基于拉盖尔-高斯光束的单光子捕获概率研究（英文）

单光子源通常采用基于高斯光束的高度衰减激光脉冲,假设激光束具有初始高斯时域脉冲波形和TEM_(01)模拉盖尔-高斯空域分布.基于折射率起伏的Rytov近似和修正von Karman谱模型,研究了大气湍流对星地量子通信单光子捕获概率的影响;建立了上行信道和下行信道的单光子捕获概率理论模型;针对低轨卫星-地面站间激光链路,对单光子捕获概率进行了分析.结果表明:上行信道的单光子捕获概率强烈依赖于地面折射率结构常数C_n~2(0),且随着C_n~2(0)的增加而减小;然而,下行信道的单光子捕获概率并不依赖于C_n~2(0),即大气湍流对其没有影响.     

基于拉盖尔-高斯光束的单光子捕获理论研究

基于TEM10模拉盖尔-高斯光束,推导自由空间量子密钥分配中单光子捕获概率表达式.针对低轨卫星-地面站间激光链路进行单光子捕获分析.结果表明:采用可高度衰减激光脉冲的TEM10模拉盖尔-高斯光束作为单光子源,单光子捕获采用前驱波参考脉冲设置时间窗口的方法,可使卫星上接收机以最大概率捕获光子.与基模高斯光束相比,采用TEM10模拉盖尔-高斯光束的优点是,不会由于卫星运动而增加单光子捕获概率的损耗.     

基于拉盖尔-高斯光束的单光子捕获理论研究

基于TEM₁₀模拉盖尔-高斯光束,推导自由空间量子密钥分配中单光子捕获概率表达式。针对低轨卫星-地面站间激光链路进行单光子捕获分析。结果表明：采用可高度衰减激光脉冲的TEM₁₀模拉盖尔-高斯光束作为单光子源,单光子捕获采用前驱波参考脉冲设置时间窗口的方法,可使卫星上接收机以最大概率捕获光子。与基模高斯光束相比,采用TEM₁₀模拉盖尔-高斯光束的优点是,不会由于卫星运动而增加单光子捕获概率的损耗。     

大气湍流中超短高斯激光脉冲的时间域展宽模型研究

穿过大气湍流的高斯激光脉冲在时间域的展宽行为与高斯脉冲通过理想高斯滤波器后的时域展宽行为相似。因此,为了激光通信信道仿真的需要,提出用级联巴特沃斯滤波器来逼近理想高斯滤波器的方法对这种时间域上的展宽行为进行建模。逼近后的高斯滤波器的3dB频率点与大气湍流的物理参数(包括折射率结构常数和湍流外尺度)和传播几何路径长度联系在一起,从而将高斯脉冲的展宽与所通过的大气湍流环境参数相结合并得到了模拟高斯脉冲展宽的解析表达式。该模型和由模型参数表示的高斯脉冲展宽的解析表达式不仅在湍流弱起伏区有效,而且在从弱起伏到中等起伏再到强起伏的整个区域中都有效。     

基于厄米-高斯光束的单光子捕获理论研究

从光波电磁场方程的TEM10和TEM01模厄米-高斯光束出发,推导了自由空间量子密钥分配的单光子捕获概率表达式.针对低轨卫星-地面站间激光链路,进行了单光子捕获分析.理论研究表明,对于低轨卫星-地面站间量子密钥分配,采用TEM10和TEM01模厄米-高斯型高度衰减激光脉冲作为单光子源是可行的.     

部分相干涡旋光束通过大气湍流上行和下行传输的比较研究

基于广义惠更斯-菲涅耳原理,以高斯-谢尔模型(GSM)涡旋光束作为典型的部分相干涡旋光束,推导出GSM涡旋光束通过大气湍流斜程传输的平均光强、均方根束宽和交叉谱密度函数的解析表达式,并用以研究了大气湍流中上行和下行对GSM涡旋光束传输和对相干涡旋的影响.结果表明,在相同条件下,GSM涡旋光束下行传输受大气湍流的影响要小于上行传输,下行传输时相干涡旋拓扑电荷守恒距离要长于上行传输.对所得结果做了物理解释. 关键词： 部分相干涡旋光束 相干涡旋 大气湍流 上行和下行传输     

