Collinear Holography E-Book

Xiaodi Tan

• Herausgeber: Wiley-VCH GmbH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 430

Veröffentlichungsjahr: 2022

Beschreibung

Collinear Holography

Provides state-of-the-art, in-depth knowledge on the principles, devices, and applications of collinear holography

In the era of Big Data, traditional magnetic and optical storage technologies are unable to satisfy the growing demand for reliable, scalable, cost-effective, and energy-efficient data storage. Holographic storage, considered the most promising technology for meeting the future storage needs of the information age, adopts a three-dimensional volume storage mode with a theoretical storage density vastly greater than conventional optical disks.

Collinear Holography: Devices, Materials, Data Storage is a comprehensive, up-to-date account of the volumetric recording technology that combines large storage capacities with high transfer rates and exceptional reliability in optical data storage systems. Written by pioneers in the field, this authoritative book provides detailed coverage of the key technological approaches, theories, applications, systems, devices, and components in the rapidly advancing field of holographic data storage.

• Explains the principles of collinear holography, its different system setups, key devices and components, and current challenges
• Describes the materials, data and media formats, servo controls, and read/write characteristics of collinear holography storage systems
• Details collinear holography in current applications such as holo-printing, ­correlation, and encryption
• Discusses futures technologies including the Holographic Versatile Disc (HVD) and the The Holographic Versatile Card (HVC)

Collinear Holography: Devices, Materials, Data Storage is an indispensable resource for applied physicists, electrical engineers, and materials and information scientists in both academia and industry.

Leseprobe

Collinear Holography

Devices, Materials, Data Storage

Xiaodi TanHideyoshi HorimaiTsutomu ShimuraXiao Lin

Authors

Prof. Xiaodi Tan

Fujian Normal University

College of Photonic and Electronic

Engineering

No.1 Keji Road, Shangjie

Minhou District

350117 Fuzhou

Fujian

China

Dr. Hideyoshi Horimai

HolyMine Corporation

2032‐2‐301 Ooka, Numazu

410‐0022 Shizuoka

Japan

Prof. Tsutomu Shimura

The University of Tokyo

Institute of Industrial Science

4‐6‐1 Komaba, Meguro‐ku

153‐8505 Tokyo

Japan

Prof. Xiao Lin

Fujian Normal University

College of Photonic and Electronic

Engineering

No.1 Keji Road, Shangjie

Minhou District

350117 Fuzhou

Fujian

China

Cover Image: © SergeyBitos/Shutterstock

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Print ISBN: 978‐3‐527‐41363‐8

ePDF ISBN: 978‐3‐527‐81045‐1

ePub ISBN: 978‐3‐527‐81047‐5

oBook ISBN: 978‐3‐527‐81048‐2

Preface

In big data era, large capacity, high speed, long life, and low‐cost data storage technology is urgently demanded. Collinear Holography is a promising candidate technology for mass data storage. The biggest feature of optical holographic storage is that it breaks through the two‐dimensional surface storage mode of traditional optical disc storage and adopts the three‐dimensional volume storage mode. As the data recording is increased by one dimension, the existing optical storage density can be increased by several orders than traditional optical disc. Data transfer is in two‐dimensional data page, so the increase of data transfer rate is also more than several orders of magnitude.

Holographic data storage technology moved out of the laboratory and went to commercialization after the collinear holography was proposed. In 2005, the paper published in the Journal of Applied Optics (edited by the Optical Society of American) is the first report that introduced the details of collinear holography. The paper had been ranked eighth of “the top 15 highly cited articles published in the journal over the last 10 years” in 2014. Collinear holography has been applied and developed not only in the area of data storage system, but also expanded to other fields of information processing systems.

Collinear holography data storage technology has now grown to become an exciting interdisciplinary field of research and development, involving device configuration, materials, opt‐electronics, and data processing. To meet the rapid development of this technology, we summarized the research results of authors in the book and emphasized the fundamental principles that can be employed for understanding the limitation of various collinear holographic systems, and to analyze their performance of data storage systems. A significant effort is made to bridge the gap between theory and practice through the use of numerical examples based on real system, materials, and page information. The book covers a very wide range of topics, including the basic physical properties of holography, the propagation of plane waves, the Fourier optics, the concept of data format, key devices of system, recording materials, system margins compared with other method, characteristics of data writing and reading. At the end of the book, the applications that use collinear technology are introduced to treat the optical transmission of information systems. In writing this book, we have assumed that the reader has been introduced to basic electromagnetic plane waves in an undergraduate course in electricity and magnetism. It is further expected that the reader has some background in elementary Fourier optics and data processing.

