Investigation of fiber Bragg gratings for high-power multi-mode XLMA-based fiber lasers

Within the EKOLAS consortium, which is part of the BmBF-funded Effilas (Efficient high-performance laser beam sources) research alliance, we are developing Fiber Bragg Gratings (FBGs) written in extra-large mode area (XLMA) fibers. By using FBGs as fiber integrated output mirrors, which is state-of-the-art in single-mode fiber lasers, we aim to reduce complexity and increase robustness and reliability of multi-mode fiber resonators. To this end, we are investigating the use of FBGs as outcoupling mirrors with a reflectivity below 10 % in XLMA high-power multi-mode fiber lasers. As a first step, FBGs are written into a passive 105/125 mm multi-mode fiber. We present their application for frequency stabilization of a resonator based on XLMA fibers, and tested the FBGs up to an output power of 150 W at 1075 nm without failure. As the next step, transition from passive 105/125 mm fibers to active XLMA fibers is currently being investigated. For FBG inscription, we use a phase mask and an ultra-fast laser system (100 fs, 800 nm). The setup is adjustable in three translation and three rotation axes. Additionally it features a modular mechanical design for fast and flexible interchanging of fiber mounts, phase mask and process optics. The FBGs are pre-characterized in transmission with a white-light source and an optical spectrum analyzer.