Cross-talk in phase encoded volume holographic memories employing unitary matrices

The cross-talk noise in phase encoded holographic memories employing unitary matrices is theoretically investigated. After reviewing some earlier work in this area, we derive a relationship for the noise-to-signal ratio for phase-code multiplexing with unitary matrices. The noise-to-signal ratio rises in a zigzag way on increasing the storage capacity. Cross-talk is mainly caused by high-frequency phase codes. Unitary matrices of even orders have only one bad code, while unitary matrices of odd orders have four bad codes. The signal-to-noise ratios of all other codes can in each case be drastically improved by omission of these bad codes. We summarize the optimal orders of Hadamard and unitary matrices for recording a given number of holograms. The unitary matrices can enable us to adjust the available spatial light modulators to achieve the maximum possible storage capacity in both circumstances with and without bad codes. PACS 42.40.Lx; 42.40.Ht; 42.40.Pa     

Two-dimensional shift-orthogonal random-interleaving phase-code multiplexing for holographic data storage

A novel phase encoding technology for phase-code multiplexing in holographic data storage (HDS) system called two-dimensional shift-orthogonal random-interleaving (2-D SORI) phase encoding is proposed. Compared with the traditional one-dimensional orthogonal phase-code multiplexing methods, the 2-D method is less sensitive to the variations of the diffraction amplitude and to the phase error of the phase mask. Phase masks for the 2-D SORI phase-code multiplexing can be generated by shifting an elaborately designed phase plate at a certain step, which can avoid the use of a high-cost phase spatial light modulator for the generation of multiple orthogonal phase masks. The cross-talk arising from the systematic phase defects of the static phase modulator is eliminated by the shift operation of the phase plate. Phase codes are interleaved under a predetermined random mapping rule to prevent unauthorized users from accessing the data and eliminate the Bragg degeneration cross-talk. A 2-D SORI phase plate with the size of 5.12 mm × 10.24 mm is designed and fabricated, from which 128 orthogonal phase patterns are generated. The feasibility of the 2-D SORI phase-code multiplexing method for HDS is experimentally demonstrated.     

Associative recall in a volume holographic storage system based on phase-code multiplexing

We present two different techniques on how to realize a content-addressed holographic memory when using phase-code multiplexing, relying on simple intensity measurements rather than phase distributions. Theoretical and experimental results of associative recall in a phase-coded system designed for digital data storage will be presented and compared to the corresponding method when using angular multiplexing. Received: 2 August 2001 / Revised version: 20 September 2001 / Published online: 23 November 2001     

Overloaded phase-code multiplexing for volume holographic storage

Overloaded phase codes for volume holographic data storage are introduced. In contrast to any previous phase-code design, overloaded phase codes enable multiplexing of a number of data pages that exceeds the number of utilized reference beams. In this way the achievable data capacity can be augmented. Overloaded codes are generated by extending multilevel phase codes based on the discrete Fourier transform. We demonstrate multiplexing of 70 analog pages by means of 64 reference beams. The analysis of reconstructed digital data pages suggests that a capacity gain of up to 15% is reasonable.     

Optical implementation of orthogonal phase-code multiplexing

We propose a new optical implementation of orthogonal phase-code multiplexing in which an arbitrary phase shift of theta or theta +pi is used in the reference beam instead of 0 or pi as in the conventional method. To compare the new and the conventional methods, we employ a 2-bit orthogonal phase code and store two binary-data images in a BaTiO(3) crystal with each method. We also employ numerical methods to simulate the 2-bit phase-code multiplexing and show that the signal-to-noise ratio in the restored images is improved by more than one order of magnitude in the new method in our experimental conditions.     

Cross talk between holograms of finite contrast in a phase-code multiplexing system

The cross talk between holograms recorded in a phase-code multiplexing system is analyzed. The signal-to-noise ratio (SNR) is derived for the original image of a finite contrast ratio in the presence of a phase-shift error of the reference beam. It is shown that one particular code in the Walsh-Hadamard code set can be the dominant cross-talk noise source. We experimentally measured the SNR of the holograms recorded in an 8-bit phase-code multiplexing system by varying the contrast ratio of the original image. We show good agreement between experiment and theory. The existence of one bad code is also shown experimentally.     

