Development Of Materials For Holographic Memory - Online Article

One of the problems standing on the way of development of holographic memory devices is a search of the appropriate material. The most of the investigations in the sphere of holography were carried out with usage of photoreactive materials (mainly the mentioned LiNBO3), but they are not suitable for data recording especially for commercial use: they are expensive, weak sensitive and have a limited dynamic range (frequency bandwidth). That's why they developed a new class of photopolymer materials facing a good perspective in terms of commercial use. Photopolymers are the substances where the light cause irreversible changes expressed through fluctuation of the composition and density. The created material have a longer life circle (in terms of storing data) and are resistant to temperature change, besides, they have improved optical characteristics and are suitable for WORM (write-once/read many).

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Lithium Niobate crystal

One of current research efforts is to explore new and better holographic materials. We design, fabricate, and investigate a photopolymer material called PQ : PMMA, in which PMMA is the host material that supports the polymer matrix and PQ the photosensitive element that is responsible for forming the refractive-index hologram. High-optical-quality samples with a dimension of 2.5 cm by 2.5 cm by 2.5 cm have been made. Multiple recording and reconstruction of 250 holograms on a single spot of a 1 centimeter-cube photopolymer block have been achieved. Preliminary experimental results demonstrate PQ : PMMA to be a promising material for volume holographic memory. The second type of materials that is under our study is sillenite-type photorefractive crystal. We investigate the doping effect of transition elements on optical and photorefractive properties of BTO (Bi12TiO20) and BGO (Bi3G4O12) single crystals. The objective is to find suitable elements for creating photorefractive centers in these materials to achieve high optical sensitivity, large dynamic recording range, and fast response time. Preliminary experiments show that holographic properties of BTO crystals can be modified by different concentrations of Ru dopants. We also study the co-and/or third doping effect with a goal to find a suitable material for non-volatile holographic data storage.

Material for Holographic Memory and Playback System

Pioneer Electronic Corp. and National Institute for Research in Inorganic Materials (NIRIM) scheduled to be renamed to Substance and Material Research Organization in 2001; have succeeded in the development of a high-performance material for holographic memory, which can perform multiplex recording of 3-dimensional (cubic) information on a recording material using laser ray, as well as a small recording / playback system. One tera-bit class recording capacity can be materialized by only one card.

Supervising researcher Mr. Kenji Kitamura et al. at NIRIM prepared the monocrystal precisely controlled in its composition ratio of niobium and lithium by a double crucible method and succeeded in writing-in 125 megabit of data per 1 spot of monocrystal. Initialization can be optionally made by ultraviolet irradiation, and recording can be made by bicolor hologram method, and data output by reference beam irradiation. The feature is capability of optional data rewriting. This process attracts attention as a potent device technology in the tera-bit class data processing age.

Holography is a recording technique using the interference effect of light. Although this technique is expected as a memory device for networks or computers, with large memory volume and increasing handling data volume, the conventional techniques had such drawbacks as long time required for memorizing and impossibility in partial erasure.

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Holographic Memory Recording and Reproducing Equipment and Recording Material

Many kinds of materials have been investigated as holographic storage media. The given table is a comparison of the properties of several that are among the best available as data storage media. Five materials are compared on the basis of optical imaging quality, scattered light level, hologram fidelity, sensitivity, M#(dynamic range), stability, and available thickness. These include the much-studied Fe-doped lithium niobate, two-color recording in reduced stoichiometric lithium niobate, and three organic materials that were chosen to typify the range of properties available from various organic materials systems. Photopolymers are very promising because of their high sensitivity and dynamic range. Phenanthrenequinone-doped polymethylmethacrylate (PQ/PMMA) has excellent optical quality and is based on a photoreaction between the dopant and polymer followed by diffusion of unreacted chromophore; this requires a long thermal treatment, which is a disadvantage from a system perspective. Finally, photo-addressable polymers are also promising but are still at an early stage of development.

Comparison of properties of prospective materials for holographic data storage media
Material Image quality Scatter Holographic fidelity Sensitivity cm2/J) M# Stability Thickness (mm)
LiNbO3 : Fe +++ +++ + 0.02 1 0 10
LiNbO3 (Two-color) ++ + 0.02 1* ++ 10
Polaroid photopolymer +++ 0 20 1.5 + 0.5
PQ/PMMA + + 0.20.5 1 ++ 2
Bayer photo-addressable polymer +++ 0 ++ 0.0020 0.1
*Values depend on writing intensity

Emerging Challenges

In the past, the realization of holographic data storage has been frustrated by the lack of availability of suitable system components, the complexity of holographic multiplexing strategies, and perhaps most importantly, the absence of recording materials that satisfied the stringent requirements of holographic data storage.

