Beam delivery by large-core fibers: effect of launching conditions on near-field output profile.

For large-core optical fibers of a few meters length, which are typical of those used in beam delivery systems for high-power Nd:YAG lasers, it is shown that the near-field profile of the output beam is a strong function of the launching conditions. The output profile depends on both the input spot size and its alignment relative to the fiber axis. A simple theoretical model is developed for step-index fiber that shows that the output profile depends on the distribution of guided power between meridional modes and groups of skew modes. A relationship is hence derived between the launching conditions and the output profile. The predictions of the theoretical model are consistent with experiment.