激光大气闪烁的分形分析

 采用分形理论分析了激光大气闪烁的统计特征。研究结果表明：在弱起伏条件下，激光大气闪烁的分形维和奇异性随光强起伏的增强而增大，而其长期相关性则减小；不同Fresnel尺度下具有相同闪烁指数的激光大气闪烁的分形特征存在着明显的差别；在强起伏条件下，有限的数据中尚未发现分形维有饱和现象，因此可以用来描述激光大气闪烁的强度。     

聚焦光束大气传输光束扩展定标规律的数值分析

选取多种典型激光传输条件下的计算参量，对平台光束聚焦大气传输湍流热晕相互作用引起的光束扩展进行了数值分析.热畸变的产生是在快速变化的湍流扩束作用之后，在此假定的基础上，建立了描述聚焦光束大气传输光束扩展与大气传输特征物理参量的定标关系.在了解激光发射系统的特征参量和大气传输效应的特征参量之后，即可由定标关系迅速判断激光大气传输的效果，从而为激光工程系统的参量优化设计及其应用效能评估提供依据. 关键词： 湍流效应 热晕效应 光束扩展 定标规律     

激光传输大气参量测量精度要求的数值分析

 就激光大气传输湍流热晕相互作用下大气参量测量误差对激光传输效果（平均功率密度）的影响进行了数值分析，并以典型的激光传输条件为例，分析了对激光大气传输影响最为敏感的3个大气参量——风速、吸收系数以及湍流结构常数的测量精度要求。综合不同传输距离下大气参量测量误差对传输效果的影响，大气风速的测量精度要求为±10%，吸收系数及湍流结构常数的测量精度要求分别为±5%和±15%。     

间歇性大气湍流中光传播问题的近Gauss极限分析

间歇性是湍流的重要特征，多年来一直是湍流研究的核心内容之一.考虑大气湍流中的光传播问题，一般回避其间歇性，假设大气介电起伏满足Gauss统计.考虑大气湍流（内）间歇性 对光波传播的影响.考虑到大气介电起伏方差较小的事实，将光场统计矩方程在Gauss场附近 展开到四阶累计量，分析其近似解.进一步，以层次结构模型为基础，着重研究了光场二阶 统计矩的间歇性效应.研究表明，大气湍流间歇性对光场的影响很小. 关键词： 光波传播 大气湍流 间歇性     

大气湍流对激光通信系统的影响

从分析激光在大气湍流场中的传输方程出发,忽略系统中的其它噪声,仅考虑由大气喘流引起的系统误码率,讨论了激光信号在传输过程中的振幅起伏以及强度起伏;推导出由大气湍流引起的光强起伏和系统误码率的关系,结果表明:在弱起伏条件下,对于系统误码率为10－9以下的要求,光强起伏应小于0.67;随着湍流强度C2n的增大,误码率增加很快;采用长波长的激光进行传输可以有效地降低系统误码率.     

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)。     

大气激光通信中多光束传输性能分析和信道建模

多光束传输技术是克服大气激光通信中大气湍流效应的有效途径之一。首先从理论上分析了大气湍流对多光束大气激光通信系统性能的影响和多光束大气传输的光强起伏特性,然后利用统计分析的方法,建立了一个以传输距离z、光束数目n、发射孔径之间的距离St、接收孔径Dr等为参量的多光束大气传输信道模型。最后,结合相关文献提供的实验结果对该信道模型进行了实验验证和误码性能分析。结果表明,当S1≥√λz或Dr远大于大气湍流相干长度ρo时,随着n的增大,接收光强将趋于对数正态分布．降低了大气激光通信系统的误码率,从而验证了多光束传输对于克服大气湍流影响的有效性。     

大气激光通信中稳定跟踪技术研究

为了研究大气激光通信中稳定跟踪所采用的器件及有效跟踪方法,分析了光束定位探测器件(电荷耦合器件和四象限光电探测器)的灵敏度特性,结合影响大气激光通信跟踪系统性能的五种大气湍流效应,分别讨论了光束漂移、光强起伏、光斑弥散、到达角起伏及光束扩展的原理.对质心跟踪算法和形心跟踪算法定位准确度的影响进行分析,得到在大气条件下形...     

