Design of the spatial filter window for digital holographic convolution reconstruction of object beam field

In the convolution reconstruction process of digital holographic object beam field, the object beam fields with different magnifications can be obtained when the reconstructing beams are spherical waves with different wave curvature radii. This paper presents a theoretical and experimental discussion on the useful spatial frequency spectrum of the hologram numerically illuminated by the spherical wave. The result shows that there would be an image in the spatial frequency spectrum of digital hologram, which is completely the same as the object, if the wave surface radius of the spherical wave is equal to the distance from the object to the CCD sensor. Taking this image as a reference and designing a filter, the position of the reconstructed images with different magnifications can be predicted correctly in the reconstructed plane. Additionally, since the spectrum distribution of zero-order diffraction can be forecasted accurately in theory, its contribution can be effectively eliminated through changing the spatial filter shape; then a reconstructed object field containing more high-frequency information can be obtained.     

重建彩色数字全息图时不同色光物光场的准确重叠研究

将数字全息检测物体表面视为散射面,球面波为参考波,使用角谱重建算法对不同波长照明情况下物平面光波场重建位置进行研究.结果表明,不考虑图像的物理意义时,衍射的一次傅里叶变换重建像中心与物光场频谱的中心相对应,以一次傅里叶变换重建像为参考,可以较好地确定物光场频谱位置,按照可变放大率的角谱重建算法实现不同色光重建场的准确重叠.     

散射光彩色数字全息光学系统及波面重建算法研究

令物体表面为散射面,参考光及重建光为球面波,详细研究了离轴数字全息物光场重建平面的光波场.导出物光、共轭物光及零级衍射光分布与光学系统参数的关系.根据彩色数字全息的特点,提出抑制零级衍射干扰的光学系统设计方法,并且对一种能够避免插值误差的波面重建算法进行了优化研究,给出了相应的实验证明.     

Eliminate the influence of zero-order diffraction for wavefront reconstruction with selectable magnification in digital holographic

The objective surface is considered as the scattering plane; frequency spectrum of a digital hologram transmission light is studied when the reference light and reconstruction wave are spherical waves. The relation between relevant parameters and the object light, conjugate object light and zero-order diffraction wave frequency spectrum distribution. The research results show that: each frequency spectrum of the diffracted wave broadens to various degrees as the radius of the reconstruction wavefront decreases, and frequency aliasing is generated under certain conditions. Based on the research results, a new method that carries out high-pass filtering processing without zero-order diffraction interference upon digital hologram is proposed and the reconstruction experimental proof for eliminating the interferential changeable magnification wave is given.     

Multiple focused-waves production by parametric mixing in a layer with hysteretic quadratic nonlinearity

Nonlinear scattering of a weak probe wave by a strong pump wave in a layer with nonlinearity hysteresis is analysed. It is demonstrated that a spherical probe wave from a point source after nonlinear process of difference frequency excitation is scattered in several foci. Different foci correspond to different frequencies of the transformed wave spectrum. In the case of a plane pump wave and under the condition that nonlinear scattering proceeds without acoustic mode conversion wave front reversal is possible. To achieve wave front reversal in a layer with hysteretic quadratic nonlinearity the pump wave should have a frequency equal to the frequency of the probe wave or to any of its subharmonics.     

光横向传播的光折变体全息透镜设计及其衍射性质

提出了一种新型的可用于光束横向传播的光折变体全息透镜。该透镜是由入射方向相互垂直的一束平面波和一束球面波在双掺杂的铌酸锂晶体平板上记录形成的，它可以将垂直入射晶体平板的输入光束产生横向传输并聚焦，或对输入光点产生横传并准直输出。该透镜除了具有一般体全息透镜的稳定性好、可靠性高的特点外，特别具有光束横向传播的功能，因而具有空占比小、更易于整个光学透镜系统的微小化集成的优点。利用局域体全息的耦合波理论，计算了该透镜衍射光的振幅分布和衍射效率。计算了该透镜的焦面上的强度分布，并与理想球面波照射矩形孔径时焦面上的强度分布进行了比较。     

Methods of Fourier optics in digital holographic microscopy

In this paper, processing methods of Fourier optics implemented in a digital holographic microscopy system are presented. The proposed methodology is based on the possibility of the digital holography in carrying out the whole reconstruction of the recorded wave front and consequently, the determination of the phase and intensity distribution in any arbitrary plane located between the object and the recording plane. In this way, in digital holographic microscopy the field produced by the objective lens can be reconstructed along its propagation, allowing the reconstruction of the back focal plane of the lens, so that the complex amplitudes of the Fraunhofer diffraction, or equivalently the Fourier transform, of the light distribution across the object can be known. The manipulation of Fourier transform plane makes possible the design of digital methods of optical processing and image analysis. The proposed method has a great practical utility and represents a powerful tool in image analysis and data processing. The theoretical aspects of the method are presented, and its validity has been demonstrated using computer generated holograms and images simulations of microscopic objects.     

