Comparison of modelling techniques for multimode fibers and its application to VCSEL source coupling

Both concepts of ray tracing and the split-step complex amplitude propagation method are widely used in modelling of multimode fibers. Both techniques work under specific assumptions: (1) Ray tracing allows modelling of non-paraxial situations but ignores diffraction-dominated propagation effects and does not handle light field information like polarization; (2) The split-step technique assumes a paraxial propagation approach and by that ignores polarization crosstalk. We propose a generalization of the ray tracing approach to an efficient geometrical optics field tracing technique, which enables the inclusion of polarization effects including crosstalk within the fiber. We discuss and compare the different techniques theoretically and through the example of the coupling of a VCSEL source into a multimode fiber (OM3, OM4). We calculate the coupling efficiency with respect to adjustment tolerances.