In addition to our four authors, many other authors have also participated in the writing of individual chapter of the book. For the editing format reason, we cannot list their names on the cover and at every chapter. The book is organized in such a way that Chapter 1 introduces the background of big data era, history of holographic data storage, and present problems. Chapter 2 introduces the collinear holographic data storage systems, in which the system control and optical standardization tester are contributed by Masaki Igarashi (New Optware) and Masao Endo (The University of Tokyo). Chapter 3 summarizes the theoretical research results of collinear holography, including the simulation results of Shuhei Yoshida (Tokyo University of Science), Jianhua Li (Beijing Institute of Space Long March Vehicle) and Liangcai Cao (Tsinghua University). Chapter 4 summarizes the key equipment and devices related to holographic data storage, including the contributions of Jinpeng Liu (Xidian University), Hiroyuki Matsuda (New Optware), Tadashi Sasada (New Optware), Yawara Kaneko (New Optware) and Masaharu Kinoshita (New Optware). Chapter 5 introduces the materials, and Yasuo Tomita (The University of Electro-Communications), Jinxin Guo (Beijing University of Technology), Lin Cao (Beijing University of Technology), Jialing Jian (Beijing University of Technology), Dayong Wang (Beijing University of Technology), Xinping Zhang (Beijing University of Technology), Fenglan Fan (Hebei Normal University for Nationalities) and Ying Liu (Beihang University) contributed all of this chapter. In Chapter 6, the data format quotes the author's research results for many years. In Chapter 7, the reconstructed characteristics of reading and writing from collinear holographic data storage system are given by the selectivity, related parameters, and symbol error. In Chapter 8, various system margins are discussed, and Jianhua Li (Beijing Institute of Space Long March Vehicle), Yeh-Wei Yu (National Central University), Liangcai Cao (Tsinghua University) and Jinpeng Liu (Xidian University) have contributed a part of them. In Chapter 9, the related applications of collinear holography are reviewed, while Kanami Ikeda (The University of Electro‐Communications) and Eriko Watanabe (The University of Electro‐Communications) described their correlation system in detail. All of the chapters contain the research results of authors' research and development in data storage and applications for more than 20 years.

With the continuous growth of storage market demand in the era of big data, holographic data storage will receive extensive attention from the industry and commercialization. Although the collinear holography has shown its infinite charm, it still needs to further improve its performance and stability, and strive to reduce costs. We hope this book can attract more readers to participate in the research and development of collinear holography to seek greater progress in collinear holography.

Xiaodi Tan

Hideyoshi Horimai

Tsutomu Shimura

Xiao Lin

Fuzhou, China

2020 August

Foreword

Collinear holography was originally proposed by the holograph inventor Dennis·Gabor in 1948; to avoid the interference of zero‐order light, off‐axis holography was adopted to realize the recording and reconstruction of three‐dimensional images until the 1960s, after laser was invented. Holographic data storage was also proposed at the same time, due to its large storage capacities and high transfer rates. Therefore, two‐beam interference holography has become a common method of traditional holography. Because two beams are easily affected by environmental vibration, holography is usually inseparable from the vibration isolator. To eliminate the commercial limitation caused by the problem of off‐axis holography, collinear holography has gained a new life and is perceived as a major breakthrough in the field of holographic data storage.

Collinear holography, in this book, is a new form of collinear holography, which combines the advantages of D. Gabor's holography and traditional off‐axis holography. The information and reference beams are divided co‐axially by a special light modulator. With this unique configuration it can produce a small, practical holographic data storage system more easily than traditional two‐axis holography. The advantages of the collinear holography have been summarized by the authors with their theoretical study to experimental results for a long term.

This book covers the data storage system, principle, basic theoretical models, key devices and components, materials, and other applications. The book includes many technical challenging works of the authors in detail. This book suggests that the collinear holography is very effective in increasing recording density and data transfer rates.

What will be the next generation of data storage technology? We do not know yet. However, I think collinear holography will increase in importance over the coming decade with the development of related technologies and devices. Collinear holography will enable us to expand its applications into other optical information systems.

