Holographic Memory

Definition

Devices that use light to store and read data have been the backbone of data storage for nearly two decades. Compact discs revolutionized data storage in the early 1980s, allowing multi-megabytes of data to be stored on a disc that has a diameter of a mere 12 centimeters and a thickness of about 1.2 millimeters. In 1997, an improved version of the CD, called a digital versatile disc (DVD), was released, which enabled the storage of full-length movies on a single disc.

CDs and DVDs are the primary data storage methods for music, software, personal computing and video. A CD can hold 783 megabytes of data. A double-sided, double-layer DVD can hold 15.9 GB of data, which is about eight hours of movies. These conventional storage mediums meet today's storage needs, but storage technologies have to evolve to keep pace with increasing consumer demand. CDs, DVDs and magnetic storage all store bits of information on the surface of a recording medium. In order to increase storage capabilities, scientists are now working on a new optical storage method called holographic memory that will go beneath the surface and use the volume of the recording medium for storage, instead of only the surface area. Three-dimensional data storage will be able to store more information in a smaller space and offer faster data transfer times.

Holographic memory is developing technology that has promised to revolutionalise the storage systems. It can store data upto 1 Tb in a sugar cube sized crystal. Data from more than 1000 CDs can fit into a holographic memory System. Most of the computer hard drives available today can hold only 10 to 40 GB of data, a small fraction of what holographic memory system can hold. Conventional memories use only the surface to store the data. But holographic data storage systems use the volume to store data. It has more advantages than conventional storage systems. It is based on the principle of holography.

Scientist Pieter J. van Heerden first proposed the idea of holographic (three-dimensional) storage in the early 1960s.

 

HolographyA hologramis a block or sheet of photosensitive material which records the interference of two light sources. To create a hologram, laser light is first split into two beams, a source beam and a reference beam. The source beam is then manipulated and sent into the photosensitive material. Once inside this material, it intersects the reference beam and the resulting interference of laser light is recorded on the photosensitive material, resulting in a hologram. Once a hologram is recorded, it can be viewed with only the reference beam. The reference beam is projected into the hologram at the exact angle it was projected during recording. When this light hits the recorded diffraction pattern, the source beam is regenerated out of the refracted light. An exact copy of the source beam is sent out of the hologram and can be read by optical sensors.

Holography was invented in 1947 by the Hungarian-British physicist Dennis Gabor (1900-1979), who won a 1971 Nobel Prize for his invention m

How Holographic Memory Will Workby Kevin Bonsor

Browse the article How Holographic Memory Will Work

Introduction to How Holographic Memory Will Work

Devices that use light to store and read data have been the backbone of data storage for nearly two decades. Compact discs revolutionized data storage in the early 1980s, allowing multi-megabytes of data to be stored on a disc that has a diameter of a mere 12 centimeters and a thickness of about 1.2 millimeters. In 1997, an improved version of the CD, called a digital versatile disc (DVD), was released, which enabled the storage of full-length movies on a single disc.

Computer Memory Image Gallery

In a holographic memory device, a laser beam is split in two, and the two resulting beams interact in a crystal medium to store a

holographic recreation of a page of data.  See more pictures of computer memory.

CDs and DVDs are the primary data storage methods for music, software, personal computing and video. A CD can hold 783 megabytes of data, which is equivalent to about one hour and 15 minutes of music, but Sony has plans to release a 1.3-gigabyte (GB) high-capacity CD. A

double-sided, double-layer DVD can hold 15.9 GB of data, which is about eight hours of movies. These conventional storage mediums meet today's storage needs, but storage technologies have to evolve to keep pace with increasing consumer demand. CDs, DVDs and magnetic storage all store bits of information on the surface of a recording medium. In order to increase storage capabilities, scientists are now working on a new optical storage method, called holographic memory, that will go beneath the surface and use the volume of the recording medium for storage, instead of only the surface area.

Three-dimensional data storage will be able to store more information in a smaller space and offer faster data transfer times. In this article, you will learn how a holographic storage system might be built in the next three or four years, and what it will take to make a desktop version of such a high-density storage system.

 

A Little Background

Holographic memory offers the possibility of storing 1 terabyte (TB) of data in a sugar-cube-sized crystal. A terabyte of data equals 1,000 gigabytes, 1 million megabytes or 1 trillion bytes. Data from more than 1,000 CDs could fit on a holographic memory system. Most computer hard drives only hold 10 to 40 GB of data, a small fraction of what a holographic memory system might hold.

Polaroid scientist Pieter J. van Heerden first proposed the idea of holographic (three-dimensional) storage in the early 1960s. A decade later, scientists at RCA Laboratories demonstrated the technology by recording 500 holograms in an iron-doped lithium-niobate crystal, and 550 holograms of high-resolution images in a light-sensitive polymer material. The lack of cheap parts and the advancement of magnetic and semiconductor memories placed the development of holographic data storage on hold.

