基于大气湍流效应的双波长激光传输特性实验研究

为研究大气湍流作用下不同波长激光信号的传输特性,建立了基于双波长条件下的激光光束传输实验测试系统,在此基础上进行了室外实验测量,得到了双波长光束同信道传输时的光束漂移和光强起伏变化的测试数据。实验结果表明,大气湍流对激光信号的传输具有较大的影响,且光强的闪烁系数与波长有很大的关系,选择波长较长的激光束可减少湍流对光强起伏的影响;光束质心漂移具有很大的随机性,且其质心变化与波长之间无明显的直接联系。实验测试数据结果与大气湍流理论相符合,对空间大气通信具有一定的参考价值。     

完美涡旋光束在大气湍流传输中的螺旋相位谱分析

基于Rytov近似,理论推导了完美涡旋光束（PVB）经过大气湍流水平信道后的螺旋相位谱解析表达式,研究了大气湍流中光束波长、半环宽、发射处轨道角动量（OAM）模态、光束半径、近地面折射率结构常数以及湍流系数对OAM模态探测概率和串扰概率的影响。结果表明：随着发射处OAM模态、传输距离、光束半径、近地面折射率结构常数以及湍流系数的增加,经大气湍流传输后的探测概率下降;随着光束波长的增加,经大气湍流传输后的探测概率增加。此外,PVB在近场的探测概率几乎不随发射处OAM模态变化,而当光束传输到远场时,探测概率随发射处OAM模态变化明显,这是因为PVB传输到远场变成类贝塞尔光束,其光束半径随发射处OAM模态变化明显。     

部分相干光束经过湍流大气传输研究进展

相较于相干光束,部分相干光束经过湍流大气传输能够有效地抑制湍流引起的光束展宽、光斑漂移及光强闪烁等扰动效应,在自由空间光通信、激光雷达和激光遥感等方面有重要的应用前景.近年来,部分相干光束湍流大气传输研究受到越来越多学者的关注.本文回顾了部分相干光束在湍流大气中传输特性研究的发展历程、理论基础及常用的理论方法,介绍了处理光束经过湍流大气传输的相位屏数值模拟方法,以及如何把该方法运用到处理部分相干光束传输.     

超连续谱激光在湍流大气中传输特性的数值仿真研究

采用多层相位屏的数值仿真方法,综合考虑大气衰减效应和湍流效应,研究了不同湍流强度和不同源相干度条件下超连续谱激光的光束扩展、光斑漂移以及闪烁特性,并对比分析了超连续谱激光传输与单色激光传输特性的差异.结果表明:湍流越强或源相干度越好,超连续谱激光的相对光束扩展越严重;湍流愈强,光斑漂移愈严重,但不同源相干度的超连续谱激光的质心漂移方差几乎相等;降低超连续谱激光的源相干度可以在一定程度上减弱光强起伏的闪烁效应,并降低闪烁指数对离轴距离的依赖性;与单色激光相比,湍流对超连续谱激光的光束扩展以及闪烁效应影响较弱,而质心漂移效应两者差异不明显.单色激光大气传输的数值仿真结果与理论解析结果基本吻合,在一定程度上说明了数值仿真过程的可靠性.该研究不仅提出了一种获得超连续谱激光大气传输特性规律的有效仿真手段,而且为超连续谱激光传输的工程应用提供参考.     

利用CCD研究激光束在大气随机信道中的传输特性

介绍了无线光通信系统中激光信号通过随机大气信道时，由于激光与大气介质的相互作用．对光信号产生的散射、吸收和湍流等效应。上述一系列效应对激光信号的传输产生巨大的影响、其中最为重要的影响效应有衰减、光束闪烁及抖动效应。通过一实验装置、利用CCD对通过大气信道的光束在不同能见度、不同大气条件和不同时刻等条件下对激光传输信号的图像的拍摄及所拍摄的图像的对比分析，验证了理论分析中的各种大气随机信道的效应。     

涡旋光束在湍流大气中的传输特性

根据广义的惠更斯-菲涅耳原理, 研究了涡旋光束在湍流大气中的传输特性。研究结果表明, 涡旋光束在湍流大气中传输时, 截面光强会从空心分布转化为高斯分布。光束所带的拓扑电荷数以及大气湍流均会影响光强分布的变化。研究结果还表明, 涡旋光束能够抑制大气湍流对光束扩展的影响, 这一现象得到了实验上的证实。通过杨氏双缝干涉的方法, 还研究了涡旋光束经过湍流大气传输后的拓扑电荷数。研究发现, 涡旋光束经过湍流大气后, 拓扑电荷数将发生波动。     

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。     

高斯涡旋光束在大气湍流传输中的特性研究

为了研究大气湍流对高斯涡旋光束传递信息的影响,理论分析了经过大气湍流的高斯涡旋光束轨道角动量(OAM)模式的径向平均功率和归一化平均功率分布、固有模式指数、初始光束半径和湍流强度;采用纯相位扰动逼近的有效性,数值模拟高斯涡旋光束在传输中的OAM模式径向平均功率分布的变化。建立传输模型并进行外场激光大气传输实验,对比分析了模拟和实测的OAM归一化平均功率分布,结果表明在弱湍流条件下,OAM模式的径向平均功率随着接收器孔径尺寸的增加而变化,逐渐趋于稳定值。对于一般常用的接收孔径,在强湍流或较小的初始光束半径条件下对OAM模式干扰十分严重。验证了用数值方法模拟OAM在湍流介质中的模式变化过程的可靠性。     

