Experimental observation of speckle instability in Kerr random media

In a disordered nonlinear medium the transmitted speckle pattern was predicted to become unstable as a result of the positive feedback between intensity fluctuations and local variations of the refractive index. We show experimental evidence of speckle instability for light transversally scattered in a liquid crystal cell, where a two-dimensional controlled disorder is imprinted by suitable illumination of a photoconductive wall and nonlinearity is obtained through optical reorientation of the liquid crystal molecules. The speckle pattern spontaneously oscillates at discrete frequencies above a critical threshold, whose dependence on the scattering mean free path confirms the crucial role of disorder in the feedback process.     

Instability of speckle patterns in random media with noninstantaneous Kerr nonlinearity

The onset of instability of a multiple-scattering speckle pattern in a random medium with Kerr nonlinearity is significantly affected by the noninstantaneous character of the nonlinear medium's response. The fundamental time scale of the spontaneous speckle dynamics beyond the instability threshold is set by the larger of times TD and tau(NL), where TD is the time required for the multiply scattered waves to propagate through the random sample and tau(NL) is the relaxation time of the nonlinearity. The inertial nature of the nonlinearity should complicate the experimental observation of the instability phenomenon.     

Magnetic domain fluctuations in an antiferromagnetic film observed with coherent resonant soft x-ray scattering

We report the direct observation of slow fluctuations of helical antiferromagnetic domains in an ultrathin holmium film using coherent resonant magnetic x-ray scattering. We observe a gradual increase of the fluctuations in the speckle pattern with increasing temperature, while at the same time a static contribution to the speckle pattern remains. This finding indicates that domain-wall fluctuations occur over a large range of time scales. We ascribe this nonergodic behavior to the strong dependence of the fluctuation rate on the local thickness of the film.     

Dynamics of phason fluctuations in the i-AlPdMn quasicrystal

We report on the study of the dynamics of long wavelength phason fluctuations in the i-AlPdMn icosahedral phase using coherent x-ray scattering. When measured with a coherent x-ray beam, the diffuse intensity due to phasons presents strong fluctuations or speckles pattern. From room temperature to 500 degrees C the speckle pattern is time independent. At 650 degrees C the time correlation of the speckle pattern exhibits an exponential time decay, from which a characteristic time tau is extracted. We find that tau is proportional to the square of the phason wavelength, which demonstrates that phasons are collective diffusive modes in quasicrystals, in agreement with theoretical predictions.     

Surface roughness measurement using spatial-average analysis of objective speckle pattern in specular direction

The speckle contrast method (SCM) and the light scattering method (LSM) are two of the most promising optical techniques for on-line surface roughness measurement of slightly-rough surface. However, due to the lack of capability in eliminating the influence from the diffuse component of scattered light, SCM and LSM are both sensitive to the variations of surface correlation length. Additionally, for LSM, the presence of speckle noise leads to fluctuations in the measuring results. To solve these problems, an approach based on the spatial-average analysis of the objective speckle pattern in the specular direction, simply called spatial-average method (SAM), is proposed. The SAM establishes the quantitative relationship between a new characteristic parameter extracted from the recorded speckle image and the rms surface roughness, eliminates to a large extent the influence of diffuse light component on the measuring results, and immunizes itself from the speckle noise. The theoretical foundation of SAM is given in details. A computer simulation is then performed to make comparisons among these three methods. Finally an experiment is presented.     

纵向运动双散射散斑场的动态特性研究

研究了慢纵向运动双散射散斑场的统计性质,得出纵向运动双散射散斑振幅自相关函数、强度涨落自相关函数,并由此得出,夫朗和费衍射区内的接收平在上任意点上的自相关函数是时间的非平稳随机过程,而在光轴附近小区域内,强度涨落自相关函数是时间的平稳随机过程,相关时间与散射体纵向运动速度或反比,理论和实验结果相一致。     

