Near-field Recording System Research Articles

Dynamic Head-Disk Interface Modeling and Adaptive Control of a Hybrid Actuator for Optical Data Storage Systems

In the near-field recording (NFR) system, the gap between the lens and disk will drop down to 100 nm. However, the disk vibration and force disturbance make it difficult to maintain the desired flying height during disk operation, and the lens-disk collision can easily occur. It is proposed in this article to design a hybrid actuator system which combines both advantages of the flying slider used in hard disk drives and the voice coil actuator used in optical disk drives. The dynamic head-disk interface model of the hybrid actuator is first developed, then an adaptive regulation approach is proposed to control the flying height at its desired value despite the unknown disturbances. Simulation and experimental results are presented to illustrate the effectiveness of the proposed flying height control approach.

Adaptive Regulation of Tracking Servo for Next Generation Optical Data Storage Systems

The tracking servo control has played an important role in the data storage servo systems. In the next generation optical data storage systems, i.e. the near-field recording system, the tracking error of the servo system should be below 10 nanometers under various unknown situations. However, higher data transfer rate and higher data density make it difficult to maintain the desired tracking precision during normal disk operation. It is proposed in this paper to use an adaptive regulation approach to maintain the tracking error below its desired value, despite unknown track eccentricity and external force disturbance. The performance of the proposed control approach is analyzed and simulation results are presented to illustrate the capability of the proposed adaptive regulator to achieve and maintain the desired tracking precision.

High-Speed and Precise Gap Servo System for Near-Field Optical Recording

We propose a high-speed and precise gap servo control system that is based on the feed-forward control method that can reduce the harmonic disturbance (RHD-FFC) on a near-field optical recording system with a solid immersion lens. An optical disk of 1.2 mm thickness rotates with about 30 µmp–p of axial run-out. The axial run-out prevents a gap servo from performing precisely at high rotational speed. The RHD-FFC method can suppress harmonic disturbances of gap error signal and can perform at high speed while keeping less than 25 nm of gap length between an optical head and a rotational disk surface. We confirmed that the gap servo applying the RHD-FFC method performed precisely and achieved at high disk rotational speed of 11000 rpm with a 1.2-mm-thick optical disk with a track pitch of 0.16 µm, corresponding to a data capacity of 100 Gbyte.

High-Speed and Precise Gap Servo System for Near-Field Optical Recording

We propose a high-speed and precise gap servo control system that is based on the feed-forward control method that can reduce the harmonic disturbance (RHD-FFC) on a near-field optical recording system with a solid immersion lens. An optical disk of 1.2 mm thickness rotates with about 30 µmp–p of axial run-out. The axial run-out prevents a gap servo from performing precisely at high rotational speed. The RHD-FFC method can suppress harmonic disturbances of gap error signal and can perform at high speed while keeping less than 25 nm of gap length between an optical head and a rotational disk surface. We confirmed that the gap servo applying the RHD-FFC method performed precisely and achieved at high disk rotational speed of 11000 rpm with a 1.2-mm-thick optical disk with a track pitch of 0.16 µm, corresponding to a data capacity of 100 Gbyte.

Near-field optical recording with nanocomposite cover-layer for numerical aperture of 1.85

Near-field optical recording media with a nanocomposite cover-layer were prepared and tested using a gap servo, near-field recording system with an effective numerical aperture of 1.85. The refractive index of the nanocomposite cover-layer, which has a smooth surface, is 1.86, yielding a successful gap servo operation. However, the recording density cannot be maximized due to the refractive index of the cover-layer being lower than the required value for the effective numerical aperture. A recording density of 95 GB per disc can be achieved with a nanocomposite cover-layer having a refractive index of 1.86. The readout signal from 110 GB is not clear due to the reduced eventual numerical aperture in conjunction with the limited refractive index of the cover-layer.

