High beam quality and high average power from large-core-size optical-fiber amplifiers

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In some embodiments, the present invention provides an apparatus, method and use for improving and merging two existing techniques (core pumping and cladding pumping) to enable high-power fiber-laser systems having excellent beam quality while using large-core (LMA) step-index gain fibers at very high optical power, wherein the core pumping includes mixing a laser seed optical signal (having a signal wavelength) with optical core-pump light (having a core-pump wavelength that is near the signal wavelength) in a manner that matches the modes of the seed optical signal and the pump light. The combined core light is mode matched to the LMA gain fiber. The core-pump light is substantially all absorbed within a short distance from the entry end of the gain fiber and provides a strong pre-amplified signal for later cladding-pumped amplification. In some embodiments, the signal wavelength and the core-pump wavelength are within a single multiplet of a rare-earth dopant.