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A liquid-crystal (LC) panel was integrated into a two-axis electromagnetic actuator in order to realize a dual-layer-compatible optical head for "Blu-ray Disc" systems. This head configuration can actively compensate the cover layer difference of up to nearly 40 µm thickness. Our novel driving method of mixing the controlling signals into four metal wire suspensions can maintain the dynamic performance of the optical head and has achieved a servo bandwidth beyond 5 kHz both in the focusing and in the tracking directions. We also confirmed its optical performance by recording and reproducing 17PP signals on a dual-layer phase-change disc and verified a 46.6 GB storage capacity. In addition, our optical head has interlayer accessibility within 20 ms and satisfies the requirement for a dual-layer disc system. By using this integrated optical head instead of a beam expander, a compact optical pickup for a dual-layer storage system can be realized.

The possibility of an areal density over 50 Gbit/in2 was examined in near-field phase-change recording. The disk structure was optimized to maximize readout signals under the land-and-groove recording condition at a tracking pitch of 160 nm. We also evaluated the signal crosstalk from adjacent tracks. Eye diagrams of 50.4 Gbit/in2 areal density were demonstrated using 1.5 NA optics and a GaN laser diode. The track pitch and linear bit density are 160 nm and 80 nm/bit, respectively. The transmission electron microscope (TEM) micrograph of recorded amorphous marks at an areal density of 50.4 Gbit/in2 is also presented.

Optical disk readout signals with a solid immersion lens (SIL) and the land-groove recording technique are calculated by use of a simplified vector-diffraction theory. In this method the full vector-diffraction theory is applied to calculate the diffracted light from the initial state of the disk, and the light scattered from the recorded marks is regarded as a perturbation. Using this method, we confirmed that the land-groove recording technique is effective as a means of cross-talk reduction even when the numerical aperture is more than 1. However, the top surface of the disk under the SIL must be flat, or the readout signal from marks recorded on a groove decays when the optical depth of the groove is greater than lambda/8.

High-density multilayer optical disc stor age was investigated by em ploying advanced signal pro cessing methods such as adaptive equalization and adaptive partial-response maximum-likelihood (PRML) detection. Preliminary experiments, recording on single-layer and dual-layer Blu-ray discs (BD), indicate storage capability of 33.3 GB/layer with adaptive 5-tap PRML. A new signal qualification method, sequence d amplitude margin quasi-error estimation (SAMQES), was introduced for high-density multilayer recording, and possibility toward a total storage capacity of 200 GB was examined by using a six-layer BD-Recordable disc. Despite coherent interlayer crosstalk and lower signal-to-noise ratio, SAMQES showed moderate increase from those in single-layer recording. Keywords : Blu-ray disc, high-density recording, multilayer, ad aptive PRML, interlayer crosstalk, SAMQES, 200 GB 1. INTRODUCTION Blu-ray disc (BD) systems have already in corporated dual-layer disc structures to increase their storage capacity up to 50 GB for both read-only and recordable applications.

We propose multi-layer near-field recording with using inner focus mode. The restricted conditions for multi-layer near-field recording are discussed from the viewpoint of both media structure and optical setup. One solution is presented for dual layer recording with a numerical aperture (NA) of 1.5 and a wavelength of 405 nm as a light source of GaN laser diode.In the proposed layer structure, a Nb2O5 material has been adopted as the intermediate layer because of its high refractive index, n = 2.4, in order to prevent the decrease of beam propagation that corresponds to NA>1. Almost the same signal characteristics can be obtained from both recording layers at the air gap of 40 nm. The adjustment of focusing position and the compensation of spherical aberrations have been implemented by the combination of an expander lens unit and a liquid crystal (LC) panel in the optical setup.

We demonstrated near-field recording with the combination of a GaN laser diode and a 1.5 NA objective lens. The realized linear bit density was less than 90 nm (corresponding to the areal recording density over 40 Gbit/in/sup 2/) regardless of using a small 1.0 mm-diameter super-hemispherical solid immersion lens (SIL). Further improvement of a near-field phase-change optical disk will enable a linear bit density of 80 nm or less. In this talk we shall also report the fabrication process of the plateau, and of the electrode on the SIL.

Recently, solid immersion lenses (SILs) have yielded a great interest in near-field optical data storage. However, convincing data on areal recording density have yet to be reported. The difficulty is attributed to the fact that an air-bearing slider needs to be kept optically contacted and also that a tiny SIL is vulnerable to aberrations caused by an air gap between a lens and a disk. The air-bearing system cannot easily achieve the condition above because its linear velocity changes the air gap. We have developed a new SIL device mounted on an actuator, which allows the air gap to be varied arbitrarily and enables optical contact independent of disk rotation. The obtained eyepattern of (l,7)-coded data on a phase-change disk demonstrates near-field recording over 1.2- numerical-aperture (NA).

