Within the EKOLAS consortium, which is part of the BMBF-funded EffiLAS (Efficient high-performance laser beam sources) research initiative, we are developing fiber Bragg gratings (FBG) directly written into multimode fibers. Fiber lasers are an established beam source for high-power materials processing due to their high efficiency and high average output power at high beam quality. By using FBG as fiber-integrated output mirrors, which is state-of-the-art in singlemode fiber lasers, we aim to reduce the complexity and increase robustness and reliability of multimode fiber resonators. Therefore, we are investigating the use of FBG as outcoupling mirrors in multimode high-power multimode fiber lasers. As a first step, we directly write an FBG into an active extra-large mode area (XLMA) fiber with <100 μm core and use the FBG as low reflective outcoupling mirror for the fiber resonator with simultaneous frequency stabilization. The setup delivers an output power of more than 800 W at 1077 nm. The output power of the system was limited by the pump laser setup and not by the FBG or its temperature. The FBG is passively cooled and the measured temperature of the fiber at the grating is below 130 °C at 800 W output power. As the next step, we set up an active XLMA-fiber (core <100 μm) with an FBG as outcoupling mirror into a laser resonator with water cooling of the resonator fiber and optimized pump coupling. This setup delivers an output power of more than 8 kW at 1077 nm without failure of the FBG.
In this paper, we present our current work towards a highly efficient XLMA (extra-large mode area) fiber-based laser, which is being performed in the EKOLAS consortium within the BMBF-funded EffiLAS (efficient high-performance laser beam sources) research alliance. To this end, the complete manufacturing process chain of the XLMA fiber was reviewed and optimized. The work started with the material composition of the active XLMA preform with the goal of improving the purity and thus the background loss. A successfully implemented fluorine co-doping process allows refractive index adjustment of the active core material which improves the beam quality of the laser fibers without changing the concentration of active ions in the glass composition. The preform is subjected to a screening in which possible scatter centers, e.g. bubbles, inclusions or contaminants, are mapped and categorized, in order to identify defects, which could lead to a failure in the drawn fiber, already at an early production stage. The subsequent fiber drawing is monitored for scattering using the emissions from the heated preform as well as for inhomogeneities of the dopants using a phase measurement technique. Finally, the fiber is tested for residual impurities and background losses using a multi-mode OTDR to ensure that the fibers are free of any defects.