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 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.     

Wander of a Gaussian-Beam Wave Propagated Through a Non-Kolmogorov Turbulent Atmosphere

Both increasing experimental evidence and some results of theoretical investigation have shown that there exist two kinds of turbulence in the aerosphere, Kolmogorov and non-Kolmogorov turbulence. Thus, it is necessary to improve the theory of optical wave propagation through atmospheric turbulence, namely, study the laser-beam propagation in non-Kolmogorov turbulence, before analyzing the joint influence of the Kolmogorov turbulence and non-Kolmogorov one on satellite laser communication. The beam wander will lead to the performance degradation of satellite laser communication systems and exert an influence on the achievement and stability of its links. In this paper, we consider a theoretical power spectrum of refractive-index fluctuations with a generalized power law in order to derive the variance of Gaussian-beam wave wander in weak turbulence for a horizonal path and analyze the influence of spectral power-law variations on the beam wander. We show that the expression for the beam-wander variance is of concise closed form and independent of the optical wavelength.     

临近空间相干激光通信链路外差效率分析

临近空间相干激光通信链路是天地一体化高速通信网络节点间连接的重要链路。围绕外差效率这一表征大气湍流扰动后信号光和本振光相干合成的指标,推导了非均匀湍流路径上的外差效率理论表达式,并结合大气折射率结构常数廓线,开展了临近空间-地面、临近空间-临近空间和临近空间-低轨卫星三类临近空间相干激光通信链路的外差效率仿真。仿真结果表明:临近空间-低轨卫星链路可以忽略大气对外差效率的影响;如果临近空间-临近空间链路距离大于500km或是临近空间-地面链路天顶角大于60°,外差效率将小于50%,有必要采用自适应光学技术进行补偿。     

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.     

Effect of non-kolmogorov turbulence on angle-of-arrival fluctuations of starlight

We revised a non-Kolmogorov turbulent power spectrum for the refractive-index fluctuations based on the consistency between the structure function and its power spectrum and the experimental data of recent lidar measurements. We investigate the joint influence of Kolmogorov turbulence from the ground up to 6 km and non-Kolmogorov turbulence above 6 km on the fluctuations in the angle of arrival (AOA) of starlight. WE show that the AOA fluctuations of starlight are mainly determined by Kolmogorov turbulence nearby the receiver. Non-Kolmogorov turbulence is responsible for 20–40% of the total AOA fluctuations for different apertures of the receiver. In addition, the AOA fluctuations induced by non-Kolmogorov turbulence depend on the receiver aperture, outer scale, and intensity of non-Kolmogorov turbulence.     

大气弱湍流下星地光链路衰落特性影响因素分析

研究了弱湍流条件下星地光通信链路的衰落概率、平均衰落次数以及平均衰落时间3个重要参数的统计特性,并通过数值仿真分析了链路天顶角、光波长以及接收机海拔高度对链路衰落特性的影响。结果表明:减小天顶角和提高接收机探测灵敏度可以改善链路的衰落特性;1.55 m激光较0.85 m、1.06 m激光更适合于星地光通信;地面接收机应尽量建在2 000 m以上的高海拔地区。     

基于拉盖尔-高斯光束的单光子捕获概率研究

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TEM01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C2n(0),且随着C2n(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C2n(0),即大气湍流对其没有影响.     

Some limitations on optical communication reliability through Kolmogorov and non-Kolmogorov turbulence

In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received signal, affecting link performance.Influence of Kolmogorov and non-Kolmogorov turbulence statistics on laser communication links are analyzed for different propagation scenarios, and effects of different turbulence spectrum models on optical communication links are presented. Statistical estimates of the communication parameters such as the probability of fade (miss) exceeding a threshold dB level, the mean number of fades, and BER are derived and examples provided. The presented quantitative data suggest that the non-Kolmogorov turbulence effects on lasercom channels are more severe in many situations and need to be taken into account in wireless optical communication. Non-Kolmogorov turbulence is especially important for elevations above the boundary layer as well as for even low elevation paths over water.     