基于MIMO的无人机斜程激光通信性能分析北大核心CSCD

研究了大气湍流信道下无人机“空-地”斜程激光通信的下行链路单输入多输出(single input multiple output, SIMO)和上行链路多输入单输出(multiple input single output, MISO)技术。基于双伽马(Gamma-Gamma)分布的大气湍流模型,利用Meijer函数推导了大气湍流和指向误差联合影响下的SIMO下行链路误码率闭合表达式;利用α-μ分布建立了Gamma-Gamma湍流模型的MISO上行链路误码率性能近似表达式。系统研究了天线数、天顶角和指向误差对系统的影响,研究结果表明,空间分集技术可以有效改善系统的误码率性能,当束散角为500μrad、下行链路接收器和上行链路发射器为3时就可以得到超过10 dB的性能优化;天顶角对“空-地”斜程激光通信误码率性能影响显著,要达到系统误码率10-6的性能要求,天顶角需控制在30°左右的较小范围内。     

湍流介质折射率结构常数C_n~2对双半高斯空心光束传输特性影响的研究

基于瑞利-索末菲衍射积分理论,利用交叉谱密度函数推导出双半高斯空心光束在湍流大气中传输时的解析表达式,并主要研究了湍流介质折射率结构常数C2n对空心光束传输特性的影响,得到了双半高斯空心光束在不同条件下传输时的光强分布.研究表明,C2n的增大加剧了近场中传输的空心光束的衍射效应,这不仅缩短了空心光束完全演变成为高斯光束时所需的传输距离,而且还增加了此后高斯光束向外扩展的程度.     

上下行传输孔径接收光强起伏特性分析

为了研究地星上行和星地下行激光链路孔径内接收光强信号的典型特征,基于大气湍流理论和位相屏方法,计算了短波和中波红外激光在特定大气相干长度条件下,孔径内接收激光功率与总功率的比值及起伏情况。根据统计结果讨论了上下行大气通道传播特性的差异,结果表明相同外界条件下,直径50 cm孔径内接收的下行激光信号强度大于上行激光信号强度1个数量级以上,其信号的起伏程度也低于上行激光;中等湍流下,下行激光孔径接收光强的概率分布函数服从对数正态分布,最大概率接收功率比与无湍流条件下的功率比值一致,分别为0.42%(1.315 m)和 0.26%(3.8 m)。     

超短脉冲厄米-高斯光束在湍流大气中的光谱移动和光谱跃变

基于广义惠更斯-菲涅尔原理，推导出了超短脉冲厄米-高斯(H-G)光束在湍流大气中的光谱传输方程，研究了其光谱移动和光谱跃变特性，并给予了合理的物理解释.研究表明，超短脉冲H-G 光束通过湍流大气传输其光谱特性由大气折射率结构常数、厄米多项式阶数、脉冲宽度和观察点位置坐标等因素共同决定.轴上点光谱为蓝移，随着离轴距离的增大，光谱由蓝移变为红移，无光谱移动点的位置坐标与脉冲宽度无关.此外，当湍流增强到一定程度时离轴点还存在光谱跃变现象.自由空间与湍流大气中的光谱跃变行为存在很大的差异，本文对此进行了详细的分析. 关键词： 超短脉冲厄米-高斯(H-G)光束 湍流大气 光谱移动 光谱跃变     

The effects of tilt corrected residual aberration on the detection probability of single-photon propagation in a slant path turbulent atmosphere

Based on the assumption of a pulse laser beam with an initial Gaussian temporal shape and a collimated fundamental-model Gaussian beam, the Rytov approximation and Kolmogorov spectrum model for the index-of-refraction fluctuation of atmosphere, the effect of turbulence on the probability density, acquisition transmittance probability, transmittance probability density, acquisition probability of single-photon propagation in atmospheric communication channel with z-tilt and centroid-tilt aberration corrected are studied theoretically. The probability density, acquisition transmittance probability, transmittance probability density and acquisition probability models for single-photon propagation in uplink path and downlink path are derived. Our results shown that the detection probability and the acquisition transmittance probability of the single-photon are obvious increase, when the beams are propagation in the z-tilt corrected communication channel.     

Angle of Arrival Fluctuations for Optical Waves Propagating Through Joint Moderate to Strong Atmospheric Turbulence

Satellite laser communication has gained wide attention both at home and abroad. The pointing, acquisition, and tracking (PAT) technology is the kernel of satellite laser communication systems, and the atmospheric layer is a part of the communication channel for satellite-to-ground links. Thus, angle of arrival (AOA) fluctuations caused by atmospheric turbulence inevitably influence long-distance satellite laser communication. Therefore, it is very important to analyze the impact of AOA fluctuations in satellite laser communication systems. According to the actual situation of satellite-to-ground links, a joint atmospheric turbulence power spectrum model is defined that includes Kolmogorov turbulence from the ground to 6 km in portions of the troposphere and non-Kolmogorov turbulence above 6 km in the stratosphere. Based on the extended Rytov theory, we derive the large-scale and small-scale variances of AOA fluctuations propagating in the uplink and downlink channels for a satellite laser communication system and analyze the influence of large zenith angle variations on the AOA fluctuations. It has long been a focus of concern that the expressions for the AOA variance obtained must be concise and of closed form.     