湍流路径积分参量与湍流大气中光的传播效应

根据光波在湍流大气中传播的理论分析了弱起伏条件下决定光波传播效应的折射率结构常数Ｃ＾２ｎ的各种路径积分参量之间的关系，导出了由光波的强度起伏所确定的等效折射率结构常数对计算大气相干长度与光波的到达角起伏，光束的扩展及漂移的相对偏差的表达式。针对折射率结构常数Ｃ＾２ｎ具有周期性，递减，递增和随     

激光收发望远镜的几何布局对光束到达角起伏相关性的影响

大气湍流的倾斜扰动会导致光束在通过大气后到达角发生起伏。对于激光测距、激光导引星等应用场合,激光分别在发射和接收时两次通过了湍流大气。激光收发望远镜几何布局方式的不同会引起激光发射和接收光路中的倾斜相关性出现差异,最终影响接收望远镜探测到的激光到达角起伏。从激光收发望远镜布局的一般几何模型出发,利用Zernike多项式波前展开,给出了受大气湍流影响的分离孔径倾斜相关函数。分析了收发共光路与非共光路情况下,不同的几何限制导致的光束到达角起伏相关性变化。最后,讨论了不同的激光收发望远镜的几何布局对接收望远镜探测到的光束到达角起伏的影响。     

复杂传播路径条件下激光光强起伏特性研究

对复杂地形情况下实际湍流大气中的激光强度起伏进行了系统的实验观测,分析了光强起伏统计特征的周日及全年变化规律。结果发现：复杂地形情况下实际湍流大气中的激光光强起伏,不论是概率密度分布还是其功率谱等统计特征均与理论上的偏离很大。     

实际大气中激光强度起伏的全年统计特征

对实际湍流大气中的激光强度起伏了系统的实验观测,详细分析了光强起伏的强度概率密度分布与功率谱的统计特征及春周日,全年变化规律。同时分析了探测器件的饱和铲应结果的影响。     

INNER AND OUTER SCALE EFFECTS ON SCINTILLATION INDEX FOR AN INFRARED LASER PROPAGATING ON EARTH-SPACE PATHS

The effects of the inner-and outer-scale of turbulent atmosphere on the scintillation index for an infrared laser beam propagating through atmospheric turbulence are discussed under the assumption that small-scale irradiance fluctuation is modulated by large-scale irradiance fluctuation on Earth-space paths. A model about the scintillation index with the inner-and outer-scale is developed. A numerical analysis is done by using this model. It is shown that the effect of the inner scale on scintillation index is larger than the outer scale effect for the lesser wavelength wave at visible and infrared band. From moderate to saturation regime, the inner scale effect becomes gradually small; however, the outer scale effect becomes gradually obvious. Under moderate to strong regime, therefore, the effects of the inner-and outer-scale on scintillation index must be considered for theoretical prediction scintillation of an infrared laser beam propagating through turbulent atmosphere on Earth-space paths.     

城市路径下激光光强起伏特性测量及链路余量估算

 在城市环境下进行了3.5 km的激光大气传输实验。实验中对光强起伏和到达角起伏进行了同步测量，分析了接收光强起伏的统计特性以及传输路径上大气折射率结构常数的特性。基于实验结果，对自由空间光通信中不同闪烁指数下的衰落冗余以及不同探测阈值下的衰落概率进行了估算，从而为空间光通信系统的设计提供可靠的实验基础。     

Saturation of laser irradiance fluctuations beyond a turbulent layer

An asymptotic theory of saturated fluctuations of laser irradiance diffracted by a thin layer of turbulent medium is built. The treatment is carried out in the phase screen approximation for the case of a power law spectrum of inhomogeneities in the layer. Asymptotic forms of the covariance of irradiance fluctuations are obtained and the character of the scintillation index approach to the asymptotic value is investigated. Calculations are made for collimated and focused laser beams.     

