| With the rapid development of information technology, network-based modern communication technology has been embedded in all aspects of human life, constituting the cornerstone of the current environment of network society. However, the security of information has been constrained and subjected to the threat of more and more factors, with the continuous development and expansion of modern network communications system. Among them, along with the full extension of optical fiber communication technology from the backbone network to the access network, how to protect the security of information in the optical domain, as the new issues under the new situation, has become increasingly prominent, and has become a hot issue and enormous challenge in the field of cyberspace and information security. With the development of the optoelectronic technology and the nonlinear dynamics theory, chaos secure communication based on semi-classical theory has received considerable attention due to its intrinsic security features and has become one of new types of secure communication solutions. Compared with the traditional encryption algorithm, chaotic laser communication technology, which supports information encryption and decryption operations in the device and the physical layer, guarantees the physical security of the information in the network infrastructure. As commonly used optical sources, semiconductor lasers (SLs) tend to exhibit rich dynamical behaviors because of the inclusion of additional degrees of freedom, and thus have received considerable attention. In particular, under proper circumstances, they can generate complex chaotic signals, which offer broad prospects for important areas, such as ultrafast random number generators (RNGs), secure communications, and reservoir computing. With the development of large-scale integrated circuits, chaos cryptography that has potential applications to information security and secure communications becomes more close to the life and social reality. Many researchers, however, have found that the chaotic laser sources contain certain security flaws. For example, the chaotic waveforms of the most widely used external-cavity semiconductor lasers (ECSLs) exhibit a weak periodic feature, which can be easily identified by means of statistical analyses. Such a weak periodic flaw not only degrades the security of optical chaos-based communication, but also significantly increases the complexity of the post-processing techniques. This work concentrates on the chaotic outputs of SLs in accordance with the great demand and hotspot of chaos secure optical communications. The aims are to explore the time-delay signature, the complexity, and statistical properties of chaotic dynamics in a global parameter space, and hence, to optimize the chaotic signals as well as enhance the performance of the above-mentioned applications, which are of great importance from both theoretical and practical perspective.This thesis is supported by the National Natureal Science Foundation of China (Study on high-speed RNGs and communication systems based on chaotic SLs; Study on chaos synchronization and polarization properties in mutually coupled SLs subjected to multi-channel time-varying injection), by the Basic Research Project of Sichuan Province (Study on ultrafast RNGs and chaotic optical communication technologies), by the Specialized Research Fund for the Doctoral Program of Higer Education of China (Theoretical study on the control and application of nonlinear dynamics in Vertical-cavity surface emitting laser systems), and by the Funds for the Excellent Ph. D. Dissertation of Southwest Jiaotong University (Chaotic time series analysis and chaos synchronization and communication based on chaotic lasers). This dissertation focuses on the chaotic signals of SLs and the corresponding studies are composed of the follows aspects:firstly, the time-delay signature, the complexity, and statistical properties of chaotic dynamics are systematically investigated, and the corresponding physical mechanisms are deeply analyzed. Secondly, based on the novel two-channel chaos synchronization scheme, two kinds of enhanced optical chaos-based communication are achieved. Thirdly, the influence of fiber transmission on two-channel chaos synchronization and communication is numerically studied, and then the multichannel and multiuser chaos communication is designed. Finally, the influence of statistical properties of chaotic signals is explored, and two approaches for ultrafast RNG based on the chaotic dynamics of a SL are proposed. The main innovative work and achievements are summarized as follows:Firstly, the symbolic time series analysis (STSA) based method is proposed to extract the feedback parameters of an ECSL, aiming at exploring new methods for cracking key parameters of high-dimensional chaotic laser systems. It is shown that, when the system structure and the corresponding time series are known, the STSA-based method works better with respect to the autocorrelation function (ACF) and the delayed mutual information (DMI) technologies, i.e., under proper selection of the control parameters in STSA method, the time delay and the feedback strength can be successfully identified. Besides, this dissertation carries out a numerical study on the characterization of time-delay signature in a chaotic vertical-cavity surface-emitting laser (VCSEL) subject to optical feedback, where the results for polarization-preserved optical feedback (PPOF) and polarization-rotated optical feedback (PROF) are presented comparatively.Secondly, in view of the security vulnerabilities resulting from the time-delay signature in ECSLs, several schemes including a SL subjected to single-chaotic optical injection, a SL with cascaded-chaotic optical injections, and a SL suffered from dual-chaotic optical injections are proposed to achieve time-delay signature concealment. Time delay signature elimination as a function of injection strength and frequency detuning is