Hideki Kitaura

Rewritable Dual-Layer Phase-Change Optical Disk Utilizing a Blue-Violet Laser

We have demonstrated for the first time the feasibility of using a rewritable dual-layer phase-change optical disk utilizing a blue-violet laser. For the first medium, we adopted a very thin recording layer with a new phase-change material Ge–Sn–Sb–Te, and a 10-nm-thick silver-alloy reflective layer to obtain a large transmittance and high-quality signals. For the second medium, we optimized the thickness of each layer to obtain both a large optical absorption of the recording layer and a small heat capacity. Carrier-to-noise ratios of more than 50 dB, erasability of more than 30 dB and recording powers of 8 mW for the first medium and 11 mW for the second medium were obtained under typical recording conditions corresponding to a capacity of 27 GB per one side of a 120 mm disk and a user data transfer rate of 33 Mbps.

Phase-change material for use in rewritable dual-layer optical disk

A thin film of Sn-doped and GeTe-rich GeTe-Sb2Te3 shows characteristics that make it suitable for use in rewritable dual-layer optical disks employing a violet laser. By increasing the GeTe component form Ge2Sb2Te5 to Ge4Sb2Te7, and Ge8Sb2Te11, optical changes were increased. By substituting Sn for a proposition of Ge in these compositions, crystallization rates are greatly increased and even a 5 nm-thick film showed a very short laser-crystallization time of less than 50 ns. The material film was successfully applied to Layer 0 of rewritable dual-layer disk: capacity of 27 GB and a 33 Mbps data transfer rate were confirmed for a disk using a conventional 0.6 mm substrate, and 45 GB capacity and the same data transfer rate were obtained for another disk using thin cover layer 0.1 mm thick.

Dual-Layer Optical Disk with Te–O–Pd Phase-Change Film

A recordable dual-layer optical disk that uses a Te–O–Pd phase-change film was studied for high-density recording. In order to reduce thermal influence to adjacent tracks, we selected the optimum composition of Te42O46Pd12 that gives low thermal conductivity. To get equivalent signal amplitude and absorbance of layers, the thicknesses of the laser incident side recording layer (Layer 1) and the other layer (Layer 2) are designed to be 20 nm and 55 nm, respectively. We confirmed that there was less than 10% bottom jitter for both layers under the following recording densities: recording bit length of 0.41 µm and track pitch of 0.74 µm. The laser powers required for recording on Layer 1 and Layer 2 were typically 10 mW and 12 mW, respectively. The jitter was less than 13% for both layers with large power tolerance of 30%p-p.

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.

Multilayer write-once media with Te-O-Pd films utilizing violet laser

Multi-layer write-once optical disks with tellurium suboxide palladium-doped phase-change recording films were designed and adopted to read/write utilizing a violet laser. A dual- layer medium showed carrier to noise ratios (CNRs) of more than 50 dB within a laser power of 7 mW at a linear velocity of 5 m/s in each the condition of numerical aperture (NA) equals 0.65 and NA equals 0.85, which corresponds to a recording capacity of 27 GB and 45 GB on a dual-layer disk of (phi) 120 mm in diameter, respectively. Furthermore, a quadruple-layer medium showed CNRs of more than 48 dB within a laser power of 12 mW at a linear velocity of 5 m/s in the condition of NA equals 0.85, which corresponds to a recording capacity of 90 GB on a quadruple-layer disk of (phi) 120 mm.

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.

Dual-layer write-once media for 1x-4x speed recording based on Blu-ray Disc format

We have developed dual-layer write-once media with Te-O-Pd based recording films on Blu-ray (BD) format. Recording capacity was 50GB with dual layers on a disk of 120mm in diameter. Rear and Front layers showed jitters of 5.8% and 7.7% at 1x speed, and 6.0% and 8.0% at 2x speed, respectively, which were good enough to satisfy the BD format. Evaluations were carried out with blue-violet laser of 405nm wavelength, objective lens NA of 0.85. Recording linear velocities were 4.92m/s at BD 1x (36Mbps), and 9.84m/s at BD 2x (72Mbps). Characteristics at 4x speed recording were also examined, and it was revealed that carrier to niose ratio at high recording linear velocity of 19.7m/s, which corresponds to BD 4x (144Mbps), was alomst as same as those of 1x and 2x. Recording mechanism was discussed and proposed a model that Te-O-Pd films were not crystallized directly through solid process, but crystallized through melting.

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.

Over 500 Years Lifetime Dual-Layer Blu-Ray Disc Recordable based on Te-O-Pd Recording Material

50GB BD-R media with Te-O-Pd recording material was estimated to have a life expectancy of over 500 years according to acceleration test under stressed conditions. This stable recording material will reduce the possibility to lose important data even in sever storage environment.

50 GB-capacity dual-layer Blu-ray Disc with GeSbTe based phase-change films

GeSbTe-based thin films showed superior properties for a recording layer of Blu-ray Disc, especially for a front-side recording layer of L1. The film can be thinned to 6 nm with maintaining a rapid phase-transition rate and a large transmittance of >50% was realized in the L1. It is remarkable that transmittances are balanced between in the amorphous and crystalline areas of L1, and it works to remove the optical interference when recording-playing on the rear-side recording layer of L0 through the L1.