Search

A multiple page fully digital holographic data storage system is demonstrated. This system is used to store and retrieve digital image and compressed video data with a photorefractive crystal. Architecture issues related to spatio-rotational multiplexing and novel error-correcting encoding techniques used to achieve low bit-error rates are discussed.

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.

A quantitative analysis of the steady-state photorefractive index perturbations caused by a given optical irradiance distribution in a periodically poled ferroelectric is presented. Axially invariant index perturbations that are due to the photogalvanic effect are reduced compared with those in a homogeneously poled crystal by approximately the square of the product of the poling-grating wave vector (K(g)) and a characteristic transverse dimension of the irradiance. This result is consistent with empirical observations in periodically poled LiNbO(3) of much higher resistance to photorefractive damage.

We report the observation of planar Y-junction waveguide splitters that are due to the bulk photovoltaic effect. The junctions are generated by multiple dark-soliton propagation in LiNbO(3) by use of low power levels (20 mW) and average intensities of 10 W/cm(2) at 488 nm. The junctions persist in the dark and can be used to divide input beams of less photorefractive sensitivity. We describe some possibilities for generating other waveguide structures in bulk and thin-film media.

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. >

We report what we believe to be the first observation of photorefractive gratings in lead barium niobate (Pb(1-x)Ba (x)Nb(2)O( 6) ; PBN). A diffraction efficiency of 9% and a beam-coupling coefficient of 9 cm(-1) are obtained. After thermal fixing a grating of 0.5% is revealed at room temperature. We can improve this diffraction efficiency by optimizing fixing temperature, timing, and material parameters.

Volume holographic data storage is a promising alternative to current surface storage technologies, since it can provide simultaneously large digital storage capacity, fast data transfer rates, and short access times [1]. This is achieved by storing multiple holographic data pages in a common recording volume, while retaining the ability to retrieve any individual page. Multiplexing of data pages can be performed either at one wavelength, using angular or phase-encoded multiplexing, or at different wavelengths, using wavelength multiplexing. This investigation focuses on phase-encoded multiplexing, which for example allows direct binary arithmetic operations on recorded pages and enables data encryption [2].

The performance of digital holographic data storage (DHDS) systems is primarily limited by the recording medium, as other components have been developed for consumer applications. In this invited paper we review state-of-the-art DHDS systems, fixing issues, and fundamental tradeoffs between materials and device characteristics. Recent results, including stored and retrieved video clips will be presented.

Starting with the work of van Heerden1, many estimates of the potential storage capacity of a holographic data storage system have been made. While some effort has been made to theoretically predict2–3 and to measure4–5 the practical limits imposed by noise sources, most results deal with fundamental limits imposed by the finite spatial frequency resolution of the recording medium and the optical system. We are interested in characterizing the noise statistics in a typical holographic storage system and using these statistics to predict the ultimate limits on system performance. Toward this end, we have developed theoretical models to describe several fundamental noise sources. These models are used to predict limits on the storage capacity and transfer rate achievable in a practical device. We have constructed an experimental system that is used to obtain data on bit-error-rate performance and to verify theoretical predictions. Finally, we are using the experimental system to develop efficient techniques for overcoming noise sources so that we can achieve performance closer to the theoretical limits.

We report on the observation of planar dark spatial solitons due to the bulk photovoltaic effect in lithium niobate, with intensities of the order of 10 W/cm2 and widths of approximately 20 \ensuremath{\mu}m. Photovoltaic solitons display a characteristic tensorial dependence on their direction of propagation, on their polarization, and on the orientation of the amplitude profile, with respect to the principal axes of the crystalline medium. The index perturbation associated with a dark soliton persists in the dark, and it can trap and guide a second beam.

Dark (bright) planar spatial solitons are predicted for photovoltaic photorefractive materials when the diffraction of an optical beam is exactly compensated by nonlinear self-defocusing (focusing) due to the photovoltaic field and electro-optic effect. These solitons may have steady-state irradiances of microwatts to milliwatts per square centimeter and widths as small as 10 [mu]m in lithium niobate. Optical control is provided by incoherent illumination, and the nonlinear index of a dark soliton may be used to trap a bright soliton by rotating the plane of polarization of the soliton field.