Both over 20 GB of capacity and higher data transfer rate more than 25 Mbps are required for the optical data storage in the era of digital broadcasting to record high-definition digital video stream.

Using a two-element objective lens, over 0.9 NA lens and long WD lens are demonstrated. Using a thin cover layer, problems for skew margin and thickness tolerance are solved. Using a red LD, a 0.85 NA lens and a 0.1 mm cover PC disk, 8 GB rewritable disk system has been developed. Using a blue-green LD and an SHG blue laser, promising results are demonstrated for 12 GB and 17 GB of capacity.

Recently, the capacity of an optical disk was increased up to 4.7 GB such as DVD with a demand of playing back a digital video. In order to record a digital video data more than 3 hours, the capacity of a rewritable disk is expected to exceed 10 GB.

A solid immersion lens attached to a conventional objective increases the effective numerical aperture (NA(eff)) of an optical pickup and yields an areal recording density proportional to (NA(eff))(2). One version of this device, with an effective (NA(eff)) of 1.7, should be capable of very high density storage but would probably need a sealed system. Another simple configuration enables the use of this method for optical data storage in an unsealed environment and extends the spatial cutoff frequency 1.5 times. Experiments with these devices are compared with the full vector field theory of this type of imaging system.

The areal density of an optical storage disk can be increased by improving the transverse resolution of the optical head which is proportional to λ/NA. In order to improve the resolution, a lot of research has been carried out to develop shorter wavelength light sources such as green or blue SHG lasers1,2) and recently ZnSe or GaN semiconductor blue lasers3). These lasers will soon become commercially available. On the other hand, it is difficult to make a high NA objective using a single molded aspherical element. Furthermore the higher the numerical aperture, the less the tolerance for variations in disk thickness, disk tilt, and focusing which are inversely proportional to the fourth, third, and second power of the numerical aperture, respectively. Other experiments have been carried out to improve the resolution using near-field optics4) such as scanning probes which can improve the areal density drastically, however, such probes seem to be tolerated only for working distances of around 100 nm.

To improve the resolution of an optical pick-up, it is desirable to decrease the wavelength or to increase the numerical aperture (NA) because the spot size is determined by diffraction to be approximately λ/NA, where λ, is the free space wavelength. Recently, Kino and co-workers1 have demonstrated an increase in the effective NA of the objective by a factor of n, the index of refraction, in an optical microscope by using a hemispherical lens which has been called a solid immersion lens (SIL) placed between the focusing objective and the sample. Furthermore, they increased the NA of the optical pick-up in an optical disk system by placing a SIL very close to the disk substrate.2–4

A solid immersion lens (SIL) attached to a conventional objective could multiply the effective numerical aperture of an optical pick-up by a factor of refractive index n. A truncated hemispherical SIL (n equals 1.5) mounted on a slider, which was designed to follow the movement of the objective, kept good contact between the SIL and a disk substrate with a slight amount of air gap. Our simple configuration enabled the use of this method in an unsealed environment and extended the spatial cut-off frequency by 1.5 times. In addition, the combination with a second-harmonic generation green laser yielded high areal density optical data storage, six times that of current CD systems.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Some time ago we described a new type of near-field imaging system using the solid immersion lens. In this paper we shall discuss how this type of lens can be employed in optical storage systems to increase the bit density, along with the research that has been carried out at Stanford in cooperation with Sony Corporation and IBM, Almaden to reach this goal. 12 ()j aimis to work with both CD ROMs and magneto-optical storage.

Coherent interlayer cross talk and stray-light intensity of multilayer read-only-memory (ROM) optical disks are investigated. From results of scalar diffraction analyses, we conclude that layer separations above 10 microm are preferred in a system using a 0.85 numerical aperture objective lens in terms of signal quality and stability in focusing control. Disk structures are optimized to prevent signal deterioration resulting from multiple reflections, and appropriate detectors are determined to maintain acceptable stray-light intensity. In the experiment, quadrilayer and octalayer high-density ROM disks are prepared by stacking UV-curable films onto polycarbonate substrates. Data-to-clock jitters ofor = 7% demonstrate the feasibility of multilayer disk storage up to 200 Gbytes.

