Masakazu Ogasawara

The Applications of a Liquid Crystal Panel for the 15 Gbyte Optical Disk Systems

We have studied the next-generation digital versatile disk (DVD) using a blue laser and investigated the application of a liquid crystal (LC) panel to the high-density disk. The LC panel can easily shift the optical path length of light beams making it possible to cancel out wavefront aberrations. First, the authors applied an LC panel to a tilt servo system. Compared with a mechanical tilt servo system, the LC tilt servo is expected to be more compact and reliable. We also developed a system for sensing the amount of disk tilt by comparing the signal levels of crosstalk from adjacent tracks. Consequently a tilt servo without any extra sensors was achieved. Another application of the LC panel is for the compensation of spherical aberrations. It was confirmed, by simulations and experiments, that the LC panel was practically useful for correcting disk thickness error which has been determined to become more serious when a blue laser was used.

Multilayer 500 Gbyte Optical Disk

To confirm the feasibility of realizing subterabyte or terabyte optical memories, we evaluated a multilayer read-only disk having 20 information layers. We propose a new disk structure, which is an alternative structure to reduce interlayer crosstalk, optimized by computer simulations. We fabricated multilayer read-only optical disks using a conventional manufacturing process for Blu-ray disk (BD) dual-layer media. Moreover, we improved the reproduction system to compensate for the large spherical aberration. From the experimental results, we obtained sufficiently low jitter values reproduced from a 20-layer disk. We confirmed the feasibility of the multilayer read-only disk with a capacity of 500 Gbytes.

Sixteen-Layer Write Once Disc with a Separated Guide Layer

We proposed a new recordable multilayer optical disc and its recording system. The new optical disc has multiple-plane-recordable layers and a separated guide layer, and we fabricated 16 recording layers made of high-transmittance inorganic recording materials stacked on the separated guide layer. The recorded signals were evaluated on all recording layers by using jitter and i-MLSE as the evaluation indicators. As a result, the technical feasibility of our proposal was confirmed to achieve the total capacities of 400 and 512 Gbytes, respectively, by the continuous recording of all 16 recording layers with a track pitch of 0.32 µm and two bit lengths. We believe that our multilayer optical disc and recording system are the most practical solutions for realizing huge-capacity data storage.

Signal Processing for 15/27 GB Read-Only Disk System

In order to realize 15/27 GB read-only disk systems using blue laser, new signal processing is required. The technology of large-scale integrated circuits has advanced considerably. Hence we can use a complicated circuit with a big gate size without increasing the system cost significantly when we fabricate the blue laser disk system. We have realized the 15/27 GB system using the two-dimensional equalizer and the limit equalizer. The two-dimensional equalizer was composed of the cross-talk cancel system and the asymmetry compensation type tangential adaptive equalizer. And could improve the deterioration due to the inter symbol interference and the cross-talk from the adjacent tracks. The limit equalizer could improve the deterioration due to the disk noise. Using these signal-processing systems, we could reduce the bottom jitter by more than 3% and expand the tilt margin.

New Coaxial Interference Method for Consumer Holographic Memory

We have developed a new coaxial interference method for the next-generation storage system. A signal beam and a reference beam are coaxially arranged and focused into a holographic medium using a bifocal lens. We can easily downsize the system because the optics is very simple. The reference beam is focused on the mirror surface of the medium, and the signal beam is defocused on this surface. A semicircular reference pattern is used to select normal playback or phase conjugate playback in a reproducing process. We have recorded a single hologram and a multiplexed hologram in a photopolymer medium with this method. These holograms are reconstructed successfully. We have found that our interference method has a 4 µm shift selectivity and excellent characteristics for the next-generation high-capacity optical memory.

Advanced Radial Position Control of a Recording Beam for Super Multilayer Disc with Separated Guide Layer

We have developed an advanced radial position control system for a super multilayer disc with a separated guide layer to realize stable recording on the plane recording layers located far from the guide layer. The developed system consists of an advanced sled servo system and a specially designed optics based on the advanced push–pull method. Recorded signals on all recording layers were evaluated by using integrated-maximum likelihood sequence error estimation (i-MLSE) as the evaluation indicator. As a result, we confirmed sufficient recorded signal qualities and feasibility at the quad-speed continuous recording with a constant track pitch of 0.32 µm, the same as that of Blu-ray disc (BD). We believe that our proposed concept is the most practical solution for the next-generation optical disc system and our control system is essential to realize stable multilayer recording with a constant track pitch.