眼底相机的均匀照明及消杂光干扰设计

给出了一种新型眼底相机照明系统的设计方案。针对现行眼底相机照明系统复杂的问题,对经典的柯勒照明光路进行改良设计,得到了一个结构简单的眼底照明系统。结构中除网膜物镜外,只需用到4片透镜,且眼底照明区域直径连续可调,充分利用了光能。通过在照明光路中添加黑点板和环形光阑,屏蔽了系统99%以上的杂散光,使眼底相机成像画面的信噪比达到20dB以上,提高了对比度。同时在Gullstrand_Le标准眼模型上,得到一个均匀度达95%以上的照明区域。     

在LiNbO3:Fe晶体中写入Y结孤子的实验研究

采用振幅掩模穿过部分空间非相干光的方法产生一暗迹,将暗迹成像在晶体前表面,得到入射暗迹半高宽为16.75 μm,入射光功率为400 μW.基于LiNbO3:Fe晶体的强光伏效应,在无背景光辐照时,在晶体内写入一对部分空间非相干Y结暗光伏孤子.实验结果表明：在无背景光辐射时可以在LiNbO3:Fe晶体内写入部分空间非相干Y结孤子.     

4π Focusing with single paraboloid mirror

Creating small bright or dark focal spots with good spherical symmetry requires 4π focusing, usually achieved by using two counter-propagating beams and two high NA lenses that need to be positioned with interferometric accuracy. We investigate a simple alternative way of focusing light to a small and uniform bright or dark focal spot. It only requires a single parabolic mirror illuminated with a single beam from one direction, resulting in a robust and compact setup. The focal spot distribution is tailored by optimizing the polarization, intensity and helicity of the incoming illumination.     

Even fluorescence excitation by multidirectional selective plane illumination microscopy (mSPIM)

Multidirectional selective plane illumination microscopy (mSPIM) reduces absorption and scattering artifacts and provides an evenly illuminated focal plane. mSPIM solves two common problems in light-sheet-based imaging techniques: The shadowing in the excitation path due to absorption in the specimen is eliminated by pivoting the light sheet; the spread of the light sheet by scattering in the sample is compensated by illuminating the sample consecutively from opposing directions. The resulting two images are computationally fused yielding a superior image. The effective light sheet is thinner, and the axial resolution is increased by square root 2 over single-directional SPIM. The multidirectional illumination proves essential in biological specimens such as millimeter-sized embryos. The performance of mSPIM is demonstrated by the imaging of live zebrafish embryos.     

Wavelength-dependent intensity distribution of a Gaussian beam scattered by a spherical particle

The wavelength-dependent intensity of a light beam scattered by an isotropic homogeneous particle illuminated with an on-axis monochromatic polarized Gaussian beam is calculated in this paper. The vector spherical harmonics expansion and T transformation matrix are used to form the theoretical basis. Numerical results show that the angular intensity distribution is symmetric for an on-axis polarized Gaussian beam illumination upon an aqueous spherical particle at 473, 532 and 660 nm. For the specified aqueous spherical particle, the scattered intensity distribution decreases with increase in wavelengths.     

尾流后向散射光功率衰减特性研究

依据Mie理论光散射在0°和180°有明显增强的结论,应用自行设计的模拟尾流后向散射光功率实验系统在632.8 nm He-Ne激光器直接照明和扩束后照明两种条件下,对模拟尾流后向散射光功率衰减特性进行了研究.结果表明,用后向散射光功率进行探测时,考虑到散射光在水中传播的衰减,对近距离分布的气泡,使用扩束光照明效果较好;对较远距离分布的气泡,使用细光束照明效果较好,其可探测的距离要比扩束光远.     

非相干照明条件下的ptychographic iterative engine成像技术

在传统多波长相干衍射成像理论的基础上提出适用于 X-射线和电子束等非相干光源照明成像的改进多波长ptychographic iterative engine 方法, 同时将小孔形状和照明光谱信息用于叠代计算, 可以在非相干照明条件下精确重建出物体的强度透射像和相位透射像, 并对光源带宽对重建精度的影响进行了分析, 对于解决如何在非相干照明条件下对大尺寸物体进行精确相位成像的问题具有较好的科研和实用价值.     

Scattering of typical particles by beam shape in oblique illumination

The expansions of the incident arbitrary beam shape in spherical, spheroidal and cylindrical coordinates in the general case of oblique illumination are presented. The scattered electromagnetic fields of typical particles of arbitrary orientation illuminated with a beam shape are provided. The numerical results of the normalized scattering cross section are evaluated.     

