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We consider holographic optical data storage systems implemented with photorefractive media. Our viewpoint emphasizes the close interaction between materials and device issues. First we discuss our current understanding of photorefractive physics as it pertains to the holographic data storage problem. Then we consider architecture issues, including angular, phase-encoded, and wavelength-multiplexing techniques, and several approaches to increasing the signal-to-noise ratio of the recordings. Finally, we discuss materials issues related to crystal growth and how crystal quality impacts the performance of data storage systems. Both bulk and fibre crystal growth techniques are reviewed.

We describe a novel method for the recall of linear combinations of stored data pages in a volume holographic memory based on phase-code multiplexing. Recall is demonstrated by use of a compound phase-and-amplitude spatial light modulator in the reference beam path.

We examine the consequences of permanent phase distortions on the replay of transmission volume holograms with a phase-conjugated reference wave. In the absence of Bragg degeneracies and significant Bragg mismatch, for negligible dispersion in the spatial frequency response and for negligible variation in obliquity, phase-conjugated reference waves compensate for the effects of phase distortions and permit cross-talk-free multiplexing by using orthogonal reference waves. Spatial frequency dispersion, however, is a source of image distortion and cross talk.

A number of spatially nonlinear-optical materials, such as photorefractive media, are suitable for volume holography. High Bragg selectivity of thick media has led to the development of applications of multiplex volume holography ranging from binary and analog data storage, to associative memory, to neural networks, to optical interconnects. An important consideration is the balance between capacity and noise. We examine here crosstalk for angular, phase-encoded, and wavelength multiplexing for holographic data storage and describe the properties of null-matched arrangement of reference waves, presenting new results for adjacent, sparse, and fractal strategies. We emphasize the impact of signal bandwidth on crosstalk and describe how crosstalk limits storage capacity. We consider first crosstalk due to Bragg mismatch (mismatch-limited crosstalk), and then relate it to other noise sources present in a holographic memory system. >

Certain memory applications based on photorefractive media require the ability to conveniently fix holograms. An attractive alternative is to sufficiently prolong the readout time. Low photoexcitation at long wavelength augmented by self-diffraction can increase readout time by a few orders of magnitude. The interaction of the writing and readout waves, both of the same wavelength, with the photorefractive space charge field has been investigated theoretically and experimentally. Dynamics of beam coupling and self diffraction, in certain geometries, can lead to an initial increase followed by non-exponential decay of the diffracted signal during readout. Here we study how the difference in gain, lifetime, photoexcitation and absorption at different wavelengths - which require changing incident angles to satisfy the Bragg condition - affect the decay dynamics of photorefractive gratings. >

Holograms may be superimposed in a holographic medium by a number of techniques such as angular, phase-encoded, and wavelength multiplexing. For typical architectures, crosstalk increases as the bandwidth of stored data increases. The number of superimposed holograms in an ideal medium is limited by its length L, the wavelength of light (lambda) , and index of refraction n, such that for crosstalk-limited storage we can superimpose up to 2nL/(lambda) holograms of high bandwidth. In practice, however, the properties of real materials depart from the ideal through index inhomogeneities and scattering centers, dispersion in the material response as a function of grating spatial frequency, and dispersion as a function of the wavevector and polarization of both incident and scattered light. We discuss the impact of these properties on holographic data storage and describe material, device, and system strategies designed to minimize these effects.

The capacity of volume holographic data storage depends on the multiplexing efficiency of input holographic data arrays or pages. We compare this page-packing capacity of two common approaches: angle, and wavelength multiplexing. For maximal page packing the angular separation between the reference and signal beam incidence in the angle and wavelength multiplexing methods should be 90 degree(s) and 180 degree(s), respectively. We find that in these optimal arrangements both methods can multiplex a similar number of paraxial data-page signals, which is comparable to conceptual storage capacity limits derived from considerations of resolution in three-dimensions. In practice, the high number of multiplexed data pages predicted here will be compromised by material-related noise sources.

