Propagating-beam model for a single-mode-fibre fused coupler

A scalar two-dimensional analysis of a single-mode-fibre fused coupler is presented. Two theoretical models (one analytical and one numerical) are implemented and compared. The first model is based on the mode coupling in the central cross-section of the coupler, the second model employs the propagating-beam algorithm (PBA). Contrary to the analytical model, the PBA is suited to the analysis of the transition from guidance by the core to guidance by the cladding taking place in the tapers, including radiation loss. The influence of the external refractive index on the power transfer is investigated. The results obtained are in qualitative agreement with the experiments presented by Hill et al.