Study of Scintillation for Infrared Laser Beam Propagating in Atmospheric Turbulence on Earth-Space Paths

On earth-space paths, by applying a modification of the Rytov method that incorporates spatial frequency filter function under strong atmosphere fluctuation conditions, a tractable model is developed for the scintillation index of infrared laser Gaussian beam wave that is valid under moderate-to-strong irradiance fluctuations. At Infrared band, based on ITU-R C ² _n model, the scintillation indexes for collimation laser beam are predicted by this analytic model. The results agree with theoretic expected scintillation. This scintillation model can be converted into a plane or a sphere wave scintillation index model on earth-space paths, and also reduced to a Gaussian beam wave model on horizontal sight paths of invariable C ² _n.     

Irradiance scintillation considering finite turbulence inner scale for optical wave propagating through weak non-Kolmogorov turbulence

The turbulence inner scale plays an important role in investigating the irradiance scintillation index for optical wave propagating through atmospheric turbulence. However, previous expressions of the irradiance scintillation index, which were derived based on the general non-Kolmogorov spectral model, did not consider the influences of finite turbulence inner scale. In this study, based on the generalized exponential spectral model for non-Kolmogorov atmospheric turbulence, theoretical expressions of the irradiance scintillation index are derived for plane and spherical optical waves propagating through weak turbulence. The new expressions have considered the influences of the finite turbulence inner scale and the receiver aperture on the irradiance scintillation index. Numerical simulations are performed to analyze these parameters’ influences.     

Effect of atmospheric turbulence on propagation of ultraviolet radiation

We investigate the effect of atmospheric optical turbulence on ultraviolet (UV) radiation with a wavelength of 253.7 nm. The normalized irradiance variance (scintillation index) was measured using a UV scintillometer with a path length of 185 m. The dependence of the UV scintillations on the atmospheric turbulence structure parameter and inner scale was determined through simultaneous measurements of these quantities made with a visible laser scintillometer. The dependence of the UV scintillation index and its probability density function on receiver aperture size was also measured. It was found that the scintillation predicted by currently available models which take into account the effects of inner scale, saturation and aperture averaging was in good agreement with the measurements made under various conditions in weak turbulence.     

Analysis of Inner-Scale Effect on Atmosphere Scintillation for Infrared Laser Beam Propagating on Earth-Space Paths

On Earth-space paths, based on ITU-R C_n² (h) model, by means of a modification of the Rytov method that incorporates an amplitude spatial frequency filter function under strong fluctuation conditions, considering atmospheric turbulence inner-scale, an expression is developed for the scintillation index of a visible or a near and middle infrared laser beam wave that is valid under moderate to strong irradiance fluctuations. The quantitative analyses of scintillation index are done by this analytic model for laser Gaussian collimated beam wave at visible or near and middle infrared wave band. The results agree with theoretic analysis scintillation. This scintillation model can be converted into a Gaussian beam wave model with finite inner-scale on horizontal sight paths of invariable C_n².     

Expressions of the scintillation index for optical waves propagating through weak non-Kolmogorov turbulence based on the generalized atmospheric spectral model

The high frequency “bump” which occurs in the turbulence spectral model just prior to the turbulence cell dissipation is important for the analysis of the irradiance scintillation for optical wave propagating through atmospheric turbulence. In this study, expressions of the irradiance scintillation index are developed from the generalized modified atmospheric spectral model for optical waves propagating through weak non-Kolmogorov turbulence. Compared with the expressions of the irradiance scintillation index derived from the general non-Kolmogorov spectral model, the new expressions can consider the influences of finite turbulence inner and outer scales and the influence of finite diameter aperture receiver. As the irradiance scintillation is caused mainly by the small scale turbulence cells' diffractive effects for weak turbulence, the turbulence outer scale's influence can be ignored. Numerical simulations show that variable inner scale values produce obvious effects on the irradiance scintillation for non-Kolmogorov turbulence.