Refractive index sensing characteristics of long-period gratings in linearly arranged three-core fiber

Abstract

We demonstrate the refractive index (RI) sensing characteristics of long-period gratings (LPGs) in linearly arranged three-core fiber (TCF), which are fabricated using arc discharge technique. Due to the asymmetric index modulation during fabrication, the RI sensitivity of TCF-LPGs in the 0° grating inscription direction is greater than that in the 90° grating inscription direction. The spectral and RI sensing characteristics of TCF-LPGs in the 0° grating inscription direction with different grating periods have been investigated experimentally. In the RI range from 1.4538 to 1.4589, the resonance at 1533.3 nm for TCF-LPGs with grating period of 465 μm could achieve the RI sensitivity of 15943.3 nm/RIU, which could be optimized to 18160.7 nm/RIU at 1676.3 nm as the TCF-LPGs with grating period of 600 μm. Moreover, the proposed TCF-LPGs exhibit notably greater RI sensitivity compared to arc-discharged LPGs in conventional single-mode fibre, presenting substantial advantage in RI sensing. And, changing the polarization state of the incident light enhances the RI sensitivity of TCF-LPGs.Additionally, the influence of bending or temperature interference on the RI sensitivity of TCF-LPGs is minimal, ensuring that interference with RI measurements can be neglected. The proposed sensor boasts cost-effectiveness, simplicity in structure, and high sensitivity to RI, showing substantial potential for applications in chemical and biological sensing.