大气激光通信链路的性能仿真

大气衰减和大气湍流严重影响着大气激光通信的链路质量。建立了大气信道的激光通信链路模型，研究了衰减信道和湍流信道中光链路的传输影响，对最大通信速率、链路功率余量和误码率进行了分析和计算。结果表明，大气湍流严重影响系统误码率，当大气闪烁指数斫是0．07时，可达到的最小误码率为10^-9。分析结果可为系统设计提供参考依据。     

沙尘信道下激光通信系统的性能分析

沙尘对激光信号的散射和吸收作用会引起激光信号的严重衰减,本文结合我国沙尘粒子的半径分布特征,在考虑含水影响的基础上,研究了沙尘对激光信号衰减特性的影响,得到了接收端信噪比的表达式;同时针对OOK调制,推导了沙尘天气下激光通信系统的误码率和信道容量。仿真分析了能见度、传输距离、含水量以及激光波长等因素对误码率和信道容量的影响。结果表明:随着含水量和传输距离的增大以及大气能见度的减小,系统的误码率会呈现出不断增大的趋势,而信道容量则呈现减小的趋势。在能见度为4 km的情况下,波长为1 550 nm、传输距离为1 km时系统的误码率可达10~(-2)数量级,且归一化信道容量为8.48 bps/Hz。     

基于MIMO的无人机斜程激光通信性能分析北大核心CSCD

研究了大气湍流信道下无人机“空-地”斜程激光通信的下行链路单输入多输出(single input multiple output, SIMO)和上行链路多输入单输出(multiple input single output, MISO)技术。基于双伽马(Gamma-Gamma)分布的大气湍流模型,利用Meijer函数推导了大气湍流和指向误差联合影响下的SIMO下行链路误码率闭合表达式;利用α-μ分布建立了Gamma-Gamma湍流模型的MISO上行链路误码率性能近似表达式。系统研究了天线数、天顶角和指向误差对系统的影响,研究结果表明,空间分集技术可以有效改善系统的误码率性能,当束散角为500μrad、下行链路接收器和上行链路发射器为3时就可以得到超过10 dB的性能优化;天顶角对“空-地”斜程激光通信误码率性能影响显著,要达到系统误码率10-6的性能要求,天顶角需控制在30°左右的较小范围内。     

弱湍流大气激光通信中的光强闪烁研究

大气激光通信中，湍流引起的光强闪烁严重影响通信系统的性能指标（误码率）。通过对湍流条件下光强分布的数值仿真，得出弱湍流时接收光强服从对数正态分布，与Rytov理论一致。用随机过程分析的方法给出了系统误码率与光强闪烁的关系，并进行MATLAB数值模拟。研究表明误码率随对数振幅方差（湍流强度）的增加而上升，且当对数振幅方差在0～0．02时，误码率随对数振幅方差的变化较快，近似呈线性递增。在工程中，可采用自适应光学或多发射机等技术抑制湍流对大气激光通信的影响。     

基于超导纳米线单光子探测器深空激光通信模型及误码率研究

高速深空通信是深空探测的关键技术之一,具备单光子灵敏度的激光通信系统将大大提高现有的深空通信速度.然而,单光子条件下的激光通信不仅需要考虑传输环境的影响,还需要考虑实际单光子探测器性能和光子数量子态的分布.本文在不考虑大气湍流影响的情况下,以光电探测模型为基础,引入超导纳米线单光子探测器(SNSPD)系统的探测效率和暗计数,建立了反应系统差错性能的数学模型,提出了系统误码率的计算公式.先对公式中的光强和激光脉冲重复频率对误码率的影响进行仿真,再通过实验结果验证仿真模型.结果表明,光强对误码率的影响最明显,随着光强从0.01光子/脉冲到1000光子/脉冲的增加,误码率从10~(-1)到10~(-7)量级明显下降;激光脉冲重复频率对误码率的影响受到不同光强的制约,但都随着脉冲重复频率的增加呈下降趋势.与此同时,当增加光强或者提高速度时,误码率高于仿真结果,约在10~(-4)量级,其原因可能是实际通信中调制光信号的消光比不足和光纤引入背景噪声提高了系统暗计数.以上模型和实验结果为进一步开展基于SNSPD的月球-地球、火星-地球等高速深空激光通信奠定了基础.     

空地激光通信跟踪精度主要外界影响因素研究

为了分析空地激光通信外界因素对跟踪精度的影响情况,将平台振动、大气湍流和背景光三个主要影响因素归结到对跟踪精度影响的统一框架下进行了研究。使用实际测量得到的平台振动数据,建立空地激光通信跟踪仿真系统,分析了不同条件下,三种外界因素对跟踪精度的综合影响情况。在文中假设基础上,分析可知:粗跟踪单元可以较好的抑制地面运动平台的振动,保证激光能够进入精跟踪视场;中强度的湍流使精跟踪残差标准差增大4倍左右,如果试验地点选择在高海拔地区,大气湍流使精跟踪残差标准差增大2倍左右。     

雾对自由空间量子通信性能的影响

为了研究雾对自由空间量子通信的影响,根据雾滴谱分布函数和消光系数,研究了雾的能见度及传输距离对链路衰减、量子通信信道容量、信道保真度及信道误码率的影响,并进行了仿真实验。结果表明,当传输距离为9 km,能见度分别为0.2 km和0.6 km时,对应的链路衰减,信道容量,信道保真度,信道误码率分别为0.76和0.25,0.24和0.4,0.91和0.97,0.01和3.26×10~(-7)。由此可见,能见度小于1 km时,雾对各项性能影响较为显著。因此,应根据雾的能见度来调整通信系统的各项参数以保证通信的顺利进行。     

大气信道对红外激光通信系统性能影响的实验研究

针对近地面、准水平红外激光大气通信系统的应用,利用微脉冲激光雷达、大口径激光闪烁仪在济南市区进行了为期一周的昼夜连续观测,获得了影响红外激光通信系统性能的关键大气参量的信息.着重分析了大气衰减信道、大气湍流信道对一公里通信链路系统性能的影响,考查了通信设备在复杂大气条件下应用的可行性及可靠性.最后就减弱大气信道对通信性能影响的方法进行了总结和探讨.     

