Holographic digital data storage using phase-modulated pixels

We propose and demonstrate the use of phase images for holographic data storage. Use of phase images as input leads to uniform diffraction efficiency of multiplexed data pages. Use of binary phase-based data pages with 0 and π phase changes produces uniform spectral distribution at the Fourier plane. This in turn facilitates better recording of higher spatial frequencies. We experimentally demonstrate a phase-based holographic data storage system using shift multiplexing in a Fe:LiNbO₃ crystal, and use it for associative retrieval. Preliminary studies indicate high discrimination capabilities of phase-based holographic data storage system over the amplitude-based system in a content-addressable memory.     

Phase modulated gray-scale data pages for digital holographic data storage

We propose a method for gray level holographic digital data storage by using three-gray levels in the phase mode. Gray level data pages are displayed on a SLM operating in the phase mode to obtain a homogenized Fourier spectrum by suppressing the extremely high intensity DC component in the Fourier spectrum of conventional amplitude based binary/gray level data pages. Holographic interferometry has been used to recover the gray level amplitude data page from phase data page. Numerical simulation results are presented for three-gray level data pages. Fourier plane homogeneity, bit-error-rate, storage density, phase modulation error of the SLM, and misalignment tolerances are investigated through computer modeling. A comparison of the present method with the amplitude-modulated gray level case with and without using a phase mask in conjunction with the data page is carried out. An experimental demonstration of the proposed three-gray level phase data page method is also presented.     

Content-addressable Holographic Digital Data Storage Based on Hybrid Ternary Modulation with a Twisted-Nematic Liquid-Crystal Spatial Light Modulator

We propose and demonstrate the use of hybrid ternary modulated digital pages for content-addressable holographic data storage. Display of binary data pages with equal number of ZEROs and ONEs by modulating both amplitude and phase of beams using twisted-nematic liquid crystal spatial light modulator, reduces strong de component and produces a more homogeneous spectral distribution at the recording plane. This technique facilitates better recording of all spatial frequencies, thus improving the discrimination capability of a content-addressable memory. Hence we get better results in associative recall in a holographic memory system, with very low number of false hits. An important advantage of the hybrid ternary modulation over pure phase data pages is that it offers a dark state for coding the undesired portion of the SLM while the search argument is small. The unique orientation of quarter wave plate and the analyzer blocks the light transmitted from OFF pixels leading to near total removal of dark signals. This in turn improves the system performance and reduces the number of false hits when the size of the search argument is small. Our experimental results show good discrimination capability and signal-to-noise ratio for a hybrid ternary modulation based content addressable memory.     

Coaxial holographic data storage without recording the dc components

A technique of recovering the data pages from Fourier holograms recorded without the dc components is demonstrated theoretically and experimentally by use of a coaxial holographic storage system. A reconstructed image is obtained by adding a phase-modulated dc component of the signal beam on reading. The bit error rate of the reconstructed image is comparable with that for the hologram recorded with the dc component as well. Since high intensities of the dc components are not recorded in this technique, the dynamic range of the recording media can be saved, which potentially contributes to increasing the number of multiplexed holograms.     

Accurate control of a liquid-crystal display to produce a homogenized Fourier transform for holographic memories

We show an accurate procedure to obtain a Fourier transform (FT) with no dc term using a commercial twisted-nematic liquid-crystal display. We focus on the application to holographic storage of binary data pages, where a drastic decrease of the dc term in the FT is highly desirable. Two different codification schemes are considered: binary pi radians phase modulation and hybrid ternary modulation. Any deviation in the values of the amplitude and phase shift generates the appearance of a strong dc term. Experimental results confirm that the calculated configurations provide a FT with no dc term, thus showing the effectiveness of the proposal.     

Optimisation of data density in Fourier holographic system using spatial filtering and sparse modulation coding

A computer model is presented that can be used to determine the image formed by a 8-f Fourier imaging system. With the model we optimised data density in a thin film Fourier holographic arrangement. Results of the simulations were experimentally verified in the holographic memory card system. We optimised the capacity in function of sparsity and Fourier filtering close to the Nyquist aperture. We achieved a data density of 1.1-bit/μm². Our results suggest that the sparse pages combined with spatially filtered Fourier spectrum lead to better performance compared to balanced codes.     

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.     

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.     

Randomly displaced phase distribution design for computer-generated binary hologram with narrow recording spots

A random phase mask is often used for Fourier transform holography to improve its recording and reconstruction characteristics. However, a conventional random phase pattern has the disadvantage of expanding the recording spot, which is caused by the high-resolution phase modulation. Previously, we proposed a random phase pattern that makes the recording spot narrower than the conventional one with maintaining a moderate reconstruction quality. In the present study, we apply the proposed random phase pattern to computer-generated binary holograms and evaluate both the hologram distribution and reconstruction quality in terms of practical holographic memory systems. The results confirm the effectiveness of the random phase in the reconstruction for an elementary data pattern.     

