Helical Long-Period Gratings in Four-Core Fiber for Multiparametric Monitoring of Directional Bending, Torsion and Temperature

We demonstrate the multiparametric monitoring of directional bending, torsion, and temperature based on helical long-period gratings (HLPGs) in a triangular four-core fiber (FCF) using CO
₂
laser irradiation. The simulations investigated the stress-relaxing effect of the stress-applying parts due to CO
₂
laser heating. In the experiments, the HLPGs with a period of 435 μm and a period number of 60 induced in FCF have been investigated for multiparametric monitoring. The directional bending sensing characteristics of FCF-HLPGs exhibit that at the curvature ranges A and B, the maximum bending sensitivities are −47.24 and 48.47 dB/m
⁻¹
obtained at −60°, respectively. And their sensitivities show the sine function behavior with curvature directions from −180° to 180°. The torsion sensitivity based on coupling efficiency variation is −0.16%/(rad/m) in the range from −10 to 10 rad/m. The temperature sensitivity based on wavelength variation is 0.123 nm/°C, and the intensity response on temperature can be negligible. The proposed FCF-HLPGs is potential in practical application for multiparametric monitoring.