Xiangang Luo

Academician of Chinese Engineering

Beijing

China

1Introduction

1.1 Big Data Era

Information has long been regarded as an important resource, and with the continuous development of information technology, information protection, mining, and data storage have become particularly important. The year 2012 was declared as the Big Data Meta Year with the sudden explosion in the amount of global data generated and the advent of the big data era has forced us to prioritize data storage. According to the research results of International Data Corporation (IDC), the total amount of data generated worldwide in 2010 was 1.2 ZB (1 ZB = 103 EB = 106 PB = 109 TB), the data volume in 2013 was 4 ZB, and that in 2018 was 33 ZB. The latest forecast of IDC predicts that by 2025, the total global data will exceed 175 ZB.

Today's data storage technology is still dominated by magnetic storage devices, such as hard disk storage and magnetic tape storage. Although this technology has matured, its data storage capacity has only increased by about 20% each year, which is far from keeping up with the current rate of data growth. Moreover, its limitation of two‐dimensional storage still persists. For further increasing the magnetic storage density, the size of the recording magnetic particles needs to be continuously reduced. However, when the magnetic particles are small enough, they are affected by the superparamagnetic effect, making it difficult for the recording particles to maintain magnetic stability even at normal temperatures. The traditional magnetic storage density has approached its theoretical limits. In practice, to ensure data redundancy, three hard disks are generally required to back up one copy of the data simultaneously, and the data in each hard disk need to be transferred to a new hard discovery four to five years; otherwise, the information will likely be lost forever. Professionally stored tape storage also needs to be transferred every 10 years or so to avoid data loss. Therefore, companies with large data centers, such as Google, require a large server scale, and the annual cost of data transfer is huge, accounting for about one‐third of the total cost of data storage. Additionally, during magnetic data writing and reading, the driver radiates a large amount of heat, and an exorbitant amount of power is consumed to cool the server. Accordingly, the power cost also accounts for about one‐third of the total cost. Therefore, the data storage cost of the magnetic storage technology is in sync with the amount of data growth, and both are exploding, and as a result, the pressure on magnetic storage technology is increasing.

Optical storage is another important data storage technology. At present, the most common optical storage technology is optical disks. If light with a wavelength of λ is used as a light source for data storage, its theoretical storage area density is nearly 1/λ2. From the earliest compact disc (CDs) to video compact (VCDs), digital video discs (DVDs), and now Blu‐ray discs, the density of disc storage is also increasing. However, the optical discs are still two‐dimensional surface storage devices based on bit storage (although some optical discs can achieve multilayer storage, but the number of layers is limited); each recording bit represents only a 0 or 1 state, and its storage density is affected by the record. Moreover, although a smaller‐sized recorded bit results in a greater data storage density, the size of the bit is limited. Each recording bit is etched by converging the energy of an incident laser light. To obtain a smaller recording bit size, the numerical aperture of the recording objective lens must be increased, and a shorter wavelength laser should be used as the recording light source. At present, the numerical aperture of the data recording objective lens in the most advanced Blu‐ray disc has reached about 0.85, and the laser wavelength has also been shortened to 405 nm. Now a single‐sided single‐layer Blu‐ray disc can have a storage capacity of 25 GB, while a double‐sided four‐layer Blu‐ray disc can exhibit a storage capacity of 200 GB. Therefore, the famous data company Facebook started to build a Blu‐ray disc‐based database with a total capacity of 1000 PB in 2014. Its data access energy consumption is 80% lower than that of the magnetic hard disks. The Blu‐ray disc can be stored for 30–50 years, and the cost of data transfer is greatly reduced. This Facebook storage system uses a very cost‐effective redundant backup method, which can achieve a data backup redundancy with a coefficient lower than two to ensure data security and not cause performance degradation due to scale expansion.

Despite the rapid development in the optical disc storage, it is still a two‐dimensional surface bitwise storage device. To further increase the storage density, the numerical aperture of the recording objective lens needs to be increased further, and the recording light wavelength should be further shortened. However, the theoretical aperture of the objective lens in air is less than or equal to one. To develop a numerical aperture greater than one, only the method of immersion can be used. The application environment for such a storage device is limited, and the implementation cost will be exorbitant. With the increase in the numerical aperture, the thickness of the protective layer on the surface of the optical disc needs to be reduced excessively, which in turn will eventually result in the loss of the protective effect. The wavelength of the recording light is currently close to the range of invisible light, and the cost of using violet or extreme ultraviolet lasers is also very high. Therefore, further increasing the storage density of the traditional optical disc is an arduous task. According to the current 200 GB data storage capacity of a Blu‐ray disc, by 2020, the number of optical discs required to store the global data is expected to exceed 500 billion, and their combined thickness will exceed the distance between the Earth and the Moon. Therefore, traditional optical disc storage is unable to meet the growing requirements of big data storage, and the discovery and development of new technologies is imminent.