High-Speed Gap Servo Control for Solid-Immersion-Lens-Based Near-Field Recording System with a Flexible Optical Disk

Higher data storage capacities and higher data transfer rates will be required in next-generation information storage devices. However, there is a limit to the rotational speeds of conventional disk structures. Hence, conventional disks will not be able to achieve high data transfer rates of over 250 Mbps that is required for next-generation storage devices. To increase the data transfer rate of a disk, flexible optical disks have been studied with the goal of stable rotation at a high speed, using a stabilizer to reduce disk oscillations. If a flexible optical disk is implemented in a near-field recording (NFR) system, simultaneous high data transfer rates and high-density recording should be possible. In an NFR system, it is very important to maintain the gap between the solid immersion lens (SIL) and the disk at distances below tens of nanometers. In this study, to simultaneously achieve high data storage capacity and high data transfer rate, we propose an improved gap servo control system for an SIL-based NFR system with a flexible optical disk. To enable robust control at a high rotational speed, a repetitive controller was designed and applied to the NFR servo algorithm. In both simulation and experiment, the newly designed gap servo controller stably maintained the gap distance in the SIL-based NFR system using a flexible optical disk.

High-Speed Gap Servo Control for Solid-Immersion-Lens-Based Near-Field Recording System with a Flexible Optical Disk

Higher data storage capacities and higher data transfer rates will be required in next-generation information storage devices. However, there is a limit to the rotational speeds of conventional disk structures. Hence, conventional disks will not be able to achieve high data transfer rates of over 250 Mbps that is required for next-generation storage devices. To increase the data transfer rate of a disk, flexible optical disks have been studied with the goal of stable rotation at a high speed, using a stabilizer to reduce disk oscillations. If a flexible optical disk is implemented in a near-field recording (NFR) system, simultaneous high data transfer rates and high-density recording should be possible. In an NFR system, it is very important to maintain the gap between the solid immersion lens (SIL) and the disk at distances below tens of nanometers. In this study, to simultaneously achieve high data storage capacity and high data transfer rate, we propose an improved gap servo control system for an SIL-based NFR system with a flexible optical disk. To enable robust control at a high rotational speed, a repetitive controller was designed and applied to the NFR servo algorithm. In both simulation and experiment, the newly designed gap servo controller stably maintained the gap distance in the SIL-based NFR system using a flexible optical disk.

Improved safety mode considering external shock direction for near-field recording system using SIL

To increase the near-field coupling efficiency in a near-field recording (NFR) system, extremely small air gap (less than 100 nm) between the solid immersion lens (SIL) and media should be maintained. However, maintaining the air gap is very important and difficult. Despite various anti-shock control methods, there are physical limitations to controlling the air gap against external shock. A safety mode that moves the actuator to the initial position for large external shock is currently being used. The existing safety mode exhibits good performance for downward shocks of 6 G amplitude and 10 ms duration time, but some problems arise for upward shocks of 3 G amplitude and 10 ms duration time. To avoid collision for upward and downward shocks, we present an improved safety mode that considers the direction of the external shock for an SIL-based near-field recording system. We analyzed the upward and downward shock responses of the NFR system. The shape of the pulse input was designed to minimize the overshoot of the actuator. Through various experiments, the amplitude and duration time of the pulse input were optimized. Even with an upward shock with 8 G and 10 ms, no collision was observed between the actuator and the media by using the improved safety mode with the optimized pulse input.

Applications of Next Generation Optical Data Storage Technologies

In recent years, there have been many research studies focusing on next generation optical data storage technologies, such as near-field recording and holographic data storage systems. Many studies from several disciplines have been perfoed and merged to realize of the next generation optical data storage. The technologies that have been developed for optical data storage are also essential for the other research areas. These technologies can be widely applied to various fields, such as near-field optics-based microscopy, small-sized imaging systems, plasmonic nanolithography using a solid immersion lens (SIL) and a nanoscale aperture, and bio-applications. In this paper, the applications and concepts of the next generation of optical storage technologies are discussed.

A Survey of Control Issues in Optical Data Storage Systems

AbstractThis paper presents a survey of control issues in optical storage data systems. Servo control has the important role of moving the optical pick to a track and regulating it to read and write data along the track. The performance of the servo system determines total system performance, such as access time, data rate, and latency. In this regard, we review servo control systems and walk readers through key issues of servo control technology. We also discuss servo control issues in some new storage technologies, such as near-field recording systems. Although optical data storage systems are highly precise mechatronic systems, they have not yet been considered extensively in the literature of the control engineering society. This review provides a description of servo control issues in optical data storage systems from specification to implementation of control theory. We thus believe that this paper will be useful for readers practicing control theory both in academia and in industry.

