Multilayer Optical Disk Research Articles

Design of a four-sided magnetic circuit for an optical pickup actuator for multilayer high-density optical disks

Large-capacity optical storage materials have reached a stage of relative maturity, but progress in the design of optical pickup actuators is currently slow. To improve the reading and writing capabilities of optical recording systems, we propose a design for a four-sided magnetic circuit for an optical pickup actuator for multilayer high-density optical disks that make full use of the space around the actuator. The basic magnetic circuit model in this design, which is derived by the equivalent magnetic charge method, can be applied to the design of all such actuators. The static and dynamic magnetic circuit evaluation parameters established in this paper can be used conveniently and directly to improve the magnetic circuit performance of the actuator. The design was verified by finite element simulation, and linear working ranges of ±2.6 mm, ±0.6 mm, and ±16° were found in the focusing, tracking, and tilting degrees of freedom. This is sufficient to compensate for errors in phase-change optical disks with more than 50 layers in high-speed rotation. The theoretical servo accuracy of the actuator can reach tens of nanometers. This can be used with a dual-objective-lens layout that can support layer-skip reading and writing of optical disks with an effective working bandwidth greater than 10 000 Hz.

Activation of fluorescence of lactone forms of rhodamine dyes by photodehydrogenation of aryl(hetaryl)pyrazolines

Aryl(hetaryl)pyrazolines are effective photogenerators of acid providing an irreversible photochemical activation of fluorescence of rhodamine group dyes. The effect of solvent on two consecutive reactions, photodehydrogenation of pyrazoline and lactone ring opening of the dye was studied. An increase in solvent polarity leads to an increase in the rate of both reactions by different degrees. The systems under study are believed to be promising for the formation of recording media for multilayer optical file type disks of ultrahigh data storage capacity with fluorescent readout.

Decoupled direct tracking control system based on use of a virtual track for multilayer disk with a separate guide layer

We developed a decoupled direct tracking control system for multilayer optical disk that uses a separate guide layer. Data marks are recorded on a recording layer immediately above the guide layer by using two spatially separated spots with different wavelengths. Accurate data mark recording requires that the relative positions of the corresponding spots on the recording layer and guide layer are maintained. However, a disk tilt can shift their relative positions and cause previously recorded data marks to be overwritten. Additionally, a two-input/two-output control system is susceptible to mutual interference phenomenon between the two outputs, which can destabilize tracking control. A tracking control system based on use of data marks previously recorded as a virtual track has been developed that prevents spot shifting and mutual interference even if the disk tilt reaches 0.7°, thereby preventing overwriting.

A high transmittance optical recording material with long-term reliability for super-multilayer discs

As a means of increasing data capacity, the multilayer optical disc is a promising approach. Because the recording layers in multilayer optical discs must have a high transmittance, they are commonly made of transparent oxide films. Moreover, the recording layer must have sufficient long-term reliability for data archival. In this work, a recording material with high transmittance and long-term reliability for use in super-multilayer discs was investigated. This paper clarifies the recording mechanism of GeBi oxide material and proposes a suitable material design that satisfies the abovementioned characteristics. Furthermore, experimental results of recording on super-multilayer discs based on GeBi oxide recording material are presented.

Dual focus diffractive optical element with extended depth of focus

A dual focus property and an extended depth of focus were verified by a new type of diffractive lens displaying on liquid crystal on silicon (LCoS) devices. This type of lens is useful to read information on multilayer optical discs and tilted discs. The radial undulation of the phase groove on the diffractive lens gave the dual focus nature. The focal extension was performed by combining the dual focus lens with the axilens that was invented for expanding the depth of focus. The number of undulations did not affect the intensity along the optical axis but the central spot of the diffraction pattern.

Multilayer optical disc system using homodyne detection

A write/read system using high-productivity multilayer optical discs was developed. The recording medium used in the system consists of planar recording layers and a separated guide layer, and is fabricated by web coating and lamination process. The recording layers in the medium are made of one-photon-absorption material, on which data can be recorded with a normal laser diode. The developed system is capable of focusing and tracking on the medium and amplifying readout signals by using phase-diversity homodyne detection. A highly layer-selective focusing method using homodyne detection was also proposed. This method obtains stable focus-error signals with clearly separated S-shaped curves even when layer spacing is quite narrow, causing large interlayer crosstalk. Writing on the medium and reading with the signal amplification effect of homodyne detection was demonstrated. In addition, the effectiveness of the method was experimentally evaluated.

