A conventional DVD encodes 0s and 1s in the form of pits, which are read by a red laser. The pit sizes and red wavelengths limit the capacity of the disks to at most 4.7 gigabytes per layer (about eight hours of video for a four-layer disk). For DVD's next step, consortiums of electronics and computer companies have shifted the laser to blue, shortening the wavelength and thereby permitting smaller pits--and thus more pits--to be etched on the disk. Two competing formats are in play: Blu-ray (25 gigabytes of storage per layer) and HD-DVD (15 gigabytes). With multiple layers, both formats can hold a high-definition movie (about 20 to 25 gigabytes).
But future movies will exceed today's high-definition resolution, and such formats will require about four times more capacity. "There will be a lot of consumer pull for optical products of hundreds of gigabytes by 2010," says Tom Coughlin, a data storage consultant in Atascadero, Calif. Simply shifting to shorter (ultraviolet) wavelengths is challenging, however, because the clear, protective part of current disks is opaque to that kind of light.
Peter T¿r¿k and his colleagues at Imperial College London have figured out a way to squeeze in more data: have individual pits encode more than one bit. They have produced asymmetric pits that look different depending on their orientation, like a lopsided crater. Shining laser light onto the pit from different angles produces different reflections and therefore can register additional bits. Last fall T¿r¿k's group reported it could distinguish the reflections from pits with simple asymmetries, and it estimates that the added information could be enough to pack a terabyte (1,000 gigabytes) onto a four-layered DVD-size disk. A single disk could hold all the episodes of all the seasons of The Simpsons.
So far they have only demonstrated the technique, called multiplexed optical data storage (MODS), on a few pits, but T¿r¿k believes that a commercial system consisting of four-layer disks could be ready between 2010 and 2015. The hardest task would be reconfiguring the lens to focus the laser correctly onto the pits, he says. The group also figures manufacturers could construct MODS disks with conventional DVD processes at pennies a disk.
The MODS disks could pose a near-term threat to the most advanced alternative for mass-storage optical technology--holography. The technique relies on a laser to reconstruct large volumes of information by focusing on a holographic "page" encoded in a polymer. Stacking the pages can result in a capacity of hundreds to thousands of gigabytes. "Just about every major consumer electronics company in data storage today is working on holographic storage," explains Brian Lawrence, optical engineer in General Electric's global research department, which is studying one type of holographic system. Holographic drives could combine terabytes of capacity with rapid data transfers. Two companies--InPhase Technologies in Longmont, Colo., and Optware in Yokohama, Japan--plan to release the first holographic drives for professional backup storage and archiving next year.
Although MODS may have the advantage of being able to use existing technology in reading data, holographic technology could probably retrieve bits faster. And after 40 years of research, the cost and engineering challenges of holographic systems are well understood, whereas MODS lacks the body of work that would allow a good comparison, notes Wolfgang Schlichting of infotech analyst group IDC. "I would caution on getting excited too early about a new technology," he advises.