We have developed a numerical simulation method on the basis of the ray tracing technique to evaluate a liquid-crystal (LC) spherical aberration compensation device for next-generation high-capacity optical memory devices. We numerically evaluated the performance and the tolerance of the LC device and showed its adequate capability to compensate the spherical aberration due to the nonuniformity of the disc cover layer thickness. Using the simulation method, we numerically examined the possibility of using LC devices to compensate the large spherical aberration due to the layer jump of the dual-layer optical disc and determined that the gradation-type LC device can be used for dual-layer optical recording.
High-density and high-speed have been the trend of developing next generation optical storage technologies. Rauch et al.[1] proposed a hybrid recording system called wave-guide hybrid advanced MEMS (WHAM) by using two kind of fields. One is an optical beam that controls the track width and the other is the magnetic field that controls bit length as shown in Fig. 1. The recorded mark is confined within the small cross-region and therefore the recording density is effectively increased. Kim et al.[2] further designed the module by integrating waveguide and two-dimensional toroidal optical elements into a substrate. It can be easily fabricated by MEMS technologies.
In advanced optical recording systems, a storage capacity of more than 20 Gbytes has been reached by adopting blue laser diodes, 0.85 numerical aperture (NA) objectives and 0.1 mm thin cover layer medium. This has been accompanied by a reduction of the optical and mechanical tolerance of the recording system. Among the parameters above, thickness error tolerance of the disc cover layer is said to be less than 3 μm for mitigating the severe disc tilt tolerance. In order to mitigate such tolerances, an aberration compensation device is strongly required. The co-refractive index liquid crystal (LC) panel is one of the promising candidates as the aberration compensation device and has been extensively studied. The refractive index of an LC depends on the applied electric field; and thus the refractive index distribution can be controlled by the electrode configuration and the applied voltage. Although the direct measurement of spherical aberration is the best method for evaluating the performance, numerical estimation methods are also required for better device design. In this paper, we report on an optical simulation method for numerically evaluating the LC aberration compensation panel and show the performance and optical tolerance of the device. By using the optical simulation method, we numerically optimize the LC aberration compensation device and show the device can be applicable to the layer jump of 25 μm for the dual layer optical recording, in next generation optical memory.
Eutectic-based InSbTe phase-change materials have been developed for low sigma-to-dynamic range (SDR) multi-level (ML) performance at linear track velocities (LTVs) of 1.9 m/s to 6 m/s. Compositions with the stoichiometry Inx(Sb72Te28)100-x (3.9<x<45) were tested. Compositions that achieved low SDRs did so at 1.9 m/s, 3.5 m/s and 6 m/s. We found two minima in the SDR at concentrations of x=10% and x=30% indium. We explain this unique finding through write-erase characteristics and crystal structure. At indium concentrations lower than 10% and higher than 30%, the favored rhombohedral crystal structure was not the major phase formed, and the SDR increased. The minima in SDR at 10% and 30%, in conjunction with a maximum at 20%, can be explained by fast solid-state crystallization time and slow melt recrystallization time.
A simple manufacturing process of multilayer recording disks has been developed to fabricate a large-capacity optical disk of more than 40 GB, using a short-wavelength laser diode and high numerical aperture (NA) objective lens. Some existing production facilities for the digital versatile discs (DVD) are applicable to this process. These disks can be utilized for long-time video recording of high-definition TV broadcasting. We have developed a dual-layer rewritable optical disk with capacity of more than 43 GB per one side of a 120 mm disk by this process. We obtained good performance of playback jitter for disks of 43 GB and 46.6 GB capacities with on-groove recording and limit equalizer at a user data transfer rate of 36 Mbps as well as good direct overwriting characteristics over 1,000 cycles.
A new theory of illumination is derived for near-field optical transducers, in which transmitted power is related to total power in the illuminating laser beam. Several transducers are compared, including simple holes in metallic films, a dielectric probe and solid immersion lens combination, and plasmon-enhanced holes. The ratio of transmitted power to incident power is a function of the illumination numerical aperture, due to the angular selectivity of the transducers. Also, a simplified collection theory is reviewed, which reveals that, when transducers are used for the collection of data signals in optical storage applications, a combination of the transducer and a solid immersion lens provides a large angular collection range, and hence a good contrast signal.
