Accurate and efficient inscription of fiber Bragg gratings in double-clad ytterbium doped fiber based on femtosecond phase mask technology

The direct inscription of fiber Bragg gratings (FBGs) in double-clad ytterbium-doped fiber (DCYDF) by fs-laser has the potential to reduce the fusion splices in fiber lasers, which is significant for developing a more compacted and stable monolithic laser system. This study demonstrates that the distinctive inner-cladding structure of DCYDF has a non-negligible influence on the focal position and intensity of the fs-laser. To realize accurate and efficient inscription of FBGs, the fs-laser is controlled to incident at a specific angle based on direct imaging of the DCYDF, and the focus of the fs-laser is determined for the first time using the 1 μm photoluminescence attributed to Yb³⁺ under fs-laser excitation, to the best of our knowledge. The results provide new insights into the influence of the cladding structure on the inscription process of FBGs and its influence on the characteristics of FBGs. This paper presents an accurate and efficient method for inscribing fiber Gragg gratings in DCYDF (YDFBGs), which is of great significance for the fabrication and application of FBGs.