Volume Holographic Memory System Research Articles

Analysis of a lens-array modulated coaxial holographic data storage system with considering recording dynamics of material.

In the first time, a simulation model with considering the recording dynamics of material is built and is used to simulate evolution of the grating strength of the recorded hologram in a coaxial volume holographic memory system. In addition, phase modulation by lens array in the reference is introduced and observed to perform better diffracted signal quality and higher shifting selectivity, in both simulation and experiment. The use of lens array is found to provide multiple advantages in volume holographic memory system. The new simulation model potentially can be used to precisely design the system to obtain higher diffracted signal quality, higher shifting selectivity, and reduction of M# consumption and increase of storage capacity.

Holography optical memory recorded with error correcting bits

A novel error correction method is proposed for volume holographic memory systems. In this method the information of two adjacent binary bits is recorded in the space between the two bits, which is used to correct the errors in the data bits. The new method is compared with (15, 5) Reed—Solomon code using the same redundancy of 200% as ours. It is shown that the new method achieves the similar bit error rate as the RS code.

Image-based numerical evaluation techniques in volume holographic memory systems

A simulation technique for a volume holographic memory system based on binary page data is proposed to evaluate quantitative diffraction efficiency, bit error rate, and signal-to-noise ratio. In this simulator, propagation of a signal and reference beams and diffraction at gratings are calculated quantitatively by use of numerical Fresnel propagation and Kogelnik's coupled-wave theory. To apply Kogelnik's wave theory to arbitrary signal and reference beams, both beams at the hologram plane are decomposed into plane waves. Numerical results show good agreement with the exact solution under the Born approximation in a transmission-type volume hologram. We also apply the simulator to reflection-type holographic memory.

Nonuniform Error Correction Using Low-Density Parity-Check Codes

This correspondence introduces a framework to design and analyze low-density parity-check (LDPC) codes over nonuniform channels. We study LDPC codes for channels with nonuniform noise distributions, rate-adaptive coding, and unequal error protection. First, we propose a technique to design LDPC codes for volume holographic memory (VHM) systems for which the noise distribution is nonuniform. We show that the proposed coding scheme has an easy design procedure and results in efficient codes for holographic memories. An important property of the proposed technique is the design of the codes that have a low error floor and low variable node degrees, while maintaining performance close to the Shannon limit. We then show that punctured LDPC codes can be studied as a special case of our design methodology for nonuniform channels. Finally, we propose a method to generate LDPC codes that can provide unequal error protection in addition to having a good overall performance. Moreover, the highly protected bits can be decoded without requiring the entire word to be decoded.

Irregular repeat-accumulate codes for volume holographic memory systems.

We investigate the application of irregular repeat-accumulate (IRA) codes in volume holographic memory (VHM) systems. We introduce methodologies to design efficient IRA codes. We show that a judiciously designed IRA code for a typical VHM can be as good as the optimized irregular low-density-parity-check codes while having the additional advantage of lower encoding complexity. Moreover, we present a method to reduce the error-floor effect of the IRA codes in the VHM systems. This method explores the structure of the noise pattern in holographic memories. Finally, we explain why IRA codes are good candidates for the VHM systems.

Low-density parity-check codes for volume holographic memory systems.

We investigate the application of low-density parity-check (LDPC) codes in volume holographic memory (VHM) systems. We show that a carefully designed irregular LDPC code has a very good performance in VHM systems. We optimize high-rate LDPC codes for the nonuniform error pattern in holographic memories to reduce the bit error rate extensively. The prior knowledge of noise distribution is used for designing as well as decoding the LDPC codes. We show that these codes have a superior performance to that of Reed-Solomon (RS) codes and regular LDPC counterparts. Our simulation shows that we can increase the maximum storage capacity of holographic memories by more than 50 percent if we use irregular LDPC codes with soft-decision decoding instead of conventionally employed RS codes with hard-decision decoding. The performance of these LDPC codes is close to the information theoretic capacity.

Bit error rate and system limitations on the storage capacity of volume holographic memory systems

The theoretical storage capacity of data in the form of volume holograms is estimated to be of the order of a terabit per centimeter cube. However, the practical limit of the capacity is set by the system parameters. A model is shown that uses system parameters such as bit error rate, data access time, number of holograms, and noise to yield practical estimates of the capacity. The model calculates the impact of different system parameters on the capacity and shows the bottlenecks for this technology. © 2001 Society of Photo-Optical Instrumentation Engineers.

Soft-decision array decoding for volume holographic memory systems.

We study the use of soft-decision array decoding in a volume holographic memory (VHM) system that is corrupted by interpixel interference (IPI) and detector noise. Soft-decision methods can unify equalization and error decoding. A highly parallel array decoder is presented in the context of two-dimensional low-pass channel mitigation and error correction. 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 resulting joint detection decoding algorithm is shown to offer VHM capacity and density performance superior to that of hard-decision n = 255 Reed-Solomon codes in concatenation with a Wiener filter.