空地激光通信系统中捕获子系统仿真

针对空地激光通信系统,推导了复合光栅螺旋扫描捕获方法所需的最大捕获时间、平均捕获时间和捕获概率的计算公式,建立了捕获性能仿真模型,分析了捕获时间和捕获概率的关系,以及空中平台的相对速度对捕获系统的影响和抑制方法.仿真结果表明,当通信终端的捕获不确定区域为50mrad,扫描重叠因子为0.12时,捕获探测器的信噪比大于6时,空地激光通信系统总的捕获概率优于95%,最大捕获时间约为36s,平均捕获时间约为12s.     

大气信道对红外激光通信系统性能影响的实验研究

针对近地面、准水平红外激光大气通信系统的应用,利用微脉冲激光雷达、大口径激光闪烁仪在济南市区进行了为期一周的昼夜连续观测,获得了影响红外激光通信系统性能的关键大气参量的信息.着重分析了大气衰减信道、大气湍流信道对一公里通信链路系统性能的影响,考查了通信设备在复杂大气条件下应用的可行性及可靠性.最后就减弱大气信道对通信性能影响的方法进行了总结和探讨.     

白光和红绿蓝光LED可调光源研制

为了提高LED光源色温和亮度的调节精度和准确度,结合色温由低向高变化时光色所呈现的渐变特点,提出了一种低色温白光LED灯珠、高色温白光LED灯珠加红绿蓝光LED灯珠补偿式调光的方法.将色温分成三个部分进行调节,每个部分选用不同的LED灯珠组合来进行调光.实验结果表明:不同组合情况下的LED光源的初始输出色温相对于目标值的偏差范围在1％以内;亮度可以在保证色温不变的情况下独立进行调节,初始输出值与目标值的偏差范围在1％以内;经过微调之后可以达到目标值;达到了色温和亮度独立调节的要求;光源发光稳定,不会因为长时间工作而影响调节精度.     

Atomic and electronic structures and stability of icosahedral nanodiamonds and onions

The relative stability of alternative carbon nanoparticles with icosahedral symmetry, such as diamond-like nanocrystallites and multiwalled fullerenes (onions), is investigated using the parametric model and the density functional tight-binding (DFTB) method. It is demonstrated that an increase in the size of particles and their hydrogenation favor the stabilization of diamond-like nanocrystallites. The formation of “intermediate” nanostructures consisting of diamond-like nanocrystallites inside the fullerene cage is revealed. The electronic spectra of icosahedral carbon nanoparticles are calculated.     

Visualization and interferometry for cylindrical and spherical sample tomography and profilometry

A technique combining image processing and laser interferometry for visualizing and detecting the deformation of transparent cylindrical and spherical sample is proposed. This deformation includes geometric deformation such as volume transition in profilometry and physical deformation such as refractive index change in tomography. Phase contour lines are used for quantitative analysis and graphical representation of the deformation. This method allows us to visually detect the spatial variation of the deformation field and to evaluate the test quality such as misalignment of optical system. A theoretical analysis using phase contour map to characterize the deformation field is described in detail. A method using phase contour map to qualify the interferometric test is proposed. Analysis of test examples is carried out. Suggestions on using phase contour line method to ameliorate test system design are finally discussed.     

Nonlocal potentials and their exact and approximate local and velocity-dependent equivalents

We show that good approximations to the exact equivalent local potential (ELP) and damping factor of a nonlocal Perey-Buck potential can be calculated in the partial wave WKB approximation of Horiuchi. The exact ELP and damping factor are obtained by means of a method previously given by one of us. We also confirm that an approximate ELP proposed by Bauhoff et al. is of comparable accuracy as the Horiuchi approximation. Thesel-dependent ELP's exhibit reduced attraction in the interior and provide a test for higher order WKB approximations. We subsequently obtain an equivalent velocity dependent potential (EVDP) which is even exactly wave function equivalent to the original nonlocal potential. This almost local potential, unlike the trivial equivalent local potential, is smooth and well-behaved and is therefore particularly useful in nuclear reactions where the off-shell behaviour of the potential is important.     

Quantum and Classical Disparity and Accord

Discrepancies and accords between quantum (QM) and classical mechanics (CM) related to expectation values and periods are generally found for both the harmonic oscillator (SHO) and a free particle in a box (FPB), which may apply generally. These indicate non-locality is expected throughout QM. The FPB energy states violate the Correspondence Principle. Previously unexpected accords are found and proven that 〈x ²〉 _CM =〈x ²〉 _QM and τ _CM =τ _QMb (beat period i.e. beats between the phases for adjoining energy states) for the SHO for all quantum numbers, n. However, for the FPB the beat periods differ at small n. It is shown that a particle’s velocity in an infinite square well varies, no matter how wide the box, nor how far the particle is from the walls. The quantum free particle variances share an indirect commonality with the Aharonov-Bohm and Aharonov-Casher effects in that there is a quantum action in the absence of a force. The concept of an “Expectation Value over a Partial Well Width” is introduced. This paper raises the question as to whether these inconsistencies are undetectable, or can be empirically ascertained. These inherent variances may need to be fixed, or nature is manifestly more non-classical than expected.