Optical Data Storage Systems Research Articles

Near-field optics: a new tool for data storage

Evanescent energy can be used to produce extremely small optical spots. Two practical implementations that use evanescent energy are aperture probes and solid immersion lenses (SILs). For data storage, the optical near field is defined in terms of evanescent coupling between the system used to read data and the recording layer. Because of the small spot size, near-field techniques are applied to optical data storage systems in order to increase recording density. Both aperture-type systems and SIL systems show good promise of achieving densities of more than 150 Gb/in/sup 2/. The characteristics and performance of several systems are compared, and future near-field technologies are discussed.

Effects of an asymmetrically molded plastic objective lens on the push–pull tracking-error signal in an optical disk drive

The effects of a plastic objective lens's astigmatism on the push-pull tracking-error signal (TES) of an optical disk data storage system were investigated theoretically and experimentally. Astigmatism of plastic objective lenses arises commonly from the asymmetric deviation from their designed shape during the molding process. By carefully studying the aberration characteristics of the objective lens and including the astigmatism of the laser diode in the analysis, we can calculate the combined effects of astigmatism of these two components on the push-pull TES. It is found, from both the simulations and the experiments, that, by rotation of the objective lens about the optical axis, the peak-to-peak value of the push-pull TES varies with the lens's rotation angle, and a change as great as 340% in its value was observed in a given optical pickup.

Pupil Plane Filtering for Optical Pickup Heads with Effective Numerical Aperture of 1.1 and 2.0

A pupil plane filtering technique is applied to data signal detection in an optical data storage system that uses a solid immersion lens (SIL) and a quadrilayer phase change recording medium. Two systems that have the effective numerical aperture (NAEFF) of 1.1 and 2.0 are investigated. A new filter is designed for the NAEFF=2.0 system on the basis of this study. We have confirmed by numerical calculations that the technique improves signal contrast, which is defined by the ratio of the difference signal to the sum signal between the homogeneous crystalline area and the homogeneous amorphous area. We have also confirmed that the technique makes the contrast less sensitive to the gap width between the SIL and the recording medium. The vectorial feature of the light is considered in the calculations. The calculated irradiance distributions and signal levels for the system with NAEFF =1.1 are confirmed by experiments.

Electronic Crosstalk Cancellation with a Quadrant Cell Detector

Single beam electronic crosstalk cancellation by a quadrant cell detector placed in the exit pupil of an optical data storage system is experimentally shown to reduce crosstalk levels by -10 dB and improve jitter by a factor of 2.5 for double density recorded compact disk rewritable media (CD-RW). Simulations show -21 dB reduction in crosstalk levels for CD-RW.

Sol-Gel Processing of Tellurium Oxide and Suboxide Thin Films with Potential for Optical Data Storage Application

A novel sol-gel processing route has been developed for the production of tellurium oxide (TeO2) and tellurium suboxide (TeOx) thin films. The process has been used to deposit thin films on glass substrates by the dip coating process. These films have been shown to undergo thermally induced, reversible changes in optical properties on heating at appropriate temperatures. Formation of TeOx was found to occur at low temperatures by direct breakdown of the sol-gel derived material even in the absence of reducing agents whilst the principal optical effects corresponded to the formation of TeOx and α-TeO2 at temperatures of <200°C and <450°C respectively. The relatively low temperatures required to induce significant optical effects are comparable with conventionally prepared TeOx and offer the potential application of these materials in optical data storage systems.

Super-resolution in optical data storage

In this paper we summarize how fundamental work on the theory of optical imaging performed by our research group over a number of years has led to experimental developments, using specially calculated image-plane masks, which have confirmed that optical resolution in high-aperture microscopy, both coherent and incoherent (fluorescence microscopy), can reach nearly twice the conventional Rayleigh limit. In particular, we give a detailed treatment of our current work on the application of these ideas to the active area of high-density optical data storage on compact discs (digital versatile discs (DVDs)). A conventional optical disc can be regarded as a coherent-light confocal scanning microscope and our general theories thus show the feasibility of `superdense' optical data-storage systems. The achievement of near twice the Rayleigh limit for DVDs in the entertainment sector is important at the present time since this would enable full-length motion pictures to be read on an uncomplicated reader rather than the two-layer or double-sided reading systems now being promoted. Optical discs which use fluorescent material are also being proposed and our resolution enhancements will also apply to these.

