Construction of an optical-carry adder

We have constructed a hybrid electronic/optical digital adder, based on a system suggested by B. Arazi [Proc. IEEE 73,162 (1985)], which uses an optical system for the carry-bits, giving fully parallel operation. Our bulk-optic breadboard system has six acoustooptic modulators as optical switches, with a single He-Ne laser source. It adds two 4-bit words at 10 MHz, with a 500-ns pipeline delay. The optical carry is based on a multistage optical path, with light added or allowed to pass at each stage, according to whether carries are generated or propagated at the corresponding column of the addition. For long addends, the system speed is limited by light loss through the carry path and by size effects.