Propagation modeling for multimode photonics

While ray optics is a valid model for many multimode optical systems, it cannot capture diffraction and interference phenomena which may be required, for example, in calculating speckle propagation in multimode fiber. Wave optics includes the effect of interference and diffraction, but it is usually much more costly computationally. We show how to bridge the gap between ray and wave optics by using a position-angle ray phase space representation of the electric field. By transporting a suitable quadratic functional of the field (the Wigner distribution) along rays, diffraction and interference are taken into account for propagation through an arbitrary transverse index profile. We show how this method allows us to propagate highly multimode fields without resorting to detailed mode calculations. We also illustrate the method by calculating the mode selective loss for a multimode graded index waveguide.