Research On The Tail Ring-shaped Cascaded Long Period Fiber Grating Sensor

The long period fiber grating is a kind of fiber grating whose refractive index is periodically modulated by the fiber core.The core mode is coupled with the forwardpropagating cladding mode,and it is a kind of transmission type fiber device.The resonance peak of long period fiber grating is sensitive,stable and has high resolution to the changes of strain,temperature and bending of the external environment.Due to the good physical properties of fiber,such as anti electromagnetic interference,high temperature resistance,corrosion resistance,light weight and low price,it can be used as an ideal sensor,which is widely used in electric power,ocean shipping and aviation Aerospace,nuclear industry and petrochemical,medical biology and other fields.In this paper,based on the mode coupling theory,transmission matrix method and finite element method,the sensitization method of long period fiber grating sensor is studied.The influence of fiber period,coupling mode,cladding thickness and core size on the refractive index sensitivity of long period fiber grating sensor is analyzed by using the control variable method.The spectral characteristics,refractive index,temperature and bending sensing characteristics of the long period fiber grating and the tail ring-shaped cascaded long period fiber grating are compared by theory and experiment.The results show that the tail ring-shaped cascaded long period fiber grating sensor has certain advantages in production process and application.The main contents of this paper are as follows:(1)The development of long period fiber gratings is briefly introduced.In this paper,different methods of fabricating long period fiber gratings are introduced,and the advantages and disadvantages of each method are analyzed.The research progress of long period fiber grating sensor in magnetic field,curvature and temperature in recent years is reviewed.(2)The mode coupling theory of long period fiber grating is analyzed.Based on the Maxwell equation,the wave equation describing the transmission of electric field and magnetic field in optical fiber is derived.The solution of field and characteristic equation is solved by combining the boundary condition.Different solutions correspond to the propagation constants of different modes,and the orthogonality of each mode is analyzed.The mode coupling theory of long period fiber grating is analyzed by using the three-layer mode theory of weakly guiding fiber.The main mode coupling,the mechanism of producing resonance peak and the transmissivity of specific wavelength in long period fiber grating are explained.(3)The refractive index sensitivity of the long period fiber grating sensor with different cladding radius,core radius and grating period is theoretical analyzed by using the control variable method,and the response of the resonance peak coupled with different order modes of a long period fiber grating to the change of external refractive index is also analyzed.The sensitivity of the sensor increases with the decrease of the cladding radius.The results show that the longer period fiber grating with larger core has higher refractive index sensitivity.The refractive index sensitivity of long period fiber grating with larger period is higher for the resonance peak coupled with the same order mode,and the sensitivity of resonant peak coupled with higher order modes is higher.(4)The interference principle and fabrication method of the tail ring-shaped cascaded long period fiber grating are analyzed in detail,and the advantages of the sensor in fabrication and application are analyzed.The influence of grating length,refractive index modulation depth and grating spacing on the transmission spectrum is studied,and the transmission spectrum of a single long period fiber grating is compared.The results show that with the increase of grating length or refractive index modulation depth,the loss intensity of resonance peak increases first and then decreases gradually,while the resonance wave hardly moves laterally.With the increase of the length of the ring fiber,the number and fineness of the interference fringes increase,and the full width of the half peak gradually narrows.(5)The refractive index,temperature and bending characteristics of long period fiber grating sensor are analyzed theoretically.The sensing characteristics of long period fiber grating sensor and tail ring-shaped cascaded long period fiber grating sensor are studied experimentally,the results show that the latter has better sensing characteristics.The measurement system,process and method are introduced in detail.