基于拉锥少模光纤长周期光栅的高灵敏度传感器

High-Sensitivity Sensors Based on Long-Period Gratings Inscribed in Tapered Few-Mode Fiber

少模光纤器件可以充分利用多个空间模式实现多参数的测量,具有精度高和检测效率高的优势,被广泛应用于光纤通信和光纤传感领域。通过仿真和实验对拉锥少模光纤长周期光栅进行研究,在不同锥腰直径的拉锥少模光纤上制备长周期光纤光栅,并对不同锥腰直径的长周期光栅的模式耦合、偏振相关损耗、温度、折射率和扭转特性进行研究。实验结果表明:随着锥腰直径减小,纤芯模式的扭转灵敏度有所提升,包层模式的折射率灵敏度和扭转灵敏度显著提升,最高灵敏度分别可以达到12166.7 nm/RIU和0.7 nm·rad^(-1)·m。拉锥少模光纤长周期光栅可以作为高灵敏度折射率传感器和扭转传感器应用于光纤传感领域。

Objective 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.Methods In this study,we change the diameter of an FMF via fiber tapering and fabricate LPFGs in the tapered FMF using carbondioxide 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 phasematching 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.Results and Discussions We fabricated tapered FMFs with different waist diameters using a carbon-dioxide laser glassprocessing 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.Conclusions 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.