TDK Corp developed a write-once optical disc with a capacity of 320 Gbytes. It has ten layers, and each layer has a storage capacity of 32 Gbytes.
The new optical disc has a larger capacity than the Blu-ray disc, which has a capacity of 25 Gbytes per layer. And it is possible to write and read data on and from the disc by using a blue-violet semiconductor laser with an oscillation wavelength of 405nm and an objective lens with a numerical aperture (NA) of 0.85, which are currently used for the Blu-ray disc.
In 2006, TDK prototyped a write-once recording medium having six layers, each of which has a capacity of 33.3 Gbytes (200 Gbytes in total).
As the number of recording layers increases, signals on each layer weaken, making it necessary to improve the transmittance. This time, the company enhanced the transmittance by readjusting the composition of used materials, it said. The transmittance of the outermost layer, which requires the highest transmittance, is more than 90%, while that of the disc prototyped in 2006 was 80 to 90%.
The structure of the new disc is as follows. A cover layer and a hard coat layer, which protects the surface from scratches and dirt, are formed on recording layers (the innermost L0 layer to the outermost L9 layer). A Si-Cu alloy layer, which is inorganic, is used for the L0 layer.
On the L1 to L9 layers, recording marks are maintained by using an inorganic material composed of bismuth peroxide and germanium dioxide. By adjusting the density of germanium dioxide, it is possible to change the transmittance of a recording layer. Also, TDK adjusted the optical properties and accumulated materials that can mitigate damage caused by heat.
The symbol error rate of the new optical disc is less than 10-4, which the company considers as the threshold of commercial viability. When data is being written on the disc, the output of the laser is less than 30mW, which is the maximum output of commercially available blue-violet lasers, on a film surface of each layer, TDK said.
The company plans to exhibit the disc at CEATEC JAPAN 2009, which will run from Oct 6 to 10, 2009, at Makuhari Messe in Chiba Prefecture, Japan.