散斑自相关分辨率测量和修正

散斑相关是许多基于散斑的光学测量和成像技术的基础,决定了光学系统的分辨率。当前散斑尺寸（颗粒度或分辨率）的理论描述不够精确,也缺乏实验验证。探究了散斑图样自相关尺寸的影响因素,与相同数值孔径物镜聚焦进行比对,揭示薄散射介质的“散射透镜”性质。通过散斑自相关和透镜聚焦尺寸的多组测量,结果表明,截趾函数会影响其分辨率,需要根据具体光路对阿贝判据做修正。对基于散斑的测量和成像技术具有一定的参考价值。     

Continual deformation analysis with scanning phase method and time sequence phase method in temporal speckle pattern interferometry

If a laser beam illuminates a continual deformation object surface, it will lead to a temporal speckle pattern on the observation plane. Recording this time-dependent speckle pattern the deformation of the surface of an object can be obtained. Two methods, scanning phase method (SPM) and time sequence phase method (TSPM), have been introduced for measuring the displacement caused by the deformation in temporal speckle pattern interferometry (TSPI). Their principle is that by capturing a series of speckle interference patterns related to the object deformations, the fluctuations in the intensity of the interference patterns can be obtained. Through scanning these fluctuations and estimating both the average intensity and modulation of the temporal speckle interference patterns, the phase maps for whole-field displacements are calculated. In this way one is capable of quantitatively measuring continual displacements simply using a conventional electronic speckle pattern interferometry (ESPI) system without phase shifting or a carrier. The elaboration on the new methods is given in this paper and experiments are performed to demonstrate their performance with a conventional ESPI system.     

The autocorrelation function of polychromatic laser speckle patterns near the image plane

The autocorrelation function of polychromatic speckle patterns produced near the image plane of a double diffraction imaging system is experimentally studied. The condition under which the polychromatic speckle field obeys Gaussian statistics is further investigated as a function of the numberN of scattering cells, using the average contrast of the speckle intensity fluctuations. The profile of the autocorrelation function is next investigated as a function ofN and the defocused distance from the image plane. It is found that the average contrast and autocorrelation function of the polychromatic speckle intensity fluctuations converge less rapidly with an increase inN than those of the monochromatic ones.     

Two-dimensional angular optical scattering patterns as droplets evolve into clusters

Remarkable changes in the two-dimensional angular optical scattering (TAOS) pattern are observed when an acoustically levitated water droplet with inclusions evolves into a dry aggregate. The emergence of polystyrene latex sphere inclusions near the droplet surface has a clear effect on the TAOS. A speckle pattern in the TAOS replaces the continuous contours associated with Mie scattering from a smooth droplet. Once the contours break up, this speckle pattern persists as the levitated droplet evaporates, making the transition from a droplet with inclusions to a dried aggregate. The initial emergence of the speckle pattern is observed for an inclusion volume fraction near 8%.     

Atherosclerotic plaque characterization by spatial and temporal speckle pattern analysis

Improved methods are needed to identify the vulnerable coronary plaques responsible for acute myocardial infraction or sudden cardiac death. We describe a method for characterizing the structure and biomechanical properties of atherosclerotic plaques based on speckle pattern fluctuations. Near-field speckle images were acquired from five human aortic specimens ex vivo. The speckle decorrelation time constant varied significantly for vulnerable aortic plaques (tau=40 ms) versus stable plaques (tau=400 ms) and normal aorta (tau=500 ms) . These initial results indicate that different atherosclerotic plaque types may be distinguished by analysis of temporal and spatial speckle pattern fluctuations.     

斜入射引入动态多重散射机制的激光散斑抑制

散斑抑制一直是激光显示技术的研究热点。对激光光束经SiO₂溶液所形成散斑的特性分析,建立了多重散射与散斑颗粒大小的联系。结合动态光散射理论,提出利用斜入射引入动态多重散射机制的激光散斑抑制方法,并构建由静态散射片和装有粒径为300 nm、摩尔浓度为3.0×10-4 μm-3的SiO₂悬浮液的光通管组成的散斑抑制装置,将其置入激光显示系统的光源部分。针对光束以不同入射角进入SiO₂悬浮液对散斑图像对比度影响进行了系统实验分析。结果表明,光束以约8°进入SiO₂悬浮液能将散斑图像的对比度从0.43降低至0.067。通过该方法实现了散斑颗粒的空间平均和散斑图像的时间平均,提高了散斑抑制的效果。散斑抑制单元无需振动装置且便于集成到激光投影系统中,不仅提高了系统的可靠性也减小了成本。     

