Distributed Optical Fiber Strain Detection System Based On The BOTDR

With the expansion of the scale of infrastructure in China,the number of infrastructure such as tunnels,bridges and dams is increasing,and the problem of security risks caused by long-term operation is also increasing;at the same time,such as landslides,surface subsidence and other geological disasters often occur due to serious environmental damage.Therefore,it is of great importance that how to carry out structural health monitoring and disaster warning for large-scale infrastructure and special landforms.Most of the structural health monitoring is achieved by detecting the strain,and the traditional strain detection technology mainly relies on strain gauge or vibrating wire strain sensor to realize point measurement.These two methods are susceptible to environmental corrosion and are not suitable for long-term testing.Meanwhile,in the application of large-scale measurement points,the point sensors have the following shortcomings,for example the layout is redundant and complex,the signal transmission is inconvenient,the resistance to electromagnetic interference is poor,which make it difficult for engineering applications.Compared with the conventional strain sensors,the distributed fiber strain sensor has many advantages such as anti-electromagnetic interference,setting transmission and sensing as one,high temperature resistance,high sensitivity,small volume,which make it particularly suitable for many harsh environments with flammable,explosive,limited space and strong electromagnetic interference,etc.In addition to the above advantages,the BOTDR-based distributed optical sensing technology also has large bandwidth,low loss,long distance sensing and so on,which can implement health monitoring and warning tasks well.Firstly,the BOTDR-based strain sensing mechanism was analyzed based on Brillouin scattering characteristics,the linear relationship between Brillouin frequency shift and strain was obtained.On the basis of theoretical analysis of coherent detection mechanism,combined with the microwave sweep technology,the overall design of strain sensor system was given.Then the characteristics of Brillouin scattering spectrum were analyzed to describes the flow and significance of the Lorentz curve fitting algorithm(L-M algorithm),and according to the detection method and the characteristics of data acquisition device,the strain demodulation software based on LabVIEW was designed.Secondly,in order to facilitate the integration of the prototype,the lab’s existing AD9467 analog-to-digital conversion chip and Kintex-7 series XC7K325 T development board were used,the AD acquisition and PCIe DMA high-speed data transmission software modules were designed based on Verilog language,and the initial verification of each program function was completed,all of these laid the foundation for the late design of a complete high-speed acquisition card.Finally,according to the whole design scheme,an experimental sensing system was build,and a tensile test was performed.0με-8000με was implemented on the optical fiber of 3.9m at the end of the sensing fiber of 10.19 km.The frequency shift error of the demodulation result is ± 1.05 MHz,and the corresponding strain measurement error is ± 24.35με,Thus,the distributed fiber strain detection system can provide a feasible detection scheme for structural health monitoring of engineering facilities and special landforms.