基于大气激光通信系统的实验测量研究

文中介绍了激光通过大气随机信道的光强分布测试系统的结构、工作原理和关键技术,在设计激光光斑测量系统基础上,进行了不同气象条件下的视距实验测量,给出了不同天气情况下的测量结果并进行了分析和讨论。用此系统可以方便、准确、形象地观测激光通过大气随机信道后光强的变化情况,为进一步建立大气随机信道模型提供了有力的依据。     

双向大气信道激光传输的信道互易性研究

在大气信道激光传输中,大气湍流对系统性能会产生较大影响,主要体现为降低传输速率和增加误码率。在具有信道互易性的双向激光传输链路中,两终端光斑信号强度的变化相关,可以在终端提取信道状态信息,以对信道影响进行补偿,从而提高传输速率。本文首先在弱湍流条件下,根据Rytov近似理论推导了平面波双向传输链路接收到的光斑信号的相关系数与传输路径的关系,并给出解析式。结果表明,两终端接收的光斑信号的光通量具有相关性,且相关系数与传输路径有关。进一步搭建了双向收发共轴激光传输系统,并进行了外场试验,试验结果不仅验证了双向大气信道激光传输链路具有互易性,且两接收端光斑信号光强的实时变化趋势相同。本文结论对实现大气信道高速率、低误码率激光传输具有重要意义。     

高能激光环形光束近场和远场传输特性

为满足高能激光环形光束在近场区和远场区的实际应用需求,从电磁波衍射积分方程出发,推导了环形光束光场分布和远场光强分布表达式,并对光场分布和光强分布进行了分析,得到光强分布与高斯光束的有限孔径大小、中心遮拦比和传输距离的关系.引入大气湍流场景,采用相位屏法对环形光束在不同湍流强度下的大气传输进行了数值模拟和分析,研究了受大气湍流影响远场光斑畸变、光斑破碎、光束扩展和漂移等的增强现象.最后开展了环形光束近场区大气传输数值模拟和实验,结果表明:随着传输距离的增加,光斑中心光强越来越强,光斑逐渐趋于均匀,平均光强呈类高斯分布,近场区环形光束扩散和光斑畸变现象受大气湍流影响而增强.     

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。     

激光在湍流大气中传输的闪烁系数及其测量

 理论研究了激光光束经过湍流大气后闪烁系数的变化规律,以ARM7的嵌入式系统构建数据采集模块,在采集激光光强因为大气湍流影响而变化的数据的基础上对存储的数据使用闪烁系数数学模型进行运算,运算结果通过网络传输,由PC端的界面显示变化曲线,从而了构造一个可以感测大气湍流的远程探测系统。用转动的相位板模拟湍流大气,利用所开发的测试系统测量激光光束通过湍流大气后的闪烁系数。实验结果显示高斯光束在湍流大气中的闪烁系数随传输距离的增加而增大,与理论模拟结果基本相符。由此可见该系统工作可靠稳定,可实时测量激光光束经过湍流大气的闪烁系数。     

湍流效应对光通信链路的影响研究与仿真

通过对大气湍流效应的分析,研究了激光在大气中传输时受大气随机信道的影响,通过软件实时显示光斑的闪烁、漂移,为进一步优化激光通信系统性能设计,最终实现全天候的大气激光通信做了有意义的探索。     

激光在雨中传输的分析与计算

本文通过分析雨滴其对激光的散射和吸收作用,研究激光在大气中传输时受雨滴作用的影响,计算不同雨滴对激光光强的影响,在不同降雨下,测试激光光强的变化,并通过软件仿真实时显示光斑的闪烁、漂移,为进一步优化激光通信系统性能设计、最终实现全天候超视距的大气激光通信做有意义的探索。     

大功率TEACO2非稳腔激光器的远场传输特性

针对大功率激光器的远场传输特性,提出了一种测量TEACO,远场光束传输特性的新方法,该法采用相对测量和绝对测量相结合的方式来获得高能激光器的绝对空间光强分布。利用该方法测试了2kW脉冲TEACO,非稳腔激光器在500m处的光强分布,并对测试结果进行了分析讨论。实验结果表明,此方法简单、易行,准确度较高,而且测量过程中考虑了大气湍流的影响,光斑强度相对分布上出现了平顶、破碎并出现旁瓣现象,实测大气透过率为81．5％,最大功率密度为1．59W／cm^2,光束的横向远场发散角达到0．64mrad,基本与理论计算结果相吻合。本研究将为评估激光系统的应用效果及系统的优化设计提供重要的参考。     

强激光远场光斑强度分布测量技术

准确测量激光远场光斑强度时空分布是分析强激光大气传输效应和评价激光系统性能的有效手段。概述了测量激光光斑强度分布的几种方法及其适用性,重点叙述了基于阵列探测法的强激光远场光斑强度分布测量技术,总结分析了量热阵列法、光电阵列法和量热/光电复合法等三类阵列探测系统应用特点。最后介绍了两种分别用于测量连续波高能激光和重频脉冲激光的光电阵列靶斑仪,系统具有结构紧凑的特点,能够满足运动靶目标上强激光参数测量要求。     

