| Brillouin optical time domain reflectometer(BOTDR)realizes fully distributed temperature and strain sensing.It has significant advantages in health monitoring of large infrastructure structures such as bridges and tunnels,oil and gas pipelines,dam slopes and so on.However,its detection method has the defect of low signal-to-noise ratio(SNR)because the measured signal of BOTDR is the spontaneous Brillouin scattering with extremely-low intensity and the sensing fiber is often placed in the harsh outdoor environments,which further increases the loss of the fiber.Increasing the probe pulse width and averaging number can enhance the SNR,however decrease the spacial resolution and measurement speed.Therefore,how to effectively improve the SNR of BOTDR and meanwhile guarantee spatial resolution and measurement speed,is an important scientific problem that needs to be solved in BOTDR performance improvement.In view of the above problem,several key technologies to improve the SNR,measurement stability and measurement speed of BOTDR were proposed in this paper based on the analysis of factors affecting the BOTDR performance.Besides,an integrated FS-BOTDR was developed,and the functions of temperature and strain sensing were verified.The main research contents and results are as follows.(1)By summarizing the research progress of BOTDR performance improvement and application,challenges and problems in BOTDR performance improvement and application were pointed out.Through the combination of simulation and experimental verification,the influencing factors of key performance parameters in BOTDR were analyzed,which provided preliminary theoretical basis and experimental guidance for the subsequent improvement of BOTDR performance.(2)By analyzing the broadening effect of Brillouin gain spectrum(BGS)in frequency-scanning measurement process and the influence of the laser linewidth on the SNR in the coherent detection,the relationship between the measurement accuracy of the Brillouin frequency shift(BFS)and the laser linewidth was clarified.Then,a method to improve the measurement accuracy of the FS-BOTDR by optimizing laser linewidth was proposed.The research result showed that the measurement accuracy of FS-BOTDR increases with the narrowness of laser linewidth within a certain range.When the laser linewidth is too narrow,the coherent Rayleigh noise increases and the measurement accuracy of BFS decreases with the further narrowness of the laser linewidth.(3)A method based on SOA gain switch modulation was proposed for improving the measurement accuracy and stability of FS-BOTDR.The optical pulse with high and stable extinction ratio can be generated by the gain switch,based on which the SNR and measurement stability of FS-BOTDR can be simultaneously improved.The research shows that,compared with the traditional electro-optic modulator,the adoption of gain switch can reduce the RMSE of BFS from 2.49 MHz to 0.78 MHz under the same measurement conditions.The effective detection distance increases from 10.75 km to 27.5km,and the system measurement stability was significantly improved.In addition,based on the pulse modulation with stable and high extinction ratio,the detection light power optimization method based on transmission spectrum analysis was proposed by analyzing the nonlinear effects in the long-distance BOTDR.The optimal pulse peak power was determined to be 300 mW with 1 m spatial resolution over 27.5 km sensing distance.(4)An FS-BOTDR based on random sequence coding detection is proposed.The gain switch with physically true random number modulation is used to generate random sequence coded optical pulse injected into the sensing fiber.The single pulse response can be restored by cross correlation operation of backscatter signal and corresponding detection light pulse sequence.Then the system decoding can be realized.Both theoretical simulation and experimental studies show that random sequence coding detection can improve the SNR of FS-botdr without changing the spatial resolution.(5)By analyzing the principle of BGS measurement based on microwave-heterodyne frequency-scanning method,the basic characteristic of the time-domain evolution of signal flow in FS-BOTDR was illustrated.Then a real-time extraction scheme of time-domain signal envelope based on fast response logarithmic detection was proposed to replace the traditional envelope extraction algorithm.This improved the measurement speed while reducing the sampling bandwidth and sampling rate requirements.The study shows that the sampling bandwidth required for 1m spatial resolution is reduced from 200 MHz to 50 MHz by using the logarithmic detection scheme with the measurement accuracy basically unchanged.(6)A parallel operation mechanism of STFT combining digital filtering and window function optimizing to realize fast BGS measurement is designed for STFT-BOTDR.The results show that the optimal BFS demodulation accuracy can be obtained by Butterworth filter and Hanning window function.Compared with serial operation mechanism,STFT parallel operation mechanism can improve the data processing speed by about 25 times on the premise of ensuring the demodulation accuracy of BFS. |
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