Abstract

Optical computing systems offer an increased information processing rate by facilitating parallel computing architectures. Previous experience with electronic computers indicates that desired accuracy can be achieved only with digital computation. Since the simplest digital arithmetic is binary, most recent work on optical computing is focused on the construction of binary optical logic gates. Many practical implementations of such logic gates have been suggested; a recent review is given by Sawchuck and Strand [1]. Most previous schemes operate on light intensity, much in the way that electronic systems operate on voltage or current. Another natural optical scheme represents the two binary states with two orthogonal polarizations of light. The optical element necessary to implement this scheme is a device with two states, one of which passes light of a chosen polarization unchanged, and the other of which converts light of the chosen polarization to its orthogonal complement. Tsvetkov et al. [1] have described a practical implementation of this logic using the now common twisted nematic (TN) liquid crystal device, which has two voltage-selected states, one of which rotates the polarization direction of appropriately oriented linearly polarized light by 90° and the other of which has no rotary power. Another implementation would use any of the variable retardation effects such as the Pockels effect. One state of the device would be chosen to have zero retardation, and the other to have half-wave retardation. In addition to either passing unchanged or imparting 90° rotation to linearly polarized light, this scheme could also work by either passing unchanged or reversing the handedness of circularly polarized light. An advantage pointed out by Lohmann [3] that any implementation of polarization-based logic has over logics based on intensity is that no light is lost in the logical operation of inversion. In intensity-based logics, it is difficult to invert an already dark input, since light has to be "recreated"; polarization-based elements, as described above, can convert the light representing either logical state to the other, making easy the realization of any desired Boolean function.

© 1987 Optical Society of America