Composite fiber Bragg gratings fabricated by femtosecond lasers for high-power fiber lasers

Fiber Bragg grating (FBG) is a key component in high-power fiber lasers. Mirror FBGs can act as cavity mirrors for fiber oscillators, and chirped and tilted fiber Bragg gratings (CTFBGs) can be used as all-fiber spectral filters to suppress nonlinear effects such as stimulated Raman scattering (SRS). Here a pair of mirror FBGs at 1080 nm are inscribed in fibers with a core diameter of 30 μm by femtosecond (fs) -lasers, and a high-power all-fiber oscillator is constructed based on the fs-written mirror FBGs. Next, a CTFBG at 1135 nm is inscribed on one side of the low reflectivity mirror FBG using fs-lasers, and the two FBGs on the same fiber form a composite FBG. SRS is effectively suppressed by CTFBG with a Raman suppression depth and width of 16 dB and 86 nm, respectively. The output power is increased to 9 kW and the Raman light ratio in the output power decreases by an order of magnitude. This work proves that composite FBG can enhance the compactness and stability of fiber oscillators and achieve effective SRS suppression, which is of significance for the development of high-power FBGs and fiber oscillators.