Experimental Study On Chaotic Optical Communication System Based On Chaos Modulation

The currently wide used RSA public key encryption system is facing with the threat of being deciphered. It is quite urgent to find a securer way of optical fiber communication. As a type of secure optical communication technology with hardware encryption in the physical layer, the chaotic optical communication has attracted great attention and been widely studied over the past decade. Plenty of fruits have been born in aspects of theoretical study and experimental exploration of chaotic optical communication. There are still some issues and difficulties to be solved for the practical use of chaotic optical communication. With the deepening of investigation and the handling of these problems, the chaotic optical communication will make a great difference in the future. In this thesis, the chaotic optical communication system based on chaos modulation has been experimentally studied from the aspect of communication system. The main works done in this thesis are as follows.(1) The chaotic optical communication experimental system with a DFB semiconductor laser as a source has been built up, and the driving circuits of semiconductor laser have been designed and made, which mainly include the power circuit, the constant current circuit and the automatic temperature control circuit. An optical feedback structure for chaotic optical signal generation though external optical feedback consists of an optical circulator, an optical coupler and a variable optical attenuator. Open-loop chaotic synchronization structure is chosen to achieve optical chaos synchronization by optical injection in the experiments. The digital signal is modulated onto chaotic optical carrier though a MZM modulator. The optical spectrum analyzer and oscilloscope are employed to observe the optical spectrum of optical signals and the waveform of corresponding electrical signals.(2) The chaotic optical communication with date rate of1.25Gb/s and transmission distance of90km is experimentally realized. At first, optical spectrum of conventional optical signal and chaotic optical signal are observed and compared on the optical spectrum analyzer. The time-domain waveform and frequency spectrum of the electrical signal, which converted from the chaotic optical signal by a photodetector, are observed though the oscilloscope. Secondly, the optical chaos synchronization based on the open-loop scheme by the optical injection has been achieved with the correlation coefficient of0.96. At last, a binary pseudorandom sequence with data rate of1.25Gb/s and period of215-1is modulated onto chaotic optical signal though an electro-optic modulator, and recovered in the receiver after transmission of about90km in an optical fiber. The Q factor of the recovered signal reaches2.1.(3) The effects of optical fiber transmission channels on recovered signal quality of chaotic optical communication system are experimentally studied. Single channel and WDM channels between the chaotic optical communication and the conventional optical communication are separately investigated. The Q factor is employed for the system performance evaluation. The factors considered in the experiments are the optical fiber transmission distance and the incident optical power. The results indicate that for the single channel chaotic optical communication system with the transmission distance less than100km, the incident optical power is the primary factor that affects the quality of recovered signal. With the proper incident optical power, the optical fiber transmission distance has little effect on the recovered signal quality. For the WDM optical fiber channel, the effect of transmission distance on the quality of recovered signal starts to appear.