Although the concept of using multiplexed holography for data storage has been considered for some time, recent advances in several critical device technologies along with developments in storage materials have greatly enhanced the likelihood of successful implementations. We review several basic architectural concepts along with various multiplexing options and associated techniques for holographic data storage.

Volume holographic memory systems: techniques and architectures

Disclosed is a multiplexing method and apparatus that allows holograms to be spatially multiplexed with partial spatial overlap between neighboring stacks of holograms. Each individual stack can additionally take full advantage of an alternate multiplexing scheme such as angle, wavelength, phase code, peristrophic, or fractal multiplexing, for example. An amount equal to the beam waist of the signal beam writing a hologram separates individual stacks of holograms. Upon reconstruction, a hologram and its neighbors will all be readout simultaneously. An filter is placed at the beam waist of the reconstructed data such that the neighbors that are read out are not transmitted to the camera plane. Alternatively, these unwanted reconstructions can be filtered out with an angular filter at an intermediate plane in the optical system that has a limited angular passband.

Polytopic multiplex holography

This chapter reviews different suggestions for applications of PRCs. Due to the high sensitivity, high diffraction efficiency, reversibility, and the ease of handling, these dynamic photosensitive media attracted the attention of researchers working in different areas of coherent optics. Early proposals on the use of photorefractive crystals and SLMs for holographic interferometry and 2D image processing are still interesting. Recently, a number of new ideas on PRC applications based on various phase-conjugate geometries have appeared, too.

http://iopscience.iop.org/article/10.1088/0034-4885/57/1/002/pdf

Applications of photorefractive crystals

We present the first, to our knowledge, demonstration of an
encrypted optical storage based on double-random phase encoding by
using angular multiplexing in a photorefractive material. Original
two-dimensional data are encrypted by use of two random phase codes
located in the input and the Fourier planes and are then stored
holographically in a LiNbO3:Fe crystal. The retrieval
of the original data can be achieved with a phase-conjugated readout
scheme. We demonstrate the encryption and the decryption of
multiple frames of two-dimensional digital data by using angular
multiplexing. We also evaluate numerically the influence of the
bandwidth of the optical system on the decrypted digital data. The
bit error rate as a function of the optical system bandwidth is
presented.

Encrypted optical storage with angular multiplexing

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.

Cross-talk considerations for angular and phase-encoded multiplexing in volume holography

We propose a volume holographic storage scheme, spatioangular multiplexing, which is a hybrid of both angular and spatial multiplexing. We describe the recording procedure, for a preliminary experiment in which 750 holograms were recorded (at room temperature) in a single photorefractive crystal of iron-doped lithium niobate and report the successful replay of all images with negligible cross talk and with an average diffraction efficiency of 0.5%.

Spatioangular multiplexed storage of 750 holograms in an Fe:LiNbO3 crystal

A novel multiplexing scheme for dense holographic storage in photorefractive crystals, spatioangular multiplexing, is described in detail. Compared with spatial multiplexing, spatioangular multiplexing increases the storage capacity by fully utilizing the volume of the storage medium. On the other hand, spatioangular multiplexing reduces the number of holograms overlapping any one hologram in a given volume and so increases the diffraction efficiency achievable as compared with angular multiplexing. Using this scheme, we succeeded in storing 756 high-resolution patterns in an Fe:LiNbO(3) crystal of volume 1 cm(3), with an average diffraction efficiency of 0.5%. This large database is designed for practical use in a novel associative-memory system, called a high-order feedback neural network.

Spatioangular-multiplexing scheme for dense holographic storage

The authors have demonstrated the use of two dimensional arrays of Fourier Transform holograms in an opto-electronic implementation of a high order feedback net (HOFNET). The nonlinearity in this net is produced by means of feedback and not by optical material nonlinearities and so the net is not limited by the switching speed, non-uniformity or availability of large arrays of opto-electronic nonlinear neurons. Spatially multiplexed holograms offer the advantages of no cross coupling, convenient addressability and, if formed in a photorefractive material can be selectively erased and updated without interfering with other memories. >

S. Tao

Papers

Spatioangular multiplexed storage of 750 holograms in an Fe:LiNbO3 crystal

Spatioangular-multiplexing scheme for dense holographic storage

Holographic high order associative memory system