Super-resolution Disk Research Articles

From the channel model of an InSb-based superresolution optical disc system to impulse response and resolution limits

The signal model of a superresolution optical channel can be an efficient tool for developing components of an associated high-density optical disc system. While the behavior of the laser diode, aperture, lens, and detector are properly described, a general mathematical model of the superresolution disc itself has not yet been available until recently. Different approaches have been made to describe the properties of a mask layer, mainly based on temperature- or power-dependent nonlinear effects. A complete signal-based or phenomenological optical channel model--from non-return-to-zero inverted input to disc readout signal--has recently been developed including the reflectivity of a superresolution disc with InSb used for the mask layer. In this contribution, the model is now extended and applied to a moving disc including a land-and-pit structure, and results are compared with data read from real superresolution discs. Both impulse response and resolution limits are derived and discussed. Thus the model provides a bridge from physical to readout signal properties, which count after all. The presented approach allows judging of the suitability of a mask layer material for storage density enhancement already based on static experiments, i.e., even before developing an associated disc drive.

Channel model for InSb-based superresolution optical disc system

A signal model of a superresolution optical channel would be an efficient tool for developing components of an associated high-density optical disc system. While the behavior of the laser diode, aperture, lens, and detector is properly described, a general mathematical model of the superresolution disc itself is not yet available. However, different approaches have been made to describe the properties of a mask layer, mainly based on temperature- or power-dependent nonlinear effects. The main problem of the modeling is that temperature-dependent material properties, such as thermal conductivity and refractive indices, are not known or not accurate enough to allow quantitative predictions. Therefore, it could be useful to define a signal-based or phenomenological model that can be calibrated with experimental data. In this contribution, we developed a complete optical channel model--from non-return-to-zero inverted (NRZI) input to disc readout signal--including the reflectivity of a superresolution disc with InSb used for the mask layer. Model parameters are derived from data measured using a static tester. The model is finally applied to a configuration appropriate for a dynamic superresolution optical drive by moving the focused spot relative to the material.

Optical nonlinear absorption characteristics of AgInSbTe phase change thin films

The AgInSbTe phase change thin films are very important as optical recording and the super-resolution mask materials in high density optical information storage. In this work, the effective nonlinear absorption coefficients of amorphous and crystalline AgInSbTe thin films were measured by the open-mode Z-scan method and no evidence of nonlinear refraction was found in the closed-mode Z-scan measurement. The effective nonlinear absorption coefficient βeff of amorphous AgInSbTe thin films is 7.53×10−3 m/W and the effective photon-absorption number n is 1.722; βeff of crystalline AgInSbTe thin films is 3.5×10−2 m/W, which is of an order lager than that of amorphous state, and the n value is 1.7011. The giant nonlinearity of AgInSbTe results from the free carrier absorption in the nanosecond time scale and this characteristic should be responsible for the mechanism of optical recording as well as the readout of super-resolution disk.

Recordable Multilayer Super-Resolution Optical Disc Realized Using Discrete-Track Three-Dimensional Pit Selection

We propose a recordable multilayer super-resolution method (discrete-track three-dimensional pit selection) to realize an optical disc with a sub-terabyte data capacity. The fundamental super-resolution characteristics of a discrete-track (DT) super-resolution disc were experimentally examined comparing to that of the conventional super-resolution disc. The experimental results show that the DT super-resolution disc exhibits high optical resolution equal to that of the conventional disc and an optical transmittance of 71%, sufficient for multilayer super-resolution. The results of the disc design indicated the possibility of a data capacity of 500 GB/disc.

3D FDTD Method Analysis of Light-beam Scattering from a RAD-MSR Disk Models

Recently, because of the increase of computer information handled by computers, higher density storage devices are expected. The magnetically super resolution (MSR) is one of the high density versions for improving the conventional magneto-optical (MO) disk. The rear aperture detection (RAD) is proposed to increase the capacity by ten times or more. The center aperture detection (CAD) is also proposed for advanced storage magneto-optical (ASMO) disks. Moreover, narrower focused beam can be realized by using blue laser light, so that the track pitch can be much more reduced. Therefore, narrower track pitches give rise to the increase of the crosstalk between adjacent tracks. In the present paper, we try to apply the three-dimensional flnite-difierence time-domain (FDTD) method to the analysis of the light-beam scattering and the characteristic of detected signal from a rear aperture detection-magnetic super resolution (RAD-MSR) disk model and examine the in∞uences of the groove depth on the crosstalk characteristics.

