In this section selection criteria for the eigenvalues effective refractive index are set.

Mathematically, the propagating mode equation, see PropagatingMode has an infinite number of eigenvalues. However, not all correspond to a mode propagating in the z-direction. There may exist purely damped modes as well as modes traveling in the -z direction. Hence, it is necessary to restrict the solver JCMsolve to compute only the modes of interest.