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Propagation modeling for multimode photonics

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