We have fabricated a plasma-etched lens about three times larger than our previous design by using a novel selectivity control technique during transfer processes, and performed phase-change optical recording for the digital-video-recording (DVR)-blue format. We have fabricated an aspherical plasma etched lens using the selectivity control transfer technique.

Near-field optical recording was implemented on a pre-grooved phase-change disk by using a φ1 mm superhemispherical solid immersion lens (SIL) of 1.5 numerical aperture. The SIL unit was mounted on a two-axis actuator for enabling both gap-height and tracking controls. The gap height was precisely adjusted with a capacitance formed between the SIL and disk materials. Groove structures were fabricated on a glass substrate by a reactive-ion-etching (RIE) process and flattened with spin-on-glass (SOG) coating for obtaining reliable push-pull tracking error signals. The phase-change layer stack was optimized by a vector diffraction analysis in order to maximize evanescent coupling at the gap height as small as 50 nm. A bit density of 80 nm was demonstrated on the land of a pre-grooved disk at the wavelength of 407 nm.

The combination of a GaN laser diode and a 0.85 numerical aperture objective has achieved an optical storage capacity of over 22 GB. Owing to sufficient modulation ability and low-noise characteristics, GaN semiconductor lasers possess adequate quality to be light sources in optical recording systems. A bit size of 130 nm×300 nm, corresponding to the areal recording density of 16.5 Gbit/in.2, was realized on a six-layered thin-cover phase-change disk with a crystallization-promoting structure. Each active layer was inversely stacked and was thermally as well as optically optimized for the blue-violet light sources. A new small electromagnetic actuator with a lightweight φ3 mm two-element lens has extended its servo-bandwidth up to 8 kHz and enabled precise focusing control at the data transfer rate of 35 Mbps.

A bit density of 125 nm was demonstrated through near-field phase-change (PC) optical recording at the wavelength of 657 nm by using a supersphere solid immersion lens (SIL). The lens unit consists of a standard objective and a φ2.5 mm SIL. Since this lens size still prevents the unit from being mounted on an air-bearing slider, we developed a one-axis positioning actuator and an active capacitance servo for precise gap control to thoroughly investigate near-field recording. An electrode was fabricated on the bottom of the SIL, and a capacitor was formed facing a disk material. This setup realized a stable air gap below 50 nm, and a new method of simulating modulation transfer function (MTF) optimized the PC disk structure at this gap height. Obtained jitter of 8.8% and a clear eye-pattern prove that our system successfully attained the designed numerical-aperture (NA) of 1.36.

A high-numerical-aperture ( NA) optical pick-up for an optical disk was developed using a two-element lens, a thin disk substrate and a spherical aberration compensation mechanism. We designed a two-element objective lens rather than a conventional singlet objective lens in order to increase the fabrication tolerance of the objective lens for high NA. A disk substrate which is thinner than that of the digital video disk (DVD) was adopted to increase the disk tilt tolerance. A spherical aberration compensation mechanism was realized by controlling the distance between the two lenses of the objective using an actuator in order to cancel the aberration due to the variations in the disk thickness. In the experiment, we fabricated a two-element lens according to our design and confirmed that it performed as a high-quality 0.8- NA objective lens.

Tripled areal density optical recording was realized with the combination of a high power laser-diode pumped intracavity frequency-doubled Nd:YAG laser (SHG green laser), a low noise GdFeCo/TbFeCo double-layered magneto-optical (MO) disk, and high sensitivity photodetectors optimized for the wavelength of the light source. A high carrier-to-noise (C/N) ratio of more than 48 dB and a bit error rate of less than 3×10-6 by (2, 7) run-length-limited (RLL) mark position recording was obtained at a mark length of 0.45 µm (0.6 µm/bit) for a disk with a 0.9 µm track pitch.

This paper proposes a dynamic zone-plate interferometer (DZPI) which can measure quite accurately both shapes and surface errors of aspherical mirrors or lenses. By changing the wavelength of the illuminating light in fine steps, the DZPI gives a series of interferograms in which fringes are shifted almost continuously. Therefore, all their orders can be identified even when the maximum order becomes greater than 100. From these many interferograms, the shape and surface error of an optical instrument under test can be calculated with high precision by computer. A simple experiment confirms the validity of the DZPI.

This paper proposes a dynamic zone plate interferometer (DZPI), which has a possibility of measuring aspherical surfaces without the need of either a standard surface or of a computer generated holograms (CGH). The accuracy of surface error measurments can be improved by using a fringe analysis method up to about λ/100. An experimantal result is also presented.