Research On Theories And Methods For Optical Chaotic Secure Communication Systems

Chaotic secure communication is a new growing field in the recent twenty years.Over the past decade, there has been tremendous interest in studying the behavior ofchaotic systems. They are characterized by sensitive dependence on initial conditions,similarity to random behavior, non-periodicity. As a result, chaos has potentialapplications in secure communication systems. Initially, scientists studied cryptosystemsbuilt with nonlinear electronic circuits, but attention has quickly shifted toward opticalsystems whose potential for higher-speed encryption and expected compatibility withlong-haul optical communications networks is promising. The possibility forself-synchronization of chaotic oscillations has sparked an avalanche of works onapplication of chaos in cryptography. Thus makes chaos synchronization, especially itssafety and robustness, become a key issue in chaos-based secure communication systems.Motivated by the above discussions, The aim of study is set to discuss chaossynchronization theory and its applications in optical secure communication systems.In conventional chaos synchronization technology, the synchronizing signalstransmitted in the channel contain the orbit features of chaotic systems, and by using thenonlinear dynamic forecasting method, the phase space of the local chaotic signal could bereconstructed from the intercepted chaotic carrier, then the hidden message can bedetected once the chaotic carrier behaviour is predicted. In this thesis, a hybridself-synchronization scheme based on impulsive stability theory without leaking thechaotic orbit information is proposed. The condition which provides quantitative relationbetween the synchronization threshold, the impulse interval and the synchronizationcoefficient is derived. A hybrid self-synchronization chaotic stream cipher is proposedbased on the aforementioned synchronization method.To overcome the drawbacks of chaotic electronic circuits, such as low-band spectrumand limited encryption speed, Impulsive synchronization of Semiconductor laser chaoticsystems is studied in this thesis. Semiconductor laser systems can be described by the rateequations governing the photon density and carrier density. Since the carrier density is not easy to observe or measure, only photon density is used to design the impulsive controller.Some sufficient conditions for the synchronization of semiconductor laser chaotic systemsvia impulsive control are derived. Numerical simulations are presented to show theeffectiveness of the method.Consider chaotic systems with fixed parameters, by using the nonlinear dynamicforecasting method or parameter estimation method, the hidden message can be detectedonce the chaotic system is cracked. Thus make these systems insecure. Time-varyingparameters may be an effective way to solve this problem. The impulsive synchronizationof the chaotic systems with time-varying parameters is discussed in this thesis. Asynchronization scheme with impulsive intervals adaptive to the time-varying parametersis presented. And the chaotic properties of nonlinear systems with time-varyingparameters are studied under the significance of Wiggins.A time-varying parameter laser chaotic secure communication system is designed. Bycomparing the advantages and drawbacks of chaotic masking, chaotic keying and chaoticmodulation, a novel chaotic keying scheme is proposed based on chaotic hybridsynchronization theory. A time-varying parameter scheme is proposed based on Shannon’s“unicity distance” theory. The counteraction between the proposed scheme andconventional schemes is discussed. The simulation shows the time-varying parameter laserchaotic secure communication system have the virtue of good security.