Stimulated Raman scattering suppressors fabricated by femtosecond laser overlap-controlled multilayer line-by-line technology

Femtosecond-laser-direct-written fiber gratings suffer from excessive scattering and high insertion loss, which restricts their application in fiber laser systems, particularly in the field of high-power lasers. To address this limitation, we propose an overlap-controlled multilayer line-by-line (OCML-LBL) technique to form planar refractive index modulation (RIM) that covers the fiber core, which can effectively reduce Type II modulation-induced scattering and further minimize insertion loss. Utilizing this approach, we fabricated low-insertion-loss chirped and tilted fiber Bragg grating (CTFBG)-based stimulated Raman scattering (SRS) suppressors and implemented them in a fiber laser system. The fabricated devices demonstrate bandwidths spanning from 38 nm to over 100 nm with efficiencies up to 99.9% at customized wavelengths, and significantly suppress the SRS in the fiber laser. Moreover, the fabricated CTFBG exhibits excellent stability, demonstrating negligible response to temperature, strain, and bending. Owing to its inherent flexibility, this fabrication method holds substantial promise for the customized fiber grating inscription of fiber lasers.