Hiroyuki Minemura

Reversible phase‐change optical data storage in InSbTe alloy films

Some characteristics of reversible phase‐change optical data storage based on an amorphous‐crystalline transformation in InSbTe alloys are given. The reversible phase change was observed in a wide region of composition. The laser amorphized spot of a ternary compound In3SbTe2 film could be crystallized using a diode laser pulse of less than 100 ns with an incident laser power of more than 10 mW. The crystallization temperature of the amorphized spot was 280 °C and the activation energy was about 1.8 eV which shows that long‐term data retention at room temperature is possible. The repetition number of static write and erase using the pulse of 50 ns reached above 105. These data show that the ternary compound film has potential for reversible optical data storage media with high‐speed erasing and long‐term data retention.

Nanosize fabrication using etching of phase-change recording films

An etching technique called phase-change etching was developed. In this technique, only crystalline regions in a phase-change recording film are selectively etched by an alkaline solution, and amorphous regions remain on the sample surface, which means that a phase-change recording film can be used as a resist for pattern formation. By combination of this technique and phase-change recording, fabrication of the dot pattern with a size of about 1∕10 of the fabricating spot was demonstrated. This result indicates the possibility of nanosize fabrication using the phase-change etching technique.

Single‐beam overwriting with melt‐erasing process in an InSbTe phase‐change optical disk

Single‐beam overwriting with melt‐erasing process was made in a 5.25‐in.‐diam phase‐change optical disk using an In22Sb37Te41 recording film. In the overwriting between 2 and 3 MHz signals at the linear velocity of 3–11 m/s, a carrier to noise ratio (C/N) more than 46 dB and an erasability less than −35 dB could be obtained. This high erasability was found to be due to the melt‐erasing process. This disk presents highly erasable overwriting and long data retention time supported by an activation energy of 2.3 eV and a temperature of 230 °C for crystallization of the amorphized part.

High-Speed Write/Read Techniques for Blu-ray Write-Once Discs

We have been developing high-speed write/read techniques for Blu-ray discs. We carried out 1–12× write/read tests on a phase-change Blu-ray write-once disc using an improved tester, an adaptive partial-response maximum-likelihood (PRML) technique, and a new write adjustment method. Sufficiently good limit equalizer jitter values of less than 6.5% were obtained under all write-speed conditions. Good bit error rates of less than 10-6 and fair power margins of more than ±10% were obtained under all write/read speed conditions.

Sub-Terabyte-Data-Capacity Optical Discs Realized by Three-Dimensional Pit Selection

To realize optical discs with the sub-terabyte data capacity, we propose the three-dimensional pit selection (3DPS) method where a single data pit to be read out in a multi-layer disc is selected three-dimensionally to obtain super-resolution in the disc plane and to reduce layer cross-talk. To examine the feasibility of this method, the phase-change pit capsule method was tested where the data pits consist of a phase-change material which melts during readout. The super-resolution effect was observed for both layers of a dual-layer disc. It was shown that a quadric-layer disc can be designed because of the high transmittance of each layer. Thus, 3DPS is considered to have the potential for a data capacity of hundreds of gigabytes with a conventional optical system.

Optical head with optical beam control using acoustic wave device

An optical head is disclosed, in which a plate-shaped optical wave guide comprises an optical system including a collimator lens, a beam splitter and an objective lens; a laser light source for emitting a laser light in the form of a light beam which is to be impinged on an optical recording medium; and electro-acoustic transducers are disposed between the beam splitter and the objective lens, the transducers being adapted to be driven by the outer electrical signal for producing surface acoustic waves in the optical wave guide, thereby establishing a condensation and rarefaction distribution in refractive index in the optical wave guide which distribution influences the direction of transmission and the degree of focusing of the light beam passing therethrough so that regulation of the tracking and the focal length can be effected without any mechanical movements.

Three‐dimensional analysis of overwritable phase‐change optical disks

Three‐dimensional computer calculations which analyze time‐transient behavior of heat conduction and phase‐change kinetics in an InSbTe overwritable disk were made. To improve the calculation accuracy, thermal conductivities of thin‐film materials and critical cooling rate for crystallizing the recording film were estimated. For realization of high‐performance overwrite, it was found that the cooling conditions of writing and erasing processes should be controlled to match the critical cooling rate in a given linear velocity range. This could be achieved by optimal design of the disk structure.

Crystallizing mechanism and recording properties of In3SbTe2 phase-change optical disks

The crystallization mechanism and its application to improvement of the carrier-to-noise ratio (CNR) for In3SbTe2 phase-change optical disks were examined by that transmission electron microscopy (TEM). The TEM images indicated: (1) mark shapes were distorted when the interval between laser irradiations was short; (2) following dc laser irradiation onto the marks, crystallization proceeded only along the periphery of the amorphous phase; and (3) mark shapes were varied corresponding to the dc laser power. We assumed that the crystallization mechanism for In3SbTe2 was dominated by crystalline growth rather than nucleation. Then we simulated the mark shapes after dc irradiation. By controlling the thermal distribution on the marks, the CNR was improved promoted. We found that dc irradiation was a simple way of improving recording properties.

Signal-Readout System for Optical Pickup with Homodyne Detection Scheme

We developed a signal-readout system suitable for optical pickups with a homodyne detection scheme, which is used to amplify signal lights using optical interference. The system consists of an optical-signal detection circuit and a readout-signal generator. The optical-signal detection circuit, which contains pairs of photodiodes connected in series, successfully cancels out large DC components arising from a reference light. This made it possible to avoid the output signal saturation of current-to-voltage amplifiers and raise the upper limit of signal gain. The readout-signal generator generates a readout signal on the basis of the phase-diversity method, which stabilizes readout signal amplitude. By using this system, a signal gain of 3.6 was obtained on a commercially available disc.

Crosstalk Cancellation for 50-GB/Layer Optical Recording

We have developed write/read technologies for post-Blu-ray Disc (BD) with a capacity of 50 GB/layer. In this report, the authors propose a crosstalk cancellation method that minimizes the error between the crosstalk canceled signal and the target signal that is generated from the binary output of an adaptive-partial response maximum likelihood (PRML) decoder. The experimental results obtained using our method show that the bit error rate is reduced to 1/100. A high-density recoding of 50 GB/layer was demonstrated using our method.