Time Delay Signature Elimination And Key Space Enhancement In A Semiconductor Laser By Dispersive Feedback From A Chirped FBG

Optical chaos secure communication is a novel technique that uses a wideband chaotic signal as a carrier for hiding information,and then use chaotic synchronization for information demodulation.It has attracted widespread attention because of its striking advantages such as hardware encryption,high transmission rate,long transmission distance and integration with the Internet.The security of chaos secure communication relies on the matching parameters of the transmitter and the receiver(i.e.,chaotic transceiver).The hardware parameters act as key parameters and the security and key space of parameters determine the degree of chaos security.Chaotic semiconductor lasers with mirror feedback are widely used as chaotic transceivers due to easy integration and simple structure.Unfortunately,its key space is limited due to the fact that optical chaos secure communication system based on mirror feedback semiconductor laser has some security risks.On the one hand,the linear feedback of the mirror leads to the obvious time delay signature(TDS)in the time-frequency characteristic of chaotic carrier,making it possible for the eavesdropper reconstruct the system.On the other hand,the TDS makes the delay unable to be used as the key parameter,resulting in the limited key space.To solve the aforementioned problems,the semiconductor laser subject to dispersive optical feedback from a chirped fiber Bragg grating(CFBG)is proposed to generate the chaotic signals without TDS.Furthermore,the chaotic source is used as the transceiver in the optical chaos security communication system,which improves the key space and enhances the security of the system.The related work and results of this paper are summarized as follows.1)We propose a novel feedback scheme in which chromatic dispersion components are utilized as reflector instead of mirror,and the nonlinear feedback caused by feedback light dispersion generates chaotic laser without TDS.In experiments,compared with mirror feedback,the TDS losses 10 d B by using a CFBG with dispersion of 2000ps/nm.The effects of the feedback strength and dispersion on the time delay characteristics are analyzed experimentally.2)The theoretical model of a semiconductor laser with chirped fiber Bragg grating feedback is established according to the conventional equation LangKobayashi.Theoretical studies have revealed the reasons that the CFBG provides additional frequency-dependent delay caused by dispersion,and thus induces external-cavity modes with irregular mode separation rather than a fixed separation induced by mirror feedback.Numerical simulation analyzes the effect of the feedback strength,feedback delay,dispersion and the laser frequency detuning of CFBG on the TDS.In addition,critical dispersion for eliminating TDS is obtained and equals to the reciprocal of the laser relaxation oscillation frequency and the spectral linewidth product.3)The complexity of a semiconductor laser subject to CFBG feedback is investigated by using the permutation entropy.With the increase of feedback strength,the permutation entropy of the chirped fiber Bragg grating feedback semiconductor laser increases continuously,approaching 1,while that of mirror feedback semiconductor lasers decreases rapidly.With the increase of dispersion,the permutation entropy of the chirped fiber grating feedback semiconductor laser rises continuously.When the dispersion exceeds 2000ps/nm,the permutation entropy always remains around 1.4)We propose and demonstrate a key space enhancement by using semiconductor laser with optical feedback from a CFBG.Theoretical analysis shows that the key space is limited due to the fact that the laser bias current and the feedback delay time cannot be used as key parameters in the semiconductor laser with mirror feedback.For the semiconductor laser with a CFBG feedback,the dimension of key space increases due to feedback delay time,grating dispersion and center frequency.Besides,chaos synchronization of CFBGfeedback lasers is more sensitive to parameter mismatch.Resultantly,the key space of semiconductor laser subject to CFBG feedback is enhanced by 244 times than that using mirror feedback with a data rate of 2.5 Gb/s and a coupling strength of 0.447.Furthermore,as the coupling strength decreases or data rate increases,the key space increases due to the fact that chaos synchronization becomes more sensitive to parameter mismatch.