The holographic disc is a high capacity, disk-based data storage device that can provide the performance for next generation mass data storage needs. With a projected capacity approaching 1 terabit on a single 12 cm platter, the holographic disc has the potential to become a highly efficient storage hardware for data warehousing applications. The high readout rate of holographic disc makes it especially suitable for generating multiple, high bandwidth data streams such as required for network server computers. Multimedia applications such as interactive video and HDTV can also potentially benefit from the high capacity and fast data access of holographic memory.

Runlength-limited (d,k) constraints and codes are widely used in digital data recording and transmission applications. Generalizations of runlength constraints to two dimension are of potential interest in page-oriented information storage systems. However, in contrast to the one-dimensional case, little is known about the information-theoretic properties of two-dimensional constraints or the design of practical, efficient codes for them. In this paper, we consider coding schemes that map unconstrained binary sequences into two- dimensional, runlength-limited (d, (infinity) ) constrained binary arrays, in which 1's are followed by at least d 0's in both the horizontal and vertical dimensions. We review the derivation of a lower bound on the capacity of two-dimensional (d, (infinity) ) constraints, for d greater than or equal to 1, obtained by bounding the average information rate of a variable-to-fixed rate encoding scheme, based upon a 'bit- stuffing' technique. For the special case of the two- dimensional (1, (infinity) ) constraint, upper and lower bounds on the capacity that are very close to being tight are known. For this constraint, we determine the exact average information rate of the bit-stuffing encoder, which turns out to be within 1% of the capacity of the constraint. We then present a fixed- rate, row-by-row encoding scheme for the two-dimensional (1, (infinity) ) constraint, somewhat akin to permutation coding, in which the rows of the code arrays represent 'typical' rows for the constraint. It is shown that, for sufficiently long rows, the rate of this encoding technique can almost achieve that of the variable-rate, bit-stuffing scheme.

We present an initial experimental evaluation of coding and signal processing tradeoffs in high-density holographic data storage. Block-based and low-pass modulation codes, predistortion of holographic pages during recording (pre- processing), and conventional equalization (post-processing) are compared using a few recorded holograms. The relative gain in number of stored holograms is obtained by measuring BER as a function of readout power: the effect on density is gauged by the size of the Fourier plane aperture in the holographic system. Results show that equalization provides a 20% density gain, and predistortion a 60% gain. The total improvement in density by combining small apertures with both of these signal processing options is greater than 100% with an 8:12 strong balanced block code, a 6:9 lowpass/sparse code, and a parity thresholding technique with 9.1% overhead.

Previously derived results for angularly multiplexed Fourier plane holograms show more noise due to interpage cross-talk at the periphery of the recalled hologram. We describe three- dimensional error correcting codes which support equal and unequal error protection for encoding two dimensional information bit arrays to be stored in a volume holographic storage system. These codes support equal error protection such that the information bits are provided the same level of error protection and unequal error protection such that selected groups of information bits are provided different levels of protection. We propose an algorithm for changing from an original equal error protection code to an unequal error protection code programmatically. For raw bit error rates ranging from 10^-5 to 10^-4, these codes provide corrected bit error rates ranging from 10^-14 to 10^-12, respectively.

Volume holographic memories (VHMs) can deliver high aggregate data rates at a slow page rate by placing on the order of one million pixels per holographic page. By reducing the number of 'on' pixels per VHM data page we may increase the diffracted power into each pixel and therefore increase the number of pages we can store in our memory; although by doing so we have reduced the amount of user information per page. A detailed analysis shows that the information capacity can be increased by 15% with proper adjustment of the binary pixel priors such that a page contains about 25% 'on' pixels. Enumeration block coding techniques allow us to adjust the data priors appropriately with a code rate near the entropy bound for long block lengths. The sparsity of 'on' pixels also helps to reduce the effects of inter-pixel crosstalk by strongly reducing the probability that worst-case pixel patterns (e.g., blocks of 'on' pixels with the center pixel 'off') will occur in the data page. In addition, enumeration coding offers low encoding/decoding latency that scales linearly with the number of pixels per page. In this paper we discuss the theoretical advantage of optimum prior selection, as well as experimental results in achieving this capacity gain. The experiments verify that it is practical to adjust the priors and that an overall capacity gain of around 17% can be achieved for a realistic VHM system.

