The highest transfer rate, 64 Mb/s, and a recording density of 0.78 micrometers ²/bit in a magneto-optic disk system have been achieved.

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 200 MB 86 mm magneto-optic disk system was developed using magnetic field modulation and pulsed laser irradiation for direct overwrite. Write/read data transfer rate was 1.26 MB/s with 60 Hz disk rotation.

The present status of waveguide-type integrated-optic disc pickups are reviewed. The authors have proposed and fabricated several types, and have demonstrated their fundamental functions. For the ROM pickup, they successfully reduced the focused spot to a diameter that is slightly larger than the diffraction-limited value, and confirmed capability of the focusing/tracking error signal detection. The pickup was extended to that for magneto-optical (MO) disc system, and the MO pickup was found experimentally to detect a polarization rotation as small as one contained in the signal beam reflected from MO medium. The pickup for parallel-data cards was also fabricated with photodiode array, and seven-track parallel readout was demonstrated. It was found by theoretical analysis that the waveguide-FGC combined type pickup has the possibility of super-resolution readout.

A compact optical head for CD-ROM drives, integrated with a laser diode chip, a photodiode chip, and a holographic optical element in a small optical module, using hybrid assembly technique, has been developed. The module size is 7.5 mm (W) X 8 mm (D) X 3 mm (H), and its weight is 0.6 g. The validity of this optical head design has been demonstrated by successful readout operation for a compact disc.

The optical components in the detection train of a conventional magneto-optical (M-O) disk head include a half-wave plate and a polarization beamsplitter. These polarization components are bulky and require specialized mounting hardware. In order to realize a more compact head, we propose that these elements be replaced by an integrated device composed of cascaded volume and surface-relief gratings. In this paper, we describe the proposed system, detail designs for the individual elements, compare theoretical and prototype element performance, and discuss the operational tolerances of these elements.

Modern military mission scenarios require very efficient access to multiple, large databases. Static `reference' databases and highly volatile databases which contain intelligence from sensors and other sources must be processed, cross referenced, and correlated. An architecture has been developed for a content addressable (associative) optical memory system. The system exploits the parallel access capabilities of optical disk memories to provide keyword correlation of free form text or structured databases within one revolution of the disk. The system consists of an optical disk drive augmented with an optical correlator and related electronics and software. The search string (keyword) is loaded into a spatial light modulator and optical matched filtering provides massively parallel readout to locate the desired data patterns on the disk. A digital degree-of-match (DOM) word is generated for each sector on the disk. Post processing based in digital electronics and software performs fuzzy computations to combine the DOMs for the current and previous keywords enabling the system to efficiently perform multi-step, content-based searches of the disk. Data stored in the best matching sectors is retrieved during the next revolution of the disk using the drive's standard read mechanism. The sustained processing rate of the optical correlator is 71 gigabits per second.

One dimensional hologram recording in both thin and thick photolayers is considered. Possibility of increasing recording density is shown. High-speed holographic memory is disclosed. Application for neural networks desighn with serial-pa.rallel processing is proposed.

