Holographic storage: Virtual reality?

A single holographic disk will be able to store 1.6 million high-resolution colour photos or over 240 hours of TV broadcast

Worldwide, the demand for storing larger and larger quantities of digital data is increasing rapidly and it is estimated that total storage capacity is now doubling every year.

Holographic data storage has been touted as the solution to the problem — such systems can theoretically store enormous amounts of data in a very small area. The technique works by storing information at high densities inside crystals or photopolymers. But the problem that engineers have faced was how to turn theory and lab demonstrations into a workable commercial system.

Many groups centred in large commercial research organisations have looked at this problem — most have given up, but a few have persevered. One such group of researchers were at Lucent Technologies, spun out in 2000 as InPhase, a company which earlier this year successfully demonstrated in public a prototype of the very first holographic data storage drive.

Dubbed the "Tapestry Drive", this is the first of a family of holographic disk drives being developed by the company and after over four years of field testing, in a range of applications, the initial commercial units will be delivered to OEMs in October 2006.

Tapestry Drive
The first drives in the family will be WORM devices with a capacity of 300GB on a single 12cm removable disk. InPhase expect to increase this to 800GB in late 2007 and to 1.6TB in 2010 — all this family of drives will be fully backward compatible. The company says that drive prices will initially be in the $12,000 to $15,000 (£7,000 to £8,500) range, but expect this to fall rapidly as sales volumes increase. "Ensuring full backward compatibility is very important in the data archiving business and we will also be maintaining this compatibility across our rewritable version when it is launched in 2007," says Nelson Diaz, chief executive of InPhase Technologies,

Having 1.6TB on a single 12cm disk promises to ease the archival data storage problem. It is, says the company, the equivalent of storing 780 million A4 pages of text, which is about the number of pages in a library with around four million books. This means that a single holographic disk will be able to store 1.6 million high-resolution colour photos, over 240 hours of TV broadcast or 18 months worth of archived radio broadcast.

Sarbanes Oxley requirements
Not surprisingly InPhase see the main market for their holographic drive in the data archival requirements of the commercial, medical, governmental and broadcast industries, in the financial world. "Holographic drives are well suited to meet the requirements of Sarbanes Oxley for high integrity, long life archiving of data in the corporate financial world," says Rich D'Ambrise, director of technology at media manufacturer Maxell.

With holographic memory techniques, lasers are used to record or "write" the information to a light sensitive polymer. However, unlike a DVD, where the data is written on the surface as a sequence of minute pits burnt into a metalised layer, holography makes use of the entire volume of the storage material. This allows holographic disks to currently store over 250GB per square inch of disk surface when using a 407nm blue laser.

One of the first major challenges that faced the developers of holographic data storage technology has been the need to develop a suitable storage media. Holographic media must satisfy stringent criteria, including high dynamic range, high photosensitivity, dimensional stability, optical clarity and flatness, nondestructive readout, millimetre thickness and environmental and thermal stability.

Polymer based media
One problem with polymer based media is that when a device writes data to the polymer the media "shrinks", or compacts, limiting the amount of data per cubic centimetre. There are several solutions to this problem, Aprilis' Cationic Ring Opening Polymerisation (CROP) chemistry is one, the "two chemistry" photopolymer developed by InPhase, for its Tapestry media is another.

InPhase's two chemistry photopolymers are fabricated from a mixture of two independently polymerisable, yet compatible, chemical components. Disks are formed by an in-situ polymerisation of one of the components...

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...to form a molecular matrix, or support medium. The other component, which is photosensitive, remains unreacted and dissolved in this matrix thus ensuring both dimensional stability during data writing and a long archival life.

$120 per disk
The Tapestry media is being produced by Maxell at their high technology media manufacturing plant in Tsukuba, Japan, using chemicals supplied by Bayer of Germany. According to Maxell's D'Ambrise, "Disk prices have not yet been set but we expect them to be about 25 cents per gigabyte, or about $120 per disk".

Recording of the holographic data occurs through a spatial pattern of polymerisation of the photosensitive polymer that mimics the optical interference pattern generated during holographic writing. The concentration gradient that results from this patterned polymerisation leads to a diffusion of the unpolymerised polymer. This creates a refractive index modulation that is determined by the difference between the refractive indices of the photosensitive component and the matrix.

The holograms are created within this special photopolymer medium, like any holographic image. This involves taking the light from a coherent laser source — in the Tapestry drive this is a 407nm blue laser of the sort developed for Blu-Ray DVD drives — and splitting it into two beams, a signal or data-carrying beam and a reference beam. The reference beam shines directly onto the recording medium that will be used to store the hologram while the signal beam passes through a spatial light modulator (SLM), before reaching the medium.

Two-dimensional matrix
The function of the SLM is to convert the light beam into an image of the data to be stored. The data is stored in blocks, each block forms a two dimensional matrix of information, typically containing one million bits and can be thought of as a chessboard. Measuring 1000x1000 bits on each side, this is referred to as a page. The SLM produces an optical image of this chessboard data block, which can then be recorded with a single laser flash.

The SLM is, in fact, a ferroelectric liquid crystal on silicon (FLCOS) microdisplay manufactured by Displaytech Inc. and is a technology that has been developed for digital TV projectors. The two beams are then recombined to form an interference pattern within the photosensitive medium, this is the pattern that constitutes the hologram of a data page within the photosensitive recording medium. By varying the reference beam's angle of incidence many different data pages can be recorded within the same volume of material. This allows the data to be multiplexed and accounts for the enormous storage capacity of holographic data storage systems.

