| With the rapid development of optical fiber sensing technology,new devices and technologies are emerging constantly.Distributed sensing technology based on optical fiber white-light interference technology has been widely used in the field of intelligent structural health monitoring,such as fire engineering,large-scale building structural health monitoring and geological hazard monitoring.In the application of these fields,ordinary optical fibers are usually used as sensing media of optical fiber sensing network to realize distributed strain and temperature sensing.However,the low efficiency of optical coupling between optical fiber sensors and the limited capacity of sensor networks seriously limit the further development of white-light interferometry based on optical fiber in the field of distributed sensing.In this thesis,the weak reflection structure,a special structure of weak reflection signal,is constructed by using the refractive index isomerism in optical fiber.This weak reflection structure has the characteristics of small size,low intensity of reflection signal,low insertion loss and high efficiency of optical signal utilization.It solves the problems of limited capacity of optical fiber white-light interference sensor network and low integration of sensors.In this thesis,the heterogeneous reflection in fiber is studied from the following aspects:Firstly,a weak reflection structure in fiber due to the heterogeneity of refractive index is proposed and several typical weak reflection structures in optical fiber are discussed.Fresnel reflection method is used to analyze and discuss the reflection characteristics of weak reflectors formed by fusion of several typical refractive index fibers,including graded index fiber,triangular index fiber and step index fiber.By theoretical simulation,the refractive index distribution at the interface of the fused fiber is calculated,and the reflectivity distribution of the weak reflective structure formed at the fused joint is obtained.The effect of various parameters on the reflective characteristics of the weak reflective structure is discussed.Secondly,the effect of thermal diffusion effect on the fusion point of optical fiber is verified by theoretical simulation and experiments.The influence of thermal diffusion of germanium ions doped in the core of single mode fiber on the weak reflection structure formed at the fusion point is discussed.The weak reflection signal produced in the fusion process of single mode fiber is very low,only 10-12 orders of magnitude,so it is difficult to make effective use of it.In this thesis,a cascaded white-light interferometric sensing technology based on heterogeneous optical fiber is proposed.The physical quantities can be sensed by using the weak reflection structure formed in the fusion process of heterogeneous optical fibers(whose reflectivity can reach 10-5 orders of magnitude).Due to the thermal diffusion effect in the fusion process of optical fibers,the weak reflection structure formed in the fusion process of heterogeneous optical fibers is a continuous changing interface.The influence of the fusion process on the fusion point of heterogenous optical fiber is verified by theoretical simulation and experiments.It provides a practical technical means for analyzing the reflection characteristics of the weak reflection structure formed at the fusion point of heterogeneous optical fibers.Thirdly,a large-capacity fiber optic integrated sensor array based on the weak reflection cascade formed by the fusion of heterogeneous optical fibers is proposed,and quasi-distributed temperature and strain sensing is realized.Three kinds of heterogenous fiber cascade sensor arrays are constructed using single mode fiber,double cladding fiber and fluoride cladding fiber.The principle of temperature and strain sensing and the weak reflection characteristics at the fusion point of heterogeneous optical fiber are analyzed and discussed,which are verified by experiments.Due to the lower reflectivity,less insertion loss and better efficiency of the weak reflection structure in the sensor array,the capacity of the sensor network is greatly improved by multiplexing more sensors in the sensor network.In addition,because the sensor array is fully integrated into an optical fiber,there are no redundant connectors.Therefore,the sensor array constructed by cascaded heterogeneous optical fibers is compact and high temperature resistant,which is very suitable for quasi-distributed temperature and strain sensing under severe conditions such as high temperature and high pressure.Finally,aiming at the problem of low sensitivity of in-fiber integrated interferometric temperature sensor,a high sensitivity in-fiber integrated high temperature sensor based on weak reflection structure is proposed.By reasonably designing the coating parameters of single mode fiber,the temperature sensing sensitivity can be improved by multiple reflections of the input light in the FP cavity composed of two single mode fiber coated with gold film.With the multiple reflection of light wave in FP cavity,the intensity of reflected signal decreases gradually,and weak reflection signal is formed.However,as the number of reflections increases,the optical path of reflected light increases gradually.Temperature sensitivity of temperature sensor based on white light interference technology is usually related to optical path,and the greater the optical path,the higher the sensitivity of temperature sensor.Therefore,the temperature sensitivity of the sensor increases gradually with the multiple reflections of light in the FP cavity.Through theoretical simulation and experiment,the influence of various parameters of the sensor on the sensor is verified to optimize it Experiments show that the optical fiber integrated high sensitivity sensor has the advantages of high sensitivity,compact structure and simple fabrication.It is very suitable for applications with limited space size,high sensitivity and temperature sensing at high temperature. |
