Sensing Characteristics Investigation And Fabrication Of Structural Modulation Ultra-long Period Fiber Grating

With the rapid development of fiber grating manufacturing,the production of new fiber grating is a hot topic in this field.The modulated fiber grating obtained the great interest of researchers because of its practical advantages such as the preparation of a flexible structure,thermal stability and the absence of hydrogen load.The present thesis based on the structural modulation of ultra-long period fiber grating(SULPG),in which we established the SULPG theoretical model based on the local coupled-mode theory and have successfully fabricated SULPG by the experiments and also we have discussed the sensing properties of SULPG.Furthermore,by using the fused biconical taper technology to inscribe the structure on the tapered fiber of ultra long period to modulate ?-phase shifted fiber grating(SPS-ULPG)and based on the characteristics of the modulated ultra long period fiber grating,four interferometers modes are constructed by combining different structures with SULPG which can be used to measure the double parameter sensing by using their spectral characteristics.The main research contents are as follows:1?We analyze the mode pattern distribution and transmission characteristics of the SULPG based on local coupled-mode theory and detailed analysis of spectral characteristics of the SULPG is presented in a conventional single mode fiber and tapered fiber as well.Also we calculated the fiber core mode and cladding mode coupling coefficients by the refractive index distribution and mode pattern distribution with the variation of fiber geometry deformation area.2?We fabricated two kinds of SULPG by means of periodic modulation optical fiber core and cladding geometrical structure to form the SULPG which is a new idea than the traditional concept that fiber grating can only be formed in the fiber core.We tested that SULPG could be inscribe in a conventional single mode fiber and also in a tapered fiber to measure the temperature,strain,bending,torsion and refractive index sensing properties.We studied the sensing properties of two kinds of structures through the experiments and found that reducing taper waist diameter of the tapered fiber can increase the average sensitivity of the external environment.3?We designed the structural ultra-long period of ? phase-shifted fiber grating with high temperature and the strain sensitivity.This technology is simple,flexible,and able to accurately control the position of the phase shift and length of tapered fiber.Moreover,the varying responses of temperature and strain are different from the wavelength and amplitude of the sensor so the simultaneous measurement of temperature and strain can be calculated in four ways.We found the sensing double parameters properties of SULPG of ultra-long period? phase-shifted fiber grating through testing the temperature sensitivity under different strain variations and obtainined the temperature and strain sensitivity cross correlation.4?SULPG can constitute four kinds of composite structures mode interferometers,at first we analyzed the formation mechanism of four mode interferometers,and achieved the simultaneous measurement of temperature and strain.Depending on the instrument specifications we used average sensitivity coefficients to calculate the resolution of the temperature and strain.