Vector diffraction from subwavelength optical disk structures: two-dimensional modeling of near-field profiles, far-field intensities, and detector signals from a dvd

Rigorous two-dimensional vector-diffraction patterns of a focused beam incident on an optical disk, specifically, a digital versatile disk (DVD), are examined both in the near field and in the far field. An efficient finite-difference frequency-domain method is developed for calculating the electromagnetic fields in the neighborhood of subwavelength dielectric and metallic structures. The results of vector-diffraction theory are compared with those of scalar-diffraction theory for pressed DVD features that consist of pits or of bumps. The sum (data) and difference (tracking) signals from a split photodetector are also calculated for different disk features and for different polarizations. The subwavelength features of a DVD result in considerable vector-diffraction effects both in the near-field profiles and in the detector signals, depending not only on the polarization of illumination but also on whether the features are pits or bumps. This paper provides important insight into the vector-diffraction effects encountered in high-density optical data storage systems.

Blue LDs begin to find their commercial niches

From research target to commercial reality, the blue laser diode has now found its way out of the lab and into end-use applications. While the most talked about application in the drive to develop blue LDs has been in optical data storage systems, the first use of the component has come in a quite different field-spectroscopy.

Pupil-plane filtering for improved signal detection in an optical data-storage system incorporating a solid immersion lens

A pupil-plane filtering technique is applied to data-signal detection in an optical data-storage system that uses a solid immersion lens (SIL) and a four-layered phase-change recording medium. We have confirmed by numerical calculations and experiment that the technique improves signal contrast and makes the contrast less sensitive to the gap width between the bottom surface of the SIL and the top surface of the recording medium. Light that is incident upon the objective lens that is used with the SIL is linearly polarized, and the full vectorial feature of the light is considered in the calculations.

Design approaches with a lenslet array and a single, high-numerical-aperture annular-field objective lens for optical data storage systems that incorporate large numbers of parallel read–write–erase channels

To achieve very high data rates (>10(9) bits/s) in optical data storage systems it is necessary to employ a large number of laser beams for parallel read-write-erase operations. Bringing all these beams to diffraction-limited focus with a high-numerical-aperture objective lens (while maintaining focus and tracking) requires techniques that are fundamentally different from those that are currently practiced in the field of optical data storage. We present two possible solutions to the problem of designing an objective lens for such systems, one involving an array of high-quality lenslets and the other based on a single, high-numerical-aperture annular-field-of-view conventional lens. Both approaches have advantages and disadvantages, on which we elaborate in the course of our discussions.

Reflection confocal microscope readout system for three-dimensional photochromic optical data storage

We propose to use a reflection confocal microscope (RCM) as a readout system for digital data stored in a three-dimensional (3D) photochromic optical memory. We describe the merits and the difficulties for 3D optical memory that are associated with the use of such a RCM.It is shown by means of 3D Fourier space analysis that successful reading of 3D data can be obtained by selection of the appropriate parameter for the RCM.The system parameters include the numerical aperture of the objective lens and the wavelength of the light used for analysis (reading). Experimental results of multilayer recording and reflection confocal reading in photochromic-molecule-doped poly(methyl methacrylate) are presented. Good-contrast images are obtained.

Elemental analysis by diode laser spectroscopy

After about 25 years of development of laser analytical spectroscopy, laser spectrochemical instruments at present work efficiently only in research laboratories. There are practically no commercial laser spectroscopic instruments for routine elemental analysis. The only laser-based instruments are Raman spectrometers with stable cw laser sources for molecular analysis and pulsed lasers with fixed wavelengths which are used for laser ablation (LA) – laser sampling of solid materials. The main difficulties of laser spectroscopic techniques are: relatively high cost, insufficient reliability, and the necessity of qualified personal. Semiconductor laser diodes are mass produced for compact disc players, laser printers, optical data storage systems and telecommunication equipment. A number of these laser diodes have excellent spectroscopic properties, which make them attractive sources for spectrochemical analysis. Presently, laser analytical instruments based on laser diodes have the highest potential for transfer from research laboratories to routine practice. Recently, the excellent analytical capabilities of diode laser spectrometry in the detection of low concentrations of analytes have been well documented. Recent advances in diode lasers and analytical trends are discussed below.

Numerical simulations of scattering in a two-photon optical data storage system

Scattering effects in a two-photon optical data storage system are numerically studied. Surface scattering analysis with a scalar, beam propagation model is performed. We analyze the problem by modeling scattering from randomly varying surfaces and also by Fourier surface decomposition. Scattering induced by propagation through multiple pages of randomly recorded data marks is also studied with a hybrid finite-difference-time-domain/angular-spectrum model. Both surface and bulk scattering are shown to influence the spatial properties of the optical beam. Results and some possible implications are presented.

