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穿过大气湍流的高斯激光脉冲在时间域的展宽行为与高斯脉冲通过理想高斯滤波器后的时域展宽行为相似。因此,为了激光通信信道仿真的需要,提出用级联巴特沃斯滤波器来逼近理想高斯滤波器的方法对这种时间域上的展宽行为进行建模。逼近后的高斯滤波器的3dB频率点与大气湍流的物理参数(包括折射率结构常数和湍流外尺度)和传播几何路径长度联系在一起,从而将高斯脉冲的展宽与所通过的大气湍流环境参数相结合并得到了模拟高斯脉冲展宽的解析表达式。该模型和由模型参数表示的高斯脉冲展宽的解析表达式不仅在湍流弱起伏区有效,而且在从弱起伏到中等起伏再到强起伏的整个区域中都有效。 相似文献
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湍流大气传输高斯谢尔光束的到达角起伏 总被引:13,自引:10,他引:3
研究了在弱大气湍流起伏环境下以窄带宽高斯谢尔光束为激光光源的大气通信问题,分析了大气湍流强度和光源空间相干度对通信光束到达角起伏的影响.采用窄带宽光场的交叉谱密度函数代替光场互相干函数的近似方法和采用包含大气湍流内外尺度的简化折射率谱密度函数,得出了湍流大气中传输高斯谢尔光束的波结构函数(WSF) 和到达角起伏方差解析近似关系.分析表明,光源的空间相干度和传输光束的湍流扩展是影响高斯谢尔光束的相位起伏结构函数和传输光束到达角起伏的重要因素. 相似文献
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采用随机表面的高斯相关模型和统计光学原理,推导出弱散射体在4f系统中的像面上光强的表达式.在此基础上提出了均方偏差粗糙度和横向相关长度的标定方法,该方法用理论结果对实验测量的像面散斑平均光强随滤波孔半径的关系曲线进行拟合,同时测量出被测样品表面的两个统计量.实验上制作了高斯相关随机表面样品,对其表面参数进行了测量,与AFM所测结果符合得较好,这表明该方法具有较高的精度.
关键词:
弱散射体
像面散斑
平均光强 相似文献
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在兰州地区夜间进行了链路长为610m的激光传输实验,研究了高斯激光在不同天气条件下的光强分布及其起伏特性.采用三维伪彩色变换方法,分析了光斑光强的空间分布规律,得到了光斑主瓣光强服从类高斯分布,并按照晴天、多云天、阴天的顺序,光斑几何中心位置附近光强分布的陡峭度依次减小,而其衰减则依次增大的结论.同时,利用实测光强值分析了大气闪烁指数,结果表明:晴天、雨后晴天和阴天下的闪烁指数分别为0.225 4、0.189 2、0.188 8,这说明晴天下的光强起伏大于雨后晴天和阴天的,且均为弱起伏.通过对归一化光强的频数分布进行非线性拟合得到了光强的概率密度分布,它们均服从对数正态分布,其中晴天下的概率密度曲线的拟合优度更是达到了0.997 50. 相似文献
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分析了高斯光束照射微粗糙圆柱在菲涅尔衍射区形成的散斑图像统计特性,给出了强度起伏自相关函数与表面曲率及粗糙程度等参数之间的关系.根据强度起伏自相关函数的离散化定义,实验并计算了圆柱轴向与径向结构有差异情况下的散斑强度起伏相关函数.结果表明,对于C1和C2圆柱沿垂直于圆柱轴向的散斑尺寸变长,强度起伏自相关函数沿平行和垂直于圆柱轴两个方向的波动相差较大;对于C3和C4圆柱两个方向上的散射特性基本相同;测量C1和C3的结果表明,散斑尺寸和形状依赖于圆柱表面的皱褶和圆柱表面曲率两个因素.研究结果对于如柱型管道、轴承等方面的机械制造的质量控制有重要的应用价值. 相似文献
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粗糙热传导表面下激光介质的热效应 总被引:1,自引:0,他引:1
在考虑激光介质与热沉不完全接触导热的情况下,用面热源自适应调整算法计算了激光介质的温度场,研究了其热效应.表面附近相位差存在起伏且深入一定深度使热效应复杂化.随抽运功率的增大,表面附近相位差的起伏增强,而起伏深度变化不明显;接触面积增大,相位差起伏减小,起伏的深度有所减小.抽运功率较小时,热致衍射损耗随抽运功率的增大近似线性增大,高斯光半径越大,增大的斜率越大,当抽运功率增加到一定程度时,热致衍射损耗增大的趋势减缓,半径大的减缓较明显.在抽运光功率变化范围内,半径大的高斯光热致衍射损耗大于半径小的.高斯光半径较小时,接触面积对热致衍射损耗的影响不明显,当高斯光半径较大时,接触面积减小热致衍射损耗增大. 相似文献
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Lixin Guo Che-Young Kim Xizheng Ke Zhensen Wu 《International Journal of Infrared and Millimeter Waves》2001,22(8):1249-1258
In this paper, according to Kirchhoff approximation, the optical backscattering enhancement of one-dimensional random rough surface, which includes fractal rough surfaces and random rough surfaces with Gaussian and exponential correlation simulated by Monte Carlo method, is obtained. It is shown that backscattering enhancement of random rough surfaces will increase with increasing the rms height of rough surface for a given