Matthew C. Bashaw

Volume Holographic Storage and Retrieval of Digital Data

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.

Holographic Data Storage Systems

In this paper, we discuss fundamental issues underlying holographic data storage: grating formation, recording and readout of thick and thin holograms, multiplexing techniques, signal-to-noise ratio considerations, and readout techniques suitable for conventional, phase conjugate, and associative search data retrieval. Next, we consider holographic materials characteristics for digital data storage, followed by a discussion on photorefractive media, fixing techniques, and noise in photovoltaic and other media with a local response. Subsequently, we discuss photopolymer materials, followed by a discussion on system tradeoffs and a section on signal processing and en/decoding techniques, succeeded by a discussion on electronic implementations for control, signal encoding, and recovery. We proceed further by presenting significant demonstrations of digital holographic systems. We close by discussing the outlook for future holographic data storage systems and potential applications for which holographic data storage systems would be particularly suited.

Encrypted holographic data storage based on orthogonal phase code multiplexing

We describe an encrypted holographic data-storage system that combines orthogonal-phase-code multiplexing with a random-phase key. The system offers the security advantages of random-phase coding but retains the low cross-talk performance and the minimum code storage requirements typical in an orthogonal-phase-code-multiplexing system.

Photovoltaic spatial solitons

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.

Optical memories implemented with photorefractive media

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.

Channel codes for digital holographic data storage

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.

Photorefractive effects in periodically poled ferroelectrics

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.

Y junctions arising from dark-soliton propagation in photovoltaic media

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.

Recall of linear combinations of stored data pages based on phase-code multiplexing in volume holography

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.

Digital holographic storage system incorporating thermal fixing in lithium niobate

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.