Gernot Berger

Hybrid multinary modulation codes for page-oriented holographic data storage

Hybrid multinary block codes for implementation in page-oriented holographic storage systems are proposed. The codes utilize combined phase and amplitude modulations to encode input data. In comparison to pure amplitude-or pure phase-modulated block code designs hybrid multinary modulation coding allows us to augment the storage density at an unchanged error rate. Two different hybrid modulation code designs are introduced. Experimental implementation is thoroughly discussed, especially concentrating on readout concepts. Phase-resolved readout is accomplished by optical addition and subtraction, using an unmodulated reference page. Experimental results indicate that the overall error rate is usually dominated by errors related to amplitude detection. The study suggests that capacity gains of up to 31% or 47% are reasonable when utilizing phase modulations in conjunction with binary or ternary amplitude modulation.

Non-volatile volume holograms in bismuth tellurite crystals

We investigate the durability of volume holograms in bismuth tellurite crystals, Bi2TeO5. This material enables self-fixing of holograms during the recording process rendering any further fixing technique unnecessary. Therefore the material overcomes the vulnerability of holograms which is a typical inconvenient property of inorganic storage crystals. The quality reduction of reconstructed analogue and digital data pages under permanent read-out is studied. In terms of digital page-oriented storage, the change of the bit error rate is investigated. For comparison, all experiments are also performed on iron doped lithium niobate crystals, LiNbO3:Fe.

Digital data storage in a phase-encoded holographic memory system: data quality and security

We review the crucial properties of a phase-encoded volume holographic storage system in terms of data quality and security, which are the key issues of any bulk memory system. Two major problems which need to be tackled in holographic storage systems in terms of data quality are the hologram erasure during readout and the data encoding schemes for error-free reconstruction. We present a novel storage material, (bismuth tellurite crystals - Bi2TeO5), which has the potential to overcome the volatility problem and avoiding the need of any further fixing. Regarding data encoding schemes, we present a general approach of gray scale modulation coding in order to improve the data capacity in comparison to normal modulation coding, while the bit error rate maintains low. Data security in a phase encoded system can be realized by exploiting its special multiplexing characteristics. We present different encryption techniques and investigate their decryption probability.

Reliability of associative recall based on data manipulations in phase encoded volume holographic storage systems

We investigate the characteristics of correlation signals accomplished by content addressing in a phase encoded volume holographic storage system under different realistic conditions. In particular, we explore two crucial cases with respect to the structure of the data. The first one deals with a scenario where only partial or defective data are available for content addressing. The second case takes similarities among the stored data sets into account, which significantly differ from their statistical correlation. For both the cases we provide, for the first time, a theoretical approach and present experimental results when employing phase-code multiplexing. Finally, we discuss the reliability of the employed methods.

Overloaded phase-code multiplexing for volume holographic storage

Overloaded phase codes for volume holographic data storage are introduced. In contrast to any previous phase-code design, overloaded phase codes enable multiplexing of a number of data pages that exceeds the number of utilized reference beams. In this way the achievable data capacity can be augmented. Overloaded codes are generated by extending multilevel phase codes based on the discrete Fourier transform. We demonstrate multiplexing of 70 analog pages by means of 64 reference beams. The analysis of reconstructed digital data pages suggests that a capacity gain of up to 15% is reasonable.

Beyond volume holographic storage: applications of phase-coded multiplexing to image processing and encryption

We present several encryption techniques for holographic data storage, using orthogonal phase-code multiplexing and random phase encoding. Our system is capable of storing page-oriented data based on the selectivity of orthogonal phase-codes in a photorefractive LiNbO3 storage crystal. In order to encrypt data, random phase multiplexing is added to the system. We proof that the combination of deterministic and random phase code multiplexing is possible with low cross-talk and take advantage of this technique for an extremely secure data encryption. Moreover, the potential of phase-code multiplexing to realize arithmetic operations is exploited for data encryption purposes.

Study of an acrylamide-based photopolymer for use as a holographic data storage medium

An acrylamide-based photopolymer formulated in the Centre for Industrial and Engineering Optics has been investigated with a view to further optimisation for holographic optical storage. Series of 15 to 30 gratings were angularly multiplexed in a volume of the photopolymer layers with different thickness at a spatial frequency of 1500 lines/mm. Since the photopolymer is a saturable material, an exposure scheduling method was used to exploit the entire dynamic range of the material and allow equal strength gratings to be recorded. From this investigation the photopolymer layers M/# was determined with regard to the recording geometry used. The temporal stability of photopolymer layers was studied in terms of diffraction efficiency and change of the reconstructed angle due to material shrinkage. In addition, the potential of the photopolymer as a holographic data storage medium was demonstrated by recording bit data-pages.

Unitary matrices for phase-coded holographic memories

Volume holographic data storage based on phase-code multiplexing is a promising technology for next generation optical storage devices. Volume holographic memories offer high storage capacities and short data access times through multiplexing and a page-oriented storage principle. Multiple holograms can be recorded in the storage media through various multiplexing techniques, such as angular, phase-code, wavelength or shift multiplexing. Among the different methods, deterministic phase-code multiplexing has been widely investigated since it entails several advantages. This method allows recording and retrieval of data pages without introducing moving parts or frequency-shifting elements into the setup, which results in a fast access speed. Crosstalk analyses have shown that the signal-to-noise ratio (SNR) of phase-code multiplexing is significantly higher than those of wavelength or angle multiplexing. In addition, phase-code multiplexing offers the potential of performing parallel optical operations of stored images, associative data search and enables powerful address based encryption techniques. this contribution, we present a novel type of matrices - unitary matrices (U-matrices) for phase-code multiplexed holographic memories.

Deterministic Non-Orthogonal Phase-Code Multiplexing

Phase-code multiplexing is used to index the reference wave in volume holographic data storage. We present unitary and deterministic non-orthogonal multilevel phase-codes that facilitate increased storage capacities.

Reliability of associative data search in phase encoded volume holographic storage systems

We investigate the characteristics of correlation signals obtained during content addressing under different realistic conditions. For two cases, either addressing with defective data or storing similar data sets, we present for the first time a theoretical approach and experimental results when employing phase-code multiplexing.