Oscillatory clusters in the periodically illuminated, spatially extended Belousov-Zhabotinsky reaction

Cluster-cluster transitions in the periodically illuminated photosensitive Belousov-Zhabotinsky (BZ) reaction-diffusion system exhibit the same scenario as in the autonomous BZ system with negative global feedback: two-phase clusters <--> three-phase clusters <--> irregular clusters <--> localized clusters. Transitions induced by changing the dark ( TD) or light ( TL) phases of the periodic external square wave illumination are dependent not only on the frequency of illumination at constant TD/TL, but also on the ratio TD/TL at constant frequency (when TD+TL = const).     

Instability and optical turbulence in the formation of spontaneous gratings in single-mode photosensitive AgCl-Ag films

In experiments on illumination of thin photosensitive AgCl films containing granular Ag by a focused laser beam (λ=633 nm), self-pulsations in the form of optical turbulence were found and studied. In the course of illumination, the turbulence was observed in the pattern of small-angle scattering in the form of “boiling” of light spots separated by dark spacings. The formation of turbulence is associated with spontaneous microgratings, whose origin involves waveguide TE modes scattered in a film, and their competition in the course of illumination, which leads to the growth of one microgratings and the decay of the other.     

α-LiIO3单晶中离子输运引起的高强度准弹性光散射

激光束通过加c向直流电压的α-LiIO₃单晶时,发生三种准弹性散射:散射光电位移矢量有特定取向且与入射光电位移矢量方向不一致者,散射光电位移矢量与入射光电位移矢量方向相同而散射带垂直c轴者以及散射光电位移矢量与入射光电位移矢量方向相同而散射带对c轴倾斜者,本文详细研究了这三种散射的空间频谱、精细结构、结构敏感性、弛豫规律以及光散射和晶体内电流密度与局部电场的关系,发现光散射发生于晶体内电流密度较大的区域,本文对三种散射的起因给予了说明:离子导体在直流电压作用下空间电荷的分布和运动引起 关键词：     

不同光照条件下水下成像背景光的建模与研究

由于光在水中传输时的衰减和散射效应, 水下成像系统通常很难达到令人满意的成像效果, 而成像过程中由于光散射产生的背景光则是造成水下图像退化的主要原因. 本文对自然光照和人工光照两种不同光照条件下的水下成像背景光进行了建模和仿真分析. 结果显示: 自然光照下的无穷远处背景光与衰减系数成反比, 与散射系数成正比; 人工光照下的无穷远处背景光则与衰减系数、散射系数和相机-光源距离有关. 两种光照条件下的背景光都可以用无穷远处背景光的指数衰减表达式来表示. 水下图像背景光的强度主要与水体光学参数、相机-景物的距离、相机-光源的距离及相机成像角等因素有关. 本文的研究结论不仅可用于水下成像系统的设计与优化, 也可用来从水下图像背景光中估计水体光学参数、相机-景物距离等图像信息.     

Electron optics for high throughput low energy electron microscopy

Novel electron-optical components and concepts aiming at improving the throughput and extending the applications of a low energy electron microscope (LEEM) have been developed. An immersion magnetic objective lens can substantially reduce e-e interactions and the associated blur, as electrons do not form a sharp crossover in the back-focal plane. The resulting limited field of view of the immersion objective lens in mirror mode can be eliminated by immersing the cathode of the electron gun in a magnetic field. A dual illumination beam approach is used to mitigate the charging effects when the LEEM is used to image insulating surfaces. The negative charging effect, created by a partially absorbed mirror beam, is compensated by the positive charging effect of the secondary beam with an electron yield exceeding 1. On substrates illuminated with a tilted beam near glancing incidence, large shadows are formed on even the smallest topographic features, easing their detection. On magnetic substrates, the magnetic flux leaking above the surface can be detected with tilted illumination and used to image domain walls with high contrast.     

Residual polarization of non-coherently backscattered linearly polarized light: the influence of the anisotropy parameter of the scattering medium

The effect of preservation of the residual polarization of backscattered light in the case of multiply scattered disordered media illumination by a linearly polarized plane wave is examined using the path-integral approach and Monte Carlo simulation. Disordered ensembles of non-interacting dielectric particles are considered as the model of scattering media. The influence of the anisotropy parameter of the scattering system on the degree of residual polarization is analysed. Experimental results obtained for various scattering systems at different wavelengths of illuminating light are in satisfactory agreement with the results of theoretical analysis and Monte Carlo simulation. The dependence of statistical properties of the polarization states of backscattered field partial components, such as probability distributions of ellipticity, on the anisotropy parameter is studied.     

