Wave optics analysis methods, such as the finite elements method (FEM) or the beam propagation method (BPM), are available for calculating ray propagation in dielectric channel waveguides. These methods, however, can only be used effectively if only one or a few modes have to be considered and the cross-section of the waveguides is not too great in relation to the optical wavelength.
In the case of multimodal step index waveguides, the cross-section of which is significantly greater than the wavelength of the radiation used, ray tracing can be carried out effectively on the basis of geometric optics.
In this process (during simulation) a single ray of predetermined direction and polarization is input into the waveguide. This either emerges directly at the end of the waveguide or is fractured on the wall of the optical channel, i.e. the interface of the indexed jump.
The incident radiation is then split into components: one reflected main ray, one transmitted main ray, a number of reflected scattered rays and a number of transmitted scattered rays. The transmitted ray parts are insignificant for further ray tracing; only their energy component is lost to the reflected rays.
A simple simulation only considers the reflected main ray, i.e. the zero order reflection, and traces its further reflection to the emergence surface. This means that by tracing a larger number of rays individually, which correspond to the characteristics of the transmitter, the beam can be determined at the waveguide exit. This method is adequate if the reflections are almost ideal, because the wall is very smooth.
If the wall is not smooth however, the method gives results which correspond only poorly to the corresponding measurements. Consideration of the reflected scattered rays however involves additional computation, which increases exponentially with the number of reflections.