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.     

全息光盘存储器的数据存取与像素匹配

全息光盘存储(HDS)技术作为一种非常具有潜力的新型信息存储技术, 是下一代光盘发展的目标。在全息光盘存储系统中, 为了实现数据精确快速地写入和读出, 降低误码率, 需要在整个高分辨率页面上实现空间光调制器(SLM)与光电阵列耦合器(CCD)之间1∶1像素匹配。在体全息存储理论和光学设计理论基础上, 研究实现SLM和CCD像素1∶1匹配的方法, 提出了实现像素匹配的要求和条件, 并且按照此要求完成该全息光盘存储器的光学系统。实验中, 分别使用随机数据掩膜版和SLM实现了对CCD的512×512精确像素匹配, 在光学系统中引入存储介质条件下, 图像质量良好, 掩膜版和SLM原始误码率分别为2.5×10-4和1.5×10-4。     

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.     

Holographic data storage with arbitrarily misaligned data pages

An improved postprocessing algorithm that can compensate for arbitrary misregistrations between a detector array and the coherent image of a pixelated two-dimensional data page is described. Previously [Opt. Lett. 26, 542 (2001)], an algorithm was reported in which both linear and quadratic interpixel cross-talk contributions are reallocated to the appropriate neighboring pixels. However, page misalignments close to +/-0.5 pixels could not be corrected to an acceptable bit-error rate because of propagation in the iterative procedure. An improved algorithm is reported in which an intentional magnification error is introduced optically and then corrected during postprocessing. Experimental results from a pixel-matched megapel volume holographic system are presented, showing that the dependence of bit-error rate on transverse detector alignment is entirely removed. This improved procedure can completely bypass constraints on page registration, optical distortion, and material shrinkage that currently hamper page-oriented holographic storage systems.     

Noise reduction of page-oriented data storage by inverse filtering during recording

Two novel techniques for eliminating deterministic noise from a page-oriented memory are presented. The first technique equalizes the output response of ON pixels by adjustment of the exposure of each pixel during the recording of each data page. A test image transmitted through the system measures the spatial nonuniformities, and the appropriate inverse filter is imposed upon the data page and recorded in the storage material. On readout, the output signal values are then spatially uniform, perturbed only by random noise sources. Experimental results of using this predistortion technique in a pixel-matched holographic storage system are shown. Under conditions of high volumetric density, raw bit-error-rate (BER) improvements of 6-8 orders of magnitude are obtained (from 10(-4) to <10(-10)). The second technique uses a phase shift during holographic storage to subtract from bright OFF pixels. Under conditions of low spatial light modulator contrast, BER improvements of 6 orders of magnitude (from 10(-2) to 10(-8)) are demonstrated.     

Compensation for misaligned data pages based on a three-pixel model in holographic data storage

To compensate misalignments between a detector array and a spatial light modulator (SLM) in page-oriented holographic data storage, a method based on a three-pixel model is proposed. Three neighboring pixels in one-dimensional direction on the SLM that make contributions to the center pixel on the detector are considered in the three-pixel model. Recursive solutions are carried out to recover the true values of the SLM pixels. Both simulation and experimental results show that the signal-to-noise ratio (SNR) can be doubled approximately by use of the compensation method based on the three-pixel model. The proposed method is appropriate for relatively small apertures, and has the similar effects with the equalization method, which effectively improves the SNR.     

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.     

Compensation for pixel misregistration in volume holographic data storage

We describe what we believe to be a novel postprocessing algorithm for compensating for misregistrations between a detector array and the coherent image of a pixelated two-dimensional data page. A lookup table of baseline local offsets is combined with the dynamically measured global offset of the received data page, producing an estimate of the total lateral shift of each small block of pixels. A serial algorithm then reallocates the appropriate portion of the signal detected by each pixel to its neighbors, accounting for both the linear and the quadratic contributions introduced by coherent illumination of square-law detectors. This procedure can relax the tight constraints on page registration, optical distortion, and material shrinkage that currently hamper page-oriented holographic storage systems. Experimental results from a pixel-matched 1-Mpixel volume holographic system are presented, showing an increase in position tolerance (for a raw bit-error rate <10(-3)) from +/-16% to +/-40% of the pixel pitch.     

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.     

体全息存储中SLM和CCD的性能对页内噪声的影响

体全息存储系统中通常采用SLM和CCD作为输入输出器件。SLM和CCD的性能包括填充因子、对比度和像素形状 ,这些性能会对像面页内像素间干涉程度产生很大的影响 ,进而影响页内噪声的大小。针对这一问题 ,首先进行了理论分析 ,然后通过计算机模拟了实际系统     

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.     

