Modal characteristics of optical stripline waveguides

A theoretical model of the optical stripline waveguide is presented. The structure consists of an underlying substrate with an index n3, a slab of thickness d having a refractive index n2, and an overlayed stripe of width 2w and index n1; the index n2≳n1, n3. Optical confinement of the fields to the slab is obtained in the vertical direction by dielectric discontinuities while confinement in the lateral direction occurs because of spatial interference of a continuum of plane waves. Modal cutoff conditions are determined as a function of the slab thickness d and the stripe width 2w for a range of index steps. The field profiles in the lateral directions are compared with experimental profiles. These profiles show that the fields decay exponentially in the slab away from the stripe. The coupling between two parallel stripes is discussed and coupling lengths are calculated. The agreement between theory and experiment is, in most cases, better than 10%.