Research On The Thermal Rebirth Mechanism And Performance Of High Temperature Resistant Fiber Bragg Grating

Fiber Bragg Grating（FBG）are optical passive devices with a periodic modulation of the refractive index in the fiber core.They have many advantages such as high sensitivity,small size,and immunity to electromagnetic interference,and can be widely used in fields such as fiber sensing and communication.However,the high-temperature resistance has always been a bottleneck for the application of FBGs in high-temperature environments.Regenerated Fiber Bragg Gratings（RFBGs）are FBGs that are regenerated by high-temperature annealing treatment after being erased by high temperatures,and can operate stably in high-temperature environments.However,research on RFBGs has not yet addressed the influence of spatial volume on FBG regeneration,and the mechanism of thermal regeneration is not clear.Therefore,based on the basic principles of FBGs and the research on the preparation and sensing characteristics of conventional FBGs,this paper focuses on the study of the thermal regeneration process of initial FBGs under different spatial volumes,and explores the possible factors and mechanism that affect RFBG regeneration.Finally,using RFBGs with superior mechanical properties,high-temperature temperature monitoring and deformation monitoring of metal structural components were achieved.The specific work mainly includes:（1）Initial FBGs with a reflectivity of about 95%were prepared using the phase mask method.Then,high-temperature annealing treatment was conducted,and it was found that the initial FBGs were thermally regenerated at 950℃,ultimately resulting in an RFBG with a reflectivity of 50.6%.The experiments on different annealing methods for preparing RFBGs were studied,and it was found that the heating rate,final temperature,and cooling rate all have important influences on the performance of RFBGs.Finally,a heating rate of 10℃/min to 950℃was determined as the annealing condition.（2）In response to the unclear regeneration mechanism of RFBGs,the thermal regeneration phenomenon of FBGs in limited spatial volumes was studied.The results showed that as the spatial volume decreases,the regeneration rate of RFBGs decreases and even the phenomenon of unregenerated initial FBGs occurs.It was concluded that a spatial volume of 5.65 cm³is the critical value for FBG regeneration.Additionally,experiments on the annealing of initial FBGs at 200℃indicated that the release of hydrogen and excessive environmental pressure during the regeneration process are factors hindering FBG regeneration.The study suggests that changes in external pressure cause changes in the internal stress distribution of the fiber,thus slowing down or eliminating the periodic stress distribution,ultimately resulting in the attenuation or disappearance of the RFBG grating.This theory conforms to the stress-induced model.（3）Regarding the issue of reduced mechanical properties of RFBG after high-temperature treatment,XRD testing revealed that the main reason for poor mechanical properties of RFBG was the increase in fiber brittleness due to the crystallization of amorphous quartz after high-temperature annealing.By studying the effect of different pressures on the performance of RFBG,it was found that pressure has a significant impact on the mechanical properties of RFBG,with lower pressure resulting in better mechanical properties.RFBG annealed at a pressure of 2.0 E^-1Torr retained 50.3%of the initial mechanical strength of FBG,and a relationship between the mechanical properties of RFBG and pressure was established as y=-0.02194x+3.605.On this basis,temperature and deformation testing was performed on RFBG with superior mechanical properties.Additionally,temperature and stress-strain measurements were carried out on rectangular metal components in a high-temperature environment using RFBG,yielding a temperature sensitivity of 16.30 pm/℃,a stress-strain sensitivity of 1.2 pm/με,and a wavelength deviation of less than 0.1 nm.