| The chaotic communication utilizes chaotic system which is sensitive to parameters toproduce broadband and noise-like chaotic signal, and uses it as the carrier to encryptinformation. This kind of physical-layer encryption mode using device parameters as keysmakes deciphering very difficult. Many scholars have made a deep study in the chaosphenomenon and chaotic communication via the modules of electronic circuit for a long time,and a large number of achievements have been made. However, two defects, that are limitedbandwidth and high attenuation, restricts electrical chaotic system to be applied in remote andhigh-speed secure communication. Chaos laser communication system uses semiconductor toproduce chaotic laser with several GHz bandwidth, and uses fiber to greatly reduce theattenuation in transmission link. Chaos laser communication system have the merits of bothchaos secure communication and fiber communication, it is able to solve the problems ofelectrical chaotic system, therefore becomes the research focus in nowadays academia.External injection and external feedback are the most common methods to cause thesemiconductor laser to generate chaotic light. In the two methods, the latter method is moresuitable for secure communication because the dimension of the light produced by it is muchhigher. Compared with the foreign research, most of domestic researches are based onexternal injection mode, thus it is necessary to study the semiconductor laser with externalfeedback and the corresponding communication system in depth. There are two typicalconfigurations for the chaos laser communication system with external feedback, close-loopand open-loop. The close-loop system and open-loop system at weak injection condition havestrict requirement on parameter matching, whereas, the open-loop system at strong injectioncondition has low requirement on parameter matching. The open-loop chaos lasercommunication system at strong injection is selected as the object that will be studied in thisarticle because it is easy to be achieved in reality.The main work and achievement of this article include five matters:(1) The dynamic characters of semiconductor laser with external optical feedback havebeen analyzed deeply. According to the equations of semiconductor laser with external opticalfeedback, the first-order disturbance equations of laser have been deduced using analytic technique based on small signal. And the dynamic characters of semiconductor laser withexternal optical feedback have been analyzed theoretically. The oscillation frequency of thelaser with feedback has been proved very near to its threshold by solving the resonancecondition of the laser, thus the equations of laser could be simplified. According to thesimplified equation, the simulation model of semiconductor laser with external opticalfeedback has been constructed using the advanced SIMULINK which was rarely used in thisresearch area. The dependence of nonlinear characters of laser on its parameters has beenanalyzed by numerical simulation.(2) The decoding method in the back-to-back chaos laser communication system understrong injection condition and in open-loop configuration has been studied. At present, mostof theoretical and experimental researches on chaos laser communication are based onclose-loop system or open-loop system under weak injection condition. However, thesynchronization of open-loop system under strong injection condition is different from theformer two systems. The common traditional subtraction decoding method is not appropriatefor decoding message in the open-loop system under strong injection because it can't trulyreflect the synchronization performance of this system. The estimation method to system gainand the adjusted subtraction decoding method using system gain have been put forward on thebasis of injection-lock synchronization in open-loop system under strong injection. It has beenproved that the system gain only depends on the injection strength, and was independentof the encryption method to message. It has also been proved that the adjusted subtractiondecoding method was markedly superior to the traditional subtraction decoding method, and itwas more suitable for the open-loop chaos laser communication under strong injection torecover the message better.(3) The parameter-mismatch performance of the open-loop chaos laser communicationsystem has been analyzed and the security of the system has been studied. It has been provedthat the maximum decoding Q factor will be obtained at proper mismatch condition in photonlifetime and carrier lifetime, and the best mismatch is negative. For a same mismatch inphoton lifetime, increasing injection coefficient would help to control wiretapping, but not tothe authorized receiving. However, for a same mismatch in carrier lifetime, increasinginjection coefficient was not conducive to both controlling wiretapping and authorized receiving. The encryption methods have been proved to have influence on theparameter-mismatch performance. Compared with CSK method, the CM method requiredstrictly on parameter matching, thus the security of the system is better.(4) The single-channel chaos laser communication system based on fiber transmissionwith dispersion compensation has been studied. At present, there are only a few researchreports on fiber-based chaos laser communication, even so, there are some problems exist inthese researches. The first is only the decoded message at a certain fiber channel is provided,whereas, the influence of fiber channel on the communication performance is rarely analyzed.The second is, dispersion compensation schemes are rarely employed, and the third-orderdispersion is neglected when second-order dispersion is near to zero. The third is, theamplified spontaneous noise (ASE) induced by erbium-doped fiber amplifier (EDFA) is oftenneglected. The last is, some results from researches on fiber-based chaos laser communicationsystem are different form that on back-to-back system. Taking into account theabove-mentioned problems, the dispersion compensation has been adopted in fiber channel,meanwhile, the third-order dispersion and ASE induced by EDFA has been considered in thisarticle. Therefore, the fiber-based chaos laser communication system which considered mostcomprehensively and was suitable for reality has been constructed. The influences of thelength ratio of two types of fiber segments in the fiber channel with dispersion compensationon the communication have been studied. It has been proved that small negative second-orderdispersion must be reserved to offset the positive frequency chirp induced by fiber nonlinearand to realize remote communication. The performances of two symmetrical dispersioncompensation schemes were always accordant. The symmetrical dispersion schemes werebetter than un-symmetry dispersion compensation schemes, and were the best dispersioncompensation schemes for the remote and high-speed communication. It has also been provedthat CM method was better than CSK method for the application of remote and high-speedcommunication, meanwhile, this result is identical with that for research on back-to-backsystem.(5) The double-channel chaos laser communication system has been established on thebasis of single-channel communication system studied before. And the disturb problembetween two channels has been analyzed. It was the first research on wavelength division multiplex of two fiber-based chaos laser communication channels. It has been proved that thebest injection power for single-channel communication system must be decreased to be fit forthe double-channel communication system in order to obtain the best decoding performance.It has also been proved that the decoding performances of two channels are associated withthe encryption method and the rate of message. In generally, two channels couldn'tsimultaneously get the best decoding performance. Besides, using CM methods for twochannels should be avoided in order to improve the performance of high speedcommunication.
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