We manufactured a high-frequency wobbled groove disk, and examined reading/writing a magneto-optical signal by using a clock generated from the wobbled groove. We also measured clock jitter and bit error rate. The results of this were clock jitter of under 2.5 ns, and a bit error rate almost equal to that of a self-clock. This wobbled groove clock method is therefore effective for higher density recording.

Tri-layer garnet film for wavelength multiplex reproduction was prepared by the laser ablation method using a new 5^th- order Nd-YAG laser, and the magneto-optical effect was measured. By changing the composition of each layer in the tri-layer, it is possible to read out signals from each layer without interference using another wavelength. Selective writing to each layer is also possible using the difference in the change in temperature of coercive force.

By nitrogen doping into a Ge-Sb-Te phase change optical disk's recording layer, we were able to significantly increase its cyclability. For example, our PD attained, at the maximum, 800,000 overwrite cycles through accurate control of nitrogen concentration. We quantified the nitrogen concentration of recording layer using secondary ion mass spectrometry (SIMS) and determined, from the viewpoint of cyclability, signal amplitude and other parameters, the optimum concentration to be around 2 - 3 at.%. From analyses by thermal desorption mass spectrometry (TDMS) and X-ray diffraction (XD) using powder, we found: (1) nitrogen atoms are mainly bound with Ge to create an amorphous phase of Ge-N; (2) as long as the nitrogen concentration remains around 5 at.%, those Ge, Sb and Te atoms which are not bound with nitrogen form NaCl type crystals. We obtained the following model by combining the results of the above analysis. Nitrogen-doped Ge-Sb-Te recording layer is composed of Ge-Sb-Te grains intermingled with a small quantity of amorphous Ge-N, which exists in the form of a thin film penetrating the grain boundary of Ge-Sb-Te. The Ge-N composing this high-melting-point material layer appears to suppress any micro-material-flow that may occur during overwrite.

Two marked effects are obtained by forming a Ge-N interface layer on either side of Ge-Sb-Te recording layer. One effect is a suppression of atomic diffusion between Ge-Sb-Te layer and protective layers, ZnS-SiO₂ representatively, which leads to a significant improvement in overwrite cyclability, and the other is the acceleration of crystallization process which leads to higher speed optical disks. A rapid-cooling type experimental disk with Ge-N layers on both sides of the Ge-Sb-Te recording layer proved to be capable of exceeding 10⁵ cycle overwrites and a recording data rate 40 Mbps at linear velocity 12 m/s. The recording conditions: bit length 0.28 micrometer and track pitch 0.6 micrometer (L/G method) using laser source with a wavelength 658 nm and a numerical aperture 0.6 correspond to a capacity 4.7 GB/(phi) 120 mm.

A near-field optical recording technique using a solid immersion lens (SIL) and a red semiconductor laser light source ((lambda) equals 650 nm) has been applied to the phase change optical recording. We have recorded and erased amorphous marks of 300 nm in diameter using a hemisphere SIL (n equals 1.84, designed effective NA equals 1.1).

The tolerances of Supersphere Solid Immersion Lens (SSIL) system in aberration were analyzed. The maximum deviation between objective lens and SSIL was given in several different optical parameters. The optical coupling efficiency of the SSIL to the air space were also calculated in these cases.

All-solid-state cw 266 nm laser operates greater than 1000 hours with diffraction-limited beam and low noise output (-130 dB/Hz), which is suitable for next-generation disk mastering.

We have succeeded in realizing a monolithic optical pick-up device capable of confocal detection. The jitter was 4.0 ns with a line velocity of 1.2 m/s. The DC offsets were less than 10% for a lens radical displacement of plus or minus 400 mm and a radical skew of plus or minus 2 degrees. These small DC offsets were due to confocal detection.

We discuss several options that might lead the way to a single-sided optical phase change disc with a capacity of 10 - 20 GB, i.e. dual layer, high numerical aperture, and blue recording.

