| Chaos-based communication technologies have received widespread attention due to their potential applications in secure communication.The complex properties exhibited by chaos,such as continuous wide-band spectrum,natural quasi-randomness,long-term unpredictability,etc.,lay the foundation for its application in secure communication.Today,in optical communication systems,chaotic signals have been used to encrypt and decrypt information to provide security.Compared with electrical chaos,the optical chaos generated by semiconductor lasers has wider bandwidth and more complex attractors.The inherent pseudo-random nature of laser chaos makes it very difficult to predicting and reconstructing dynamics.The extreme sensitivity of laser chaos to initial parameters enables the system to generate a huge number of spreading code sequences.Therefore,optical chaotic signals can meet the needs of optical multiple access systems and become a new candidate for spread spectrum sequences.With the deepening of research,it has been found that chaotic laser light sources will expose security flaws.The traditional laser chaos generated by optical feedback and external optical injection,with the introduction of system feedback,will increase the possibility of attackers by using time-delay information to reconstruct the semiconductor laser system.The time delay signature（TDS）introduced by system feedback can be extracted from the chaotic time series by using standard delay time identification techniques,making it vulnerable to be attacked by eavesdroppers,and the randomness of the chaos is also significantly reduced.Once these internal and external parameters are exposed,the chaotic attractors can be easily reconstructed,eavesdroppers can illegally intercept messages,the security of the chaotic system will be compromised.Therefore,it is of great significance to study the delay signature hiding technology to prevent the reconstruction of chaotic dynamics and enhance the randomness of chaos.Combining with the current hotspots,this paper takes the security communication technology based on synchronous chaotic semiconductor lasers as the research object,and focuses on the chaotic signal source optimization of phase conjugate feedback（PCF）semiconductor lasers and the design of chaotic spread spectrum security communication systems.Two chaotic spread spectrum communication schemes are proposed.The specific research contents and results are as follows:（1）Firstly,a spread-spectrum secure communication system based on chaotic synchronization of semiconductor lasers is proposed.The system consists of a semiconductor laser chaotic synchronization system,a spread spectrum modulation module,an optical fiber link,and a despreading module.Using the phase conjugate feedback and the opto-electronic feedback loop to excite the semiconductor laser to generate chaotic signal as the spread spectrum code of the communication system.At the receiving end,the correlation operation is performed on the transmitted signal and the local synchronized chaotic signal to recover the signal.By calculating the largest lyapunov exponent（LLE）and the attractor of the chaotic optical power time series,it is proved that the system can generate high-complexity and high-dimensional chaotic signals.By changing some controllable parameters,analyzing the autocorrelation function（ACF）and the delayed mutual information（DMI）characteristics of the optical power signal,it is shown that this dual feedback mechanism can completely hide the time delay signatures（TDS）and ensure the security of the system.By numerically simulating the encryption and decryption of 5 Gbit/s baseband signal,it is shown that the modulated signal spectrum can be extended to 23.45 GHZ,and the system can recover the transmitted signal without internal and external noise and interference.In addition,the effects of dispersion and noise on the cross-correlation coefficient and bit error rate（BER）are simulated,and it is proved that the correlation demodulation scheme has better anti-jamming performance than the detecting-synchronization-error demodulation scheme.The results show that the chaotic laser spread spectrum system can matain a BER of 10-8when the noise level as low as-3d B,and has good communication performance and security.（2）Then,this paper presents a spread spectrum secure communication system based on two chaotic laser system.The major semiconductor laser（MSL）with phase conjugate feedback injects the chaotic output generated by the slave semiconductor laser（SSL）as the address code of the encrypted"1"bit signal.Based on the chaotic system structure,an injection-coupled chaotic synchronous laser system is designed as the address code of the"0"bit signal.Two electro-optical（EO）switches are used for chaotic spreading.At the receiving end,the local synchronized chaotic signal and the received signal are used for correlation demodulation to recover the original signal.By calculating the LLEs,attractor and bifurcation diagram of the optical power time series,the chaotic dynamics of the system is studied,which shows that the system can generate chaotic signals with high complexity and high dimensionality.By changing the controllable parameters and analyzing the amplitude change of the delay feature in the ACF and the DMI curves,it is proved that the chaotic laser system can hide the delay feature,making it very difficult for eavesdroppers to reconstruct its chaotic dynamics.The good correlation characteristics of the chaotic signals are proved by plotting the cross-correlation curves of the two chaotic signals.The spread spectrum communication of the system is simulated by using the 2 Gbit/s baseband signal,and the spectrum of the numerical simulation baseband signal is broadened to 20.36 GHz after modulation.Finally,the BER of the system under different noise levels is studied,and the results show that the dual-chaotic spread spectrum communication system can achieve high-quality secure communication with a SNR as low as 7.4 d B.In summary,this paper combines the theory of spread spectrum communication with the theory of optical chaotic communication,and proposes two chaotic spread spectrum communication systems based on synchronized semiconductor laser chaos,and carries out theoretical analysis and numerical simulation in terms of dynamical characteristics,TDS concealment performance,information transmission and bit error rate,which is an exploration and innovation of chaotic laser communication research,and is of great significance for enriching and perfecting the theory of laser chaotic communication. |
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