Research On Fiber Strain And Curvature Sensor Based On Mode Interference

With the rapid development of urbanization,lost of construction projects are being carried out in many places.While the engineering buildings may subject to large deformations during construction and use because of the problems in scheme design,construction quality and daily maintenance,etc.,which induce the safety accidents eventually.Structural health and safety monitoring is related to the safety of people's lives,thus optical fiber sensing technology is gradually becoming an important means in structural health monitoring.Curvature and strain are two key indicators for structural health and safety monitoring.Among them,the optical fiber strain and curvature sensor based on mode interference has the advantages of simple structure and anti-electromagnetic interference.Therefore,many scholars have conducted extensive research on this.The main research works of this paper are as follows:(1)The three-core fiber with a triangular core distribution is fusion spliced with the thin-core fiber(TCF),and the middle part of the three-core fiber is tapered to form a Mach-Zehnder interferometer based on the three-core-thin-core fiber structure for strain monitoring.Through experiments,the optimal length of three-core optical fiber and thin-core optical fiber are explored,besides,the best parameters of the fusion splicer for the taper of three-core optical fiber and the influence of temperature change on the sensing structure are also studied.The maximum sensitivity of the sensing structure in the strain measurement range of 0-1600??is-3.47 pm/??,having high linearity.(2)Using the manual fusion splicing method of the optical fiber fusion splicer,the four-core optical fiber(FCF)and the single-mode optical fiber(SMF)are eccentrically spliced to obtain an optical fiber Mach-Zehnder interferometer.The Rsoft software simulates and analyzes the mode transmission and light field distribution in the sensing structure to get the best eccentric welding distance.In the experiment,the response of the sensor to the curvature in different bending directions is tested,and it is found that the sensors in different bending directions have different sensitivity.The maximum sensitivity in the curvature range of 0.042-0.163 m^-1is-18.75 nm/m^-1,and the linearity can reach to0.9743.The influence of temperature changes on the curvature measurement is explored,and a two-parameter coefficient matrix is constructed to eliminate the interference of temperature changes.(3)A Mach-Zehnder interference structure based on the eccentric fusion splicing of the FCF and the no-core fiber(NCF)is fabricated.By adjusting the splicing advancement distance,a thick fusion splice is formed in the structure so that more light can couple into the FCF.In the experiment,the temperature and refractive index sensing characteristics of the structure are explored,and it is found that the temperature mainly modulates the wavelength of the monitoring spectrum,and the curvature mainly affects the intensity of the spectrum,proving that the sensor can realize the dual-parameter measurement of curvature and temperature.The maximum linear sensitivity of curvature and temperature is48.83 d Bm/m^-1and 53.81 pm/?,respectively.(4)The Michelson fiber curvature sensor is constructed from seven-core fiber and NCF.A layer of silver film is coated on the end face of the seven-core fiber by physical vapor deposition(PVD)coating equipment,and encapsulated by UV glue and capillary tube.The monitoring of reflection spectrum help reduce the size of the optical fiber sensing structure.Experimental test results show that in the curvature range of 0.0419-0.1390 m^-1,the sensitivity of the sensor is about-22.76 nm/m^-1,and the linearity can reach to 0.9823.