A new thermal model to study the laser induced temperature profile of a multilayered phase change optical recording disk is proposed. The new model considers the thermal effect generated by both the transmission and reflection light. The calculation formulae are listed. The model is used to simulate the Ge₂Sb₂Te₅ phase change optical disks with five layers structure. In order to study the differences between the new and existing models, simulations are carried out using both models and the differences are compared. The differences get larger as the phase change layer becomes thinner. It is also revealed that the differences get larger as the wavelength becomes shorter. The idea proposed in this paper is also suitable for the analysis of magneto-optical disks as well as for improved accuracy in the measurement of thermal parameters.

A scanner-based storage system employs a head mounted on a scanner which oscillates over the moving media. The head moves in an approximately sinusoidal path relative to the media at a high frequency, time-multiplexing the read/write signals of several tracks. The resulting multi-channel readback can yield higher data rates over a conventional system with a head that moves linearly relative to the media. Scanner-based storage systems are not commercially available at present. We are envisioning a system that uses an opto-electronic scanner, developed at CMU, in which the deflection of a laser beam is controlled by an input voltage. Since no mechanical motion is involved, this scanner has a high bandwidth which makes it well suited to our application.

An optical disc system has been proposed that stores data holographically in three dimensions as microholograms combining different multiplexing techniques. Such microholograms can be stored overlapping in the same volume by using angle multiplexing, wavelength multiplexing or the combination of both. Storage of several information bits in one single position on the disc increases the data rate so that the total storage capacity could reach more than 100 GBytes on a DVD sized disc. In contrast to photorefractive memory systems, the storage media are made of cheap and mass- producible photopolymer layers instead of expensive crystals. Furthermore, the microholographic storage method can be downward compatible with today's Compact-Disk (CD)- and Digital-Versatile-Disk (DVD)-systems. In this paper we report on wavelength multiplexing of microscopic reflection holograms and on thermal processing of the photopolymer.

A two-dimensional channel model for phase change recording media was developed based on two simple models. An adiabatic model neglecting thermal diffusion was used during mark generation and a geometric optics model was assumed during the readout process. The channel model contains a nonlinear mapping between two linear systems and thus the whole channel is inherently nonlinear.

A 4-focal length, telocentric holographic storage system architecture usually employs an aperture stop. The aperture plays an important role in determining the areal density of storage as well as the extent of inter-symbol interference. We propose a figure-of-merit for such an aperture. For simplicity, we assume that a zero-forcing equalizer is used to compensate for inter-symbol interference caused by the point spread function.

We have developed a loading mechanism of DVD-RAM drive for DVD-RAM, PD, DVD-ROM, and CDs. To load both a cartridge and a bare disc, a pair of disc holding members is equipped in a cartridge holder. The front half of the cartridge holder is drawn out of the drive, and a cartridge or a disc is inserted from the front, and so the drive can be used in both horizontal and vertical setup directions. The drive mechanism height is minimized to 31.6 mm by pulling the cartridge holder into the drive after opening the cartridge shutter. It provides enough space for the electric circuit board below the mechanism in the height of 41.3 mm of the half height drive. When a disc is inserted, the cartridge holder is pulled into the drive without pushing the cartridge holder using sensors in the cartridge holder.

The read-out signal patterns by MAMMOS were classified and suitable read-out parameter (laser power, magnetic field) was estimated. By adjusting these parameters, both 0.2 micrometer size packed domain and modulated domains (T equals 0.2 micrometer) could be reproduced. The MAMMOS read-out from variously lengthened domains and eye pattern were obtained by application of a magnetic field synchronized with clock marks embossed on the disk.

Calculation method for focal positions in a multi-beam optical head using a multi-beam laser diode is introduced. In this calculation model, focal positions were so calculated that a light source of each laser beam with a specific source height and an astigmatic difference was imaged by optical lenses and a beam shaping prism. Calculated results show that four focal positions are located each other with defocus along the optical axis due to the curvature of field of lenses. Astigmatic differences in focal spots caused by the laser diode characteristics can be decreased by almost zero with a displacement of a collimator lens along the optical axis.