Optical data storage that exploit thick holograms have been investigated extensively in the area of neural networks and optical signal processing[1]. To overcome the low capacity of the thin holograms, a volume holographic interconnection scheme using volume holograms has been proposed[2]. Volume holographic interconnections exploit the third spatial dimension of the volume hologram to superpose a large number of gratings that are used to implement data storage. The angular selectivity arising from volume holograms has been recognized early on by Van Heerden to increase the storage capacity dramatically[3]. Light diffraction from volume sinusoidal gratings has been studied by many authors[4-8]. However, the number of gratings they cionsidered was not more than two. On the other hand, the number of gratings in the volume holographic interconnects is hundreds or billions. In this case, we need a simple and systematic method of handling many gratings to assess the crosstalks. Crosstalk effects due to adjacent gratings among a large number of superposed gratings was shown to limit the maximal data storage capacity of a volume hologram[2]. This result was based on a simple coupled-mode theory[9]. Recently, more detailed analysis using iteration method of integral equation of Maxwell's wave equation was carried out to study the crosstalk effects[1O]. However, this analysis has the following limitations. First, the analysis was limited to the case of transverse optical polarazations. Furthermore, the wave vectors of the incident light and volume index gratings were assumed to be in the same plane. These two assumptions eliminated the possibility of polarization mixing and graddivE term in the Maxwell's wave equation was automatically disappeared. In real situation, incident light waves propagate in many different directions. Volume index gratings also have different wave vectors and they are not in the same plane. Therefore, polarization mixing of the light waves should be considered to obtain correct results on crosstalk effects. Second, the analysis did not account for backward diffraction from volume index gratings. In this paper, we use a rigorous method of perturbative integral expansion to study the crosstalk effects due to superposed gratings. The method by Tu et. al. was based on calculating perturbative integrals in wave vector space. However, in our method, the integral is calculated directly in the space domain. Our method can be applied to arbitrary interaction geometries of optical waves and volume index gratings. It also accounts for backward as well as forward diffractions simultaneously.

The wavelengths of light used for the Read/Write/Erase processes in Electron Trapping Optical Memory have been shown to be tunable. Simulation and excitation efficiency spectra of nine samples of these IR stimulable phosphors were measured. Tuning was accomplished through compositional modifications of the ETOM^TM pseudobinary host lattice. The host lattice systems investigated were CaS, SrS, BaS, Sr_xCa_1-xS, and Sr_xBa_1-xS (0 <EQ X <EQ 1). Shifts in the read wavelength of over 125 nm and in the write wavelength of 5 nm were achieved.

The linear amplitude response of electron trapping optical storage materials provides the capability of utilizing multiple amplitude symbols in the data channel code sets. M-ary channel coding has received much interest from information theorists but, has had little acceptance for practical storage systems. Conventional optical and magnetic storage devices are limited by the marking process to codes requiring no more than two, or possibly three, symbol amplitudes. Electron trapping materials offer an excellent opportunity to exploit the increased capacity and transfer rates characteristic of M-ary code sets.

Key factors in designing the optics, lens actuator, and head base for commercial magneto- optical heads are discussed in this paper. The design and performance of two types of magneto-optical disk heads developed at Mitsubishi are presented.

Signal quality in an optical storage device greatly depends on the optical system transfer function used to write and read data patterns. The problem is similar to analysis of scanning optical microscopes. Hopkins and Braat have analyzed write-once-read-many (WORM) optical data storage devices. In this paper, we discuss transfer-function analysis of magneto-optic (MO) data storage devices with respect to improving transfer-function characteristics. Several authors have described improving the transfer function as super resolution. However, none have thoroughly analyzed the MO optical system and effects of the medium. In our development, we discuss both the optical system transfer function and effects of the medium.

The use of continuous pregroove in optical disks creates various servo issues. This paper discusses some examples.

Two kinds of high-speed lens actuators and a positioning control method have been developed for a 3-beam overwritable magneto-optical disk drive, which has a data transfer rate of 2.1 MB/s and an average access time of less than 50 ms for ISO standard 130 mm disks. Both of these lens actuators, the relay lens type and the rotary type, have higher-order mechanical resonances of more than 35 kHz, and offer stable focusing and tracking at 5400 rpm. The 3- beam positioning control method consists of a direct track-counting seek and relative position control. The velocity control for the direct track-counting seek switches from the positioner to the lens actuator when the track counter reaches a few score so as to provide stable positioning onto the track.

The effects that the base bending resonance exert on the stability of the tracking servo system and the base design method considering the stability of the servo system are investigated using the simple two-degree-of-freedom model of base. As a result, a fundamental method of base design which does not make the servo system unstable is acquired.

