Studies on relationships between single beam overwrite characteristics and physical and chemical properties of the phase change recording media and their mechanisms are reviewed. Improvements in overwrite characteristics based on these studies, especially improvements in carrier to noise ratio(C/N) and the life of recorded dots by the addition of one of the transition metal elements or Tt are also reported. The maximum C/N was 61dB and the estimated life of the recorded dots is longer than 100 years at 60°C.

A single-beam overwrite was achieved by an In-Sb-Te phase-change optical disk. The carrier to noise (C/N) ratio of more than 50 dB and the overwrite modulation of -40 dB were obtained for overwriting frequencies of 2 MHz and 3 MHz using a low power laser diode of 30 The high C/N ratio and the excellent overwrite modulation were due to the large reflectivity difference between the written and erased phases and the use of suitable write/erase power.

A new single beam overwrite method for a phase-change optical disk is proposed to obtain both high carrier to noise ratio (CNR) and high overwrite erasing ratio. Contrary to ordinary method is, both writing and erasing process are controlled over the melting point of a phase-change recording film. To realize new overwriting method, we discuss desired thermal characteristics of a disk and crystallization properties of recording film theoretically. Necessary conditions to realize this method are (1) optical disks must have high thermal conductivity to control cooling rate, (2) recording material may have adequate crystallizing tine within dynamic range of disk's cooling rate. Examination on this method was carried out by using newly developed In-Sb-Te phase-change optical disk. CNR higher than 50dB and erasing ratio higher than 40dB were obtained.

GeTe-Sb2Te3-Sb pseudo ternary components alloy shows laser induced rapid crystallization and amorphous change phenomena. Films of non stoichiometric GeTe-Sb2Te3-Sb can be crystallized using laser pulses less than 100ns duration. The crystalline structure shows the cubic structure. The time it takes to laser crystallize varies Sb concentration. And increasing the Sb concentration leads to increased crystallization temperature. The crystallization temperature of 180 C and more than 1200h stable of acceleration test of 80C 80% RH environment. Thin film disk structure of 20nm active layer produce more than million cycle over-write BER(bit error rate) stability. I would like to discuss the degradation model of pinhole generation in phase change disk media.

Newly developed automated clean process has made it possible to produce 12" W-0 type optical disks with average defect rate of 3 x 10^-7(1/bit). Using these disks with ultra low defect rates, accelerated life tests were performed. No disks showed any degradation in bit error rate during the high temperature and high humidity test(60°C;90%RH) of one year and the heat cycle test up to 1000 times between -40°C;3 hours and 80°C;3 hours. The acceleration coefficient for 60°C;90%RH referred to 25°C;90%RH was estimated to be more than 100.

The excellent stability of Hoechst Celanese WORM media was demonstrated in accelerated aging studies. CNR data was measured as a function of exposure time for disks exposed to 65°/80% RH, 80°/80% RH, and 90°/80% RH. Disks were also exposed to a high intensity xenon lamp to compare the relative stability to visible and ultraviolet radiation. The stability of the polycarbonate substrate and the recording layer were investigated.

Statistical analysis on a life time probability of optical recording media is studied. Byte error rate change under accelerated environments was applied to plot on an Arrhenius' scale. Assuming an error generation model where an error is caused by combination of noise generated in a disk and drive, following equation can be obtained; BER = Ao exp [ C exp( - Ea / kT ) ) • t 1. The analysis result suggested that the life time probability distributed normally on an Arrhenius' time scale. Experimental results on high temperature and high humidity accelerated test were in good agreement with the analysis. The life time of 99.9% our magneto-optical media were expected to be longer than 25 years in an office environment.

