Fiber Specklegram Sensor Based on Reflective Mode Rotation in Few-Mode Fiber

In this article, a fiber specklegram sensor (FSS) is formed from reflective mode rotation in few-mode fiber (FMF) with tilted fiber Bragg gratings (TFBGs). The FMF-TFBG with different tilt angles has been fabricated and could realize the high-efficient reflective mode conversion from fundamental mode to high-order core modes (LP11, LP21, LP02, LP31, and LP12 modes) simultaneously when the tilt angle is 1.6°. With the twist induced in FMF-TFBG, the recorded LP11, LP21, and LP31 mode specklegram rotated with fiber twisting. The specklegrams’ rotation angle is detected using speeded-up robust feature algorithm combined with Canny edge detection algorithm to improve the accuracy. When fiber twist angle changes from −300° to 300°, the specklegram rotation angle of LP11, LP21, and LP31 mode is linearly related to fiber twist angle, with a linearity of 0.515, 0.544, and 0.528, respectively. Therefore, the proposed mode rotation from FMF-TFBG with advantages of low cost, high accuracy, and excellent performances could give rise a new generation of FSS.