Dependence of output near-field beam profile on launching conditions in graded-index fibers used in delivery systems for Nd: YAG lasers

For graded-index optical fibers of a few meters in length and of large core diameter, 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 launch conditions and laser-beam characteristics. The output profile depends on the input spot size and its alignment relative to the fiber axis as well as on the dependence of the beam divergence on the position within the input spot. A theoretical model has been developed to demonstrate how the output profile depends on the distribution of guided power between meridional modes and groups of skew modes, which are excited by a particular launch condition. The predictions of the theoretical model are consistent with the experiment. © 1993 Optical Society of America.