Flexible and fast inscription of parallel-integrated fiber Bragg gratings by femtosecond laser through spatial light modulation

Fiber Bragg gratings (FBGs) are essential passive photonic devices widely used in many important application fields such as optical sensing and communication. Compared to traditional ultraviolet (UV) laser-based fabrication, the direct femtosecond (FS) laser writing provides significantly higher flexibility and thermal stability. However, the conventional inscription technique of complex FBG relies on complicated mechanical scanning of precision stages, resulting in low production efficiency. To overcome this limitation, this study proposes a beam-shaping method of the FS laser based on a spatial light modulator (SLM) for fabricating parallel-integrated fiber gratings (PI-FBGs). Using the weighted Gerchberg-Saxton (GSW) algorithm to compute phase distribution, multiple focal spots (e.g., 4 or 8 spots in a ring array) are simultaneously generated via a single exposure. This enables fast inscription of a parallel grating structure, and significantly improves the processing efficiency and flexibility. This work also systematically investigated the influence of the key production parameters, including the number of grating units and their radial distance from the fiber core center. We successfully inscribed the PI-FBGs in a single-mode fiber based on this method. Experimental results demonstrated that this method can fabricate ultra-short PI-FBGs with a length of only 500 µm, and a high reflectivity exceeding 90%. Furthermore, the fabricated PI-FBGs exhibited excellent performance in high-temperature tests up to 800°C. The temperature sensitivity is 15.18 pm/°C during the heating process and 15.11 pm/°C during the cooling process. Moreover, the PI-FBG is insensitive to axial strain. The proposed FS laser direct-writing technique, assisted by the SLM, effectively overcomes the efficiency and flexibility limitations of traditional mechanical scanning methods. It enables parallel processing of complex patterns, offering an efficient and flexible solution for manufacturing high-performance, miniaturized fiber sensors.