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 into different layers of the disk, it is still currently limited to around 4.7 Gb per layer. Now Peter Török and colleagues at Imperial College in the UK and co-workers at the University of Neuchâtel in Switzerland have realized that by giving the pits an angular sub-structure they could hold at least ten times more data.

To do this, the physicists needed to find a way of reading this angle rapidly, so as not to compromise the optical drive's data rate. The solution was a combination of polarized light, a quadrant detector and light scattering analysis. The team has built a prototype using a 405-nm laser and 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," says Török. "The orientation is between 0 to 180°, but we can resolve 330 different orientations within the 0 to 180° angular range."

Funded as part of the EU's SLAM (Super Laser Array Memory) programme, the three-year project, which also involved scientists at Aristotle University of Thessaloniki in Greece, ended in May this year. Török believes that if his team can attract further funding, the first MODS disks, with a storage potential of around 250 Gb 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 25 Gb per layer, five times the capacity of current DVDs, are expected to be released towards the end of 2005 for the home market.