Femtosecond Tilted Fiber Bragg Grating Preparation And Refractive Index Sensing Characteristic Research

Titled Fiber Bragg Grating(TFBG)is an optical passive device with a special structure.It is characterised by a grating plane inscribed inside the fiber that is tilted at a certain angle with respect to the axial direction of the fiber.This structure allows complex inter-mode coupling between the core mode and the cladding mode or radiation mode of the fiber.As a result,changes in external parameters such as temperature,strain,refractive index(RI),etc.can be detected.It has attracted much attention in the field of sensing.This paper firstly introduces the current state of research and applications of Fiber Bragg Grating(FBG)and TFBG sensors at home and abroad.Next,coupled mode theory(CMT)and transmission matrix theory are used to analyze the mode coupling patterns,transmission spectral characteristics and sensing properties of small angle tilted gratings(8°)and large angle tilted gratings(81°).This paper then simulates and analyses the 8° TFBG with different grating parameters,summarises the effect of grating parameters on the spectral characteristics of the 8°TFBG,and simulates its sensing characteristics.Then,the principle and advantages of TFBG preparation by ultrafast femtosecond laser direct writing technique are analyzed.The effects of laser processing parameters and different grating structure parameters on the spectral properties of 8° TFBGs are investigated,including laser power,laser scribing speed,tilt angle,grating period,grating length and other parameters.The spectral properties of the 8° TFBG are summarised and the optimum grating parameters based on this femtosecond laser processing system are finally obtained.Finally,liquid RI sensors are prepared using tilted gratings as the main body,and then combined with other gratings or interferometric structures to produce dual-reference sensors,such as Mach-Zehnder Interferometer(MZI),FBG and 45°-TFG,to simultaneously measure liquid temperature and RI comparison and analysis of sensor structure,performance and spectral characteristics,summarising the characteristics of a variety of tilting gratings,the 8°TFBG liquid RI sensor has the advantage of a wide range and high sensitivity sensing,the 45°-TFG can be used as an all-fiber structure starter,and the 81°-TFG liquid RI sensor has the advantage of high resolution and accuracy.