The Beam Propagation Method relies on the [[slowly varying envelope approximation]], and is inaccurate for the modelling of discretely or fastly varying structures. ItBasic isimplementations are also inaccurate for the modelling of structures in which light propagates in large range of angles and for devices with high refractive-index contrast, commonly found for instance in [[silicon photonics]]. Advanced implementations, however, mitigate some of these limitations allowing BPM to be used to accurately model many of these cases, including many silicon photonics structures.
The BPM method can be used to model bi-directional propagation, but the reflections need to be implemented iteratively which can lead to convergence issues.
