Lensless complex amplitude demodulation based on deep learning in holographic data storage

To increase the storage capacity in holographic data storage(HDS),the information to be stored is encoded into a complex amplitude.Fast and accurate retrieval of amplitude and phase from the reconstructed beam is necessary during data readout in HDS.In this study,we proposed a complex amplitude demodulation method based on deep learning from a single-shot diffraction intensity image and verified it by a non-interferometric lensless experiment demodulating four-level amplitude and four-level phase.By analyzing the correlation between the diffraction intensity features and the amplitude and phase encoding data pages,the inverse problem was decomposed into two backward operators denoted by two convolutional neural networks(CNNs)to demodulate amplitude and phase respectively.The experimental system is simple,stable,and robust,and it only needs a single diffraction image to realize the direct demodulation of both amplitude and phase.To our investigation,this is the first time in HDS that multilevel complex amplitude demodulation is achieved experimentally from one diffraction intensity image without iterations.

Growth and Holographic Storage Properties of Sc, Fe Co-Doped Lithium Niobate Crystals

The holographic storage properties of Fe （0.03% （mass fraction） Fe2O3）：LiNbO3 doped with Sc at different levels （0, 1%, 2%, 3%） were investigated. The Sc threshold concentration in Fe：LiNbO3 was implied to be about 3% （mole fraction） because O-H vibration absorption peak of Sc （3%）：Fe：LiNbO3 was at 3508 cm^-1, compared with 3484 cm^-1 of crystals with lower Sc doping level. Sc（3%）：Fe：LiNbO3 exhibited higher optical damage resistance ability. The threshold intensity （wavelength 488 nm） of Sc （3%）：Fe：LiNbO3 was 2.2 ×10^2 W ·cm^-2, two orders of masnitude higher than that of Fe：LiNbO3. Holographic storage properties of the crystals were determined in an extraordinary polarized laser of wavelength 632.8 nm by a two-wave coupling method. It was found that in terms of holographic storage properties, the optimal doping concentration of Sc was 2% （mole fraction） among this crystal series.

Holographic storage of three-dimensional image and data using photopolymer and polymer dispersed liquid crystal films

We present holographic storage of three-dimensional（3D） images and data in a photopolymer film without any applied electric field.Its absorption and diffraction efficiency are measured,and reflective analog hologram of real object and image of digital information are recorded in the films.The photopolymer is compared with polymer dispersed liquid crystals as holographic materials.Besides holographic diffraction efficiency of the former is little lower than that of the latter,this work demonstrates that the photopolymer is more suitable for analog hologram and big data permanent storage because of its high definition and no need of high voltage electric field.Therefore,our study proposes a potential holographic storage material to apply in large size static 3D holographic displays,including analog hologram displays,digital hologram prints,and holographic disks.

Collinear holographic data storage technologies

In the era of information explosion,the demand of data storage is increased dramatically.Holographic data storage technology is one of the most promising next-generation data storage technologies due to its high storage density,fast data transfer rate,long data life time and less energy consumption.Collinear holographic data storage technology is the typical solution of the holographic data storage technology which owns a more compact,compatible and practical system.This paper gives a brief review of holographic data storage,introduces collinear holographic data storage technology and discusses phase modulation technology being used in the holographic data storage system to achieve higher storage density and higher data transfer rate.

Holographic Storage Properties of In:Fe:Mn:LiNbO_3 Crystals

In：Fe：Mn：LiNbO3（LN） crystals were grown in air atmosphere by Czochralski method with different concentration of In （0, 1, 2, 3 mol%） in the melts, while the contents of Fe2O3 and MnO were 0.1 and 0.5 mol%, respectively. The location of doping ions was analyzed by Ultravioletvisible absorption spectra and differential thermal analysis. The diffraction efficiency （η）, writing time （τw） and erasure time （τe） of the crystals were measured by two-beam coupling experiment. The dynamic range and photorefractive sensitivity have also been calculated. The results showed that with the increase of In ions in the melt, the absorption edge of In：Fe：Mn：LN crystal shifts to the violet firstly and then makes the Einstein shift, the Curie temperature of crystal increases firstly and then decreases, the storage ratio speeds up, diffraction efficiency decreases, and dynamic range and photorefractive sensitivity increase. The mechanism of holographic storage properties of In：Fe： Mn：LN crystal with different doping concentration of In^3＋ was investigated, suggesting the In： Fe：Mn：LN crystals are excellent holographic storage materiel with better synthetical properties than Fe：Mn：LN crystals.

