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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.

We present a novel digital holographic data storage system based on wavelength multiplexing in 90 degrees geometry, using an automated tunable diode laser and a lithium niobate crystal. The automatic storage and retrieval of a 60-kbyte data file, as well as the limitations and future implementations of the system, are discussed.

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.

We study detuning effects that modify the Bragg condition in photorefractive crystals and analyze the influence of these effects on the retrieval of information that is stored in such crystals. We examine and integrate several mechanisms that induce Bragg detuning, including the effects of temperature changes, applied electric fields, and polarization changes, taking into account the anisotropic nature of photorefractive crystals. Transfer functions based on these detuning effects are defined and computed to examine and quantify the signal quality that is obtained during the readout of image-bearing holograms. The different recording configurations and geometries that are typically used in photorefractive applications are considered and compared. Finally, we study more specifically the consequences of temperature-induced detuning in the context of a holographic data storage system that uses thermal fixing. We show theoretically and experimentally that distortions (in this case limited field of view) occur on readout, and we propose solutions to improve the quality of the retrieved images.

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.

An encryption method and apparatus for holographic data storage are disclosed. In a system using orthogonal phase-code multiplexing, data is encrypted by modulating the reference beam using an encryption key K represented by a unitary operator. In practice, the encryption key K corresponds to a diffuser or other phase-modulating element placed in the reference beam path, or to shuffling the correspondence between the codes of an orthogonal phase function and the corresponding pixels of a phase spatial light modulator. Because of the lack of Bragg selectivity in the vertical direction, the phase functions used for phase-code multiplexing are preferably one dimensional. Such phase functions can be one-dimensional Walsh functions. The encryption method preserves the orthogonality of reference beams, and thus does not lead to a degradation in crosstalk performance.

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.