Fast phase retrieval in slightly off-axis digital holography

In this study, three efficient algorithms are proposed for fast phase retrieval in slightly off-axis digital holography using spectrum cropping, spatial multiplexing, and complex encoding. In the first algorithm, the real spectral order of the subtracted hologram is filtered and cropped, and the number of pixels is decreased in the subsequent retrieval operations. In the second algorithm, two sequential subtracted holograms are digitally phase shifted and spatial multiplexed into one synthetic hologram, and thus only one inverse Fourier transformation is then required. In the third algorithm, two sequential subtracted holograms are encoded separately into the real part and the imaginary part of a complex hologram. Two cross-correlations can be used to reconstruct the phase, thereby improving the utilization of the spectrum. The three new algorithms speed up our previously proposed retrieval method with the assistance of specimen-free holograms. Our experiments demonstrated the validity and improved time requirements of the proposed methods.     

Off-axis phase-only holograms of 3D objects using accelerated point-based Fresnel diffraction algorithm

A method for calculating off-axis phase-only holograms of three-dimensional (3D) object using accelerated point-based Fresnel diffraction algorithm (PB-FDA) is proposed. The complex amplitude of the object points on the z-axis in hologram plane is calculated using Fresnel diffraction formula, called principal complex amplitudes (PCAs). The complex amplitudes of those off-axis object points of the same depth can be obtained by 2D shifting of PCAs. In order to improve the calculating speed of the PB-FDA, the convolution operation based on fast Fourier transform (FFT) is used to calculate the holograms rather than using the point-by-point spatial 2D shifting of the PCAs. The shortest recording distance of the PB-FDA is analyzed in order to remove the influence of multiple-order images in reconstructed images. The optimal recording distance of the PB-FDA is also analyzed to improve the quality of reconstructed images. Numerical reconstructions and optical reconstructions with a phase-only spatial light modulator (SLM) show that holographic 3D display is feasible with the proposed algorithm. The proposed PB-FDA can also avoid the influence of the zero-order image introduced by SLM in optical reconstructed images.     

Review of Fresnel incoherent correlation holography with linear and non-linear correlations

Fresnel incoherent correlation holography(FINCH) is a well-established incoherent imaging technique. In FINCH, three selfinterference holograms are recorded with calculated phase differences between the two interfering, differently modulated object waves and projected into a complex hologram. The object is reconstructed without the twin image and bias terms by a numerical Fresnel back propagation of the complex hologram. A modified approach to implement FINCH by a single camera shot by pre-calibrating the system involving recording of the point spread function library and reconstruction by a nonlinear cross correlation has been introduced recently. The expression of the imaging characteristics from the modulation functions in original FINCH and the modified approach by pre-calibration in spatial and polarization multiplexing schemes are reviewed. The study reveals that a reconstructing function completely independent of the function of the phase mask is required for the faithful expression of the characteristics of the modulating function in image reconstruction. In the polarization multiplexing method by non-linear cross correlation, a partial expression was observed, while in the spatial multiplexing method by non-linear cross correlation, the imaging characteristics converged towards a uniform behavior.     

High speed holocinematography using spatial multiplexing for image separation

Up to eight holograms are successively recorded at maximum repetition rates of 20kHz. A multiply Q-switched ruby laser is used to produce the series of coherent light pulses for hologram exposure. All holograms are recorded on the same plate. Image separation of different holograms is achieved by spatial multiplexing using a rotating disk with apertures directly in front of the holographic plate. The performance of the technique is demonstrated by reconstructions of a hologram series taken of laser produced cavitation bubbles following optical breakdown in water.     

基于液晶空间光调制器的全息显示

在传统的纯相位全息显示系统中, 一般基于快速傅里叶变换(FFT)算法来计算相位全息图, 在FFT的计算中需要遵循Nyquist采样定理, 因此, 重建图像的尺寸往往受限于空间光调制器的固定采样率. 这个限制可以通过卷积算法或者两步菲涅耳衍射算法来解决, 但是需要使用多个FFT的计算, 导致计算量增大. 鉴于此, 提出了一种基于透镜的纯相位全息图计算方法. 在全息图的计算中, 通过透镜的成像原理建立一个采样率可变的虚拟全息面, 通过调节相应的距离参数使得在全息图的计算中可以任意调节原始图像的采样率, 摆脱了传统方法中液晶空间光调制器带宽积对重建图像尺寸的限制, 并且这种算法只需使用一次FFT就能达到变采样率的衍射计算, 大幅提高了全息图的计算速度. 数值模拟及光学实验结果证明了此方法可以在全息显示光学系统中清晰地重建不同尺寸的图像. 同时该系统可以有效地消除由空间光调制器的像素化结构带来的零级衍射.     

Dual beam encoded extended fractional Fourier transform security hologram with in-built repositioning

This paper describes a simple method for making dual beam encoded extended fractional Fourier transform (EFRT) security holograms. The hologram possesses different stages of encoding so that security features are concealed and remain invisible to the counterfeiter. These concealed and encoded anticounterfeit security features in the security hologram can only be read through a key hologram. Key hologram also facilitates in-built repositioning of security hologram. The method of fabrication, the principle of reconstruction and the experimental results are presented.      

