Femtosecond inscription of large-area fiber Bragg gratings for high-power cladding pump reflection

A new, to the best of our knowledge, method for inscribing fiber Bragg gratings inside a fiber's cladding based on the motorized rotation of the fiber is reported. By minimizing the aberrations induced by the fiber curvature on the femtosecond writing beam, this technique based on a phase mask allows to cover large transverse areas of a standard high-power fiber's cladding. With this approach, a first-order Bragg grating was inscribed in the pure-silica inner cladding of a 20/400-µm fiber. It was then implemented as a pump reflector at the end of a 36-m-long Yb-doped fiber laser reaching 600 W of output power, confirming the power handling capabilities of such a component. Comparison of the laser performances with and without the pump reflector showcases its great potential for increasing pump absorption inside cladding-pumped fiber lasers, which paves the way for significantly reducing their active fiber length.