Yoshihisa Fukushima

Single-beam overwrite optical disk memory using a phase-change medium

A single beam overwrite optical disk memory using a rewritable phase change medium has been developed for digital data storage. The overwrite function was realized by using the same simple optical head as that of write-once memory. An 86mm pregrooved disk has the large capacity of 280MB on both sides. A higher data transfer rate of 6Mbps was achieved. A split type optical head having a 9gr. moving portion realized 42ms seek time with low power consumption. Overwrite operation is done by making amorphous marks and crystallized (erased) area with single laser beam. As the read-out signal is obtained from the difference of their reflectivities in the same way as write-once media, the optical head has become simple and compact. The overwriting temperature of the active layer is much higher than the operating temperature. This optical disk memory can execute reliable write and read operations with a wide tolerance for the operating temperature. Rewritable media were evaluated under practical perating conditions and could be overwritten more than 100,000 cycles on the same physical sectors. Implementing effective defect management, the practical or logical cyclability of the optical disk memory was improved up to one million cycles.