The servo system, designed and developed by InPhase, regulates both radial and rotational movement of the media and the angle of the reference beam. During a read operation, feedback from the hologram provides information to the servo system to optimise the recovery of the data with the best signal to noise ratio.

On the Tapestry drive each data page is located at a unique address within the material and several hundred pages of data, each with their own unique address, are recorded in the same location of the medium. Multiple pages of data are referred to as a book, approximately 12MB of data is stored in each book location.

To read the data from a holographic disk the reference beam is used to diffract off of the recorded holograms and thus reconstruct the image of the stored chessboard array of bits. This reconstructed array can then be projected onto a CCD photo-detector that reads the data in parallel.

In the Tapestry drive this photo-detector is an ultrasensitive and ultrafast CMOS image sensor manufactured by Cypress which has a pixel count of 1696x1710, a pixel pitch of 8 microns and most importantly is capable of very high-speed reading of data, over 500 frames per second. This allows the drive to have a data transfer rate of 20MB per second.

This high data transfer rate means that holographic drives are particularly well suited to applications such as the storing and replaying of video in real time, the professional film and video industry is seen as a major market for such drives.

Real-time video
"We have the need to archive high-definition movies as large files and, yet, be able to retrieve them quickly when needed for air. Until now, there has been no cost-effective, practical way to meet our volume and throughput requirements. Holographic storage appears to be the perfect answer," says Ron Tarasoff, vice-president of Broadcast Technology & Engineering at Turner Entertainment Networks.

To make the removable disks easier to use, a special media cartridge has been developed by InPhase. The media cartridges can be loaded and unloaded automatically using a mechanism designed and developed by ALPS Electric Co. Ltd.

The Tapestry drive has a SCSI interface that uses the Pegasus Disk Technologies Windows device driver and InPhase is working with Pegasus to provide file system connectivity for the Windows operating system...

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...as part of a total data storage solution. From a system perspective, the device presents itself like a drive letter with complete random access, in less than 200 milliseconds, to any file on the holographic disk.

The photopolymer holographic media that will be used by InPhase in the first generation of drives is a WORM media. Because of the high stability of the polymers used these are ideal for archiving applications since, unlike DVD and other optical technology, there is no problem of oxidation of a metal layer. This means that the manufacturers are currently quoting a 50 year archival life of the media and expect to increase this to 100 years plus in the next two years.

A 300GB recordable media holographic disk drive is scheduled to be launched in 2007 by InPhase. "These recordable drives will open up major consumer markets for products using holographic drives. They will also replace tape in backup applications where the fast random access nature of such drives substantially reduces restore time, which is currently reaching critical levels with tape systems," says InPhase's Diaz

Upper storage limit
In the longer term holographic drive capacities will continue to increase and, according to Diaz, "the theoretical upper storage limit of a 12cm holographic disk using our polymer technology is a massive 17TB".

Although InPhase technology drives look set to be the first commercially available holographic storage systems, there are other approaches which show distinct commercial potential. One comes from the Japanese company Optoware — they have demonstrated a holographic data storage drive that uses a technology known as polarised collinear holography.

This uses separately polarised objective and reference beams that are bundled on the same optical axis as if they were one light beam. The technique relies upon the fact that the holographic recording materials they are using are only sensitive to green light. This allows them to use the two lasers on the same optical axis, a red one for the servo, and a green one to read and write the holographic data.

Red laser
This has the distinct advantage in that it makes the optical system as compact as those of DVD and CD disk systems and allows use of the cheap mass produced servo systems used in existing optical disk systems. It also means that the red laser can be used to read data on standard DVD and CD disks.

The Optoware disk is preformatted with address pits and tracking grooves on the reflecting layer at its bottom. The 12cm 200GB disk has a 1.2mm-thick sandwich structure consisting of a 0.5mm glass substrate, a 0.2mm volumetric recording layer and a 0.5-mm glass substrate coated with a reflection layer.

Data is recorded as an interference fringe in the "volumetric" material layer and uses a 30,000 bit page, which in the demonstration system gave 16.25MB per second transfer rate.

Following a recent fundraising exercise the company, which now counts Toshiba amongst its shareholders, has said that it will be launching a 200GB disk drive and a 30GB holographic data card drive in June 2006.

Holographic data storage devices are thus finally on the verge of becoming a commercial product and look well positioned to solve the burgeoning data storage needs of a number of different applications. In the short term simply for data archiving but in the longer term with the arrival of rewritable disks as a replacement for tape in backup applications, according to InPhase's Diaz. "By 2010 I see very little tape being used and holographic disks will really be taking over in the backup space."

The development of high capacity holographic drives also comes at a time when new consumer applications, such as streaming high definition television feeds, online photography and video archiving and retrieval, are starting to emerge. These will become a more practical and economic reality with this technology

Companies mentioned in this article
Formed in 2000 to exploit developments at Lucent's Bell Laboratories, it is part owned by Hitachi-Maxell Ltd., has a drive development partnership with Sanyo and a media development partnership with Bayer.

Formed in 1999 as a spinout from Polaroid, the company is shipping its holographic storage media to customers including Sony, Pioneer, Panasonic and Samsung.

Japanese company, formed by ex-Sony engineers, has venture capital funding from Toshiba and others. Is aiming to develop a 1TB capacity HDS.