Hartmut Richter

Effective spherical aberration compensation by use of a nematic liquid-crystal device

The next generation of optical data storage system beyond DVDs will use blue laser light and an objective lens with a high numerical aperture of 0.85 to increase storage capacity. Such high numerical aperture systems have an inherent higher sensitivity to aberrations. In particular, the spherical aberration caused by cover layer thickness tolerances and--more obvious--by dual-layer disks with a typical separation of approximately 20 microm between the two layers must be compensated. We propose a novel transmissive nematic liquid-crystal device, which is capable of compensating spherical aberration that occurs during the operation of optical pickup systems.

Two-dimensional modulation for holographic data storage systems

Page-oriented holographic data storage systems (HDSSs) generally use spatial light modulators (SLMs) to generate two-dimensional (2D) digital patterns, so-called data pages. These data pages are stored via interference patterns of the object and reference beam in the holographic medium and are retrieved from the medium by exposing it to the reference beam. The reconstructed data pages are then detected by a matrix detector. One important challenge in designing an HDSS is to develop a suitable modulation, which takes into account the specific characteristics of the transfer channel and enables high data capacity, high data transfer rate and low symbol error rate (SER). In this paper, we present a new method for modulating data in an HDSS. The main idea is to adapt the general concept of 2D run-length limited modulation (RLL) to a numerically more efficient implementation usable for HDSS. We demonstrate that this 2D block coding method with a constant weight and a sparse code increases the amount of user data per data page, while the SER remains low compared with standard sparse modulation coding. This results in a higher data rate and higher data density.

System Aspects of Dual-Layer Phase-Change Recording with High Numerical Aperture Optics and Blue Laser.

In this publication, key technologies for blue laser recording and dual layer phase-change disks with a total capacity of 50 Gbytes are explained and system margins are evaluated. Key challenges are the phase-change recording layer design, a robust read and write system with low intrinsic noise and the optical pickup design including, e.g., high numerical aperture (NA) objective lens, spherical aberration compensation devices and beam shaping optics. Blue-laser dual-layer technology will be emerging as the next-generation optical data storage standard beyond compact discs (CDs) and digital versatile discs (DVDs).

Test of key elements for common path holography

Holographic devices are expected to have a much larger capacity than conventional optical storage systems such as CD, DVD or blue diode laser based HD-DVD or BD. Recent developments in the field of dedicated recording materials and advanced optical enabling technologies are now opening the door for the realization of commercial products. One of the major technical challenges is the development of a robust and reliable system concept, which allows easy exchangeability of the medium. We developed a holographic tester system with common paths for the reference and the signal beams based on a single mode blue laser diode and a commercial CMOS detector. The system will be used to evaluate various multiplexing schemes, to investigate the influence of system tolerances on the reading performance and to estimate fundamental system limitations.

Material consumption and crosstalk characteristics of different holographic storage concepts

Holographic data storage is considered to be one of the most promising technologies for high-capacity data storage. Several holographic concepts have been suggested and investigated in detail by many companies. The concepts differ in the method of superposing object and reference beams inside the holographic medium. At present, the most relevant concepts are the plane wave concept, the collinear concept, and a concept with counterpropagating beams. We compare all three concepts, with regard to their beam overlap, efficiency of material consumption, diffraction efficiency, and crosstalk characteristics. The investigation is performed by numerical simulations, which offer well-defined conditions in all setups and are independent of experimental uncertainties such as the nonlinear behavior of medium sensitivity and the effects of light scattering or reflection.

Shift Selectivity Calculation for Finite-Volume Holograms with Half-Cone Reference Beams

Analytical shift selectivity equations assume an infinite hologram size. When developing a compact holographic storage system, one uses small holograms. From a practical viewpoint, it is better to use reflection type discs, in which the entire optical system, writing and reading objectives, among others, are on the same side of the disc. For reflection-type holographic discs, it is important to use half-cone-shaped reference beams to avoid generating ghost images caused by phase conjugate readout. Our calculations show significant discrepancy between the shift selectivity curves corresponding to the approximated analytic equation and the numerically calculated shift selectivity curves.

4GOOD - Technology and Prototype for a 4th-Generation Omni-Purpose Optical Disc System

Digital optical data storage has become firmly established through CD, DVD and their derivatives. Future mobile and stationary multimedia applications demand for ever higher storage density, even beyond HD-DVD and Blu-ray Disc with 50 GB maximum storage capacity (dual layer). While holographic recording is in the far future for the consumer domain, the 4th generation of optical storage is being heralded by a leap in technology. Within a European initiative in the scope of the EUREKA project MobileDRIVE and the project 4GOOD (4th-generation omni-purpose optical disc-system), funded by the German Ministry of Economy and Technology, the fundamental technologies are being developed for high-density optical data storage (60...100 Gbit/ inch2) in order to achieve at least 200 GB on a 12 cm disc, or e.g. > 5 GB on a 3 cm miniaturised version (single layer). The contribution describes the technology objectives, challenges, concepts and project status in the key areas of disc development, drive including laser, optics, acceleration sensors for mobile operation, and signal processing.

Compact optical pickup with a two-wavelength laser diode

The DOE in combination with TWIN laser diodes shown in this paper is an effective scheme to simplify the optical path and to reduce manufacturing costs of two-wavelength optical pickups. The presented optical design offers a high optical efficiency for both wavelengths. Operation with only one four-quadrant photo detector is possible. By adjusting the position of the TWIN laser diode and of the DOE, our DOE can handle variations of the distance between the laser sources of about ±20 μm.

Data Detection Methods for Holographic Data Storage Systems

We suggest a new method for data detection based on direct data identification and compare this new method with three commonly used methods based on decimation using linear transformations. To compare these methods, the complete procedure from data page creation, sending data through an optical channel, and data detection is simulated numerically. This is performed by varying the noise level, sampling factor, and spatial filter. The proposed method requires a higher computational complexity, but for a broad range of realistic system parameters, it enables lower symbol error ratios.

Investigation of a reflective counter-propagating holographic setup

A reflective counter-propagating holographic setup for optical data storage is presented. The system makes efficient use of the laser light by using the reference beam to generate the signal beam. After passing through the holographic medium, the reference beam is sent onto a spatial light modulator that directs the modified reference beam back into the holographic medium, where it interferes with the original reference beam. Thus 100% of the available laser power can be used for the reference beam. Furthermore, a special random phase mask and a corresponding data page format are introduced. The phase mask has an improved alignment tolerance of ±0.5 pixels in contrast to a conventional binary phase mask that has a tolerance of only ±0.1 pixels. Moreover, the mask still improves the shift-selectivity and eliminates the strong intensity peak in the Fourier plane. We investigate the shift selectivity and compare experimental and simulated results that were obtained with a two-dimensional fast-Fourier-transform (2D-FFT) volume integral method.