Suppressing Side-Scattering on Laser-Written Bragg Gratings for Back-Reflection Engineering in Fibers

Abstract

Laser direct writing (LDW) is versatile in structuring fibers with micro-sized functional elements such as fiber Bragg grating (FBG) and backscattering centers by finely manipulating back and side scattering from laser-induced refractive index modified (RIM) points. However, the side-scattering is a lesser-explored property in laser-structured fibers. In this work, a concise physical model is established to understand the side-scattering as a combined effect of microstructure and geometry of RIM points. Based on a single-pulsed LDW method, the parametric decoupling between scattering loss (α) and coupling strength (κ) coefficients of FBGs is reported, whose cross-section is customized to have a flattened ellipse with thoroughly positive RIM, enabling controllable reflectivity from −21.33 dB to −0.0018 dB while maintaining narrow bandwidth and low loss. Exemplarily, a designed FBG realizes ultra-low loss of 0.008 dB with a resonance attenuation of 10.81 dB, exhibiting a record-breaking κ/α of 2083. Using this FBG as the high-reflective mirror of a home-made bismuth-doped fiber laser, narrow-band lasing with a high optical signal-to-noise ratio of ≈43 dB is achieved, demonstrating flexibilities of the proposed approach in customizing both back- and side-scattering in fibers and opening up wide opportunities for combining multifunctional components into optical fibers and realizing all-fiber networks.