面阵扫描型警戒系统目标探测方法

根据红外面阵探测器的成像特点,提出了一种面阵扫描型警戒系统,该系统的新颖之处在于面阵的旋转会带来目标探测有效像元数目的增加,并使得目标探测信噪比得到提高.描述了使用面阵探测器扫描型警戒系统的系统参数和工作过程,推导了该系统的点扩散模型,分析了目标的成像特点,重点描述和分析了针对本系统特点的目标探测算法,并介绍了其信号处理的硬件结构.实验结果表明,基于面阵探测器的红外搜索系统具有很高目标探测概率,并且能够在强杂波环境中检测弱目标.     

The ellipsoidal area ratio: an alternative anisotropy index for diffusion tensor imaging

In the processing and analysis of diffusion tensor imaging (DTI) data, certain predefined morphological features of diffusion tensors are often represented as simplified scalar indices, termed diffusion anisotropy indices (DAIs). When comparing tensor morphologies across differing voxels of an image, or across corresponding voxels in different images, DAIs are mathematically and statistically more tractable than are the full tensors, which are probabilistic ellipsoids consisting of three orthogonal vectors that each has a direction and an associated scalar magnitude. We have developed a new DAI, the "ellipsoidal area ratio" (EAR), to represent the degree of anisotropy in the morphological features of a diffusion tensor. The EAR is a normalized geometrical measure of surface curvature in the 3D diffusion ellipsoid. Monte Carlo simulations and applications to the study of in vivo human data demonstrate that, at low noise levels, EAR provides a similar contrast-to-noise ratio (CNR) but a higher signal-to-noise ratio (SNR) than does fractional anisotropy (FA), which is currently the most popular anisotropy index in active use. Moreover, at the high noise levels encountered most commonly in real-world DTI datasets, EAR compared with FA is consistently much more robust to perturbations from noise and it provides a higher CNR, features useful for the analysis of DTI data that are inherently noise sensitive.     

熊猫型保偏光纤中应力区失配对光纤性能影响的研究

采用弹性力学理论和复变函数法导出了熊猫型保偏光纤应力双折射的近似解析表达式.利用有限元法数值分析了熊猫型保偏光纤的双折射特性,结果表明,与采用纤芯中心应力相比,采用芯区平均应力表示光纤的双折射较为精确,且结果与解析法导出的芯区中心应力双折射计算结果相比误差小于0.18%.在此基础上进一步分析了应力区失配对熊猫型保偏光纤的双折射所产生的影响,并通过实验测试证明了分析结果的正确性. 关键词： 偏振保持光纤 应力区失配 双折射 拍长     

七芯光子晶体光纤结构优化的数值分析

从耦合模方程出发,推导了描述七芯光子晶体光纤模场特性的本征方程,得到了这种结构光纤的本征值和本征矢.使用频域有限差分法数值模拟,通过改变传输波长、纤芯间距、包层孔大小分析了纤芯之间的强度分布与耦合系数的关系,分别实现了模式整形和高阶超模截止.掌握了具有强耦合特性的七芯光子晶体光纤设计规律,和这种结构光子晶体光纤的同相位超模的选取方法. 关键词： 光子晶体光纤 大模场面积光纤 多芯结构 频域有限差分法     

小麦生长过程中光能利用率和光化学反射指数的相关性研究

以不同品种的小麦为材料,同时测量小麦生长过程的光谱反射率和光能利用率,研究了小麦整个生长期内光化学反射指数和光能利用率之间的关系变化,讨论二者之间关系的时间效应.实验结果表明,对小麦的整个生长期,光化学反射指数并不能作为估算光能利用率的一个有效指数.由于叶片叶绿素含量和叶面积指数的影响,在小麦的生长初期和末期,光化学反射指数和光能利用率之间的相关性不高,相关系数R2分别为0.222 4和0.210 6.对于小麦生长力旺盛的阶段,光化学反射指数可以作为估算光能利用率的一个可行方法,能够获得一定准确度的光能利用率,相关系数R2分别为0.612 7、0.641 2和0.601 1.     

