Practicability of determining the modal power distribution by measured near and far fields

Under certain assumptions the modal power distribution in optical multimode fibers can be derived from either the near- or the far-field. We measured both intensities and compared the calculated mode spectra. From these results, it becomes clear which assumptions are necessary and how to control them. The influence on the modal power distribution by leaky modes and some error sources such as the profile exponent, the fiber end quality, and misalignments are shown.