Highly sensitive refractive index and magnetic field sensor based on long-period fiber grating inscribed in tapered optical fiber

A refractive index (RI) and magnetic field sensor based on tapered long period fiber grating (LPFG) is proposed. We prepared two different periods of LPFGs (LPFG1 and LPFG2) using high-frequency CO2 laser point by point method on tapered single-mode fiber. Firstly, LPFG1 is used to measure the RI of sodium chloride solution. LPFG1 is immersed in sodium chloride solutions with different RI to measure the sensitivity of LPFG to RI. The experimental results indicate that LPFG1 is particularly sensitive to changes in environmental RI, with a sensitivity of 381.820 nm/RIU. Due to the special sensitivity of tapered LPFG to environmental RI, in order to eliminate the influence of residual sodium chloride on the surface of LPFG1 on magnetic field measurement, we use LPFG2 with similar parameters to measure the environmental magnetic field. LPFG2 was encapsulated in a glass capillary tube filled with the magnetic fluid (MF). After the environmental magnetic field changed, the RI of MF changed, causing the resonance peak wavelength of LPFG2 to drift. We measured the magnetic field using LPFG2 within the magnetic field range of 0.66 mT to 20 mT. Only in the magnetic field range of 1.95 mT to 4.6 mT, good linear fitting was achieved, with a magnetic field sensitivity of 1.242 nm/mT and a fitting coefficient of 0.9852. The experimental results indicate that LPFG inscribed in tapered optical fiber has high sensitivity to environmental RI and magnetic field. The sensor has a simple structure, small volume, easy production, and high sensitivity, which has certain practical application prospects. © 2023 Elsevier GmbH