Comparison of single mode slab and rib waveguide planar evanescent wave transducers

Abstract

It is well known that a homogeneous sensitivity of a single-layer slab waveguide, for given polarization, is maximized for a certain thickness of a waveguide film. Transition from a slab waveguide, referred to as a parent-slab, to a rib waveguide consist in partial constraint of the waveguide film in the direction parallel to its interfaces and perpendicular to the direction of propagation. Therefore it is appropriate to normalize the rib waveguides homogeneous sensitivity, which depends on the two additional parameters: rib width and height, to the sensitivity of the optimized slab waveguide. The article discuss dependence of rib waveguides normalized, homogeneous sensitivity on a parent slab thickness, rib width and rib height. High aspect ratio rib waveguides, whose rib width is several times longer than the parent slab thickness, are considered. It is shown, using the film mode matching method, that the sensitivity of a rib waveguide can exceed the sensitivity of the optimized slab waveguide and that sensitivity characteristics as a function of rib height may have maximum like it is in the case of a homogeneous sensitivity of the slab waveguide as a function of its thickness. Analysis of a principal electric field component spatial distribution, carried out for the fundamental EH mode, showed that for the optimized rib waveguide, its sidewalls are the place where changes of the ambient refractive index affect the guided mode to greatest extent.