Optimization of Refractive Index Sensitivity in Nanofilm-Coated Long-Period Fiber Gratings Near the Dispersion Turning Point

In this article, we present detailed investigation of the high-performance long-period fiber gratings (LPFGs) based surrounding refractive index (SRI) sensors. A comprehensive design approach is proposed to optimize the sensitivity in a specific SRI range by combining adequately both dispersion turning point (DTP) and mode transition (MT) effects. The SRI sensitivity enhancement induced by the DTP and the MT is discussed respectively. The relative importance of the mechanisms in different SRI range is revealed. The mode transitions of the HE _1,18 and HE _1,20 are investigated in a single LPFG, where two high sensitivity ranges are obtained. The atomic layer deposition (ALD) technology is used to deposit the titanium dioxide (TiO ₂ ) nano-film on the LPFG surface. The SRI sensitivities of 1 × 10 ⁴ nm /RIU in the SRI range 1.336-1.3397 and 4.2 × 10 ⁴ nm/RIU in the range 1.4526-1.4561 are achieved, respectively. The experimental results are well consistent with the theoretical analysis.