High-density Optical Storage Research Articles

High-Density Optical Storage with Nanospheres on Surface Relief Structure

We report optical storage media using fluorescent nanospheres on a surface relief structure. By using a nanosphere as one recording bit, we can carry out high-density recording. We have developed a new recording medium by arranging and fixing a large number of nanospheres on a two-dimensional surface relief grating (SRG).

Spectroscopic properties of azo-dye doped PMMA films studied by multi-photon absorption

Summary The fluorescence spectra and their decay profiles of novel azo-dye doped PMMA films (ADP) have been studied by using infrared femtosecond pulsed laser as multi-photon excitation source. The cis -isomer fluorescence emission of the azo-dye molecules has been observed stronger than trans -isomer emission. The experiment result shows that the analyzed azo-dye compound isomerizes obviously under two-photon absorption, which may be potentially useful in high-density optical storage.

Signal Non-Linearities And Their Effect Upon The Data Recovery Process In High-Density Optical Storage

To predict the performance of optical read channels readout waveforms generated using linear superposition techniques are often used. The readout waveform is, however, often degraded by non-linear components particularly at high storage densities. In this paper the origins of such non-linearities are explained and their effects on read channel performance investigated. It is found that channel bit error rate can be properly optimised only if the signal non-linearity is taken into account.

Polymeric nanostructured material for high-density three-dimensional optical memory storage

The unique properties of a polymer photonic crystal are examined with respect to applications as a medium for high-density three-dimensional optical data storage media. The nanocomposite material was produced from core-shell latex particles, in which the latex cores contained dye-labeled polymer. Nonfluorescent latex shells were attached to the core particles. Upon annealing, the close-packed core-shell particles formed a nanostructured material with the fluorescent particles periodically embedded into the optically inert matrix in a hexagonal close-packed structure. A two-photon laser scanning microscope was used to write bits of information into the material by photobleaching the optically sensitive particles and, under much lower fluence, read out the resulting image. Relative to conventional homogeneous storage media, the nanostructured periodic material is shown to increase the effective optical storage density by at least a factor of 2 by spatially localizing the optically active region and imposing an optically inactive barrier to cross-talk between bits. This polymer photonic crystal has the potential to dramatically improve performance further through the improved capabilities to optimize the photochemical processes and more fully exploiting the periodic nature of the information domains in the image processing.

Fabrication of narrow surface relief features in a side-chain azobenzene polyester with a scanning near-field microscope

Abstract We show that it is possible to fabricate topographic submicron features in a side-chain azobenzene polyester with a scanning near-field optical microscope. Through irradiation at a wavelength of 488 nm at intensity levels of 12 W/cm 2 , topographic features as narrow as 240 nm and as high as 6 nm have been reproducibly recorded in a thin film of the polyester. These observations are consistent with the fact that at low intensities peaks are produced evolving into formation of trenches at high intensities in the case of amorphous side-chain azobenzene polyesters. This may find applications in high-density optical storage and high-resolution lithography.

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.

Investigation of changes in absorber position in the 650 nm AlGaInP self-pulsating laser for optical storage applications

Self-pulsating laser diodes operating at 650 nm are required in high-density optical storage devices. By growing saturable absorbing quantum wells in the p-doped cladding layer of a laser it is possible to obtain the interplay between gain and absorption that is required for pulsation. A self-pulsating AlGaInP laser is modelled in order to gain an insight into how the distance between the active and absorber layers affects the laser output. Results indicate that the device performance can be altered significantly as the distance changes, showing that this design feature is a key parameter for optimum performance.

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 densityoptical 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.

Nanometric aperture arrays fabricated by wet and dry etching of silicon for near-field optical storage application

We fabricated nanometric aperture arrays in order to apply to an optical probe in high-density near-field optical storage for increase of data-transmission rate. The aperture arrays were fabricated by forming concave pyramidal grooves on a silicon-on-insulator wafer with electron beam lithography and wet anisotropic etching. Modification of the apex shape of the grooves by re-oxidation and subsequent reactive ion dry etching was able to increase the uniformity of the aperture size remarkably. The light transmission efficiency of the fabricated apertures was measured to be ∼10−3 when the aperture size was 100 nm, which was higher than that of a conventional optical fiber probe by several orders of magnitude.

Silicon planar-apertured probe array for high-density near-field optical storage

We propose a novel, to our knowledge, silicon planar-apertured probe array as an optical head for high-density near-field optical storage. In comparison with a conventional fiber probe employed for near-field optical storage the apertured probe array has a higher readout data-transmission rate and better mechanical durability. A probe array with an aperture size of 100 nm was fabricated by use of photolithography and wet etching of a silicon wafer. Subwavelength-readout capability was demonstrated by use of one aperture of the probe array. Furthermore, we achieved a 16 times increase in the light-transmission efficiency of the probe array by installing glass-sphere microlenses on each aperture. The increase was confirmed by measurement of the near-field optical intensity.