Over the past decade, the Defense Advanced Research Projects Agency (DARPA) and high-tech giants IBM and Lucent's Bell Labs have led the resurgence of holographic memory development.

The Basics

Prototypes developed by Lucent and IBM differ slightly, but most holographic data storage systems (HDSS) are based on the same concept. Here are the basic components that are needed to construct an HDSS:

Blue-green argon laser Beam splitters to spilt the laser beam Mirrors to direct the laser beams LCD panel (spatial light modulator) Lenses to focus the laser beams Lithium-niobate crystal or photopolymer Charge-coupled device (CCD) camera

When the blue-green argon laser is fired, a beam splitter creates two beams. One beam, called the object or signal beam, will go straight, bounce off one mirror and travel through a spatial-light modulator (SLM). An SLM is a liquid crystal display (LCD) that shows pages of raw binary data as clear and dark boxes. The information from the page of binary code is carried by the signal beam around to the light-sensitive lithium-niobate crystal. Some systems use a photopolymer in place of the crystal. A second beam, called the reference beam, shoots out the side of the beam splitter and takes a separate path to the crystal. When the two beams meet, the interference pattern that is created stores the data carried by the signal beam in a specific area in the crystal -- the data is stored as a hologram.

Images courtesy Lucent Technologies

These two diagrams show how information is stored and retrieved in a holographic data storage system.

An advantage of a holographic memory system is that an entire page of data can be retrieved quickly and at one time. In order to retrieve and reconstruct the holographic page of data stored in the crystal, the reference beam is shined into the crystal at exactly the same angle at which it entered to store that page of data. Each page of data is stored in a different area of the crystal, based on the angle at which the reference beam strikes it. During reconstruction, the beam will be diffracted by the crystal to allow the recreation of the original page that was stored. This reconstructed page is then projected onto the charge-coupled device (CCD) camera, which interprets and forwards the digital information to a computer.

The key component of any holographic data storage system is the angle at which the second reference beam is fired at the crystal to retrieve a page of data. It must match the original reference beam angle exactly. A difference of just a thousandth of a millimeter will result in failure to retrieve that page of data.

Desktop Holographic Data Storage

After more than 30 years of research and development, a desktop holographic storage system (HDSS) is close at hand. Early holographic data storage devices will have capacities of 125 GB and transfer rates of about 40 MB per second. Eventually, these devices could have storage capacities of 1 TB and data rates of more than 1 GB per second -- fast enough to transfer an entire DVD movie in 30 seconds. So why has it taken so long to develop an HDSS, and what is there left to do?

When the idea of an HDSS was first proposed, the components for constructing such a device were much larger and more expensive. For example, a laser for such a system in the 1960s would have been 6 feet long. Now, with the development of consumer electronics, a laser similar to those used in CD players could be used for the HDSS. LCDs weren't even developed until 1968, and the first ones were very expensive. Today, LCDs are much cheaper and more complex than those developed 30 years ago. Additionally, a CCD sensor wasn't available until the last decade. Almost the entire HDSS device can now be made from off-the-shelf components, which means that it could be mass-produced.

Although HDSS components are easier to come by today than they were in the 1960s, there are still some technical problems that need to be worked out. For example, if too many pages are stored in one crystal, the strength of each hologram is diminished. If there are too many holograms stored on a crystal, and the reference laser used to retrieve a hologram is not shined at the precise angle, a hologram will pick up a lot of background from the other holograms stored around it. It is also a challenge to align all of these components in a low-cost system.

Researchers are confident that technologies will be developed in the next two or three years to meet these challenges. With such technologies on the market, you will be able to purchase the first holographic memory players by the time "Star Wars: Episode II" is released on home 3-D discs. This DVD-like disc would have a capacity 27 times greater than the 4.7-GB DVDs available today, and the playing device would have data rates 25 times faster than today's fastest DVD players.

For more information on holographic memory and related topics, check out the links on the next page.

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Bell Labs: High-Density Holographic Data Storage Hand-Drawn Holograms Holographic data storage (IBM, Journal of Research and Development) IBM: Holographic Data Storage Holographic Memory

The keys to holographic data security: Encrypted optical memory systems based on multidimensional keys for secure data storage and communication

Osamu Matoba and Bahram Javidi

There is a growing demand to store large sizes of data due to the development of digital computer technologies and digital television for the next generation. Up to now, only digital versatile disk (DVD) and compact disk (CD), which are bit-oriented storage methods, have been developed. Optical technology can provide a number of ways to solve the problem of large storage and fast transmission of data. The holographic technique discovered in 1948 records and reconstructs a complex-valued optical wave by using the interference between two coherent light beams. Using the holography technique, two- or three-dimensional images can be stored and reconstructed. Thus, development is expected of a three-dimensional television or a two- or three-dimensional image storage system operating at high speed that cannot be implemented in a magnetic-based storage system.

