Exact Efficiency Evaluation in Loaded Diffused Optical Waveguide Modulators

In this paper, we consider an overlayed erfc diffused LiNbO3 ferroelectric crystal in order to design a planar-type modulator by the mode conversion method. The multilayer structure is analysed into two configurations; in the first the optical axis is perpendicular to the electrode plane, in the second is parallel. In both cases the effects on the coupling coefficient of the loading film refractive index and thickness and of the coplanar electrode spacing are described. The mapping of the modulating electric field components has been carried out by a finite element computer program by accounting the anisotropy and inhomogeneity of the structure. This exact evaluation of the r.f. field, along with the larger optical energy confinement at the surface due to the diffusion and to a high refractive index overlay, has allowed to obtain with the second configuration a modulation efficiency improvement of the 45%, the first being equal to 11.5 m-1.