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Journal of the Optical Society of America B

Optimization of selective erasure in photorefractive memories

Marı́a Aguilar, M. Carrascosa, Fernando Agulló-López, E. Serrano

Published Jan 1, 1997• 29 citations
Publisher

Abstract

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.

DOI
10.1364/josab.14.000110

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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.
We present a method for optical encryption of information, based on the time-dependent dynamics of writing and erasure of refractive index changes in a bulk lithium niobate medium. Information is written into the photorefractive crystal with a spatially amplitude-modulated laser beam which when overexposed significantly degrades the stored data making it unrecognizable. We show that the degradation can be reversed and that a one-to-one relationship exists between the degradation and recovery rates. It is shown that this simple relationship can be used to determine the erasure time required for decrypting the scrambled index patterns. In addition, this method could be used as a straightforward general technique for determining characteristic writing and erasure rates in photorefractive media.