ABSTRACT
Multi-mode waveguide devices have shown their advantages for only large fabrication tolerance, easy optical coupling and, resultantly, low-cost devices. These understandings are mostly true, but it is noteworthy that positive utilizations of mode interference in a multi-mode waveguide were also proposed to manipulate image formation by O. Bryngdahl and R. Ulrich. And multi-mode interference (MMI) waveguides have been spotlighted as compact functional optical devices, and various optical devices such as couplers, combiners, splitters and optical switches have been proposed and demonstrated for higher performances. The chapter describes the operational principle, theoretical treatment and fundamental characteristics of mode interference in the MMI waveguides. Optimal lengths of the MMI waveguides for various applications such as a connector, a switch, a splitter and a combiner are designed by taking the beat length into consideration, and that is why the beat length is one of the very important factors for characterization of MMI waveguides.
