Optical Fiber Bending Sensor Based On LP₂₁Mode

As a new mode of bending sensor, optical fiber bending sensor is featured by strong anti-electromagnetic interference capacity, corrosion resistance, light weight, small size, etc. and is widely applied. The available optical fiber bending sensor can achieve certain sensitivity and resolution; however it cannot distinguish the bending direction and also requires spectral analysis device or subsequent image processing. There is also a cross sensitivity problem and the accuracy is greatly affected by temperature. Therefore, the practical application is subject to certain restrictions.This paper introduces a kind of specklegram bending sensor based on fiber LP21mode. When the sensor bends, the embedded optical fiber will twist correspondingly and the output LP21mode speckle will rotate in the same direction. The twist angle of the optical fiber will be obtained by inspecting the rotation angle of the speckle through CCD and further get the bending value of the sensor. Compared with the traditional optical fiber bending sensor, LP21mode has a simpler structure which is free from the interference of ambient temperature; meanwhile, the bending direction is able to be determined according to the rotation direction of the speckle. Even more remarkable is that during the bending of optical fiber outside the sensing region, the LP21mode will neither deform nor rotate, which will get rid of the influence from the stress outside the optical fiber sensing region. Owing to this feature, LP21bending sensor has a very strong expansibility.From the aspect of the research on sensor and LP21theoretical mode as well as the design of relevant test methods, this paper mainly covers the following parts:First of all, in-depth understanding and studying the relevant knowledge of bending sensor, summarizing the available bending sensor theories and development status and analyzing the merits&demerits of various bending sensors and major existing technically problems.Second of all, studying LP21mode communication theory under the influence of elasto-optical effect, analyzing the characteristics of LP21mode when the optical fiber bends and twists simultaneously, studying the screw rod mode theory and figuring out the relationship between the sensor bending and the bending&twist angle of optical fiber.In the next place, describing the design and manufacture of sensor components and introducing the precise CNC engraving technique; completing the sensor and corresponding testing platform under laboratory conditions; utilizing LP21specklegram obtained through Matlab or other analysis to test the performance of bending sensors with different optical fiber structures; and testing the sensitivity of sensor to temperature. The test results show that this optical fiber bending sensor, with very high accuracy and sensitivity, is able to identify the bending direction and is not sensitive to temperature changes.In the last place, analyzing the shortages of the system, optimizing, improving and expanding the sensor to improve its accuracy and sensitivity and realize simultaneous measurement of several parameters.