Nonlinearity of effective index versus bulk index in multilayer dielectric waveguides: Large incremental effective index sensitivity*

Abstract

A device for modulating the effective refractive index of a guided wave can be viewed as having a nonlinear input-output characteristic, with the effective refractive index as output and the bulk refractive index of the active (e.g., electrooptic) medium as the input. As with any such device, proper “biasing” of the input can result in a large incremental, or small-signal, sensitivity. In a multilayer dielectric waveguide, obtaining a large incremental effective index sensitivity (IEIS) is equivalent to the problem of synthesizing total internal reflecting structures with prescribed dependence of phase shift on angle of incidence. The required dependence can be achieved with five dielectric layers, over a narrow range of effective index. In this case, the IEIS is limited primarily by thickness tolerances and introduction of additional guided modes.