Unitary matrices for phase-coded holographic memories

We propose a novel type of unitary matrix for phase-code multiplexed holographic memories, which could be quickly generated from geometric sequences. Our analysis shows that the phase-code matrices are unitary rather than orthogonal. The new matrices have complex elements. The order of unitary matrices can be any positive integer, so that we can accommodate the available spatial light modulators to obtain the maximum possible storage capacity. The cross-talk noises in phase-encoded memories with unitary matrices and with Hadamard matrices are of the same order of magnitude, which are much lower than those in holographic memories with wavelength multiplexing or angle multiplexing.     

Multilevel phase and amplitude modulation method for holographic memories with programmable phase modulator

The utilization of spatial quadrature amplitude modulation (SQAM) signals with amplitude and phase modulation is a simple method used to improve storage capacity in a holographic data storage system. We propose a multilevel phase and amplitude modulation method for holographic memories with a programmable phase modulator (PPM). In this method, holographic page data is recorded by a two-step exposure process for different phase-modulated data. There is no need to adjust the positions of spatial light modulators (SLM) with high accuracy because we use only one spatial modulator. We estimate the quality of 16 SQAM signals produced by our technique.     

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 .     

Transmission type twisted nematic liquid crystal display for three gray-level phase-modulated holographic data storage systems

Gray-level data pages enhance the storage capacity of holographic data storage systems. A balanced three gray-scale data page in phase mode produces a homogenized Fourier spectrum which is regarded to be necessary for suitable exploitation of the holographic recording medium. A commercially available transmission type twisted nematic liquid crystal display has been characterized for use as a phase spatial light modulator with phase modulation in the range 0–3π/2 at 532 nm wavelength, such that it may be used for holographic data storage with binary as well as three gray-level phase-modulated data pages. Experimental results of the phase and amplitude modulation characteristics of the SLM, Fourier plane homogeneity, and recording of phase-modulated binary, three gray-level data pages and their reconstruction with a real-time holographic interferometric method are presented.     

高密度光学全息存储技术的新进展：向光盘存储挑战

光全息存储的独特优点，尤其是体全息存储的高容量和高数据传输速率，使光全息存储最有希望成为下一代海量存储技术。通过与现有的光盘和磁盘存储技术的对比，介绍了全息存储的原理和材料，以及在这一活跃的研究领域中的新进展，特别是为了提高存储容量和抑制串象噪音所发展起来的各种全息复用技术。对全息存储技术取得实用性进展的前景也作了简略讨论。     

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     

New technique to record and playback holographic color video memories

A low-cost and simple color-encoding technique for holographic video memories is proposed and designated as a “stripe-coded” method. The playback system requires a laser of a single wavelength, a black-and-white vidicon installed TV camera and a common color TV monitor. One frame can be stored in a hologram of 4-mm diameter, and the storage density is several times higher than that of the conventional TV films. Color images of satisfactory hue and saturation were successfully restored.     

Optimization of a holographic memory setup using an LCD and a PVA-based photopolymer

Holographic data pages were stored in a polyvinyl alcohol (PVA)/acrylamide (AA) photopolymer. This material is formed of AA photopolymers which are considered interesting materials for recording holographic memories. A liquid crystal device was used to modify the object beam and store the data pages in the material. During the storage process, some parameters like exposure time, beam ratio and reading beam intensity were controlled to obtain high image quality after the reconstruction process. The bit error rate (BER) was calculated fitting the histograms of the images to determine what parameters improve the quality of the images.     

Fast processing method to generate gigabyte computer generated holography for three-dimensional dynamic holographic display

Two different methods from graphic processing unit(GPU) and central processing unit(CPU) are proposed to suitably optimize look-up table algorithms of computer generated holography(CGH). The numerical simulations and experimental results show that we can reconstruct a good quality object. The computation of CGH for a three-dimensional(3D) dynamic holographic display can also be sped up by programming with our proposed method. It can optimize both file loading and the inline calculation process. The phase-only CGH with gigabyte data for reconstructing 10 MB object samplings is generated. In addition, the proposed method effectively reduced time costs of loading and writing offline tables on a CPU. It is believed the proposed method can provide high speed and huge data CGH for 3D dynamic holographic displays in the near future.     

Impulse response and shannon number of holographic optical systems

The impulse response of a holographic optical system for arbitrary geometry and with consideration of the film O.T.F. was obtained in an explicit form. The result is more general in comparison to an analoguous one given earlier by Kozma and Zelenka. Starting from that operational form of the impulse response, the holographic system was compared with a conventional optical system and the Shannon number of the general holographic system was found. It was also proved, that Kelly's model in the instantaneous-frequency approximation implies a product form of the effective pupil function of the system     

New photopolymerizable holographic recording material based on polyvinylalcohol and 2-hydroxiethylmethacrylate (HEMA)

We have developed a new polyvinylalcohol-based photopolymeric holographic recording material. The recording is obtained by the copolymerization of acrylamide and 2-hydroxiethylmethacrylate (HEMA). Diffraction efficiencies near 70% are obtained with exposures of ≈65 mJ/cm² in materials of 110-μm thickness. The addition of HEMA increases the thickness of the film, which results in improved storage capacity for holographic optical storage. Received: 7 March 2002 / Published online: 2 May 2002     

体全息存储技术

体全息存储技术以其存储密度和数据读出率高及相关内容寻址等特点而具有广阔的应用前景。但体全息存储技术要同其它已经成熟的技术竞争 ,以在存储市场上占有一席之地 ,就必须注重发挥自己的优势。另外体全息存储技术要实现商品化 ,还有两个重要的问题需要解决。一是以较低的价格实现激光、空间光调制器和探测器阵列的对准。其次是要寻找合适的存储材料。对体全息存储来说寻找合适的存储材料仍是一个尚待解决的问题 ,至今还没有一种材料具有性能、容量和价格的综合优势     

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.     