correlation length. The angle width of backscattering enhancement is directly proportional to incident wavelength and inverse proportional to correlation length of rough surface. Complex phase of scattering field from superposed rough surface is uniformly distributed, none of the directions is of more overweight. The backscattering enhancement is also studied by wavelet analysis. The numerical results show good consistent with that of the relative references. 相似文献
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刻蚀波导粗糙面散射的矩量法数值分析 总被引:2,自引:2,他引:0
用矩量法对刻蚀波导粗糙面的散射特性做出了精确的数值分析基于MonteCarlo方法建立高斯粗糙面数学模型,证明矩量法在分析刻蚀波导散射问题时,是可以应用于不同起伏高度、不同入射角的一种精确而高效的数值方法对不同起伏高度的两种偏振态,分别在小角度入射和大角度入射两种情况的随机散射特性应用矩量法做出了分析并以基于二氧化硅波导的EDG器件为例,分析了由于波导壁粗糙对于器件频谱响应特性的影响. 相似文献
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Hybrid method for investigation of electromagnetic scattering from conducting target above the randomly rough surface 总被引:1,自引:0,他引:1 下载免费PDF全文
A current based hybrid method (HM) is proposed which combines the method
of moment (MOM) with the Kirchhoff approximation (KA) for the analysis
of scattering interaction between a two-dimensional (2D) infinitely
long conducting target with arbitrary cross section and a
one-dimensional (1D) Gaussian rough surface. The electromagnetic
scattering region in the HM is split into KA region and MOM region.
The electric field integral equation (EFIE) in MOM region (target)
is derived, the computational time of the HM depends mainly on the
number of unknowns of the target. The bistatic scattering
coefficient for the infinitely long cylinder above the rough surface
with Gaussian roughness spectrum is calculated, and the numerical
results are compared and verified with those obtained by the
conventional MOM, which shows the high efficiency of the HM.
Finally, the influence of the size, location of the target, the rms
height and correlation length of the rough surface on the bistatic
scattering coefficient with different polarizations is discussed in
detail. 相似文献
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Study of MPI based on parallel MOM on PC clusters for EM-beam scattering by 2-D PEC rough surfaces 下载免费PDF全文
This paper firstly applies the finite impulse response filter (FIR)
theory combined with the fast Fourier transform (FFT) method to
generate two-dimensional Gaussian rough surface. Using the electric
field integral equation (EFIE), it introduces the method of moment
(MOM) with RWG vector basis function and Galerkin's method to
investigate the electromagnetic beam scattering by a two-dimensional
PEC Gaussian rough surface on personal computer (PC) clusters.
