Chaotic Communication And Time Delay Signatures In A Semiconductor Laser With Double Optical Feedback

The time delay signatures of external cavity are used as one of the key in a semiconductor laser based chaotic secure communication systems. In recent years, with the development of nonlinear analysis techniques, the extraction of time delay parameters becomes more and more feasible from the chaotic output of external cavity feedback semiconductor laser system. It is become a pressing problem that how to well hide the time delay signatures of external cavity feedback semiconduction laser system. The time delay signatures of a semiconductor laser with single optical feedback can be hided through adjusting the feedback strength, but it is difficut to achieve higher chaos correlation dimension and wider chaotic carrier bandwidth. Because the double optical feedback semiconductor laser systems have more controllable parameters, it is possible to hide the time delay signatures of external cavity, at the same time has higher-dimensional and larger-bandwidth, further more it can improve the system's security and the transmission rate of signal. Therefore, study the characteristics of the semiconductor laser with double optical feedback systems has practical value.This article build a semiconductor laser systems with double feedback chaotic, experimental and theoretical research for the effect of the time delay signatures and feedback strength for systems's chaotic output. The results show that, (1)for the case of the two cavities with identical feedback strengths, time delay feedback signatures of chaotic output can be suppressed efficiently when the cavity length of the M2 are approximately equal (but can not strictly equal) to the cavity length of M1 (or half of the cavity length of M1); Fixed the length of M2 to approximately equal to the cavity length of M1 (or half of the cavity length of M1), the time delay feedback signatures suppression can be further improved through adjusting the feedback strength of M2 carefully. (2)time delay signatures of chaotic output can be suppressed efficiently through adjusting the feedback strength of one cavity carefully and fixing the feedback strength of another cavity. Meantime, there is a big time shift which is the deviation between extracted delay time and real system delay time. Theory simulations is accord with experimental result.Taking a semiconductor laser with double optical feedback as a chaos transmitter, a unidirectional chaotic optical communication system has been constructed and the performances of this system have been investigated theoretically. The results show that, via by selecting reasonable parameters of this system, the time delay signatures of chaotic carrier generated by the semiconductor laser with double optical feedback can be suppressed efficiently. Through strongly injecting from transmitter to receiver, the perfect synchronization can be realized between transmitter and receiver and the synchronization performances have higher tolerance to frequency detuning between transmitted laser and received laser; Under the additive chaos modulation (ACM) encryption scheme, the 500Mbits/s encoded message can be hided in the chaotic carrier and can be successfully extracted at the receiver.