Investigation Of Broadband Chaos Secure Communication Based On VCSELs

With the rapid development of science and technology, the social informatization has been improved, which pushes the increasing requirements on the capacity, the speed and the security of communication, so that the new secure communication system with high speed, large capacity and high degree of security has great application prospects. The features of chaotic signal such as the unpredictability, high complexity, easy to implement, make the chaos communication security when it is used as the chaotic carrier. Especially, the chaos secure communication based on semiconductor lasers (SLs) has been a research hotspot in this field because of the high transmission rate supported in this system and good compatibility with existing optical fiber communication system. The introduction of time-delay mechanism during the chaos producing causes the apparent time-delay signature (TDS) in chaos signal, which may make communication system reconstructed and hence threaten the security of chaotic secure communication. Moreover, since the chaotic intensity oscillation in SL is usually dominated by laser’s relaxation oscillation, the effective bandwidth of optical chaos is normally restricted to several gigahertz. The consequence is that the message transmission rate in chaos-based communication will be limited. Therefore, the TDS suppressed and bandwidth-enhanced chaos carrier has great significance for achieving the high capacity and secure chaos communication.Compared with traditional edging-emitting semiconductor lasers (EELs), vertical-cavity surface-emitting lasers (VCSELs) possess numerous advantages such as single longitudinal-mode operation, low threshold current, circular output beam with narrow divergence, low cost and easy large-scale integration into two-dimensional arrays etc. Especially, the outputs of VCSELs generally include two orthogonal polarization components (i.e., x polarization component and y polarization component) due to weak material and cavity anisotropies, which can offer more perturbation sources and some novel polarization-related perturbation patterns such as polarization-preserved optical feedback, polarization-rotated optical feedback, variable-polarization optical feedback and corresponding optical injections, etc. Therefore, it’s reasonably necessary to study the TDS suppression and bandwidth enhancement of optical chaos based on VCSELs and its application in chaos secure communication.In this paper, after briefly introducing the basic theories of chaos and VCSELs, we focus on time-delay signature suppression and bandwidth enhancement of polarization-resolved chaos based on the dual-path mutually coupled VCSELs (DPMC-VCSELs) system and compare that with single-path mutually coupled VCSELs (SPMC-VCSELs) system. Using self-correlation function (SF) and permutation entropy (PE) to identify the time-delay signature, the effect of coupling delay time on TDS suppression has been studied and analyzed. The results show that the TDS can be well suppressed and has a roughly robustness to coupling strength as long as the difference of two coupling delay times satisfies a half relaxation oscillation period of the DPMC-VCSELs. Under the optimal coupling delay time, the influences of frequency detuning and coupling strength on both TDS suppression and chaos bandwidth enhancement have also been investigated, meanwhile, combining the TDS evolution and bandwidth maps, the optimal parameters to obtain the TDS suppressed and bandwidth-enhanced polarization-resolved chaos can be determined. What’s more, based on the polarization-resolved chaos synchronization between twin 1550 nm vertical-cavity surface-emitting lasers (VCSELs), a novel long-haul dual-channel bidirectional chaos communication system is proposed. In this system, a time delay signature (TDS) suppressed chaotic signal, generated by a driving VCSEL (D-VCSEL) under double external cavity feedbacks (DECFs), simultaneously injects into twin VCSELs by variable-polarization optical injection (VPOI) to synchronize them and enhance the chaos output bandwidth of the two VCSELs. The polarization-resolved synchronization performances between these two bandwidth-enhanced chaotic carrier signals are investigated, and the robustness of the synchronization performances to some mismatched inner parameters is also discussed. Finally, based on the high-quality polarization-resolved chaos synchronization between VCSELl and VCSEL2, the chaos communication performances have been analyzed for different kinds of fiber channels. The simulated results show that four 10 Gb/s messages can be decrypted effectively over 15 km SMF channels or 140 km DSF channels.