激光脉冲湍流大气瞬态传输机理及对FSO通信的影响研究

Laser pulses belong to transient optical fields whose statistics is dependent on the time. The propagation of a laser pulse in turbulent atmosphere causes the changes of its transient characteristics, such as the shape distortion, temporal broadening, arrival-time fluctuations, time-varying intensity scintillations, and so on. Different from the low-speed FSO communication system using IM/DD scheme, if the speed increases beyond several Gbps, the traditional models for evaluating the effects of turbulent atmosphere on the FSO communication performance becomes invalid, in which the changes of transient characteristics of laser pulses need to be considered. First, the change mechanism of the transient characteristics of laser pulses will be investigated and the theoretical models will be derived. Second, numerical simulations of laser pulses propagating in turbulent atmosphere will be studied, and the results can be used to validate and correct the theoretical models from a different physical perspective. Third, the models with respect to the relationship between the decision signal-to-noise ratio (SNR) of FSO communication receiver and the turbulence-induced changes of transient characteristics of laser pulses will be proposed to evaluate and analyze the turbulence-induced effects on the FSO communication performance. Finally, experiments of the laser pulse propagation in turbulent atmosphere and the testing of the corresponding FSO communication bit-error-rate will be carried out, and the theoretical models can be further validated and corrected according to the comparisons between the results obtained by experiments and theoretical models. This project can provide support for the evaluation and optimization design of FSO communication systems in turbulent atmosphere.

激光脉冲属于瞬态光场，其光场统计特性依赖于具体时间位置。湍流大气传输使激光脉冲发生形状畸变、时间扩展、到达时间起伏、时变光强闪烁等瞬态特性变化。与低速率IM/DD模式FSO通信系统不同，当速率达到几十Gbps以上时，传统的基于连续光波传播理论的FSO通信湍流大气影响模型不再可靠，在模型中必须考虑激光脉冲瞬态特性变化的影响。首先研究激光脉冲湍流大气传输的瞬态特性变化机理，建立激光脉冲的瞬态特性解析理论模型；然后开展激光脉冲湍流大气传输的数值模拟研究，从不同的物理角度来检验和修正解析理论模型；接着研究湍流大气导致的激光脉冲瞬态特性变化与FSO通信判决信噪比的模型关系，以便可靠地评估和分析FSO通信中的湍流大气影响；最后开展激光脉冲湍流大气传输瞬态特性及FSO通信误码率测量实验研究，通过分析和比较实现对理论模型的检验和修正。本项目研究可为湍流大气信道FSO通信系统的性能评估与优化设计提供支持。

自由空间光通信系统通常用光脉冲来携带数据信息，建立激光脉冲在大气湍流中的瞬态传输特性理论模型有利于分析大气湍流对自由空间光通信的影响。本项目首先通过理论解析法和数值模拟手段，研究自由空间光通信系统的光脉冲大气湍流瞬态传输特性，然后再利用实验测量方法来研究实际激光脉冲经地球大气传输后的瞬态特性变化及其对自由空间光通信的影响。本项目完成了：(1) 建立了基模高斯脉冲光场在广义幂律谱湍流大气中传输时的双频互相干函数模型、基模高斯脉冲光场在广义幂律谱湍流大气中传输时的近/远场区四频交叉相干函数模型、基模高斯脉冲光场在广义幂律谱湍流大气中传输时的近/远场区系综平均脉冲光强模型、基模高斯脉冲光场在广义幂律谱湍流大气中传输时的近/远场区二阶强度矩模型，推导出基模高斯脉冲光场的时间依赖闪烁指数解析表达式；(2) 建立了高斯波束光脉冲经弱各向异性广义幂律谱湍流传输后的双频互相干函数的解析理论模型；(3) 建立了高斯波束光脉冲经极强各向异性广义幂律谱湍流传输后的双频互相干函数的解析理论模型，推导出均方脉冲时间宽度的闭型解析表大式；(4) 推导出在光束大气湍流传输二阶统计量求解中使用小间隔渐近双源球面波结构函数的前提条件；(5) 提出基于对比接收面平均光强分布的解析理论结果和数值模拟结果来验证数值模拟结果正确性的方法，完成了高斯波束光脉冲大气湍流传输的蒙特卡洛数值模拟研究工作；(6) 开展了激光脉冲大气湍流传输特性及湍流信道参数实验测量研究工作；(7) 研究了根据信道状态自适应调节光脉冲峰值发射功率的方法；(8) 分析了宽谱高斯-谢尔模态光束经大气湍流传输后的空时相干特性变化规律，并从多个不同的角度出发，研究了大气湍流对自由空间光通信的影响。发表学术论文18篇，申请发明专利7项，出版著作2部，获软件著作权1项，培养了6名研究生。