We report on the holographic storage of multiple digital data pages in photopolymer media. The results demonstrate the potential of these materials for high density data storage.

Fixing of holograms recorded in a Fe-doped LiNbO3 crystal of 5 mm in thickness was achieved by heating with a CO₂ laser. A series of 4 laser pulses at a power level of 7 watts and a total fixing time of 80 seconds was used. In this first demonstration of in situ postrecording fixing by laser heating a fixing efficiency of 30% was obtained.

We report a new technique for high-density, high-speed optical memory based on two-photon coherence excited spin echoes in solids. Unlike conventional echo-based optical memory techniques, a two-photon coherence excited spin echo memory has potential for Tbits/cm² in memory density and THz in data processing speed at high temperatures.

Tilt is a major source of distortion for the DVD channel. This paper is aimed at understanding the effect of tilt on the readback signal. Equalized signals were collected from DVD-ROM media on an optical spin stand with a variable tilt stage. A simulated version of duty cycle correction (DCC) was applied to each signal to cancel the baseline drift. The pulse response, which characterizes the signal under the linear superposition assumption, was extracted for each signal. The pulse response of the DVD-ROM channel is shown to be highly affected by tangential tilt while the effect of radial tilt is minimal.

Using a two-element objective lens, over 0.9 NA lens and long WD lens are demonstrated. Using a thin cover layer, problems for skew margin and thickness tolerance are solved. Using a red LD, a 0.85 NA lens and a 0.1 mm cover PC disk, 8 GB rewritable disk system has been developed. Using a blue-green LD and an SHG blue laser, promising results are demonstrated for 12 GB and 17 GB of capacity.

We investigate the readout signal with near field SIL by using vector diffraction theory. Readout signal from phase-change medium and optical ROM is analyzed. Air gap dependence and polarization dependence is discussed.

We introduce an adaptive equalization technique that compensates for the channel nonlinearities causing asymmetric eye-patterns of RF signals in a digital versatile disk (DVD) system. Based on the observation that a DVD channel can be modeled as a cascade of modulation transfer function (MTF) and a zero-memory nonlinearity (ZNL), an adaptive maximum likelihhood sequence estimation (MLSE) receiver that estimates the nonlinearities and incorporates them in its metric calculation is developed. Computer simulation results demonstrate that the proposed receiver can outperform conventional MLSE when signal asymmetry is present.

We developed a phase change media that has less cross-erase at a track pitch of less than 0.6 micrometer. The phase change media has an absorptivity ratio A_c/A_a of 1.5 (A_c:absorptivity in a crystalline state, A_a:absorptivity in an amorphous state), while maintaining the rapid cooling structure. There was no significant cross-erase even at a track pitch of 0.52 micrometer. Feasibility of recording capacity of about 5 GB on a 120 mm disk has been confirmed.

The complex refractive index of phase-change Ge₂Sb_2+xTe₅ media fabricated by DC magnetron sputtering has been determined by using spectroscopic ellipsometry and atomic force microscopy. The composition of Ge₂Sb_2+xTe₅ analyzed by Inductively Coupled Plasma/Atomic Emission Spectrometer and X-ray Fluorescence was in the range 0.08 less than or equal to x less than or equal to 0.58. The complex refractive index and the reflectivity have been verified nearly constant with Sb-addition at 780 nm, 650 nm, and 410 nm, respectively.

