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Angular pits boost DVD storage

29 Sep 2004

European scientists develop an optical method for encoding up to one Terabyte (1000Gb) of data on to a single DVD.

An optical storage method that could squeeze one Terabyte (1000Gb) of data on to a DVD sized disk is being developed by researchers in Switzerland, Greece and the UK. Their angular multiplexing technology is back-compatible with existing DVDs and CDs, and could fit all ten series of the US TV sitcom Friends on a single disk.

DVDs store information in the form of simple, steep-sided pits each holding 1 bit of data. Although the storage capacity can be increased by writing the pits in to different layers of the disk, it is still currently limited to around 4.7Gb per layer.

Now however, Imperial College (UK) scientist Peter Török and his colleagues have given the concept of optical data storage a new twist. He realized that by giving the pits an angular sub-structure they could hold at least ten times more data.

For the idea to take-off the team had to find a way of reading this angle rapidly, so as not to compromise the optical drive's data rate. The solution turned out to be a combination of polarized light, a quadrant detector and some clever light scattering analysis, performed by Török's PhD student Peter Munro.

Working with Martin Salt, at the University of Neuchâtel's Institute of Microtechnology (Switzerland) and scientists from Aristotle University of Thessaloniki the team has built a prototype using a 405 nm laser. The scheme, dubbed Multiplexed Optical Data Storage (MODS), is now being patented.

Although based initially on straight edge features, it turns out that the system also works with other pit geometries. "They do not have to be steps as long as they have a suitable asymmetry," Török told Optics.org. "The orientation is between 0-180 deg, but we can resolve 330 different orientations within the 0-180 angular range."

Török believes that if his team can attract further funding, the first MODS disks, with a storage potential of around 250GB per layer, could be on the shelves between 2010 and 2015.

Despite having only a tenth of the storage capacity of MODS technology, it will be Sony's BluRay that is the first to challenge DVDs' domination of the audiovisual optical disk market. BluRay disks storing 25GB per layer, five times the capacity of current DVDs, are expected to be released towards the end of 2005 for the home market.

James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.

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