Ultra-High Capacity Holographic Memories

Abstract : This report summarizes the achievements at Georgia Institute of Technology in the area of ultra- high capacity holographic memories funded by Air Force Office of Scientific Research (AFOSR) during the period of December 1, 2001 to October 1, 2004. The primary goal of this research has been to enhance the technologies that will enable the design of ultra-high capacity bulk memories based on the unique properties of volume holographic storage. Such memories have a short access time, a high data transfer rate, and a high storage capacity. In these memories, data is stored in two-dimensional arrays of ones and zeros that are recorded as pages in a holographic material. These data pages can be multiplexed to occupy the same holographic volume. The individual data pages are read by selecting either the angle or the wavelength of the reference beam. Furthermore, the readout from these memories is extremely fast because of the parallelism that is inherent in their large data pages. To take advantage of the advancement in making spatial light modulators with ultra-small pixels, we need to solve the problem of limited spatial bandwidth of the storage material. This limitation puts an upper bound on the smallest pixel size that can be recorded. However, by modulation of the pixel size (while keeping the size above the smallest acceptable value) or modulation of the pixel position using constrained coding, we can improve the number of available bits per page. On the other hand, the data cannot be read out without errors and the bit- error rate varies as a function of the position of the data within the crystal. To get the overall bit- error rate below an acceptable level for bulk storage (less than 10 exp-12) will require error control coding.

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