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Abstract
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.
Highlights
The femtosecond laser direct writing technique is a nice choice for fabricating fiber Bragg gratings (FBGs)
The point-by-point (PBP) method is the most common way of directly writing FBGs [5,6,7]; point-by-point fiber Bragg gratings (PBP-FBGs) are often with high insertion loss [8], which limits their applications in many fields, such as fiber lasers
Studies indicated that off-center writing resulted in more complex cladding mode for coupling inscribed in the center
Summary
The femtosecond laser direct writing technique is a nice choice for fabricating fiber Bragg gratings (FBGs). The point-by-point (PBP) method is the most common way of directly writing FBGs [5,6,7]; point-by-point fiber Bragg gratings (PBP-FBGs) are often with high insertion loss [8], which limits their applications in many fields, such as fiber lasers. Huang et al reported that a phase-shifted FBG fabricated by the LBL inscription could be applied in a fiber torsion sensor for its strong birefringence [11]. Another kind of phaseshifted FBG inscribed on both sides of a fiber taper with the LBL technique was proved to be effective in axial strain detection [12].
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