Short-wavelength semiconductor lasers will potentially lead to major improvements in the performance of optical data storage systems. The various approaches to making visible semiconductor lasers are reviewed, including II-VI and III-V compound semiconductor lasers, and frequency-doubled infrared lasers. Although it is anticipated that diode lasers constructed from II-VI compounds will become important in the future, high-performance diode lasers of AlGaInP, a high-bandgap III-V alloy, are presently available. Lasing wavelength is 650nm<2.<680nm, and typical threshold current densities are 1-3 kλ/cm². Recent advances in the growth of quantum well device structures have resulted in significant performance improvements.

Insite Peripherals introduces a new generation of data storage devices featuring a marriage of optical and magnetic memory technologies in the Insite 325 Floptical disk drive.

A high reflectivity write-once disc with specifications satisfying the CD Standard has been developed. The new disc realizes a reflectivity greater than 70% and employs a simple configuration with a substrate, photo-absorption layer, reflection layer and protection layer. Since there is no space between layers, high durability is assured. Recording is performed when the substrate resin expands and intrudes into the photo-absorption layer, mixing with the decomposed dye materials. The specifications of the new disc are completely compatible with the current CD Standard, while the disc's simple configuration realizes low cost processability.

We propose the detection of magneto-optic signals using an external cavity laser diode suitable for constructing a small optical head. A magneto-optic medium is used as one of the cavity mirrors. This method is based on detecting the frequency-shift signal, instead of the conventional amplitude-modulated signal, and is expected to raise the signal-to-noise-ratio.

Application of Bernoulli Principle to optical recording is described. Bernoulli technology stabilizes the flexible disk so that a light, fixed focus optical head can be used.

A multi-beam optical disk drive, which improves the data transfer rate for the magneto-optical disk systems, has been developed. It employs a newly developed image rotating actuator for tracing multi-tracks simultaneously, and photodiode arrays for simplifying signal detection circuits. Four-channel parallel read/write operation has been accomplished. The carrier-to-noise ratio is greater than 55dB, and the crosstalk between channels is lower than -36dB. A data transfer rate, four times that for an ordinary single-beam optical disk drive, has been real ized.

Development of high data rate, high capacity optical disk buffer technology has reached demonstration of the basic modular breadboard. Test and operational data are presented.

A Spaceflight Optical Disk Recorder (SODR) is being developed by NASA. A primary goal of the SODR program is to develop a modular SODR system architecture which can address a wide range of Mass Storage applications from spaceflight to ground based systems and a modular performance configurability from small capacity and low data rates to very large capacity and very high data rates. First application on EOS, a polar orbiting platform of the Space Station system, has been used to drive the SODR design. This established a range of performance requirements for the system. A modular architecture was then developed which allows an SODR system to be configured from a suite of modules to meet any storage capacity, I/O data rate and thruput rate in increments governed by each module's performance.

Compact holographic heads using the spot size detection of double beams with linear photodetector-elements are proposed to attain highly reliable performances under considerable optical misalignment. Acceptable amount of misalignment could be expanded by a factor of ten compared with that in an astigmatic system. Mechanical accuracy of the integrated head is also discussed.

A compact magneto-optical disk head using a reflection polarizing holographic optical element has been developed, which has polarizing beam splitter function and error detection optics functions. As a result of introducing the element into an magneto-optical disk head, a high carrier to noise ratio has been achieved, while simple optics has been used.

We have developed a magneto-optical head to provide both fast access and large memory capacities for optical library systems. It features a pair of optical heads which face each other on opposite sides of the disk, and a new tracking method which enables a wide tracking range. The bias coil to generate the magnetic field is mounted around the objective lens of each optical head. The two opposing bias coils can generate an interlocking magnetic field of more than 250 Oe. We call this bias-coil driving method the push-pull driving. The power consumption needed for a 250 Oe field is only 2 W. A new tracking method makes it possible to use a stepping motor for coarse access. Two stepping motors are used not only for on-media access but also for media-to-media access, which help to miniaturize the drive. A good magneto-optical readout signal was obtained. Its carrier-to-noise ratio is more than 50 dB for a mark length of I gm. We confirmed that this signal level is sufficient for recording on magneto-optical disks.