A commonly used tracking method on pre-grooved magneto-optical (MO) media is the push- pull technique, and the astigmatic method is a popular focus-error detection approach. These two methods are analyzed using DIFFRACT^c, a general-purpose scalar diffraction modeling program, to observe the effects on the error signals due to focusing lens misalignment, Seidel aberrations, and optical crosstalk (feedthrough) between the focusing and tracking servos. Using the results of the astigmatic/push-pull system as a basis for comparison, a novel focus/track-error detection technique that utilizes a ring toric lens is evaluated as well as the obscuration method (focus error detection only).

We discuss the origin of focus servo feedthrough in optical storage drives and show modeling and experimental feedthrough results for various aberrations.

We investigated the effect of the Viterbi detection of partial response in a magneto-optical disk system using a magnetic field modulation recording, and achieve a wide phase margin at 0.4 micrometers /bit.

This paper presents a general method of measuring the carrier-to-noise ratio (C/N) on a sample servo (SS) media. A media spectra model was also developed and it is in good agreement with our experimental results.

Signal and noise characteristics are investigated in a magneto-optical signal detection method employing the Kerr effect and optical dichroism. In this detection method, elliptically polarized light is focused on the recording medium. The relationship between Kerr rotation angle and Kerr ellipticity for maximum carrier level is considered and experimentally investigated.

Magnetic force microscopy as a tool for characterization of magneto-optical recording media with submicron resolution is investigated. Experimental data are analyzed using a model which takes account of the micromagnetics of the needle.

A new, simplified readout waveform equation for an elliptical mark is analytically derived and confirmed. This approach easily treats an elliptical mark by using an equivalent rectangular mark having (delta) function width and equivalent readout beam spot. The equation is verified by comparison with precise numerical calculations. Good agreement is obtained for practical dimensions of optical recording. The equation gives the characteristics of the readout waveform directly from the elliptical mark.

In order to gain a more reliable description of the magnetic state in the exchange-coupled films, experiments on the Hall effect in multilayer structures and computer simulations of the observed Hall effect behavior, using the distributed domain wall model, were performed. It was demonstrated that this approach can give insight into the effect of exchange interaction on the magnetization directions in TbDyFeCo/TbFeCo exchange-coupled double layer films with various extra intermediate layer.

The transition jitters of a double-masked rear aperture detection (D-RAD) readout were calculated as a function of defocusing, disk tilt, and recording mark width. The modeling results successfully explain our experimental results. Large jitter tolerances and a low byte error rate were obtained using 0.3 micrometers bit length recording.

Measurement of the magneto-optical polar Kerr effect is performed on rare earth-transition metal (RE-TM) amorphous films using in-plane fields. From this measurement and the measurement of the saturation magnetization using a vibrating sample magnetometer (VSM), the magnetic anisotropy constants are determined. In this paper, we present the temperature dependence of the magnetic anisotropy in the range of -175 degree(s)C to 175 degree(s)C. The results show a dip in the anisotropy near magnetic compensation. This anomaly is explained based on the finite exchange coupling between the rare earth and transition metal subnetworks.

Exchange coupled MO films like (PtCo/TbFeCo) and TbFeCo/TbFeCo) enable a very small external field of only 50 Oe for erase and write. Those are suitable media for a field modulation overwriting. In plane magnetized PtCo or low perpendicular anisotropy TbFeCo acts as the external field enhancing layer through exchange coupling with the TbFeCo recording layer.

The material classes currently used or discussed as magneto-optic (MO) media, namely rare- earth transition-metal (RE-TM) alloys, Pt/Co multilayers (ML) and garnets are reviewed with emphasis on those properties which limit their use for MO recording. A comparison between RE-TM and Pt/Co with respect to thermal sensitivity and recording with shorter wavelengths is performed. We conclude that RE-TM based disks are the more sensitive medium and are also good candidates for `blue' recording due to their fine microstructure. We then discuss the energy balance for the stability of domains in exchange-coupled layers, media and stack designs to obtain stable domains, and the role of interface wall motion in the domain formation in such layers.