Improvement of Shock Resistance for Solid Immersion Lens-Based Near Field Recording System Using Air-Bearing Surface

A solid immersion lens (SIL)-based near-field recording (NFR) device is regarded as the next-generation optical storage device that can achieve an areal density of more than 100 Gbyte/in.2. However, the NFR system is very weak to external shock because of the extremely small gap between the SIL and the disc. To solve this problem, we proposed a new method of using an air-bearing surface (ABS) to avoid collision between the SIL and the disc, and investigated the effect of the ABS on shock resistance in our previous study. In our current study, we optimize an initial ABS model using the results of our previous study and achieve an optimal ABS model with an improved shock resistance. We also verify the improvement of shock resistance for the optimal ABS model by finite element (FE) analysis, by applying the ABS model to a gap servo control system, and by experimental shock testing.

Improved Antishock Air-Gap Control Algorithm with Acceleration Feedforward Control for High-Numerical Aperture Near-Field Storage System Using Solid Immersion Lens

A near-field storage system using a solid immersion lens (SIL) has been studied as a high-density optical disc drive system. The major goal of this research is to improve the robustness of the air-gap controller for a SIL-based near-field recording (NFR) system against dynamic disturbances, such as external shocks. The servo system is essential in near-field (NF) technology because the nanogap distance between the SIL and the disc is 50 nm or less. Also, the air-gap distance must be maintained without collision between the SIL and the disc to detect a stable gap error and read-out signals when an external shock is applied. Therefore, we propose an improved air-gap control algorithm using only an acceleration feedforward controller (AFC) to maintain the air-gap distance without contact for a 4.48 G at 10 ms shock. Thus, the antishock control performance for the SIL-based NF storage system in the presence of external shocks is markedly improved. Furthermore, to enhance the performance of the antishock air-gap control, we use the AFC with a double disturbance observer and a dead-zone nonlinear controller. As a result, the air-gap distance is maintained without contact for a 6.56 G@10 ms shock.

Theoretical study of optical recording with a solid immersion lens illuminated by focused double-ring-shaped radially-polarized beam

Based on the Richards–Wolf vector diffraction theory, the intensity distributions in the recording sample near a solid immersion lens are calculated for two different radially-polarized beams ( R- TEM 11 ∗ and R- TEM 01 ∗ modes). Numerical results show that a double-ring-shaped R- TEM 11 ∗ mode focusing has some excellent features in near-field optical storage, compared with a single-ring-shaped R- TEM 01 ∗ mode focusing. The recording density is markedly improved, the focal depth of the near-field recording system is substantially increased, and a subsurface recording is effectively obtained using the R- TEM 11 ∗ mode focusing.

Improved gap servo system using a brake pulse for a SIL-based near-field recording system

A mode-switching servo is widely used in solid immersion lens (SIL)-based near-field recording (NFR) systems to control the air gap between the bottom surface of the SIL and the top surface of the rotating media. In the mode-switching servo, it is very important to reduce the initial overshoot in the near field region, as this overshoot can cause collisions between the SIL and the media, and increase the settling time required for the SIL to be initially moved to the final air gap. In this paper, the mode-switching servo with a brake pulse is proposed to reduce the initial overshoot and settling time. The effectiveness of the proposed method is proved by experiments of which results show that the minimum air gap due to the initial overshoot is improved from 6.86 to 18.73 nm and the settling time is reduced from 30 to 4.5 ms.

Effect of Air Bearing Surface on Shock Resistance of Optical Head for Solid Immersion Lens Based Near Field Recording System

A solid immersion lens (SIL)-based near field recording (NFR) device is regarded as the next generation optical storage device that can achieve an areal density over 100 GB/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . However, the NFR system is very weak for the external shock due to the extremely small gap between a SIL and a disc. Many researches using a gap servo control were carried out, but the results were too insufficient to satisfy needs of the industry. To solve this problem, we propose a new method using air bearing surface (ABS) to avoid a collision between the SIL and the disc occurred by the external shock. In this paer, we check a feasibility of a ABS on shock resistance in NFR system due to many reasons such as a large mass of moving part, a protrusion of the SIL and a low stiffness of wire. We also design an ABS to avoid effectively the collision between the SIL and the disc and to improve shock resistance of SIL based NFR system. Finally, we investigate the effect of the ABS on shock resistance of optical head for SIL based NFR system.