激光通信中大气闪烁引起探测噪声的计算机模拟

大气瑞流效应是光通信系统中不可忽视的一个重要问题，主要表现为大气闪烁，光束抖动、光束漂移等，这些现象都给探测系统带来附加噪声，使系统的接收灵敏度下降，本文主要讨论激光大气传输中强度闪烁效应（大气闪烁效应）的理论基础，以及强度闪烁的计算机模拟。     

湍流中漫射目标偏振部分相干激光的闪烁特性

采用广义惠更斯-菲涅耳原理和矩阵光学理论,研究了湍流大气中偏振部分相干激光波束从发射机到目标和从目标到接收机双程路径的闪烁特性。将产生任意偏振光束的琼斯矩阵和ABCD传输矩阵进行结合,围绕接收机处波场四阶矩展开推导,得出双程路径下偏振部分相干激光波束在接收机处的闪烁指数表达式,数值分析了大气折射率结构常数、激光波束的波长、束宽、相干长度对接收机处光强闪烁指数的影响。结果表明:偏振部分相干激光波束的闪烁指数随着目标与发射机之间距离呈现先增大、到达峰值后逐渐减小的变化趋势;相干性差的光束产生的闪烁指数小,相干长度微小的变化将会产生较大的闪烁指数变化,相干性好的光束产生较大的闪烁指数,但是相干长度的变化对闪烁指数的影响很小。     

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.     

激光大气闪烁的间歇特征

 以小波变换法观察了湍流大气中激光光强起伏的间歇特征, 并用奇性测度进行了定量的描述。根据间歇性参量的周日变化规律发现：激光大气闪烁的间歇性存在于所有的时间和起伏条件下,并且保持稳定,间歇性参量约为0.1,基本不随起伏条件和时间变化。     

Influence of non-kolmogorov turbulence on intensity fluctuations in laser satellite communication

Satellite laser communication holds the potential for high-bandwidth communication, but the atmosphere can significantly affect the capability of this type of communication systems for satellite-toground and ground-to-satellite data links to transfer information consistently and operate effectively. Usually the influence of atmosphere on satellite laser communication is investigated based on the Kolmogorov turbulence model. However, both increasing experimental evidence and theoretical investigations have shown that the Kolmogorov theory is sometimes incomplete to describe the atmospheric statistics properly, in particular, in some portions of the atmosphere. Considering a non-Kolmogorov turbulent power spectrum with power law −5 that describes the refractive-index fluctuations in the atmosphere above 6 km, we calculate the scintillation index of a lowest-order Gaussian-beam wave under the weak-fluctuation condition. Then, considering a combined power spectrum of refractiveindex fluctuations and using the expression obtained, we analyze 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 used in ground-to-satellite and satellite-to-ground laser communication links. We show that the scintillation index in satellite laser communication is equal to the sum of the scintillation indices induced by the Kolmogorov turbulence from ground to 6 km and that caused by the non-Kolmogorov turbulence above 6 km. Also we investigate variations of the scintillation index with the beam radius on the transmitter, wavelength, the radial distance, and zenith angle. Finally, comparing the scintillation index induced by these two turbulences with the conventional results, we show that the scintillation index induced by these two turbulences is a bit smaller than the conventional results.     

Twist phase-induced reduction in scintillation of a partially coherent beam in turbulent atmosphere

The scintillation index of a Gaussian Schell-model beam with twist phase (i.e., twisted GSM beam) in weak turbulent atmosphere is formulated with the help of a tensor method. Variations of the scintillation index of a twisted GSM beam on propagation in turbulent atmosphere are studied in detail. It is interesting to find that the scintillation index of a twisted GSM beam can be smaller than that without twist phase in weak turbulent atmosphere. Thus, modulation of the twist phase of a partially coherent beam provides a new way to reduce turbulence-induced scintillation.     

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。     

Scintillation index of elliptical Gaussian beam in turbulent atmosphere

A tensor method is used to formulate the on-axis scintillation index for an elliptical Gaussian beam (EGB; astigmatic Gaussian beam) propagating in a weak turbulent atmosphere. Variations of the on-axis scintillation of an EGB are studied. It is interesting to find that the scintillation index of an EGB can be smaller than that of a circular Gaussian beam in a weakly turbulent atmosphere under certain conditions and is closely related to the ratio of the beam waist size along the long axis to that along the short axis of the EGB, the wavelength, and the structure constant of the turbulent atmosphere.     

Scintillation properties of dark hollow beams in a weak turbulent atmosphere

The on-axis scintillation index for a circular dark hollow beam (DHB) propagating in a weak turbulent atmosphere is formulated, and the scintillation properties of a DHB are investigated in detail. The scintillation index for a DHB reduces to the scintillation index for a Gaussian beam, an annular beam and a flat-topped beam under certain conditions. It is found that the scintillation index of a DHB is closely related to the beam parameters and can be lower than that of a Gaussian beam, an annular beam and a flat-topped beam in a weak turbulent atmosphere at smaller waist sizes and longer propagation lengths. PACS 42.25.Bs; 42.68.Ay     

Scintillation index of astigmatic annular beams in a turbulent atmosphere

The scintillation properties of astigmatic annular beams in a weak turbulent atmosphere are investigated. Expression for the on-axis scintillation index of an astigmatic annular beam is derived. It is found that the scintillation index of an astigmatic annular beam can be smaller than that of a Gaussian beam, an elliptical Gaussian beam and a stigmatic annular beam in a weak turbulent atmosphere under certain conditions. The scintillation properties of astigmatic annular beams are closely controlled by its beam parameters.