Holographic memories that use photorefractive materials are attractive due to their high storage capacity, high-speed access to data, and rewritability [1-6]. Unlike bit-oriented optical memories such as DVD and CD, two-dimensional data is stored as a hologram by illuminating the interference pattern formed by an object beam and a reference beam. In a photorefractive crystal, the intensity distribution of the interference pattern is stored as a refractive-index distribution [7, 8]. Using angle, wavelength, and phase multiplexing techniques, one can store multiple images at the same position. Maximum storage capacity using angular multiplexing techniques is estimated to be V/l3 (1500 CD-ROMs per cm3), where V is the volume of the photorefractive material and l is the wavelength of light. The readout rate can be more than 1 Gb/s, operating at 1 kHz readout rate with a 1 Mb image.

In practical systems, data security is an important issue. Optical encryption techniques provide a high level of security [9-15] because there are many degrees of freedom with which to encode the information, such as amplitude, phase, wavelength, and polarization. This article discusses three encrypted optical memory systems based on multidimensional keys.

Encryption Overview

Numerous methods have been described to use optical encoding schemes [10, 16-18] in order to secure data in holographic memory systems. One way to protect the stored information is to encrypt the data. Here the encryption means that the original data is converted into stationary white-noise data by key codes, and unauthorized users cannot obtain the original data without knowledge of the key code. Original data may be encoded optically by using encryption techniques such as double random phase encryption [10] or exclusive OR (XOR) operation [16, 17]. Another method is based on protecting the access to memory from unauthorized users by encoding the reference beam [18]. These techniques can be implemented by using a set of uncorrelated reference beams generated by orthogonal phase codes, such as random phase masks, or generated by speckle patterns from an optical fiber.

1. (a) Block and (b) schematic diagrams of the secure communication system using secure holographic memory. ST and TS denote the space-to-time and the time-to-space converters, respectively.

Encrypted memory using double random phase encryption can be used in a secure communication network using ultrashort pulses, as shown in Fig. 1 [19]. Figure 1(a) and Fig. 1(b) show block and schematic diagrams of the secure communication system using the encrypted memory and spatial-temporal converters. In this system, the original data is stored in an encrypted memory system. The encrypted data read-out from the memory is converted into a one-dimensional temporal pulse using the space-to-time converter [20] and then is transmitted to users via optical fibers. At the receivers, the temporal signal is converted again into the spatial

signal by the time-to-space converter. The authorized users can decrypt the data using the correct key. This system can be expected to communicate at an ultrahigh speed of more than 1 Tb/s.

In the following sections we will discuss three encrypted optical memory systems using multidimensional keys [21-23]. Original data is encrypted by the multidimensional keys, which consist of two random phase masks, their three-dimensional positions, and the wavelength of the recording beams. It makes it difficult to decrypt the data without the key information because the total number of the multidimensional keys becomes extremely huge.

2. Encrypted holographic memory system using multidimensional keys.

 

Encrypted Memory Using Double Random Phase Encryption

Figure 2 shows an illustration of the encrypted optical memory systems used in this article. The encoding methods used in the proposed memory systems are based on the double random phase encryption technique [10]. We briefly review encryption and decryption of encrypted holographic memory using double random phase encryption [21]. Let gi(x,y) denote the ith positive real-valued image to be encrypted. Here, x and y denote the spatial-domain coordinates. The original data is converted into a white-noise-like image by using two random phase masks, exp{-jni(x,y)} and exp{-jhi(n,h)}, located at the input and Fourier planes. Here, ni(x,y) and hi() are two independent white sequences that are uniformly distributed on the interval [0,2]. Note that and denote Fourier domain coordinates. The original data is illuminated by a collimated light beam and multiplied by a random phase function exp{-jni(x,y)}. The Fourier transform of the input data is multiplied by another random phase function H()=exp{-jhi ()} and is given by

(1)

where

(2)

In Eq. (2), F[•] denotes the Fourier transform operation, is the wavelength of the light, and f is the focal length of the Fourier-transform lens. Each encrypted data frame is obtained by taking another Fourier transform:

(3)

where denotes convolution. Equation (3) shows that the two phase functions, ni(x,y) and hi(), convert the original data into a stationary-white-noise-like data [10].

