Mode Selection In Optimization Design Of Refractive Index Sensing Based On Fiber Grating

Long period fiber grating(LPFG)has the advantages of simple preparation,low insertion loss,easy integration,low cost,and sensitivity to multiple environment parameters.In recent years,LPFG becomes one of the most important waveguide devices in optical fiber sensing.In the viewpoint of coupled-mode theory,the core guided mode couples with the co-propagating cladding modes in the LPFG,resulting in a set of resonance peaks in the transmission spectrum.The resonance wavelength and the depth of each resonance peak are related to ambient refractive index.The advantages of traditional coupled-mode theory,such as concise mathematical form and intuitive physical concepts,have been fully reflected in the study of discrete guided mode couplings in LPFG.When exploring the couplings involving continuous radiation modes,solving radiation modes is more complicated,and the treatments of both orthogonality and normalization are more difficult.To solve these problems,the complex coupled-mode theory is presented to extend the traditional coupled-mode theory to the complex domain by using discrete quasi-leaky modes to express the radiation modes equivalently and then use the complex coupled mode equations to describe the couplings of guided and radiation modes.By means of solving analytically the complex coupled mode equations of LPFG,'loss factor' is defined by the attenuation constant of the leaky mode participating in the coupling and the coupling coefficient of the mode coupling,and the influence of the 'loss factor' on the sensing performance is deeply explored,thereby the sensing optimization based on the radiation mode coupling in LPFG is proposed.As a new type of fiber grating,chiral fiber grating(CFG)has frequency selection similar to the conventional fiber gratings,and has greater flexibility in selecting fiber materials,which can avoid the structural damage caused by strong radiation such as ultraviolet radiation or high temperature.Owing to the prominent characteristics and the potentials in sensing application,CFG gets widespread attention.The systematic theoretical analysis of the characteristics of the refractive index sensing based on chiral long period fiber gratings(CLPFGs)will necessary for sensing design.Based on solving analytically the CLPFG complex coupled mode equations,exploring the physical meaning of the loss factor and its effect on the coupling between modes will help to discover new phenomena,reveal new mechanisms,and stimulate new applications.In this paper,based on mode selection,the optimization design of refractive index sensor based on both conventional and chiral LPFGs will be studied.The thesis mainly includes four aspects as followings:1)By solving analytically the LPFG complex coupled mode equations,the impacts of the loss factor on the transmission characteristics are investigated deeply.Starting from mode selection,the LPFG parameters are optimized further.Meanwhile,combined with the analysis of the previous experiment results of LPFG high refractive index sensing,an improved optimization is proposed,and the sensing performance based on radiation mode coupling in LPFG is improved in both sensing sensitivity and sensing linearity.2)Based on the improved optimization of LPFG high refractive index sensor,the influences of the angular order of cladding mode on the grating parameter design and then the sensing performance is discussed emphatically.Besides,the changes of the sensing performance before and after using the improved strategy are analyzed comparatively.When the ambient refractive index changes from 1.47 to 1.55,the theoretical sensing resolution is increased from the original order of magbitude of 10-4 to the present 10-5.3)In the critical region where the cladding guided mode changes to the radiation modes,the effective refractive index changes abruptly with the change of environment refractive index.Using this characteristic,and by selecting modes,the high sensitivity are realized for both low and high refractive index sensor based on LPFG.4)Using mode selection,low refractive index sensing based on CLPFGs are analyzed.By solving analytically the CLPFG complex coupled mode equations,three different coupling states are defined according to different value ranges of loss factor,and the transmission characteristics of CLPFG at different coupling states are explored,which lays a theoretical foundation for the optimization of high refractive index sensing based on radiation mode coupling in CLPFG.