Preparation And Sensing Characteristics Of Long Period Fiber Grating Based On Dislocation Fusion

Long period fiber grating(LPFG),as a common passive optical device,has the advantages of wavelength selectivity and thorough processing technology.LPFG can undertake the measurement of parameters such as temperature,strain,refractive index,humidity,etc.However,the common preparation methods of LPFG often require the use of expensive lasers,such as ultraviolet lasers,carbon dioxide lasers,femtosecond lasers,etc.,which increases the preparation cost of LPFG to a certain extent.In this thesis,we fused multiple segments of quartz single mode fiber with misalignment fusion,and effective refractive index modulation is achieved by using the refractive index difference between the fiber core and the cladding.Therefore,a low-cost LPFG is developed.In this thesis,the structural parameters of LPFG are designed and optimized using the beam propagation method(BPM).Two types of LPFG,grating period fixed and chirped,were experimentally fabricated based on the misalignment fusion method.The sensing applications of temperature,strain,and refractive index are investigated.The main research contents of this thesis include:1.The classification,main preparation methods of LPFG and their advantages and disadvantages are described.The Helmholtz equation is derived from Maxwell’s equations and the matter equation.Based on the dispersion equation in the fiber,the mode field distributions of cladding modes in the core,inside and outside the cladding of a single mode fiber are analyzed,and the sensing characteristics of environmental refractive index,strain,and temperature of LPFG are analyzed.2.A LPFG sensor based on dislocation fusion is proposed.The transmission spectrum of LPFG structure was simulated using beam propagation method,and the parameters such as grating period,dislocation distance,and grating length have been optimized.The fabrication of LPFG sensors with dislocation fusion was realized experimentally using fiber fusion splicing through dislocation fusion method.And the sensing characteristic of its environmental refractive index,strain,and temperature were measured.The experimental results showed that the refractive index,strain and temperature sensitivities of the misalignment spliced LPFG sensor is determined to be 992.8 nm/RIU,-7.4 pm/με,and-31 pm/°C,respectively.3.A chirped long period fiber grating(CLPFG)strain sensor based on misalignment fusion is proposed.The structural parameters of CLPFG were optimized using the beam propagation method,and the shape change of different optical fiber dislocation structures under tension was simulated using the finite element method.The strain and temperature sensing characteristics of CLPFG sensor were measured.The experimental results show that chirped structures have a significant improvement in strain measurement sensitivity,which is about 71 times higher than non-chirped structures.The specific strain sensitivity is-104.6 pm/ με,and the temperature sensitivity is-10 pm/°C.In this thesis,the preparation of LPFG and CLPFG based on dislocation fusion was achieved through dislocation fusion method,and the corresponding sensing characteristics of refractive index,strain,and temperature were studied.The dislocation fusion method provides a new processing method for the preparation of LPFG,which achieves high sensitivity sensing while reducing the preparation cost of LPFG.