To increase the recording bit density of a magneto-optical disk using a shorter-wavelength laser, readout characteristics of disks with Pt/Co multilayered films were investigated by using a 532 nm SHG laser. The experiment using a (lambda) /4 - (lambda) /2 plate differential detection system showed that the disk noise of the Pt/Co disk was caused by a fluctuation of Kerr rotation angle, and that this noise component was reduced by sputter etching the surface of a SiN under-layer. A C/N ratio greater than 40 dB with a 0.45 micrometers domain length was obtained from this noise-suppressed Pt/Co disk.

Multilayer films such as (Co/Pt) are considered to be potential candidates for future high density optical recording media, since they exhibit a strong magneto-optic Kerr activity at wavelengths less than about 500 nm. The magnetic properties of those multilayer films, however, are very sensitive to the way the films are grown, and thus it is the key for achieving better performance to understand how they are correlated to the microstructure. Very little understanding has ever been achieved as to the correlation between the microstructure and the magnetic properties. This is particularly so for the coercivity, which is the important factor governing the domain shape and its size. In view of this aspect, we have systematically examined how the fabrication condition affects the microstructure of both Co/Pt and Fe/Pt multilayer films and in turn influences the magnetic properties.

A new and comprehensive dielectric tensor characterization instrument is presented for characterization of magneto-optical recording media and non-magnetic thin films. Random and systematic errors of the system are studied. A series of TbFe, TbFeCo, and Co/Pt samples with different composition and thicknesses are characterized for their optical and magneto- optical properties. The optical properties of several non-magnetic films are also measured.

A fast data transfer and high density recording was realized, using a 30 mw visible laser and a GeTeSb phase-change medium. Fast laser power control technique was used for suppressing the laser light power fluctuations, and a RIN of -123 dB was obtained. Using this technique, a CNR of 56 dB and an erase ratio of 25 dB was obtained with a mark pitch of 1.25 micrometers for an overwritable phase change medium.

The possibility of ultrahigh density recording has been shown using an overwritable phase change optical disk with a Ge-Te-Sb recording film. It was found that recording marks of 0.2 micrometers length can be formed stably and that high contrast ratio can be obtained in the wide range of wavelength. From these results, about six times as high linear density as the existing format is possible by using mark-edge recording or MCAV method. It was also found that the crosstalk of PCE optical disk is so small that double track density recording is possible by track pitch reduction or groove/land recording. These results show a possibility of large capacity of 10 GB for 5.25 inch diameter disk.

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.

Phase change optical storage is attractive for several reasons. However, the number of times the media can be reversibly cycled between the amorphous and crystalline state is in general less than magneto-optic media. It is the purpose of this paper to explore the effect of a metal layer on the ability to cycle the recording layer. Experiments were conducted which attempted to separate the thermal effect of the metal layer from other factors. We used ZnS as the capping dielectric, (GeTe)₈₅Sn₁₅ as the recording layer and aluminum as the metal layer.

Thermal deformation is calculated to clarify the cause of material flow due to multiple overwriting in a phase-change optical disk. The results suggest that the molten material of the recording layer moves along the recording track by thermal deformation of layers. Moreover, the direction of the material flow depends on the Al reflective layer thickness.

A general method for making optical media by a continuous roll process has been developed. The process can accommodate a variety of optical structures, although it has been optimized for those with smaller form factors and thin cover sheets. A test disk was used to demonstrate process capabilities and evaluate media made by this method.

An investigation into the fundamental properties of the photolithographic optical disc mastering process is presented. It is shown that, despite the extreme differences in processing conditions, some of the models used in IC-lithography are valid for this process. This opens the possibility of introducing some of the techniques for resolution enhancement, proposed in IC-lithography, into the optical disc mastering process.

Characteristics of optical read-only media (O-ROM) having both grooves for tracking and pits that represent data by length were studied. Servo signals were observed over a wide bandwidth up to the data clock frequency and, in addition, the surface structures of the disks were observed. These observations showed that servo signals depend not only on the depth of grooves but also on that of the embossed data pits. Optimizing the depths of pits and grooves based on the results of pit density dependency of servo signals is necessary to ensure good characteristics of pit and groove signals.

