Fabrication And Characteristic Analysis Of Two-mode Fiber Bragg Grating

Fiber Bragg Grating（FBG）is a very important optical fiber sensor.With the continuous improvement of FBG sensing network technology,FBG has been widely used in health monitoring of bridges,oil wells,tunnels and other projects.Traditional FBG can only be used to measure single physical quantities,such as temperature and stress,and cannot respond to multiple physical quantities at the same time,which limits the development of FBG sensing applications to a certain extent.At the same time,it is difficult to solve the problem of interference between different physical quantities,which hinders the application of grating sensing.Because of its own core modes,few-mode fibers have certain differences in different modes,which is of great research value in multi-parameter sensing.Therefore,a two-mode FBG is proposed and its characteristics are analyzed theoretically and experimentally.Based on the study of the reflection peaks formed by self-coupling of LP₀₁ and LP₁₁1 modes at FBG,a two-parameter sensing of temperature and curvature is realized.The research contents and results are as follows:（1）In this paper,based on single-mode optical fibers,COMSOL finite element analysis software is used to increase the number of modes by increasing the core diameter.Finally,a two-mode optical fiber with a diameter of 200μm is fabricated by drawing tower.Then,the reflection spectrum of two mode FBG is simulated by using MATLAB and OptiGrating software.It is shown that the three reflection peaks of two mode FBG are formed by the coupling of the core modes.Because the effective refractive index of LP01 and LP11 modes is obviously different,the three reflection peaks formed by self-coupling and mutual coupling of LP₀₁ and LP₁₁ modes have obvious separation at wavelength;（2）In this paper,the two-mode fiber is fabricated by drawing towers,and the writing of fiber grating is completed based on phase mask method.The reflection peaks of fiber grating with different diameters are measured,and the relationship between fiber structure and grating spectrum is studied experimentally.When the diameter of the fiber is 200μm,the grating has three distinct reflection peaks,which are located at1550 nm,1551 nm and 1552 nm,respectively.The experimental results verify the theoretical calculation results.（3）In this paper,the bending losses of LP01 and LP11 modes in two-mode fibers are calculated theoretically.It is found that under the same bending conditions,the bending losses of LP₁₁ modes are obviously larger than those of LP₀₁ modes.Based on this feature,the temperature and curvature of the two-parameter sensing system using dual-mode fiber grating is pm/℃,and the curvature sensitivity is-0.21 dB/m^-1.The fabrication of the two-mode fiber grating array with a length of 100 m and a grating spacing of 1 m is fabricated.By using this grating array,a quasi-distributed temperature sensing system is constructed,which can also respond to large curvature(>50 m^-1)region.It provides a feasible direction for expanding the application of grating array sensing.