非均匀拉盖尔-高斯关联光束及其传输特性

提出了一种新型特殊关联部分相干光束即非均匀拉盖尔-高斯关联光束,其在x和y方向上的关联结构函数分别为非均匀关联和拉盖尔-高斯关联函数.基于广义惠更斯-菲涅耳衍射积分公式,推导了这种光束交叉谱密度函数在自由空间以及大气湍流中的传输式,并计算了该光束经过自由空间和湍流大气传输的光强分布和关联结构函数分布演化特性.研究发现,该光束光强分布在传输过程中展现出自聚焦、自偏移和自分裂等奇异特性,同时发现控制关联结构函数参数可以有效地降低湍流大气的影响.关联结构调控为操控光束传输行为提供了一种新颖而有效的手段,在大气激光通信、微粒操控等领域具有重要的应用前景.     

大气湍流信道中聚焦涡旋光束轨道角动量串扰特性

携带轨道角动量的涡旋光束作为传输信息的载体能有效提高信息传输效率,然而在传输过程中受大气湍流影响轨道角动量会发生串扰.基于螺旋谱分析理论,推导得到了聚焦拉盖尔高斯光束在各向异性大气湍流中传输时的螺旋谱解析表达式,并对比分析不同湍流和光束参数对聚焦与非聚焦拉盖尔高斯光束接收功率的影响,最后利用多相位屏法进行模拟验证.结果表明:随着传输距离、湍流强度、拓扑荷数的增大以及湍流内尺度、光束波长的减小,接收功率减小,轨道角动量串扰增大;接收孔径到达一定值时对轨道角动量串扰的影响非常小;聚焦光束比非聚焦光束的轨道角动量串扰要小.这些结果将对提高自由空间光通信的质量有一定意义.     

水下拉盖尔-高斯涡旋光束及其叠加态传输特性

通过实验研究了拉盖尔-高斯涡旋光束及其叠加态在水下湍流中的传输特性,充分考虑了不同温度差和盐度差的水流扩散产生的湍流对4种光束（高斯光束,阶数为0、拓扑荷数为6的拉盖尔-高斯涡旋光束,阶数为1、拓扑荷数为2与阶数为0、拓扑荷数为6的拉盖尔-高斯涡旋光叠加,阶数为1、拓扑荷数为2与阶数为0、拓扑荷数为10的拉盖尔-高斯涡旋光叠加）传输的影响,并对4种光束的漂移方差和闪烁指数进行深入讨论与分析。实验结果表明：随着湍流强度的增大,4种光束的漂移方差和闪烁指数都增大,相比其他3种光束,拉盖尔-高斯涡旋光束的漂移方差和闪烁指数较小;在较弱的湍流强度下,两种涡旋光叠加态的漂移方差和闪烁指数与拉盖尔-高斯涡旋光束相近。     

非局域非线性介质中光束传输的拉盖尔-高斯变分解

光束在非局域非线性介质中的传输过程由非局域非线性薛定谔方程描述.1+2D非局域非线性薛定谔方程可以转化为圆柱坐标系下的变分问题.通过展开介质响应函数并合理假设试探解求解变分方程,得到光束在强非局域非线性介质中的拉盖尔-高斯解.满足一定条件时,拉盖尔-高斯光束将形成光孤子或退化为高斯光束. 关键词： 非局域非线性介质 强非局域性 变分法 拉盖尔-高斯光束     

非Kolmogorov大气湍流对部分相干厄米高斯光束传输因子的影响

基于非Kolmogorov谱模型,利用广义惠更斯-菲涅耳原理和维格纳分布函数的二阶矩定义,推导出部分相干厄米高斯(H-G)光束在非Kolmogonov大气湍流中传输因子的解析表达式,并用以研究了非Kolmogorov大气湍流对部分相干H-G光束传输因子的影响。结果表明,部分相干H-G光束在非Kolmogorov大气湍流中传输时,传输距离、湍流外尺度、广义结构常量和空间相关长度越小,湍流内尺度和光束阶数越大,光束传输受非Kolmogorov大气湍流影响越小,光束质量越好。当广义指数取3.11时,部分相干H-G光束在传输过程中表现的光束质量最差。     

利用液晶空间光调制器产生高阶拉盖尔高斯光束

本文采用反射式纯相位液晶空间光调制器产生了实验需要的高阶拉盖尔高斯光束。理论上详细计算了生成高阶拉盖尔高斯(Laguerre-Gaussian)光束的最优参数选择,用MATLAB编写程序,制作产生高阶LG光束的相息图。通过CCD采集图像数据,观测了以平面波入射生成拉盖尔高斯光束的强度分布,并利用干涉法验证了产生高阶拉盖尔高斯光束的螺旋相位分布。在此基础上,对实验测得一维强度轮廓进行拟合分析并估算了实验产生LG光束的纯度。     