We consider a volume holographic memory (VHM) system that is corrupted by interpixel interference (IPI) and detector noise. We compare hard-decision Reed-Solomon (RS) decoding with both hard- and soft-decision algorithms for 2D array decoding. RS codes are shown to provide larger VHM storage capacity and density as compared with array codes when hard-decision methods are employed. A new likelihood-based soft-decision algorithm for 2D array decoding is described. The new decoding algorithm is motivated by iterative turbo-decoding methods and is capable of incorporating a priori knowledge of the corrupting IPI channel during decoding. The new algorithm is shown to offer VHM capacity and density performance superior to hard-decision RS methods.

In this work, a method of decision-feedback equalization is developed for a digital holographic channel that experiences moderate-to-severe imaging errors. Decision feedback is utilized, not only where the channel is well-behaved, but also near the edges of the camera grid that are subject to a high degree of imaging error. In addition to these effects, the channel is worsened by typical problems of holographic channels, including non-uniform illumination, dropouts, and stuck bits. The approach described in this paper builds on established methods for performing trained and blind equalization on time-varying channels. The approach is tested on experimental data sets. On most of these data sets, the method of equalization described in this work delivers at least an order of magnitude improvement in bit-error rate (BER) before error-correction coding (ECC). When ECC is introduced, the approach is able to recover stored data with no errors for many of the tested data sets. Furthermore, a low BER was maintained even over a range of small alignment perturbations in the system. It is believed that this equalization method can allow cost reductions to be made in page-memory systems, by allowing for a larger image area per page or less complex imaging components, without sacrificing the low BER required by data storage applications.

The wavelength selectivity of the hologram using the reference beams with random phase codes and multiple wavelengths is calculated and compared with experimental results. If the angle between signal and reference beams goes to 180 degrees, the effect of random phase code does not significantly affect the wavelength selectivity. Cross-talk noise analysis is Fourier holographic memory using random phase codes and multiple wavelengths is performed, and the signal-to-noise ratio is shown to depend on the angle between signal and reference beams as well as the wavelength difference.

The physical properties of photopolymer grating formation are, for the first time, investigated elaborately with respect to I, and (Lambda) . The dynamics of holographic recording with constant exposure energy (15mJ/cm²), are evaluated for a wide range of different I (mW/cm² - W/cm²), and for a few typical (Lambda) (0.5 - 3.5 micrometer), in a material utilizing cationing-ring-opening polymerization (Polaroid CROP ULSH-500B). Diffusion was evaluated to limit the photo- initiated recording sensitivity at high I(greater than W/cm² approximately (Lambda) ^-2). At the same time, however, the significant post-exposure grating development observed for diffusion limited recordings, was identified to allow eventually for equally high sensitive final gratings (approximately 3 - 5 cm/mJ) without reciprocity, or diffusion limitations. Based on these observations, a new physical model was developed that describes more accurately holographic recording utilizing photo-initiated polymerization, and accounts successfully for the observed physical properties of grating formation.

Pulsed illumination of lithium-niobate crystals with green light excites electrons from deep traps into the intrinsic defect Nb_Li⁵⁺ (Nb on Li site in the valence state 5+) and creates Nb_Li⁴⁺ centers (small polarons). The electrons trapped in this more shallow center increase the light absorption in the red and near-infrared. The dark decay of the polaron concentration is observed by monitoring the relaxation of these absorption changes. Iron- doped lithium-niobate crystals with different concentrations of Nb_Li are investigated for various illumination conditions. The relaxation shows a stretched-exponential behavior which is in disagreement with the predictions of the standard rate-equation model. The observed lifetimes of the polarons range from tens of nanoseconds to some milliseconds. Computer simulations reveal that all results can be explained considering distance-dependent excitation and recombination rates, i.e. the lifetime of an individual polaron depends on the distance to the next available deep electron trap. Based on the new insights, tailoring of lithium-niobate crystals for non-volatile holographic storage becomes possible.