Growth and Holographic Storage Properties of Fe∶LiTaO_3 Crystal

Using Si Mo bar as heater and doping Fe 2O 3 in LiTaO 3, Fe∶LiTaO 3 crystal was grown by Czochralski method. The curie temperature of the crystal was measured by thermal analyze method. The lattice constants of the crystal were calculated by measuring of X ray spectra. The response time, exponential gain coefficient and diffraction efficiency of Fe∶LiTaO 3 were obtained by two wave coupled technique. Compared with Fe∶LiNbO 3, the response time of Fe∶LiTaO 3 is six times shorter, the storage time of Fe∶LiTaO 3 is ten times longer and the photo scattering resistance ability of Fe∶LiTaO 3 is four times higher. Fe∶LiTaO 3 is an excellent holographic storage material.

Readout Properties of Polychromatic Reconstruction for Nondestructive Holographic Data Storage

Polychromatic reconstruction(PCR) is a novel nondestructive readout method that utilizes a spectrally broad light source for the probe beam. The stored image can be completely reconstructed even though the probe wavelength is very different from the recording one. The large spectral width of the polychromatic probe beam also causes adverse effect on the storage density. But this can be overcomed if an additional optical component is inserted in the imaging system. Therefore, PCR has a great potential to achieve nondestructive readout and large storage density simultaneously. In addition, PCR enables us to design a simple memory system because its readout tolerance is quite large as compared to the conventional monochromatic readout. Unique and attractive features of the polychromatic reconstruction method was theoretically and experimentally demonstrated.

Modified Holographic Exposure to Fabricate Varied Bragg Grating in an Identical Chip

A new fabricating method is demonstrated to realize two different Bragg gratings in an identical chip using traditional holographic exposure. Polyimide is used to protect one Bragg grating during the first period. The technical process of this method is as simple as that of standard holographic exposure.

A new photopolymerizable holographic recording material based on acrylamide and N-hydroxymethyl acrylamide

A new polyvinylalcohol-based photopolymeric holographic recording material has been developed. The recording is obtained by the copolymerization of acrylamide and N-hydroxymethyl acrylamide. Diffraction efficiencies near 50% are obtained with energetic exposure of 80mJ/cm^2. N-hydroxymethyl acrylamide can improve the optical quality of the film. With the increase of the concentration of N-hydroxymethyl acrylamide, the flatness of the film increases, scattering reduces and the straight image is clearer with a small distortion. The postexposure effect on the grating is also studied. The diffraction efficiency of grating increases further during postexposure, gradient of monomer exists after exposure.

POLYMER SCAFFOLDS BEARING AZOBENZENE-POTENTIAL FOR OPTICAL INFORMATION STORAGE

The fundamental optical storage mechanism of the laser light eddressable azobenzene moiety is briefly introduced.A modular and flexible synthesis design furnishes polyester matrices covalently integrating cyanoazobenzene in regularlyspaced side chains. Thin films of these materials are particularly well suited for holographic storape. Notable figures of meritsof liquid crystalline polyesters are response time to blue-green laser light of the order of nanoseconds, storage capacityexpressed as 5000 lines/mm, and high, permanent (almost nine years) diffraction efficiency of the order of 50% or greater,and erasability, The implications of the main chain nature for polyester morphology and for the permanency of the inducedanisotropy are discussed, The design and methods of preparation of other significantly different polymer scaffolds supportingcyanoazobenzene are elaborated. Oligopeptides always result in amorphous materials, whereas copolymethacrylates anddendritic or hyperbranched polyesters provide some materials that exhibit liquid crystallinity. However, none of these scaffolds affords materials that result in permanent anisotropy when exposed to interfering laser light.

