TDK showcased an optical disc with a capacity of 1 Tbyte at Ceatec Japan 2010 trade show.The disc is formed by 16 recording layers, each of which has a capacity of 32 GB.
The problem with multilayer optical discs is the difficulty to obtain clear signals from each recording layer in a stable manner due to crosstalk from adjacent layers, transmission loss and spherical aberration.
Each layer should be seperated from the rest using reflective materials, which would not allow optical laser to reach to innermost layers, but also dielectric materials, which would minimize the transmittance loss when the laser has to reach specific layers.
TDK has developed a dielectric material as the reflection film that offers a high light transmittance in order incident light to efficiently reach as far as innermost layer and produce a clear eye pattern (read-out signal).
The result is impressive: While the light transmittance of a single layer is 95.1%, it becomes just 72.6% for the total 16 layers of the disc. The specific material has been already used in the currently available Blu-ray discs, according to TDK.
In addition, data on the disc can be accessed using a laser diode with a a numerical aperture of 0.85, which is also used for the Blu-ray Disc.
According to measurements made by TDK, the symbol error rate for reading out data stored on the disc ranges from 1 x 10-5 to 1 x 10-4, which is acceptable and low enough for commercialization of the disc.
However, the disc is thicker than what the specifications for the Blu-ray discs demand (<100μm). The recording layer of the new disc is 260μm, increasing the influence of spherical aberration. This means that althpugh the disc could be read using a BD drive pickup, an improved objective lens and a wide range compensator would be requierd in order to effectively decrease spherical aberration.
TDK has not provided information on how it managed to reduce the interlayer crosstalk. Hovever it is assumed that it stacked two different thickness of layers alternatively.
TDK expects that the new optical disc will be used as a consumer recording medium, a backup medium and a medium for a broadcasting service. However, its commercialization depends on disc makers.