Photopolymerizable materials are capable of recording high- efficiency volume holograms by changing the refractivity of the layer. An attractive feature of these media is that they allow multiple permanent holographic storage. Chemical composition, conditioning and pre-irradiation of the reactive mixture developed in the Mulhouse laboratory were optimized for the sequential recording of several permanent holograms into the same sample, with fair diffraction efficiency and without degradation of the spatial resolution. Since the species involved in the initiation mechanism are gradually consumed as the hologram builds up, the schedule (i.e., the successive exposure times and incident intensities) must be determined, to take into account the degree of conversion of the different components and reach full completion of the reaction at the end of the ultimate imagewise exposure. Examples of more than twenty multiplexed gratings or holograms of a target will be shown, the images being recorded at the same location in the polymer at ca one degree angular separation intervals. Applications for holographic data storage can be expected in view of these results, the simplicity of use (self-processing and self-fixing material) and the possibility of short single-pulse recording.

Hologram writing and fixing mechanisms are examined in disordered conjugated polymer/glass composites. The conjugated polymers used were alkoxy substituted poly(phenylenevinylne) analogs and the glass matrices were zirconia-organosilica xerogels. Hologram formation mechanism is shown to be a photochromic process consisting of light induced photo- oxidation (bleaching) of the embedded conjugated polymer resulting in the formation of an absorption grating and a phase grating. IR and Raman spectroscopy show that the chemical transformations upon photo-bleaching involve chain scission and oxidation of the polymer at the vinylic position of the conjugated polymer. Oxygen removal increases hologram formation time by more than an order of magnitude and halves the total hologram efficiency. The oxygen dependence was also highly correlated with photo-bleaching of the samples and beam interaction of the writing beams. Light sensitivity was compared for several polymer/glass composites showing that the new composites and film preparation techniques are promising for blue and ultraviolet sensitive holographic materials. A hologram fixing method based on a PMMA coating, applied on the film after hologram formation is demonstrated and shown to increase hologram erasure times by four. These important findings show that conjugated polymer/glass composites based storage media can be manufactured and fixed efficiently for a long term based on this method.

Optical Dynamic RAM (ODRAM) is a high capacity, low latency optical memory architecture based on persistent spectral hole burning in frequency selective materials. This paper describes the basic ODRAM architecture and progress towards realization of a high capacity, low latency, tabletop demonstration unit. In particular, a new technique, Spatially Distributed Spectral Storage (SDSS) is introduced and demonstrated to provide over two orders of magnitude improvement in spectral capacity for materials that experience excitation induced frequency shifts. Finally, the relative strengths and weaknesses of ODRAM are emphasized in a competitive analysis that includes currently available memory technologies such as semiconductor DRAM and magnetic disks.

We demonstrate one-to-one pixel-matching of phase-conjugate digital volume holographic data storage, with data pages as large as a megapel (1024 X 1024 pixels). A self-pumped phase-conjugate mirror in BaTiO₃ is used to provide a phase-conjugate reference beam, which reconstructs the data- bearing object beam from a LiNbO₃:Fe crystal using the 90 degree geometry. The systems tradeoffs of phase-conjugate readout are described, and two methods of generating phase- conjugate reference beams are described and compared.

Angularly multiplexed volume holography is considered for use in optical memory systems because of its compactness, capacity, fast parallel access, and associative readout capability. The associative nature makes holographic memories especially attractive to database applications. We define three characteristics, recall, precision, and fallout, and propose a figure of merit to measure the accuracy of associative recall and evaluate the performance of holographic memory systems with different types of data. The effect of a post-processing technique on the quality of the system's response is presented as well. This technique can significantly improve the performance of associative searches involving analog data.

Phase-coded multiplexing for volume holographic memories is a promising alternative to conventional addressing methods. We present a first realization of digital data storage by means of phase-coded multiplexing. The system concept, its implementation, and its performance is described. The potential of image processing is demonstrated experimentally. Data encryption of the stored data using an additional random phase key is discussed and experimental results are shown.

