Optical directional couplers based on SOI-wire waveguides have been modelled by a semi-analytical approach
based on the Coupled Mode Theory and Finite Element Method. The modelling is used to obtain analytically optical
power at the parallel and cross ports by utilizing numerically calculated coupling coefficients. Geometrical dimensions of
the couplers have been optimized to obtain a polarization-independent behaviour. The influence of non-vertical sidewalls
on the coupler performance has also been addressed.