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.

In this invited paper we discuss recent advances in holographic data storage using ferroelectric SBN fibers as the recording medium. An optical architecture involving an array of SBN fibers is discussed that potentially allows 2-3 orders of magnitude faster access times than for conventional magnetic data storage devices of Gbyte-size . To realize this potential we are studying the interplay between fundamental materials issues related to fiber growth and photorefractive processes underlying storage and readout processes in SBN.

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.

We analyze self-trapping of one-dimensional optical beams in photorefractive, photovoltaic media for open- and closed-circuit realizations. We show that a passive load (resistor) in the external circuit can be used for switching of dark photovoltaic solitons. Dark solitons in a short-circuited crystal can be obtained for a much smaller nonlinearity than in open-circuit conditions. Shorting the crystal affects bright solitons very little.

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 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.

The increase in photorefractive damage resistance in both MgO-doped and nondoped Li-deficient LiNbO3 is studied by two wave mixing measurements. Holographic diffraction measurements demonstrated that crystals with high optical damage resistance increase photoconductivity and response time for erasing photorefractive grating. The erasure speed is linearly proportional to the MgO dopant and Li-site vacancy concentrations. The erasure time of holographic grating shows an Arrhenius dependence on temperature from which activation energy is obtained. We proposed an energy level model of the light-induced transport for LiNbO3:MgO dopants introduce donor levels 0.11 eV below the conduction band, whereas, Li-site vacancies in nondoped crystal introduce acceptor levels 0.29–0.44 eV above the valence band.

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 present a comprehensive analysis of the evolution of a grating envelope, in both transmission and reflection geometry, during the erasure process in various readout configurations in bulk photorefractive media. We derive a single partial differential equation that describes grating evolution. This equation shows that under certain conditions the grating envelope propagates and shares several features with either bright or dark solitons. Details of the grating envelopes directly relate to the dynamics of the observed diffraction efficiency. We derive analytical expressions for diffraction during nondestructive readout. Finally, we discuss the effects of nonzero absorption, finite dark conductivity, fringe bending, fanning, and nonlinear responses on the propagating envelope.

We introduce the concept of traveling grating envelopes to study the erasure dynamics in bulk photorefractive media. This new technique provides considerable physical insight into the observed photorefractive transients. We propose and demonstrate experimentally, in a Ce-doped SBN:60 crystal, two novel ways of extending the grating lifetime by trapping the grating envelope inside the medium. The incident beam ratio and the readout geometry have the largest effect on how long the grating can be read out.

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.

A two-center photorefractive model permitting all center-to-band transitions and electron–hole charge transport unifies several models describing subsets of these transitions. Using the quasi-steady-state approximation for low continuous-wave illumination, the model derives the first-order Fourier harmonic of the steady-state space-charge distribution and fields and the corresponding response rates. The model describes electron–hole competition, complementary gratings, double-exponential time-dependent behavior, and nonlinear dependence of response rates on intensity. The model can be used to make new predictions of photorefractive behavior of crystals in which two species of impurity center are present; an example is presented for BaTiO3.

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.

Bismuth silicon oxide is one of the most sensitive photorefractive materials available today. It is therefore important to develop an understanding of how its photorefractive properties can be altered by adding impurities during growth. In this paper results from analyses of doped bismuth silicon oxide crystals grown by the addition of impurities to the melt during Czochralski growth are presented, and the influence of these impurities and their concentrations on the photorefractive effect are determined. Doped crystals are used in a holographic arrangement, and the effects of dopants and temperature on grating formation (erasure time and diffraction efficiency) are reported. Data on the effect of dopants on long-term retention time are also presented. The apparent effects of certain impurities on traps are reviewed. The results are discussed and compared with current models.