To fully exploit the high bandwidth and inherent parallelism of optical memory systems, it is necessary to perform correspondingly parallel computations at or near the interface to the memory system. In this paper, we present a system in which a dynamically reconfigurable processor is built at the optical memory interface. Dynamically reconfigurable processors exploit parallelism at the level of individual machine instructions. They are based on the time multiplexing of gate array logic between various processor configurations, each of which is matched to a particular required computation. This paper is an analysis of the performance of an optically reconfigurable processor in comparison to conventional multiple instruction issue processors. We will show that the volume of configuration data required makes these systems difficult to build in electronic implementations but ideal for implementations with optical memory.

By controlling the pixels of a liquid crystal display (LCD) electronically, we fabricated a real-time moving window on a LCD, through which light passes. Using the moving window and multi-focusing lens, we suggested a non-mechanical spatio- angular multiplexed holographic memory system and demonstrated its feasibility through optical experiments. The principle of the proposed method and optical experimental results are also presented.

The potential for implementing a three-dimensional space-time correlator employing spatio-spectral holographic materials is discussed. The constraints on various configurations are derived and discussed, as well as possible enhancements. The analysis shows that time-space images on the order of 10,000 X 1,000 X 1,000 can be searched for at frame rate exceed 10 GHz.

Three-dimensional parallel readout of 2-photon multilayer optical disks can simultaneously offer high capacities (greater than 100 GB/disk) and high data transfer rates (greater than 1 Gb/s). The robust system tolerances should enable cost effective storage systems with capacities and transfer rates that are scaleable to match various application requirements.

Most optically accessed memory technologies resort to correlative address schemes. These addresses tend to be lengthy, inaccurate, and to produce crosstalk between fields. Alternative to full-length orthogonal, binary amplitude address codes are discussed here. Lookup table approaches could potentially reduce word length for address specification, but the complexity of the correlation process would still pertain. As the investigation of this topic proceeded its emphasis shifted so that the paper should more appropriately should be titled 'Short Addresses for Correlative Optical Memory Access.'

Several new photochromic fluorescent materials have been developed to be used as write, read, erase media for 3D two- photon optical memory devices. These materials are of the substituted fulgides family and were specially synthesized by us for 3D memory devices. Both the read and write forms of these materials were found to be stable at room temperature and their read, colored form, emit strong fluorescence, while the write, colorless form, does not fluoresce. Their spectroscopic and kinetic properties were measured and the quantum efficiency for writing reading and erasing were determined. In addition the kinetics of the photoprocesses involved in the write-read-erase cycles were also measured. These data suggest that these materials are potentially suitable for use in rewritable optical memory devices.

We report on room-temperature photochromism of Zn- tetrabenzoporphyrine-doped polymethylmethacrylate. A one- quantum, thermo-reversible photoreaction is initiated with the 633 nm line of a HeNe-laser. The quantum yield depends strongly on the concentration of an electron acceptor which is doped into the matrix. The optical density of the sample can be reduced by up to a factor of 5 via irradiation, leading to refractive index changes of about 0.015.

For frequency domain storage rare earth doped sulfide show the most promising characteristics of any spectral holeburning material known. They provide the desired density of storage with a remarkable stability of memory against storage, read- out, and thermal surges. Problems associated with the fabrication of a prototype device not only require a careful consideration of design parameters but also raise some interesting questions regarding the material characteristics and the holeburning mechanism. This paper gives the status of our efforts in the direction of making thin films of MgS and CaS doped with Eu suitable for spectral holeburning based optical storage using the Pulsed Laser Vapor Deposition (PLD) technique.

The possibility of using Raman excited spin coherences to increase the operating temperature of spectral hole burning memories and processors is being explored experimentally. The approach is to store and/or process data using spin coherences excited by optical Raman transitions. This is motivated by the fact that spin coherence lifetimes are much less sensitive to temperature than optical coherence lifetimes. However, direct microwave excitation of spin coherences does not give a high storage density because of the large microwave wavelength. By using optical Raman fields to excite the spin coherences, full optical spatial resolution can be achieved. Initial experiments in Pr doped YSO demonstrated potential for higher temperature operation. Current experiments are concentrating on further increasing the operating temperature using this and other materials.