Effect of the outer scale on the angle of arrival variance for free-space-laser beam corrugated by non-Kolmogorov turbulence

It is well known that atmospheric turbulence causes significant variations of the arrival angle of laser beams used in free-space communications. Usually, angle-of-arrival fluctuations of an optical wave in the plane of the receiver aperture is calculated by Kolmogorov’s power spectral-density model. Unfortunately, recently increasing experimental evidence has shown that atmospheric turbulence statistics does not obey Kolmogorov’s power spectrum model in some parts of the troposphere and stratosphere. These experiments have prompted investigations of the optical-wave propagation through atmospheric turbulence described by nonclassical power spectra. In this paper, employing a new approach and considering a non-Kolmogorov power spectrum with a generalized power law instead of the constant standard power-law value 11/3 and a generalized amplitude factor instead of the constant value 0.033, we derive the variances of the angle-of-arrival fluctuations of the plane and spherical waves in a weak turbulence for the horizontal path. The concise closed-form expressions are obtained and used to analyze the influence of spectral power-law variations on the angle-of-arrival fluctuations. In addition, the outer scale effect is also analyzed.     

Improved free space optical communications performance by using time diversity

A laser beam propagating through turbulence experiences random amplitude and phase fluctuations, which can severely degrade the performance of free space optical communication systems. It this letter, time diversity is demonstrated as a technique which can decrease turbulence influence. Statistically, laser propagation along an atmospheric path is uncorrelated with an earlier-time path for a time interval greater than the atmospheric turbulence correlation time. To estimate time diversity system performance, a 2.2-km optical link is set up for comparing the fade probability of a system using time diversity with a system not using time diversity. The experimental results obtained under different turbulence conditions are shown which are in good agreement with the theory.     

星地激光通信中分布式接收阵列的特性研究

星地激光通信中,发射机对准误差及大气湍流会引起接收信号衰落,对带前置光放大的阵列接收机,分析了分布式接收阵列的抗衰落性能。研究表明,分布式接收阵列对发射机对准误差引起的衰落具有一定抑制作用,阵列中各子接收孔径之间的距离可根据发射机对准误差及湍流的强弱进行优化设计。与传统式阵列相比,分布式阵列可以采用更窄的发射光束宽度,降低对发射功率的要求。分布式阵列的另一个重要特性还在于:当实际发射机对准误差标准差大于设计值时,通过调整阵列各子接收孔径之间的距离,可以在很大程度上降低发射机对准误差引起的功率损失。     

Free-space laser communication performance in the atmospheric channel

In spite of the tremendous technical advancement of available components, the major limitation of free-space laser communication (lasercom) performance is due to the atmosphere, because a portion of the atmospheric path always includes turbulence and multiple scattering effects. Starting from a fundamental understanding of the laser communications system under diverse weather conditions, this chapter provides a comprehensive treatment of the evaluation of parameters needed for analyzing system performance. The significance of higher-order statistics of probability density functions of irradiance fluctuations due to turbulence to performance analysis is explained. Starting from link analysis, the necessary expressions relating link margin, bit-error-rate, signal-to-noise-ratio, and probability of fade statistics are presented. Results for laboratory-simulated atmospheric turbulence and multiple scattering are presented. Example numerical results for simulations of lasercom systems operating under various atmospheric conditions are presented for various scenarios such as uplink-downlink (e.g., between ground and satellite, aircraft or UAV) and horizontal (terrestrial) link. Both turbulence and multiple scattering effects have been included in the analysis with both on-off keying and pulse-position modulation schemes. Statistical estimation and computation of communication parameters presented in this chapter will be useful in designing and optimizing lasercom systems that are reliable under all weather conditions.     

Analysis of marine atmospheric turbulence effects on infrared imaging system by angle of arrival fluctuations

For the long-range infrared imaging system, the marine atmospheric turbulence degrades seriously the probability of object recognition and tracking. In this study, the angle of arrival fluctuations of an optical wave, which describes the distortion effects of marine atmospheric turbulence on an infrared optical imaging system, is investigated in detail both analytically and numerically. Analytic expressions of the angle of arrival fluctuations are derived for optical plane and spherical waves propagating through weak marine atmospheric turbulence with horizontal path, and they consider simultaneously finite turbulence inner scale, turbulence outer scale, wavelength, and aperture diameters. Numerical calculations are conducted to analyze the influence of marine weak turbulence on the infrared imaging. The results are useful for understanding the potential impact of deviations from the terrestrial turbulence.     

