Invisible anti-cloak with elliptic cross section using phase complement

Based on the theory of phase complement,an anti-cloak with circular cross section can be made invisible to an object outside its domain.As the cloak with elliptic cross section is more effective to make objects invisible than that with circular cross section,a scaled coordinate system is proposed to design equivalent materials of invisible anticloak with elliptic cross section using phase complement.The cloaks with conventional dielectric and double negative parameters are both simulated with the geometrical transformations.The results show that the cloak with elliptic cross section through phase complement can effectively hide the outside objects.     

Cornu's spiral in the Fresnel regime studied using ultrasound: a phase study

A simple experimental technique for measuring the phase and amplitude of diffracting ultrasound wave [A. Hitachi and M. Takata, Am. J. Phys. 78, 678 (2010)] has been applied to diffracting objects with straight edges as a demonstration of the Cornu spiral. Babinet's principle is studied observing the ultrasound field behind a slit and a complementary strip obstacle and is verified directly by comparing vectors (phasors) in the complex plane. The phase of the diffracted wave observed in the geometrical shadow of the straight screen has the form of a cylindrical wave originating at the edge of the straight screen as the boundary diffraction wave proposed by Young. In addition, the incident wave has a phase delay of π/4 behind the wave passing through on the center line of the slit, the plane of symmetry, has been observed as predicted by Huygens-Fresnel diffraction theory.     

基于螺旋相位调制的非相干全息点扩散函数研究

分析了菲涅耳非相干相关全息(Fresnel incoherent correlation holography,FINCH)系统中纯相位空间光调制器(spatial light modulator,SLM)加载螺旋相位掩模时的点扩散函数.以氙灯为照明光源搭建了FINCH系统,电荷耦合器记录的点源全息图与点扩散函数模拟结果一致.采用该系统分别在SLM上加载双透镜掩模和螺旋相位调制双透镜掩模两种情况下对分辨率板和非染色洋葱细胞成像,给出了成像对比结果.结果表明:采用螺旋相位调制的FINCH系统可以在几乎不牺牲分辨率的情况下提高图像的边缘对比度;同样,对相位物体也可以实现图像的边缘提取和识别.该方法在实时监测活细胞的分裂、形变等方面具有重要应用前景.     

Enhanced wavefront reconstruction by random phase modulation with a phase diffuser

A phase retrieval technique for enhanced wavefront reconstruction using random phase modulation and a phase diffuser is proposed. The speckle field generated is sampled at multiple axially displaced planes and the speckle patterns are used in an iterative algorithm based on the optical wave propagation in free space. The presentation of this technique is carried out using two setups. In the first setup, a diffuser plate is placed at the image plane of a metallic test object. The benefit of randomizing the phase of the object wave is the enhanced intensity recording due to high dynamic range of the diffusely scattered beam. The use of demagnification optics will also allow the investigations of relatively large objects. In the second setup, a transparent object is illuminated using a wavefront with random phase and constant amplitude by positioning the phase diffuser close to the object. The benefit of phase-only modulation is the increased resolution of the phase structures of the transparent test objects.     

Acoustic scattering by a three-dimensional elastic object near a rough surface

The ensemble-averaged field scattered by a smooth, bounded, elastic object near a penetrable surface with small-scale random roughness is formulated. The formulation consists of combining a perturbative solution for modeling propagation through the rough surface with a transition (T-) matrix solution for scattering by the object near a planar surface. All media bounding the rough surface are assumed to be fluids. By applying the results to a spherical steel shell buried within a rough sediment bottom, it is demonstrated that the ensemble-averaged "incoherent" intensity backscattered by buried objects illuminated with shallow-grazing-angle acoustic sources can be well enhanced at high frequencies over field predictions based on scattering models where all environmental surfaces are planar. However, this intensity must compete with the incoherent intensity scattered back from the interface itself, which can defeat detection attempts. The averaged "coherent" component of the field maintains the strong evanescent spectral decay exhibited by flat interface predictions of shallow-angle measurements but with small deviations. Nevertheless, bistatic calculations of the coherent field suggest useful strategies for improving long-range detection and identification of buried objects.     

用于实现散射介质中时间反演的数字相位共轭的相干性

抑制散射介质对光的散射,调控光在散射介质中的传输,是光通信、生物光子学、光镊等领域的重要课题.设计并实现了基于宽谱光源和数字相位共轭的可调控光在散射介质中传输的时间反演实验系统.实验获取了不同相干长度下物光和参考光束之间的光程差与干涉图样、相位图及时间反演信号之间的关系,分析了光源相干性对调控光在散射介质中传输的影响.实验结果表明,基于宽谱光源的相干特性和数字相位共轭技术,通过调节光程差能选择性获取同一散射角度及相同传输路径的光束的相对相位,再利用空间光调制器对参考光束进行调控,实现光束的反向传播,从而选择性实现对同一散射角度及相同传输路径的光的时间反演.     

