Twist Sensor Based on Long Period Grating and Tilted Bragg Grating Written in Few-Mode Fibers

A twist sensor based on a few-mode long period grating (LPG) and a tilted fiber Bragg grating (TFBG) is proposed and demonstrated. The LPG, inscribed in a step index few-mode fiber by a CO
₂
laser, is utilized to excite the LP
₁₁
core mode. The followed TFBG written in graded index a few mode fiber with the LP
₁₁
core mode as exciting light is served as sensing element. By analyzing transmitted intensity difference of two dips in transmission spectrum of the TFBG induced by the mode couplings from forward-propagating LP
₁₁
core mode to backward-propagating LP
₁₁
and LP
₂₁
core modes, a twist sensitivity of 1.074 dB/(rad/m) from -5.23 rad/m to 5.23 rad/m is achieved. The proposed sensor has advantages of easy operation and simple structure.