High-Order Coherence And Randomness Of Chaotic Laser For High-Speed Secure Communication

In the field of modern communications,with the increasing requirements for communication speed and security,building a more secure and high-speed communication network has become the goal of information security in various countries,but it also faces challenges.Due to its inherent randomness,chaotic light has important application prospects in the field of high-speed secure communications.At present,chaotic light has been applied in the research of various communication fields such as chaotic high-speed optical fiber communication,chaotic key distribution,and high-speed physical random number generation.In the research of high-speed secure communication,the measurement of the randomness of chaotic entropy source has always been a research hotspot,but it still faces some difficulties and problems to be solved.The previous research on chaotic light field mainly characterizes and measures the randomness and complexity of the light field from the macroscopic level,such as the bifurcation diagram,autocorrelation function,macroscopic intensity statistics,permutation entropy and other methods.However,there are still some unsolvable problems in the above methods.For example,there is a large difference between the theoretical and experimental intensity statistical distribution,the chaotic light field is difficult to distinguish from other noisy light fields,and the source of randomness of the chaotic light field exists dispute.Microscopic photon measurement can provide a new perspective to explore and measure the randomness of chaotic light field.It is expected to use quantum statistics to accurately measure the statistical characteristics of chaotic light field,and further analyze and study its randomness.Microscopic quantum statistical research of light field mainly includes photon statistical distribution and measurement of higher-order coherence.Analyzing the photon statistical distribution can study the distribution characteristics and randomness of chaotic light field under different experimental conditions at the microscopic quantum level,and provide a basis for measuring and extracting the randomness of chaotic light.The second-order coherence of the light field can not only reflect the bunching characteristics of photons,but also distinguish different light fields such as coherent and incoherent.At present,the method of analyzing the second-order coherence based on the Hanbury Brown-Twiss(HBT)scheme has been widely used in the research of spatial interference,efficient single photon detection,spatiotemporal ghost imaging and so on.However,the second-order coherence is not enough to reveal all the information contained in the light field,such as the information of the non-Gaussian scattering process and the time asymmetry.The coherence above second-order can provide more information about the quantum characteristics of multiphoton radiation.For example,the third-and fourth-order coherence of the light field can reflect the skewness and kurtosis of the photon number distribution,respectively.In experiments,due to the need for high time resolution accuracy,fast detection response,and large amount of data analysis and calculation,there are certain difficulties in measuring the coherence characteristics above second-order.When using the extended HBT scheme to measure the higher-order coherence of the light field,the system efficiency and background noise will affect the measurement results.At the same time,the resolution time of the experimental measurement system and the counting rate of the incident light intensity are also important factors in the measurement process.However,comprehensive consideration of various influencing factors to accurately measure the high-order coherence of the light field is still a problem in current researches.In response to the above problems,the main contents of this study are as follows:1)Construct a model theoretically,analyze the photon statistical distribution of chaotic light field with optical feedback,and study the second-order coherence of chaotic light field under the different conditions of injection current and feedback strength.The results show that as the feedback strength increases,the photon number statistics of the chaotic light field gradually transitions from Poisson distribution to Bose-Einstein distribution.When the feedback strength is large and the injection current is small,the value of the second-order coherence of the chaotic light field is larger,the photon bunching effect is stronger,and the indistinguishability between multiphotons is stronger.2)Considering the influence of Gaussian random noise on the photon statistical characteristics of chaotic light field,under the influence of Gaussian random noise,the second-order coherence of the chaotic light field near threshold is theoretically analyzed and experimentally measured.The analysis found that under the influence of Gaussian random noise,the second-order coherence of the chaotic light field exhibits a Gaussian random distribution,which is different from the second-order coherence determined by the chaotic light field without Gaussian random noise.The experiment results are in good agreement with the theory.It shows that the chaotic light field is a non-linear amplification of the initial value of Gaussian random noise,which further proves that the chaotic laser generated in the experiment originates from random noise.3)The traditional HBT scheme is expanded into the Double HBT scheme for measuring the high-order coherence of the light field.Taking into account the overall efficiency and background noise of the system,the results of the second-,third,and fourth-order coherence of the coherent,chaotic thermal,squeezed vacuum,and Fock states are theoretically analyzed.Through theoretical analysis,it is proved that comprehensive consideration of the system efficiency,background noise and the mean photon number of incident light,can accurately obtain high-order coherence of different light fields.In experiments,considering the effects of counting rate and resolution time of the system,the second-,third-,and fourth-order coherence of chaotic thermal light and coherent light are measured using Double HBT scheme.Under the optimal experimental conditions of counting rate of 80 kc/s and resolution time of 2¹⁰ ns,the accurately measured third-and fourth-order coherence of the chaotic thermal light is 5.99±0.02 and 23.9±0.2.Secondly,the results of the third-and fourth-order coherence of the chaotic thermal light with different delay times are measured.The above results indicate that the comprehensive analysis and research of various influencing factors such as system efficiency,background noise,counting rate and resolution time can accurately measure the high-order coherence of the light field using the Double HBT scheme.