The Fourier-transformed pattern of the encrypted data that is described in Eq. (1) is stored holographically together with a reference beam in a photorefractive material. To store many frames of data, angular multiplexing is employed. The interference pattern ) to be stored in a photorefractive material is written as

(4)

where M is the total number of stored images and Ri() is a reference beam at a specific angle used to record the ith encrypted data. Here we briefly describe the mechanism of the photorefractive effect when the carriers are electrons. In a photorefractive crystal, illumination with a sufficient wavelength content excites the electrons in the conduction band from the donor level between the valence and the conduction bands. The donor level is created by impurity ions or defects. The photoexcited electrons can move in the crystal by the diffusion, the drift, and the photovoltaic effect and then get trapped in the ionized donors. At the steady state, the space charge density is proportional to the interference pattern in the diffusion-dominant region. This space-charge density creates the space-charge field that can cause the refractive-index change via the electro-optic effect. The created refractive-index distribution is proportional to the interference pattern and can be stored for a long time (more than two months) in the dark in LiNbO3 crystal. Since a volume hologram is created in a photorefractive material, an appropriate angular separation between adjacent stored data can reduce the cross talk among the stored frames of data. Thus, we can store many frames by using angular multiplexing.

In the decryption process, the readout beam is the conjugate of the reference beam. The readout using the conjugate of the reference beam offers advantages. It is able to use the same random phase mask in the encryption and the decryption process, and it eliminates the aberration of the optical system. The data of the ith-stored image can be reconstructed when the readout beam is incident at the correct angle. The reconstructed data in the Fourier plane (FP), Di(), is written as

(5)

where

(6)

The asterisk in Eq. (5) denotes the complex conjugate, and ki() is a phase key used in the decryption process. We can reconstruct the image by Fourier transforming Eq. (5). The reconstructed ith image, di(), is written as

(7)

where

(8)

In Eq. 8, denotes correlation. When phase key ki() = hi(), the conjugation of the original data is successfully recovered because Eq. (8) becomes a delta function. The random phase function in the input plane, exp{-jni()}, may be removed by an intensity-sensitive device, such as a charge-coupled device (CCD) camera. In a practical system, operating at high-speed detection of the reconstructed data, the parallel detection at each pixel of two- dimensional data is desirable. When one uses an incorrect phase key, ki()� hi(), the original data cannot be recovered.

We describe an encrypted memory system based on double random phase encryption [21]. Figure 3 shows the experimental setup. A 10 X 10 X 10 mm3 LiNbO3 crystal doped with 0.03 mol.% Fe is used as the recording medium. The c axis is on the paper and is at 45° with respect to the crystal faces. The crystal is mounted on a rotary stage and a three-dimensionally movable stage. An Ar+ laser beam of wavelength 514.5 nm is used as a coherent light source. The light beam is divided into an object and a reference beam by a beamsplitter (BS1) for holographic recording. The reference beam is again divided into two reference beams. One of the beams is used for the conjugate readout by another beamsplitter (BS2). An input image is displayed on a liquid-crystal display that is controlled by a computer. The input image is multiplied by an input random phase mask (RPM1) and is then Fourier-transformed by lens L1. The Fourier-transformed input image is multiplied by another random phase mask (RPM2) at the Fourier plane. The Fourier-transformed image is imaged at a reduced scale in the LiNbO3 crystal by lens L2. The encrypted image is observed by a CCD camera (CCD1) after the Fourier transform is produced by lens L3. The focal lengths of L1, L2, and L3 are 400 mm, 58 mm, and 50 mm, respectively. For holographic recording, the object and reference beams interfere at an angle of 90� in the LiNbO3 crystal. All of the beams are ordinarily polarized due to the creation of an interference fringe pattern. Shutters SH1 and SH2 are open, while SH3 is closed.

In the decryption process, the readout beam is the conjugate of the reference beam used for recording. Shutters SH1 and SH2 are closed, while SH3 is open. If the same mask is located at the same place as the one used to write the hologram, the original image is reconstructed at

CCD2. This is because the ideal reconstructed beam read out by using the conjugate of the reference beam eliminates the phase modulation caused by the random phase mask. Otherwise, the original data may not be recovered. In the experiments, we use a pair of counterpropagating plane waves as the reference and the conjugate beams.

