Mode conversion in planar dielectric separating waveguides

Separating optical waveguides are expected to become important in integrated optics. They occur whenever a branching waveguide is used (Fig. 1a) or whenever two parallel waveguides in close proximity are separated (Fig. 1b). These situations will arise in virtually all applications of electrooptic switches and waveguide couplers that depend on evanescent coupling. Using quasi-normal modes and a step transition model we have studied theoretically the mode conversion behavior of separating planar dielectric waveguides as a function of taper slope and mode synchronism, which is adjusted by branch asymetry. When the taper slope is large in a near symmetric structure, considerable mode conversion will occur and the structure will act as a power divider (Fig. 2b). Incident power concentrated in the upper and lower parts of the structure will end up in the upper and lower arms of the branch, respectively. This behavior is usually required for conventional evanescent couplers. However, in a more asymmetric structure with smaller taper slope, mode conversion is negligible´ and the structure will act as a mode splitter (Fig. 2a). Mode power is then transferred to one arm of the branch or the other. The transition boundary between a power divider and mode splitter (Fig. 3) is conveniently described approximately by: Δβ/βy>;<;.43.Here Δβ is the difference in mode propagation constant for large waveguide separation, e is the taper slope, and y is the decay constant of the field in the separating region. This result will allow quantitative design of mode splitters or power dividers, as desired, and should aid in the design of separating waveguides with minimum radiation losses.