Tatsunori Ide

Spectroscopic Ellipsometry Studies on Optical Constants of Ge2Sb2Te5 Used for Phase Change Optical Disks.

Spectroscopic ellipsometry has been applied to Ge2Sb2Te5, which is commonly used as a recording layer for phase change optical disks, and the optical constants have been measured for a wavelength range from 400 to 900 nm. We have improved the accuracy of this measurement by means of inserting a transparent film between a Ge2Sb2Te5 film and a Si substrate. A calculation of the reflectivity change, due to the phase transition between the amorphous and the crystalline states, shows that the change is sufficient for optical optimization of the disk structure, down to the 400 nm wavelength.

Jitter improvement in mark edge recording for phase change optical disks with optical phase encoding

We have developed a novel absorptivity control method for phase change optical disks for improving overwriting jitter in mark edge recording. In order to control absorptivity more easily, we have designed a disk with a small difference in reflectivity between the amorphous and the crystalline states, and with a large optical phase difference between the reflected light from those two states, which produces a high carrier to noise ratio. We have obtained remarkably reduced overwriting jitter, down to 2.47 ns from 5.69 ns for the leading edge, and this has meant greatly increased applicability of mark edge recording to phase change optical disks.

Optimization of Write Strategy in a Partial-Response-Maximum-Likelihood System for High-Density Recording

It is difficult to optimize the write strategy by conventional methods in high-density recording systems with very severe intersymbol interference. We have proposed a new method using the index accounting for the linearity of the channel, which is very important in the partial-response-maximum-likelihood detection scheme. It has been confirmed that the minimum bit error rate can be obtained when the linearity of the channel becomes maximum. We have shown the feasibility of a recording capacity of over 20 GB with 120-mm-diameter phase-change media using the blue-violet laser diode with an objective lens whose numerical aperture is 0.65.

Optimization of write strategy in a PRML system for high density recording

Recently, there has been much research on high density recording using an NA = 0.85 objective lens . The use of PRML (partial response-maximum-likelihood) is an efficient way to handle inter-symbol interference (ISI) that degrades the bit-error-rate at a very high recording density. We have shown that the recording capacity of about 32 GB is achieved with the use of PR channel with a constraint length of 5, i.e., PR (12221) and Viterbi detection in the NA = 0.85 system. This paper describes the method for optimizing a write strategy in the PRML system. We have confirmed that the write strategy can be optimized in terms of a linearity of the channel. It has been found that the recording density near the optical cut-off frequency can be achieved by using the optimized write strategy and PRML.

High-Density Recording Systems Using Partial Response Maximum Likelihood (PRML) with Blue Laser Diode (LD)

This paper describes our two newly developed high-density recording systems, which consist of a 35 GB system and a 20 GB system using partial response maximum likelihood (PRML) with blue laser diode (LD), and their respective evaluation systems. The 20 GB system consists of a 0.6 mm substrate disk system (System #1) with a numerical aperture (NA) 0.65 objective lens. On the other hand, the 35 GB system consists of a 0.1 mm cover layer disk system (System #2) with a NA0.85 objective lens. Moreover, we considered the two advantages and disadvantages, and future technical progress. Consequently, we considered that System #1 is fit for personal computer (PC) applications, and System #2 is fit for video disk recorder applications, such as prolonged high definition (HD) video recording of broadcasting satellite (BS) digital broadcasting.

High-density phase-change optical disk with a Si reflective layer

For high-density recording at a wavelength of 690 nm, we developed a phase-change optical disk with a Si reflective layer. We estimated the effect of interference layer by calculating optical properties. The absorption control required for mark edge recording and a 2 dB C/N improvement was obtained by forming a ZnS-SiO2 interference layer on the Si layer. Under recording conditions with a minimum bit length of 0.335 micrometers and a track pitch of 1.2 micrometers , a sufficient C/N and a BER less than 10-4 were confirmed. This result indicates that the recording capacity of the new disk is more than 4 GB.

High-Deposition-Rate Dielectric Thin Film for Phase Change Optical Disc

To improve the productivity of high density digital versatile disk-rewritable media (HD DVD-rewritable media), a silicon-nickel oxynitride (SiNiON) film that can be sputtered with a SiNi target in an atmosphere of mixed argon, oxygen, and nitrogen gases has been developed. The SiNiON film had a deposition rate almost the same as that of the ZnS-SiO2 film widely used in phase change recording media, and its refractive index was sufficiently low, that is, 1.54. Its structure was a mixed matrix of Si, Ni, O, and N, rather than a mixture of SiO2 and Si3N4 clusters. HD DVD-rewritable media using the SiNiON film showed almost the same excellent read/write characteristics as those of the media using the SiO2 film.

Phase Change Optical Disks Having Recording Marks with Large Optical Phase Difference Suitable for Mark Edge Recording

We have developed a novel absorptivity control method for phase change optical disks with the aim of reducing overwriting jitter in mark edge recording without degradation of overwriting cyclability. To control absorptivity more easily, we have designed a disk with a small difference in reflectivity between the amorphous and the crystalline states, and with a large optical phase difference between the reflected light from those two states, which produces a high carrier-to-noise ratio. We have obtained significantly reduced overwriting jitter, down to 2.5 ns for the leading edge, as well as a sufficient overwriting cyclability of up to 105 times.

Phase-change media for high linear velocity and high recording density

Overwrite characteristics have been studied for Ge-Sb-Te phase change media under high linear velocity conditions, ranging from 11.3 m/s to 22.6 m/s, with mark-edge-recording (MER). The Ge1Sb4Te7 recording layer composition was chosen for the rapid cooling structure to obtain a sufficient erase ratio under high linear velocity conditions. The optical optimization for the disk structure and the narrow-grooved substrate have been applied to improve erase characteristics with MER. The optical phase-difference-reproduction (PDR) has been studied to realize a high carrier to noise ratio (C/N). The pulse-width-reduction (PWR) recording compensation has been developed for high recording density. A 44.8 dB C/N was obtained for 0.68 micrometers minimum recording mark length. A -26.8 dB erase ratio was obtained at 22.6 m/s linear velocity.

High-density recording systems using PRML with blue LD

In high-density optical disk recording systems using blue laser diodes, a NA 0.85 thin cover layer disk system (I. Ichimura et al., Jpn. J. Appl. Phys., vol. 39, pt. 1, no. 2B, pp. 937-942, 2000) and a 0.6 mm-thick substrate disk system (S. Ohkubo et al., Optical Data Storage 2000, PD5, 2000) have been developed. However, the demand for huge capacity with higher data bit rate recording for HD-video recording, and PC application requests (e.g. no cartridge and easy compatibility with DVD/CD-ROM) have been growing rapidly. In this paper, we describe our newly developed two high-density recording systems, which are System 1 (0.6 mm substrate disk system with NA 0.65 objective lens) and System 2 (0.1 mm cover layer disk system with NA 0.85 objective lens), with advanced PRML technology respectively. Moreover, we examined a fingerprint influence and arranged various advantages and disadvantages over each high-density recording system, and considered future applications.