Effect of non-Kolmogorov turbulence on beam spreading in satellite laser communication

In the past half a century, satellite laser communication has caught the attention of scientists due to its distinct advantages in comparison with conventional satellite microwave communication. For ground-to-satellite and satellite-to-ground data links, the atmosphere is a part of the communication channel; thus, atmospheric turbulence severely degrades the performance of satellite laser communication systems. In general, the Kolmogorov turbulence model is used to study the effect of atmosphere turbulence on satellite laser communications since it has been confirmed by numerous direct measurements of temperature and humidity fluctuations in the atmospheric boundary layer. However, increasing experimental evidence and theoretical investigations have shown that the Kolmogorov theory is sometimes inadequate to describe atmospheric statistics properly, in particular, in some domains of the atmosphere. We analyze the joint influence of Kolmogorov turbulence from the ground to 6 km and non-Kolmogorov turbulence above 6 km on the spot size associated with the uplink and downlink propagation channels for a satellite laser communication system in the geosynchronous orbit, using a power spectrum of non-Kolmogorov turbulence with power law ?5 that describes the refractiveindex fluctuations in the atmosphere above 6 km and considering the combined power spectrum of Kolmogorov and non-Kolmogorov turbulence. Before this analysis, we study the joint influence of the Kolmogorov turbulence from the ground to 6 km and non-Kolmogorov turbulence above 6 km on the scintillation indices of laser beams.     

Influence of Non-Kolmogorov Turbulence on Bit-Error Rates in Laser Satellite Communications

In satellite–ground laser communications, atmospheric turbulence in the link degrades the performance of the system, so it is very important to estimate its influence on the performance of the system. The bit-error rate (BER) is an important parameter to indicate the communication performance of the system, which is directly used to judge the quality of the communication system. Using the results of our two previous papers, we analyze the joint influence of Kolmogorov turbulence from the ground up to 6 km and non-Kolmogorov turbulence above 6 km on the BER. These kinds of turbulence are associated respectively with the uplink and the downlink propagation channels for the geosynchronous orbit satellite communication system.     

湍流大气中激光上行和下行传输时相位奇点的演化

利用激光大气传输四维程序数值模拟了激光在湍流大气中上行和下行传输时产生的相位奇点的变化过程。由模拟结果可知,当光束自地面向空中垂直上行传输时,相位奇点数密度随传输高度的变化有一个从无到有、从快速增加到缓慢增加、达到峰值后又减小的过程;湍流越强,畸变光场中产生的相位奇点数密度越大,达到的峰值越高,且达到峰值后减小的幅度也越大,但达到峰值时对应的传输高度越低;当激光自空中某一位置垂直下行传输时,相位奇点数密度随传输距离的增加有一个从无到有、从缓慢增加到快速增加且在接近地平面处急剧增加的过程。另外,通过对模拟结果的曲线拟合发现,激光在湍流大气中上行传输时产生的相位奇点数密度与传输高度的关系符合黑体辐射公式;当激光在湍流大气中下行传输时,相位奇点数密度随传输距离的增加呈指数增加。     

大气湍流对空间光耦合至单模光纤的影响

 根据弱湍理论，考虑湍流强度闪烁、孔径平均效应、湍流波前畸变及耦合系统跟踪误差的影响，导出了空间光到单模光纤的平均耦合效率及耦合功率起伏方差模型。根据Hufnagel-Valley 5/7折射率结构模型，对下行连接及上行连接的平均耦合效率及耦合功率起伏进行了研究。仿真结果表明，对于下行连接，随着地面接收天线孔径的增大，相对功率起伏先是由于孔径平均效应而减小，而后由于湍流波前畸变的影响而逐渐增大；对于上行连接，可忽略湍流波前畸变及孔径平均效应的影响。     

Detection probability of single photons in a slant path atmospheric turbulence communication channel

Using Rytov approximation and modified von Karman spectrum model of the index-of-refraction, the detection probability of single-photon beam propagation in a atmospheric turbulence communication channel is investigated and developed. In this research, the single-photon beam is assumed as a pulse beam which has an initial Gaussian temporal shape of the pulse and a Laguerre-Gaussian fundamental-model spatial distribution. It is found that in order to obtain the messages probability greater than or equal 0.1, we should encode and dispatch 66680 photons in identical information carriers at the transmitting side, and the radius of the detector at the receiving plane must be at least up to 0.4 m when the photons propagate in a turbulent atmosphere communication channel.