TDK Corp. has prototyped a recordable Blu-ray Disc medium with its capacity increased to 200 GB by stacking six recording layers. The new medium boasts the capacity approximately eight times that of the existing single-layer Blu-ray Disc medium.

TDK prototyped a 100 GB recordable medium having four 25 GB recording layers stacked on one another in 2003. This time, the company employed a different signal processing scheme to increase the capacity per layer to 33.3 GB, while increasing the number of the recording layers to six through the adoption of a novel recording material.

The bit error rate at 36 Mbps recording/playback fell within the range between 5.0 x 10-5 and 2.3 x 10-6 depending on the layer.

The prototyped medium has a layered structure in which the recording layers are formed from the bottom L0 layer to top L5 layer, and a 0.1 mm cover layer and a hard coat layer to protect the medium from scratches and dirt are stacked on these recording layers. As for the L0 layer, an inorganic material, i.e. a commercialized Si-Cu alloy layer is used. For the L1 to L5 layers, a novel inorganic material consisting of bismuth peroxide and germanium oxide is employed. When bismuth peroxide is irradiated with laser light, oxygen is deposited to form air bubbles when the temperature reaches 690 K. The presence or absence of these air bubbles is detected based on the difference in reflectance of the laser light.

Germanium oxide has a property that it can freely change the transmittance of the recording layers in the range between 73 and 88% by varying its concentration. Thus, the transmittance of the recording layers--a key factor in stacking technology--can be precisely set, which has led to the realization of six layers.

Naoki Asakawa, Nikkei Electronics