High-density Optical Storage Systems Research Articles

Three-Dimentional Optical Storage Medium Using Arranged Nano-spheres

In this study, we have developed a multi-layered optical storage medium with nano-spheres as the recording medium. We made a three-layered medium of spheres and thereby realized a three-dimensional high-density optical storage system. Using a reflection confocal optical system, we read out clock signals from the medium. The system records data and reconstructs it jitter-free.

Improved performance in coaxial holographic data recording

We describe a coaxial holographic recording system for achieving high recording density. We implement several techniques, such as an objective lens with high numerical aperture (NA), high capacity page data format, a random binary phase mask, and an optical noise reduction element. Our system successfully realizes a hologram recording/retrieving at a low diffraction efficiency less than 2.0 x 10(-3) and achieves a raw data density of 180 Gbit/in.(2), thus demonstrating the potential of a coaxial holographic system for high-density optical storage systems.

High-density optical storage system with minute sphere disk

High-density optical storage system with minute sphere disk

Performance of run-length limited (4, 18) code for optical storage systems

In this paper, the authors investigate the performance of recently presented run-length limited (4, 18) code for high density optical storage systems. The construction of the code is described simply. The code has code rate R = 1/3 and density ratio (DR) = 1.67. The bit error rate (BER) performance for decision feedback equalizer (DFE) and partial response maximum likelihood (PRML) detector are simulated, considering signal-to-noise ratio (SNR) and optical channel jitter. The result shows that the performance of the code is acceptable. The encoder and decoder of the code are implemented by complex programmable logic device (CPLD) chip and the hardware resources required for encoder and decoder are␣low.

Run-Length-Limited (4, 13) Code for High Density Optical Storage Systems

In this paper, we propose a new (4, 13) run-length-limited (RLL) code for high-density optical storage systems. The code rate of this code is 1/3, and the resulting density ratio is 1.67, which is larger than that of EFMPlus code used for digital versatile disc (DVD) systems. The bit error rate (BER) performance of the code is analyzed considering noise and channel jitter. The (4, 13) RLL code has very low complexity of hardware since the mapping rule is simple. The encoding and decoding circuits of the code are implemented using a complex programmable logic device (CPLD) chip.

A New Error Correction Algorithm for High-Density Optical Storage Systems

We propose a new error correction technique for high-density optical storage systems using a syndromechecking long-burst error detection scheme. The advantages are (1) easy code rate adjustment and (2) simple code construction (less inner code encoding and decoding time).

Modified Partial Response Maximum Likelihood Detection Using the Relative Probability of the 17PP Code for Optical Storage Systems

In this paper, we propose a modified calculation method of the branch metric in partial response maximum likelihood (PRML) detection for high-density optical storage systems. The scheme is related to the relative probability of the run-length limited code. Hence, we also present the relative probabilities of the 17PP coded sequences for 2nd-, 3rd- and 4th-orders of PR targets. The modified PRML detection using the relative probability of the 17PP coded sequence has better bit error rate (BER) performance than conventional PRML detection.

A Cost-Effective Scheme to Improve Radial Tracking Performance for Higher-Speed Optical Disk Drives

The periodic disturbances caused by the inherent eccentricity and unbalancing in compact disk systems is one of the prominent radial-tracking problems of high-speed and high-density optical storage systems. To compensate these periodic disturbances, an adaptive feedforward compensation technique is presented and investigated with the aim of having cost-effective optical disk drive manufacturing. Computer simulations and experimental evaluation on a high-speed CD-ROM product show that the proposed adaptive feedforward compensator is very effective and provides a way to significantly improve the radial tracking performance by reducing the radial error by 85%.

Design of Domain Wall Displacement Detection Disk with Groove Recording for High Density and Wide Recording Power Tolerance

Domain wall displacement detection (DWDD) magneto-optical (MO) recording is the most useful method for achieving high linear density recording without using a higher numerical aperture (NA) objective lenses. Securing a wide recording power tolerance is very important to realize a high density optical storage system. From this point of view, we designed the DWDD disk with groove recording using blue laser and NA 0.60 optics. We optimized the heat sink structure and the displacement layer for groove recording, and we could improve the bit error rate with a track pitch of 0.44 µm and a high linear density of 0.07 µm/bit, which corresponds to 21 Gbit/inch2. Additionally, we developed groove substrates using reactive ion etching (RIE) and O2 plasma treatment. We found that the land shape was very important for the recording power tolerance, and we could obtain a wide recording power tolerance with a bit length of 0.09 µm. We confirmed that we could achieve higher density by using a narrower track pitch while maintaining sufficient recording power tolerance.

Readout Signal Analysis of Optical Disk Based on Approximated Vector Diffraction Theory

A simple approximation method is proposed for readout signal simulation of an optical disk based on a vector diffraction model. Tensor reflectivity is introduced to deal with the boundary condition at the recording surface. Preliminary calculations were carried out for stadium-shaped recording pits periodically arranged on a recording surface. Calculations of the push-pull signal were also carried out for guide grooves with a trapezoidal intersection. The results of the frequency response, readout signal waveform, and push-pull signal showed reasonable polarization characteristics. The calculation time was only several tens of times (typically 20–80 times) that required for the scalar model calculation. This method is expected to be useful for extensive studies of readout characteristics necessary for the development of high-density optical storage systems.