Use of spatial filtering in imaging through a randomly inhomogeneous medium

The possibility of improving the image quality of a coherently reflecting object, viewed through a randomly inhomogeneous medium, is investigated theoretically. Three cases of coherent illumination of the object by a plane wave are considered: illumination by unperturbed wave, illumination and viewing through different inhomogeneities of a medium, illumination and viewing through the same inhomogeneities. The spatial spectrum of the average intensity is investigated. It is shown that, when the fluctuations of the intensity in the plane of the receiving lens aperture are strong, the use of a suitable spatial filter located in the focal plane of the lens always gives an improvement of the image, but the quality of the image is better when illuminating and receiving optimal systems coincide.     

Focusing for laser hole drilling

The power intensity distribution of a laser beam across the focal plane is important for producing good hole geometry. To examine the power intensity distribution in the region of the image plane of a practical lens, a diffraction integral method is employed. Lens aberration is accounted for by the intensity of the appropriate phase changes. In the analysis, a free-running pulsed multimode solid-state laser is considered and the lens is assumed to have aberrations. Since an on-axis object is examined, only spherical aberration is taken into account.     

Use of spatial filtering in imaging through a randomly inhomogeneous medium

Abstract

The possibility of improving the image quality of a coherently reflecting object, viewed through a randomly inhomogeneous medium, is investigated theoretically. Three cases of coherent illumination of the object by a plane wave are considered: illumination by unperturbed wave, illumination and viewing through different inhomogeneities of a medium, illumination and viewing through the same inhomogeneities. The spatial spectrum of the average intensity is investigated. It is shown that, when the fluctuations of the intensity in the plane of the receiving lens aperture are strong, the use of a suitable spatial filter located in the focal plane of the lens always gives an improvement of the image, but the quality of the image is better when illuminating and receiving optimal systems coincide.     

基于液晶空间光调制器的空间滤波实验

空间滤波实验可以实现对输入物体的频谱分析及图像的边缘增强、噪声消除等各种图像处理。液晶空间光调制器可以作为衍射波面变换器件来使用。在空间滤波实验系统的滤波面上放置了液晶空间光调制器,并用CCD数码相机进行输出图像的观察和记录,分别给出了低通滤波、高通滤波、方向滤波等的仿真实验结果。该实验实现了输入数字物体的实时改变,并能够实时观察采用不同空间滤波器对输出像的影响。     

Phase retrieval and coherent diffraction imaging by a linear scanning pinhole sampling array

We propose a new method for phase retrieval and coherent diffraction imaging by a specially designed pinhole sampling array (PSA) based on a liquid crystal spatial light modulation. We demonstrate that the phase and the amplitude of the wave front passing through a pinhole sampling array plate can be directly extracted from the inverse Fourier transform of the far-field diffraction intensity pattern. Scanning the whole surface of the wave front by such a series of the PSA plates, we can assemble the extracted complex amplitude to a two-dimensional discrete distribution of the sampled wave front covering the entire PSA plate plane in the scanning consequence. We called it linear scanning pinhole sampling array (LSPSA). Thus the wave front can be reconstructed which avoids any iterative algorithms. Furthermore, it provides a feasible approach for lensless coherent diffraction imaging in real-time.     

Backscattering from different objects in turbulent media

Abstract

The connection between diffraction characteristics of the scatterer and distribution of average backscattered intensity of a spherical wave is considered. In experiments with an ‘infinite’ plane mirror it is shown that the distribution of average backscattered intensity coincides with the correlation function of the intensity fluctuation of a virtual point source located at the mirror and observed from the real source plane. Non-monotonic dependence (with a minimum at the Fresnel number of scattered mirror?1) between the enhancement factor and the size of reflected mirror is observed in experiments.     

