Keikichi Andoo

Analyses of signals from dual-layer phase change optical disks

The signals obtained from dual-layer optical disks were analyzed experimentally and theoretically with emphasis on reducing layer cross talk (LCT) and spherical aberration (SA). Using the dual-layer phase change optical disk, the DC element of the LCT was measured and phase change marks were written on both layers. Diffraction calculations were used to explain the experimental results and to determine the amount of LCT and signal degradation caused by SA as functions of spacer layer thickness and optical detector size. Analytical methods to estimate LCT and SA were proposed, and their predictions turned out to agree well with the experimental and diffraction calculation results. These findings indicate that for a blue laser, the conventional techniques cannot simultaneously keep the LCT and SA to within practical values. Accordingly, designs for disks and drives for dual-layer optical disks are discussed.

High performance phase change media for DVD-RAM

A rewritable digital versatile disk (DVD-RAM) has eight times more capacity than a CD-ROM disk, which makes it promising for use as a computer file memory device. Media compatibility is realized because a DVD-RAM uses the same write/read wavelength as the DVD-ROM, the same substrate thickness (the DVD-ROM disk uses a thin 0.6 mm-thick substrate so it can use a focusing lens with a large NA), and the same error correction method. The DVD-RAM uses phase-change recording media, so that rewriting by overwriting is easy, and reading and writing can be done with an optical head similar to that used for DVD-ROM. The authors demonstrate that 10/sup 5/ overwrites in mark edge recording and long storage life can be achieved on a new phase-change disk that has a double-reflective-layer structure and a recording film that contains a high-melting-point component.

Application of Domain Transfer to Magneto-optical Recording: Domain Stabilization during Readout

The domain transfer phenomenon has been applied to light intensity modulated overwrite in order to obtain large power margin. By applying a suitable magnetic field to the multilayer before readout, recorded domains on the memory layer are transferred onto the writing layer. The application of domain transfer before readout improved the read power margin. However, an increase in noise was observed after irradiation of high read power. The reason for the noise increase was found to be an incomplete domain transfer. The incompleteness was successfully explained by introducing a domain nucleation model.

OVERWRITE REPEATABILITY OF MAGNETO-OPTICAL EXCHANGE-COUPLED MULTILAYER

Overwrite repeatability of exchange‐coupled multilayer for light‐intensity‐modulated overwrite (OW) was studied by using Arrhenius analysis. A computer simulation showed that typical OW disks and a conventional nonoverwrite (NOW) disk have similar maximum temperature during recording. Nevertheless, the overwrite repeatability of the OW disks was smaller than that of the NOW disk. The degradation mode characteristic to the OW disks was found to have lower activation energy than the conventional NOW single‐layer disk. The degradation mode was caused by initialization noise resulting from domain‐wall fluctuations under a large initializing field, and was reduced by using a transition‐metal dominant memory layer with high‐domain‐wall coercivity. An overwrite repeatability of more than 106 was achieved.

REVERSIBLE PHASE-CHANGE OPTICAL RECORDING BY USING MICROCELLULAR GeSbTeCo RECORDING FILM

A microcellular recording film is proposed to prevent material flow due to multiple overwriting in phase-change optical disks. The GeSbTeCo recording film, divided into cells of approximately 20-30 nm in diameter, is prepared by sputter-etching using a Ti polycrystalline film as a mask. Reversible phase-change occurs in the microcellular GeSbTeCo film. Moreover, single beam overwriting is achieved at a linear velocity of 7 m/s.

Progress of Phase-Change Single-Beam Overwrite Technology

Studies on relationships between single beam overwrite characteristics and physical and chemical properties of the phase change recording media and their mechanisms are reviewed. Improvements in overwrite characteristics based on these studies, especially improvements in carrier to noise ratio(C/N) and the life of recorded dots by the addition of one of the transition metal elements or Tt are also reported. The maximum C/N was 61dB and the estimated life of the recorded dots is longer than 100 years at 60°C.

Analysis of Reproduced Waveform in Phase-Change Single-Beam Overwrite

Reproduced wave forms for the mark-center-detection and the mark-edge-detection recording methods are analysed for a phase-change optical disk with a Ge–Sb–Te–Co recording film which certifies a high C/N and single-beam overwrites of more than 105 times. The result showing that the disk has high-density recording capability. Signal demodulation for the mark-edge-detection recording method is possible at a small error-generation probability when the recording laser power is in the range of 60% to 140% of the most preferable power level and the erase ratio is greater than 25 dB.