Sharp, TDK and a Japanese government agency have developed a low-cost technology that, researchers say, allows a 12cm optical disc to hold 25 times more data than a 4.7GB DVD disc.

The National Institute of Advanced Industrial Science and Technology (AIST), working with research and development centres at the two companies, developed the technology which uses a red laser beam similar to those currently used for making optical discs.

In optical disc systems, data is stored as a series of dots on a recording surface using a red laser. Currently, laserbeams are discharged directly on to the disc and the size of each dot is determined by the type of laser used. This is because the size of the dots is proportional to the wavelength of the laserbeam.

If engineers manage to make this dot size smaller, they can put more dots on the disc and it can hold more data.

Until now, engineers needed to use shorter-wavelength lasers, such as blue lasers, to get smaller dots. However, this requires new optical materials and more expensive lasers that are still under development.

The Japanese team has succeeded in registering a dot size as small as 85 nanometres, using a common red laserbeam. The same type of laser, if directly discharged on to the disc, would create a 400 nanometre dot, according to Junji Tominaga, a chief developer for AIST's development team.

Engineers from AIST, Sharp and TDK managed to narrow the size of laser spot by using heat generated by the laserbeam without changing the laser's wavelength.

A film layer, which creates heat by absorbing the laserbeam, coats the base of the disc. A registering layer, which chemically reacts to the heat and creates patterns from the heat on its surface, is laid on top of that.

When a red laserbeam is aimed at a spinning optical disc, the film layer absorbs the beam and creates heat. By carefully controlling the temperature at which the registering layer reacts, the engineers were able to record smaller dots on the disc. This control is achieved by manipulating the speed of disc revolutions and the thickness of the film layer.

Engineers hope to go one step further. So far, they have demonstrated that the technology can store up to 125GB on a 12cm disc. Theoretically, they say, the technology should allow 12cm discs to hold up to 200GB of data and, if used with blue laserbeams, could allow even more data to be stored, Tominaga said.