Research On The Performance Improvement Method Of BOTDR Temperature Measurement System

With the continuous development of China’s technological level,temperature detection has played an important role in large-scale infrastructure construction.In recent years,optical fiber temperature sensing technology has been widely used.The quasi-distributed fiber grating technology adopts single-point detection but the cost of long-distance laying is high.The temperature detection technology based on Raman scattering used multi-mode fiber yet it’s inevitable to cause frequency dispersion during long distance measurement,and the measurement accuracy is not highly guaranteed.The BOTDR temperature measurement system based on Brillouin scattering uses single-end pumping and has a long measurement distance.When a single-mode sensing fiber breaks at a certain position,the part before the break point can still be detected,which is suitable for practical engineering applications.However,due to the weak detection signal of the system,it is often placed in harsh environments with large fiber losses.In the long-distance and large-scale detection,the performance improvement of the BOTDR temperature measurement system has become an urgent problem to be solved.Therefore,this article mainly conducts research from the following aspects:(1)This article firstly studies the temperature measurement mechanism of the system through selecting the appropriate detection and demodulation technology and building a BOTDR temperature measurement system.Through experimental analysis,it is concluded that there is a compromise between the pulse width and peak power on the system spatial resolution and measurement accuracy.The coding technology based on the two modes of linear combination and correlation operation is analyzed theoretically to improve the system signalto-noise ratio effectively without reducing the spatial resolution.(2)A BOTDR temperature detection method based on true random code sequence pulse modulation is introduced.Starting with the theoretical introduction of the auto-correlation characteristics of true random code sequences and the principles of encoding and decoding,and derive the coding gain formula.Through simulation,the feasibility of the scheme is demonstrated in two dimensions of unit impulse response and Brillouin gain spectrum.Aiming at the distortion of the coding sequence amplification existing in the optical path presystem,a continuous optical amplification method is proposed to eliminate the distortion.(3)A FPGA-based true random code drive generator and corresponding impedance matching amplifier circuit board is designed and meanwhile an experimental system based on the frequency sweep method is built to test the randomness of true random codes under actual conditions,then the experimental results are analyzed properly.It is proved that using the true random code for modulation under the same parameter conditions can effectively improve the system’s signal-to-noise ratio and the measurement distance.(4)Aiming at the problem of long processing time of the frequency sweep method BOTDR system,the theory of short-time Fourier transform algorithm is studied.A fast demodulation method of BOTDR temperature measurement system based on short-time Fourier algorithm is proposed,and an experimental system is built.The optimization methods of digital filter,window function and operation points are proposed to improve the measurement accuracy of the system,suppress the frequency leakage and fence effect caused by the algorithm,and provide experimental guidance for rapid demodulation accuracy improvement.