Research On Performance Of Modulation Format And Fiber-optic Channel For Chaotic Secure Optical Communications

High dimensional, broadband chaotic optical signal can be generated by a semiconductor laser with external disturbance, which is an ideal carrier for high-speed secure communications. Semiconductor laser has the features such as small size, low cost, mature technique, widely usage, and so on. Therefore, during the past decade chaotic optical secure communications based on semiconductor lasers has aroused widespread concern, and has been rapidly investigated. The development of the chaotic optical secure communications will be eventually based on optical fiber as the transmission medium to transmit high-speed data. Focuses on the performance of chaotic optical secure communication system with encrypting the message by different ways such as chaos modulation (CMO) and chaos masking (CMS), the analyses of what the influences on the system due to employing optical fiber transmission are presented. The main works of this thesis are listed as follows:(1) The back-to-back chaotic secure communications system is established. The sinusoidal and digital information are transmitted confidentially based on the chaos synchronization between the receiver and transmitter. The performance of chaotic optical secure communications systems are numerically studied under encrypting the message by the CMO or CMS. The variation of the signal to noise ratio (SNR) and Q-factor are demonstrated when sinusoidal and digital information are respectively transmitted by CMO or CMS. The results indicate that different modulation depth (MD) or modulation frequency/rate will affect system performance, also show that the performance of CMO system is better than that of CMS.(2) The chaotic secure communications system which with optical fiber channel transmission is also established. With optical fiber transmission the chaos synchronization of the system will be reduced due to the chaos carrier will be affected by the loss, dispersion and nonlinearity, and other physical characteristics of the fiber. Correlation coefficient and synchronization error respectively as a function of the transmission distance for different dispersion values and nonlinearity values are analyzed. Q-factor is demonstrated when different rate as 1Gb/s,1.25Gb/s and 2.5Gb/s are respectively transmitted by CMO. The results indicate that synchronization of the system decreases with the fiber length going, and the greater dispersion or nonlinearity values, the greater the degree of synchronization decrease. The results also show that the Q factor decreases with the fiber or the rate increasing for CMO system.(3) In order to restrain the fiber loss, EDFA is employed to amplify the chaos carrier. To the dispersion influence, symmetric and asymmetrical dispersion compensation maps are proposed to improve the system performance. The results show that the proposed schemes can effectively improve the system performance, and the symmetric scheme is more effective than the asymmetrical.