干涉高斯光诱导的表面等离子激元驻波场的分析

考虑到实际入射光强的空间分布不均匀,基于Kretschmann模型并采用角谱方法分析模拟了两束高斯光干涉诱导表面等离子激元(SPP)驻波场。与理想平面波不同,高斯光诱导的SPP干涉条纹幅值大小不等,分布复杂,这表明光强空间非均匀程度会严重地影响到曝光深度的分布。还分析了金属薄膜的厚度、损耗以及光刻胶的介电常数对SPP驻波场的影响,并得出不恰当的金属板厚和细微损耗都会极大削弱SPP驻波场,而如果光刻胶的介电常数过大则有可能产生不了表面等离子体共振的结论。     

Recording and reconstruction of digital Gabor hologram

This paper looks at some as yet undisclosed observations in the recording and reconstruction of digital Gabor hologram. Small particles on the specimen plane diffract the illuminating spherical wave to form the object wave which interferes with the un-diffracted reference to form the in-line hologram. The interference is similar to that between two spherical wavefronts. It is shown that resolution enhancement can be achieved in the recording process through the use of wavefronts with different curvature. Numerical reconstruction of this hologram with a conjugate of the illuminating (reference) wave provides the in-focus real image at the object plane. It is exemplified that the proper choice of the reconstruction wavefront can optically magnify the reconstructed image enabling one to better visualize the resolution enhancement. Both theoretical analysis and experimental confirmation are provided for these observations.     

Numerical reconstruction in in-line digital holography by translation of CCD position and gradient operator method

We propose an approach based on axial translation of a charge-coupled device (CCD) and gradient operator method for numerical reconstruction in in-line digital holography. The object wave in the hologram (or CCD) plane is determined using one position translation of CCD, and a phase-shifting device can be fully avoided. Gradient operator method is further applied to detect the focal plane of an object, and the object wave is reconstructed by an angular spectrum algorithm with the detected focal distance. The feasibility and effectiveness of the proposed method are demonstrated by numerical simulations.     

Experimental evaluation of entropy for transmission holographic optical elements

An experimental study of transmission holographic optical elements is presented. Several important aspects related to the diffractional analysis of these elements have been considered. The case of the spherical object wave is analyzed in detail. The emphasis is on obtaining the entropy from the intensity distribution on an image plane. We use the calculated entropy to locate experimentally the position of the best image plane in the sense of minimal aberrations for a holographic lens. The comparison of experimental and theoretical results is presented and good agreement between theory and experimentation has been obtained.Part of this paper was presented at the IS & T/SPIE symposium in Electronic Imaging, Science and Technology (Practical Holography IX) February 4–10, 1995, San José, CA, USA     

散射光波场卷积重建的三种方法研究

将数字全息检测物体的表面视为散射面,当物体投影尺寸远大于CCD面阵尺寸时用球面波为重建波,对物平面光波场卷积重建过程进行了理论研究.导出了重建物光场放大率与重建参量的关系.介绍了通过补零扩大CCD平面以及对物体局域重建组合的另外两种卷积重建方法.通过实验对三种方法进了行比较.结果表明,球面波为重建波的方法优于另外两种方法.     

水下目标对幅度高斯相关海面环境噪声场扰动特性的研究

理论探讨了水下目标受海面环境噪声场激发产生的散射场对海洋环境噪声场的扰动特性,基于幅度高斯型相关海面噪声源模型,利用任意声源分布声场的积分表达式和单极子源半无限空间格林函数的球面波展开式,获得了刚性球目标在海面单极子和偶极子源海洋环境噪声“照射”下的直达噪声场、散射噪声场、总噪声场,以及二接收点直达、散射和总噪声协方差的理论表达式,表明总噪声场除了和目标阻抗特性、接收点的方向有关外,还受到海面噪声源相关特性,以及直达与散射噪声场之间干涉的较大影响。数值计算结果给出较近距离范围内,刚球目标声学可见度约为4~5dB,并指出在海洋环境噪声场中,刚球目标散射的方向特性明显不同于平面波入射。      

二步合成平板周视彩虹全息术

提出一种将物光波前先分解记录，再合成的二步平板周视彩虹全息术。波前的分解记录与合成彩虹全息图是分别通过转动物体和干板来实现的。记录这种周视全息图，不需要大孔径和其它特殊光学元件，即可获得周视半径大的无象差立体再现象。该方法可直接用于模压全息图母板的制作。