Acquiring High Complex Degree Of Optical Chaos Based On A Semiconductor Laser With Double Optical Feedback

In recent years, semiconductor laser has been studied frequently for its applications in various fields such as secure communications, chaotic radar, fast physical random bit generation etc. In normal circumstances, with one or more external disturbances such as optical feedback, optical injection and optoelectronic feedback, the output of semiconductor laser is chaos. Compared with other methods, the device of optical feedback is relatively simple and easy to get complex chaotic state, so optical feedback gradually become one of the major way to obtain high-quality chaotic laser based on the semiconductor laser currently. With the increase of computer calculation speed, it has become possible to recover the entire time series form a part of the time series with lower complexity, but the more complexity of the time series, the harder or impossible to recover the entire time series. The research of how to get high complexity chaotic output from the optical feedback semiconductor laser system is of great significance for not only the security of chaotic secure communication but also improving the statistical performance of fast physical random bit generation.In this paper, firstly, the theories of definition of chaos, the basic characteristics of chaos and the route to chaos have been introduced. Secondly, we systematic elaborate the tools to evaluation the complexity of time series. Then, the output of semiconductor laser with external disturbances has been analyzed. Furthermore, by adopting the permutation entropy (PE) analysis, we proposed a high complex optical chaos generation scheme based on a semiconductor laser with double optical feedbacks. The simulated results show that, the delay times and the strength of feedbacks have a great influence on the complex degree of output. When the delay times of two optical feedbacks are close to each other but have a difference of about a half of the SL relaxation oscillation period, that τ1-τ2≈±1/2τRO, the characteristic value of PE reaches a maximum, and then the most complex chaotic output can be obtained. With the increase of the strength of two optical feedbacks, the characteristic value of PE firstly experiences a rapid increasing process and then decreases gradually. The regular pattern of single feedbacks is similar to double optical feedbacks. Thus, the high characteristic values of PE and high complexity chaos output can be founded within a certain range of feedback strength. Under the same feedback strength setting, the complexity of output for double optical feedback system is always higher than that of single optical feedback system, moreover, when the strength of feedback is range from8ns-1to36ns-1, the complexity of output for double optical feedback system is larger than the maximum of single optical feedback system. Compared with single optical feedback system, double optical feedback system is more conducive to get higher complexity chaotic laser output and better ensure the safety of the system under a certain extent.