Round-trip imaging through scattering media based on optical transmission matrix

Traditional one-way imaging methods become invalid when a target object is completely hidden behind scattering media. In this case, it has been much more challenging, since the light wave is distorted twice.To solve this problem, we propose an imaging method, so-called round-trip imaging, based on the optical transmission matrix of the scattering medium. We show that the object can be recovered directly from the distorted output wave, where no scanning is required during the imaging process. We predict that this method might improve the imaging speed and have potential application for real-time imaging.     

基于相位信息的边缘检测在目标识别中的应用

在分析利用相位信息进行图像边缘检测原理的基础上,探讨了基于小波域相位信息的边缘检测方法,并将其应用到环境光强变化的目标识别系统中。该算法不需要设定阈值即可对目标边缘进行可靠的检测。实验结果表明,基于相位信息的边缘检测算法受图像的光照和对比度的影响小,在不同光照度的环境下具有良好的目标识别能力     

Image edge-enhancement in optical microscopy with a phase mismatched spiral phase plate

We present a spiral phase filtering system with a large tolerance for edge enhancement of both phase and amplitude objects in optical microscopy.The method is based on a Fourier 4-f spatial filtering system.A phase mismatched spiral phase plate (SPP) fabricated by electron beam lithography is employed as the radial Hilbert transform for image edge enhancement.Compared with holography,SPP is simple,economical,reliable,and easy to integrate.     

Generalized symmetry transformation with Gaussian phase weight function

The generalized symmetry transformation (GST) is a symmetry operator detecting objects by using edge gradient directions. Conventional GST uses the cosine function to define the phase weight function (PWF), which represents the symmetry of two gradient directions. The cosine function of PWF leads to a good performance in detecting symmetrical objects. However, the weights of gradient pairs, which are considered to be asymmetrical, are relatively high, so side effects appear near the symmetry pick regions. (Note that side effects disturb the multiple object detection.) In this paper, we use the Gaussian function in calculating the symmetric weights of gradient pairs. The Gaussian function can suppress the weights of less symmetric gradient pairs. In addition, the symmetry for elliptically shaped objects can be more emphasized by controlling the width of the Gaussian function. The proposed GST is evaluated through experiments on synthetic images, which include various bright and dark plane figures, and on real images, which requires the detection of elliptical shapes.     

散射介质超衍射极限技术研究进展

综述了已有散射介质超衍射极限聚焦和成像技术的研究现状及进展。首先介绍了这一领域的研究背景及意义,以及已有超衍射极限成像技术的发展现状;然后给出了应用于超衍射极限成像的散射介质定义;其次分析了时间反演技术在声学、微波领域聚焦上的应用,介绍了时间反演法在光学领域超衍射极限聚焦应用中的实现方法,总结了散射介质加入到光学系统中的作用,分析了利用反馈控制调节和光学相位共轭方法进行散射介质超衍射极限聚焦方法的特点;讨论了基于空域和空频域传输矩阵测量的散射介质宽场成像方法及在该目的下的散射介质制备方法;最后给出了散射介质光学超衍射极限成像技术研究前景及展望。     

Reconstructing spatial phase distributions and object images from speckle intensity patterns of the diffraction field

A method of reconstruction of the spatial phase distribution and the image of a scattering object from a recorded speckle pattern of the diffraction field has been developed and verified on test objects. The proposed method is based on an assumption that, for objects of a certain class, the phase difference between adjacent speckles in the far zone is equal to π. Digital records of Fourier speckle patterns and the corresponding digitally reconstructed images are presented.     

Numerical reconstruction of an object image using an X‐ray dynamical diffraction Fraunhofer hologram

A numerical method of reconstruction of an object image using an X‐ray dynamical diffraction Fraunhofer hologram is presented. Analytical approximation methods and numerical methods of iteration are discussed. An example of a reconstruction of an image of a cylindrical beryllium wire is considered. The results of analytical approximation and zero‐order iteration coincide with exact values of the amplitude complex transmission coefficient of the object as predicted by the resolution limit of the scheme, except near the edges of the object. Calculations of the first‐ and second‐order iterations improve the result at the edges of the object. This method can be applied for determination of the complex amplitude transmission coefficient of amplitude as well as phase objects. It can be used in X‐ray microscopy.     