High-efficiency detour-phase holograms

We suggest a new approach to the fabrication of detour-phase holograms. By combining the concept of the detour phase with highly efficient carrier gratings it is possible to overcome the major drawback of detour-phase holograms, i.e., their low efficiency. We call the new hologram type a kinoform detour-phase hologram (KDPH). KDPH's facilitate the implementation of diffractive elements with high phase complexity. Especially if off-axis reconstruction is desired, the number of degrees of freedom for the design of the phase component is increased significantly.     

利用反射全息实现计算全息三维显示

计算全息和光学全息都可应用于三维显示,但各有自己的优势和缺陷.将计算全息和光学反射全息相结合,可以突破光学全息对记录物体的限制,进行虚拟物体或自然场景的全息图的制作,同时可以实现白光再现.本文首先用三维扫描仪获得实际物体的三维数据,用"点云算法"模拟得到其菲涅耳全息图透射率数据,采用计算全息打印机将其输出于全息记录介质,得到可光学再现的菲涅耳计算全息图H₁.然后将H₁作为光学全息的记录物体进行反射全息记录,将平面全息转化为体全息,实现了计算全息白光再现.     

Multi-wavelengths digital holography: reconstruction, synthesis and display of holograms using adaptive transformation

A method based on spatial transformations of multiwavelength digital holograms and the correlation matching of their numerical reconstructions is proposed, with the aim to improve superimposition of different color reconstructed images. This method is based on an adaptive affine transform of the hologram that permits management of the physical parameters of numerical reconstruction. In addition, we present a procedure to synthesize a single digital hologram in which three different colors are multiplexed. The optical reconstruction of the synthetic hologram by a spatial light modulator at one wavelength allows us to display all color features of the object, avoiding loss of details.     

Digital off-axis holography without zero-order diffraction via phase manipulation

By means of manipulating the phase information of the object beam in an off-axis digital holographic setup, we show that it is possible to fully eliminate the zero-order diffraction (ZOD) from numerically reconstructed holograms. Two different approaches are presented. In the first method, we introduce a ground glass on the object path beam to provide a random phase illumination. The subtraction between two holograms recorded with different positions of the ground glass yields a ZOD free hologram. In the second approach, a piece of window glass inserted on the path of the object beam produces a constant phase shift. The subtraction of two holograms, one recorded with and one without window glass generates a new hologram whose numerical reconstruction is ZOD free. Theoretical models and experimental results are shown to validate our proposals. They show that the proposed methods totally remove the ZOD without ruining the object information.     

利用反射全息实现计算全息三维显示

计算全息和光学全息都可应用于三维显示,但各有自己的优势和缺陷.将计算全息和光学反射全息相结合,可以突破光学全息对记录物体的限制,进行虚拟物体或自然场景的全息图的制作,同时可以实现白光再现.本文首先用三维扫描仪获得实际物体的三维数据,用"点云算法"模拟得到其菲涅耳全息图透射率数据,采用计算全息打印机将其输出于全息记录介质,得到可光学再现的菲涅耳计算全息图H1.然后将H1作为光学全息的记录物体进行反射全息记录,将平面全息转化为体全息,实现了计算全息白光再现.     

Phase-only stereoscopic hologram calculation based on Gerchberg–Saxton iterative algorithm

A phase-only computer-generated holography(CGH) calculation method for stereoscopic holography is proposed in this paper.The two-dimensional(2D) perspective projection views of the three-dimensional(3D) object are generated by the computer graphics rendering techniques.Based on these views,a phase-only hologram is calculated by using the Gerchberg–Saxton(GS) iterative algorithm.Comparing with the non-iterative algorithm in the conventional stereoscopic holography,the proposed method improves the holographic image quality,especially for the phase-only hologram encoded from the complex distribution.Both simulation and optical experiment results demonstrate that our proposed method can give higher quality reconstruction comparing with the traditional method.     

全息图的数字重现

计算机数字重现全息图可消除像差，噪声及记录过程中底片非线性因素的影响，与可见光重相现比有更大的灵活性。对短波和全息如Ｘ射线全息，电子全息来说，数字重现可大大提高其重现分辨率。本文给出了全息图数字重现的理论公式，利用理枋全息图的强度分布数据成功地重现了同轴和离轴全息图，并对重现结果作了分析。     

Single-exposure two-dimensional superresolution in digital holography using a vertical cavity surface-emitting laser source array

We present a new implementation capable of producing two-dimensional (2D) superresolution (SR) imaging in a single exposure by aperture synthesis in digital lensless Fourier holography when using angular multiplexing provided by a vertical cavity surface-emitting laser source array. The system performs the recording in a single CCD snapshot of a multiplexed hologram coming from the incoherent addition of multiple subholograms, where each contains information about a different 2D spatial frequency band of the object's spectrum. Thus, a set of nonoverlapping bandpass images of the input object can be recovered by Fourier transformation (FT) of the multiplexed hologram. The SR is obtained by coherent addition of the information contained in each bandpass image while generating an enlarged synthetic aperture. Experimental results demonstrate improvement in resolution and image quality.     

