The Time Delay Concealment And Unpredictability Characteristics Of Chaotic Systems Based Vcsel

Chaotic signal has been used as carrier for the optical secure communication, due to its potential properties, such as continuous broadband spectrum, inherent ergodicity, noise-like et al. For the chaotic optical communication system, security should be paid more attention. Vertical-cavity surface-emitting lasers (VCSEL) has the rich polarization property, which can be utilized to enhance the security of chaotic communication system. This paper focuses on the security of the VCSEL communication system, including concealing time-delay in the intensity and phase series of master-slave VCSEL via dual-path variable polarization optical feedback, and the unpredictability of mutually coupled VCSEL by optical injection. The details are as follows:(1) The concealing time-delay of chaotic signal is generated by slave VCSEL subject to master VCSEL optical injection with dual-path variable polarization feedback. By adopting auto-correction function (ACF), chaotic time series of intensity and phase, which have the information of feedback time delay, are investigated numerically. It is shown that although the dual-path method is better than single-path in concealing time-delay, it can also recover the system feedback time-delay, when two rotating polarization degrees of the dual-path are set 45 degrees. The selection of 50 and 40 degrees of rotating polarization,3ns and 5ns of feedback time-delay for the master-slave VCSEL with dual-path feedback not only conceals the time-delay, but also makes the intensity and phase time-delay have distinctly shift, which cannot exactly reflect of feedback time delay of master-slave VCSEL and is highly desirable for security-enhanced chaotic communication system.(2) The chaotic system model of mutually coupled VCSEL is established by adding another injected VCSEL with variable polarizer optical feedback (VPOF), in order to enhance the unpredictability of mutually coupled VCSEL. The permutation entropy(PE) method, based on an information-theory, is used to quantitatively evaluate the unpredictablity of the chaotic signal. It is shown that the chaotic signals PE of mutually coupled VCSEL system can be increased, when the chaotic system is driven by another VCSEL. That is to say, the unpredictablity of coupled VCSEL system by the third VCSEL injection can be better than that with no optical injection. When the feedback strength, injection strength and rotating polarizer degrees of the driving VCSEL are set at 10ns-1,80 ns-1 and 45 degrees, and the coupled strength of coupled VCSEL is selected 80 ns-1, the PE of coupled VCSEL driven by the third VCSEL can be increased. In addition, choosing unequal feedback time-delay and coupled time-delay, and proper detuning frequency between coupled VCSEL and driving VCSEL, can effectively contribute to unpredictability-enhanced chaotic signals, which can be used to promote the security of chaotic optical communication.