investigated experimentally and numerically in the first two schemes, where the results show that, for large parameter regions of injection strength and frequency detuning, time delay concealment can be achieved; the concealment of time delay signature of chaotic signals, which are generated by the last scheme, is investigated numerically, and it is shown that, two scenarios of time delay concealment are identified by adjusting the injection strength and frequency detuning, that is, both the time delays of master semiconductor lasers can be masked simultaneously or only one of them can be eliminated.Thirdly, sample entropy is introduced to analyze chaos dynamics and chaos communications, as well as provides the quantitative information of the complexity of chaotic dynamics generated by an ECSL. This measure can be directly applied to distinguish different dynamic behaviors for its superior advantages, such as fast computation (available for short data), extreme simplicity, robustness, and even more efficiency in the presence of a certain amount of noise. More particularly, when employed to discriminate the encryption performance of a chaos modulation scheme, the sample entropy algorithm seems to be a powerful and complementary tool for detecting and quantifying the presence of a message within a chaotic carrier. Furthermore, the modified SampEn could be an alternative to quantify the underlying dynamics of an ECSL under the condition that the dimension, the radius, and the time delay of the delayed vectors are properly selected. The numerical results are supported by the earlier numerical studies using the permutation entropy and the Kolmogorov-Sinai entropy. In addition, SampEn also shows certain robustness to the additive observational noise.Fourthly, by constructing an experimental setup and a simulating system of an ECSL, the validity of Lang-Kobayashi model is demonstrated based on the analysis of statistical properties of chaotic signals. The first- and second-order statistics of the optical intensity of a chaotic external-cavity semiconductor DFB laser in fully-developed coherence-collapse are investigated experimentally and theoretically. The second-order statistic is characterized by the autocorrelation, where consistent experimental and theoretical results are achieved over the entire parameter range considered. For the first-order statistic, it is found that the experimental probability-density function is significantly more concentrated around the mean optical power and robust to parameter changes than theory predicts.Fifthly, two types of schemes for the security-enhanced two-channel optical chaos-based communication are proposed, the influence of the fiber transmission on the two-channel chaos synchronization and communication is analyzed, and the potential application to multichannel and multiuser system is explored. In the first scheme, a semiconductor laser (SL) subjected to polarization-rotated optical feedback serves as the drive semiconductor laser (DSL), and two identical or near-identical SLs act as the transmitter semiconductor laser (TSL) and receiver semiconductor laser (RSL). The TSL and the RSL receive identical polarization-rotated optical injection from the DSL. Two channels are used, and the TM mode of the DSL, after being rotated by 90°, serves as the perturbation quantity. The communication performance is numerically studied. More importantly, the high fidelity and high degree of privacy of the proposed scheme are verified in detail, and the failure of two potential attacker scenarios is demonstrated, even if the attacker can get access to both channels simultaneously. In the second scheme, the time-delay eliminated chaotic signal generated by a SL with dual-chaotic optical injection is employed to drive twin semiconductor lasers, and then security-enhanced two-channel optical chaotic communication using isochronous synchronization is successfully achieved. In order to gain more insight into two-channel chaos synchronization and communication, the influence of fiber dispersion, loss and nonlinearity is studied for the first time, where the bit-error rate of the recovered message is evaluated for different fiber lengths and message bit rates, respectively. In particular, acceptable communication performance can be achieved for a bit rate up to 8Gb/s when the fiber length is approximately 60km. Finally, the two-channel scheme is extended to multichannel and multiuser communication system, in which the results show that the matched users can stably achieve high-quality chaos synchronization and independent messages can be simultaneously encrypted and decrypted since the message does not affects the synchronization performance.Sixthly, multi-bit extraction schemes for fast random bit generation using an ECSL are numerically investigated in detail. The simulation shows that the statistical properties of the chaotic signal significantly influence the generation of random bit sequences. More importantly, bit sequences with verified randomness at hundreds of Gb/s, even up to the order of Tb/s could be numerically generated utilizing some effective post-processing techniques even though the original statistical distribution substantially differs from a symmetric distribution. And then, the experimental investigation of two different approaches to random bit generation based on the chaotic dynamics of a semiconductor laser with optical feedback is carried out. By computing high-order finite differences of the chaotic laser intensity time series, time series with symmetric statistical distributions that are more conducive to ultrafast random bit generation are obtained. The first approach is guided by information-theoretic considerations and could potentially reach random bit generation rates as high as 160Gb/s by extracting 4 bits per sample. The second approach is based on pragmatic considerations and could lead to rates of 2.2Tb/s by extracting 55 bits per sample.
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