Aspherical Solid Immersion Lens of Integrated Optical Head for Near-Field Recording

New solid immersion lenses (SILs) have been studied for a high-density optical storage system by the near-field process that can overcome the far-field diffraction limit. We have proposed the aspherical SILs, named elliptic SIL (ESIL) and Cartesian SIL (CSIL) according to geometrical optics. The SILs have a high numerical aperture (NA), for instance, the NAs of the ESIL and the CSIL are over 1, with a refractive index of 1.56 of the disc cover layer. The SILs that include the function of the objective lens are able to read/write the signals inside the disc substrate. The optical heads employing an internal recording method are expected to be utilized in an unsealed environment. Experimental results of the application of an ESIL are presented. The replicated ESIL (RESIL) has been proposed to solve critical issues such as the problems of the thickness error of the SILs or the disc substrate. These problems need to be solved for the commercialization of the near field recording (NFR) technology.

Run-length limited (4, 18) code for high density optical storage systems

A run-length limited (RLL) code for high density optical storage system is presented. The density ratio of the code is 1.67, which is larger than that of EFMPlus code used for DVD systems. The fix-length (4, 18) RLL code with code rate m/n=1/3 is attractive for storage systems.

100-mW kink-free blue-violet laser diodes with low aspect ratio

400-nm-band GaN-based laser diodes (LDs) operating with a kink-free output power of over 100 mW and having a low aspect ratio of 2.3 have been successfully fabricated for the first time. A new ridge structure, in which the outside of the ridge is covered with a stacked layer of Si on SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and the ridge width is as narrow as 1.5 μm, is applied to realize high kink-free output power with a wide beam divergence angle parallel to the junction plane. A new layer structure around the active layer is demonstrated to be quite effective for obtaining a narrow beam divergence angle perpendicular to the junction plane, maintaining low threshold current. Ten LDs with low aspect ratio have been operated stably for over 1000 h under 30-mW continuous-wave operation at 60°C. Relative intensity noise measured under optical feedback with high-frequency modulation is as low as -125 dB/Hz. These results indicate that this LD is suitable for next-generation high-density optical storage systems.

GaN-based blue laser diodes

We report our recent progress on GaN-based high-power laser diodes (LDs),which will be applied as a light source in high-density optical storagesystems. We have developed raised-pressure metal-organic chemical vapourdeposition (RP-MOCVD), which can reduce the threading-dislocation density inthe GaN layer to several times 108 cm-2, and demonstratedcontinuous-wave (cw) operation of GaN-based LD grown by RP-MOCVD. Furthermore,we found that the epitaxial lateral overgrowth (ELO) technique is useful forfurther reducing threading-dislocation density to 106 cm-2and reducing the roughness of the cleaved facet. By using this growthtechnique and optimizing device parameters, the lifetime of LDs was improvedto more than 1000 hours under 30 mW cw operation at 60 °C. Our resultsproved that reducing both threading-dislocation density and consumption poweris a valid approach to realizing a practical GaN-based LD. On the other hand,the practical GaN-based LD was obtained when threading-dislocation density inELO-GaN was only reduced to 106 cm-2, which is a relatively smallreduction as compared with threading-dislocation density in GaAs- andInP-based LDs. We believe that the multiplication of non-radiative centres isvery slow in GaN-based LDs, possibly due to the innate character of theGaN-based semiconductor itself.

GaN-based high-power laser diodes

We report our recent progress on GaN-based laser diodes (LDs) which will be applied as a light source in high-density optical storage systems. Recently we achieved a lifetime of more than 500 hours under continuous-wave operation with a constant power 20 mW at 25°C using GaN-based LDs with a standard ridge structure. We also report the potential of GaN-based LDs with another structure of a buried-ridge. The far-field pattern of the LDs with a buried-ridge structure strongly depended on the Al content of the Al x Ga 1− x N burying layer. This dependency showed that the device characteristics change from gain-guiding to refractive index-guiding. The critical point was around x=0.30 of an Al content which corresponds to Δ n=0.007 of a lateral index step. It was, therefore, found that the optical transverse mode can be controlled by adjusting the Al content of the Al x Ga 1− x N burying layer.

Tracking Learning Feedforward Control for High-Speed CD-ROM

The periodic disturbances caused by the inherent eccentricity and unbalancing in compact disc systems is one of the prominent radial tracking problems in high-speed and high density optical storage systems. To compensatethese periodic disturbances, a learning feedforward compensation (LFF) method is presented and investigated. Computer simulations and experimental evaluation on the high speed CD-ROM product show that the proposed LFF provides a effective way to improve the radial tracking performance by reducing the radial error by 85%.

Run-length limited (3, 11) code for high density optical storage systems

A run-length limited (RLL) code for high density optical storage systems is proposed. The minimum and maximum run-length constraints are d = 3 and k = 11, respectively. The code rate is 2/5, and the resulting density ratio is 1.6, which is larger than that of eight-to-fourteen (EFM) and EFMPlus codes.

High-temperature operation of 650-nm wavelength AlGaInP self-pulsating laser diodes

Low-coherence self-pulsating laser diodes operating at a wavelength of 650 nm and at temperatures in excess of 70/spl deg/C are required for high density optical storage systems. We report on AlGaInP lasers operating at this wavelength which exhibit stable self-pulsation up to a temperature of 100/spl deg/C. The lasers are 50-/spl mu/m-wide oxide-isolated stripe devices in which the saturable absorption necessary for pulsation is provided by multiple-quantum wells placed within the p-doped cladding layer. The pulsation frequency of the devices increases linearly with increasing drive current and is present up to 1.5 times, lasing threshold.

High-numerical-aperture lens system for optical storage

We describe a new type of optical pickup for use in high-density optical storage systems. By changing a conventional pickup design to include a glass element between the objective and the disk, the transverse resolution of the system can be increased by as much as a factor of 1.5. This system is also less sensitive than conventional systems to variations in disk thickness and disk tilt. We demonstrate how the system has been used as a microscope to inspect an optical disk and describe possible design configurations for an optical pickup.