System aspects of dual-layer phase-change recording with high-NA optics and blue laser

The ever-increasing demand for higher capacities driven by data intensive applications such as high-quality video has been met through steady increase of the areal bit density of the optical media. Very recently, the so-called Blu-ray Disc (BD) standard was announced, offering a capacity of up to 27 Gbytes per 12 cm disk. The BD format is designed as a groove-only format, which allows easier ROM, R and dual layer disk standard extension since no prepit headers are required. Another approach to increasing the effective storage capacity is quite unique for optical memories: multi-layer recording. Thin phase-change layers are one of the most promising recording materials (T. Akiyama et al, Jpn. J. Appl. Phys. vol. 40, pp. 1598, 2001; E.R. Meinders et al, Proc. SPIE vol. 4342, p. 64, 2001). However, due to high absorption, the number of recording layers is currently limited to two layers only. In this paper we discuss the required technologies to realize a dual-layer BD type system. This includes primarily the dual-layer phase-change disk technology, beam shaping to enhance the light efficiency of the optical head unit and spherical aberration control to compensate cover layer thickness variations.