Optical holographic data storage is an optical storage technology that follows the principle of holography for data storage and reproduction. The most noticeable feature of the optical holographic storage is that it breaks through the two‐dimensional surface storage mode of the traditional optical disc storage and adopts the three‐dimensional volume storage mode. Further, its theoretical storage density is 1/λ3. By increasing the storage density by one dimension, the existing optical storage density can be increased by several orders of magnitude.

The optical holographic data storage technology records the amplitude and phase information of objects in the form of holograms in holographic materials. As shown in Figure 1.1, during the recording process, the light passes through a spatial light modulator (a two‐dimensional optical element that can display the two‐dimensional pattern that you want to upload), and the information carrying the two‐dimensional pattern is called the object light. This object light interferes with another beam of the known light field (reference light) in the holographic material to form a complex light field distribution. The holographic material responds to the light fields of different intensities accordingly, resulting in changes in the material and finally forming some kind of stable structure, namely a hologram. This hologram records the object light information in the holographic material. During the reading process, only the hologram in the material needs to be irradiated with the same reference beam in the same state as that used during the recording process, and the light energy is coupled to the object light through the coupling effect of the holographic structure on the reference light. In the field, diffraction of light occurs, and the diffracted light is also called as the reconstructed light, which is actually consistent with the distribution of the original light field, enabling readout of the object light information.

Figure 1.1 Schematic illustration of holographic data storage: (a) writing process and (b) reading process.

Source: Xiao Lin.

Figure 1.2 Comparison between traditional data storage and holographic data storage.

Source: Xiao Lin.

Optical holographic storage can get rid of the one‐dimensional data storage limitation of bit storage. Each recording position represents a two‐dimensional coding pattern, as shown in Figure 1.2. Further, because the holographic storage has the characteristics of reusable recording, the recording bit and its area can be superimposed on each other, which leads to a large recording holographic storage density of the order of TB/in2. Simultaneously, the data reading conversion rate can also reach the order of 10 GB/s, because each recording position readout is a two‐dimensional encoding pattern. Additionally, the performance of the materials used for holographic storage is significantly improved. The materials, viz. the early photorefractive crystals to the current photopolymers, are easy to prepare and cost effective, and the life of data can be as long as 50 years. Therefore, holographic storage is considered to be the most promising and a key optical storage technology in the era of big data.

1.2 History of Holographic Data Storage

As early as 1948, Dennis Gabor proposed the concept of holography, involving only the method of wavefront reconstruction. This method did not link holography with data storage, but used holographic technology for the magnification of X‐ray images [1, 2]. The primary drawback of this method was the unavailability of a good coherent light source; thus, conjugate images appeared in the coaxial wavefront reconstruction systems. Previously, Gabor [3] and other scientists, such as Kirkpatrick and Hussein El‐Sum [4, 5], Baez [6], Gordon Rogers [7], and others, attempted to solve this problem, albeit with poor results. With the discovery of lasers in the early 1960s, a good coherent light source was discovered, and holography entered a stage of rapid development. The recording and reconstruction of clear images became a reality, and because of the excellent coherence of the light source, E.N. Leith and J. Upatnieks proposed a reference light off‐axis holographic recording system [8, 9] that overcame the drawback of Gabor hologram, in which the reconstructed conjugate images are aliased in one place. In 1963, van Heerden formally proposed the concept of holographic data storage [10]. He attributed holographic data storage to three‐dimensional solid‐state optical information storage and estimated that its storage density limit was V/λ3 (V is the volume of the recording material, and λ is the wavelength of the recording light). Subsequently, van Heerden also discussed the possibility of multiplexing with the reference light angle and the wavelength in holographic data storage. Leith et al. also proposed and validated the technique of recording disk rotation multiplexing in the early developmental stages of holographic data storage [11]. At this time, the holographic technology had been quite embryonic, but owing to the lack of effective recording materials, most of the research on holographic data storage stayed was confined only to the discussion of methods. In 1966, Ashkin et al. of Bell Labs unexpectedly discovered the photorefractive effect while performing frequency doubling experiments with lithium niobate crystals [12]. These crystals have been extensively studied as photorefractive crystals and applied to holographic data storage [13–16], making three‐dimensional volume holographic data storage once again a research hotspot. Due to the development of materials, a series of verification experiments on holographic data storage have been published, and more reuse technologies have been studied.

