Dual dispersion turning-points-enabled highly sensitive refractive index detection using tri-core fiber helical long-period gratings

This paper proposes a dual dispersion turning points (DTPs) enabled tri-core-fiber (TCF) helical long-period gratings (HLPGs) sensor, which is experimentally investigated for the first time, to the best of our knowledge. Numerical simulations and analysis show that dual DTP-related resonances can be generated within a single grating by optimizing the grating period and that the 2nd DTP-related resonance can be excited by increasing the surrounding RI. Subsequently, TCF-HLPGs samples were fabricated using a CO₂ laser and the RI sensing behaviors are experimentally investigated, confirming the realization of dual-DTPs, thereby enabling dual-region RI detection. The sensor achieves a maximum wavelength-separation sensitivity of 6653.9 nm/RIU and a transmission-contrast sensitivity of -139.1 dB/RIU, with a low temperature cross-sensitivity of 0.00017 RIU/°C. The proposed TCF-HLPGs can realize dual-DTPs-enhanced RI sensing within a single grating, providing a compact and reproducible structure for high-sensitivity and wide-range RI detection.