The details of the parallel conjugate gradient method (CGM) for
solving the matrix equation are also presented and the numerical
simulations are obtained through the message passing interface (MPI)
platform on the PC clusters. It finds significantly that the
parallel MOM supplies a novel technique for solving a two-dimensional
rough surface electromagnetic-scattering problem. The influences of
the root-mean-square height, the correlation length and the
polarization on the beam scattering characteristics by
two-dimensional PEC Gaussian rough surfaces are finally discussed. 相似文献
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采用土壤介电常数的四成分模型表示实际地面的介电特性,应用指数型分布粗糙面模型和Monte Carlo方法模拟实际的粗糙地面,运用时域有限差分方法研究了微分高斯脉冲波照射时粗糙地面与上方目标的宽带电磁散射问题,得出了后向复合散射系数的频率响应曲线,计算了后向复合散射系数随粗糙地面高度起伏均方根、相关长度、土壤湿度、目标尺寸、高度、倾角等几何参数及介电参数等的变化情况,并做了详细分析与讨论,得到了指数型粗糙地面与上方目标宽带电磁散射特性. 相似文献
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Three types of statistical fourth moments of acoustic waves forward scattered by a randomly rough ocean surface are derived and numerically evaluated. The first one is related to the scintillation index which characterizes intensity fluctuations. The second one is the two-position intensity correlation function which describes the spatial correlation of wave intensity. The third is the fourth-moment two-position coherence function which carries information on the phase fluctuations of the scattered wave. In the range of weak scattering, the ratio of the absolute value of the fourth-moment two-position coherence function over the two-position intensity correlation exactly describes the mean-square fluctuation of the relative phase between the two positions. The acoustic frequency is high so that the Kirchhoff approximation can be used. Two types of spectral functions for surface-height fluctuations are considered: a Gaussian spectrum and the Donelan-Pierson spectrum. The latter is obtained from a model for the fluctuations of the ocean surface height which are controlled by the wind speed at the ocean surface. 相似文献
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Akira Ishimaru Jei S. Chen Phillip Phu Kuniaki Yoshitomi 《Waves in Random and Complex Media》1991,1(3):S91-S107
This paper Presents numerical simulations, theoretical analysis, and millimeter wave experiments for scattering from one-dimensional very rough surfaces. First, numerical simulations are used to investigate the effects of roughness spectrum, height variation, interface medium, polarization, and incident angle on the backscattering enhancement. The enhanced backscattering due to rough surface scattering is divided into two cases; the RMS height close to a wavelength and RMS slope close to unity, and RMS height much smaller than a wavelength with surface wave contributions. Results also show that the enhancement is sensitive to the roughness spectrum. Next, a theory based on the first- and second-order Kirchhoff approximation modified with angular and propagation shadowing is developed. The theoretical solutions provide a physical explanation of backscattering enhancement and agree well with the numerical results. In addition to the scattering of a monochromatic wave, the analytical results of the broadening and lateral spreading of a pulsed beam wave scattering from rough surfaces are also discussed. Finally, the existence of backscattering enhancement from one-dimensional very rough conducting surfaces with exact Gaussian statistics and Gaussian roughness spectrum is verified by a millimeter-wave experiment. Experimental results which show enhanced backscattering for both TE and TM polarizations for different angles of incidence are presented. 相似文献
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研究了不同粗糙度下的非均匀和分区均匀不稳定表面粗糙导体目标在太赫兹波段的散射特性。区别于采用经验公式的建模方法,提出把随机粗糙面的建模理念应用到太赫兹波段的非均匀和分区均匀不稳定表面粗糙导体目标的建模中,用描述粗糙面的均方根高度和相关长度两个物理量来调节目标表面的粗糙度变化。提出表面粗糙目标的分类形式并给出具体模型,然后用随机高斯粗糙面来模拟非均匀和分区均匀不稳定粗糙目标的表面,再采用物理光学和等效电流相结合的方法来进行仿真计算,分别对不同入射角、不同频率和不同粗糙度的非均匀和分区均匀不稳定表面粗糙导体目标的太赫兹波散射特性进行分析,最后得出了相关的结论。 相似文献
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In this paper, the backscattering coefficient of a two-dimensional randomly rough perfectly-conducting surface is investigated using the Kirchhoff approach with a shadowing function. The rough surface height/slope correlations assumed to be Gaussian are accounted for in this analysis. The scattering coefficient is then formulated in terms of a characteristic function for the integrations over the surface heights, in terms of expected values for the integrations over the surface slopes. Numerical comparisons of Kirchhoff's approach (KA) with the stationary-phase (SP) approximation are made with respect to the choice of the one-dimensional surface height autocorrelation function and the shadowing effect. For an isotropic surface the results show that SP underestimated the incoherent backscattering coefficient compared with KA. Moreover, when the correlation between the slopes and the heights is neglected, the shadowing effect may be ignored. 相似文献
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《Waves in Random and Complex Media》2013,23(4):419-439
Abstract Three types of statistical fourth moments of acoustic waves forward scattered by a randomly rough ocean surface are derived and numerically evaluated. The first one is related to the scintillation index which characterizes intensity fluctuations. The second one is the two-position intensity correlation function which describes the spatial correlation of wave intensity. The third is the fourth-moment two-position coherence function which carries information on the phase fluctuations of the scattered wave. In the range of weak scattering, the ratio of the absolute value of the fourth-moment two-position coherence function over the two-position intensity correlation exactly describes the mean-square fluctuation of the relative phase between the two positions. The acoustic frequency is high so that the Kirchhoff approximation can be used. Two types of spectral functions for surface-height fluctuations are considered: a Gaussian spectrum and the Donelan-Pierson spectrum. The latter is obtained from a model for the fluctuations of the ocean surface height which are controlled by the wind speed at the ocean surface. 相似文献