Optically switchable directional couplers can function as logically complete building blocks for constructing all-optical
computational engines. Logically, such devices operate as all-optical five-terminal gates, where two output signals are
logical functions of two guided input signals and an input control signal. Such an optically controlled exchange element is
a promising functional unit for constructing general-purpose digital optical logic circuits.
We have implemented such all-optical five-terminal gates using Ti:LiNbO eleciro-optic directional couplers, with singlemode
optical fiber for all gate interconnections. Effectively DC-coupled control circuitry converts short, low-power (<5ns,
-25 dBm) 1 .3 mm optical pulses into electrical pulses capable of switching the transfer state of low-voltage lumped
electrode directional couplers. These optical logic gates, while containing significant electronics in the control arm, are
logically identical to more fundamental optically controlled exchange elements, and utilize mature technology developed
for the optical fiber telecommunications industry, simplifying construction of robust optical logic hardware. In addition,
the use of significant electronics in the control terminal input offers considerable advantages over more fundamental optical
switching effects, i.e. greater noise immunity, wider dynamic range, adjustable detection threshold, simple phase
compensation, and conirol over the quiescent switching state.
We have demonstrated the use of such Ti:LiNbO3 five-terminal optical logical gates in the construction of various simple
circuits, such as oscillators and divide-by-N circuits. Such circuits demonstrate many of the issues arising in the
construction of all-optical digital computing systems, and are fundamental subsections of an all-optical bit serial computer
design, which we are currently consiructing.