Modulation instability of bright photovoltaic solitons on a partially incoherent background

We present the observation of incoherent anti-dark photovoltaic solitons in LiNbO₃:Fe crystal. This new class of soliton states involves bright photovoltaic solitons on a background beam meeting ?? > 1, where ?? is the ratio of background illumination photovoltaic constant to that of soliton beam. For ?? < 1, dark photovoltaic solitons are generated. Furthermore, this novel type solitons are investigated experimentally by injecting coherent light and partially coherent background of infinite extent. In case of spatial coherence of the background lower than the threshold of incoherent modulation instability, these results indicate that bright photovoltaic solitons can propagate in a stable fashion.     

Amplified backward scattering in LiNbO3:Fe

We study the characteristics of the amplified back-scattered light in a LiNbO₃:Fe crystal cut normal to the optic axis when it is illuminated with a linearly polarized parallel beam of monochromatic light. The scattered light forms different patterns depending on the direction of vibration of the incident beam. The incident beam and the back-scattered beams interfere and generate numerous ‘noisy’ gratings in the volume of the crystal which diffract and amplify selectivity the weak scattered beams. This study is of practical importance since a normally cut LiNbO₃:Fe crystal may be used as a self-starting non-linear adaptive mirror in laser cavities.     

利用后向散射光空间谱强度分布探测尾流气泡的实验研究

利用自行设计制作的尾流模拟系统产生气泡,应用散射光空间频谱测量分析系统,对水中气泡群的后向散射光空间谱强度分布进行测量和分析,研究了不同探测距离的散射光衰减对其强度分布的影响.结果表明,在气泡密度、大小分布不变的情况下,扩束光照明时,由于参与光散射的气泡数目增加对散射光在水中传播衰减的补偿,散射空间谱强度分布的峰值和半宽值均不随探测距离变化;而细光束照明时,散射谱强度分布峰值随探测距离的增加而减小,半宽值则逐渐增大.所以在远距离探测时,细光束的探测效果好.     

均匀椭球粒子对拉盖尔-高斯光束的散射特性研究

基于广义Mie理论, 研究了椭球粒子对在轴入射的拉盖尔-高斯光束的散射特性. 通过局域近似法求解椭球坐标系中的波束因子, 计算得到了波束因子之间满足的普遍关系. 对散射强度随椭球粒子不同尺寸参数和扁圆程度的变化特性进行了数值计算, 并针对不同拓扑荷时的散射强度进行了对比分析. 结果表明: 当椭球粒子尺寸在与入射光波长可比拟的范围内变化时, 散射强度随尺寸参数的增大而增大, 随椭球长短轴之比和拓扑荷的增大而减小. 本文的理论研究能够为拉盖尔-高斯光束在粒径测量、大气激光通信、 大气遥感等领域的应用提供更准确的粒子模型和参考价值. 关键词： 椭球粒子 拉盖尔-高斯光束 波束因子 散射强度     

A theoretical study on the visibility of a speckle pattern based on a Gabor hologram and speckle photography

The visibility of the speckle pattern resulting from the scattered radiation of transmitted beams through a diffusing surface has been studied for a Gabor hologram and speckle photography. The cases of strictly- and quasi-monochromatic light are also considered. Mathematical expressions for the dependence of the visibility on the distance from the centre of the undiffracted illuminating beam, its spectral half-width Δf and the amplitude ratio m between the reference beam and the integrated scattered beams are obtained. A graphical method is found for determining m and Δf from the visibility as a function of the distance from the centre of the undiffracted illuminating beam.     

A method for fringe normalization by Zernike polynomial

A new approach for fringe normalization by Zernike polynomial fitting to cancel background illumination in an interferogram is proposed. With this method, background illumination can be suppressed, high frequency noise is immunized and the contrast is improved by normalization. The main idea for this paper is to use the Zernike polynomial fitting interferogram to cancel background illumination; the high frequency noise is then filtered by a Wiener filter. Finally, the paper uses the method of local region contrast modulation to enhance fringe contrast. This method can overcome the problem of the non-uniformity illumination of fringe patterns resulting from the marginal reflection of optical components and a non-uniformity light source