RSM
Radiation Spectrum Method : a modal BPM (Beam Propagation Method)
... : refractive index plot, field propagation plot and modes spectrum plot. This permits, with the interpretation of the guided and radiation modes spectrum, a physical understanding of the propagation mechanisms in the integrated optical device under evaluation. The complex geometry of the component is discretized in a stack of multilayer dielectric waveguides.
See : https://ipp.icube.unistra.fr/index.php/Photonics_Modeling_and_Simulation:RSM