Twenty-Layer Optical Disc Fabricated by Web Coating and Lamination

We developed a new fabrication method for multilayer optical discs for the high-throughput production of such discs. We used web coating and lamination to prepare a stacked unit. The stacked unit was a layered structure consisting of a recording layer, a UV resin layer, a recording layer, and a pressure-sensitive adhesive layer. We obtained a 20-layer disc simply by laminating the stacked units 10 times. The transmittance of the 20 recording layers was 87% owing to the high transparency of the two-photon recording material. A scanning electron microscopy (SEM) image of the disc showed a clear multilayer structure. The recording layers of the disc were recorded using a pulse laser without interlayer cross write. The thickness variation of the transparent part of the disc was within ±2 µm, and the tilt angles of the disc satisfied the Blu-ray disc (BD) specifications.

A Feasibility Study of 25 GB/Layer Dynamic Recording Using Two-Photon Material and Heat-Mode Bump Formation Method

We developed a novel heat-mode bump formation method using a two-photon material, and its feasibility for high-capacity recording was experimentally demonstrated. This unique recording method exhibits a very high recording resolution and uses a material with excellent optical transparency. These characteristics are very appropriate for realizing high-capacity multilayer optical discs. We also examined the high-density recording feasibility of a Blu-ray-equivalent format and we obtained a jitter value of 22%. We also fabricated quad-layer media and their recording performance was examined. All the layers exhibited almost identical performance characteristics (2T random pattern; jitter, 14.5 to 16.5%) under the same recording and reading conditions.

Optical polymer materials with photocontrolled fluorescence

The results of the development of optical fluorescent solid film materials for applications as recording media for two-photon three-dimension bitwise optical memory with nondestructive fluorescent readout are presented. It was shown that photochromic diarylethenes in combination with organic fluorophore phenalenone or inorganic quantum dots CdSe/ZnS provide preparation of multilayer optical disks for working optical memory. Polymer layers based on irreversible light – sensitive chromones are acceptable for making optical disks of the archival type.

Reduction of Interlayer Crosstalk of Multilayer Optical Disc by Using Phase-Diversity Homodyne Detection

We theoretically and experimentally studied the effects of phase-diversity homodyne detection on the interlayer crosstalk of a multilayer optical disc by comparison with those of conventional intensity detection. From analytical studies, we clarified the differences in interlayer crosstalk of both detections. Interlayer crosstalk consists of two noises, the intensity of the stray light N1 and the interference between the signal and stray light N2. The noise N1, which is dominant between these two, drastically decreases with layer spacing in phase-diversity homodyne detection owing to mismatch in the phase distribution between reference and stray light compared with that in intensity detection. Simulations and experiments on a dual-layer Blu-ray DiscTM having a layer spacing less than 10 µm demonstrated that phase-diversity homodyne detection provided higher tolerance to interlayer crosstalk than the conventional intensity detection.

Reduction of Interlayer Crosstalk of Multilayer Optical Disc by Using Phase-Diversity Homodyne Detection

Reduction of Interlayer Crosstalk of Multilayer Optical Disc by Using Phase-Diversity Homodyne Detection

Compact Optical Module of Homodyne Detection

Homodyne detection is a detection technique that significantly amplifies optical signals. It is useful for the readout of multilayer optical discs, where the signal level is quite low. The improvement of optical disc readout signal quality by homodyne detection has been demonstrated experimentally. However, homodyne detection requires a complicated optical system, which causes large-scale implementation and high cost, which prevents practical use of the technique in optical disc systems. In this report, we demonstrated compact implementation of homodyne detection. We proposed and assembled two types of optical module for homodyne detection. The sizes of the modules were 10×30 and 10×40 mm2, which were comparable to that of the current optical pickup. Jitter of the attenuated Blu-ray DiscTM recordable (BD-R) readout signals was improved from 15 to 7.8% by applying one of the modules.

Sixteen-Layer Write Once Disc with a Separated Guide Layer

We proposed a new recordable multilayer optical disc and its recording system. The new optical disc has multiple-plane-recordable layers and a separated guide layer, and we fabricated 16 recording layers made of high-transmittance inorganic recording materials stacked on the separated guide layer. The recorded signals were evaluated on all recording layers by using jitter and i-MLSE as the evaluation indicators. As a result, the technical feasibility of our proposal was confirmed to achieve the total capacities of 400 and 512 Gbytes, respectively, by the continuous recording of all 16 recording layers with a track pitch of 0.32 µm and two bit lengths. We believe that our multilayer optical disc and recording system are the most practical solutions for realizing huge-capacity data storage.

33.4 Gbyte/Layer Recording with Adaptive Write Strategy for 100 Gbyte Rewritable Triple-Layer Disc

For multilayer rewritable optical discs, we developed a new adaptive write strategy named the S-22 (sequential 2T–2T) write pulse control method, which controls the laser pulse of the 2T-mark according to the combination of both the preceding and succeeding space lengths. By applying this write strategy, a rewritable triple-layer disc based on Blu-ray disc optical systems with a capacity of 100 Gbyte was realized. In particular, the advantage of this write strategy is significant for a transparent information layer. The bit shift of sequential 2T–2T patterns, which is a dominant error in 100 Gbyte rewritable discs, was well compensated. We experimentally confirmed that adopting the S-22 write pulse control method is effective to improve symbol error rate.

