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Design of adiabatic tapers for high-contrast step index waveguides

Tapered waveguides are frequently used to obtain a good matching between the modes of two butt coupled waveguides having different guiding properties. The design of these tapered waveguides is in most cases conservative because of the lack of precise modelling guidelines and accurate modelling tools. This paper discusses in the first place a quantitative approach for taper design by considering the whole taper with arbitrary shape as a succession of short linear taper fragments. As a result that radiation loss per relative thickness reduction as a function of index contrast, taper angle and thickness is obtained. These simulations require a highly accurate beam propagation method (BPM) able of handling large index contrasts and thereby principally exceeding the applicability range of paraxial algorithms. Therefore a novel BPM which solved directly the non-paraxial 2D Helmholtz equation by decomposing an arbitrary input field in local eigenmodes of the waveguide is used.