100-MHz optical counter that uses directional coupler switches

We have constructed what is to our knowledge the first speed-of-light 100-MHz digital optical counter using directional coupler switches and single-mode fibers. The counter has operated with both 4- and 6-bit counts, with the use of two different counter designs. In addition, we have demonstrated operation of two simultaneous and independent 4-bit counters running on the same hardware by time-division multiplexing the hardware. This approach allows effective clock rates many times the individual machine clock rate and is limited only by the switching speed. For large latency systems, this approach offers the promise of gigahertz clock rates for digital optical computers.