Stability and Signal Quality Enhancement of Super-Resolution Disc by Pre- and Post-Curings of Readout Layer

By applying pre- and post-curings to the fabrication of a readout layer of a super-resolution disc that uses a phase-change material for the readout layer, the short-term readout stability and the signal quality were greatly improved. We obtained a disc whose signal degradation does not occur after the curing and the noise level was reduced by 1–2 dB, carrier-to-noise ratio (CNR) was enhanced by 2–3 dB, and the jitter was enhanced by 2–3%. Moreover, for long-term readout stability, we chose GeN interfacial layers between the readout and dielectric layers and obtained stable signal quality in terms of CNR of up to over 0.2 million readings.

On a thermally induced readout mechanism in super-resolution optical disks

We have simultaneously measured the carrier-to-noise ratio (CNR) as well as the transmitted and reflected light intensities from platinum oxide based super-resolution near-field structure (PtOx super-RENS) disks. The the reflected and transmitted light intensities were found to decrease and increase, respectively, as the CNR value increased. The phase-change material AgInSbTe (AIST) used in PtOx super-RENS disks was found to exhibit a strong optical nonlinearity with respect to readout laser power. AIST becomes transparent at higher laser powers. To ascertain whether the presence of Pt nanoparticles is important to the readout mechanism, a super-RENS disk was fabricated in which the PtOx layer was replaced with a metal-free phthalocyanine (H2Pc) layer and the CNR of the H2Pc disk was measured. From the observation that the CNR value was equivalent to that of a disk made using PtOx, we conclude that the presence of nanoparticles does not play an important role in the super-RENS readout mechanism. Finally, we also investigated the use of Si and the alloy Ge2Sb2Te5 in lieu of AIST in a super-RENS disk and simple three layer structure disks. The super-resolution effect was observed for all disk types. Based upon these observations, we discuss the possibility of a thermal origin for the super-resolution effect in all super-resolution disks.

Super-resolution disk technology for optical recording

Super-resolution disk technology for optical recording

A New Type Center Aperture Detection Magnetically Induced Super-resolution Disk Using Exchange-Coupling Force

A new type center aperture detection (CAD) magnetically induced super-resolution (MSR) disk is described. A magneto-static type CAD disk was already reported last year, and it improves readout stability withou deteriorating the readout characteristics such as readout resolution by using a gadolinium intermediate layer instead of a non-magnetic layer. Our group studied the intermediate layer further, and realized the exchange coupling type CAD disk that has high-resolution and readout stability simultaneously by using GdFeCo intermediate layer. It has large tolerance to the external magnetic field at readout. The tolerance became about three times larger than the magneto-static coupling type CAD reported before.

Read-Out Signal Simulation of an Optical Disk Having an Oxide Super-Resolution Film

The read-out signal simulation of an optical disk having a cobalt-oxide super-resolution film was performed to find the optimum condition for a higher recording density of the super-resolution disk. A simple model of the complex refractive index change in a super-resolution film due to its temperature was applied, which can explain experimental data. It was found that this type of super-resolution disk must increase its reflectivity as the incident laser-beam intensity rises. To confirm this experimentally, a new super-resolution disk with an added Ge–N layer was prepared, and its read-out signals were measured, which agreed well with the calculated results.

High-Density Optical Memory using Photochromic Diarylethenes

The great interest in photochromic materials for high-density optical memory has been increased for the past decade. A super-low power readout method has been proposed to solve a problem about degradation of recorded information by readout process. About 106-time readout was demonstrated by the method. New high-density optical memory, a super-resolution disk with a photochromic mask layer, was introduced. The super-resolution method has enabled us to increase the recording density of conventional optical disks. Ultra-high density near-field photochromic memory as a new innovative storage technology was also discussed.

Recording and readout using clock marks premastered by groove wobbling

A new recording and readout technique for land and groove recording on a magnetic super-resolution (MSR) disk is described. The technique uses specifically premastered clock marks. To generate a stable clock signal, the clock marks are fabricated with short bursts in grooves by wobbling at a different frequency from address information. The clock marks and the address information can be separated from a track error signal. When the extracted clock signal is applied to precise recording and readout of the magneto-optical data marks, the bit error rate becomes lower than that of the conventional clock recording/readout system. Additionally, the crosstalk of the wobbled address information to the magneto-optical signal can be canceled out electronically. This new format is not only suitable for high-density recording but also convenient in the disk manufacturing process.

Applying an Objective Lens of 0.7-Numerical Aperture to a Center-Aperture-Detection Type of Magnetically Induced Superresolution Disk

We studied the performance of the center-aperture-detection type of magnetically induced superresolution (CAD-MSR) disk using an objective lens with a numerical aperture (NA) of 0.7. We confirmed the practicability of a 10 Gbyte user capacity and the feasibility of an 11.5 Gbyte user capacity on a 120-mm-sized disk by the evaluation of the system margins. We also evaluated system margins by using an objective lens with an NA of 0.6. We have confirmed that the performance of the CAD-MSR disk depends on the NA of the objective lens.

