Masahiko Kaneko

Multilayered Magneto-Optical Disks for Magnetically Induced Superresolution

Magnetically induced superresolution (MSR) has been realized in magneto-optical disks using exchange-coupled magnetic multilayer film. Two new detection methods have been developed. In front aperture detection (FAD), the heated area in the light spot is optically masked. In rear aperture detection (RAD), a signal is read out only from the heated area. For higher readout power in RAD, the aperture area is sandwiched by double masks, since another mask is generated in the highest temperature region. The cut-off spatial frequency in both types of detection is more than two times higher than that in conventional detection. A high C/N of 42 dB is obtained in the MSR disks by both methods for a mark length of 0.3 µm.

Premastered optical disk by superresolution using rear aperture detection

Premastered optical disk by superresolution (PSR) using rear aperture detection (RAD) has been realized. The PSR disk has a chalcogenide material. Using PSR by RAD with a current CD system, we demonstrated the feasibility of increasing both the linear density and the track density of read only memory (ROM) optical disks. By superresolution in the linear direction, a high C/N of 47 dB was obtained for a pit length of 0.3 ?m which is beyond the optical limit. Furthermore, by superresolution in the track direction, a low crosstalk of -38 dB and a wider margin of radial skew angle compared to conventional detection were obtained for a track pitch of 1.1 ?m.

The interface wall structure of magnetic triple-layer film for overwriting by light intensity modulation

The direction of the spin in the interface wall of magnetic double-layer and triple-layer films has been calculated using the variation method. The calculation as well as experimental results have shown that the interface wall of magnetic double-layer film is unstable in most cases unless it is supported by the applied magnetic field, while the wall in magnetic triple-layer film is stabilized at the intermediate layer. In addition to the stability of its wall, triple-layer film has been found to have advantages for overwriting by light intensity modulation in that the initializing field is as small as 2.5 kOe and the total magnetic layer thickness is only 1200 A.

High-density magneto-optical disk system using magnetically induced super resolution

Magnetically induced super resolution (MSR) is a new technology developed by us. It more than doubles the readout resolution limit of a conventional optical disk system without shortening the laser wavelength or increasing the numerical aperture (N.A.) of the objective lens. We examined the characteristics of the readout signals of an MSR-by-RAD (rear aperture detection) disk and obtained a high C/N of more than 49 dB and good stability of the edge position at a mark length of 0.4 µm. We investigated the high-density MO disk system using the MSR-by-RAD disk. Owing to optimization of the writing process and waveform equalization, the system achieved a byte error rate of less than 10-4 at a bit length of 0.3 µm. This bit length corresponds to a linear density more than three times higher than that of the conventional 130-mm MO disk system of the ISO standard.

A Simulation of Magneto-Optical Signals in Near-Field Recording

The diffraction from two-dimensional magneto-optical marks has been calculated using the plane wave expansion method and the Fourier modal method. The modulation transfer function (MTF) in near field magneto-optical recording has been simulated for the first time on the basis of the strict vector diffraction theory. The simulated result proved that the magneto-optical signal is reduced by only 3 dB at a spatial frequency of NA/λ, for an air gap of 50 nm between a solid immersion lens with an effective numerical aperture (NA) of 1.5 and a magneto-optical disk surface.

High Density Magneto-Optical Recording with a Second-Harmonic Generation Green Laser

Tripled areal density optical recording was realized with the combination of a high power laser-diode pumped intracavity frequency-doubled Nd:YAG laser (SHG green laser), a low noise GdFeCo/TbFeCo double-layered magneto-optical (MO) disk, and high sensitivity photodetectors optimized for the wavelength of the light source. A high carrier-to-noise (C/N) ratio of more than 48 dB and a bit error rate of less than 3×10-6 by (2, 7) run-length-limited (RLL) mark position recording was obtained at a mark length of 0.45 µm (0.6 µm/bit) for a disk with a 0.9 µm track pitch.

High-Density Overwriting on a Magneto-Optical Disk with Exchange-Coupled Triple-Layer Film

The characteristics of high density recording on an overwritable magneto-optical disk using light intensity modulation were investigated by means of (2, 7)-RLL mark position detection. We examined the overwritable region of optimum high level, (PH) and low level, (PL) by varying the recording pulse width and film structure in the overwritable magneto-optical disk, in order to minimize thermal interference between closely recorded marks. A wide margin of ±30% was obtained using low power at a bit length of 0.8 µm.

1.3-GByte 130-mm direct-overwritable MO disk system using light-intensity modulation

We have investigated the possibility of a direct overwritable MO disk system which uses light intensity modulation and a triple-layered disk of 130 mm to provide 1.3 GByte of capacity using both sides. The writing and reading performance was experimentally evaluated using an overwritable disk with 1.4 micrometers track pitch and 0.8 micrometers /bit linear density to give a 1.4 times increase in recording density. This recording format is the same as the ISO standard [(2,7)-RLL Code, mark position recording], so that we can maintain compatibility with the current MO disk systems. We also introduce a laser power off-time period after each recording mark in order to reduce thermal interference. The laser power margin is improved by this method from about +/- 17% to over +/- 30%. We confirmed that this is a sufficient margin for a practical system. We demonstrated the feasibility of a 1.3 GByte direct overwritable MO disk system by combining the higher recording density, the introduction of a laser off-time period, and zone bit recording techniques.

A new write compensation pulse for high-density recording on magnetically induced super resolution (MSR) media

We have studied the write power margin for an magnetically induced super resolution (MSR) by front aperture detection (FAD) disk system and investigated a method for evaluating jitter which uses XY-mapping visualization. Using this method, we evaluated the effect of conventional write compensation on the shifts of the leading and trailing edges and developed a new write compensation pulse that is optimized for a high density system such as the ISO 8X. We confirmed the advantages of using the new write compensation pulse for MSR.

A Thermal Analysis of a Magneto-Optical Recording Mechanism with the Magnetic Field Modulation Method on a Land and Groove Substrate

The power tolerance in recording is one of the most significant issues in the case of rewritable optical disk systems. Magneto-optical land/groove recording has been investigated by magnetic field modulation with pulse irradiation. The lower limit and the upper limit of the power tolerance are determined from the overwriting and the crosswrite characteristics, respectively. The experimental dependence of the power tolerance on track pitch, pulse duty, and optical spot size has been in good agreement with our model based on the thermal analysis. According to our model, the mark width is exactly the width of a recording track for the lower limit of power tolerance, and it spreads into the neighboring track for the upper limit. The deep groove substrate has been found experimentally and theoretically to be effective for improving the power tolerance.