Abstract
AlxGa1-xAs electro-optic (EO) zero-gap directional couplers (ZGDCs) offer orders of magnitude shorter device lengths than AlxGa1-xAs conventional directional couplers (CDCs). Due to the small EO activity of AlxGa1-xAs, however, the length of the AlxGa1-xAs ZGDC becomes voltage limit dependent. To operate at TTL levels, the interaction channel length of the active AlxGa1-xAs ZGDC needs to increase by 25 times over the optimum coupling length of the passive device. For the passive device, the coupling percentage can be easily controlled to within +/- 10% using standard lithographic and growth techniques demonstrating a fabrication tolerant device. The coupling percentage between channels in the longer active devices, however, is much more sensitive to the accuracy of the interaction length which, in turn, is controlled by the accuracy of processing and growth. If one could control the interaction length even to within 1 micrometers , holding the voltage independent phase change to within 1%, the coupling percentage could vary by as much as 45%. Because precise accuracy could not be achieved, the active AlxGa1- xAs ZGDC fabricated for this experiment required a 30% higher switching voltage than optimum. Control of both material growth and fabrication processing is needed unless one is willing to continuously apply a voltage.