Research On Distributed Temperature Sensing System Of BOTDR Based On The Sideband Of Brillouin Gain Spectrum Demodulation

At present,the real-time monitoring system has not been used to monitor coalbed methane,oil pipelines and other underground pipelines widely in China,as a result,potential security problems for the pipelines are gradually highlight.Distributed optical fiber sensing technology based on Brillouin optical time domain reflectometry(BOTDR)has the advantages of anti-electromagnetic interference,high temperature resistance,corrosion resistance,easy to installation and so on.Moreover,the system features long detection distance,simple installation,one-end access,and measures the broken point.What is more,it can realize a full range of continuous measurement to temperature and strain of objects in time and space simultaneously.Thus the BOTDR system very suitable for detecting the leakage,tensile pull and other real-time structure health monitoring along the long distance pipeline.Measurement temperature accuracy is one of important indexes to evaluate the performance in a distributed temperature sensing system of BOTDR.To improve it,a distributed temperature sensing system of BOTDR based on demodulation of the sideband of Brillouin gain spectrum(BGS)is proposed and demonstrated in this paper.And in this way,the signal-to-noise ratio(SNR)of the system is increased,thus a higher measurement temperature accuracy is obtained.On this basis,we further analyze the main factors affecting the performance of the BOTDR system in depth.Our main works are shown as follows:(1)A distributed temperature sensing techniques by demodulating the sideband of BGS in the BOTDR system is proposed.The generating mechanism of the BGS sideband and the reason why demodulating the sideband of BGS can improve temperature measurement accuracy both are analyzed in theory(2)The experimental setup of the distributed temperature sensing system of BOTDR based on the sideband of BGS demodulation is built.And an in-depth analysis about the selection of main devices in the system is made.In additions,the demodulation of Brillouin frequency shift(BFS)along the fiber under test(FUT)based on Levenberg-Marquardt algorithm and the correlative interface display are completed by LabVIEW.(3)On the basis of the sideband demodulation of BGS,the factors that affect the performance of the system are analyzed in theory and verified by experiments.In order to improve temperature measurement accuracy,firstly,the sideband of BGS demodulation is proposed,and further investigated due to the contribution of the reference light by coherently detecting and eliminating the effect of the coherent Rayleigh noise,the method can enhance the accuracy effectively;secondly,the influences of the laser linewidth and the peak power of the probe optical pulse on the temperature accuracy are also researched,respectively;Finally,the contribution of the central frequency of the narrow-band pass filter(NBPF)in the scanning frequency part of the radio-frequency signal receiving terminal to temperature measurement accuracy is also studied.To enhance the spatial resolution,the influences of the probe optical pulse width,the bandwidth of NBPF and the response rate of the radio-detector in the scanning frequency part of the radio-frequency signal receiving terminal are analyzed and investigated selectively.(4)On the basis of the above,a series of temperature sensing experiments are carried with the usage of BOTDR system we build.When the left sideband of BGS is scanned and demodulated,and then the time domain waveforms acquired are processed by LabVIEW,the temperature measurement accuracy of ±0.5? is achieved over a 10.2 km sensing fiber.Moreover,a spatial resolution of 2.79 m is obtained over a sensing fiber of 10.8 km by injecting the probe optical pulse with about 30-ns width.Due to the limited available temperature range of the thermostat,the temperature measurement range of the BOTDR system reaches-30?~90? currently.