Specialty Optical Fiber Bragg Grating Bending Sensor

Directional bending sensing has great significance in many fields,such as the aerospace and structural health monitoring,robot pose sensing,the deformation in the composite materials.The optical fiber sensors have received a great deal of interest due to its merit of small size,light weight,immunity to electromagnetic interference,low loss,low cost,ease of multiplex,embedability and so on.As a very importance sensing component,fiber Bragg grating(FBG)has been used in many optical fiber sensing field.Basing on the study of fabrication and sensing principle of specialty optical fiber Bragg grating,such as the eccentric-core fiber(ECF)FBG and multicore(MCF)fiber FBG,we propose several bending sensors based on FBG,which can be used for directional bending sensing with different functions.The main content of this paper are as follows:1.The transmission property of the multicore fiber is analyzed based on the coupledmode theory and the transmission property of the twin-core fiber is analyzed in details.The transmission property of bending multicore fiber is investigated by using the coupled-mode theory and coupled-power theory.2.The fabrication of the ECF FBG,twin-core fiber FBG,and triple-core fiber FBG by using the 248 nm UV laser and phase mask technique,the FBGs are fabricated at different inscribe directions,and the influence of the inscribe direction on the characteristic parameters of the FBGs is discussed.3.Basing on the investigation of the different bending responses of the ECF FBG and the twin-core fiber FBG at different bending directions,four different composite structures for one-axis directional bending sensing are proposed,the sensing principles of them are analyzed theoretically,and their sensing characteristics are experimentally demonstrated.These four structures are:(1)The one-axis directional pure bending sensor based on the cascading of ECF FBG and standard single mode fiber(SMF)FBG is proposed.This sensor has two kinds of structure,one is the two FBGs inscribed on the two side of the ECF-SMF fusing junction,and the other one is the two FBGs inscribed on the fusing junction and fabricated simultaneously.(2)The one-axis directional bending sensor based on twin-core fiber FBGs.The axial strain and temperature cross-sensitive with the bending can be eliminated by this sensor.Andits bending sensitivity is twice of that of the ECF FBG with the same material,geometry,and core-offset.(3)The Fabry-Perot(F-P)interferometer composed of two FBGs inscribed in one ECF is proposed.This sensor has the similar one-axis directional bending response with the ECF FBG,and can be used for one-axis bending sensing.If some material is coated on the ECF FP cavity,the F-P interferometer will have different strain and temperature sensitivities with the ECF FBGs,and the sensor can be used for one-axis directional bending and temperature simultaneous measurement,or axial strain and temperature simultaneous measurement.(4)The sensor based on ECF FBG cascaded with air cavity F-P interferometer is proposed,which can be used for one-axis directional bending and temperature simultaneous measurement.The ECF air cavity F-P interferometer can be used as one-axis directional bending sensor that eliminates the temperature cross-sensitive.4.The two-axis directional bending sensors based on ECF FBG and triple-core fiber FBG are proposed,the sensing principles of them are analyzed theoretically,and their sensing characteristics are experimentally demonstrated.The two structures are:(1)The two-axis bending sensor based on cascaded two ECF FBGs with their core-offset directions orthogonal to each other is proposed.The two-axis bending sensing can be realized by using the different bending responses of the two ECF FBGs at different bending directions.(2)The two-axis directional bending sensor based on triple-core fiber FBGs.The axial strain and temperature influence on bending measurement can be eliminated by this sensor.5.Two ECF Mach-Zehnder(M-Z)interferometers structures utilizing the asymmetric structure of the ECF are proposed and can be used of one-axis directional bending,refractive index,or temperature measurement.The two structures are:(1)The M-Z interferometer composed of a section of ECF lateral-offset spliced between two pieces of standard SMFs is proposed.This M-Z interferometer can be used for one-axis directional bending,refractive index,or temperature measurement,and the highest bending sensitivity of this sensor is much higher than that of the ECF FBG.(2)The composite structure based on ECF FBG cascaded with ECF M-Z interferometer is proposed.The ECF FBG is sensitive to the temperature but insensitive to the refractive index of the environment,and the ECF M-Z interferometer is sensitive to both the temperature and the refractive index of the environment,therefore,the simultaneousmeasurement of the temperature and refractive index of environment can be realized by this sensor.