Femtosecond laser fabrication of large-core fiber Bragg gratings for high-power fiber oscillators

In this paper, a fs-laser phase mask inscription system based on a galvanometer scanning strategy is designed and set up for the fabrication of large-core fiber Bragg gratings (FBGs). Based on this setup, a homogeneous cross-sectional refractive index modulation can be achieved in the core of a large-mode-area fiber, and a pair of FBGs are fabricated in fibers with a core diameter of 30 µm. To investigate the performance of the fabricated FBGs, a high power all-fiber oscillator is built using a pure backward pumping structure. The FBGs work well, and the maximum output power of 7920 W is achieved with an optical–optical conversion efficiency of 77.3%. To the best of our knowledge, this is the highest power of all-fiber oscillators based on fs-written FBGs. This work provides a flexible, stable, and economic scanning strategy for large-core FBG inscription and exhibits excellent performance for high power fiber lasers.