Design Of Strain Sensor Base On BOTDR

With the acceleration of the urbanization in China,the development mode and industrial structure of the city are constantly upgrading.In order to meet the demands of urbanization,the number of subways,bridges and urban comprehensive pipe corridor and other large infrastructure facilities grows explosively.However,as we all know,structural damages of these might cause accidents,which seriously threats the national property and the safety of the people.The detection of health status and the prevention of accidents on existing infrastructure sites,of which the characteristics of large scale,wide coverage,complex geological conditions and long service cycle,and so on,are a great challenge.Therefore,a large-scale and real-time monitoring method is urgently needed.In view of shortcomings of the current Large-scale infrastructure health monitoring technology,such as high maintenance cost,limited measurement range,poor corrosion resistance and long information feedback period etc.,a distributed fiber strain sensor based on Brillouin Optical Time Domain Reflector(BOTDR)was designed by utilizing single fiber as the sensing and signal transmission unit simultaneously.For this technology,the electro-optic modulator is susceptible to environmental temperature and other factors,causing the instability of the pulse extinction ratio.Therefore,a method of pulse modulation based on SOA was proposed to solve the stability of BOTDR strain detection.And a strain sensor based on the new approach proposed by us was designed,which provided a technology used for large-scale,on-line stability monitoring of structural damage.The sensor has the advantages of maintenance-friendly,large measuring range,stability,intrinsic safety,anti-electromagnetic interference,and corrosion resistance,etc.This paper mainly studies the following aspects:(1)The modulation characteristics of the acousto-optic modulator(AOM)and the electro-optic modulator(EOM)was theoretically analyzed.According to the slow modulation rate of AOM and instability of EOM,a pulse modulation characteristic of SOA was in-depth researched,and a BOTDR strain detection system based on SOA pulse modulation was proposed.(2)A strain detection scheme based on SOA pulse modulation was designed,and the key devices were analyzed and selected.The monitoring software based on MFC was designed,and the experimental system was built to collect Brillouin scattering signals online and achieve stable demodulation of strain information.(3)The influence of the extinction ratio stability on the signal-noise ratio(SNR)of the system was experimentally studied.The characteristics of modulated pulse and stability of AOM,EOM and SOA were compared,and the influence of these on Brillouin scattering timing curve and strain detection was tested and verified.(4)The influencing factors of the spatial resolution,SNR and measurement accuracy were theoretically deduced,and the effects of the pulse width,peak input power,band-pass filter,and average times on system performance were experimentally verified.(5)Through the above research,the parameters of the device were matched,and the system performance and response rate optimization system were weighed.Finally,the BOTDR strain sensor was integrated and the system performance was tested.The experimental results showed that the designed strain sensor could achieve stable strain detection with a spatial resolution of 1m,the farthest sensing distance up to 27 km,and when the sensing distance was 10 km,the accuracy of strain detection was ±60??.