Synchronous Trigger Detection for Pulse Laser Readout on Super Resolution Erasable Optical Disks

The laser pulse readout scheme is advantageous in reading thermal super resolution disks, but the light modulation on the RF signal confuses the detection channel. A synchronous trigger detection scheme, using the pulse laser trigger signal to pick up the desired signal, is proposed to suppress the adverse effects of the light modulation. A 5-dB increase of the signal on an magneto-optical super resolution center aperture detection disk by the synchronous detection system was demonstrated experimentally.

Noise and coupling in magnetic super-resolution media for magneto-optical readout

Interfacial magnetic coupling, transient thermal response, and carrier and noise levels are investigated for two central aperture detection magnetic super-resolution disks. In one of the disks, the two magnetic layers are exchange-coupled, while in the other the coupling is of magneto-static nature. For the exchange-coupled disk, the coupling between the two magnetic layers is fairly strong, and the Kerr loop of the readout layer does not have a square shape. For the magneto-statically coupled disk, the strength of coupling depends on the nonuniformity of the magnetization of the storage layer. The readout layer has a square Kerr loop, but its perpendicular magnetization in the hot region under the focused spot has random orientation if the stray field from the storage layer is weak. This random orientation of magnetization within the readout layer gives rise to a high level of noise during readout.

Effects of Aperture Wall on the Readout Signal of Erasable Thermal Phase Change Super Resolution (EPSR) Disks

A “below-diffraction-limited” aperture within the readout spot for high-density recording can be achieved on erasable thermal phase change super resolution (EPSR) disks. Compared to the large aperture wall formed by the DC detection method, the small aperture wall formed by pulse readout can not only increase the signal amplitude, but also suppress readout noise when detecting the EPSR disks. The small aperture wall width generated by pulse readout in the EPSR disk resulted in an 8 dB higher in carrier-to-noise (CNR) than that by DC readout in the conventional single-recording-layer phase change disk when reading the below-diffraction-limited marks of 0.4 µm.

Feasibility Study of CAD-MSR with 7 Gbit/in2

Feasibility of achieving an areal density over 7 Gbit/in2 was examined in a center aperture detection type of magnetically induced super resolution disk. We confirmed land and groove recording with a 0.45 μm track pitch was available with practical margins by the use of a conventional optics which consists of a 640 nm wavelength laser and a 0.6 numerical aperture lens. The available areal density reached to 6.8 Gbit/in2 and over 9 Gbyte user-capacity in a 120 mm diameter single side disk can be realized.

Pulse–read on erasable thermal phase-change superresolution disks

New erasable thermal phase-change superresolution (EPSR) disks composed of mask and recording layers can increase recording density by the detection of the below-diffraction-limited marks within the readout spot. The formation of the aperture and the readout signal on the EPSR disk were analyzed. The feasibility of optically designed EPSR disks was evaluated by thermal simulation. A carrier-to-noise ratio of 32 dB at a mark size of 0.4 mum, 8 dB higher than that of a conventional disk, was obtained by application of a pulse-read method to the EPSR disks at a wavelength of 780 nm and a numerical aperture of 0.55.

Magnetic Domain Expansion Phenomena in a Double Mask Type Magnetically Induced Super Resolution Magneto-Optical Disk

The readout mechanism in a trilayer double mask type rear aperture detection (DRAD) magnetically induced super resolution (MSR) disk was studied by readout waveform analysis. A signal enhancement phenomenon was observed, which was ascribed to magnetic domain expansion. Two kinds of magnetic domain expansion mechanisms for the erase direction readout field (Hr) and for the write direction Hr were found. In the case of write direction Hr, a magnetic domain can expand toward the rear mask before the mark contacts with the rear mask from the estimation of mark size and mask position. Edge jitter was evaluated in each readout mode and characterized according to the readout mechanism. Both high-resolution power of MSR and signal amplitude enhancement due to magnetic domain expansion contributed to improvement of DRAD disk readout signal qualities.

Photochromism and Its Application to a High-density Optical Memory

In order to solve problems of photochromic memory, a super-low power readout method and a crosstalk reduction method for multi-wavelength recording were proposed. In the demonstration of 2-wavelength recording using photochromic diarylethenes, the crosstalk between multiplexed channels, which is caused from broad absorption bands of compounds, was effectively reduced by the method. A new high density optical memory, a super-resolution disk with a photochromic mask layer, was also introduced. The super-resolution method has enabled us to increase the recording density of conventional optical disks.