高功率纳秒光脉冲在单模光纤中的受激喇曼散射及抑制研究

利用包含喇曼增益、抽运消耗、自相位调制、交叉相位调制和群速度色散效应的耦合非线性薛定谔方程组,对千瓦级高功率纳秒脉冲在单模光纤中的传输特性进行了模拟计算.分析了输入脉冲和斯托克斯脉冲的演化过程,研究了大模面积光纤对受激喇曼散射的抑制作用.研究结果表明,采用模面积为530 μm~2的光子晶体光纤作为惯性约束聚变脉冲整形系统的传输介质,能有效抑制受激拉曼散射,不但信号衰减较小,而且中心凹陷程度较低.     

温度对铷原子D2线饱和吸收光谱的影响

从理论上分析了铷原子蒸汽温度对87Rb和85Rb D2线饱和吸收光谱在消除了多普勒背景情况下的光谱相对强度的影响.由于有效原子数密度、多普勒加宽、电极化率受温度变化的影响,每一条超精细谱线的相对强度随着温度的升高都会出现先增大后减小的变化趋势,相对强度最高时的温度称为最佳温度.对于87Rb或85Rb来说,不同超精细谱线最佳温度不同的主要原因是由于烧孔面积和跃迁几率不同造成的,而87Rb和85Rb不同超精细谱线最佳温度不同的主要原因是由于它们在自然界中丰度不同造成的.     

Four-wave mixing analysis in WDM optical communication systems with higher-order dispersion

Four-wave mixing (FWM) is one of the major limiting factors in WDM optical fiber communication systems. In this paper, we analyze the individual and combined effect of second-, third-, fourth- and fifth-order dispersion parameters on FWM at different input channel powers and core effective areas, which have not been calculated earlier. FWM power versus channel power graphs for individual and combined effects of dispersion parameters have been presented, and it has been observed that FWM reduces for combined effect of dispersion parameter.     

Designing a graded index depressed clad non-zero dispersion shifted optical fiber for wide band transmission system

Non-zero dispersion shifted fibers (NZ-DSFs) find extensive use in wavelength division multiplexed (WDM) system as it reduces the non-linear effects like four-wave mixing (FWM) generation. A depressed clad graded index fiber with a central dip in the refractive index profile, as well as without dip, has been modeled to perform as an NZ-DSF using the spot size optimization technique. The performance characteristics of the proposed NZ-DSF have been studied by changing different fiber parameters; such as inner core radius (a), relative refractive index differences (Δ⁺), normalized end position of depressed clad (C), depression parameter (ρ), etc. for a given value of Petermann-2 spot size . By suitably adjusting the fiber parameters, the effective core areas (A_eff) as simulated here are very large (80 μm²) so that the effect of non-linearities upon them can be minimized. These NZ-DSFs have also been optimized for WDM transmission system. The dispersion slopes of the proposed fibers with and without dip have been estimated which are comparable with the reported results.     

A two-step strategy for numerical simulation of radiative transfer with anisotropic scattering and reflection

This article presents a two-step procedure for the computation of radiative heat transfer with anisotropic scattering and reflection. It is based on a concept that the coincident processes of absorption and scattering/reflection can be separated factitiously. All medium elements and wall surfaces are supposed to be pure-absorbing when receiving incident radiation. Afterwards they emit the scattered/reflected radiations. The absorption of both the initial and the secondary radiations can be assessed by the direct exchange area. It is needed to repeat the processes for a few times until the radiations are substantially absorbed. For anisotropic scattering/reflection, a vector summation obtains the directional distribution of emissive power. The method is validated by several benchmark computations in terms of emissive power and heat transfer coefficients. It is shown that the method gives more accurate solution than the isotropic scaling for the heat transfer in anisotropically scattering media.