The polarization properties of high numerical aperture holographic objectives were experimentally and theoretically investigated. A local planar grating model was combined with one-dimensional coupled wave theory to determine the full aperture s and p diffraction efficiency characteristics of this holographic optical element (HOE). To verify the modelling approach, diffraction efficiencies for s and p reconstruction waves were measured at local regions of the HOE aperture and compared to corresponding planar gratings. Good agreement was obtained between the theoretical model and the measured full aperture diffraction efficiency. Results indicated that the control of polarization sensitivity, along with the high diffraction efficiency obtainable with volume holograms suggests promising applications for holographic components in magneto-optic head designs.

The current status of erasable phase-change recording materials is re-viewed. Using GeTeSn-based materials as examples, dynamic perform-ance data are presented along with discussions of the basic physical mechanisms determining media performance. For applications requiring limited number of cycles, phase-change materials have been developed to a very practical level.

Ge-Te media are markedly improved upon alloying with small amounts of Ti. Improved are the recording sensitivity and noise yielding CNR greater than 62 dB.

The optical recording properties such as erase speed, lifetime of written spots, maximum number of write/erase cycling of In47Se53 and In47Se51Pb2 thin films are reported. The factors whiCA influence the erase speed and stability of the written spots are discussed based on the theory of glass forming ability (GFA). It is found that the glass transition temperature Tand the reduced temperature Tg/Tm can be used to estimate the stability and erase speed or the written spots respectively. Since the elements with high Tm such as Fe, Co, Ni and A.1 possess relatively high Tg and small Tg/Tm, it is expected that the introduction of these elements into phase change optical recording materials will improve both the erase speed and stability significantly. The role of Pb on the erase speed and stability of phase change optical recording materials is similar to :hat of Tl. Its Tg/Tm value is smaller than those of In and Se, but its T is lower than those of In and Se. Therefore, the introduction of Pb into In-Se alloys will improve the erase speed and decrease the stability.

Pits made in organic WORM media have been examined using an STM. The relationships of STM observations to media performance and properties are discussed.

Low conductivity, laser-irradiated thin films such as the chalcogenides employed for optical data storage can display manifestations of signific ant through-thickness thermal gradients. A combined Laplace-transform, Fourier-integral method was utilized to derive the temperature distributions in laser-irradiated, low conductivity thin films, and the results were used to examine the effects of some marking parameters on through-thickness thermal gradients. These were found to be strongly influenced by the laser incidence direction, the coefficient of optical absorption, and the film thickness, and influe:aced to a somewhat lesser degree by the scanning velocity.

Experimental measurements and theoretical computations presented in this paper indicate that for optimal tracking error signals organic based optical recording media require grooves that are narrower and deeper than those required for metallic based media.

The effect of birefringence in polycarbonate substrates on the dynamic performance of magnetooptical disks has been investigated. We have observed a monotonic decrease of the carrier-to-noise ratio with increase in the normal retardation for retardation levels greater than about 15 nm.

This paper describes magneto-optical disk employing the contact printing method for a glass substrate. This method brings high productivity to a disk preparation process and high dynamic performance.

Calculated Kerr rotation spectra for bilayer and multilayer media with Fe are presented. Kerr rotation enhancement and the effect of the plasma reflection edge of adjacent metal reflector layers are studied.

Kerr rotation enhancement is important for improving the signal-to-noise ratio (SNR) in magneto-optical recording media. Multilayer designs have been studied as a means of increasing the SNR through interference effects. Here, for the first time, we discuss, with illustrations, the effectiveness of rare-earth hexaboride films as reflector layers to enhance the polar Kerr rotation. The plasma reflection edge of these materials, which is near 800 nm, interacts favorably, producing a larger Kerr rotation around this wavelength.

