Power-Interrogated Torsion and Strain Sensor Based on a Helical Long-Period Fiber Grating Written in a Thinned Four-Mode Fiber

A helical long-period fiber grating (HLPG)-based temperature-insensitive torsion and strain sensor is proposed and experimentally demonstrated, which was realized by using a second-order HLPG written in a thinned four-mode fiber. Simultaneous measurement of the torsion and strain was accomplished by using the power-interrogated scheme, i.e., the torsion- and strain-induced changes in transmission spectrum of two linear regions were especially used as the testing objectives. The maximum torsion sensitivity is 665 pm/(rad/m) under the torsion ranging from −22.4 to 22.4 rad/m and the maximum strain sensitivity is −5.42 pm/ $\mu \varepsilon $ under the strain ranging from 0 to $1588 \mu \varepsilon $ , which are about 2-fold and 10-fold higher than those of the conventional long-period fiber grating (LPG)-based fiber sensors, respectively. In addition, the temperature sensitivity of the proposed sensor is $\sim $ 13.7 pm/°C at a temperature ranging from 20 to 70°C, which is almost 4-fold lower than that of conventional LPGs.