Abstract: We have developed a femtosecond laser plane-by-plane direct writing method based on beam shaping, enabling single-step fabrication of the large-area fiber Bragg grating (FBG) planes. Using this innovative approach, we successfully fabricated high-reflectivity (HR) and low-reflectivity (LR) FBG pairs in 20/400-m double-clad fibers and implemented them in an all-fiber oscillator. Under a pump power of2262W, the oscillator achieved near-single-mode laser output of1664W with an optical–optical conversion efficiency of 73.47%, a beam quality factor M² of 1.46, and a 3-dB spectral bandwidth of 3 nm without stimulated Raman scattering peaks. The results indicate that femtosecond laser direct writing technology offers both fabrication flexibility and thermal management advantages for high-power grating fabrication, and establishes a key technological pathway for enhancing the performance of all-fiber laser systems.