Long-period fiber gratings are placed in a predetermined pattern along a double-clad optical fiber having an outer cladding, an inner cladding and a core. The core is doped with an optically active material such as, for example, a rare earth ion or other laser ion. The long-period fiber gratings couple light between a mode of the inner cladding and a mode of the core. As a consequence of increased coupling into the core resulting from the use of long-period fiber gratings, the length of double-clad optical fiber needed to transfer light between the inner cladding and the core is reduced.