The specification describes optical fiber gain devices, such as lasers and amplifiers, wherein losses due to a large step transition between an input section and a gain section are reduced by inserting an adiabatic transformer between the input section and the gain section. In the preferred case the adiabatic transformer comprises a GRadient INdex (GRIN) lens. The lens serves as an adiabatic beam expander (reducer) to controllably increase (reduce) the modefield of the beam as it travels through the step transition.