In a double-clad fiber there are two waveguides – the Yb-doped core that forms the signal waveguide and the inner cladding waveguide for the pump light. The inner cladding of the active fiber is often shaped to scramble the cladding modes and increase pump overlap with the doped core. The matching of active and passive fibers for improved signal integrity requires optimization of the core/clad concentricity, and the MFD through the core diameter and NA, which reduces splice loss. This is principally achieved by tightening all of the pertinent fiber specifications.Oulundsen, G., Farley, K., Abramczyk, J. and Wei, K. "Fiber for fiber lasers: Matching active and passive fibers improves fiber laser performance", Laser Focus World, Vol 48 Jan 2012. http://www.nufern.com/library/item/id/391/