Color holographic display based on azo-dye-doped liquid crystal (Invited Paper)

A multiplexed holographic display video has been achieved by using a passive azo-dye-doped liquid crystal （LC） cell. Holograms formed in this cell can be refreshed in the order of several milliseconds. By angular multiplexing technique, dynamically multiplexed holographic videos are realized. Moreover, the reconstructed RGB images are merged into a color image, which illustrates the possibility of a color holographic three-dimensional （3D） display by holographic multiplexing of the LC cell.     

Shift multiplexing with a two-dimensional lens array

We propose a new shift multiplexing method to reduce the cross-talk noise and increase the holographic storage density. We use spherical waves emerging from a two-dimensional lens array as reference beams to multiple holograms. The theoretical results indicate that the sidelobes of the diffraction intensity can sharply be reduced to near-zero and the spatial separation of adjacent holograms can also be reduced, resulting in higher holographic storage density than conventional shift multiplexing methods.     

利用菌紫质薄膜进行角度复用和偏振复用全息存储实验研究

全息光存储以其高密度、大容量、高速并行数据存取而成为光存储领域的一个重要研究方向。生物光致变色材料———菌紫质是一种新型可擦重写全息记录介质。实验证明了使用菌紫质薄膜进行角度复用和偏振复用全息存储的可行性。利用菌紫质的光致变色特性,采用90°角度复用全息存储光路,在BR-D96N薄膜样品同一位置上实现了6幅全息图存储,并分别读出了无串扰的再现像。利用菌紫质薄膜的光致各向异性进行了偏振复用全息存储,在BR-D96N薄膜样品的同一位置上存储了两幅正交偏振光记录的图像,用原参考光再现和偏振片选择,可分别读出这两幅图像。     

散斑全息存储中的散斑尺寸优化

本文分别给出散斑全息存储中复用选择性和读出衍射效率与散斑尺寸的关系,依此定义了一个用于优化散斑尺寸的判据参数并进行优化计算.在均衡散斑尺寸对复用选择性和存储介质动态范围的不同影响下,散斑存储应选取适当的大小以获得高密度存储.     

Reconstruction resolution enhancement of EPISM based holographic stereogram with hogel spatial multiplexing

We investigate how the splicing mode of a holographic element (hogel) affects the reconstruction of a 3D scene to improve the reconstruction resolution of a holographic stereogram fabricated using the effective perspective image segmentation and mosaicking method (EPISM). First, the effect of hogel spatial multiplexing on holographic recording and reconstruction is studied based on the mechanism of recording interference fringes in the holographic recording medium. Second, combined with the influence of multiple exposures on the hologram's diffraction efficiency, the diffraction efficiency of the holographic stereogram is analyzed in the spatial multiplexing mode. The holographic stereogram is then regarded as a special optical imaging system. The theory of spatial bandwidth product is adopted to describe the comprehensive resolution of the holographic stereogram, which explains why hogel spatial multiplexing can significantly improve the reconstruction resolution of a holographic stereogram. Compared with the traditional printing method under the same parameters in optical experiments, hogel spatial multiplexing has a lower diffraction efficiency but a higher quality of reconstructed image, consistent with the theoretical analysis.     

Optimization of a 1 mm thick PVA/acrylamide recording material to obtain holographic memories: method of preparation and holographic properties

Information holographic storage is a very promising technique due to its high theoretical capacity. One of the key factors in developing holographic memories is the need for a suitable recording material which must have certain specific characteristics. In particular, in order to achieve a high storage density it is necessary to work with great thicknesses. One of the essential requirements for holographic memories to be competitive is that the material must have a thickness of 500 μm or more, but it is not easy to find such thicknesses with the photopolymers currently available. In this study, we develop a method of preparing layers of a polyvinyl alcohol/acrylamide based photopolymer approximately 1 mm thick. Optimization of this material makes it possible to obtain good results for the main holographic parameters; diffraction efficiency 70% and energetic sensitivity 50 mJ/cm². PACS 42.70.Ln; 42.40.Pa; 42.40.Ht An erratum to this article can be found at .     

Numerical simulations on the focus-shift multiplexing technique for self-referential holographic data storage

For increasing the data density of holographic data storage (HDS), combining more than two multiplexing techniques is effective. This is also true in self-referential holographic data storage (SR-HDS) that enables holographic recording purely with a single beam. In this paper, a focus-shift multiplexing technique is applied to \(xy\)-shift multiplexed SR-HDS, the feasibility of which has been shown in our previous work. The focus-shift multiplexing technique enables the multiplexing of datapages by slightly changing the focal length of the objective lens. However, the required focus-shift distance for multiplexing and the implementation method of the focus-shift have not been clarified. First, the focus-shift selectivity is investigated by the numerical simulations. In the case where the focus-shift multiplexing technique is applied to \(xy\)-shift multiplexed SR-HDS, the inter-page crosstalk properties are clarified to decide the recording layout that can achieve a low-crosstalk readout. Second, the technique of displaying an additional phase pattern onto the spatial light modulator (SLM) is introduced, which is a focus-shift method without any special optical components, such as varifocal lenses. Finally, we investigate the relationship between the accuracy of the focus-shift and the parameters of SLM.     