Data integrity is an important factor for optical disks. When a long burst error is detected during verification which follows writing, the same data is re-written in a spare sector. Although this is a powerful error control method, its reliability depends strongly upon the error detection capability of the error correcting code format. The upper bound of misdetection was calculated on the basis that all code words are separated from each other by the minimum Hamming distance. The upper bounds of the probability of misdetection were 6.59×10-51, 3.29×10-45 and 1.15×10-39 for replacement criteria of 3, 4 and 5, respectively, when the symbol error rate is 10-5. However, the weight of most code words is far greater than the minimum one. The average probability of misdetection was also estimated. Although it is very difficult to obtain the experimental value of the misdetection since it is extremely small, the estimation indicates that the defect management is very reliable.
An aspherical single lens is widely used as an objective lens for optical discs. These lenses are designed as aplanats or are very close to an aplanat design. We have analyzed the lens with NA of 0.85 and determined general equations that predict paraxial parameters of such lenses. The equations are very useful for determining lens specifications. Applying the equations we have determined the specifications of a new lens whose focal length is 2 mm. The wavefront aberration was 24 mλ and well below the diffraction limit. The experimental results on an optical pickup with a 405 nm LD are also shown. The jitter value of the read-back signal using a 23.3 Gbytes phase change disk was as small as 9.0%. This jitter value is sufficient for achieving a low error rate after decoding.
We studied the characteristics of the 4.7 GB recordable digital versatile disc (DVD-R) for a high data transfer rate higher than 44 Mbps. Using simulation, analytical pulse sequence and atomic force microscopy (AFM) measurement, we analyzed the thermal interference. We also used originally synthesized dye with low thermal interference and realized good recording characteristics with jitter values of around 8% at 66 Mbps recording and 11% at 89 Mbps recording.
We have investigated high-density near-field readout using a solid immersion lens with a high refractive index. By using a glass material with a high refractive index of 2.08, we developed an optical pick-up with the effective numerical aperture of 1.8. We could observe a clear eye pattern for a 50 GB capacity disc in 120 mm diameter. We confirmed that the near-field readout system is promising method of realizing a high-density optical disc system.
The tribological properties of a near-field optical disk with diamond-like carbon (DLC) protective and lubricant films were evaluated. DLC protective films were prepared under different sputtering conditions and their mechanical and optical properties were investigated. Near-field recording (NFR) disks were prepared on several different substrates, and the substrate material effects on a head-disk interface (HDI) were measured by a tribological test. The heat-treated NFR disks with a glass substrate showed the best durability.
A dual-layer disc is developed for blue-laser phase-change recording. The two recording stacks are based on fast-growth phase-change materials (FGMs). Both layers show good recording performance at a total disc capacity of 46.6 GB and with a slightly high jitter at 50 GB. The thicknesses of both the cover and spacer layers are controlled such that the deviations from the reference thicknesses of both recording layers are less than 2 µm. This eliminates the need for dynamical spherical aberration correction in the drive. The absolute difference in transmission between the written and unwritten states of the upper layer is only 2%. It is shown that this transmission difference causes no problem for readout and writing of the lower layer.
For reading and writing on a lower targeted recording layer of volumetric optical disks, transmitted laser power is decreased by absorption and reflection of the upper layers. Thus, a layer of thermo-chromatic material, such as silver oxide, has been added as an optical switching layer next to the recording layers to modulate the optical characteristics of the recording layers by the diffraction limited laser beam during writing. The optical switching structure can reduce peak writing laser energy by 42%. Consequently, optical switching layer is promising for volumetric recording to optimize R/W performance.
In optical pickup for recording, push-pull method is popularly used to remove offset because 3-beam method creates offset in recording. Also that is used to playback the disk with pit structure. If pits on the disc are created very poorly and they are out of specification, signals measured by Push-Pull method become very low level and it increases the difficulty in track servo control, while the signals from 3-beam method don't show any difference.
