The classical optical matrix-vector multiplier, or crossbar, offers a number of obvious advantages over its electronic counterpart when the data transmission rate is very high. However, there are two principal limiting factors in its design, namely, insertion loss and crosstalk. Insertion loss is unavoidable and sets a loss-limited upper bound to the switch dimensions due to the finite optical power budget. More important, the low-contrast ratio of present-day spatial light modulators (SLMs) and wiring crosstalk caused by aberrations in the imaging optics also sets a limit to switch dimensions. Fortunately, this crosstalk limit is dominant in the lower transmission rate systems which are of less interest. This paper examines the relative magnitudes of the loss and crosstalk limit and their effect on switch dimensions and how the switch performance can be improved by tailoring the input and output optical field shapes. It is also shown how the crossbar can be made bidirectional without loss or crosstalk penalty. A bulk-optic 32 × 32 bidirectional crossbar with a transmission rate of 140 Mbit/s is proposed which uses multimode optical fiber inputs and outputs and a twisted-nematic liquid crystal SLM cross-point matrix.
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