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High-density multilayer optical disc stor age was investigated by em ploying advanced signal pro cessing methods such as adaptive equalization and adaptive partial-response maximum-likelihood (PRML) detection. Preliminary experiments, recording on single-layer and dual-layer Blu-ray discs (BD), indicate storage capability of 33.3 GB/layer with adaptive 5-tap PRML. A new signal qualification method, sequence d amplitude margin quasi-error estimation (SAMQES), was introduced for high-density multilayer recording, and possibility toward a total storage capacity of 200 GB was examined by using a six-layer BD-Recordable disc. Despite coherent interlayer crosstalk and lower signal-to-noise ratio, SAMQES showed moderate increase from those in single-layer recording. Keywords : Blu-ray disc, high-density recording, multilayer, ad aptive PRML, interlayer crosstalk, SAMQES, 200 GB 1. INTRODUCTION Blu-ray disc (BD) systems have already in corporated dual-layer disc structures to increase their storage capacity up to 50 GB for both read-only and recordable applications.

We demonstrated near-field recording with the combination of a GaN laser diode and a 1.5 NA objective lens. The realized linear bit density was less than 90 nm (corresponding to the areal recording density over 40 Gbit/in/sup 2/) regardless of using a small 1.0 mm-diameter super-hemispherical solid immersion lens (SIL). Further improvement of a near-field phase-change optical disk will enable a linear bit density of 80 nm or less. In this talk we shall also report the fabrication process of the plateau, and of the electrode on the SIL.

Recently, solid immersion lenses (SILs) have yielded a great interest in near-field optical data storage. However, convincing data on areal recording density have yet to be reported. The difficulty is attributed to the fact that an air-bearing slider needs to be kept optically contacted and also that a tiny SIL is vulnerable to aberrations caused by an air gap between a lens and a disk. The air-bearing system cannot easily achieve the condition above because its linear velocity changes the air gap. We have developed a new SIL device mounted on an actuator, which allows the air gap to be varied arbitrarily and enables optical contact independent of disk rotation. The obtained eyepattern of (l,7)-coded data on a phase-change disk demonstrates near-field recording over 1.2- numerical-aperture (NA).

Both over 20 GB of capacity and higher data transfer rate more than 25 Mbps are required for the optical data storage in the era of digital broadcasting to record high-definition digital video stream.

Using a two-element objective lens, over 0.9 NA lens and long WD lens are demonstrated. Using a thin cover layer, problems for skew margin and thickness tolerance are solved. Using a red LD, a 0.85 NA lens and a 0.1 mm cover PC disk, 8 GB rewritable disk system has been developed. Using a blue-green LD and an SHG blue laser, promising results are demonstrated for 12 GB and 17 GB of capacity.

A solid immersion lens (SIL) attached to a conventional objective could multiply the effective numerical aperture of an optical pick-up by a factor of refractive index n. A truncated hemispherical SIL (n equals 1.5) mounted on a slider, which was designed to follow the movement of the objective, kept good contact between the SIL and a disk substrate with a slight amount of air gap. Our simple configuration enabled the use of this method in an unsealed environment and extended the spatial cut-off frequency by 1.5 times. In addition, the combination with a second-harmonic generation green laser yielded high areal density optical data storage, six times that of current CD systems.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Some time ago we described a new type of near-field imaging system using the solid immersion lens. In this paper we shall discuss how this type of lens can be employed in optical storage systems to increase the bit density, along with the research that has been carried out at Stanford in cooperation with Sony Corporation and IBM, Almaden to reach this goal. 12 ()j aimis to work with both CD ROMs and magneto-optical storage.