Femtosecond laser inscription of FBGs in ytterbium-doped fiber based on fluorescence detection techniques

Directly inscribing fiber Bragg gratings (FBGs) in ytterbium-doped fiber (YDF) using fs-laser can reduce the melting point in fiber lasers, which is significant for developing efficient fiber laser systems. In this paper, the photoluminescence at 1 µm band excited by the fs-laser in YDF was first reported to assist the FBG inscription, to the best of our knowledge. Moreover, we demonstrate that when the fs-laser scans across the fiber, the unique inner-cladding structure of YDF has a non-negligible effect on the focus position and intensity, thereby presenting a challenge to achieve high reproducibility when inscribing FBGs. The refractive index modulation characteristics of fs-laser incident from different angles are also analyzed.