非傍轴矢量拉盖尔-高斯光束的二阶矩表示

基于Porras提出的光传输的非傍轴矢量矩理论，推导出初始圆偏振的非傍轴矢量拉盖尔-高斯(LG)光束的特征参数，包括束宽、远场发散角和M²因子等的公式，并表示为级数求和形式.非傍轴矢量高斯光束公式作为特例给出.研究表明，基于二阶矩定义的束宽按双曲线规律传输，当w₀/λ→0(w₀为束宽，λ为波长)时，远场发散角θ趋于90°，大于非傍轴标量理论预示的值63.435°.非傍轴矢量LG光束的M²因子不仅与模指数p有关，而且还与w₀/λ有关.最后，对非傍轴矢量LG光束和非傍轴标量LG光束的传输作了比较，结果表明在w₀/λ较小时，矢量效应对远场发散角的影响十分显著.对θ→90°引起的问题和非傍轴矢量矩理论的适用范围，以及解决问题的可能途径作了分析和讨论. 关键词： 非傍轴矢量拉盖尔-高斯光束 圆偏振 非傍轴矢量矩理论 光束参数     

在大气湍流斜程传输中拉盖高斯光束的轨道角动量的研究

大气湍流引起大气折射率随机变化,导致空间不均匀性.拉盖高斯光束在大气湍流中传输时,空间不均匀性会使光子波函数改变,引起轨道角动量的变化.本文讨论了拉盖尔高斯光束在大气斜程传输时,湍流介质改变光子轨道角动量而形成不同的光子态.计算了螺旋谐波各分量所占光束总能量的权重,分析了拉盖高斯光束的轨道角动量的变化规律.     

高斯-谢尔模型光束在海洋湍流中偏振传输特性

基于广义惠更斯-菲涅耳衍射原理和光束的偏振相干统一理论,研究了高斯-谢尔模型光束在Kolmogorov海洋湍流中传输时偏振特性的变化规律,得出了保持偏振特性不变时的条件.在此基础上,研究了完全偏振线偏振光与圆偏振光在海洋湍流中的偏振传输特性,结果表明:在Kolmogorov海洋湍流下,完全偏振的高斯-谢尔模型光束的偏振特性不受海洋湍流影响.     

多厄米-高斯子波束的传输特性研究

基于拉盖尔-高斯模和厄米-高斯模之间相互转换的关系,将拉盖尔-高斯涡旋光束展开成多厄米-高斯子波束,给出了模式转换关系的强度分布图。采用惠更斯-菲涅尔原理,推导了非傍轴条件下多厄米-高斯子波束在真空中传输时接收面处的场分布表达式。采用数值分析方法对真空传输过程中不同传输距离和拓扑荷下的多厄米高斯子波束的强度和相位分布进行了分析。结果表明:在短距离传输范围内,多厄米-高斯子波束在主扇位区域具有高光强能量集中,可以用于精确的光学操控。随着传输距离的增大,子波束场的等相位线由直线变为平滑的弧线。在远距离传输时,非傍轴近似多厄米-高斯子波束的相位变化远远小于傍轴近似拉盖尔-高斯光束的相位分布。由此可见,非傍轴近似条件下多厄米-高斯子波束更适合远距离传输。所研究的结果可为高拓扑荷、宽发散角拉盖尔-高斯涡旋光束的传输提供理论基础。     

Capacity of wander and spread beams in log-normal distribution non-Kolmogorov turbulence optical links

We model the average channel capacity of optical wireless communication systems in weak turbulence horizontal channels, using the log-normal distribution models. The effects of beam wander and spread, pointing errors, turbulence inner scale, turbulence outer scale and the spectral index of non-Kolmogorov turbulence on system's performance are included. The model can evaluate the influence of the atmospheric turbulence conditions in the performance of a ground-to-train optical wireless communication system.     

Capacity for non-Kolmogorov turbulent optical links with beam wander and pointing errors

The performance of optical wireless communication links depends strongly on the atmospheric conditions and the parameters of the link such as the propagation distance, the operation wavelength, jitter variance, attenuation coefficient and effective beam spot radius at the receiver. The analytical expression for the evaluation of the average capacity of optical wireless communication systems is derived, using the gamma-gamma distribution in the non-Kolmogorov atmosphere turbulence. The impact of atmospheric attenuation, beam wander and pointing errors on the average of the optical wireless communication link is investigated. It is shown that the capacity has a fluctuation curve, when power law α increases, and the power law α of minimum point in fluctuation curve is bigger as the non-Kolmogorov turbulence strength is stronger.     

Scintillation of a laser beam propagation through non-Kolmogorov strong turbulence

Atmospheric turbulence causes strong irradiance fluctuations of propagating optical wave under the severe weather conditions in long-distance free space optical communication. In this paper, the scintillation index for a Gaussian beam wave propagation through non-Kolmogorov turbulent atmosphere is derived in strong fluctuation regime, using non-Kolmogorov spectrum with a generalized power law exponent and the extended Rytov theory with a modified spatial filter function. The analytic expressions are obtained and then used to analyze the effect of power law, refractive-index structure parameter, propagation distance, phase radius of curvature, beam width and wavelength on scintillation index of Gaussian beam under the strong atmospheric turbulence. It shows that, with the increasing of structure parameter or propagation distance, scintillation index increases sharply up to the peak point and then decreases gradually toward unity at rates depending on power law. And there exist optimal value of radius of curvature and beam width for minimizing the value of scintillation index and long wavelength for mitigating the effect of non-Kolmogorov strong turbulence on link performance.     