Magneto-optical multilayers are of interest to the optical data storage community as a possible second-generation medium of the future. The important Co/Pt-superlattice system is introduced in this respect, and an extensive reference listing is provided to previous research. Magneto-optical modeling studies of Co/Pt are presented, and it is concluded that the interfacial Pt is magnetized and is magneto-optically active at the short wavelengths of interest (approximately 4 eV) for applications. Magneto-optics in the ultrathin limit are discussed, and an additivity law is presented and verified experimentally utilizing data for epitaxial Fe/Ag(111) superlattices. Finally, the surface magnetic anisotropy that provides the vertical easy axes of magnetization in candidate superlattice systems is discussed and illustrated experimentally using ultrathin epitaxial films of Fe grown on a variety of substrates. It is concluded that magneto-optic multilayers will provide many stimulating basic and applied challenges in the years ahead.

The free energy associated with the formation of the crystalmelt interface ("interfacial tension") is an important parameter in the theory of homogeneous nucleation. In the proximity of a crystal plane the liquid is more ordered than in the bulk. Static structural modeling1, as well as molecular dynamics simulations2, show that the liquid atoms are localized to a varying degree in planes parallel to the interface. The loss in entropy associated with the ordering of the liquid is the main factor contributing to the interfacial tension1 . Density functional calculations on hard sphere systems have confirmed this3.

The surface oxidation processes of thin films of magneto-optic materials, such as the rare- earth transition metal alloys, have been studied starting in ultrahigh vacuum environments using surface analysis techniques, as a way of modeling the oxidation processes which occur at the base of a defect in an overcoated material at the instant of exposure to ambient environments. Materials examined have included FeTbCo alloys, as well as those same materials with low percentages of added elements, such as Ta, and their reactivities to both O₂ and H₂O compared with materials such as thin Fe films coated with ultrathin adlayers of Ti. The surface oxidation pathways for these materials are reviewed, and XPS data presented which indicates the type of oxides formed, and a critical region of Ta concentration which provides optimum protection.

Environmental stability is particularly important for thin film materials since small changes in surface structure and chemistry may result in significant degradation of the magnetic or optical properties. Two aspects of the corrosion chemistry of these materials are discussed. Dealloying processes may be important for rare earth/transition metal systems due to the thermodynamic driving force for selective dissolution of the rare earth elements. Corrosion resistance of many metals and alloys is related to the formation of an interfacial oxide layer. The formation and stability of these films are important for both alloy and multilayer systems.

A narrower track pitch for magneto-optical recording is evaluated by simulation and experiment for a higher-capacity disk subsystem. This experimental disk subsystem uses a sampled servo format with currently available optical components. The simulation estimates the recording track pitch limit for conventional laser wavelengths of 780 nm, and the effects of narrower track pitch recording is examined in actual sampled servo format disks. The reproduced signals are evaluated from the jitter in the time domain, and erase characteristics are evaluated from the decrease in signal amplitude. A track pitch of 1.0 micrometers is shown feasible by the experimental results if the recording pulse waveform is optimized and a pulsed erase method is used to reduce cross-erase.

Methodologies for determining extrapolated life expectancy values for optical disk media were investigated at the National Institute of Standards and Technology (NIST). These investigations showed that the life expectancy values of optical disk media vaiy greatly due to several factors, including the method used to derive the value of the quality parameter (the byte error rate), the areas measured in the disks, the written patterns used and the criteria for data analysis. Vendor's life expectancy claims can be properly assessed by prospective users if standard methods for calculating and reporting extrapolated life expectancy values are implemented.

