conclusions are summarized as: the size of the feedback strength determines the nonlinear dynamics of the system,when its value exceeds a certain threshold,bifurcation diagram and the positive Lyapunov index shows that system can enter a chaotic state;the generated chaotic signal has high complexity;the chaotic synchronization of the laser system is robust;the parameter mismatch has some effects on the chaotic synchronization;these schemes can realize bidirectional communication and multiple-access bidirectional communication well.Thus,these studies can provide some ideas and methods for the design of secure communication,enrich the theory of chaotic secure communication system,and promote the application of the chaotic communication. |
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