Naoyasu Miyagawa

Land and Groove Recording for High Track Density on Phase-Change Optical Disks

We have proposed a high-track-density optical recording method, which records signals on both the land area and groove area. From a computer simulation based on the scalar diffraction theory, we found that crosstalk between recording tracks depends on the groove depth, and is most reduced at the groove depth of 80 nm~90 nm for a laser wavelength of 780 nm, a numerical aperture of 0.55 and a track pitch of 0.8 µm. Applying this method to a phase-change optical disk, we experimentally confirmed that crosstalk depends on the groove depth and obtained a crosstalk level of less than -32 dB for a groove depth of 64 nm to 78 nm and 0.9 µm length marks. In order to prove that this recording method can be used for rewritable media, we also measured change of carrier-to-noise ratio (CNR) when signals were repeatedly recorded on an adjacent track. The reduction of CNR after 1000 repetitions of writing was less than 2 dB for a linear velocity of 5 m/s.

Phase-Change Optical Disk Having a Nitride Interface Layer

A thin nitride layer formed at the interface of a Ge–Sb–Te recording layer and a ZnS–SiO2 protective layer successfully suppresses the phenomenon that reflectivity or signal amplitude becomes markedly small due to repeated overwrites. Based on secondary ion mass spectrometry (SIMS) observations, the 5-nm-thick interface layer was found to restrain sulfur atoms in the ZnS–SiO2 layer from diffusing into the Ge–Sb–Te layer and from changing the optical characteristics of the layer. Among several nitride materials, germanium nitride (Ge–N) sputtered film is found to have the most suitable properties as an interface layer: high barrier effect and good adhesiveness with Ge–Sb–Te and ZnS–SiO2 layers. The optical disk having the Ge–N interface layer achieves more than 5×105 cycles of overwrites with almost no changes in signal amplitude, reflectivity and jitter based on DVD-RAM specifications. The disk shows no degradation such as cracking, peeling, and corrosion after exposure to accelerated environmental conditions of 90°C and 80% RH for 200 h.

High-Transmittance Phase-Change Media with TiO2 Film for Rewritable Dual-Layer Blu-ray Discs

We have developed a semitransparent layer structure that is ideally suited to the laser-incident-side (front-side) recording layer of a rewritable dual-layer Blu-ray Disc (BD-RE). A TiO2 top film and a memory film of GeTe–Sb2Te3 characterize this layer structure. The TiO2 film has a large refractive index, which works to increase the transmittance of the entire recording layer to more than 50%. It simultaneously increases the reflectivity contrast, defined by (Rcry-Ramo)/(Rcry+Ramo), where Rcry and Ramo (Rcry>Ramo) are the reflectivities of the optical disc in the crystalline and amorphous parts, respectively. The GeTe–Sb2Te3 memory film has optical constants that are appropriate for balancing the transmittance between the crystalline part (Tcry) and the amorphous part (Tamo) in the laser wavelength region of approximately 405 nm. Our experimental dual-layer disc shows high transmittances of Tcry=51.6% and Tamo=52.0% and a high reflectivity contrast of 0.71 in the laser-incident-side layer. Dynamic examination showed that both recording layers have a good jitter value, satisfying the BD-RE specifications; more specifically, the laser-incident-side layer jitter value is 7.4% (spec.: 8.5%) and the back-side layer jitter value is 6.4% (spec.: 6.5%) through the front-side layer.

Over-500-Mbps Data Recording on Write-Once Media with L-Shaped Write Strategy

We have confirmed the necessary conditions to achieve data recording at a channel bit rate of over 500 Mbps. We discuss the approaches and experiments adopted to realize such high-speed data recording by way of present available techniques. In order to clarify the necessary conditions for them, we have investigated the laser response speed required for recording, optimum write strategy and suitable media for high-speed mark formation. As a result of these studies, the over-500-Mbps data recording was realized by applying a new L-shaped write strategy with a laser pulse of 0.7 ns rising time to Te–O–Pd write-once media of the Blu-ray Disc format.

High-density recording on phase-change optical disks

Current status of high density recording on phase change optical disks is summarized. More than 4.1 Mbits/mm2 density of direct overwriting on phase change disks was achieved using red (680 nm) high power (35mW) LDs, large NA (0.6) optics, thin (t0.6 mm) substrates, mark-edge recording with write compensation, and land-groove recording technologies.

Rewritable Dual-Layer Phase-Change Optical Disk with a Balanced Transmittance Structure.

A rewritable dual-layer phase-change optical disk with a balanced transmittance structure for the layer located at the laser beam incident side (Layer 0) was developed. In this disk structure, transmittance of Layer 0 is almost constant, whether the layer is recorded or not. This structure was realized by adopting Ge–Sb–Te film that has appropriate optical constants, and optimizing the thickness of the dielectric layers. It is proved that this disk structure is effective in suppressing the influence of the recording state of Layer 0 on the layer beneath (Layer 1). Practical performances of the disk with this structure were confirmed for both layers. The feasibility of up to 55 GB capacity was examined for this rewritable dual-layer phase-change optical disk.

Phase Change Optical Disk Using Land and Groove Method Applicable to Proposed Super Density Rewritable Disc Specifications

We have developed high-recording-density phase change optical disks applicable to the proposed Super Density Rewritable Disc (SD-RAM) specifications using the land and groove method. From experiments we confirmed that GeSbTe phase change optical disks employing this recording method have the following tolerances ; ±12/±9 mrad for radial tilt during reading/writing ; ±11/±8 mrad for tangential tilt during reading/writing ; more than ±0.07 μm for offtrack free from cross track erase.

33.4 Gbyte/Layer Recording with Adaptive Write Strategy for 100 Gbyte Rewritable Triple-Layer Disc

For multilayer rewritable optical discs, we developed a new adaptive write strategy named the S-22 (sequential 2T–2T) write pulse control method, which controls the laser pulse of the 2T-mark according to the combination of both the preceding and succeeding space lengths. By applying this write strategy, a rewritable triple-layer disc based on Blu-ray disc optical systems with a capacity of 100 Gbyte was realized. In particular, the advantage of this write strategy is significant for a transparent information layer. The bit shift of sequential 2T–2T patterns, which is a dominant error in 100 Gbyte rewritable discs, was well compensated. We experimentally confirmed that adopting the S-22 write pulse control method is effective to improve symbol error rate.

One-Path Calibration Method of Write Strategy using Simultaneous Linear Equations

Write strategy optimization is desirable for expanding system margins. However, this process is very complex and time-consuming. We propose one-path calibration method of write strategy using simultaneous linear equations for Blu-ray disc. In this method, write pulse compensation on mark deviation is expressed clearly by two simple linear equations. Experimental results using these linear equations show that the write strategy is optimized after only one-path test recording. Thus, with the new method, it is possible to reduce the number of test recordings and to save test recording areas.

Ten-Times-Speed Recording on Dual Layer Blu-ray Disc Recordable Media

To clarify the potential of our 1×–4× speed dual layer Blu-ray disc recordable media (BD-R) in higher speed recordings, we evaluated the recording performance of each layer on over 4× speed recording. The approaches are simulations and experimentations under the condition of 2T mark recordings at 4×, 6×, and 10× speeds. As a result, we have successfully improved the speed dependence of dual layer BD-R by the write strategy. It is confirmed that the recording power ratio Ps/Pw, where Ps is the space power and Pw is the peak write power, is one of the key parameters for achieving high-speed recording. It is expected that our dual layer BD-R could cover 1×–10× speeds.