Edge detection readout signal and cross talk in phase-change optical data storage

Readout signal, noise, and cross-track cross talk were investigated for edge detection in a phase-change optical data storage system. Both theoretical and experimental results indicate that edge detection has a performance superior to the conventional detection of reflectance variations, especially when the amorphous marks are shorter than the size of the focused spot. More than 50 dB of carrier to noise ratio for marks of 0.36 μm in length is obtained using light at a wavelength of 690 nm and an objective lens of 0.6 numerical aperture. Diffraction analysis on the cross talk has shown that, in the scheme of land-groove recording, there is no optimum groove depth which can cancel the cross talk from adjacent tracks.

Two-Photon Single-Beam “Bit-by-Bit" Writing and Fluorescent “Page-by-Page" Reading 2.5D Femtosecond Optical Data Storage System

A novel scheme of rewritable multilayered (2.5D) optical data storage based on single-beam femtosecond two-photon consequent writing and fluorescent parallel readout has been developed. The table-top prototype is created to demonstrate writing-reading-erasing the information in dual-layer polymer photochromic matrices with equivalent information density of 50 Mbit/cm2 per layer.

Ion channeling studies of GaN layers on c-oriented sapphire

The successful growth of epitaxial GaN films by metal organic vapor phase epitaxy (MOVPE) or gas source molecular beam epitaxy (GSMBE) has opened up new applications in short wavelength photonic devices for displays and optical data storage systems. The large lattice mismatch of 14% between GaN and sapphire, usually used as the substrate, and the different thermal expansion coefficients generally lead to high densities of structural defects. In this paper we investigate the defect structure of MOVPE- and GSMBE-grown GaN layers on sapphire by Rutherford backscattering and ion channeling measurements. In addition to axial c-axis channeling, which reveals χ min values as low as 1.2%, channeling measurements along the (2 1 1 0) and (1 1 0 0) crystal planes were performed in order to improve the sensitivity to dechanneling by crystalline defects. Angular scans around the c-axis indicate clearly the hexagonal symmetry of the GaN lattice. Dechanneling results were compared to observations of defects by transmission electron microscopy (TEM). This comparison suggests that the cross-section of dechanneling by edge dislocations is about a factor of 4 larger than the dechanneling cross-section of screw dislocations.

Photoinduced Switching in Self-Assembled Multilayers of an Azobenzene Bolaamphiphile and Polyelectrolytes

The reversible cis–trans photoisomerization of azobenzene and its derivatives has potential applications in optical data storage systems. But can the photoisomerization occur also in multilayers? It is shown that differences in photochemical activity can be ascribed to the different packing of azobenzene derivatives in the layers, free volume, such as illustrated in the Figure, being necessary to enable the photoisomerization to take place.

Photoinduced Switching in Self-Assembled Multilayers of an Azobenzene Bolaamphiphile and Polyelectrolytes

The reversible cis–trans photoisomerization of azobenzene and its derivatives has potential applications in optical data storage systems. But can the photoisomerization occur also in multilayers? It is shown that differences in photochemical activity can be ascribed to the different packing of azobenzene derivatives in the layers, free volume, such as illustrated in the Figure, being necessary to enable the photoisomerization to take place.

Computer simulations of effects of disk tilt and lens tilt on push-pull tracking error signal in an optical disk drive

We quantify the effects of disk tilt and objective lens tilt on the push-pull tracking error signal of an optical disk data storage system. For a grooved disk, such as a recordable compact disk that operates at a laser wavelength of lambda, it is found that disk tilt produces a tracking offset of 0.05lambda per degree of tilt, whereas objective lens tilt produces an offset of 0.012lambda per degree of tilt. The amplitude of the tracking error signal decreases by 2.5% at the disk tilt angle of 0.3 degrees and by 5% at the objective lens tilt of 0.3 degrees . We achieved these simulations with the computer program Diffract, which performs a combination of diffraction and ray-tracing calculations through the entire optical path, from the light source to the detectors.

Free-Space Integrated Optics Realized by Surface-Micromachining

A surface-micromachined free-space micro-optical bench (FS-MOB) technology has been proposed to monolithically integrate micro-optical elements, optomechanical structures, micropositioners, and microactuators on the same substrate. Novel three-dimensional micro-optical elements have been fabricated by surface-micromachining techniques. The optical axes of these optical elements are parallel to the substrate, which enables the entire free-space optical system to be integrated on a single substrate. Mocro-scale Fresnel lenses, refractive microlenses, mirrors, beam-splitters, gratings, and precision optical mounts have been successfully fabricated and characterized. Integration of micro-optical elements with translation or rotation stages provides on chip optical alignment or optomechanical switching. This new free-space micro-optical bench technology could significantly reduce the size, weight, an cost of most optical systems, and could have a significant impact on optical switching, optical sensing and optical data storage systems as well as packaging of optoelectronic components.