Multi-clad optical fibers and fiber amplifiers are disclosed. Various embodiments include multi-clad, large core fiber amplifiers. In various implementations mixing of pump modes is enhanced relative to that obtainable with conventional double-clad fibers. In some embodiments end terminations are provided with increased length of end-cap fiber. In at least one embodiment a multi-clad fiber is provided, with a pump cladding formed by stacking a layer of low index rods in the preform. Various embodiments include a multi-clad fiber amplifier system. The system includes a pump source to pump said fiber amplifier. The system also includes an optical fiber having a core and a cladding, wherein the cladding includes a pump cladding having a corrugated boundary. In various embodiments the pump cladding is formed by rods in a preform, which are disposed to mix the pump modes and/or scatter or reflect pump energy into the core.