Apodized fiber Bragg grating inscribed by femtosecond laser point-by-point method

Fiber Bragg Grating (FBG) is an important type of fiber devices, which has been widely applied in optical filtering, fiber laser, and fiber sensor networks. Due to its uniform Refractive Index (RI) modulation, the conventional FBG usually behaves with prominent sidelobes in the reflection spectrum. In this paper, we propose a simplified dual-path approach utilizing femtosecond laser point-by-point direct writing method, to fabricate apodized FBGs with both high reflectivity and side mode rejection ratio. We adjusted the area of the femtosecond laser applying on the fiber core to achieve different overlapping integrals in different areas using a dual-path approach. By linearly translating the laser focus on the fiber core through two symmetrical paths, two identical Gaussian apodization functions was introduced to fabricate apodized FBG in YOFC single-mode fiber with target wavelengths in the range 1520–1570 nm using focused 515 nm femtosecond laser pulses and a fiber-guiding system with sub-micrometer transverse control. Following such simplified fabrication method, a new apodized grating with both enhanced reflectivity (⪆90%) and side mode rejection ratio (⪆ 13 dB) has been achieved. Such improved spectral quality allows the fabricated FBGs to fulfill the requirements of the applications in optical communication and sensing systems.