Recently the development of practical components for holographic systems has rekindled interest in this technology. While the development of the needed components has been accomplished largely in fields outside the storage industry, the volume of these markets is expected to lead to low-cost, reliable components for holographic data storage. Frequency-doubled, diode-pumped Nd : YAG green lasers, used in the medical, cable TV, and printing industries, are attractive recording sources due to their small size, ruggedness, and low cost. Digital micro-mirror devices appearing in new types of displays are ideal spatial light modulators with their large numbers of pixels (~ 1 million), fast frame rates (2000 Hz) and high optical contrast. The CMOS active pixel detector arrays emerging in digital photography exhibit the rapid access and data transfer properties required for holography.

The Bell Labs has invented a multiplexing geometry that yielded a simple, easily implementable architecture for holographic storage systems. Spurred by this development, they focused on the long-standing problem of the lack of suitable storage materials and invented new high-performance recording media with demonstrated high density data storage capabilities. Their work serves as the foundation for a practically realizable, high capacity storage system with fast transfer rates and low-cost, removable recording media.

Technologocal Breakthrough

New Multiplexing Methods

The methods used to overlap or multiplex holograms determine the complexity and architecture of the recording system. In the past, multiplexing methods have required large optical systems and moving optical parts. Bell Labs have developed a method known as correlation multiplexing where an optically complex reference beam, created by a fixed set of optics, encodes the position of the hologram in the recording medium. Large numbers of holograms can therefore be multiplexed in essentially the same volume of the recording medium through only micron-size spatial translations of the medium relative to the reference beam. This "fixed optics" method enables construction of a simple holographic storage system based on a spinning disk architecture used throughout much of the storage industry.

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Correlation Multiplexing

New Photopolymer Recording Media

One of the major challenges in the area of holographic data storage has been the development of suitable storage materials. Holographic media must satisfy stringent criteria, including high dynamic range, high photosensitivity, dimensional stability, optical clarity and flatness, nondestructive readout, millimeter thickness, and environmental and thermal stability.

To meet the needs of high-density holographic data storage, researchers at Bell Laboratories have designed a new type photopolymer, a "two-chemistry" system, which yields high response, high photosensitivity media in millimeter-thick, optically flat formats. The media exhibit the some of the highest dynamic range of any holographic material and currently represent one of the few recording systems appropriate for high density digital holographic storage applications.

The media are fabricated from mixtures of two independently polymerizable yet compatible chemical systems. Recording disks are formed by an in-situ polymerization of one of the components to form the matrix or support of the medium. The other component, which is photosensitive, remains unreacted and dissolved in this matrix. Recording of holograms occurs through a spatial pattern of polymerization of the photosensitive species that mimics the optical interference pattern generated during holographic writing The concentration gradient that results from this patterned polymerization leads to diffusion of the unpolymerized species which creates a refractive index modulation that is determined by the difference between the refractive indices of the photosensitive component and the matrix. This approach allows flexibility in tailoring the media to the particular needs of high density holographic data storage.

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Two - Chemistry Photopolymer

In these materials, a storage densities of 31.5 channel Gbits/inch (a density that would yield ~45 Gbytes on a 5 " disk) have been demonstrated by recording and retrieving >3000 digital data pages. Newer "two-chemistry" materials have the capability to store densities at least five times higher. With these photopolymer materials meeting the critical performance requirements for holographic mass storage, have removed much of the risk associated with the development of holographic technology.

Future Prospects of Holographic Memeory

It is believed that the substantial advances in recording media, recording methods, and the demonstrated densities of >30 channel Gbits/inch described earlier, coupled with the recent commercial availability of system components remove many of the obstacles that previously prevented the practical consideration of holographic data storage and greatly enhance the prospects for holography to become a next-generation storage technology.

Bell Labs has recently entered into an agreement with Imation Corporation, one of the leading data storage companies, for the development of holographic data storage system. They are currently working with the Lucent Technologies New Ventures Group to explore avenues that would lead to commercialization of the technology.

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Real-time information fusion is impossible without several orders of magnitude improvement in data storage capacity and processing speed. Exponential increases in both these domains of computing power continue in the commercial sector. Rome Laboratory's C3I Technology Area Plan articulates the issues of storage and speed as a thread in virtually all of its thrusts.

Reliable holographic storage media using a stacking system and photoreactive polymers are creating one gigabyte in a 2.5-square inch-area. Professor Lambertus Hesselink's work with optical fibers of strontium barium niobate suggests that one million bits of data can be encoded on a rod smaller than a straight pin. He plans for an array that handles 120 gigabytes in one square centimeter within the next few years. Negroponte and Gates argue that growth in memory is driven today by the commercial sector, and society (military, government, and education) will ride upon their coattails, gaining virtually unlimited memory storage capacity in miniature form.

Holographic media are most efficient in maximizing this feature, using optical storage and retrieval systems. Simultaneous reading and writing of "pages" of data and multiplexing allows storage of large numbers of pages in the same location. Consequently, saving and accessing data is much quicker.

Holographic memory has also proved its utility in the field of electronic warfare. The Army Land Warrior Program is scheduled to provide each combat soldier with a wearable computer to assist with the processing of sensor and targeting data, situational awareness displays, and communications. As the use of graphical formats to facilitate the assimilation of information in real time increases, the Army will have a growing need for computer memory capacity on the battlefield.

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