The Study Of Coding Technology Based On BOTDA Optical Fiber Sensing System

The first low-loss fiber was officially launched in 1970.Since then,distributed fiber sensing technology has emerged as a new type of sensing technology.Sensing systems with this new technology not only have the advantages of common fiber-optic sensing systems,such as small size,light weight,high sensitivity,a wide range of measurement objects,and good anti-electromagnetic and chemical stability,but also can be real-time along the fiber to acquire temperature and strain information,achieve distributed monitoring,and far better than other fiber-optic sensing systems in terms of measuring distance and measurement accuracy.Therefore,the monitoring has been extremely widely used in the health and safety of reservoirs,dams,mountains,mines,tunnels and other geology and construction.The Brillouin optical time domain analysis(BOTDA)distributed optical fiber sensing system can not only be applied to a standard single-mode fiber,but also realizes a long-distance transmission of a large dynamic range.Sensing,and sensitive to changes in temperature and strain,can simultaneously measure temperature and strain throughout the fiber.The main limitation of the current BOTDA system is that,in the process of detecting pulsed light and continuous detection light,due to factors such as noise,the signal-to-noise ratio of the system is very low,and useful signals are basically submerged in noise.To improve the signal to noise ratio of the BOTDA system,the measurement of system performance mainly depends on the pulse width,spatial resolution,signal-to-noise ratio,and other indicators.These indicators show a mutual restraint relationship,by increasing the pulse width,the signal-to-noise ratio can be improved,but the spatial resolution of the system will decrease.In order to solve this problem,it is considered that the optical pulse signal is encoded and then input into the sensor fiber so as to achieve the purpose of not only reducing the spatial resolution but also improving the signal-to-noise ratio.The article first introduces the characteristics and research status of the distributed optical fiber sensing system,and then briefly explains the three fiber sensing systems based on Rayleigh,Raman and Brillouin scattering.Next,the article elaborates on the BOTDA system from the aspects of stimulated Brillouin scattering theory,Brillouin scattering spectrum,the main performance index of BOTDA system,and the relationship between temperature and strain and Brillouin frequency shift.The BOTDA fiber-optic sensor measurement system for this experiment was designed.Then the coupling equation of the stimulated Brillouin scattering is introduced in detail,and the transient equation of the stimulated Brillouin scattering is solved.The system is simulated in 3D with MATLAB,and the noise in the fiber in the actual BOTDA system is simulated by adding Gaussian white noise.The system response curve under single-pulse operation was obtained.Based on this,for the requirement of high SNR for BOTDA system,several encoding methods for improving system SNR are introduced in detail.The encoding and decoding principle of each encoding is explained,and encoding of different encodings is deduced.Gain,a combination of CCPONS and Walsh pulse coding was proposed.Based on the in-depth study of the codec principle of combined coding,the performance of the combined coded BOTDA system was analyzed,and the introduction of the combined pulse coded BOTDA was deduced.System SNR improvement.Finally,the corresponding system response curves of different coding methods were simulated and analyzed.The improvement of signal-to-noise ratio of different pulse codes on the system was analyzed and verified.