Hoe/lensless matched spatial filter wavelength-scaling correlator

A scaling correlator optical pattern recognition system is described in which a lensless matched spatial filter (with the second Fourier transform lens and the matched spatial filter recorded on the same plate) is used with a first Fourier transform lens that is also an holographic optical element. The matched spatial filter is recorded at one wavelength and correlation is obtained at a second wavelength. Experimental demonstration and output correlation SNR data are reported, together with a comparison of the system's noise level using conventional optics and holographic elements.     

Gray-scale data pages for digital holographic data storage

The prospects for gray-scale (or multilevel) digital holographic data storage are theoretically and experimentally investigated. A simple signal-to-noise ratio (SNR) partitioning argument shows that when SNR scales as 1 over the number of holograms squared, five gray levels (log(2) 5bits/pixel) would be expected to result in a 15% capacity increase over binary data pages. However, the additional signal-dependent noise sources present in practical systems create a baseline SNR that reduces both the optimal number of gray levels and the resulting gain in capacity. To implement gray-scale recording experimentally, we adapt the predistortion technique previously developed for binary page-oriented memories [Opt. Lett. 23, 289 (1998)]. Several new block-based modulation codes for decoding gray-scale data pages are introduced. User capacity is evaluated by an experimental technique using LiNbO(3) :Fe in the 90 degrees geometry. Experimental results show that a balanced modulation code with three gray levels provides a 30% increase in capacity (as well as a 30% increase in readout rate) over local binary thresholding.     

双色相干场高次谐波

通过数值求解一维含时薛定谔方程，研究了激光频率对谐波辐射的影响以及双色相干场（基频光与其二倍频光或三倍频光）作用于原子时的谐波辐射行为。结果表明，在其它条件相同时，用低频入射光可获得更高次谐波，但转换效率较低，高频入射光转换效率较高，但只能获得较低次谐波，基频光及其三倍频光同时作用于原子，谐波次数和转换效率都有所提高。基频光及其二倍频光同时作用于原子，不但谐波次数增多，转换效率提高，重要是的除了通     

Deterministic Phase Encoded Holographic Data Storage Using Lenticular Lens Array

This work presents a novel optical holographic encrypted data storage approach based on a phase encoding multiplexed scheme. In the proposed data storage scheme, patterns to be encrypted are stored holographically in a photorefractive LiNbO₃:Fe crystal using a lenticular lens array (LLA) sheet phase-encoded multiplexing. Experimental results reveal that rotating an LLA placed as a phase modulator in the path of the reference beam is a simple but effective method of increasing the phase addresses for holographic memory in a crystal. Combining this rotational multiplexing with two-axis rotating multiplexing provides further data storage and data encryption capacity.     

Compact, integrated dynamic holographic memory with refreshed holograms

An innovative architecture for compact, integrated volume holographic memories is described. It is based on phase-conjugate readout and on a modulator-detector-memory array implemented in a silicon integrated circuit. The lensless memory module sustains dynamic read-write holograms by periodic refreshing. The integrated circuit is described and experimentally characterized. Holograms were stored in a prototype storage module that uses a 30 degrees -cut BaTiO(3) crystal and the 90 degrees recording geometry. As many as three angularly multiplexed holograms were periodically refreshed and subjected to >40% decay from exposure to the reference beam over 50 to 100 cycles. Experimental data are presented.     

Tomosynthesis: A holographic method for variable depth display

A circular symmetric tomosynthetic arrangement has been investigated using holographic storage techniques. With a set of 24 single radiographs a 3D image of a human orbita region has been generated by image integration leading to a continuously variable depth display of arbitrary layers. The received resolution in a particular layer was about 3 1p/mm. An estimate will be given for the number of necessary radiographs to compare the blurr quality of discrete sample procedures in tomosynthesis with a continuous motion in tomography. Finally a short outline will be given for a possible real-time procedure.     

Optical memories implemented with photorefractive media

We consider holographic optical data storage systems implemented with photorefractive media. Our viewpoint emphasizes the close interaction between materials and device issues. First we discuss our current understanding of photorefractive physics as it pertains to the holographic data storage problem. Then we consider architecture issues, including angular, phase-encoded, and wavelength-multiplexing techniques, and several approaches to increasing the signal-to-noise ratio of the recordings. Finally, we discuss materials issues related to crystal growth and how crystal quality impacts the performance of data storage systems. Both bulk and fibre crystal growth techniques are reviewed.