We present an overview of our research effort on volume holographic digital data storage. Innovations, developments, and new insights gained in the design and operation of working storage platforms, novel optical components and techniques, data coding and signal processing algorithms, systems tradeoffs, materials testing and tradeoffs, and photon-gated storage materials are summarized.

Holographic data storage

Research into and the development of data storage devices is a race to keep up with the continuing demand for more capacity, more density, and faster readout rates. Improvements in conventional memory technologies-magnetic hard disk drives, optical disks, and semiconductor memories-have managed to keep pace with the demand for bigger, faster memories. However, there is strong evidence that these two-dimensional surface storage technologies are approaching fundamental limits. An alternative approach for next-generation memories is to store data in three dimensions. This article describes developments in holographic 3D memories, in which high density is achieved by superimposing many holograms within the same volume of recording material. Holographic storage is a promising candidate for next-generation storage. Research has demonstrated that holographic storage systems with desirable properties can be engineered. The next step is to build these systems at costs competitive with those of existing technologies and to optimize the storage media.

/pdf/holographic-data-storage-i6ionpw23n.pdf

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.

/pdf/holographic-data-storage-systems-3glg5sf5v9.pdf

Holographic Data Storage Systems

The promise of new architectures and more cost-effective miniaturization has prompted interest in molecular and biomolecular electronics. Bioelectronics offers valuable near-term potential, because evolution and natural selection have optimized many biological molecules to perform tasks that are required for device applications. The light-transducing protein bacteriorhodopsin provides not only an efficient photonic material, but also a versatile template for device creation and optimization via both chemical modification and genetic engineering. We examine here the use of this protein as the active component in holographic associative memories as well as branched-photocycle three-dimensional optical memories. The associative memory is based on a Fourier transform optical loop and utilizes the real-time holographic properties of the protein thin films. The three-dimensional memory utilizes an unusual branching reaction that creates a long-lived photoproduct. By using a sequential multiphoton process, paral...

Biomolecular Electronics: Protein-Based Associative Processors and Volumetric Memories

The invention is embodied in a method of recording successive holograms in a recording medium, using at least a fan of M waves along at least a first axis with a separation angle between adjacent waves and directing the fan of M waves as a reference beam along a reference beam path onto the recording medium, successively modulating a wave with a succession of images to produce a succession of signal beams along a signal beam path lying at a propagation angle relative to the reference beam path so that the signal and reference beams intersect at a beam intersection lying within the medium, the beam intersection having a size corresponding to beam areas of the reference and signal beams, producing a succession of relative displacements in a direction parallel to the first axis between the recording medium and the beam intersection of the signal and reference beam paths in synchronism with the succession of signal beams, each of the displacements being less than the size of the intersection whereby to record successive holograms partially overlapped along a direction of the displacements.

/pdf/holographic-storage-using-shift-multiplexing-40g2ichnjh.pdf

Holographic storage using shift multiplexing

A peristrophic multiplexing system directs a signal light beam and reference light beam onto a selected recording spot in the recording medium. The light beams collectively define a plane of interaction. Either the recording medium or the signal/reference beam are rotated relative to the other through a succession of peristrophic multiplexing angles. Those angles cause relative rotation about an axis that is not perpendicular to the plane of selectivity. The rotation occurs contemporaneously with the modulating of the hologram on the medium.

Method for holographic storage using peristrophic multiplexing

An iterative method is introduced for determining the exposure schedule for multiplexing holograms in saturable recording materials, such as photopolymers. This method is designed to share all or part of the available dynamic range of the recording material among the holograms to be multiplexed. Using exposure schedules derived from this method, the authors find that the diffraction efficiency of DuPont’s HRF‐150 38‐ and 100‐μm photopolymer scale is (2.2/M)^2 and (6.5/M)^2 respectively, where M is the number of holograms recorded. Finally, 1000 holograms were multiplexed at a single location in the 100‐μm thick photopolymer using an exposure schedule derived with this method.

/pdf/exposure-schedule-for-multiplexing-holograms-in-photopolymer-uvzk4cjyz9.pdf

Exposure schedule for multiplexing holograms in photopolymer films

The cross talk between wavelength-multiplexed holograms is analyzed. The signal-to-noise ratio (SNR) is calculated for a recording schedule that places the center of each image at the null (in wavelength space) of the adjacent hologram. An asymptotic closed-form expression for the minimum SNR is derived in a general reflection geometry. The reflection geometry with counterpropagating signal and reference beams is shown to have the best SNR.

/pdf/cross-talk-in-wavelength-multiplexed-holographic-memories-4o75081czt.pdf

Cross talk in wavelength-multiplexed holographic memories

The correlation speed and storage capacity of an optical disk-based correlator is enhanced by employing a thick (several hundred micron) photo- polymeric film (or other thick holographic media) on the disk as the recording media to permit volume holography and angular multiplexing of holograms in each spot on the disk. For example, if 100 holograms are multiplexed at one spot, 100 1-dimensional correlation functions can be read in parallel off of the disk while illuminating it with a single input image. The diffraction of the image beam by the recorded holographic patterns occurs at the holograms in the disk. The full 2-dimensional correlation function for each one of the holograms stored in a given spot on the disk is generated line-by-line a follows: By imaging in the along-track direction of the disk and Fourier transforming in the radial direction of the disk when both recording the template image hologram and presenting the input image to the recorded hologram, disk rotation generates the two-dimensional correlation functions between stored template images and the input image. All correlation functions for holograms angularly multiplexed at a given spot are generated line-by-line and detected in parallel along adjacent line detectors in the off-disk correlation plane.

Disk-based optical correlator and method

The cross talk between holograms multiplexed with Walsh-Hadamard phase codes is analyzed. Each hologram is stored with a reference beam that consists of N phase-coded plane waves. The signal-to-noise ratio (SNR) is calculated for a recording schedule for which the center of each stored image coincides with the nulls of the selectivity function for the adjacent plane-wave components of the reference beam. The SNR characteristics for phase coding with Walsh-Hadamard phase codes are then compared with the SNR for angle and wavelength multiplexing.

/pdf/cross-talk-in-phase-coded-holographic-memories-3ngz7if31f.pdf

Kevin R. Curtis

Papers

Method for holographic storage using peristrophic multiplexing

Exposure schedule for multiplexing holograms in photopolymer films

Cross talk in wavelength-multiplexed holographic memories

Disk-based optical correlator and method

Cross talk in phase-coded holographic memories