Limitations of BPM
The Beam Propagation Method relies on the slowly varying envelope approximation, and is inaccurate for the modelling of discretely or fastly varying structures. It is also inaccurate for the modelling of structures in which light propages in large range of angles and for devices with high refractive-index contrast, commonly found for instance in silicon photonics.
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.
More rigorous alternatives include Eigenmode Expansion and FDTD.
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