菌紫质高密度偏振全息光数据存储实验研究

实验研究了基因改性菌紫质BR_D96N薄膜在不同偏振光记录下的全息存储特性，比较了不同 偏振态记录光和读出光对衍射像光强及信噪比的影响. 实验结果表明，与其他偏振全息记录 相比，正交圆偏振光记录可实现衍射光偏振状态与散射噪声偏振状态的分离，得到高信噪比 的衍射像，同时还具有高的衍射效率. 以 He_Ne 激光器（633nm，3mW）为记录和读出光源 ，用空间光调制器作为数据输入元件，CCD作为数据读出器件，采用傅里叶变换全息记录的 方法，在 BR_D96N 薄膜样品60μm×42μm的面积上进行了正交圆偏振全息数据存储，达到 了2×10⁸bit/cm²的存储面密度，并实现了编码数据的无误读出与 还原. 关键词： 菌紫质 偏振全息 光致变色 光致各向异性 高密度光存储     

体全息存储中二维交错编码技术的研究及实现

为了克服和纠正体全息存储系统中大量的随机错误和突发错误,数据在按"页面"为单位存入系统时,通常采用纠错编码、交错和调制相结合的编码方式来进行信息组页.文中提出了一种二维交错技术用于克服数据"页面"存在的突发错误,以提高纠错编码的纠错能力,并通过FPGA实现了块交错RS(255,223)编码码字在数据"页面"中的均匀分布.     

Selective phase masking to reduce material saturation in holographic data storage systems

Emerging networks and applications require enormous data storage. Holographic techniques promise high-capacity storage, given resolution of a few remaining technical issues. In this paper, we propose a technique to overcome one such issue: mitigation of large magnitude peaks in the stored image that cause material saturation resulting in readout errors. We consider the use of ternary data symbols, with modulation in amplitude and phase, and use a phase mask during the encoding stage to reduce the probability of large peaks arising in the stored Fourier domain image. An appropriate mask is selected from a predefined set of pseudo-random masks by computing the Fourier transform of the raw data array as well as the data array multiplied by each mask. The data array or masked array with the lowest Fourier domain peak values is recorded. On readout, the recorded array is multiplied by the mask used during recording to recover the original data array. Simulations are presented that demonstrate the benefit of this approach, and provide insight into the appropriate number of phase masks to use in high capacity holographic data storage systems.     

Hybrid multinary modulation using a phase modulating spatial light modulator and a low-pass spatial filter

We propose a method for performing binary intensity and continuous phase modulation of beams with a spatial light modulator (SLM) and a low-pass spatial filtering 4-f system. With our method it is possible to avoid the use of phase masks in holographic data storage systems or to enhance the phase encoding of the SLM by making it capable of binary amplitude modulation. The data storage capabilities and the limitations of the method are studied.     

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.     

Recording media that exhibit high dynamic range for digital holographic data storage

A general strategy for fabricating thick, optically flat photopolymer recording media with high dynamic range (M/#) that exhibit low levels of recording-induced Bragg detuning for holographic data storage is presented. In particular, media with M/# values as high as 42 in 1-mm-thick formats are obtained. We believe that these results are the first demonstration of a holographic storage medium with a dynamic range of this magnitude. In addition, we report the holographic recording and recovery of high-capacity (480-kbit) digital data pages in these media, further illustrating their data-storage capabilities.     

A review of aspects relating to the improvement of holographic memory technology

Results of studying a holographic memory to write/read digital data pages are presented. The research has been carried out in Novosibirsk, Russia. Great attention was paid to methods of improving recording density and the reliability of data reading, the development of ‘dry’ photopolymers that provide recording of superimposed three-dimensional phase holograms, and the designing of parallel optic input large-scale integration (LSI) for reading and logical processing of data arriving from the holographic memory.     

Non-interferometric phase retrieval for collinear phase-modulated holographic data storage

Optical Review - Because of its simple optical system setup and robust noise tolerance, non-interferometric phase retrieval is an important technique for phase-modulated holographic data storage....     

Optical noise reduction for dc-removed coaxial holographic data storage

A method of reconstructing positive and negative images from Fourier holograms recorded without the dc components is demonstrated by use of a coaxial holographic storage system. Reconstructed images are obtained by adding a phase-modulated dc component of the signal beam on reading. Contrast reversal of the reconstructed images can be achieved by reversing the readout reference pattern. This method can realize not only optical noise reduction but also less consumption of the dynamic range of the recording medium, potentially contributing to increasing the number of multiplexed holograms.     

Hologram multiplexing in a highly photosensitive photopolymerizable material in a sol-gel matrix

In this paper, we present a photopolymerizable material in a sol-gel matrix suitable for recording a large number of multiplexed diffraction gratings with low total exposure energy. The matrix of this material is an inorganic porous silica glass, a material that does not shrink when radical photopolymerization is initiated. The photosensitive component is based on acrylamide, N,N′-methylenebisacrylamide, triethanolamine and yellow eosine as a photoinitiator. In the reported study, 50 holograms were angularly multiplexed with diffraction efficiencies between 0.1 and 1.9% resulting in a dynamic range of M/# = 3.9. The total exposure was 4.5 mJ/cm² and the energy used in recording each hologram was 95 μJ/cm². This indicates a very high sensitivity for this material in the range of 3.3 to 15.5 cm/mJ. Due to this good holographic performance, the material is suitable for holographic data storage applications.     

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.