Study On Time-Delay And Bandwidth Of Chaos In Semicondcutor Lasers

As commonly used optical sources, semiconductor lasers (SLs) tend have rich dynamical behaviors because of the inclusion of additional degrees of freedom, and thus have gained considerable attention. In particular, under proper circumstances, they can generate complex chaotic signals, which can find applications in ultrafast random number generators (RNGs), secure communications, and reservoir computing. With the development of large-scale integrated circuits, chaos cryptography that has potential applications to information security and secure communications becomes more close to the life and social reality. Many researchers, however, have found that the chaotic laser sources contain time delay signature which caused by external cavity, this signature can be easily identified by means of statistical analyses, which not only degrades the security of optical chaos based communication, but also significantly increase the complexity of the post processing techniques. Hence, it is great importance from both theoretical and practical perspective to conceal the time delay signature. In this work, we establish two methods to conceal time delay signature and enhance bandwidth based on the external cavity semiconductor lasers. The details are as follows:(1) Based on the structure of external cavity semiconductor laser (ECSL) single chaotic optical injection, a method to conceal time delay signature by introducing parameter mismatch to semiconductor lasers is established. The effect of parameter mismatch between the master and slave lasers on the concealment of time delay signature is investigated numerically, in terms of autocorrelation function, as well as the combined effect of parameter mismatch, injection and frequency detuning. It is shown that, the parameter mismatch is helpful in concealing time delay signature. In particular, when the parameter mismatch value is within the range of -0.4～0, the injection strength is limited to 10 ns-1～20ns-1 and the frequency detuning between the two lasers is controlled around -20GHz, the time delay signature can be more effectively concealed. Therefore, the chaotic signals generated by semiconductor lasers subject to single chaotic optical injection can enhance security of chaotic carrier significantly.(2) Using by external cavity semiconductor laser (ECSL) and semiconductor ring lasers (SRLs), an external injected semiconductor lasers system is proposed. The system is composed by three SLs, a master external cavity laser (M-SL), a solitary intermediate semiconductor ring laser (I-SRL), and a solitary slave semiconductor ring laser (S-SRL). Then the system is investigated numerically and the result shows that the system can generate wideband chaos with time delay signature concealment for a wide region of injection strength and frequency detuning. The physical mechanism is that M-SL and I-SRL are used for time delay signature concealment, and the I-SRL and S-SRL are used for bandwidth enhancement. Hence the SL system is highly desirable for security-enhanced chaotic communication as well as the generation of high speed random number generation.