A semiconductor laser system with optical injection scheme is investigated, specifically its synchronization characteristics and the application in chaotic communications. Both experimental and theoretical approaches are carried out to address this subject. It is known that a semiconductor laser subject to optical injection has a route to chaos through period-doubling bifurcation. When two injection-locked semiconductor lasers, as a transmitter and a receiver, are unidirectionally coupled, the dynamics of the receiver laser can synchronize to that of the transmitter laser if both the intrinsic laser parameters and operating condition satisfy the requirement of chaos synchronization theory. Since the system is a fully optical system, when chaos synchronization is achieved, the entire chaotic optical field of the receiver laser duplicates that of the transmitter laser. This requires that the fast-varying optical phase, the slowly-varying phase and the amplitude of the chaotic envelope are simultaneously synchronized. In the experiment, a good chaos synchronization has been observed. The condition satisfies what the synchronization theory requires. Furthermore, two other types of synchronous phenomena different from chaos synchronization are observed experimentally. These three types of synchronous phenomena could demonstrate all the synchronous phenomena between two nonlinear oscillators occurring in nature. The application of chaos synchronization in private communications is also investigated. Different message encoding and decoding schemes at 10 Gbits/s are investigated and compared. The numerical analysis shows that two types of synchronization error, namely the synchronization deviation and the desynchronization bursts, appear in this system. A low-pass filter can decrease the synchronization deviation, but not the desynchronization bursts. Therefore, the system performance in high-speed communication at 10 Gbits/s depends on the occurrence probability of the desynchronization bursts.
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