We developed a generalized field-propagating model for active optical fibers that takes into account mode beating and mode coupling through the amplifying medium. We applied the model to the particular case of a few-mode erbium doped fiber amplifier. Results from the model predict that mode coupling mediated by the amplifying medium is very low. Furthermore, we applied the model to a typical amplifier configuration. In this particular configuration, the new model predicts much lower differential modal gain than that predicted by a classical intensity model.