A write-once data storage media has been developed which is suitable for optical tape applications. The media is manufactured using a continuous film process to deposit a ternary alloy of tin, bismuth, and copper. This laser sensitive layer is sputter deposited onto commercial plastic web as a single-layer thin film. A second layer is sequentially deposited on top of the alloy to enhance the media performance and act as an abrasion resistant hard overcoat. The media was observed to have laser write sensitivities of less than 2.0 njoules/bit, carrier-to-noise levels of greater than 50 dBs, modulation depths of approximately 100%, read-margins of greater than 35, uniform grain sizes of less than 200 angstroms, and a media lifetime that exceeds 10 years. Prototype tape media was produced for use in the CREO drive system. The active and overcoat materials are first sputter deposited onto three mil PET film in a single pass through the vacuum coating system, and then converted down into multiple reels of 35 mm X 880 m tape. One mil PET film was also coated in this manner and then slit and packaged into 3480 tape cartridges.

Characteristics of the bit-capsule recording are studied. Every information bit is recorded on phase change erasable material in an enclosed bit-capsule using sample-servo technique. It is confirmed that the bit-capsule is effective on size-limited mark formation or low distortion recording, which is inevitable for high density recording.

Multilayered refractive data storage allows serial optical recording at data densities exceeding 10¹²bits/cm³. Data may be recorded in a write once mode using two-photon excitation of a photopolymer film. Data is read with minimal cross-talk between layers by wavefront shearing interferometry, for example by differential interference contrast microscopy. The readout signal from such a multilayered refractive memory initially increases with the refractive index contrast. However, high index contrast may eventually lead to excessive scattering which could reduce signal quality. We report here an empirical method for quantitatively assessing the readout performance of multilayer refractive memory media using laser scanning differential interference contrast microscopy and simple image processing. Signal level distributions for a multilayered refractive write once data storage structure are presented.

We have developed a 428 nm blue laser source based on resonator-enhanced frequency doubling of a GaAlAs diode laser and have used this source to demonstrate writing and reading on an MO disk. The power, beam quality, and noise of the blue laser are sufficient for high-density optical recording.

We developed a 5 inch form factor magneto-optical disk drive with overwrite and real-time verification functions. A 2.1 MB/s data transfer rate for both reading and writing and an average seek time of less than 35 ms have been achieved. The 3-beams drive uses 130 mm ISO-standard magneto-optical disks. This high-performance magneto-optical disk drive provides high-speed operation and low-cost storage and is suitable for high-end systems such as optical libraries and image-data processing systems which require a high transfer rate and huge data capacity.

It is found that an equivalent temperature profile and mark size produces one million cycle stable overwrite characteristics. The linear velocity at constant angular velocity (CAV) disk rotation mode varies by a factor of two between the outermost track and the innermost track. Varying the laser pulse width can achieve an equivalent mark formation process at both inner and outer track positions. The configuration of the phase change disk media features a quadrilayer rapid cooling structure. Our disk has a new structure, consisting of a dielectric layer combination of a ZnS-SiO2 mixture layer and an SiO2 layer. The equivalent thermal profile was determined using a heating and cooling process simulation method. A large pulse width of 70 ns on the inner track gives the equivalent temperature profile of a smaller 50 ns pulse width on the outer track. This method produces million-cycle-overwrite characteristics through the whole linear velocity range on the disk in a large capacity MCAV system.

A recordable compact disc (CD-R), developed by the authors in 1988, is now widely used for professional audio and data applications. It is a write-once optical disc consisting of a polycarbonate substrate, a light absorbing layer of dyes, a light reflecting layer of metal, and a protective layer. The main features of CD-R compared with other recordable optical discs are its high reflectivity of more than 70% and a large modulation amplitude of around 75% for compatibility with existing compact disc (CD) systems. The disc structure for achieving such high reflectivity and some analyses of the recording mechanism were reported by the authors in 1989. Further discussions on the recording mechanism were made by Holtslag et. al. in 1991. However, those papers did not clarify the mechanism of obtaining the large signal modulation. This paper reports that both the decomposition of dye in the light absorbing layer during recording and the deformation of the substrate surface are the key factors causing the large modulation of the CD-R.

This paper describes the features, specifications, and control of a three-beam magneto optical disk drive, which has write data transfer rate of 2.1 MB/s with verification and average seek time of less than 35 ms.