Strain-independent fiber torsion and displacement sensor based on acoustically-induced fiber grating

We present a strain-independent torsion and displacement fiber sensor using acoustically-induced fiber grating (AIFG) in dual-mode fiber (DMF). By tuning the radio frequency of driving signal, LP01 and LP11 modes generated by the AIFG can be exploited to measure the dynamic displacement and torsion variations, respectively. Both the twist angle and the twist direction can be monitored via image detection facility at the end of DMF. Between torsion angles of −80° and 80°, the highest twist sensitivity reaches 15 pixel/°. The average displacement sensitivity is 5 pixel/µm within the recorded two-dimensional movement of 100um × 80um. The stable property of sensor is verified when the strain is varied from 100 με to 1500 με.