A strong adaptable autofocusing approach of off-axis infrared digital holography under different quality conditions of holograms

In this paper, we present an innovative autofocusing criterion for the reconstruction of infrared digital holograms. This criterion has the advantages of fast, efficient and precision when determining the reconstruction distance of off-axis digital holography. This criterion is a mean-free high frequency calculation process. We focus on the problem of mean value drifting found in previous published methods and design our new approach to solve it. Unlike the previous methods perform well only with high quality holograms, our method is effective for both high and low quality holograms. Even when hologram is degraded by destructive interference, our method still performs well. This method helps to automatically determine the precise reconstruction distance, and we are sure that this technology can be applied in industrial applications in the future.     

Engineered pixels using active plasmonic holograms with liquid crystals

Digital holography requires arrays of small reconfigurable elements to achieve complex reconstruction of the hologram with common systems based on pixels utilizing liquid crystal on silicon (LCoS) technology. The backplane of a typical pixel element is potentially underutilized and thus relatively large physical areas exist in which information can be stored and exploited to give additional functionality to pixel elements. Polarisation and wavelength dependent optical properties can be achieved in small areas using the plasmonic effects of optical antennae. The integration of LCs with optical antennae‐based plasmonic holograms allows active modulation of the far field pattern. The work here demonstrates the concept that conventional LCoS pixel elements can be greatly enhanced with the integration of plasmonic holograms, composed of optical antennae patterned on the surface, giving rise to new levels of modulation capability for holographic pixel elements. Using LCs, polarisation dependent effects in plasmonic holograms can be switched. ‘Engineered pixels', with sub‐wavelength multiplexing over both polarisation and wavelength, can increase the channel capacity of a typical LC display device. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Volume hologram scheme using optical fiber for spatial multiplexing

Speckle patterns from optical fibers are used for reference beams in writing and reading volume holograms. The photorefractive volume hologram with the scheme shows good spatial selectivity for spatial (shift) multiplexing. Experimental results are presented and compared with numerical analysis.     

Hilbert reconstruction of phase-shifted second-harmonic holographic images

New techniques are presented that make phase-shifting holography viable for second-harmonic generation (SHG) holography with weak object fields. We developed an intrinsic phase shift calibration of SHG holograms, an algorithm that extracts the reference and object intensity directly from a set of phase-shifted holographic data, and a more robust phase-shifting holography reconstruction algorithm based on π-shifted hologram pairs that permits self-calibration of the phase shift and recovery of the complex field through a Hilbert transform.     

彩色全息显示中液晶空间光调制器位相调制偏差的矫正方法

采用三基色激光照明并结合时分复用或空间复用技术,可实现基于液晶空间光调制器(LC-SLM)的彩色全息显示.但由于在不同激光波长入射条件下LC-SLM的位相调制特性曲线不同,难以同时满足多波长入射条件下2π线性位相调制的要求,致使彩色全息再现结果受到共轭像和零级斑的干扰.针对该问题,本文通过实验测试获得LC-SLM在不同波长入射时的位相调制特性曲线,并分析了RGB激光器各分量的位相调制特性曲线非线性偏差和调制幅度偏差对多阶位相型傅里叶变换相息图再现效果的影响.根据"查表法"建立了各波长入射条件下满足2π线性位相调制的灰度映射关系,并对RGB分量相息图进行修正.通过对修正前后RGB分量相息图的数值模拟再现和光电再现实验与分析,结果表明:该方法有效地克服了LC-SLM位相调制特性曲线偏差的不利影响,从而改善了彩色全息显示中各分量相息图的再现质量.     

光学扫描全息术研究进展

光学成像技术极大地拓展了人类的视觉极限,提高了人们观察和理解现实世界的能力。越多地获得目标的光学信息,对其的认识越充分。数字全息术是一种可以将样本的三维信息以二维全息图的形式编码记录下来的一种成像技术。通过获得由携带物体信息的物光波和参考光波叠加产生的干涉图案,可以以数字化的方式实现多种重建模态,例如图像恢复、相位成像和切片成像等。光学扫描全息术是一种独特的数字全息成像技术,通过主动式二维化扫描对三维物体进行成像,其完整的波前信息可以被单像素探测器记录,并基于光外差检测进行信号解调,从而恢复出复数全息图。对光学扫描全息术的最新进展进行介绍。首先,基于双光瞳成像系统,通过特殊的硬件和算法设计,提高光学成像系统的性能,如提高空间分辨率、缩短扫描时间。其次,基于计算成像原理,通过改进和优化全息像重建算法,实现高质量的图像恢复,主要涉及切片成像和三维成像等重建模态。第三,介绍光学扫描全息术的其他研究方向,并讨论该领域未来可能的发展方向。