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

The low-pass nature of the optical systems (both coherent and incoherent) used for volume optical storage results in the presence of intersymbol interference (ISI) at the output of these systems. Since ISI can seriously degrade retrieved data fidelity, we consider the design of linear, minimum-mean-square-error equalizers for two-dimensional finite-contrast optical ISI channels. Signal models are developed and filter design is conducted for various operating environments associated with particular implementations of page-oriented optical memories (POM’s). Specifically, we consider optically incoherent systems dominated by either postdetection thermal or photon-shot noise, and coherent systems are treated subject to either postdetection thermal or coherent speckle noise. Simple locally connected postdetection filters (equalizers) are designed to reduce the impact of ISI and finite contrast on retrieved data. It is demonstrated how these simple ISI mitigation algorithms may be used to improve the fidelity (i.e., bit error rate) of retrieved data and also to enhance the space– bandwidth-product (SBP), the storage density, and the memory capacity of POM systems. The notion of a fidelity-based SBP is quantified and shown to depend strongly on the receiver processing. The fidelity-based SBP of thermal-noise-dominated incoherent imaging systems operating at the Rayleigh resolution is shown to improve by 28% through the use of equalization, and a 48% SBP increase is found in the shot-noise-dominated case. More dramatic gains are found for thermal-noise-dominated coherent systems operating at the Rayleigh resolution, with 116% SBP gains typical in the infinite-contrast case and 30% gains possible for low-contrast (C ! 4) cases. Equalization is also shown to facilitate a capacity increase for holographic POM systems, providing a 47% increase in the number of stored pages and the storage density for a system operating at the Rayleigh resolution. The maximum storage density in holographic POM is increased by 20% through the use of equalization. © 1999 Optical Society of America [S0740-3232(99)01403-9] OCIS codes: 210.0210, 200.0200, 210.2860, 110.4280, 100.2550.