We explain that it is possible in principle to detect a change in light transmittance and phase shift of transparent materials without exposing even a single photon on them. Based on such interaction-free measurement, novel rewritable optical memories can be constructed with a photo-sensitive medium. In this case, even if the stored data are vulnerable to light, they can be read out without (or, at least a reduced) data impairment, because there is no (or, at least a reduced) interaction between the photon and the medium. We also discuss the effect of two system imperfections, the system loss and the low quantum efficiency of photon detectors, when such memories are to be implemented.

High-resolution spectroscopic studies of Eu³⁺ doped Y₂SiO₅ revealed certain new aspects, which were not known in the past. Our studies indicate that europium occupies several distinct sites in this host material. Temporal evolution revealed unusual behavior.

The design issues for the technique of continuously programming a coherent transient spatial-spectral optical signal processor are discussed. The repeated application of two spatially distinct optical programming pulses to a non- persistent hole-burning material writes an accumulated, spatial-spectral population grating with low intensity optical pulses as compared to single shot programing. An optical data stream is introduced on a third beam, resulting in a processor output signal spatially distinct from all the input pulses. Programming and processing take place simultaneously, asynchronously and continuously. For accumulated gratings, the frequency stability of the optical source is an important consideration. Assuming a sufficiently stable optical source, simulations show that an accumulated (and maintained) grating in steady state, for both storage of a true-time delay and/or pattern waveform, can be highly efficient using currently available materials, on the order of that predicted for a perfect photon-gated device. An experimental demonstration of the continuous programming concept for true time delays programmed with chirped pulses is presented, showing the accumulation of the grating with low area pulses over time until it reaches steady state, for times longer than the persistence of the material.

The Boolean logic operations such as OR, AND and NOR are demonstrated using photostimulated luminescence (PSL) phenomenon in the imaging plate (IP) which is made by BaFBr:Eu photostimulable phosphor. The PSL in Eu and Sm co-doped SrS (SrS:Eu,Sm) phosphor ceramics is studied in order to develop a smart erasable and rewritable optical memory utilizing PSL phenomenon, because the SrS:Eu,Sm phosphor ceramics which are excited with UV-light or visible light exhibits an intense PSL, compared with that in UV-irradiated IP. Intense PSL with a peak at about 600 nm is observed in SrS:Eu,Sm phosphor ceramics which is stimulated with infrared light after irradiation with ultraviolet-(UV) light or visible-light. The PSL characteristics of SrS:Eu,Sm phosphor ceramics as a photostimulable material for the optical memory are reported.

Techniques of volume holographic associative storage and wavelet transform are combined to develop a system for classification and quality inspection of industry products. Wavelet transform is introduced to improve the recognition accuracy. The Comparison with the conventional correlation is studied. Multichannel correlation characteristic of the system is adopted to classify products. According to the position of the correlation output with highest intensity, a manipulator can classify the input product. Localization of products is used to solve invariance of the system in application. Variation of the highest correlation intensity is used to inspect quality of products. Curves of each product between correlation intensity and deformed degree are measured in advance. A critical intensity value is setting to judge a product is good in quality or not. In the inspection, the highest correlation output is detected and compared with its own curve. The unqualified product is rejected by the manipulator and the qualified one is accepted and classified. The classification and quality inspection of standard elements are used as an example in our experiment.

I pioneered the use of incoherent ultraviolet light to fix holograms in photorefractive materials. I discovered a reversible hologram fixing process that eliminates hologram erasure during readout and dark decay in photorefractive materials. Holograms fixed with this process are read out indefinitely with high and constant diffraction efficiency and do not exhibit any apparent dark decay. Thus, information stored in a photorefractive material is always retrieved without any loss in quality and is preserved in the material without any degradation. Plane wave holograms were fixed in lithium niobate with incoherent ultraviolet light. The holograms exhibited high and constant diffraction efficiency when they were read out continuously for many hours with the reference beam and did not register any apparent dark decay for over a year. The fixed holograms were erased with incoherent ultraviolet light, thus making this all-optical hologram fixing process reversible.