Research On The Sensing Properties Of Vector Bending Sensor Based On Multi-Core Fiber

Vector bending characteristic is an important detection index,which has important application value in the fields of building structure health monitoring,mechanical bending angle measurement,human posture detection and aerospace,etc.Fiber optic bend sensor inherits many advantages,such as small size,anti-electromagnetic interference,flexibility,light weight,corrosion resistance and compact structure.Furthermore,fiber optic bend sensor also has the advantages of high reliability,high safety,stable structure and easy fabrication.Therefore,since the optical fiber micro-bend sensor was first proposed in 1980,optical fiber bend sensor has been widely studied and applied in the engineering field.Multi-core optical fiber has been widely used in various optical sensor devices because of its good mode transmission characteristics.Different structures of inter-mode interference sensors can be made using multi-core optical fiber fusion spliced with single-mode and multimode fiber,which used to measure the curvature magnitude and direction of bending.The main contents of this paper are listed as follows:(1)Based on different measuring principles,the bending sensors are classified,and the principles of bending sensors based on inter-mode interference,FBG and LPFG are analyzed,respectively.The development status of bending sensors at home and abroad is summarized.The development status and different types of multi-core optical fiber sensors are summarized.(2)The dislocation mode excitation principle of optical fiber is analyzed.The theory of double-beam interference is expounded,and the relationship between optical fiber mode interference and double-beam interference is analogized.The working principle of mode interference is explored.The principle of bending and temperature sensing is analyzed.The influence of structure bending and temperature change on output spectrum is analyzed.(3)A bending sensing system and a temperature sensing system were built.Different types of vector bending sensors were designed by cascading three-core optical fibers and singlemode optical fibers in different fusion methods.The finite difference beam propagation method is used to optimize the length of the three-core optical fiber.A vector bending sensor based on three-core optical fiber is fabricated.The bending sensing characteristics and temperature disturbance to the sensor are investigated.The experimental results are compared and analyzed.The experimental results show that the curvature sensitivity of the vector bending sensor with dislocation-taper cascaded SMF-TCF-SMF structure reaches-12.82 nm/m-1 in the positive ybending direction and-5.82 nm/m-1 in the negative y-bending direction.The temperature experiment is carried out to analyze the disturbance of temperature change to bending measurement.It is concluded that temperature sensitivity is three orders of magnitude lower than that of bending sensitivity.So,in a small range of temperature,the disturbance of temperature to bending detection can be neglected.(4)The sensing characteristics of a seven-core fiber-based vector bending sensor were explored.The structural parameters are simulated based on the finite-difference beam propagation method to optimize the length of the seven-core fiber and multimode fiber.The curvature sensing characteristics and temperature disturbance analysis were conducted based on a seven-core fiber fusion spliced with multimode fiber,a dislocated cascading single mode fiber,and an asymmetric cascaded fiber.The curvature sensitivity of the vector bending sensor is improved compared to the sensor based on three-core fiber.For the asymmetric cascaded SMF-SCF-SMF sensor,its sensitivity reaches-35.51 nm/m-1 in the range of 0.544-1.44 m-1 in the positive y-bending direction,and 35.86 nm/m-1 in the range of 1.445-2.323 m-1 in the negative y-bending direction.Temperature experiments were carried out to analyze the disturbance of temperature variation on the bending measurement.It is concluded that temperature sensitivity is three orders of magnitude lower than that of bending sensitivity.So,in a small range of temperature,the disturbance of temperature to bending detection can be neglected.In this paper,the sensor structure parameters is optimized using the sensing characteristics of three-core fiber and seven-core fiber.Different sensing structures are fabricated,and different length sensing structures are discussed.The optimized sensing structure is selected to achieve the curvature detection and the measurement of two opposite bending directions.Temperature experiments are carried out to exploring the effect of temperature on the sensing performance.