Angularly multiplexed recording of four digital images is demonstrated. One of the original digital images is shown in Fig. 4(a). This image consists of 32 X 32 randomly generated pixels. The size of the liquid crystal display that shows the input image is 28.5 mm X 20 mm. Two diffusers are used as the random phase masks, RPM1 and RPM2. Figure 4(b) shows the intensity distribution of the encrypted image. Random-noise-like images were observed. In the recording process, the optical intensities of the object and the reference beams were 78 mW/cm2 and 1.4 W/cm2, respectively. The exposure time of each image was 60 s. Angular multiplexing was achieved by rotating the LiNbO3 crystal in the plane of Fig. 2. The angular separation between adjacent stored images was 0.2�. This angular separation is enough to avoid the cross talk between reconstructed images. Figure 5(a) shows the reconstructed images obtained using the correct key. The resolution of the reconstructed image is determined by the crystal size and the space-bandwidth product of the optical system. This key is the same as the phase mask in the Fourier plane used to record the hologram. This result shows that the stored images were reconstructed successfully. No noise due to the cross talk between the reconstructed images was observed. After the binarization of the reconstructed images, we confirmed that there is no bit error in the four-output digital data. Figure 5(b) shows the reconstructed images when incorrect keys were used. No part of the original image can be seen. The average bit-error rate obtained using incorrect keys was 0.384.

3. Experimental setup. RPM denotes random phase mask; BS denotes beamsplitter; L denotes lens; M denotes mirror; BE denotes beam expander; SH denotes shutter; CCD denotes CCD camera.

4. (a) Original image and (b) encrypted image.

5. (a) Reconstructed image using correct phase key and (b) reconstructed image using incorrect phase key.

6. Experimental results. (a) Original image, (b) encrypted image. (c) and (d) are reconstructed images when positions of the phase masks are correct and incorrect, respectively.

7. (a) Reconstructed image using correct phase key and (b) reconstructed image using incorrect phase key.

Encrypted Memory Using Three-Dimensional Keys in the Fresnel Domain

We can make the memory system more secure by using random phase masks in the Fresnel domain. In addition to the phase information, the positions of two phase masks are used as encryption keys. Even if the phase masks are stolen, the unknown positions of the masks can protect the data. The positions of the masks have as many as three degrees of freedom. We have demonstrated encryption and decryption of three binary images by angular multiplexing [22]. The experimental setup is the same as that shown in Fig. 3. Figure 6(a) shows one of the three original images. RPM1 and RPM2 were located at a distance of 100 mm from L1 and at the center of L1 and FP, respectively, as shown in Fig. 3. Figure 6(b) shows an encrypted image of Fig. 6(a). Random-noise-like images were observed. In the recording process, the optical powers of the object and the reference beams were 4 mW/cm2 and 500 mW/cm2, respectively. The exposure time was 110 s. Figure 6(c) shows one of the reconstructed images obtained by using the same masks located at the same positions used in the recording. This result shows that the original image was successfully reconstructed. Figure 6(d) shows the reconstructed image when the two phase masks were incorrectly located. We can see that the reconstructed image is still a white-noise-like image.

We estimate the available number of three-dimensional positions of two random phase masks. Let the dimensions of random phase masks be Lx X Ly, and x and y be correlation lengths of the random phase mask along the x- and y-axes, respectively. The x-, y-, and z-axes are defined as shown in Fig. 3. When a number of Nz resolvable positions along the optical axis can be used for the encryption key, the total number of three-dimensional positions to be examined in a three-dimensional key, P, is written by,

(9)

(10)

where f is the focal length of L1 in Fig. 3 and z is computed according to the sensitivity of the decryption to the shifts of the keys along the z axis. Since two three-dimensional keys are used in the system, the total number of three-dimensional positions to be examined is given by

(11)

In the memory system shown in Fig. 3, N = 3 X 1018 when Lx = Ly = 25 mm, L = 400 mm,x =y = 6 m, and z = 4 mm. Note that x and y were calculated from the measurement of an autocorrelation function of the phase mask used in the experiments. When one searches 106 positions per second, it takes 95 years to finish the whole search. It is practically impossible to decrypt without the knowing the positions of two three-dimensional keys.

Encrypted Memory Using Wavelength-Code and Random Phase Masks

The wavelength of recording beams can be used as a key for security in a holographic memory system [23]. The wavelength code increases the key space by one dimension. Since an optical storage medium, such as a photorefractive material doped with impurities, has broad spectral sensitivity, we can use many wavelengths of light emitted from tunable laser sources, such as a dye laser. In this memory system, shown in Fig. 3, one original data frame is stored by using a set of two random phase masks at the input and Fourier planes as well as a wavelength key. The wavelength key can protect the decrypted data even if the phase masks have been illegally obtained. When the wavelength of the readout beam is different from that of the recording beam, the wavelength mismatch modifies the scale of coordinates at the Fourier plane in the readout process. Due to the incorrect wavelength, the phase modulation at the Fourier plane is not completely canceled because of a scale mismatch. If a substantial part of the phase modulation at the Fourier plane is not canceled, the original image cannot be recovered. We note that the wavelength mismatch results in decreased diffraction efficiency due to the Bragg condition, because the volume grating structure of the hologram is complex.

