Fiber Bragg Grating Inscribed in Large Mode Area Double-Clad Fiber Using Femtosecond Laser Multi-Layer Line-by-Line Technology

We demonstrate the fabrication of a high-quality fiber Bragg grating (FBG) in a large mode area passive double-clad fiber (DCF) using femtosecond laser multi-layer line-by-line inscription. By optimizing the number of layers and the length of each line, the femtosecond laser-induced refractive index modulation can cover the entire fiber core area. A uniform FBG (UFBG) and a chirped FBG (CFBG) with high reflectivity and low insertion loss (IL) are fabricated. The UFBG has a reflectivity of 99.95% at 1060.42 nm with an IL of less than 0.2 dB. The CFBG has an average reflectivity of 99.97%, a 3-dB bandwidth of 2.5 nm, and an IL of less than 0.6 dB. Furthermore, the CFBG is employed within a fiber laser system, featuring a temperature rise coefficient (TRC) of 0.106 °C/W. The laser system achieves an output power of 133.8 W with a beam quality of M² = 1.145. Thanks to the flexibility of proposed multi-layer line-by-line inscription technology, it is expected to be used to fabricate FBGs in DCF with varying core diameters and special wavelengths. This method holds significant potential for applications in high-power all-fiber oscillators with different output power levels and wavebands.