We investigate the resonant two-photon (two-step) processes of photorefractive grating formation and identify the regimes in which nonvolatile holography is possible. We develop a charge-transport model to describe this behavior for a photorefractive crystal with a single active impurity species under continuous-wave illumination. For the cases that allow for nondestructive reconstruction of gratings, we evaluate the maximum refractive-index perturbation and the response rate with respect to illumination intensities and impurity characteristics. We evaluate the importance of the impurity intermediate-state occupancy. Holographic data storage system issues are also discussed. The present results are consistent with previously reported photorefractive behavior and predict additional properties that characterize these resonant processes. The analysis may be used further to study other related two-photon phenomena of interest in holographic storage systems.

We investigate photorefractive media for which quasi-stabilized ionic gratings can be used to prolong readout lifetime. We use coupled-transport-mode theory to describe the coevolution of photorefractive gratings that arise from free-electron transport and ionic transport. We evaluate in detail the differences between low-temperature and high-temperature recording for typical conditions required by multiplex holography. We provide general normalized examples for simple diffusion transport and specific examples for photovoltaic LiNbO3. We introduce a common formalism to compare widely varying results present in the literature and to guide the materials and system development processes.

Unwanted erasure during readout of holographic data can be reduced or eliminated by use of a different wavelength for reading than that which was used for writing. To prevent distortion and Bragg mismatch that would be unacceptable for digital data storage, one can format data to account for the wavelength difference. Techniques to format data and the results of this formatting are presented. Varying the formatting parameters is investigated to optimize diffraction efficiency.

We derive the relationships for coded wave fronts in volume holography for reference and signal waves coded with two-dimensional wave fronts. We develop estimates for cross talk that is due to grating degeneracy, nonorthogonality, and momentum-mismatch reconstruction. We compare a number of multiplexing techniques on the basis of cross talk and other noise sources in a system.

We describe a digital holographic data storage system that uses in situ thermal fixing to achieve nonvolatile readout. The system was used to store and fix 530 holograms representing 1.7 MB of digital data. The system demonstrates that fixing by heating after recording gives adequate performance for multiplex holography in the perpendicular recording geometry. The postrecording heating procedure is preferred over high-temperature recording in the perpendicular geometry to achieve Bragg matching for the entire signal angular bandwidth.

Various channel codes, including binary, gray-scale, threshold, and differential techniques, are compared for digital holographic data storage. The tradeoffs among bit error rate, storage capacity, and system complexity are discussed.

The selection of reference beams for cross-talk-limited wavelength- and angular-multiplexed volume-holographic data storage is discussed. It is shown that sparse selection of the reference waves can lead to a significant improvement in the cross-talk-limited signal-to-noise ratio in a wavelength-multiplexed system. In a high-capacity angular-multiplexed system sparse selection offers essentially no advantage.

We compare the characteristics of cross talk for angular multiplexing and several phase-encoded multiplexing strategies for volume holography. We discuss the implications of these characteristics for holographic data storage and compare noise arising from cross talk with noise arising from scattering that is due to inhomogeneities within a holographic medium.

Growth-induced striations in photorefractive media constitute phase gratings that scatter transmitted light and therefore degrade the quality of reconstructed images in holographic applications. A simple angular multiplexed holographic memory arrangement in a striated strontium barium niobate sample is considered. For forward-propagating reconstruction the striations distort the readout images and introduce ghost images. For counterpropagating readout, inhomogeneous image reconstruction and interimage cross talk can occur. We investigate these effects experimentally for different reconstruction methods and demonstrate the cross talk and the ghost images with a simple model based on coupled-wave theory. Readout with a counterpropagating plane wave eliminates the ghost images and the distortions but suffers degradation from cross talk and inhomogeneous image intensity. Only reconstruction with a phase conjugate of the transmitted reference beam eliminates cross talk and distortions. The model indicates that negligible material dispersion is a necessary requirement for high-quality phaseconjugation readout.

Growth-induced striations in photorefractive media scatter transmitted light, degrading their performance in holographic applications. We model this phenomenon by considering the striations to be a permanent hologram within a medium and compare it with experimental results in a striated strontium barium niobate crystal. We show that the construction of the phase-conjugate signal beam by a phase conjugate of the transmitted reference beam alleviates the effects of distortions that are due to striations.