Figure 7(a) and Fig. 7(b) show an original and an encrypted image. The encrypted image is stored holographically using recording beams at a wavelength of 514.5 nm. In the decryption process, we use two readout beams at wavelengths of 514.5 nm and 632.8 nm. Note that in both cases the Bragg conditions are satisfied. Figure 7(c) and Fig. 7(d) show reconstructed images when wavelengths of 514.5 nm and 632.8 nm are used, respectively. When the wavelength of the readout beam is the same as that of the recording beam, and when the same mask is located at the

same place as that used to record the hologram, we can obtain the reconstructed original image. The wavelength selectivity depends on the pixel size of the random phase mask at the Fourier plane. We can use many wavelengths by utilizing the small pixel size of the random phase mask.

Conclusions

We have presented three encrypted holographic memory systems. These systems are secure because the total number of mathematical possibilities of the multidimensional keys, which consist of two-dimensional phase masks, their three-dimensional positions, and wavelengths of light, is extremely large. The experimental results are very encouraging. We expect the encrypted memory system to play an important role in ultrafast secure communication systems using the spatial-temporal converters with ultrashort pulsse that enable communication at ultrahigh speed of more than Tb/s [19, 24].

Osamu Matoba is a research associate at the Institute of Industrial Science of the University of Tokyo, Japan. E-mail: matoba@ iis.u-tokyo.ac.jp. Bahram Javidi is a professor with the Department of Electrical and Computer Engineering of the University of Connecticut in Storrs, Connecticut, USA. E-mail: bahram@ engr.uconn.edu.

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By Maksim Len

The upcoming problems requiring very huge computing power make us today looking properly for new technical solutions not only in terms of CPU enhancement but also in terms of other PC components. Regardless of the technology used for CPU production, the data number transferred for processing are determined also by possibilities of other subsystems. Capacity of modern devices of mass memory reflects this tendency. CDs discs allow storing up to 700 MBytes, the developing technology of DVD-ROM - up to 17 GBytes. Technology of magnetic recording develops quickly as well - for the last year the typical capacity of a hard disc in the desktop computers has increased up to 15-20 GBytes and higher. But in the future computers are to process hundreds of gigabytes and even terabytes - much more than any current CDs or hard discs can accommodate. Servicing of such data volumes and their transfer for processing by ultraspeed processors requires completely new approaches when creating storage devices.

Holographic memory

Wide possibility in this case are provided by technology of optical recording, it's known as holography: it allows high record density together with maximum data access speed. It's achieved due to the fact that the holographic image (hologram) is coded in one big data block which is recorded at one access. And while reading this block is entirely extracted out of the memory. For reading and recording of the blocks kept holographically on the light-sensitive material (LiNbO3 is taken as the basic material) they use lasers. Theoretically, thousands of such digital pages, which contain up to a million bits each, can be put into a device measuring a bit of sugar. And theoretically they expect the data density to be 1 TBytes per cubic cm (TBytes/cm3). In practice, the developers expect around 10 GBytes/cm3, what is rather impressive when comparing with the current magnetic method which allows around several MBytes/cm2 - and this without the mechanism itself. With such recording density an optical layer which is approx 1 cm in width will keep around 1TBytes of data. And considering the fact that such system doesn't have any moving parts, and pages are accessed parallel, you can expect the device to be characterized with 1 GBytes/cm3 density and higher.

Exceptional possibilities of the topographic memory have interested many scientists of universities and industrial research laboratories. This interest long time ago poured into two research programs. The first of them is PRISM (Photorefractive Information Storage Material), which is targeted at searching of appropriate light-sensitive materials for storing holograms and investigation of their memorizing properties. The second program is HDSS (Holographic Data Storage System). Like PRISM, it includes fundamental investigations, and the same companies participate there. While PRISM is aimed at searching the appropriate media for storing holograms, HDSS is targeted at hardware development necessary for practical realization of holographic storage systems.

How does a system of holographic memory operate? For this purpose we will consider a device assembled by a task group from the Almaden Research Center.

At the first stage in this device a beam of cyan argon laser is divided into two components - a reference and an object beam (the latter is a carrier of data). The object beam undergoes defocusing in order it could entirely illumine the SLM (Spatial Light Modulator) which is an LCD panel where a data page is displayed in the form of a matrix consisting of light and dark pixels (binary data).