1 , 2 A magneto optic (MO) rewritable disk drive has been put to practical use. The ISO standardization activity on a 130 mm rewritable disk is in its final stage and it will be standardized in 1989. An MO disk drive must he equipped with bias magnetic field generating functions in addition to the same characteristics as that of a write once read many (WORM) disk drive. The key factor for the broad acceptance of MO disk drives by users is that they conform to the ISO standard and that the drive size is of such a widely adopted size as that of a magnetic disk drive.

High speed accessing using a split optical head with a linear actuator is presented. A new optical head, which can detect and compensate for the displacement between the optical axis and the optical head moving part, was developed for the linear actuator. The moving part weight for this disk drive is less than 30g. The 21 msec average coarse seeking time can be realized.

In the optical data storage technology the optical servo crosstalk represents an undesired phenomenon. The optical crosstalks between the tracking push-pull and focus error signals are analyzed for various focusing techniques. The conditions minimizing these crosstalks are drawn. It is also shown that a simple far field diffraction method can be used for the evaluation of the focus error signals which are detected in the Fresnel dif-fraction domain.

Direct overwriting on a magnetic triple-layer magneto-optical disk has been performed by modulating the light intensity. Previously, direct overwriting had be performed only on a double-layer disk. The initializing field was reduced to 3.5 kOe for the triple-layer disk, compared with more than 5 kOe for a double-layer disk. A high C/N ratio of 50 dB was obtained for an overwritten bit length of 0.75 Am without any residual former signal. Margins of the light power for bits to be overwritten at an acceptable error rate are more than ±24 % for both the high and the low light intensity. The magnetic triple-layer disk is a promising medium for direct overwriting by light intensity modulation.

Magneto-optic direct overwrite using the resonant bias coil technique has been demonstrated for field frequencies as high as 12 MHz. The technique is compatible with multi-channel recording and allows larger coil-to-media spacing than obtained using conventional field modulation techniques. Suitable run length limited codes are investigated which satisfy an even consecutive zero constraint. The performance of these codes appears to be competitive with the performance of more traditional (d,k) codes.

Laser-feedback noise in a GaAIAs laser was investigated as a function of optical-path length and modulation frequency.

In a magneto-optic head with differential detection, the light level on the data detectors is determined by the ratio of a semi-polarizing beam splitter. This paper presents a way of measuring the various noise sources with a common set of values and uses this data to choose the beam splitter ratio that optimizes the SNR.

The test methods for carrier to noise ratio (CNR) and for cross talk were applied concurrently to typical TMO optical disks. The behavior of CNR and cross talk as functions of incident write laser energy and bias magnetic field were measured. Experimental results are shown and discussed.

We have investigated the edge shift of written domain in magnetic field modulation recording for magneto-optical disk. The edge shift between the timing for the recording and reproducing signal edges is inevitable. We have proposed an edge shift compensation method for the purpose of applying the magnetic field modulation method to embedded clock system.

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.

In the field of optical recording much has been written on the general theory of operation of the optical recording and playback system.1'2'3 However, one of the areas that has received comparatively little attention is the detailed understanding of optical interaction of the media surface with the focused laser beam and the resultant intensity profile in the detector plane of the optical head. While much has been published on the interactions involved in the thermal-physical writing process for the many forms of media under development today, the optical interactions involved in reading, detection, and servoing have largely been derived by applying physically sound, but generalized, arguments to intense experimental investigations.

We have shown a possibility to store data at more than 600x10 8 bits/cm 2, by forming a double-layered PHB system. This memory density is the highest one reported so far.

In this paper we report the measurements of the optical absorption spectrum , diffraction efficiency, recording sensitivity , erasure behavior in the poled single crystals LiNb03 with Pr and with Ce and Fe as new impurities respectively, We show associative holographic analog to the Hopfield neural network model in algorithm and present a real-time parallel optical associative memory system implemented with a CeiFesLiNb03 crystal as recording medium and a Fe,LiNb03 crystal amplifier to provide feedback, threshotding and gain . In our experiment, the retrieval of complete image when only a partial image addresses the system is demonstrated.