Femtosecond laser inscription of chirped and tilted fiber Bragg gratings in double-cladding ytterbium-doped fiber

Stimulated Raman scattering (SRS) is a key factor that limits the power scaling of high-power fiber lasers, and the chirped and tilted fiber Bragg gratings (CTFBGs) are essential components for SRS suppression. In this study, we report the first fabrication of CTFBGs in ytterbium-doped fiber (YDF) using the fs-laser phase mask technique with precise fiber rotational alignment. The CTFBG exhibits a maximum depth of 25 dB before annealing and a 3 dB bandwidth of 14 nm. The integration of the CTFBG within a high-power fiber amplifier system facilitates a Raman threshold enhancement of 350 W, resulting in a laser output power of 2.5 kW with minimal impact on the laser beam quality and power slope efficiencies. Theoretically, CTFBGs can be fabricated at any position to suppress SRS without the introduction of additional fusion splices, enhancing flexibility and reliability, which is of critical importance for practical high-power fiber laser applications.