The both beams go into the light-sensitive crystal where they interact. So we get an interference pattern which serve a base for a hologram and is recorded as a set of variations of the refractive exponent and the reflection factor inside this crystal. When reading data the crystal is illuminated with a reference beam, which interacts with the interference factor and reproduces the recorded page in the image of "chess-board" of light and dark pixels (the holograms converts the reference wave into the copy of the object one). After that, this image is transferred into the matrix detector where the CCD (Charge-Coupled Device) serves a base. While reading the data the reference beam must fall at the same angle at which the recording was made; alteration of this angle mustn't exceed 1 degree. It allows obtaining high data density: measuring the angle of the reference beam or its frequency you can record additional pages of data in the same crystal.

However, additional holograms change properties of the material, and such changes mustn't exceed the definite number. As a result, the images of holograms become dim, what can lead to data corruption when reading. This explains the limitation of the volume of the real memory which belongs to this material. The dynamic area of the medium is defined by the number of pages which can be virtually housed, that's why PRISM participants are investigating limitations to the light sensitivity of substances.

The procedure in 3-dimensional holography which concludes in enclosure of several pages with data into the same volume is called multiplexing. Traditionally the following multiplexing methods are used: of angle of dip of the reference beam, of wavelength and phase; but unfortunately they require complicated optical systems and thick (several mm) carriers what makes them unfit for commercial use, at least in the sphere of data processing. But lately Bell Labs have invented three new multiplexing methods: shift, aperture and correlative, which are based on the usage of change in position of the carrier relative to the beams of light. The shift and aperture multiplexing use a spherical reference beam, and the correlative uses a beam of more complicated form. Besides, considering the fact that the correlative and shift multiplexing enable mechanical moving elements, the access time will be the same as that of the usual optical discs. Bell Labs managed to build an experimental carrier on the same LiNBO3 using the technology of correlative multiplexing but this time with 226 GBytes per square inch.

Another problem 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).

One more problem concludes in the complexity of the used optical system. For holographic memory the LEDs based on semiconductor lasers used in traditional optical devices are not suitable, since they have insufficient power, give out a wide beam angle, and at last it's too difficult to get a semiconductor laser generating radiation in the middle range of the visible spectrum. There you need as powerful laser as possible which gives the most exact parallel beam. The same we can say about the SLM: yet some time ago there were no any such devices which could be used in the holographic memory systems. But time flies and today you can get inexpensive solid-state lasers; besides, there appeared the MEM technology (Micro-Electrical Mechanical). The devices on its base consist of the arrays of micromirrors around 17 micron in size, they suit very much for the role of SLM.

Since the interference patterns fill up the whole substance uniformly, it gives another useful property to the holographic memory - high reliability of the recorded information. While a defect on the surface of the magnetic disc or tape destroy important data, a defect in holographic medium doesn't cause a loss of information, it leads only to tarnish of the hologram. The small

desktop HDSS-devices are to appear by 2003. Since the HDSS equipment use an acoustooptical deflector for measuring angle of dip (a crystal which properties change with a sound-wave passing through it), the extraction time for adjacent data pages will constitute 10ms. Any traditional optical or magnetic memory device needs special means for data access of different tracks, and this access time constitutes several milliseconds.

The holographic memory is not a completely new technology since its basic conceptions were developed about 30 years ago. The only that has changed is availability of the key components - the prices considerably fell down. The semiconductor laser, for example, is not unusual. On the other hand, SLM is a result of the same technology which is used in production of LCD-screens for notebooks and calculators, and the CCd detector array is taken right from a digital video camera.

Well, the new technology has more than enough highlights: apart from the fact that information is stored and recorded parallel, you can reach very high data rate, and in some cases high speed of random access. And the main advantage is that mechanical components are practically absent (those that typical for current storage devices). It ensures not only a fast data access, less probability of failures, but also lower power consumption, since today a hard disc is one of the greatest power-consuming elements of a computer. However, there are problems with adjustment of optical devices, that's why at the beginning the data of the device will probably "fear" exterior mechanical effects.

Molecular memory

Another approach in creation of storage devices is a molecular method. A group of researchers of the "W.M. Keck Center for Molecular Electronic" with Professor Robert R. Birge as a head quite a long time ago received a prototype of memory subsystem which uses digital bits of a molecule. These are protein molecules which is called bacteriorhodopsin. It's purple, absorbs the light and presents in a membrane of a microorganism called halobacterium halobium. This bacterium lives in salt bogs where the temperature can reach +150 °C. When a level of oxygen contents is so low in the ambient that to obtain power breathing (oxidation) is not enough, it uses protein for photosynthesis.

Bacteriorhodopsin was chosen because a photocycle (a sequence of structural changes undergone by a molecule when reacting with light) makes this molecule an ideal logically storing element of "&" type or a type of a switch from one condition into another (trigger). According to Birge's investigation, bR-state (logical value of the bit "0") and Q-state (logical value of the bit "1") are intermediate states of the molecule and can remain stable during many years. This property (which in particular provides a wonderful stability of protein) was obtained by an evolutional way in the struggle for survival under severe conditions of salt bogs.