In 1973, researchers at the American RCA Corporation designed and verified a holographic memory that could read and write 106 bit data [17]. In 1973, the Japanese company NEC proposed a holographic data storage system (HDSS) with read‐only holographic encoder disks, but stored in two dimensions, on a 128 mm × 128 mm surface, with a storage capacity of 2.5 × 105 bits at a resolution of 1 line/mm [18]. In 1974, the American 3M company, Strehlow, and others designed a 7 MB holographic memory [19]. In 1973, Huignard and coworkers first demonstrated the law of angular multiplexing, and proposed that recording 100 holograms in one location can reach a total storage capacity of 1011 bits [20], and then experimentally demonstrated the angular multiplexing scheme in iron‐doped niobium. Ten holograms were reused in the lithium acid crystals [21]. In 1975, Amodei and Staebler of RCA Corporation, for the first time, recorded 500 holograms in 1 cm3 of iron‐doped lithium niobate crystals [14]. In 1978, Andrei Mikaeliane described a rewritable holographic storage system based on iron‐doped lithium niobate crystals [22]. In 1976, Hitachi, Japan reported a holographic video disc, which stored a 30 minutes color video in a disc of 300 mm diameter; a total of 54 000 holograms were recorded on the disc [23]. In 1980, Kubota et al. of NEC Company developed a one‐dimensional Fourier transform hologram optical disc for recording audio information; this disc could achieve a data conversion rate of 256 Mb/s [24].

Although there was a rapid development of the holographic storage theory in the 1970s, it was still limited by the backward effects of recording materials, modulators, and detectors at that time, and a satisfactory storage density could not be achieved in the early works.

Until the early 1990s, with the development of recording materials, spatial light modulators, charge‐coupled device detectors, and other key materials and devices, the storage density of the HDSSs could be greatly increased, and the optical holographic data storage entered a period of accelerated development. Overall, the materials used and the reuse theory have been comprehensively improved.

In 1991, Mok et al. stored 500 holograms of military vehicle shapes in 1 cm3 of iron‐doped lithium niobate crystals [25], and only two years later, F.H. Mok achieved the storage of 5000 images in 1 cm3 of iron‐doped lithium niobate crystals, indicating a 10‐times‐increased storage capacity [26]. In 1994, Hesselink and coworkers from Stanford University showed a full digital holographic storage system that converts images and videos into multiple data pages, and the bit error rate can reach 10−6[27]. In 1994, IBM and Stanford University and other seven companies and university research groups formed a joint agency under the partial sponsorship of the United States Defense Advanced Research Projects Agency (DARPA). They expect to develop a HDSS with 1013 bit storage capacity and 1 Gb/s data conversion rate within five years. The material test system is provided by IBM. This system can not only store and reconstruct holograms of large data pages, but also perform bit error rate analysis on the reconstruction results [28]. The world's first complete HDSS was jointly established by Stanford University, Siros, IBM [29] and Rockwell, Thousand Oaks [30], and other companies. Researchers at Caltech and Lucent also completed similar system demonstrations [31].

In 1997, CIT's Allen Pu and D. Psaltis used a spherical reference light to obtain a volume holographic storage with an area density of 100 bits/μm2 on a 1‐mm‐thick iron‐doped lithium niobate crystal via shift multiplexing [32]. In 1998, Bell Labs' K. Curtis and coworkers used a similar multiplexing technology to store the areal density in iron‐doped lithium niobate crystals at a rate exceeding 350 bits/μm2[33].

In terms of recording materials, in addition to photorefractive materials that have been studied since the 1970s [21, 22, 34], in 1994, the United States DuPont company developed a radical polymerizable photopolymer [35]. Pu et al. conducted an in‐depth study on the holographic storage characteristics of this photopolymer material product, and performed holographic storage experiments using cyclic and angular multiplexing techniques. Subsequently, Allen Pu and Demetri Psaltis converted this material into a holographic disc, and multiplexed and stored 32 holograms in each flat cell area, and obtained a storage area density of 109 bit/cm2[36]. Even though the sensitivity of the photopolymers is one to two orders of magnitude higher than that of the photorefractive crystals, the problem of material shrinkage is also severe. Therefore, the dynamic response range and shrinkage of the material must be balanced, as explained in detail by T. Bieringer [37]. The researchers at the Polaroid and Bell Labs took different approaches to materials research. Polaroid used experimental cationic ring‐opening materials for the experimental verification [38, 39], whereas Bell Labs and Lucent used free radiating media [33]. Later, Stanford University and Aprilis Company (continued to develop after Polaroid) used a cationic open‐loop material to record 250 GB of information on a DVD‐sized disc with a data transmission rate of 10 Gb/s. In 2003, Waldman et al. used a photopolymer material, HMD‐050‐G‐C‐400 manufactured by Aprilis Company, with a thickness of 400 μm to achieve a holographic storage area density greater than 100 bits/μm2[40].