Influence of Phase Drift on Optical Coherence Tomography Using a CCD Camera-Based Heterodyne Detection Technique

The performance of a two-dimensional heterodyne detection technique in optical coherence tomography (OCT) was studied. This technique, which is based on the frequency synchronous detection method, enables the use of an imager such as a charge coupled device (CCD) camera as a heterodyne sensor array, so that horizontal cross-sectional image can be acquired in real time without lateral scanning. OCT measurements of scattering media including a biological object were demonstrated. To evaluate the influence of phase fluctuations on the present technique, we measured and analyzed the statistical relative-standard-deviation of heterodyne signal intensity as a function of the random phase shifts between two consecutive CCD frames. Practical limitations in the signal stability and possible solutions are discussed.     

Generating particlelike scattering states in wave transport

We introduce a procedure to generate scattering states which display trajectorylike wave function patterns in wave transport through complex scatterers. These deterministic scattering states feature the dual property of being eigenstates to the Wigner-Smith time-delay matrix Q and to the transmission matrix t(?)t with classical (noiseless) transmission eigenvalues close to 0 or 1. Our procedure to create such beamlike states is based solely on the scattering matrix and successfully tested numerically for regular, chaotic, and disordered cavities. These results pave the way for the experimental realization of highly collimated wave fronts in transport through complex media with possible applications such as secure and low-power communication.     

Label-Free Microscopic Imaging Based on the Random Matrix Theory in Wavefront Shaping

Wavefront shaping technology has mainly been applied to microscopic fluorescence imaging through turbid media,with the advantages of high resolution and imaging depth beyond the ballistic regime. However, fluorescence needs to be introduced extrinsically and the field of view is limited by memory effects. Here we propose a new method for microscopic imaging light transmission through turbid media, which has the advantages of label-free and discretional field of view size, based on transmission-matrix-based wavefront shaping and the random matrix theory. We also verify that a target of absorber behind the strong scattering media can be imaged with high resolution in the experiment. Our method opens a new avenue for the research and application of wavefront shaping.     

A two-step phase-retrieval method in Fourier-transform ghost imaging

A two-step phase-retrieval method, based on Fourier-transform ghost imaging, was demonstrated. For the complex objects, the phase-retrieval process was divided into two steps: first got the complex object’s amplitude from the Fourier-transform patterns of the squared object function, then combining with the Fourier-transform patterns of the object function to get the phase. The theoretical basis of this technique is outlined, and the experimental results are presented.     

Real-time measurement of nano-particle size using differential optical phase detection

We demonstrate a size sensing technique for nano-particles using optical differential phase measurement by a dual fiber interferometer through phase-generated carrier(PGC) demodulation. Nano-particle diameters are obtained from the differential phase shift as a result of adding an optical scattering perturbation into two-beam interference. Polystyrene nano-particles with diameters from 200 to 900 nm in a microfluidic channel are detected using this technique to acquire real-time particle diameters. Compared with amplitude sensing with over 10 mW of laser irradiance, particle sizing by PGC phase sensing can be achieved at a laser power as low as1.18 mW. We further analyze major sources of noise in order to improve the limits of detection. This sensing technique may find a broad range of applications from the real-time selection of biological cell samples to rare cell detection in blood samples for early cancer screening.     

一种快速暗通道去雾算法

基于暗通道的去雾算法计算初始透射率需要进行大量的数据比较,优化透射率时需要计算融合矩阵,这两个过程耗时巨大,使其难以投入实际应用。针对这一问题,提出了一种快速暗通道去雾方法。首先利用分区最小表的数据结构,提高初始透射率的计算速度;接着,在优化透射率时,采用基于形态学梯度的彩色图像边缘检测算法提取图像边缘信息,减少优化范围,再利用边缘信息与像素的空间信息优化透射率,避免计算复杂矩阵,从而加快了优化速度;最后,运用最小可觉差模型补偿前两个步骤导致的图像画质下降,提高图像清晰度。实验证明,快速暗通道去雾算法在保持恢复图像效果基本不变的基础上,很大程度提高了算法速度。     

全光纤激光多普勒测振仪空气耦合振动检测

采用外差探测方式, 建立了全光纤结构的激光多普勒测振仪,用来测量经空气传播耦合引起的散射体振动,以期感知振源振动信息。实验探索了散射体材料、固定方式和相对振源位置等对空气耦合振动的影响,得到了200~2700 Hz频率范围内6种散射体振动响应特性曲线。结果表明:通过空气耦合引起的散射体振动,在相同振源振幅下,散射体振动的振幅一般随频率升高而升高; 在材料、固定方式和相对振源位置等因素中,散射体材料对振动响应特性的影响是主要的,后两者只影响局部细节。