Birge estimated that the data recorded on the bacteriorhodopsin storage device must "live" around 5 years. Another important feature of the bacteriorhodopsin is that these both states have different absorption spectra. It allows easily defining the current state of the molecule with the help of a laser set for the definite frequency.

They built a prototype of memory system where the absorption spectrum stores data in 3-dimensional matrix. Such matrix represents a cuvette (a transparent vessel) filled up with polyacryde gel, where protein is put. The cuvette has an oblong form 1x1x2 inch in size. The protein which is in the bR-state is fixed in the space with gel polymerization. The cuvette is surrounded with a battery of lasers and a detector array based on the device using a principle of CID (Charge Injection Device), they serve for data recording and reading.

hard disk

The primary computer storage medium, which is made of one or more aluminum or glass platters, coated with a ferromagnetic material. Most hard disks are "fixed disks," which have platters that reside permanently in the drive. Almost all computers have an internal hard disk, and external units can be plugged in for additional storage or backup.

The other type of hard disk is a "removable disk" encased in a cartridge, allowing data to be ejected from the drive for external storage or transfer to another party. Before high-speed Internet connections were common, removable SyQuest, Jaz and Zip cartridges were routinely shipped via the post office (see removable disk).

Three Major Categories: PATA, SATA and SCSIMost hard disks are Parallel ATA (PATA), Serial ATA (SATA) or SCSI. SCSI drives have traditionally been found on servers and high-performance workstations and were the first drives used in fault-tolerant RAID systems. Today, ATA drives are widely used for RAID arrays. See IDE, PATA, SATA, SCSI and RAID.

Hard drives are low-level formatted at the factory, which records the original sector identification on the platters (see format program). See hard disk defect management.

Fast RotationHard disks provide fast retrieval because they rotate constantly at high speed, from 5,000 to 15,000 RPM. Either to preserve battery life in laptops or to promote longevity, hard disks can be configured to turn off after a defined period of inactivity.

It Started in the Mid-1950sIn 1956, IBM introduced the RAMAC hard disk with platters two feet in diameter that held the equivalent of 100,000 bytes. In the 1980s, desktop computer hard disks were introduced with 5MB using 5.25" platters (see ST506). Today's entry-level drives have at least 8,000 times more capacity. Platter size was reduced to 3.5" for desktops, 2.5" for laptops and 1" for handhelds. In 2004, Toshiba introduced the 0.85" drive (see below). See magnetic disk, floppy disk, Microdrive, drop protection and CAV.

TYPES OF HARD DISKS

Transfer Type of Encoding Rate Range of Interface Method** (Per sec) Capacities

SATA (IDE) RLL 150-300MB 40GB-1.2TB

PATA (IDE) RLL 3-133MB 500MB-400GB

SCSI RLL 5-320MB 20MB-300GB

Older Interfaces

IPI RLL 10-25MB 200MB-3GB ESDI RLL 1-3MB 80MB-2GB SMD RLL 1-4MB 200MB-2GB IDE RLL 1-8MB 40MB-1GB

ST506 RLL RLL 937KB 30MB-200MB ST506 MFM 625KB 5MB-100MB

** Most disks use RLL, but encoding methods are not prescribed by all interfaces.

Hard Disk Measurements

Capacity is measured in bytes, and speed is measured by transfer rate in bytes per second (see above) and access time in milliseconds (ms). Hard disk access times range from 3 ms to about 15 ms, whereas CDs and DVDs range from 80 ms to 120 ms.

Non-Removable Internal Hard Disk

Hard disks use one or more metal or glass platters covered with a magnetic coating. Although there has been a variety of removable hard disks over the years, a computer's primary hard disks are fixed inside the drive. The entire unit is removed only to be replaced or repaired. In this drawing, the cover is removed.

First Hard Disk

Part computer, part tabulator, in 1956, IBM's RAMAC was the first machine with a hard disk, which was extraordinary technology of the times. Each of its 24" diameter platters held a whopping 100,000 characters (they were not bytes then) for a total of five million characters. (Images courtesy of International Business Machines Corporation. Unauthorized use not permitted.)

First Microcomputer Hard Disk

Seagate introduced the first hard disk for personal computers in 1979. At 5MB, the ST506 held 10 times as much as the RAMAC at a fraction of its size. (Image courtesy of Seagate Technology, Inc.)

Four Decades Later

Entry level these days, but in 1998, this Seagate drive's 47GB was impressive. Four decades of research and development let us store 100,000 times as much on the same platter surface. Even more impressive is that this much data are stored on one side of only one platter today. (Image courtesy of Seagate Technology, Inc.)