Weak localization of light in a disordered microcavity

We report the observation of weak localization of light in a semiconductor microcavity. The intrinsic disorder in a microcavity leads to multiple scattering and hence to static speckle. We show that averaging over realizations of the disorder reveals a coherent backscattering cone that has a coherent enhancement factor > or =2, as required by reciprocity. The coherent backscattering cone is observed along a ring-shaped pattern due to confinement by the microcavity.     

Compton散射对五阶非线性下零色散附近调制不稳定性的影响

应用多光子非线性Compton散射模型和非线性薛定谔方程,研究了Compton散射对五阶非线性零色散附近调制不稳定性的影响.将入射光和Compton散射光作为产生调制不稳定性的机制,分析了光纤损耗、四阶色散和五阶非线性对增益谱的影响.结果表明:散射下的正或负五阶非线性分别使零色散附近的增益谱宽和峰值比散射前增大得更大或...     

Fourier magnitude of the field incident on a random scattering medium from spatial speckle intensity correlations

Spatial speckle intensity correlations are used to determine the spatial Fourier magnitude of a field incident on a random scattering medium. The patterned beam is scanned across the scattering medium, and the speckle pattern on the opposite side is imaged at each beam position. A theory based on a Green's function representation is used to reconstruct the spatial Fourier magnitude of the patterned incident field.     

Dynamic behaviour of speckle cluster formation

In this work, we analyse the speckle cluster structure generated when a coherently illuminated diffuser is imaged by introducing a multiple aperture pupil mask in front of the lens plane. We demonstrate that the speckle cluster originates from the complex speckle modulation generated by multiple interferences among the wavefront passing through each aperture. The auto-correlation function of the intensity distribution when using a multiple aperture pupil arrangement is calculated. Besides, we demonstrate that the autocorrelation function and the intensity corresponding to a single scattering element of the input are coincident. This result allows interpretation of the dynamics behaviour of the speckle cluster formation by considering the result obtained by a single scattering element. Then, we determine the pupil mask geometrical parameters that control the cluster behaviour and therefore the condition for obtaining a highly repetitive cluster structure that we define as a 'regular cluster'. The theoretical simulations based on the random walk model are in agreement with the experimental results supporting the validity of our approach.     

一种基于多重散射的光学Hash函数

本文提出了一种基于光与多重散射介质相互作用的光学Hash函数构造方法.该方法创新性地利用多重散射介质对相干调制光的天然随机散射作用,实现了对调制光的“混淆”和“扩散”,从而满足了Hash函数的核心功能要求:高安全强度的单向编码/加密.所设计的光电混合系统能有效地模拟Hash函数中的“压缩函数”,结合具有特征提取功能的Sobel滤波器,能实现将任意长度的输入数据压缩并加密为固定长度为256 bit的输出(即Hash值).一系列仿真结果表明:该方法所构造的光学Hash函数具有良好的“雪崩效应”和“抗碰撞性”,其安全性能可比拟当前最为广泛使用的传统Hash函数(MD5和SHA-1).     

Effects of imaging properties on dynamic speckles produced by a set of moving phase screens

Space-time correlation functions have been derived for the time-varying intensities of speckle patterns in image space produced by a series of moving random-phase screens. The time-correlation length, which measures the changing rate of intensity fluctuations, is evaluated from the correlation functions to examine the effects of the imaging properties on the temporal behaviour of the speckles resulting from the multiple scattering. It is shown that the average velocity and velocity dispersion of the phase screens can be measured from the time-correlation length by using imaging systems which have a small and a large point spread at the front focal plane of the systems, respectively. Theoretical results for the relationship between the focal position of the imaging lens and the time-correlation length are also presented.