We have studied the characteristics of super resolution optical disks with an organic dye used for the recording layer. It was demonstrated that cyanine dyes with either high decomposition temperature or low absorption show good readout durability and high carrier-to-noise ratio (CNR).
A mark simulation tool for fast-growth phase-change materials was developed to predict the formation and erasure of amorphous marks in phase-change discs that are based on doped Sb2Te compositions. One of the main challenges of such a simulation tool is the accurate determination of the input parameters. The thermal conductivity of the thin films in the recording stack was determined from melt-threshold experiments in combination with numerical modelling. The crystallisation process, modelled as crystal growth from a crystalline-amorphous interface, is governed by the temperature-dependent velocity of crystal growth. In this paper, we discuss a procedure for measuring the low-temperature velocity of crystal growth from the size of an amorphous mark and the time required for complete erasure under isothermal conditions. Furthermore, to determine the velocity of crystal growth in the temperature range encountered during recording conditions, we performed time-resolved erasure experiments in combination with numerical modelling. The derived time-dependent growth velocity was used in a numerical model to simulate the formation of amorphous marks. Transmission electron microscopy (TEM) measurements of recorded marks were compared to predicted mark shapes to validate the numerical model.
A hemi-paraboloidal solid immersion mirror is successfully fabricated with the ELID (electrolytic in-process dressing) grinding technology and the spot size obtained was 150% of that for diffraction limited processes. Small sized optical head would be possible with the HP-SIM, which size is comparable to that of a magnetic head for HDDs.
Introduction There has been lower the tilt margin by the increase of coma aberration due to the reduction of wavelength and the increase of numerical aperture for high density recording. The RF signal degradation is increased as the decrease of tilt margin. To compensate the tilt margjn, the optical pick-up actuator for high-speed CD/DVD re -writable system should be able to drive the radial-tilt motion. We announce that the Saxis drive actuator for high-speed CD/DVD rewritable system And what was more, the DUAC sensitivities of this actuator are suitable for the low-speed playability, the high-speed readability and the high-speed rewritability.
1. Introduction Recent rapid advances in information society demand large-capacity and high-speed access capabilities even for optical data storage. Near-field surface recording technology, which utilizes an aperture-mounted head slider, has been vigorously studiedp3 due to its high density recording potential. One of the problems of near-field recording system is its low signal-to-noise ratio, and an optical aperture is required to realize high signal output and high spatial resolution simultaneously. Recently, in order to solve this problem, various configurations of novel near-field aperture have been proposed and their performances are validated in numerical simulationq6. In this paper, we propose a new aperture configuration, a near-field optical head with a triangular aperture, and perform numerical simulations to reveal its optical performance through our developed three-dimensional finite-difference time-domain (3D-FDTD) method. 2. Analysis Model Figure 1 shows the schematic diagram of the analysis model including a triangular aperture head and a metal-coated medium. The shape of the triangular aperture head is a pyramid of an equilateral triangle with a taper angle of 45 degrees, and its shading material is Aluminum. The length of the side of the triangular aperture at the end of the head is 200 nm. Incident light of the linearly polarized plane wave with a wavelength of 680 nm normally illuminates the triangular aperture head. A recording medium, which consists of glass substrate and 40-nm thick Chromium layer having a 100-nm wide space, is placed a few tens of nm (separation: h) apart from the aperture, and moves in lateral direction. The electric energy distribution (=I El *) i s calculated for a measure of the near field energy, and the transmitted optical energy is obtained by integrating the normal Poynting vector component, considering it as a far-field sensed signal. 3. Simulation Results without a Recording Medium First we performed calculations, without considering head-medium interactions to reveal fundamental characteristics of the triangular aperture head. Figure 2 shows the energy distributions of the triangular aperture for X-polarization and Ypolarization on the XY plane just below and 30 nm apart from the aperture. We can see that the energy distribution of the triangular aperture strongly depends on the polarization state, and especially in the case of X polarization, the energy is enhanced at the only one side of the triangular aperture due to the edge enhancement effect. While the circular and rectangular aperture usually show the energy enhancement at both edges of the …