We report on the holographic recording and read-out of microscopic Bragg-reflectors in DuPont Omnidex HRF-800 photopolymer films with 20 micrometer thickness. A Helium-Neon laser operating at (lambda) equals 632.8 nm was employed as light source. Lenses from Digital Versatile Disk (DVD) pick- ups were used to realize a beam waist with submicron diameter. The local distribution of the diffraction efficiency was investigated by applying a confocal scanning microscope setup. Holograms with a 1/e²-radius of 1.8 micrometer and 12 micrometer depth (FWHM) have been recorded. Due to shrinkage, the maximum diffraction efficiency was 0.4%, which can be improved by adjusting the read-out wavelength.

Optical pick-ups offering DVD, CD and CD-R/RW compatibility begin to use non-conventional optical components. Possible influences to HF and servo signals caused from these components are discussed. And the newly developed compatible pick-up is introduced.

An estimator is shown to accurately count tracks during a seek in a DVD or CDROM storage device. The estimator states are position and velocity. Although the gains can be adapted via the recursive Kalman equations, the track count accuracy suffers little degradation if the gains are fixed. The position estimate's bandwidth can be matched to the seek system poles affording optimum filtering. Although the input is a nonlinear function of position, the estimator is constructed in such a way as to maintain linearity.

Correlation techniques are used widely in communications, sonar, radar, medical and other signal processing systems. The most desirable property of correlation as a Digital Phase Detector (DPD) is the ability to measure the time delay between two similar signals. This paper proposes the use of correlation techniques to generate DPD signals which can be used in place of conventional phase detectors and filters. The proposed correlator generates two quadrature position error signals and a plus or minus 1/2 track position error signal. The correlator length is adaptively adjusted to provide the appropriate signal bandwidth.

High density optical recording disk using vacuum evaporation deposited organic dye film is newly developed. As we investigate the signal characteristics of the vacuum evaporated disk in comparison with the disk fabricated by conventional spin-coat method, it is found that the vacuum evaporated disk has some features: to keep uniformity of tracking signal easily, high modulation, and wide power margin. Furthermore, it is clarified that the recording mechanism of the evaporated disk includes the hole formation by sublimation of the dye material. We confirm that the performance of the vacuum evaporated disk is high potentiality for DVD-R.

In this paper, use of recurrent neural network equalizer (RNNE) in place of linear equalizer (LE) to combat both linear and nonlinear distortions corrupting the Magneto-optical (MO) readback signal is discussed. It is shown that RNNE can outperform LE without introducing significant complexity. RNNE is used to equalize the MO recording readback signal corrupted by transition jitter, intersymbol interference (ISI) and additive white Gaussian Noise (AWGN) at a density of 50 kbpi. The MO signal is equalized to a partial response (PR) (1 + D) using either the RNNE or the LE and the equalizer's mean- squared-error (MSE) performance is compared. Then, the equalized signal is passed through a detector and it is shown that a signal equalized to a PR (1 + D) shape can be detected using either a bit-by-bit type of detector (BD) or a sequence detector implemented via Viterbi Algorithm (VA). The bit-error-rate (BER) performance of BD is compared to that of the Viterbi detector and it is shown that PR equalization of MO readback signals using RNNE improves MSE performance over linear equalizer, allowing use of BD rather than LE + Viterbi Algorithm with comparable BERs.

In general, phase change media have to be initialized prior to recording. Initialization for phase change optical disks involves crystallization of as-deposited amorphous recording films. An initialization-free technology will lead to a low production cost, which is very important for commercialization phase change optical disks. A crystallization assisted layer (CA layer) was deposited just before the sputtering of a recording layer GeSbTe. The CA layer is assist to crystallize GeSbTe film during sputtering. We have confirmed by X-ray diffraction method that the recording layer was crystallized just after sputtering. Our proposed disk with CA layer show the similar performance to the 2.6 GBytes digital video disk- random access memory (DVD-RAM) disk.

