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Optics Letters

Dynamic photorefractive optical memory

Hajime SASAKI, Yeshaiahu Fainman, Joseph E. Ford, Yoshinao Taketomi, S. H. Lee

Published Dec 1, 1991• 45 citations
Publisher

Abstract

We investigate an architectural approach to dynamic three-dimensional volume storage that circulates holograms between two photorefractive crystals. Introduction of an optical amplifier into the system increases the effective write-erase asymmetry of the crystals and permits the amplification of the recalled images. This memory architecture is experimentally shown to provide write-erase storage with robustness to multiple optical readouts.

DOI
10.1364/ol.16.001874

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We investigate an architectural approach to dynamic three-dimensional volume storage that circulates holograms between two photorefractive crystals. Introduction of an optical amplifier into the system increases the effective write-erase asymmetry of the crystals and permits the amplification of the recalled images. This memory architecture is experimentally shown to provide write-erase storage with robustness to multiple optical readouts.
We propose and demonstrate a new long-term readout holographic memory using two photorefractive crystals. Images are stored in the first crystal and reconstructed with low-intensity beams, then amplified in the second crystal operating in a high-gain low-noise configuration. By using combinations of LiNbO(3)/Bi(12)SiO(20) and BaTiO(3)/Bi(12)SiO(20) as storage/amplification crystals, continuous readout of the angularly coded memories for more than 20 h and 40 min, respectively, is experimentally achieved. A lower bound for the index modulation to be allocated to one stored image in LiNbO(3)is estimated.
The speed of modern supercomputers is primarily limited by the relatively slow speed of memory. Research over the past years has focused on developing optical memory systems that, by nature of optics, promise to achieve high speed and high memory capacity. Three- dimensional optical memory systems are well known for their storage capacity and achievable short access time. But the early attempts to exploit 3-dimensional media were limited to photorefractive crystals such as lithium niobate, due to the unavailability of any other suitable medium (alternative media are precluded by their diffraction limited storage densities and non- erasable characteristics). Although volume phase holograms can be recorded in photorefractive crystals, these materials have many obvious flaws such as strong scattering, phase distortion, self diffraction, fast decay, and low refractive index modulation. There have also been continuous efforts to develop new 3-D media (such as photon echo, photorefractive polymers, bacteriorhodusphin, memory access cache, etc., but none can be applied in real world 3-D device because these materials are still in the early stage of development.
Photorefractive-based optical memory systems have received considerable attention in recent years [1-6] In these systems, numerous volume holograms are stored within a single photorefractive crystal, with the writing and recall selectivity generally being performed via angle [1] wavelength [2,3], or phase [4] multiplexing. As well, non-photorefractive-based optical memory systems [7,8] have been proposed. In this paper, we present a hybrid system [6] that utilizes a combination of wavelength and angle multiplexing in a photorefractive medium. Such a hybridization is driven by photonic source generation and manipulation device limitations, as well as the desire to expand information throughput rates in optical memory systems via spectral parallelism [9]. Our design is essentially the summation over wavelength of many angularly multiplexed volume memory systems. We therefore compare our system’s parameters to those of purely wavelength or angle multiplexed ones.
A new implementation of an all-optical associative memory is presented. It is realized in a nonresonant cavity containing a photorefractive and a nematic liquid crystal. The images, which are stored on slides, modulate the intensities of the signal beams circulating in the cavity. In a photorefractive KNbO3 crystal the signal images are compared with an arbitrary input image on the pump beam by multiple two-beam coupling. In the nematic liquid-crystal cell the signal images are thresholded by a feedback-induced optical bistability. Associative readout, error correction, and image completion are demonstrated for two and three stored black-and-white images. The performance of the new scheme is estimated to be comparable with that of nonlinear holographic associative memories based on thin holograms.
We propose and demonstrate a long-term readout photorefractive memory. It is made up of two photorefractive crystals: the first where the images are stored and the second where the signal is amplified by use of a low-noise self-bent pump-beam technique, making it possible to use very low illumination of the storage crystal. A LiNbO(3)/BaTiO(3) configuration is used with angular multiplexing; this technique permits continuous readout for more than 60 days. The long-term storage capacity and the maximum number of stored images are estimated.