Research On Fabrication And Characterization Of Long Period Fiber Gratings Based On Structural Perturbation

Long-period fiber grating?LPFG?is a widely used sensing device.In this thesis,two kinds of LPFG composite structures based on structural disturbance are proposed.One is a composite structure cascaded with LPFG,which has the characteristics of wave division by using the microstructures such as melt torsion and taper torsion.Part of the energy in the core is injected directly into the grating cladding as it passes through the microstructure.Thus the degeneracy of phase velocity between the original modes is destroyed.The microstructures and the LPFG constitute Mach-Zehnder Interferometer?MZI?,to obtain the transmission spectrum which is easy to measure by adjusting the distance between the microstructure and the LPFG,and the sensing characteristics of torsion,taper torsion and so on are measured.The other is the composite structure of LPFG with local melting ball,torsion,cone and so on.From the fabrication of melting fiber ball,torsion and cone,it can be known that they will lead to the deformation of the core and break the degeneracy of the phase velocity between different modes.Then the temperature,stress and bending characteristics of the LPFG composite structure sensor fabricated by the above two methods are measured,and good experimental results and practical application value are obtained.First of all,the coupling mode theory,Mach-Zehnder theory based on fibers and phase-shifted LPFG principle are simply analyzed and summarized,and the theoretical approach of LPFG in the research process is summarized.The theory of partial sensing characteristics of LPFG which is based on the coupled mode theory is briefly discussed,including Mach-Zehnder principle and LPFG phase shift principle.Based on the above theory,two LPFG composite structures based on structures energy perturbation are proposed.Secondly,the experimental results show that the cascaded LPFG composite mechanism has directional correlation,the wavelength moves of the two resonant peaks to the direction of long wave when it twists clockwise,the sensitivity is 0.058nm/(rad?m^-1)and0.117 nm/(rad?m^-1),respectively;When the sensor is twisted counterclockwise,the wavelength moves to the direction of shortwave,the sensitivity is0.055 nm/(rad?m^-1)and0.262 nm/(rad?m^-1),respectively.The measured torsion and temperature sensitivity are substituted into the matrix of simultaneous measurement is obtained:when twists clockwise,the resolution of torsion and temperature is 0.04 rad/m and 0.49°C,respectively;When twists counterclockwise,the resolution of torsion and temperature is 0.01 rad/m and 0.43°C,respectively.It is verified that the proposed structure performs well in the resolution of temperature and torsion.At different temperatures,the average sensitivity standard deviation when twisting clockwise of two resonant peaks is 0.0027nm/(rad?m^-1)and 0.0049nm/(rad?m^-1),respectively and the average sensitivity standard deviations when twisting counter-clockwise of two resonant peaks are 0.0013nm/(rad?m^-1)and 0.0222nm/(rad?m^-1),respectively.In addition,the temperature and strain characteristics of another cascaded LPFG structure,the fused cone twisted cascade LPFG structure,are tested respectively.It is found that the temperature sensitivity of the two peaks of the structure is similar.When the strain is applied,the wavelength sensitivity and amplitude sensitivity of the two resonant peaks are quite different,so it can be proved that the structure can be measured with two parameters.Finally,three kinds of LPFG composite structures based on phase shift principle are proposed.The local microstructures used are melt ladle,torsion and cone respectively.The temperature,bending and strain characteristics of the three composite structures were measured.The experimental results show that all the three structures can disturb the cladding of the original LPFG and split the transmission spectrum into two peaks.The results show that the local microstructure does not interfere with the temperature characteristics of LPFG.However,among the bending and strain characteristics of the three structures,one peak has a great improvement on the LPFG characteristics before the introduction of the microstructure,which indicates that the local application of the microstructure has an effect on the bending direction and tensile strength.Because of the great difference between bending and strain characteristics,but the difference of temperature sensitivity is small,it can be used to measure the three kinds of structures with multiple parameters.