Department of Electrical and Computer EngineeringState University of New York at BuffaloAmherst, N. Y. 14260ABSTRACTThe concept of using a low dielectric medium in a parallel -plate H -guide toobtain velocity- matching between optical and microwave signals for the LiNb03modulator is analyzed in detail. From solutions of the wave equations, we haveobtained characteristics of the fundamental mode and the design parameters whichlead to velocity- matching. The mode is guided by the electrooptic substrate anddecays exponentially outside the LiNb0 substrate. The parameter which determinesits properties is 2a /A