Design And Performance Optimization Of BOTDR Prototype

The distributed optical fiber sensing technology,which takes optical fiber as the sensing unit and transmission medium,possess the characteristics of long transmission distance,strong anti-electromagnetic interference ability and comprehensive access to the parameter information of the monitoring target body.It can realize long-term online distributed monitoring,which is suitable for real-time monitoring of large buildings.Therefore,this technology has a wide range of application scenarios.As a distributed optical fiber sensing technology,the Brillouin Optical Time Domain Reflectometer(BOTDR)based on Spontaneous Brillouin Scattering can measure temperature variation and strain simultaneously,and possess the advantages of simple single-end measurement structure,so it has a great research value.Aiming at the problems of unstable operating point and low extinction ratio of electro-optic modulator in existing schemes,a pulse modulation scheme based on SOA(Semiconductor Optical Amplifier)is adopted to improve the SNR(Signal-to-Noise Ratio)of the system.Meanwhile,since the Spontaneous Brillouin Scattering signal is faint,the method of cumulative average noise reduction can improve the SNR of the system,but the drawback is that it takes a long time.Therefore,this paper proposes a noise reduction scheme combining the lifting wavelet threshold and cumulative average.The feasibility and effectiveness of the system are verified by theoretical derivation,simulation analysis and experimental test.The prototype is developed and assembled according to the modular design idea of the system.The main research contents and innovation points of this paper are as follows:Firstly,the development status and existing problems of BOTDR technology are introduced,the generation and sensing mechanism of Spontaneous Brillouin Scattering in optical fiber are analyzed,and the Brillouin scattering signal is simulated to provides theoretical guidance for the development of BOTDR prototype.Second,in view of the performance indicators of the BOTDR prototype,such as the system signal-to-noise ratio,spatial resolution,measurement time and measurement accuracy,a heterodyne coherence detection scheme based on microwave was designed to select the key devices in the optical path and signal detection and acquisition parts.In the existing schemes,the working point of electro-optic modulator is easy to drift and the extinction ratio is low.Aiming at this disadvantage,a measurement scheme based on SOA pulse modulation.Thirdly,the demodulation scheme based on short time Fourier transform is adopted to reduce the measurement time of BOTDR prototype and improve the measurement speed.By analyzing the time-domain cumulative average denoising scheme and time-domain cumulative average denoising scheme,a combining lifting wavelet threshold with cumulative average is proposed to further improve the SNR and measurement distance of the system.Finally,a Brillouin optical time domain reflectometer based on the experimental scheme of microwave heterodyne coherence detection technology is developed with the idea of modular design and used for temperature measurement experiments.