In 2002, Suzuki et al. first doped TiO2 nanoparticles into methacrylate photopolymer films and found that the volume shrinkage of the materials during the holographic exposure was suppressed [41]. Since then, the method of nanoparticle doping to modify materials has been the focus of many studies. In 2008, Goldenberg et al. studied the doping of metal nanoparticles and modified gold nanoparticles. It has been proven that nanoparticles, when mixed with carboxyl functional acrylate monomers, can affect the structure of the material, inhibit shrinkage, and increase the stability of the material [42]. In 2010, Koji Omura and Yasuo Tomita studied the properties of ZrO2 nanoparticle polymer composite films under a 404 nm laser and proposed that the shrinkage of the material was suppressed owing to the increase in the gel point [43]. In 2011, Hata et al. introduced thioolefin monomers into silica nanoparticle polymer composites, investigated their photopolymerization kinetics and volume holographic recording characteristics, and found that the material shrinkage and thermal stability were greatly improved [44]. In 2014, the research group of Chengming Yue and others proposed a holographic kinetic model to represent the kinetics of the hybrid grating in bulk polymer doped with photopolymers of gold nanoparticles quantitatively and described the polymerization of the gold nanoparticles. Further, because of the multicomponent diffusion behaviors, a mixed circular polarization‐angle volume holographic multiplexing recording was then achieved in the prepared gold nanoparticle‐doped phenanthrenequinone/polymethyl methacrylate (PQ/PMMA) photopolymer [45, 46]. In 2016, Tomita et al. introduced hyperbranched polymers, with ultrahigh refractive indices, as organic nanoparticles, and prepared nanoparticle–polymer composite holographic gratings, achieving a significant increase in the refractive index modulation and diffraction efficiency (close to 100%) at a wavelength of 532 nm [47]. In 2018, Liu et al. prepared a new material by dispersing silver nanopillars in a photopolymerizable mixture, and demonstrated the grating of this nanopillar‐doped polymer composite under the exposure of an ultrafast nanosecond laser. To study the formation, they analyzed the causes of reciprocity failure and the improvement in the polymer holographic properties by the doped silver nanoparticles; the diffraction efficiency of the optimized polymer was as high as 51.4% [48].

In addition to incorporating nanoparticles into photopolymers, many optimization attempts have been made on photopolymer systems. In 2012, Ortuño et al. developed a new dry photopolymer called bio‐photoelectronics, which has a low toxicity. This photopolymer was developed with the aim to counter the toxicity and poor environment compatibility exhibited by most of the photopolymer components. Additionally, its high thickness makes it ideal for holographic data storage applications [49]. Subsequently, in 2013, Ortuño et al. also proposed a new chain transfer agent 4,4′azo‐bis‐(4‐cyanovaleric acid) (ACPA) with a polyvinyl alcohol (PVA)/acrylamide (AA) photopolymer, whose performance can be improved by the initiator ACPA [50]. In 2016, Cody et al. studied a novel composition of a low‐toxic water‐soluble holographic photopolymer and obtained a bright reflection grating with a recording diffraction efficiency of up to 50% [51]. Later, the research team of Fan Fenglan and others proposed to modify the material components via chemical methods to prepare PQ‐supported dimonomer photopolymer materials, which improved the solubility of the photoinitiator in the photopolymer, thereby achieving material optics, as an improvement of features [52]. In 2018, Liu et al. proposed a new type of photosensitizer‐doped photopolymer and proposed an optimized three‐step thermal polymerization preparation method. The thermal initiators with different concentrations and the photoinitiator characteristics of the photopolymers [53] were studied in detail.

By doping nanoparticles and optimizing the polymerization methods, the current recording materials have been significantly improved to better meet the needs of optical holographic data storage. The next step is to steadily increase the diffraction efficiency while further enhancing the material stability and corresponding rate as well as improving the data storage density.

The supreme advantage of volume holographic storage is its ability to use a variety of multiplexing technologies to increase the storage density. The multiplexing technology of volume holographic storage was well developed in the 1990s [27, 54, 55].