A fiber amplification system is provided for amplifying a laser pulse signal, e.g., an oscillator signal of an oscillator device. The fiber amplification system includes a fiber pre-amplification system having a short, fundamental-mode and step-index fiber configured to pre-amplify the laser pule signal to generate a seed signal and a main amplification system having a large core fiber configured to amplify the seed signal. The short, fundamental-mode step-index fiber can have a length no longer than about 30 cm, and a mode field diameter no less than about 30 μm, e.g., in a range from 30 μm to 60 μm, as well as a high doping concentration needed to provide an absorption length no more than about 30 cm, for providing the seed signal for the large core fiber with low non-linearity.