Seiji Yonezawa

Chalcogenide thin films for laser‐beam recordings by thermal creation of holes

The shapes of holes thermally created by a laser beam in thin films of wide range of materials are studied. The most cleanly shaped holes, which measure up to recent requirements for direct‐read‐after‐write video recordings by submicron dots, are obtained in some compositions of amorphous chalcogenide thin films such as As‐Te system, Ge‐Te system, As‐Se system, Ge‐Se system, and Sb‐S system thin films. The experiments clarify several important conditions helpful in obtaining cleanly shaped holes. The most important condition is that the viscosity of the material in liquid or softened state should be high. The required incident laser‐beam power for recording in an As‐Te system thin film on a polymethyl methacrylate substrate rotating at 1800 min−1 is about 10 mW. Addition of Se to the As‐Te system thin films greatly increases film stability. A signal‐to‐noise ratio of about 45 dB is obtained in reproduced color video signals.

Thermodynamic simulation of magnetic-field-modulation methods for pulsed laser irradiation in magneto-optical disks

Direct overwrite magnetic-field modulation recording with dc and pulsed laser irradiation in a magneto-optical disk was simulated thermodynamically. Because the laser light heats the magnetic film, which allows the magnetization to be reversed, the waveform of the laser light strongly affects the shape of the magnetic domain. The dynamic temperature changes in magnetic thin films are simulated for dc and pulsed laser irradiations. With pulsed irradiation the heat is constrained within a limited area, which results in jitter-free magnetic domains, high optical efficiency, and high tracks-per-inch and bits-per-inch recordings.

Key Technologies to Realize Magneto-Optical Storage of Over 7 Gbytes in CD-Sized Disk.

Many types of recording technologies have been reported for future magneto-optical (MO) storage. However, they have not been well discussed from the viewpoint of total recording technology including the recording and readout methods, the pick-up technology and the signal processing technology. We propose key technologies for realizing MO storage of over 7 Gbytes in a CD-sized disk using a red laser, and describe the analytical and experimental results pertaining to each key technology.

Amplitude detection for the focus error in optical disks using a birefringent lens

A lens made of a birefringent material shows two focal points. If the polarization of incident light is switched periodically and a detector is placed between the two focal points, this property can be utilized to detect a focus error signal equivalent to that of a dual detection optics. The proposed system offers the possibility to achieve amplitude detection for the focus error. It results in a compact optical pickup with reduced adjustment requirements. A second possible pickup uses a birefringent lens and a Wollaston prism. In this case, a compact dual detection method is achieved.

Domain observation during direct overwrite on magneto‐optical recording films

Domain patterns were observed with a polarized microscope during direct thermomagnetic overwrite at a fixed bias magnetic field. The size of a recorded domain can be changed reversibly by irradiation of laser pulses with various durations. By scanning a rapidly pulsed laser beam on a film, a stripe domain can be written, erased, and manipulated. This technique may be used in a new type of rewritable memory device. These overwrite behaviors can be explained with a model which assumes that an initial domain formed during laser pulse heating shrinks during cooling.

Sector Servo Data File Optical Disk Memory

Sector servo optical disk mass data storage system was designed. In this method each track is divided into 1024 sectors. Each sector consists of the preformated servo field and data field where data are recorded. One forth wavelength depth preformated servo field contains wobbled pits array from which tracking servo signal is generated. This tracking error signal is sampled and hold in the data field. This system has advantages of its compactness of optical drives and higly improved optical stability.

Short-marks recording characteristics of laser-pumped magnetic-field-modulation recording in a narrow track pitch and on magnetically induced superresolution disks

We evaluate laser-pumped magnetic-field-modulation (LP-MFM), continuous laser-irradiated magnetic-field-modulation, andlight-intensity-modulation recording in magneto-optical disks, usingtemperature calculations and experiments. LP-MFM is useful forrecording short marks: One can record magnetic domains as small as 0.15-mum marks with LP-MFM by using a spin-polarized scanningelectron microscope. LP-MFM is also useful for reducing the trackpitch. Experiments showed that no cross write occurred at a 0.6-mum track pitch, and calculation showed that the heatdistribution is narrow. We confirmed the advantages of LP-MFM forrecording magnetically induced superresolution disks by investigatingthe difference in the shapes of the recorded domains.

Bit Shift Measurement and Overwriting in a Sampled Servo Format Magneto-Optical Recording

A new direct overwrite method, which uses oscillating laser pulses and magnetic field pulses modulated by code data, is demonstrated and the characteristics of bit shifts are discussed. A bit shift correction method is also demonstrated. In this method, preamble pits are written in the open space of the header area when data pits are recorded. In reconstruction, the phase of PLL clocks is adjusted automatically using the signal from the preamble pits.

Laser diode uses for optical disks

Summary form only given, as follows. Semiconductor lasers are key devices in optical disks. Optical properties of transverse mode-controlled diode lasers, such as far-field beam pattern, output power, spectrum, astigmatism, polarization, lifetime, and temperature dependencies for these characteristics are presented in this paper. Their use in optical heads has been improved in three respects: (1) Longitudinal mode instability causes noise in the output power. Therefore an appropriate length between the diode and theoretical disk was determined. (2) For diode laser recording, beam profile conversion from an elliptic to a circular profile was accomplished by using two prisms without optical loss. (3) The objective lens was actuated radially and vertically for tracking and focusing, instead of actuating it wholly. This has several advantages, such as easier laser diode cooling, high speed data recording and playback, and simple objective lens configuration. A means to reduce tracking offset error, due to radial displacement of an objective lens, is discussed.

New recording thin film for optical video disk

range of wavelengths, generally including that of the GaAs diode laser. This means that in the case of the antireflection trilayer structure, we can easily reoptimize the antireflection condition to the diode laser wavelength simply by adjusting the thickness of the dielectric layer. In order to improve on the sensitivity of the titanium trilayer, we have investigated the recording characteristics of trilayers fabricated with several alternative low melting point metals. We have found that in the majority of cases the playback video SNR is limited by film surface quality to a value below that for broadcast use. The fully encapsulated tellurium trilayer, however, was found to give playback SNR 3 50 dB for the recording levels of only 6 mW incident using the Ar’ ion laser system. We shall review the results described above as well as present our initial results of the recording characteristics of the tellurium trilayer using a GaAs diode laser (A = 820 nm).