To confirm the feasibility of realizing subterabyte or terabyte optical memories, we evaluated a multilayer read-only disk having 20 information layers. We propose a new disk structure, which is an alternative structure to reduce interlayer crosstalk, optimized by computer simulations. We fabricated multilayer read-only optical disks using a conventional manufacturing process for Blu-ray disk (BD) dual-layer media. Moreover, we improved the reproduction system to compensate for the large spherical aberration. From the experimental results, we obtained sufficiently low jitter values reproduced from a 20-layer disk. We confirmed the feasibility of the multilayer read-only disk with a capacity of 500 Gbytes.

We realized an inorganic write-once disk using a Bi-Ge nitride alloy recording layer for an optical recording system for the Blu-ray disc format. We developed a recording system, which is composed of two types of metal nitride as a recording material. One metal nitride is decomposed easily by the heat of the laser beam during recording. The other metal nitride is not decomposed at that temperature. The recording signal modulation depends mainly on the reflectivity change of the recording layer caused by the decomposition of nitride. The non-decomposing nitride controlled the reaction and prevented deformation of the layer. Thus we could obtain a well-formed minute mark. We confirmed the reliability of our hypothesis on the recording mechanism by using another material, which is composed of Sn-N and Ti-N. By this method, we confirmed that this recording mechanism can be widely applied to any inorganic recording material.

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