Research On The High Temperature Sensor Based On Fiber Bragg Gratings With Low Reflectivity

Fiber Bragg grating sensors have been widely applied in many fields,because of obvious advantages such as high sensitivity,reusability,anti-electromagnetic interference and so on,in particular,researchers have paid more and more attention on distributed sensing.Distributed sensing system based on ? type fiber gratings are only applicable in atmospheric temperature,because the grating reflection intensity will decline gradually until completely erased with the temperature and sensing time increasing over critical temperature.This feature has been greatly limited the application environments,therefore,research of fiber gratings that can work in high temperature have broad prospects.The Regenerated fiber Bragg gratings can work stable at high temperature,and the reflective spectrum is consistency well with the initial grating,which is a perfect candidate for distributed optical fiber grating sensor.Based on the background and application requirement before-mentioned,the research object was about the regenerative weak fiber grating,and focus on the technology of production,methods of annealing and sensing properties for high temperature.The main work and results are as follows:(1)The current progress of high temperature resistant fiber grating was introduced,mainly focused on the regeneration of fiber gratings and its protection.(2)The basic principle of weak fiber grating,mainly including transmission theory and sensing theory was summarized.The reflective features and sensing characteristic were simulated by MATLAB software with the theory mentioned.The on-line writing weak fiber Bragg grating arrays by Draw-tower was described,the type of fiber which would be used in the experiment and the fabrication method of seed gratings was determined.The decay mechanism and regeneration mechanism were followed at last.(3)Fiber grating regeneration scheme was established.First,the seed gratings with reflectivity about 1%~3% were annealed after hydrogen loading,which were fabricated on single-mode fiber,boron germanium co-doped fiber and high germanium doped fiber,and regenerated gratings were obtained from seed gratings with reflectivity about 2% and above.Furthermore,regenerated grating with higher physical strength was also grown from seed grating by means of fast thermal which was coated 100 nm gold film by magnetron sputtering.Finally,a regenerated grating array composed by two gratings was made,and the weak Bragg grating regeneration mechanism was analyzed.(4)The regenerated gratings were tested to explore its temperature sensing characteristics,including temperature response,high temperature stability,high and low temperature cycle stability.The results show that regenerated gratings have perfect temperature sensing characteristics no matter which annealing process.Regenerated gratings show a higher sensitivity above 300? with linearity fitting more than 99%,the grating reflected intensity begin to decay over 1000?,and undetectable at 1200?.The regenerated gratings keep stable after three cycles of 600? heat preservation experiment for 5 hours,but after working for 5 hours at 1000?,the reflectivity reduced 4.5 dBm and it was irreversible decay.Experimental results show that regenerated gratings are stable at 600?,fluctuating up and down at 800? for high and low temperature cycle.Furthermore,the central wavelength of regenerated gratings increased and then keep steady after repeating high temperature test cycles about 4 times.In a word,the regenerated weak gratings have a good temperature sensing performance under 800?.