大气环境下用于瞄准的激光波束传播特性研究

针对激光瞄准过程中的光斑偏移现象,结合修正的Von Karman谱的折射指数起伏和Hufnagel-Vally湍流模型上的近似积分,研究了高斯脉冲光束在湍流大气中远场水平以及斜程传播时的脉冲展宽和闪烁指数,分析了强湍流条件下1.06 μm准单色光斜程大气传输光强分布与脉冲展宽的关系,对数值结果进行比较,发现远距离传输中波长和距离对波束瞄准偏差影响较大.从理论和实验上对大气环境下激光光斑瞄准偏差进行了分析研究,研究结果表明:将激光光斑全场数据及分析结果应用到现有偏差补偿算法中,可以实现激光瞄准偏差的有效补偿,在大气能见度1 km～3 km范围内,激光瞄准偏差测量误差σA≤0.1 mrad.     

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.     

Comparison of theory and experiment in electron-beam-controlled CO2 laser oscillators

A four-temperature model describing the electron-beam-controlled CO₂ pulsed laser has been developed and used to study the time evolution of a laser pulse and the influence thereon of experimental parameters. This paper explains the experimental results of two different electron-beam CO₂ pulsed laser systems which have been built in the laboratories at Frascati.     

Investigation of a novel hybrid process of laser drilling assisted with jet electrochemical machining

Recast layer and spatter are two inherent defects commonly associated with holes produced with laser drilling. This paper reports a novel hybrid process of laser drilling assisted with jet electrochemical machining (JECM-LD) that aims to minimize such defects and improve the quality of laser-drilled holes. The process based on the application of a jet electrolyte, being aligned coaxially with the focused laser beam, on the workpiece surface during laser drilling. The effect of the jet electrolyte mainly is an electrochemical reaction with materials. The jet electrolyte also cools the workpiece and transports debris during the process. On the basis of a measurement of laser attenuation in electrolyte, an experimental apparatus system is made and JECM-LD experiments have been performed on 0.5-mm-thick 321S20 stainless steel with two lasers at wavelength of 1064 and 532 nm. It is shown that recast layer and spatter have been effectively reduced during the JECM-LD compared with laser drilling in ambient atmosphere conditions.     

Parallel computing in experimental mechanics and optical measurement: A review

Because of the merits of non-destruction, high speed, and high sensitivity, optical techniques have been developed for experimental mechanics and optical measurement. In commercial optical systems, the speed performance becomes more important and real-time systems are pursued. Among many acceleration methods, using parallel computing hardware is proven effective. In this paper, the main principles of parallel computing at an application level are introduced; the hardware platforms that support parallel computing are compared; the applications of parallel computing in experimental mechanics and optical measurement are reviewed. Parallel hardware platforms are seen to be useful for the acceleration of various problems. When the computation is time-consuming or real-time performance is required, hardware acceleration is a possible approach for consideration.     

低仰角蒙气差精密修正的新方法

由于蒙气差随着大气密度、温度和压力等条件的变化而变化,为降低蒙气差对光电测量设备测量精度的影响，提出了精密恒星回扫的新方法。在标准大气条件下，通过理论计算得到的蒙气差与实际测量值存在偏差，采用精密恒星星库，引导测量设备对观测目标附近的恒星进行事后回扫测量，对回扫测量数据进行拟合可以得到蒙气差的修正量。分析了恒星回扫方法中影响修正精度的轴系、编码器等因素，通过外场实验，在大气湍流变化均匀的情况下，采用该方法对20°仰角以下星体进行观测，测量精度可以达到1″。     

新疆实验次声台阵设计

次声测量与数据是次声各项研究的基础,而次声台阵是现代次声测量的主要形式,为此设计并建立了两个无人值守式实验次声四元台阵。描述了次声台阵的结构,对台站的仪器构成进行了说明,重点对其降噪管和通信系统进行了描述。降噪管为十字玫瑰花瓣型降噪管,给出了其降风噪性能。台站通信系统基于VPN进行架构,实现了数据的实时自动传输和FTP传输功能。实测结果表明实验台阵的噪声主要是由微气压、山背波和台阵附近的随机干扰噪声构成。台阵记录到了大量当量较小的区域地表采矿、采石微差爆破次声信号,也记录到了2013年2月15日俄罗斯陨石大爆炸事件相关的清晰的次声信号。分析结果表明,该次声台阵具有良好的性能,具有一定的推广价值。     

基于阿秒XUV-飞秒IR泵浦-探测的冷靶反冲离子动量成像谱仪

利用多道可见光谱探测系统测量了Hα、CⅢ(464.7nm)和OⅡ(441.5nnm)谱线的时间行为,得出了碳、氢和氧元素的入射通量。在简化模型下算出了氧碳间的化学溅射率,结果表明HT-6M托卡马克边界杂质产生机制主要是氢氧间的化学溅射和氧碳间的化学溅射,因此控制氧杂质尤其重要。