Jitter-free multi-layered nanoparticles optical storage disk with buffer ring

A multi-layered nanoparticles optical disk has been developed for a jitter-free high-density data storage system. The disk has nano structures composed of 300-nm-diameter photosensitive particles and 30-nm-width non-photosensitive buffer rings around them. With the buffer rings into the nanoparticles disk, a conventional confocal microscope equipped with a low numerical aperture (NA) objective picked up a particle's shape signal to generate a synchronous signal on its own. In the three-dimensional structured disk proposed, no electronically-produced reference signal is necessary for clock data recover (CDR); no jitter occurs in data decoding.

Photochromic and irreversible photofluorescent organic materials for 3D bitwise optical memory

This paper presents latest own results in the development of reversible photochromic and irreversible photofluorescent polymer systems providing fabrication of multilayer recording media for 3D optical memory with super high information capacity. It was shown that synthesized thermal irreversible photochromic diarylethenes into plastic binders allow to prepare photochromic polymer layers providing nondestructive refractive read-out of optical information. These photochromic polymer layers were used for preparation of six-layer recording media tested with the positive results using the framed optical device. This device imitated layer-by-layer writing, erasure and read-out of optical signals. Experimental evidences for making photochromic polymer layers based on a mixture of photochromic diarylethene and dye — phosphor with nondestructive fluorescent read-out are presented too. Polymer systems based on light-sensitive chromones manifest an irreversible photochemical transformations of these non-fluorescing compounds into the fluorescent products under UV irradiation. Received results open perspectives for making multilayer optical discs for bitwise working (based on photochromic systems) and archival (based on irreversible photofluorescent systems) optical memory with information capacity more 1 TB.

Interlayer crosstalk reduction of a multilayer Blu-ray Disc using a grating in a three beam optical system

In multilayer optical discs, light reflected by out-of-focus layers, which we call interlayer crosstalk, causes the tracking error signal to fluctuate, making the readout signal unstable. We previously proposed a novel method to use a grating along the optical axis in the return path of a pickup to suppress the fluctuation of a differential push-pull (DPP) signal. We develop a pickup and evaluate its performance to stabilize the DPP signal experimentally. DPP signal fluctuation is suppressed to one-third (6% to 2%), and also satisfactory readout jitters (about 8%) are obtained for a triple-layer Blu-ray Disc (BD), which demonstrate the validity of this method to reduce interlayer crosstalk of multilayer optical discs.

Multilayer 500 Gbyte Optical Disk

To confirm the feasibility of realizing subterabyte or terabyte optical memories, we evaluated a multilayer read-only disk having 20 information layers. We propose a new disk structure, which is an alternative structure to reduce interlayer crosstalk, optimized by computer simulations. We fabricated multilayer read-only optical disks using a conventional manufacturing process for Blu-ray disk (BD) dual-layer media. Moreover, we improved the reproduction system to compensate for the large spherical aberration. From the experimental results, we obtained sufficiently low jitter values reproduced from a 20-layer disk. We confirmed the feasibility of the multilayer read-only disk with a capacity of 500 Gbytes.

Amplification of Optical Disk Readout Signals by Homodyne Detection

We experimentally demonstrated the amplification of optical disk readout signals by homodyne detection. This technique uses optical interference to amplify the signals. We further applied phase-diversity detection to reliably obtain the amplified readout signal. The optical system was carefully designed so that a sufficiently amplified readout signal can be obtained. In particular, we applied a corner cube prism as a reflection mirror to achieve sufficient stability of the interferometric optical system. We experimentally demonstrated a 3.6 times amplification of a Blu-ray Disc readout signal. The estimated signal-to-noise ratio (SNR) improvement for an assumed eight-layer optical disk readout signals by applying homodyne detection on the basis of the observed amplification was +7.9 dB, which significantly enables reliable readout of recorded signals. The present technique will be essential for the real commercialization of next-generation multilayer optical disk because of its outstanding ability of SNR improvement.

Multilayer Disk Reduced Interlayer Crosstalk with Wide Disk-Fabrication Margin

To reduce interlayer crosstalk caused by the ghost spot which appears in a multilayer optical disk with more than three information layers, a multilayer disk structure which reduces interlayer crosstalk with a wide disk-fabrication margin was proposed in which the backward reflectivity of the information layers is sufficiently low. It was confirmed that the interlayer crosstalk caused by the ghost spot was reduced to less than the crosstalk from the adjacent layer by controlling backward reflectivity. The wide disk-fabrication margin of the proposed disk structure was indicated by experimentally confirming that the tolerance of the maximum deviation of the spacer-layer thickness is four times larger than that in the previous multilayer disk.