富勒烯掺杂的光致聚合物的全息存储特性研究

研究了富勒烯对六官能团脂肪族聚氨酯丙烯酸酯/环氧树脂(RJ423/EPIKOTE 828)体系光致聚合物的全息性能的影响,分析了富勒烯(C_(60))材料以及曝光光强对光致聚合物衍射效率的影响。通过吸收光谱结合X射线衍射图谱分析,掺杂的富勒烯既没有和聚合物中的其他成分发生化学反应,也不影响材料本身的结构和结晶性能。实验结果表明,富勒烯掺杂提高了单体聚合反应的速率,并且参与活性单体分子之间的扩散。在光强20 mW/cm^(2)、时间40~50 s、厚度200µm时,光致聚合物的衍射效率提升到86%,感光灵敏度达到1.32 cm^(2)/J,收缩率降低了约80%。由于C_(60)促进聚合,抑制体积收缩,从而增强全息存储的稳定性,经过全息图像存储实验对比,证明该光致聚合物掺杂富勒烯之后具有优秀的全息存储性能,同时也表明富勒烯掺杂的光致聚合物在全息储存领域具有较大的应用前景。

PQ/PMMA聚合物应用于全息光存储的皱缩特性及影响研究

基于全息光栅的角度优化工艺提升光存储性能的方法,相比引入新型组分改善存储材料,更简单且具有一定适用性。采用了热致聚合法制备菲醌掺杂的聚甲基丙烯酸甲酯(PQ/PMMA)光致聚合物,研究其在全息存储系统中材料厚度、记录光强及干涉夹角下的衍射性能及对应的体积皱缩率。结果表明,材料厚度与衍射性能线性相关,最佳记录光强也随厚度增加而增加,PQ/PMMA响应的最大空间频率约1285 lp/mm。通过布拉格曲线偏移推算发现,全息光栅体积皱缩表现为厚度、记录光强与皱缩量的负相关,与空间频率呈正相关。

计算成像在全息存储相位恢复中的应用研究进展

全息存储技术作为一种三维体存储、二维面数据传输的数据存储技术,具有存储密度高、数据传输快等特点,是解决海量数据长期存储的有力方案之一。传统全息存储方法受到光电探测器只对强度响应的限制,通常采用纯振幅编码进行调制,但仅利用振幅信息无法完全发挥全息技术本身优势,如何简单快速、稳定精确地解码相位信息是全息存储技术面临的现实问题。计算成像因其算法多变、高感知维度等特点为全息存储技术的相位恢复问题提供了新的思路。本文主要从迭代计算相位恢复和深度学习相位重建角度回顾近年来利用计算成像技术解决全息存储相位恢复问题的一些工作,从存储密度提升、数据读取速度提升以及数据读取稳定性等角度对工作进行了分析,并对该方向未来发展做出展望。

全息存储中的纠错码研究综述

大数据时代对于高密度大容量的存储技术需求与日俱增。与传统存储技术的按位记录方式不同,全息存储以二维数据页为读写单位,采用三维体存储模式,凭借存储密度高、数据转换速率快、节能安全以及超长期保存等优势,成为海量冷数据存储的有力竞争者。重点介绍了相位调制的同轴全息存储,分析了目前面向全息存储的纠错码研究现状,并详细介绍了一种参考光辅助的低密度奇偶校验LDPC码优化方案。

