Two-dimensional equalization in coherent and incoherent page-oriented optical memory

The low-pass nature of the optical systems (both coherent and incoherent) u sed for volume optical storage results in the presence of intersymbol inter ference (ISI) at the output of these systems. Since ISI can seriously degra de retrieved data fidelity, we consider the design of linear, minimum-mean- square-error equalizers for two-dimensional finite-contrast optical ISI cha nnels. Signal models are developed and filter design is conducted for vario us operating environments associated with particular implementations of pag e-oriented optical memories (POM's). Specifically, we consider optically in coherent systems dominated by either postdetection thermal or photon-shot n oise, and coherent systems are treated subject to either postdetection ther mal or coherent speckle noise. Simple locally connected postdetection filte rs (equalizers) are designed to reduce the impact of ISI and finite contras t on retrieved data. It is demonstrated how these simple ISI mitigation alg orithms may be used to improve the fidelity (i.e., bit error rate) of retri eved data and also to enhance the space-bandwidth-product (SBP), the storag e density, and the memory capacity of POM systems. The notion of a fidelity -based SEP is quantified and shown to depend strongly on the receiver proce ssing. The fidelity-based SEP of thermal-noise-dominated incoherent imaging systems operating at the Rayleigh resolution is shown to improve by 28% th rough the use of equalization, and a 48% SEP increase is found in the shot- noise-dominated case. More dramatic gains are found for thermal-noise-domin ated coherent systems operating at the Rayleigh resolution, with 116% SEP g ains typical in the infinite-contrast case and 30% gains possible for low-c ontrast (C = 4) cases. Equalization is also shown to facilitate a capacity increase for holographic POM systems, providing a 47% increase in the numbe r of stored pages and the storage density for a system operating at the Ray leigh resolution. The maximum storage density in holographic POM is increas ed by 20% through the use of equalization. (C) 1999 Optical Society of Amer ica [S0740-3232(99)01403-9].