Fiber laser based brightness converters enable diode laser beam sources to access a superior beam quality of better than 10 mm × mrad in combination with multi kW output power. A design of a fiber laser that is based on a single active optical converter fiber that is pumped by a direct diode is presented. Due to the high transfer efficiency of such brightness converters an electrical/optical efficiency > 25% can be achieved. The current status with an output power > 4 kW in combination with a beam quality of < 5 mm × mrad will be described. The principal design of such diode laser based fiber brightness converters will be presented and building blocks of such lasers will be outlined. As an application example laser welding will be presented of both the fiber converter laser and direct diode laser using optical light guides with identical core diameters on both lasers for comparison. Additionally, fibers with a core diameter of 200μm will be used on the fiber converter laser to perform remote welding. The weld results will be compared regarding welding depth and surface quality of the weld samples to determine the optimum power/brightness levels for different aluminum and steel materials.