Research On Curvature Sensing Technology Based On Seven Core Fiber

With the continuous development of science and technology,optical fiber curvature sensor can be applied in a variety of complex environments because of its small structure,light weight,strong anti-electromagnetic ability,corrosion resistance,good safety and other excellent characteristics.Curvature is not easy to be observed as a physical quantity to measure the deformation of an object,and sensors play an important role in the accurate measurement of curvature.Optical fiber based curvature sensor has been studied in robot motion control,medical device shape tracking,limb joint motion monitoring,underwater object deformation detection and so on.Aiming at the above engineering problems,this paper designs and develops fiber grating type fiber curvature sensor and Mach-Zehnder interferometer fiber curvature sensor respectively by using seven core fiber,and carries out theoretical analysis and curvature experiment research on them.The main work is as follows:（1）Based on the introduction of the structure of seven core fiber,the curvature sensing principle and temperature response characteristics of seven core fiber Bragg grating are analyzed.Using excimer laser,fiber Bragg grating is carved into seven-core fiber by phase mask method.The seven-core fiber Bragg grating sensor is studied by directional curvature sensing experiment and temperature response experiment.The results show that seven cores have different curvature sensitivities in the curvature range of 2.53-8.62m^-1,among which the maximum curvature sensitivity reaches 0.195nm/m^-1.The seven cores responded uniformly to temperature in the 30-70℃range,with an average temperature sensitivity of 9.97pm/℃.In addition,the central wavelength of FBG in the intermediate core is responsive to temperature change,but not to curvature change.The FBG central wavelength of the six surrounding cores varies with temperature and curvature.Therefore,the temperature can be measured directly through the FBG of the intermediate core,and the cross influence of temperature change on curvature measurement can be eliminated at the same time,and the temperature compensation function of the intermediate core can be realized.（2）Based on the curvature sensing principle of seven core fiber Bragg grating（FBG）,the direction sensing of the sensor is studied experimentally.The results show that the wavelength of the six cores in the 0-360°bending direction under the same curvature has a regular drift trajectory.The theoretical models of curvature and direction are reconstructed,and then the experiments of temperature,curvature and direction are verified.The results show that the relative errors of temperature,bending direction and curvature are 2.37%,5.17%and 7.92%respectively.There is little difference between the theoretical results and the actual results,which indicates that the reconstructed model is feasible.The low temperature response characteristics of a seven core fiber Bragg grating sensor are studied.The results show that the average temperature sensitivity is 8.74pm/℃.The sensor has the advantages of simple preparation and easy operation,good stability,small size,low cost,and has a good development prospect in various fields in the future.（3）A Mach-Zehnder fiber curvature sensor with seven core fiber fused with multi-mode fiber was designed and fabricated.The preparation process of the sensor is introduced,and the structure of the sensor with different parameters is optimized.The sensor structure formed by the welding of the seven-core fiber with the size of 1.5cm and the multi-mode fiber with the size of 1.5cm is selected.The curvature sensing experiment and the temperature response experiment are carried out.The results show that when the curvature increases gradually,the spectrum shifts to the shortwave direction,and the curvature sensitivity is 6.493nm/m^-1in the curvature range of 2.273-3.610m^-1.As the temperature gradually increases,the spectrum shifts towards the long wave direction,and the temperature sensitivity is 0.0569nm/℃in the temperature range of 30-80℃.Through linear fitting,it can be seen that the response of the sensor to curvature and temperature has good linearity.The difference between temperature and curvature sensitivity is 2 orders of magnitude.It can be considered that in general,the sensor is less affected by temperature and can be applied in practical measurement.（4）A Mach-Zehnder fiber curvature sensor was prepared by cascade connecting seven core fiber with no core fiber.By optimizing the structure of different parameters,the sensor structure formed by cascade of 2cm seven-core fiber and 2cm coreless fiber was selected,and the curvature sensing experiment and temperature response experiment were carried out.The results show that blue shift occurs when the curvature increases gradually,and the curvature sensitivity is 5.758nm/m^-1within the curvature range of 2.273-3.789m^-1.When the temperature gradually increases,the spectrum is redshifted,and the temperature sensitivity is 0.162nm/℃in the temperature range of 30-80℃.In general,the sensor is also affected by temperature crossing when measuring curvature.