Page 497 - Introduction to Information Optics

482 9. Computing with Optics
combinations of the coded signals can be generated to provide a set of output
functions. The combination is specified by the connection pattern in the AND
and OR gate arrays. In optics, AND logic is readily performed when light is
transmitted through the modulators representing the inputs, and an OR
operation is performed by collecting the light from several inputs to a common
detector. An optical programmable logic array can be constructed by using
AND and OR logic arrays interconnected by optics. The logic arrays are
programmed by enabling or disabling certain interconnections in the arrays,
arid any logic functions can be implemented in the form of sum of products.
Such an optical programmable logic array processor can be constructed by
using 2D SEED arrays interconnected by a multistage interconnection net-
work such as Banyan, crossover, or perfect shuffle, as shown in Fig. 9.5. All
products are formulated by several stages of AND gate arrays and sums of the
particular products are generated by several stages of OR gate arrays. Binary
masks are used to select different optical paths among the gate arrays.
Feedback paths are imaged back to the system with a vertical shift so that each
row is imaged onto a different part of the masks at each iteration. However,
this implementation requires a large number of gates. To alleviate this problem,
space-invariant optical interconnections may be used.
The AND operation can also be realized by NOT and NOR operations. For
example, using De Morgan's law, the logic function operation F = AB + CD

Interconnect Maik

Fig. 9.5. A digital optical computing system composed of optically interconnected programmable
logic arrays [58].