Team makes optical data storage breakthrough

Sharp Corp., TDK Corp. and a Japanese government agency have developed a low-cost technology that, researchers say, allows a 12-centimeter optical disk to hold 25 times more data than a 4.7G-byte DVD disk.

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

In optical disc systems, data is stored as a series of dots on a recording surface using a red laser. Currently, laser beams are discharged directly onto 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 laser beam.

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 nanometers, using a common red laser beam. The same type of laser, if directly discharged onto the disc, would create a 400-nanometer 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 laser beam without changing the laser’s wavelength.

A film layer, which creates heat by absorbing the laser beam, is coated onto the base of the disk. A registering layer, which chemically reacts to the heat and creates patterns from the heat on its surface, is coated on top of that.

When a red laser beam 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 disk 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 125G bytes on a 12-centimeter disk. Theoretically, they say, the technology should allow 12-centimeter disks to hold up to 200G bytes of data and, if used with blue laser beams, could allow even more data to be stored, Tominaga said.

“Commercialization is possible within four years, as long as the market demands it,” said Tominaga.

Besides the technology developed by the Japanese research team, another technology also holds out promise for putting more data on optical discs. Other reaserch teams are developing technologies using electron beams and a procedure called electron beam recorder (EBR) for use with next-generation recording discs. Until the development of the technology based on red lasers, EBR was the only way to register dots with size of less than 100 nanometers, said Tominaga. However, EBR is more complicated and requires new facilities and larger equipment, according to the AIST.