Anti-Shock Air Gap Control for SIL-Based Near-Field Recording System

The aim of this research is to improve the robustness of the air gap controller for solid immersion lens (SIL)-based near-field recording (NFR) servo systems against dynamic disturbances such as external shocks. Stable control is essential in these systems because the air gap distance between the SIL and the rotating disk is less than 100 nm. To detect gap error and read-out signals for the SIL and the disk, the air gap control performance must be increased to avoid collisions due to external shocks. We propose an anti-shock air gap control system using a disturbance observer (DOB) and a dead-zone nonlinear controller. Experimental results demonstrate that a DOB with a Q filter bandwidth of 500 Hz reduced the minimum air gap distance due to an external 1 g@10 ms shock by 70.1%. A DOB and a dead-zone nonlinear controller maintained the optical head distance for a 2 g@10 ms shock. Thus the control performance in the presence of external shocks was improved.

Near-Field Recording Media With Nanocomposite Cover-Layer

A nanocomposite cover-layer and a hybrid cover-layers consisting of a nanocomposite and a dielectric protective film are coated on the media for cover-layer incident near-field recording (NFR). The refractive index of the cover-layer should be matched with the effective numerical aperture of a solid immersion lens (SIL) system. The nanocomposite composed of polymer binder and nano particles is a suitable material to increase the refractive index. A collision problem in the interface between the SIL and the nanocomposite cover-layer in the NFR media yields damages after gap servo tests using a homemade gap servo NFR system. The scratch problem owing to the damages is attributed to a rough surface and brittle characteristics of the nanocomposite cover-layer. Coating a dielectric protective film on the nanocomposite layer results in a stable gap servo, in which the mechanical properties of the cover-layer are compensated by the dielectric film. However, the surface roughness problem not only in the nanocomposite but also in the hybrid cover-layers should be improved.

Improvement of Protection Process Using Observer for Soild Imersion Lens-Based Near-Field Recording

In the near field recording (NFR) system, there always exists the possibility of collision between solid imersion lens (SIL) and media owing to dust, scratches on the media, and external shock because of the extremely small gap distance. The kinetic energy associated with damage from shock is directly proportional to the square of velocity, and the phase of velocity is faster than the phase of displacement with 90°. Thus, it is effective to prevent a collision on the basis of velocity information. The velocity of the actuator in the NFR system should be estimated by an observer without using a velocity sensor. In this paper, we propose an improved protection process with a mode switching servo method using a Luenberger observer. The gap distance state can be estimated earlier by detecting the velocity state yielded by external shock. Through simulations and experiments, we confirm that the protection process based on velocity and gap distance is more powerful than the protection process based only on the gap distance.

Design of Compatible Optics for Solid-Immersion-Lens-Based Near-Field Recording System and Blu-ray Disc Using Relay Lens

We designed compatible optics for a solid-immersion-lens (SIL)-based near-field recording (NFR) system and a Blu-ray disc (BD). The working distance, numerical aperture (NA), and cover layer thickness of the SIL-based NFR system differ from those of BD. Therefore, for the compatibility of a cover layer incident type NFR system of whose NA is 1.46 and BD, we use a relay lens that is composed of two low-NA lenses. The optical tolerances such as decenter, tilt, and SIL thickness tolerances are considered to ensure manufacturability. The decenter and tilt tolerances of each optical component are greater than 1.0 µm and 0.1°, respectively. The tolerance of the SIL thickness range is 14 µm at the criterion of the wavefront aberration of 0.035 λrms. We design the optical layout and simulate each focus error signal of BD and the SIL-based NFR system. From the practical design results, we confirm that the SIL-based NFR system can be compatible to BD with a simple relay lens.

Solid Immersion Lens Optical Head for High-Numerical-Aperture Cover-Layered Incident Near-Field Recording

For increasing data recording density and reducing spherical aberration in cover-layered incident near-field recording (NFR) systems, a high-refractive-index cover layer is necessary and the assembly and evaluation technologies of a solid immersion lens (SIL) optical head for a high-numerical-aperture (NA) cover-layered incident NFR system are also required. To assemble a SIL optical head for the high-NA cover-layered incident NFR system, a modified Twyman–Green interferometer is developed. In this paper, we present the design and assembly results for a SIL optical head with a high-refractive-index cover-layered disk. We also compare evaluation results with those of a simulation to confirm the feasibility of the assembly. Through this research, we can improve the effective NA to 1.84, which is the highest NA reported for a cover-layered incident NFR system, and consequently, the data recording capacity per layer can be increased.