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Is holography the future for storage?

Antony Adshead

This summer will see the launch of the world's first storage devices to use holography to write data to disc. The company behind the pioneering launch is InPhase, a spin-off company from Alcatel-Lucent's Bell Labs.

InPhase is one of about 20 companies worldwide working on the technology. The first generation of holographic data technology promises 300Gbytes of storage per standard disc - about six times more than Blu-ray or HD-DVD - and that is only the first stage of an evolution that promises 1.6Tbytes of data on a single disc within three years.

As well as a huge storage capacity, holography offers incredible levels of data integrity. Tape needs to be carefully stored and re-recorded every five to seven years, and disc arrays have maintenance issues if platters are not kept spinning constantly.

With holographic media, users write once to disc and the data is then stable for 50 years. Holographic storage also wins over tape by allowing random access, meaning users can immediately access data anywhere on the device.

So, how does the technology work, how far down the development cycle is holographic storage, and when can we expect the technology to become the standard method for data storage?

The idea of using holography for storage has been around for at least 40 years. What has made the technology usable in recent years is the advancement of laser technology that has come about through the development of DVDs and the complementary metal-oxide semiconductor (CMOS) detectors that are used in digital cameras.

Additionally, improvements have been made in the production of the holographic medium, resulting in the possibility of more stable data storage.

Where existing optical disc technologies work by refracting light from what are essentially microscopic bumps in the disc material, holography works by creating a three-dimensional image within the medium.

Light from a single laser is split into two beams: the data-carrying signal beam and a reference beam. Data is encoded into the signal beam by converting zeros and ones into more than a million light and dark areas in the holographic image.

The hologram, which is like an optical chessboard on photosensitive material, is formed by the interference created when the signal and reference beams intersect on the recording medium.

The big advantage of holographic storage, says Dennis Szubert, principal analyst at analyst firm Quocirca, is that unlike existing optical methods of storage, holographic data is written in three dimensions throughout the volume of the disc and not just on its surface.

"Holographic storage systems promise discs that initially will provide 300Gbytes of storage capacity," he says. "But ultimately, they will be able to store more than a terabyte of information. That is 200 times more than a single-sided DVD and 20 times more than a current double-sided Blu-ray disc.

"This is partly due to storing holograms in overlapping patterns, while a DVD basically stores bits of information side by side. It also uses a thicker recording layer than DVD, storing information in almost the entire volume of the disc, instead of just a single thin layer," Szubert says.

Holographic storage has improved durability over magnetic devices such as tape drives in which the media and drive head are in contact. This proximity can cause problems with both the media and the read heads, and recovering data stored on tape can be problematic if the tape is old or has been used many times.

Because of this mechanical wear placed on drive heads and the storage medium, magnetic forms of storage typically have a life of about five years. Holography has no head-to-media contact as data is recorded and read by shining a light on the media. As a result, none of the wear issues associated with magnetic storage apply.

In theory, holography promises incredibly high transfer rates. Up to 1Gbyte per second - 40 times faster than DVD - is the kind of figure bandied about. This is because a holographic system stores and retrieves an entire page of data - about 60,000 bits of information - in one pulse of light. A DVD can only transfer one bit of data per pulse of light.

This is, however, only a theoretical transfer speed, and current holographic storage technology falls down on data-transfer speed compared with existing media. Although InPhase's disc capacities and data integrity are remarkable, transfer rates are slow - 20Mbytes a second - compared with tape, for example, which offers 120Mbytes a second.

The current generation of holographic storage relies on existing lasers and CMOS for image-processing. This technology is called "pagewise" because it allows the writing or recording of one "page" of holographic chessboard patterns at a time. Writing to disc involves a process where the disc is rotated by small increments at a time, hence the slow transfer rates.

What the industry is looking towards is a data-transfer method called "bitwise", where data can be written constantly to a revolving disc, thus giving transfer rates of up to 1Gbyte a second.

This method requires next-generation laser and CMOS technology with the ability to process images many times more quickly than is possible at present. When the industry overcomes this technological hurdle, the road will be clear for holographic storage to compete in data-transfer rates as well as in capacity and integrity.

David Waldman, chief scientist at holographic data storage systems supplier Aprilis, explains the obstacles. "There are hurdles to overcome in form, factor and manufacturing capability," he says.

"Holographic storage needs a semi-conducting laser, for which there is not yet mass-manufacturing capability. It also needs encoding and reading devices that work at a speed of thousands of frames per second, both of which are far higher in performance than those found in existing digital cameras."

Despite the present limitations of the technology, there is no doubt that InPhase's forthcoming product release is pioneering. Initial shipments of the drive and discs will be sent to beta customers in the film and broadcast-media sectors, as well as to various electronics manufacturers.

The product being launched by InPhase is the Tapestry 300R drive and media, which has a capacity of 300Gbytes and a transfer rate of 20Mbytes a second. The first-generation product is write once, read many, and a single drive will cost about £9,000. One disc will cost about £90. By way of comparison, an enterprise-class tape drive costs about £15,000 and a mid-range tape drive costs about £2,000.

InPhase expects to develop and release three generations of drives and media. From the initial specifications outlined above, it hopes to move to a second-generation device in 18 months to two years, which will provide 800Gbytes of capacity and a transfer rate of 80Mbytes a second. A third system with 1.6Tbytes of capacity and a transfer rate of 120Mbytes a second should be available in three to four years.

Bill Wilson, chief scientist at InPhase, says, "What we are launching is the final wave of the first generation of product development. The decision we took was to take the simplest type of holographic technology and try to make a very rugged drive product using it."

He adds, "It is an enterprise model aimed primarily at the data and video archiving space, such as broadcast companies interested in large capacities, a high transfer rate and random access.

"Right now they often use tape, but that is a medium best suited to catastrophic recovery, not dynamic use. They want something that is random access and an archiving format so that they do not have to manage it."

The InPhase product is potentially well suited for the archiving requirements of media businesses, whose requirements are quite unique. These companies have to store vast amounts of image data, require high levels of data integrity and longevity, and need data-transfer rates similar to video transfer speeds. InPhase believes that the current generation of holography is compatible with these requirements.

Greg Schultz, founder and senior analyst at analyst firm Storage IO, says, "Currently, the cost comparison between holographic storage and existing technologies is not very good, at least for the broad commercial market.

"However, for extreme cases where large amounts of data are retained for long periods, such as news or movie archives and other data vaults, the cost and future capability warrant early trials and perhaps some early adoption to address very specific scenarios. In other words, there is a pain point for a certain type of user performing a particular task using traditional media such that they are happy to pay the price."

But use by media businesses is quite a limited application of holographic storage. What of the future? Before holography can have a truly widespread appeal, significant technological hurdles must be overcome.

Although industry analysts seem to be convinced that holography will be the next leap forward in data storage, there remain two or three generations of Blu-ray and HD-DVD to be worked through. And given the investment the major electronics firms have put into these storage technologies, they will not be too keen to see them superseded anytime soon.

Even so, most industry analysts expect holographic storage to emerge as a commercially viable product for business users in about three years' time.

Wolfgang Schlichting, research director at analyst firm IDC, says, "We forecast steady growth and expect that it will have a chance to succeed existing media about 2010 and appeal to the high-end business user.

"Surface-based recording media are coming to an end with Blu-ray, and holographic storage is one of the more plausible follow-on technologies. All the major drive manufacturers are working on it, but are keeping it secret as they want to see out the next generations of Blu-ray."

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Holography to enable terabyte disc capacity >>

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