An improved method of detecting and processing for the beacon in satellite-to-ground laser communication links

In satellite-to-ground laser communication links, the motion and vibration of the satellite are important factors that affect the real-time requirements of pointing, acquiring, and tracking. In order to acquire again the beacon quickly and accurately after losing it for a short time, we propose an improved method of detecting and processing for the beacon. In contrast to the conventional design of processing systems for the beacon, the advantage of the system we designed is provided by the spatial environment, where the optical communication terminal is located, and the motion trace of the beacon can be predicted effectively. We perform numerical simulations and simulation experiments, with a sampling frequency of the CCD in a system of 60 Hz and frequency and amplitude of the simulation vibration signal of 1 Hz and 0.1 mrad, respectively. We assume the motion of the satellite to be of translation type. The prediction precision is not more than 6 pixels of the CCD after the algorithm converges and the experimental results correspond fairly to the numerical simulation.     

部分相干光在大气湍流中的到达角起伏

研究了部分相干高斯谢尔光束在大气湍流中的到达角起伏.主要采用湍流内外尺度的修正Von Karmon谱模型及广义惠更斯-菲涅尔原理和交叉谱密度函数推导出了部分相干光在大气湍流中的到达角起伏表达式.对比分析了湍流内外尺度、湍流强度、传输距离、源相干参数以及波长等参数对部分相干光在大气湍流水平路径上传输时的到达角起伏的影响.结果表明:随着传输距离的增加,到达角起伏越来越小;随着大气湍流内外尺度和源相干参数的增加,到达角起伏也越来越大;与部分相干光相比,完全相干光的到达角起伏受湍流影响很小;随着波长和湍流强度的减小,到达角起伏越来越小.     

自由空间宽谱部分相干光通信系统

大气湍流对自由空间光通信系统所造成的影响是不可忽略的，为了减弱湍流对空间光通信系统带来的影响，实验搭建了一套通信距离为900 m的真实大气信道宽谱部分相干光通信系统。系统采用皮秒脉冲泵浦高非线性光纤产生超连续谱并滤波得到部分相干宽谱脉冲，对其调制后完成通信。在测试过程中，实验专门设置了一条参考链路，保证了测试环境的一致性。实验结果表明，在中等湍流条件下，系统光强闪烁指数为0.035 8，相比窄线宽通信系统提升了23%，最低探测灵敏度达到了-23.35 dBm，相比窄线宽通信系统提升了42%。与窄线宽激光通信系统相比，宽谱部分相干光通信系统可以明显降低湍流引起的光功率抖动，并提升自由空间光通信系统通信性能。     

基于空间光调制器的MIMO大气传输特性研究

大气湍流对无线光通信系统的影响不可忽视,为了更准确地反映实验室模拟多输入、多输出(MIMO)大气湍流信道的实际特征,提出了一种利用相位屏来模拟MIMO大气湍流信道的方法,并针对基于液晶空间光调制器（LC-SLM）的液晶调制法展开研究,通过实验验证该方法的可行性。实验结果表明：通过相位屏模拟MIMO大气湍流信道的激光光斑发生不同程度的畸变,湍流环境下两路激光发射系统比单路发射激光系统功率稳定性好,在前向纠错误差极限（3.8×10-3）下,单个发射单个接收系统的链路代价为10.5 dB,2个发射2个接收的MIMO系统的链路代价为9.3 dB。该项研究对于实验室模拟MIMO大气湍流信道实验方法提供一种新思路。     

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心算法的跟踪误差小于质心误差的结论,并通过实验进行了验证.     

Chaotic free-space laser communication over a turbulent channel

The dynamics of errors caused by atmospheric turbulence in a self-synchronizing chaos-based communication system that stably transmits information over an approximately 5 km free-space laser link is studied experimentally. Binary information is transmitted using a chaotic sequence of short-term pulses as a carrier. The information signal slightly shifts the chaotic time position of each pulse depending on the information bit. We report the results of an experimental analysis of the atmospheric turbulence in the channel and the impact of turbulence on the bit-error-rate performance of this chaos-based communication system.