Improved associative recall of binary data in volume holographic memories

A new technique is presented that improves the results of associative recall in a volume holographic memory system. A background is added to the normal search argument to increase the amount of optical power that is used to reconstruct the reference beams in the crystal. This is combined with post-processing of the captured image of the reference beams. The use of both the background and post-processing greatly improves the results by allowing associative recall using small arguments. In addition, the number of false hits is reduced and misses are virtually eliminated.

Crosstalk in volume holographic memory

Crosstalk noise, due to the finite dimension of the recording medium, is a fundamental limitation to storage capacity in volume holographic memory. We present a general theoretical analysis on the crosstalk noise in angle- and wavelength-multiplexed volume holographic memory systems. Results on storage capacity and its dependence on hologram separation and required signal-to-noise ratio are obtained and discussed. Crosstalk noise may also arise from the expansion or shrinkage of the storage medium during and after the hologram recording process. Its effects, including Bragg mismatch, pixel displacement and impulse broadening, are analyzed for Fourier-plane holographic memories. Complete compensation and optimization to eliminate or reduce these effects are presented and discussed.

Interleaving and error correction in volume holographic memory systems

We study the use of error-correction coding (ECC) and two-dimensional interleaving for volume holographic memory (VHM) systems suffering from both random and systematic errors. The bit-error rate (BER) is used as the data-fidelity measure and as a design metric for optical 4f systems. The correlated error patterns arising from both lens aberrations and misalignment errors are analyzed, and we discuss the information theoretic storage capacity of VHM in the presence of such correlated error patterns. The performance of interleaving and ECC is analyzed from both BER and storage-capacity perspectives. Magnification, rotation, tilt, and defocus errors are also studied, and an experimental demonstration that combines ECC with two-dimensional interleaving is included.

Absorption effects in photorefractive volume-holographic memory systems I Beam depletion

We consider how absorption-induced beam depletion alters the index-modulation profile, the diffracted beam profile, and the Bragg selectivity of volume holograms. We derive theoretical equations that consider beam depletion (in the absence of two-beam coupling) and compare these with experimental observations. We also derive a smart scheduling technique whereby such absorption effects can be compensated for during the scheduled recording of multiple holograms. In our smart scheduling approach, during recording the profile of the input signal beam is modified such that during recall its output profile is uniform.

Sparse-wavelength angle-multiplexed volume holographic memory system: analysis and advances

We present and analyze our hybrid wavelength-and-angle-multiplexed volume holographic memory system. The hybridization of wavelength and angle multiplexing relaxes demands on spectral-tuning sources, angle-tuning devices, and optical system numerical apertures while maintaining a large K-space addressability. We consider realistic properties of our volume holographic memory system, addressing practical issues such as storage density and material-dependent photon-limited information throughput. Finally, we present experimental results of the storage of 2000 sparse-wavelength angle-multiplexed volume holograms in a 1.86-cm3 volume of lithium niobate.

Volume holographic memory systems: techniques and architectures

Although the concept of using multiplexed holography for data storage has been considered for some time, recent advances in several critical device technologies along with developments in storage materials have greatly enhanced the likelihood of successful implementations. We review several basic architectural concepts along with various multiplexing options and associated techniques for holographic data storage.

Error-correcting code for volume holographic storage of a relational database

We present an error-correcting code suitable for encoding relational database information to be stored in a volume holographic memory system. The code makes use of the page-oriented format of holographic memories as well as the tabular arrangement of database records. The performance metrics discussed here show that the code results in industry-standard bit error rates and permits simple and fast decoding.

Effects of cross talk on fidelity in page-oriented volume holographic optical data storage

Using numerical simulation, we quantitatively examine the effects of cross talk on the recall fidelity of stored binary data in a page-oriented volume holographic memory system. We discuss the trade-off between the signal-to-noise ratio of the reconstructed bits and the optical throughput of the system (i.e., the fraction of the incident beam that is diffracted to the detector plane). We show that significant gains in the signal-to-noise ratio can be achieved with modest decreases in optical throughput in the region where the highest throughput occurs. The magnitude of this trade-off is influenced by both beam degeneracy and coherent recording cross talk. At low optical throughputs an upper limit to the signal-to-noise ratio is set by the cross talk caused by angular sidelobe overlap, which in turn is a function of the angular isolation of the angularly multiplexed data pages.

Cross-talk-limited storage capacity of volume holographic memory

We consider the storage density limited by the cross talk between stored pages during readout. We review some earlier work done in this area and present new theoretical work that characterizes the storage capacity limitations due to the cross talk. The results show that because of the presence of degeneracy noise, storage capacity does not benefit from expanding the reference points from one dimension to two dimensions. An optimum configuration that fully utilizes storage capacity and completely eliminates degeneracy noise is given for an angularly multiplexed volume holographic memory system.