An optical system for an optical amplifier, the optical system comprising: a pump coupler or combiner (1682, 1782, 1882, 2182) having one or more pump input ports and an output port; an active optical fiber (1684, 1784, 1884, 2184) having a pump wavelength and an amplified wavelength, the active optical fiber having a first end operatively coupled to the output port of the pump coupler or combiner; one or more pump sources (1688, 1788, 1888, 2188) for producing optical radiation having the pump wavelength; and one or more mode mixing optical fibers (1600, 1700, 1887) for delivering the optical radiation, each of the mode mixing optical fibers having an input end operatively coupled to one of the one or more pump sources and an output end operatively coupled to one of the input ports of the pump coupler or combiner, each mode mixing optical fiber configured to be mode mixing for the pump wavelength and comprising: a centerline (104, 304); a core (110, 310, 410, 510, 610, 710, 810) for propagating the optical radiation, the core having a refractive index profile; and a cladding (320, 520) disposed about the core, wherein the core of the mode mixing optical fiber is configured to support at least twenty guided modes at the pump wavelength; wherein the core comprises one or more substantially doped regions (715, 815a, 815b, 815c, 815d) disposed asymmetrically around a centerline of the core, each of the substantially doped regions being substantially up-doped or substantially down-doped so that the mode mixing optical fiber is configured to distribute a fraction of the optical radiation propagating therein from its lower-order guided modes to its higher-order guided modes, such that at least 20% of the optical radiation is guided in a mode other than the fundamental mode. a pump coupler or combiner (1682, 1782, 1882, 2182) having one or more pump input ports and an output port; an active optical fiber (1684, 1784, 1884, 2184) having a pump wavelength and an amplified wavelength, the active optical fiber having a first end operatively coupled to the output port of the pump coupler or combiner; one or more pump sources (1688, 1788, 1888, 2188) for producing optical radiation having the pump wavelength; and one or more mode mixing optical fibers (1600, 1700, 1887) for delivering the optical radiation, each of the mode mixing optical fibers having an input end operatively coupled to one of the one or more pump sources and an output end operatively coupled to one of the input ports of the pump coupler or combiner, each mode mixing optical fiber configured to be mode mixing for the pump wavelength and comprising: a centerline (104, 304); a core (110, 310, 410, 510, 610, 710, 810) for propagating the optical radiation, the core having a refractive index profile; and a cladding (320, 520) disposed about the core, wherein the core of the mode mixing optical fiber is configured to support at least twenty guided modes at the pump wavelength; wherein the core comprises one or more substantially doped regions (715, 815a, 815b, 815c, 815d) disposed asymmetrically around a centerline of the core, each of the substantially doped regions being substantially up-doped or substantially down-doped so that the mode mixing optical fiber is configured to distribute a fraction of the optical radiation propagating therein from its lower-order guided modes to its higher-order guided modes, such that at least 20% of the optical radiation is guided in a mode other than the fundamental mode. a centerline (104, 304); a core (110, 310, 410, 510, 610, 710, 810) for propagating the optical radiation, the core having a refractive index profile; and a cladding (320, 520) disposed about the core, wherein the core of the mode mixing optical fiber is configured to support at least twenty guided modes at the pump wavelength; wherein the core comprises one or more substantially doped regions (715, 815a, 815b, 815c, 815d) disposed asymmetrically around a centerline of the core, each of the substantially doped regions being substantially up-doped or substantially down-doped so that the mode mixing optical fiber is configured to distribute a fraction of the optical radiation propagating therein from its lower-order guided modes to its higher-order guided modes, such that at least 20% of the optical radiation is guided in a mode other than the fundamental mode.