Phase change optical disks having "Thermally balanced structure" have the so-called slow-cooling structure with low thermal diffusing efficiency from the reason on optical designing. We tried to improve overwrite cycling characteristics of them by replace a part of ZnS-SiO_2 layer sandwiching Ge-Sb-Te recording layer by SiO_2 layer(40nm)having smaller thermal expansion coefficient. The test disk having 6 layers shows about 10 times larger cycle number as compared with the conventional disk of 4-layers type. The cycle number reaches to half million (0.65um/bit) with wide power tolerance of >20% (λ : 680nm, N.A. : 0.60) at 12m/s of liner velocity.

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We have developed phase change optical disk with a Si reflective layer suitable for both mark edge recording under high linear velocities and land/groove recording with red Laser. Optical enhancement layer on a Si reflective layer not only improves C/N ratio but makes optical properties appropriate for land/groove recording. Minimum bit length of 0.33 μm and 0.6 μm track pitch have been achieved without influence of cross-erase.

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We have fablicated an optical video disk recorder using a φ120mm phase-change disk with silicone refrection layer. The structure of disk was optimized for land-groove recording, therefore we have attained 10.4 GB storage data capacity on both disk sides. A small beam spot less than 1 μm of an optical head, and land-groove progressive recording have been adopted. Furthermore, (1,7)RLL code and PRML reprodusing were applied, 3.2Gb/in^2 of the areal recording density has been achieved. Combining a high performance video codec at 25Mbps with disk recoder, high quality video signal of 40 minutes on both disk sldes has been attained.

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AlGaInP laser diodes with a shorter wavelength, a low threshold current, a high maximum operating temperature and low noise are strongly desired as light sources for high-density optical disc systems. A 630-nm band AlGaInP red laser diode, which fulfills these demands, was realized, for the first time, by introducing strain-compensated MQW active layer with compressively strained quantum barriers and tenslle strained quantum wells. The following characteristics have been obtained : a threshold current of 33mA in the single mode laser diode, and a threshold current of 48 mA and relative intensity noise (RIN) below 7×10^<-14>Hz^<-1>in the sellf-pulsating laser diode.

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Characteristic of Mark Edge recording, of which minimum mark length was 0.53μm, on rewritable phase change optical disk including the recording layer of the four component alloy system has been investigated. Leading jitter and trailing jitter are below 10% of window width after 10^4 overwrite cycles. Characteristic of Land and Groove recording, of which track pitch was 0.7μm, on it has been investigated. Jitters are below 10% of window width after 10^4 overwrite cycles on adjacent tracks, because of little influence of cross-talk and cross-erase.

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Single-Spiral Land/Groove(SS-L/G) recording method, which is suitable disc format for rewritable type of digital video disc, was introduced. Inherent problems of SS-L/G that is applied for ZCLV format, such as reliable detection of connecting points of land tracks and groove tracks and settling latency of disc rotation during crossing the border area of zones, were investigated. Combination of L/G pointer information in preformatted ID, deviation of ID pits detected by tracking error signal and sophisticated zone mark pattern realizes the reliable detection of L/G switching points. Transition areas defined in the outer rim of each zone also enables completely continuous read/write operation that is required for long time and real-time video recording, which is the most attractive feature of SS-L/G disc format.

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We have experimentally investigated write-once optical data storage media using metal island films. From the measurements of optical characteristic changes, we show that silver and copper island films are the most efficient media among gold, silver, copper, and aluminum. We also demonstrate laser writing with 820 nm-laser diode. It seems that the metal island films are one of the most potential media for CD-compatible write-once optical disks.

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