Line Position-Dependent Effect in Line-by-Line Inscribed Fiber Bragg Gratings

• Published in: Sensors (Basel, Switzerland) Vol. 21; no. 21; p. 7231
• Main Authors: Li, Hongye, Zhao, Xiaofan, Rao, Binyu, Wang, Meng, Wu, Baiyi, Wang, Zefeng
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 30.10.2021; MDPI
• Subjects: apodization; Bandwidths; Bragg gratings; Cladding; cladding mode; Communication; Coupled modes; Direct laser writing; Experiments; femtosecond laser; fiber Bragg gratings; Inscriptions; Lasers; Spectrum allocation; Writing
• ISSN: 1424-8220; 1424-8220
• DOI: 10.3390/s21217231

Line-by-line direct writing by femtosecond laser has been proved to be a simple and effective method for the fabrication of low-loss fiber Bragg gratings (FBGs), and is more flexible compared with the traditional ultraviolet exposure method. In this paper, the line-position-dependent characteristics of cladding modes coupling in line-by-line FBGs have been studied, to the best of our knowledge, for the first time. Both theoretical and experimental results show that off-center inscribing could compress the bandwidth of the Bragg resonance and excite more abundant cladding mode coupling, in which the core-guided fundamental mode would couple to the cladding-guided LP0n and LP1n simultaneously. By aligning the line positions across the core region, the first apodized line-by-line FBG was achieved. This work enriches the theories of line-by-line FBGs and provides an inscription guidance to meet different application requirements.