90 mW 660 nm distributed feedback lasers

High efficiency single spatial and spectral mode, 662 nm wavelength distributed feedback lasers have been fabricated from a GaInP/AlInP laser structure. Output powers of up to 90 mW at room temperature were achieved without any spectral mode jumps. These devices should be useful for short and/or visible wavelength applications that require spectral mode stability, such as spectroscopy, interferometry, and high-density optical storage.

Theoretical optimization of self-pulsating 650-nm-wavelength AlGaInP laser diodes

Self-pulsating laser diodes operating at a wavelength of 650 nm are attractive for high-density optical storage. The main candidate for such a device is an AlGaInP laser diode including an epitaxially integrated saturable absorber. The characteristic self-pulsation occurs due to the interplay between gain in the active region and the absorption within the structure. In the paper, we calculate the dynamics of self-pulsation in this type of AlGaInP laser diode, including a detailed description of gain and absorption within the relative sections. In particular, we identify how, by modifying the structure of the epitaxial absorber layers, we can alter the operating characteristics of these laser diodes.

Strongly Luminescent Poly(ethylene glycol)-2,2′-bipyridine Lanthanide Ion Complexes

A highly efficient luminescent material is produced from a 2,2′-bipyridine-lanthinide-ion coordination complex. The authors describe how this coordination complex can be stabilized by ethylene glycol oligomers. The material, obtained by simple mixing of the components, emits blue, green, and red photoluminescence by single near-UV excitation. Additionally, the complex can be mixed with silica through a sol-gel process, producing transparent soft solids that preserve all photophysical characteristics of the original liquid mixture. These materials may find applications in electroluminescent displays, lasers, or high-density optical storage.

Strongly Luminescent Poly(ethylene glycol)-2,2′-bipyridine Lanthanide Ion Complexes

A highly efficient luminescent material is produced from a 2,2′-bipyridine-lanthinide-ion coordination complex. The authors describe how this coordination complex can be stabilized by ethylene glycol oligomers. The material, obtained by simple mixing of the components, emits blue, green, and red photoluminescence by single near-UV excitation. Additionally, the complex can be mixed with silica through a sol-gel process, producing transparent soft solids that preserve all photophysical characteristics of the original liquid mixture. These materials may find applications in electroluminescent displays, lasers, or high-density optical storage.

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.

Persistent high density spectral holeburning in CaS:Eu and CaS:Eu,Sm phosphors

Persistent spectral hole-burning has been reported for singly, Eu-doped, and doubly, Eu- and Sm-doped, CaS phosphors. Efficient photon gated holeburning in the 4f7 (8S7/2)−4f65d1 transition of Eu2+ is a result of photoionization of Eu2+ to Eu3+. These holes have a width of <5 GHz (2 K), survive thermal cycling of the phosphor up to the room temperature, 300 K, and have no detectable deterioration over more than a day of storage time at low temperature (2 K). Although self-gated holeburning is observed with the reading laser at higher powers, the photon budget for reading these holes is so small that in excess of 1000 reading cycles can be performed without destroying the optical signal. The nature of holes burned by photon-gating is found to be very different from the self-gated holes. The characteristics for the holeburning are the same in singly and doubly doped phosphors, suggesting that under the conditions of our experiments, Sm traps do not play any significant role in spectral holeburning. Possibilities of high density optical memory storage using photon-gated holeburning in this THz broad transition are discussed.

High density optical storage based on superresolution techniques employing optical path filtering and saturable absorption

Optical superresolution techniques to increase the density of data storage by source and medium modifications are explained. It is shown that the light distribution at the source can be modified using a phase filter. This results in overall energy efficiency and improvement in the frequency response of the optical read-out. The frequency response of the optical readout with source modification Is good at medium and high frequencies. The technique of superresolution through source modification, and with saturable absorber in the medium is proposed.

A dual probe differential interference scheme for multi-layered high-density optical storage

We present a new approach for multi-layered high-density optical storage using transparent dielectric films. The proposed scheme encodes the digital bits in the storage layer by varying its optical thickness and recovers the signal by performing differential interference between two probe beams that traverse two adjacent data bits. We demonstrate the feasibility of this approach by scanning two focused laser spots across the edge of an optical step and analysing the interference pattern using a matched grating filter. The system's sectioning capability which permits multi-layered data recovery has also been demonstrated by scanning a stack of two such optical steps. The use of dual probe interference offers the advantage of inherent vibration isolation as both beams pass through a common path except at the focal region where the storage medium is positioned.

Application of dual probe direct interference tomultilayered high density optical storage: A proposal

The author proposes an alternative approach for optical data storage in transmission holograms. Digital codes are embedded in the transparent storage medium by varying its optical thickness. The medium is sandwiched between two transparent dielectric layers of smaller refractive index. The detection scheme uses signals obtained from the direct interference between two focused optical probe beams. When the medium is scanned under the probes the stored ‘bits’ will be registered as intensity modulation in the interference signal. The use of a transparent medium in optical disks offers the possibility of stacking a large number of storage layers, thereby greatly increasing the storage density.