Holographic storage: Virtual reality?

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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