The next generation of ground-based gravitational wave detectors requires several hundred watts of optical power to increase the detector sensitivity. In addition, high demands are placed on the beam quality and noise properties of the laser beam. Optical amplifiers based on ytterbium-doped optical fibers are being researched as a promising technology for power scaling. In this thesis, a monolithic fiber amplifier concept has been developed for use in gravitational wave detectors. These amplifiers rely on fiber components for fiber-based pump and signal coupling to the gain fiber, which enables alignment-free and low-maintenance operation. Excellent beam characteristics at the 200 W optical power level are demonstrated and conclusively prove for the first time that fiber amplifiers can produce a high power beam that is in accordance with the requirements for the freerunning laser system for gravitational wave detectors. Based on the amplifier development further power scaling via coherent beam combining up to the 400 W power level and beam characterization is shown.weiterlesen