Fiber Bragg grating(FBG)is a kind of passive fiber device with periodic modulation and distribution of refractive index.It has many advantages such as high sensitivity,small size and antielectromagnetic interference.It can be widely used in fiber sensing and fiber communication.However,it is found that the grating characteristics of general FBG will gradually deteriorate or even be completely erased when it works in high temperature environment for a long time,which greatly limits the application of FBG in some special fields such as industrial production,petroleum and electric power,aerospace and so on.By annealing at high temperature,FBG can be erased at high temperature and regenerate into regenerated Fiber Bragg grating(RFBG)that can work stably at high temperature.However,in the process of FBG thermal regeneration,fiber embrittlement will occur,which leads to a serious decline in its mechanical properties and makes it impossible to carry out stress-strain related sensing applications.Therefore,it is a great significance to study the effects of high temperature annealed on the performance of RFBG.In this paper,starting with the basic principle of FBG,the fabrication,packaging and sensing characteristics of general FBG are studied.Furthermore,the evolution process of thermal degradation and thermal regeneration of FBG under high temperature was studied,and the high-temperature embrittlement mechanism of RFBG was discussed.Finally,simultaneous temperature and stress-strain measurements of structural components of pressure vessel in an extended temperature range up to 900℃ were achieved with the optimized RFBG.The main research contents are as follows:(1)The writing system of Fiber Bragg Grating and the real-time testing system of RFBG thermal regeneration and high temperature reflection spectrum are built.Based on a 248 nm excimer laser,an initial FBG with a reflectivity of 97.8% is produced by the phase mask method.It is found that FBG achieves thermal regeneration at 950℃,and an RFBG with a reflectivity of 50.6% is obtained.(2)Through theoretical analysis,the reasons for the mechanical properties of RFBG after high temperature treatment are speculated,and it is found that the annealing process after high temperature thermal regeneration has a great influence on the properties of RFBG.The post-regeneration FBGs were annealed in different ways and implemented experimentally.Fiber tensile test results show that the mechanical strength of RFBGs treated in rapid cooling and argon atmosphere reaches 49% and56.5% of the reference FBG respectively,the mechanical strength of RFBG is nearly 36 % higher than that of ordinary method,indicating that rapid cooling or air isolation is beneficial to the enhancement of the mechanical strength of RFBG.The high-temperature embrittlement mechanism of RFBG is further explored according to XRD patterns.It is considered that the crystallization of amorphous quartz is the main cause of fiber embrittlement during the process of high-temperature annealing of RFBG,and the defects induced by the interface mismatch between crystalline regions and amorphous regions lead to the reduction of mechanical properties of RFBG.Further tests on thermal cycling and thermal stability of the RFBG treated with natural cooling in an argon atmosphere.The results show that the results of the three heating cycles of RFBG at 150℃~1050℃ completely overlap,the temperature sensitivity is 16.30 pm/℃ and a wavelength drift at high temperature of less than 0.1 nm.(3)Simultaneous temperature and stress-strain measurements of structural components of pressure vessel in an extended temperature range up to 900℃ were achieved with the RFBG.The corresponding temperature sensitivity and stress-strain sensitivity are 15.90 pm/℃ and 1.21 pm/με,respectively.The packaging of RFBG was explored,304 stainless steel tube and single component ultra-high temperature adhesive were selected as the packaging material.The temperature sensitivity of RFBG after packaging was measured to be 16 pm/℃,which not only maintained good temperature characteristics but also protected the grating well. |