Few-mode fiber (FMF) components can fully utilize multiple spatial modes to achieve multiparameter measurements while offering high accuracy and detection efficiency; thus, they are widely used in optical communications and fiber sensors. This study investigates long-period fiber gratings (LPFGs) inscribed in a tapered FMF with different waist diameters, both numerically and experimentally. The mode coupling, polarization-dependent loss, temperature, refractive index, and torsion characteristics of the tapered FMF-LPFGs with different waist diameters were investigated experimentally. The experimental results show that the torsion sensitivity of the core mode can be improved, whereas the refractive-index sensitivity and the torsion sensitivity of the cladding mode increase significantly, with maximum values of 12166.7 nm·RIU^-1 and 0.72 nm·rad^-1·m achieved, respectively, as the waist diameter decreases. The tapered FMF-LPFGs can be applied as a high-sensitivity refractive-index sensor and torsion sensor for fiber sensing.

In this study, we change the diameter of an FMF via fiber tapering and fabricate LPFGs in the tapered FMF using carbon-dioxide laser. We use the COMSOL Multiphysics simulation software to calculate the propagation modes and effective refractive indices supported by the FMF with different waist diameters in different wavebands. Based on phase matching, we plot the phase-matching curves for different modes of the tapered FMF-LPFGs. The fundamental mode is coupled to the higher-order core modes in the FMF with a larger waist diameter, whereas it is coupled to the cladding mode in the FMF with a smaller waist diameter. By reducing the waist diameter, the sensitivity of higher-order core modes and cladding modes can be improved.

We fabricated tapered FMFs with different waist diameters using a carbon-dioxide laser glass-processing system. Subsequently, the LPFGs with different periods were inscribed in the tapered FMFs with different waist diameters using a carbon-dioxide laser. We observed the mode field distribution of the tapered FMF-LPFGs. The resonance wavelength and mode field distribution are consistent with the simulation results, as shown in Figs. 1 and 2. The tapered FMF-LPFG presents a lower polarization-dependent loss and temperature sensitivity. Figure 7 shows the refractive-index characteristics of the LP₁₂ cladding mode. As the waist diameter decreases, the refractive-index sensitivity increases significantly. Figures 8, 9, and 10 show the torsion characteristics of the LP₁₁ and LP₂₁ core modes and the LP₁₂ cladding mode, respectively. As the waist diameter decreases, the torsion sensitivities of the three abovementioned modes improve significantly.

The three core modes and the cladding mode can be excited by the tapered FMF-LPFGs. The refractive-index sensitivity and torsion sensitivity of the cladding mode can reach 12166.7 nm·RIU^-1 and 0.72 nm·rad^-1·m, respectively, which can benefit fiber sensing. Therefore, the proposed tapered FMF-LPFG can be used as a high-sensitivity refractive-index sensor and torsion sensor.