The exact field solution of a step-index profile fiber was used to determine the excitation efficiency of a distribution of sources in the core of an optical fiber. We have compared previous results of a thin-film cladding source distribution to its core source counterpart. The behavior of power efficiency Peff with the fiber parameters was examined and found to be similar to the behavior exhibited by cladding sources. It was also found that a core source fiber is two orders of magnitude more efficient than a fiber with a bulk distribution of cladding sources. This result agrees, qualitatively, with previous ones obtained experimentally.