| Fiber Bragg Grating(FBG) sensors have undergone a rapid development in application of the optical communication and fiber sensing in recent years due to their prominent advantages over conventional sensors, for instances, small size, low splice loss, corrosion resistance, immunity to light-intensity variation, realization to multipoint distribution measurement, facilitation to single source of WDM and electromagnetic interference. Additionally, the peak wavelength of FBG is sensitive to the change of physical quantity, such as temperature, strain. However, optical fiber(mainly composed of SiO2) is so fragile that it is easily to be destroyed in the application. What is more, the temperature sensitivity and strain sensitivity of bare FBG are too low to satisfy the requirement of high temperature and strain resolution for the application. Hence, it is necessary to protect the FBG prior to the application and enhance the temperature or strain sensitivity. In order to enhance the temperature and strain sensing characteristics, as well as successfully package FBG into the inside or on the surface of metal structure, the related work needs further explored. In this study, the encapsulating process and positioning erection technology were systematically studied. The main contents of this study are as follows.1) To enhance temperature sensitivity of the bare FBG, a capillary encapsulating method was developed. Bare FBG sensors were encapsulated by zinc capillary and the associated temperature sensing characteristics were studied. The central wavelength, transient response speed, stability of central wavelength and temperature sensitivity of the FBG before and after capillary encapsulating was systematically studied. Results show that:(i) The central wavelengths of the encapsulated FBGs were shifted 0.1~0.2 nm;(ii) The temperature transient response speed of the encapsulated FBG remained the same as before;(iii) The central wavelength stability of the encapsulated FBG was slightly reduced, resulting measure fluctuations about0.3 °C;(iv) The temperature sensitivities of the FBGs encapsulated by different polymer and zinc capillary were 18.25 pm/°C?21.87 pm/°C?35.13 pm/°C, which were enhanced about 1.9, 2.3, 3.6 times as that of a bare FBG, respectively. Atemperature sensing model of the encapsulated FBG was presented. Comparing the experimental results with the calculated values, the relative errors were less than 10%,which verified the feasibility and reliability of the temperature sensing model. Based on the sensing model, the effects of encapsulation material characteristics on the temperature sensitivity were analyzed.2) The electroless-electro plating method for FBG metallization process was studied in this part. The Cu-coating, Ni-coating and Co-coating were electroless plated on FBG. Additionally, the Cu-coating and Ni-coating were electroplated on metal coated FBG. To study the effects of metal coatings on the temperature sensitivity, the temperature sensing experiment was conducted. Results show that:(i)The temperature sensitivities of Cu-, Ni-, Co-coated FBG were 23.83 pm/°C?19.22pm/°C and 11.95 pm/°C, respectively;(ii) The failure temperature of the metal coated FBG is checked, which is about 100°C higher than that of bare FBG, showing a level of up to 900 °C;(iii) Within a certain range, the temperature sensitivity increases as the thermal expansion coefficient, Poisson's ratio or elastic modulus increases,respectively.3) FBG sensors were chemical plated and electroplated with Ni-coating, and then mounted on surface of the copper substrate, TC4 substrate and aluminum substrate by using brazing/soldering. The microstructure of brazing/soldering joint was observed. In addition, the central wavelength and spectral characteristic were investigated. Results show that:(i) the central wavelength shifts were 3.281 nm,8.252 nm, 5.784 nm, respectively;(ii) the temperature sensitivities were enhanced about 3 times as that of a bare FBG, showing a level of up to 26.9 pm/°C, 22.77pm/°C, 28.65 pm/°C, respectively. A finite element model(FEM) simulation for the FBG packaging by using brazing/soldering was investigated. The focus was on the brazing/soldering embedding processes, their impacts on the wavelength change of FBGs during the processes. Simulation results show that, from 230 °C to 30 °C, the central wavelength of surface mounted FBG shifted down a level of 4.17 nm, which was similar with the experiment result of 3.281 nm. As a result, the simulation analysis is reliable to analyze the central wavelength shift after mounting FBG on different metal substrate, which is helpful for calculating the residual stress of themetal coated FBG after welding.4) An electroplating method for mounting metal coated FBG on the surface of metal substrate was presented. The Cu-coated FBG and the Ni-coated FBG were mounted on the surface of Copper substrate and Q235 substrate by using the electroplating method, respectively. Additionally, the mechanical property of the electrolytic joint is investigated and the interface of the joint is checked. Moreover,the temperature and strain sensing characteristics as well as the failure temperature of the surface attached FBGs are systematically studied. Results show that the strength of the electrolytic joint was higher than that of the optical fiber with Cu-coating,which is about twice larger than that of fibers with acrylate. Therefore, the good mechanical property of the joints is confirmed. The spectrum changes little, without waveform distortion. What is more, the central wavelength shifted down within 1 nm,which indicated that the residual compressive stress was small. The temperature sensitivity was 30.6 pm/°C, which was enhanced about 3 times as that of a bare FBG.The strain sensitivity was much higher than that of a bare FBG, reaching to 4.998 ×103 pm/??. The conducted experimental results show that the electroplating method for attaching the FBGs on the surfaces of the metal structures is effective and holds great potential application in high temperature harsh environments. |
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