Abstract: A fiber optic structure for efficient pumping of a rare-earth element fiber laser, such as an Ytterbium fiber laser, operating at the three-level transition. In order to operate at the three-level transition, a large amount of pumping energy must be effectively coupled from the fiber's inner cladding into the rare-earth element-doped core. The present invention discloses the use of eccentric asymmetric, and non-cylindrical inner claddings to maximize the overlap of the pumping energy of multi-mode field distribution patterns with the fiber core and to maximize the number of intersections of the pumping optical ray trajectories with the core. While the cladding surrounding the core is eccentric with respect to the core, the core itself is coaxial with the outer cladding for compatibility in coupling to standard transmission fibers. Also disclosed is the use of air claddings and methods for optimal sizing of the capillaries.