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The potential advantages of volume optical storage were recognized almost 30 years ago and since that time many research efforts have focused on improving the underlying materials and devices for use in such systems. Recent progress in these supporting technologies have made possible several system-level demonstrations of volume optical storage. These recent demonstrations have verified the feasibility of volume memory systems for offering large volumetric storage capacities, fast access times, and very high data transfer rates realized via the parallel two-dimensional (2D) nature of the stored data. The success of these volume storage testbeds have served to ignite additional research into supporting two-dimensional or page-oriented interface technologies such as parallel 2D data detection and error correction. This application therefore provides a strong impetus to study traditional communication theoretic topics such as signaling, equalization, coding, etc. , in the context of highly parallel 2D channels. The storage and retrieval processes associated with holographic optical memory systems are complex. In order to model a specific memory architecture (e.g. , 90 degree Fourier plane photorefractive memory), many physical parameters must be specified (e.g., focal lengths, material constants, modulator and detector details, etc.). We have elected to utilize a simplified model of these systems for equalizer design purposes, in which the recording and retrieval physics are captured through the behavior of a "channel." We consider a coherent optical channel (i.e., linear in electric field) suffering from finite-contrast and post-detection additive white Gaussian noise (AWGN) . This coherent system produces a measured intensity that represents a quadratic function of the stored data, complicating the simple 151 channel model that is familiar to traditional communications applications.
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