A fiber amplifier comprises an amplifying double-clad fiber (DCF). The fiber has a core having a first refractive index, an inner cladding surrounding the core and having a second refractive index lower than the first refractive index and an outer cladding surrounding the inner cladding. The core is doped with Erbium (Er) and co-doped with Ytterbium (Yb), and further co-doped with Cerium (Ce). The Ytterbium (Yb) enables pump energy transfer from Ytterbium (Yb) ions being in the excited state to Erbium (Er) ions being in the ground state (4I15/2). The Cerium enables a resonant energy transfer between the Erbium (Er) excited state (4I11/2) and Cerium (Ce) ground state (4F7/2). This leads to a lower population of the Erbium 4I11/2 state and thereby increases energy transfer from Ytterbium to Erbium.