A novel fiber-optic bending sensor based upon the propagation of LP21 mode is demonstrated. The sensor, comprised of an S-bend fiber on an elastic film, measures LP21 mode specklegram rotation, which increments linearly with bending angle by the stress-optic effect. The sensor is capable of experimentally achieving a sensitivity as high as 4.13 rad/m(-1). The theoretical analysis of the sensor, which is a combination of fiber coupled-mode theory and elastic-optic theory, validates the accuracy of the sensor. The sensor is also shown to be temperature-immune, and can detect both bending direction and bending angle with a large dynamic range. Furthermore, the sensor implementation incorporates inexpensive single-mode fiber at 650 nm for few-mode operation, enabling low-loss transmission and compatibility with existing interfaces.

Twisting and bending characteristics of low-multimode LP21 mode propagation in optical fibers is presented for the first time. Theoretical fiber mode modeling, combining geometrical rotation with opto-elastic effects, demonstrates that the propagation of the LP21 mode is bending-effect-immune. Experimental testing verifies that the LP21 mode specklegram rotates 0.9112 of the fiber twist angle in a fused silica fiber, independent of any fiber bending. This characteristic allows for the LP21 mode to be highly applicable in fiber specklegram sensors.