High density collinear holographic data storage system

Holographic data storage system （HDSS） has been a good candidate for a volumetric recording technology, due to their large storage capacities and high transfer rates, and have been researched for tens of years after the principle of holography was first proposed. However, these systems, called conventional 2-axis holography, still have essential issues for commercialization of products. Collinear HDSS, in which the information and reference beams are modulated co-axially by the same spatial light modulator （SLM）, as a new read/write method for HDSS are very promising. With this unique configuration, the optical pickup can be designed as small as DVDs, and can be placed on one side of the recording media （disc）. In the disc structure, the preformatted reflective layer is used for the focus/tracking servo and reading address information, and a dichroic mirror layer is used for detecting holographic recording information without interfering with the preformatted information. A 2-dimensional digital page data format is used and the shift-multiplexing method is employed to increase recording density. As servo technologies are being introduced to control the objective lens to be maintained precisely to the disc in the recording and reconstructing process, a vibration isolator is no longer necessary. Collinear holography can produce a small, practical HDSS more easily than conventional 2-axis holography. In this paper, we introduced the principle of the collinear holography and its media structure of disc. Some results of experimental and theoretical studies suggest that it is a very effective method. We also discussed some methods to increase the recording density and data transfer rates of collinear holography.

Bleaching effect and nonvolatile holographic storage in doubly doped LiNbO_3:Fe:Cu crystals

The bleaching effect, i.e. the crystal shows that decoloration after it is illuminated by ultraviolet light, has been observed in congruent LiNbO3:Fe:Cu crystals. Based on this bleaching effect, a new technique including the recording phase by two interfering red beams and fixing phase by both UV light and a coherent red beam has been experimentally investigated to realize nonvolatile holographic storage in LiNbO3:Fe:Cu. The results of proof-of-concept experiments confirm that bleaching effect becomes an alternative physical mechanism for nonvolatile holographic storage with high recording sensitivity and weak light-induced scattering noise.

A two-dimensional constant-weight sparse modulation code for volume holographic data storage

In order to simplify the threshold determination,reduce the inter-pixel cross-talk,and improve the storage density for high-density volume holographic data storage,a two-dimensional constant-weight sparse modulation code is proposed.The evaluation criteria and design rules are investigated based on the page-oriented optical data storage system.Coding parameters are optimized to achieve large channel capacities.An 8:16 modulation code is designed to reduce the raw bit error rate and its performances are experimentally evaluated.A raw bit error rate of the magnitude of 10 4 is obtained with a single-data-page storage and 10 3 with multiplexing.

A rapid response photochromic diarylethene material for rewritable holographic data storage

A novel diarylethene,namely 4,5-(2,5-dimethyl thiophene) phthalimide,was synthesized and successfully introduced to rewritable holographic data storage.Upon the alternative illumination of UV and visible light(>400 nm),this compound underwent rapid,reversible inter-conversion between colorless open-ring isomer and yellow-green ring-closed form in both solution and polymethyl methacrylate(PMMA) film.Subsequently,we investigated the characteristics of volume homographic recording of the diarylethene-doped PMMA film(1 m thick).The maximum refractive index modulation(0.87‰) of the film during recording could be reached within just 120 s which gave the ability of fast recording.The high quality reconstruction after 50 write/erase cycles demonstrated its excellent fatigue-resistance and high resolution.All those results indicated that this molecule was a reliable fast write/erase holographic storage material.

Recent advances in holographic data storage

Nowadays, big-data centers still rely on hard drives. However, there is strong evidence that these surface-storage technologies are approaching fundamental limits that may be difficult to overcome, as ever-smaller bits become less thermally stable and harder to access. An intriguing approach for next generation data-storage is to use light to store information throughout the three- dimensional （3D） volume of a material. Holographic data storage （HDS） is poised to change the way we write and retrieve data forever. After many years of developing appropriate recording media and optical read-write architectures, this promising technology is now moving industriously to the market. In this paper, a review of the major achievements of HDS in the past ten years is presented and the key technique details are discussed. The author concludes that HDS technology is an attractive candidate for big data centers in the future. On the other hand, there are many challenges ahead for HDS technology to overcome in the years to come.