Quantitative characterization of holographic displays based on liquid-crystal-on-silicon spatial light modulator

This report discusses the effect of speckle size on the quality of holographic images based on a liquid-crystal-on-silicon (LCoS) spatial light modulator (SLM). Further, it proposes methods of quantifying the average speckle size and holographic image resolution. These methods enable both characteristics to be compared using the same unit (the number of pixels in the holographic image), providing an intuitive and effective comparative analysis method. In particular, by varying the LCoS resolution ratio, the change in the resolvable minimum pixels of the holographic image is interpreted in conjunction with the average speckle size; moreover, an analysis of the correlation between the latter two is presented. This approach, based on LCoS resolution division, could provide useful insights into single-SLM-based, full-color holographic displays using space division. Furthermore, it could be extended to other components, including more advanced LCoS SLMs, and used to identify the relative effects on image quality with speckles.     

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.     

体全息存储系统图像质量因素MTF数值模拟分析

影响体全息存储系统中读出图像误码率的因素很多,其中输入与输出器件SLM、CCD的性能以及它们信号匹配关系起着关键的作用。针对这一实际问题,利用CCD成像系统的调制传递函数理论和数值模拟方法,通过对4f系统中周期性SLM像素分布的CCD输出响应分析,研究了SLM和CCD的填充因子、对比度、位相匹配关系对读出图像误码率的影响,并给出计算机仿真结果。     

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.     

Unequally spaced four levels phase encoding in holographic data storage

Holographic data storage system is a candidate for the information recording due to its large storage capacity and high transfer rate. We propose an unequally spaced four levels phase encoding in the holographic data storage system here. Compared with two levels or three levels phase encoding, four levels phase encoding effectively improves the code rate. While more phase levels can further improve code rate, it also puts higher demand for the camera to differentiate the resulting smaller grayscale difference. Unequally spaced quaternary level phases eliminates the ambiguity of pixels with same phase difference relative to reference light compared to equally spaced quaternary levels. Corresponding encoding pattern design with phase pairs as the data element and decoding method were developed. Our encoding improves the code rate up to 0.875, which is 1.75 times of the conventional amplitude method with an error rate of 0.13 % according to our simulation results.     

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.     

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     

Spatiotemporal multiplexing and streaming of hologram data for full-color holographic video display

We propose a new approach to spatiotemporally multiplex sub-holograms and stream hologram data in parallel by using multiple spatial light modulators (SLMs) to increase the spatiotemporal resolution of holographic display system. With the proposed approach, we have achieved a spatiotemporal resolution of 4.5 × 10¹⁰ pixel/s, as compared with 1.89 × 10⁹ pixel/s of a single SLM. Each frame of computer-generated hologram video has a pixel count of 378 Mpixels and is divided into 288 sub-holograms, each of 1.31 Mpixels. During the playback of holographic video, these sub-holograms are spatiotemporally multiplexed and streamed in parallel to form the integrated hologram within 16.67 ms, which enables full-color holographic video display with a 10-in. diagonal at a hologram data frame rate of 60 frames per second (fps). New SLM devices with higher spatiotemporal resolution need to be developed in order to meet the data bandwidth requirement of about 10¹²–10¹⁴ pixel/s for future 3D holographic displays.     

Correction to: Generation of amplitude- and phase-modulated signal beam with a phase-only spatial light modulator and its detection by the transport of intensity equation method for holographic data storage

This study evaluates a novel holographic data storage (HDS) that uses a phase-only spatial light modulator (SLM) for the multilevel complex amplitude modulation of a signal beam and the transport of intensity equation (TIE) method to detect the signal beam without interferometry, to increase the capacity of the HDS, simplifying its optical system, and improving the stability of the signal beam modulation and detection. Both the amplitude and phase of the signal beam were modulated by a computer-generated hologram displayed in a phase-only SLM, a 4-f optical system, and a pinhole placed in the Fourier plane. The complex amplitude-modulated signal beam generated by this scheme does not always perfectly match the target complex amplitude, and deviations from the amplitude and phase of the target complex amplitude may exist. It is unclear whether the TIE method, which is sensitive to the state of the beam intensity and the phase distributions to be detected (such as zero-intensity points and phase discontinuities), can accurately detect a signal beam whose complex amplitude is modulated by the modulation scheme with a phase-only SLM. Here, we demonstrate via numerical simulations and experiments that several methods of complex amplitude generation using a phase-only SLM can achieve multilevel modulation of the amplitude and phase of a signal beam and are suitable for detection by the TIE method in HDS.