多路角度复用体全息三维显示技术

为满足高分辨率真三维大数据显示的空间带宽积要求,提出一种基于多通道角度复用模式的体全息三维显示技术。通过对三维场景进行波前编码,获得相位计算全息图,并将计算全息图依次按照不同角度复用记录到掺杂金纳米颗粒体全息光致聚合物材料的同一区域,获得复用体全息图,再现时可以在不同角度观察三维场景。在体全息三维显示实验系统中,实现体全息材料记录区域的单点像素总数为120×1 920×1 080,显示的空间带宽积达到了2.5×108,相对于空间光调制器显示提升了120倍。     

二芳烯材料在重复擦写体全息存储中的应用研究

二芳烯材料是一种性能优异的光致色变材料,具有很好的抗疲劳性能,一般能重复擦写上万次,而不损失其性能。通过体全息光栅记录实验、角度复用体全息存储实验、重复擦写及强光记弱光读等实验,对二芳烯材料在重复擦写体全息存储中的应用进行了系统研究。由于二芳烯材料具有很高的折射率调制度(Δn=1.15×10-3)、可多次重复擦写、强光记弱光读的准非易失等性能,在可重复擦写的体全息存储中具有广阔的应用前景。     

A simple technique for holographic multiplexing

A technique for holographic multiplexing is proposed which utilizes a combination of theta modulation and carrier frequency multiplexing. Twenty two binary transparent object were multiplexed with this technique and a resolution of 10 lines/mm with good diffraction efficiency was obtained. The technique can be used for studying the sequential events at discrete time intervals.     

A wideband sensitive holographic photopolymer

A novel wideband sensitive dry holographic photopolymer sensitized by rose bengal (RB) and methylene blue (MB) is fabricated, the holographic storage characteristics of which are investigated under different exposure wavelengths. The result shows that the sensitive spectral band exceeds 200 nm in visible light range, the maximum diffraction efficiency under different exposure wavelengths is more than 40% and decreases with the decrease of exposure wavelength, the exposure sensitivity is not change with the exposure wavelength.This photopolymer is appropriate for wavelength multiplexing or multi-wavelength recording in digital holographic storage.     

Gigabyte capacities for holographic memories

The principle of beam-addressed holographic memories are outlined and existing data memories are considered in relation to future holographic memories. The next objective is to expand the capacity to the gigabyte region while maintaining read—write access times of about 100 μs. In view of the limitations imposed by conventional storage techniques, two basic approaches to fast gigabyte memories are discussed. On reasonable approach is to design modular systems comprising a number of storage modules of variable capacity. The other is to reach the gigabyte region by using multiple exposure techniques. Besides block-organized memories, bit-organized and byte-organized holographic memories with transported storage media are considered as possible alternatives to conventional memories with magnetic tapes, discs, and drums.     

Polytopic multiplexing

Polytopic multiplexing is a new method of overlapping holograms that, when combined with other multiplexing techniques, can increase the capacity of a volume holographic data storage system by more than a factor of 10. This is because the method makes possible the effective utilization of thick media. An experimental demonstration of this technique is also presented.     

记录飞秒级超快动态过程的脉冲数字全息技术

采用脉冲数字全息技术实现对飞秒级超快动态过程的数字显微全息记录.其中，全息记录系统将单脉冲分割成具有飞秒至皮秒量级时间延迟的角度相同的物光子脉冲序列和具有同样时间延迟的角度不同的参考光子脉冲序列，并以空间角分复用的方式在普通CCD的一帧图像上记录下包含多张子全息图的复合全息图，然后通过数字傅里叶变换和数字滤波的方法，分别再现出每张子全息图所记录的图像.通过对飞秒激光激发空气电离过程的全息记录，获得了具有飞秒时间分辨的等离子体形成和传播过程的动态图像. 关键词： 超短脉冲激光 飞秒全息 脉冲数字全息 空间角分复用