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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Photorefractive dynamic memories for optical processing: performances and limitations (Invited Paper)

Gilles Pauliat, Gérald Roosen

Published Oct 30, 1992• 0 citations
Publisher

Abstract

After a rapid introduction that presents the main reasons for the renewal of interest for photorefractive holographic memories, we first discuss their potentialities and limitations. We then continue by a presentation and analysis of novel techniques that permit considerable facilitation of the memory operation and improve its capabilities. In conclusion, we indicate envisioned applications for photorefractive holographic memories.

DOI
10.1117/12.131202

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After a rapid introduction that presents the main reasons for the renewal of interest for photorefractive holographic memories, we first discuss their potentialities and limitations. We then continue by a presentation and analysis of novel techniques that permit considerable facilitation of the memory operation and improve its capabilities. In conclusion, we indicate envisioned applications for photorefractive holographic memories.
One can construct a memory having associative characteristics using optical resonators with an internal gain medium. The device operates on the principle that an optical resonator employing a holographic grating can have user prescribed eigenmodes. Information that is to be recalled is contained in the hologram. Each information entity (e.g. an image of a cat) defines an eigenmode of the resonator. The stored information is accessed by injecting partial information (e.g. an image of the cat's ear) into the resonator. The appropriate eigenmode is selected through a competitive process in a gain medium placed inside the resonator. With a net gain greater than one, the gain amplifies the field belonging to the eigenmode that most resembles the injected field; the other eigenmodes are suppressed via the competition for the gain. One can expect this device to display several intriguing features such as recall transitions and creativity. I will discuss some of the general properties of this class of devices and present the results from a series of experiments with a simple holographic resonator employing photorefractive gain.
A theoretical analysis for the optimization of the selective-erasure process in holographic photorefractive memories is presented. It is based on the solution of the standard material equations under a linear approximation (low modulation depths). Specific expressions for the optimum phase shifts and erasure rates are obtained. The approach includes all transport processes and so applies to photovoltaic materials such as LiNbO3. The different behavior with regard to nonphotovoltaic materials is discussed. Some additional strategies to improve the overall erasure process are proposed.
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.