Elegant Laguerre-Gaussian beam in a turbulent atmosphere

Paraxial propagation of an elegant Laguerre-Gaussian beam (ELGB) in a turbulent atmosphere is investigated in detail. Analytical formulae for the average intensity and effective beam size of an ELGB in a turbulent atmosphere are derived based on the extended Huygens-Fresnel integral. The average intensity and spreading properties of an ELGB in a turbulent atmosphere are studied numerically and compared with those of a standard Laguerre-Gaussian beam (SLGB). Our results indicate that the propagation properties of an ELGB in a turbulent atmosphere are much different from its properties in free space, and are closely related to its beam parameters and the structure constant of the atmospheric turbulence. The ELGB with higher mode orders is less affected by the turbulence. The SLGB spreads more rapidly than the ELGB in a turbulent atmosphere under the same conditions. Our results will be useful in long-distance free-space optical communications. ©2010 Elsevier Science B.V. All rights reserved.     

Average capacity for non-Kolmogorov turbulent slant optical links with beam wander corrected and pointing errors

The effects of beam wander corrected and pointing errors on the average capacity of a non-Kolmogorov turbulent atmosphere communication channel are studied. The gamma–gamma irradiance probability density function model has been considered to include the effects of non-Kolmogorov turbulence. An expression for the average capacity under beam wander corrected, pointing errors and the spectral index of refractive-index fluctuation. Simulation results for effects of circular detection aperture of radius, atmosphere turbulence strength and wavelength on average capacity are presented. Our results show that the average capacity has an oscillation in the case of short propagation distance, but in the case of long propagation distance, it has gradually reduced curves. And the turbulence strength has a strong influence on average capacity. In general, turbulence reduces the average capacity; it can reduce average capacity by increasing of the power law α. We can ignore the little effects of wavelength increases on the average capacity.     

LIDAR systems operating in a non-Kolmogorov turbulent atmosphere

ABSTRACT

It is well known that in free atmosphere the Kolmogorov power spectrum of the refractive index might not properly describe the actual turbulence behavior. In this paper, we use general non-Kolmogorov power spectrum for theoretical investigation of laser beam propagation in the double-passage problem: transmitter–target–receiver. The major application of our work is the Light Detection And Ranging (LIDAR) system operating at high altitudes, where non-Kolmogorov turbulence may be present. On confining ourselves to the weak turbulence regime, we show that the long-term average beam intensity profile, the long-term beam spread and the scintillation index are substantially affected by the non-Kolmogorov turbulent channels. Our analysis is valid for both bi-static and mono-static configurations, the latter leading to the enhanced backscattering effects.     

Propagation factor of electromagnetic concentric rings Schell-model beams in non-Kolmogorov turbulence

We derive an analytical expression for the propagation factor(known as M2-factor) of electromagnetic concentric rings Schell-model(EM CRSM) beams in non-Kolmogorov turbulence by utilizing the extended Huygens–Fresnel diffraction integral formula and the second-order moments of the Wigner distribution function(WDF). Our results show that the EM CRSM beam has advantage over the scalar CRSM beam for reducing the turbulence-induced degradation under suitable conditions. The EM CRSM beam with multi-rings far-fields in free space is less affected by the turbulence than the one with dark-hollow far-fields or the electromagnetic Gaussian Schell-model(EGSM) beam. The dependence of the M2-factor on the beam parameters and the turbulence are investigated in detail.     

Propagation properties of partially coherent Hermiteben Gaussian beams through non-Kolmogorov turbulence

The propagation properties of partially coherent Hermite—Gaussian beams through non-Kolmogorov atmospheric turbulence are studied. The effects of non-Kolmogorov turbulence and beam nonparaxiality on the average intensity evolution and the beam-width spreading are stressed. It is found that the evolution of the average intensity distribution and the beam-width spreading depends on the generalized exponent factor, namely, on the non-Kolmogorov turbulence strength for the paraxial case. For the non-paraxial case the effect of the turbulence is negligible, while the beam-width spreading becomes very large. The analytical results are illustrated numerically and interpreted physically.     

Average capacity of free-space optical systems for a partially coherent beam propagating through non-Kolmogorov turbulence

The performance of partially coherent free-space optical links is investigated in the moderate to strong fluctuation regime of non-Kolmogorov turbulence. The expressions for large- and small-scale log-irradiance flux variance are obtained in non-Kolmogorov turbulence. By employing the gamma-gamma distribution of irradiance fluctuations, the effects of spatial coherence of the source, index of non-Kolmogorov spectrum, and size of the receiver on channel capacity for horizontal links are discussed. Results show that channel capacity presents fluctuating behaviors with the variation of alpha for longer links and increases for alpha values higher than 11/3.