Analysis of Sensing Characterictics of a Helical Long-Period Fiber Grating Written in a Graded-Index Few-Mode Fiber and its Temperature Sensor Application

The unique sensing characteristics of a helical long-period fiber grating (HLFG) written in a graded-index few-mode fiber (GI-FMF) was analyzed in the paper. Simulation results show the dispersion factors of the designed HLFG can keep higher values in a wide wavelength rage than those of traditional fibers, resulting in the unique characteristics of the ultra-wide linear range and the ultra-high sensitivity for sensing temperature. Experimental results show the temperature sensitivity of $122.2 \ \mathrm{pm}/^{\circ} \mathrm{C}$ can be obtained by using the designed HLFG, indicating the temperature sensing potential application of this device.