XLMA fibers based on Yb-doped bulk silica possess an excellent refractive index and doping level homogeneity . To
achieve the highest optical-to-optical efficiency and long-term operation without degradation we simulated the effect of the brightness conversion factor of different core dopant compositions of such XLMA fibers. We also investigated the beam quality of a multi-kW single XLMA fiber laser system and its long-term stability. The current state-of-the-art
XLMA single fiber laser has 5 kW maximum output power and a degradation rate of about 0.5 % / 500 h at 4 kW
measured over a period of 1700 h. Several application tests demonstrate the excellent performance of the XLMA fiber
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.
The quality of Yb-doped fused bulk silica produced by sintering of Yb-doped fused silica granulates has improved
greatly in the past five years [1 - 4]. In particular, the refractive index and doping level homogeneity of such materials
are excellent and we achieved excellent background fiber attenuation of the active core material down to about 20 dB/km
at 1200 nm. The improvement of the Yb-doped fused bulk silica has enabled the development of multi-kW fiber laser
systems based on a single extra large multimode laser fiber (XLMA fiber).
When a single active fiber is used in combination with the XLMA multimode fiber of 1200 μm diameter simple and
robust high power fiber laser setups without complex fiber coupling and fiber combiner systems become possible. In this
papper, we will discuss in detail the development of the core material based on Yb-doped bulk silica and the
characterization of Yb-doped fibers with different core compositions.
We will also report on the excellent performance of a 4 kW fiber laser based on a single XLMA-fiber and show the first
experimental welding results of steel sheets achieved with such a laser.