For realizing 5.2 GB/side recording capacity Multimedia video file (MMVF), we have been developing mastering technologies, which is requested to fabricate the fine preformat patterns (L/G pitch 0.578 micrometer, minimum pit length 0.39 micrometer), optimizing exposure processes through the verification by simulation technique. With an optimized, i.e. elliptical exposure beam profile, 2T pits of minimum pit length for ideal resolution by simulation can be fabricated. Optimizing the exposure beam shift makes it possible to easily fabricate a header whose shift difference from ideal shift (0.289 micrometer) is much smaller than the plus or minus 0.05 micrometer difference that begins to decrease prepit amplitude.

Phase-change optical data storage is basically based on the difference in optical properties between amorphous and crystalline phases of a recording medium. It has been known that the physical properties including reflectance and thermal conductivity of a material basically depend on the chemical composition of materials and their microstructure. Lately, the manipulation of chemical composition in GeSbTe-based chalcogenide that is one of the most promising candidate materials for phase-change recording medium was tried by doping a small amount of nitrogen in Ge-Sb-Te phase-change optical disks. From the earlier work, an enhanced media cyclability was achieved in PPM recording at the wavelength of around 780 nm. The goal of this research is to investigate the variation of thermal, optical and dynamic properties, and their correlation to microstructure in nitrogen-doped Ge₂Sb₂Te₅ phase-change recording media. Primary endeavors focused on the dependence of nitrogen doping on crystallization temperature, reflectance, initializing power level, CNR and overwrite jitter at the wavelength of 650 nm and pulse-width modulation (PWM) recording, leading to higher storage density.

Differential phase detection (DPD) is a method of generating a tracking error signal from rotation of the irradiance pattern formed in the pupil of an optical data storage system. The amount of rotation is related to the distance between the scanning spot and the center of the track being scanned. Media parameters, such as reflectivities of the marks and land and the groove depth affect the DPD signal. The representation of mark and land reflectivity vectors on a complex plane diagram proves to be an elegant technique for designers to optimize a given medium for DPD.

In general, phase change media have to be initialized prior to recording. Initialization for phase change optical disks involves crystallization of as-deposited amorphous recording films. An initialization-free technology will lead to a low production cost, which is very important for commercializing phase change optical disks. A five-layered phase change optical disk, which has a thin metal layer between the substrate and the lower dielectric layer, has a good overwriting jitter at high linear velocity. We propose to use the higher reflective amorphous state of a five-layered phase change optical disk as the prerecording state without initialization. Our proposed new concept of phase change optical recording has some advantages, such as initialization- free, large signal amplitude of a header field, servo stability and high performance of overwriting jitter. Our proposed disk shows the performance to the 2.6 GBytes digital video disk-random access memory (DVD-RAM) disk.

Work is in progress for improving the switching times of erasable as well as DOW type of optical memories. As it has been already established that the erasure time (ayieldsc) is dominated by incubation time ((tau) ), hence (tau) needs to be explored more in depth. In the present work the mass, molecular diameter and density effects have been shown in addition to the temperature effects. The earlier calculations could only estimate the value of (tau) using a range of critical radii values, whereas the present model can be used to calculate the exact values of critical radii using shape factor calculations for grain boundary, grain edge and grain corner nucleation separately. Knowing the material parameter of a system, one can calculate variation of (tau) with temperature, mass or composition.

A new optical dada storage technology based on the surface- enhanced Raman scattering (SERS) effect has been developed for high-density optical memory and three-dimensional data storage. With the surface-enhanced Raman optical data storage (SERODS) technology, the molecular interactions between the optical layer molecules and the nanostructured metal substrate are modified by the writing laser, changing their SERS properties to encode information as bits. Since the SERS properties are extremely sensitive to molecular nano- environments, very small 'spectrochemical holes' approaching the diffraction limit can be produced for the writing process. The SERODS device uses a reading laser to induce the SERS emission of molecules on the disk and a photometric detector tuned to the frequency of the RAMAN spectrum to retrieve the stored information. The results illustrate that SERODS is capable of three-dimensional data storage and has the potential to achieve higher storage density than currently available optical data storage systems.