摘要
提出一种基于布里渊光时域反射(BOTDR)系统的寻极大值法,在快速傅里叶变换(FFT)和短时傅里叶变换(STFT)的基础上,实现了频移的快速定位及系统空间分辨率的增强。对时域信号进行FFT处理,利用寻极大值法判断是否存在温变或应变信息,再通过逐级选取对应长度的时域信号来确定温变或应变频移发生的位置范围;对时域信号进行STFT处理,构建三维布里渊增益谱,通过寻极大值法构建布里渊频移分布来检测短距离温变或应变,从而提高系统空间分辨率。在实验中,利用基于等分FFT的寻极大值法,对2 km待测光纤中130 m段加热光纤的位置范围进行快速定位,系统运算时间缩短到原来的1/8。同时在设置探测光脉冲宽度为100 ns的条件下,利用基于STFT的寻极大值法,在2 km待测光纤上检测到0.6 m光纤长度的温度变化,实现了亚米级别的空间分辨率。与传统BOTDR系统相比,基于寻极大值法的STFT-BOTDR系统提高了实际应用中的检测速度与空间分辨率,工程实用性得以提高。
Objective In the past three decades,Brillouin optical time domain reflectometry(BOTDR)has attracted widespread attention from researchers and has been applied to health and safety monitoring in various engineering structures.BOTDR based on shorttime Fourier transform(STFT)performs signal processing on the broadband signal of the Brillouin scattering spectrum.The acquisition time of the broadband signal is shorter than that of the frequency sweep system,and therefore system response is swifter.Spatial resolution and frequency resolution are two important performance parameters of the STFTBOTDR system.The former,spatial resolution,is proportional to optical pulse width and related to the form and length of the window function.The latter is related to the signaltonoise ratio of the electrical signal,step length of frequency,center frequency of the Brillouin gain spectrum,and full width at half maximum.The two resolutions correlate with each other.Meanwhile,the computing time of the STFT is related to the set parameters of frequency step length and of the sliding window.As a result,improving the system frequency resolution will increase the system′s computing time.How can we optimize the photoelectric design and improve the efficiency of the demodulation algorithm under the current system of typical BOTDR to obtain highly enhanced spatial resolution by using economic optical pulses of common width(instead of using narrow pulses and other costly photoelectric modules)?The solution to the question is essential to the extensive and largescale application of BOTDR in the engineering field.Methods We propose a maximumseeking method based on the BOTDR system,which realizes the rapid positioning of frequency shift and the enhancement of system spatial resolution based on fast Fourier transform(FFT)and STFT.The maximumseeking method based on the equal division FFT process first performs FFT processing on the timedomain signal and linear fitting and then maximumseeking processing on the spectrum within the frequency range of 100 MHz on both sides of the Brillouin center frequency.Then,by using the judgment Eq.(5),it determines whether there is temperature variation or strain information and then continuously divides the timedomain signal.Finally,it selects the corresponding length of the timedomain signal to determine the temperature variation or strain frequency shift range,thus realizing the rapid positioning of frequency shift and reducing the system′s operation time.The maximumseeking method based on STFT first processes the timedomain signal with STFT to construct a threedimensional Brillouin gain spectrum and then builds a Brillouin frequency shift distribution through the maximumseeking method.The Brillouin frequency shift curve is corrected by using the judgment Eq.(7)in different situations,determining the length of the shortdistance temperature variation or strain segment,thereby improving the system′s spatial resolution.Results and Discussions In the experiment,we design a BOTDR system based on STFT and quickly locate the heated fiber in a section of 130 m the fiber of 2 km under test.We use the spectrum constructed with equal division FFT(Fig.6)to determine the position of temperature occurrence based on whether the frequency shift peaks appear in each segment of the spectrum.The traditional STFTBOTDR system detects fiber temperature variation data of 130 m in 12800 groups of data,with a system operation time of 482 s.By using the maximumseeking method based on equal division FFT,the system operation time for detecting the temperature variation information of 130 m is reduced to 68 s,which is 1/8 of the original time.The calculation speed is much improved.At the same time,to verify the enhancement of spatial resolution by using the maximumseeking method based on STFT,we design test fiber 2(Fig.7)with heating section temperatures set at 40°C and 50℃.Under the condition of setting the probe light pulse width to 100 ns,we use the traditional peak search algorithm and the maximumseeking method to process the constructed Brillouin frequency shift distribution(Fig.9).From the experimental data of Brillouin frequency shift distribution(Fig.9),it can be seen that after using the STFTbased maximumseeking method,the system′s spatial resolution is optimized from 12.8 m to 1.2 m under the heating section at 40°C and from 4.6 m to 0.6 m under the heating section at 50℃.Conclusions We propose a new method to achieve rapid frequency shift positioning and spatial resolution enhancement in the BOTDR by using the maximumseeking method.By continuously dividing the original signal and performing FFT processing,the maximumseeking method processes the twodimensional Brillouin gain spectrum to determine the position range of temperature variation or strain segments,reducing the system′s computing time.At the same time,the threedimensional Brillouin gain spectrum obtained from STFT is processed by using the maximumseeking method to construct Brillouin frequency shift distribution,reducing the minimum detectable temperature variation or strain segment length and enhancing the system′s spatial resolution.In the experiment,an STFTbased BOTDR system is designed.By using the maximumseeking method based on equal division FFT,the heated fiber of 130 m in the test fiber of 2 km is quickly located,reducing the system′s operation time to 1/8 of the original and improving calculation speed.Simultaneously,under the condition of setting the probe light pulse width to 100 ns,a spatial resolution of 0.6 m is achieved on the test fiber of 2 km.The experimental results show that this method can further improve the performance of existing STFTBOTDR systems without sacrificing other sensing performance parameters.By using the maximumseeking method based on STFT,the submeter level spatial resolution is achieved.Compared with the traditional BOTDR system,the STFTBOTDR system based on the maximumseeking method has faster detection speed and better spatial resolution in engineering applications.Besides,this method helps to obtain higher system performance under limited system cost,making it easier for lowcost and highprecision BOTDR systems to be used in larger quantities at construction sites,bridges,and other occasions,thereby accelerating the engineering and largescale application of distributed fiber optic sensing technology.
作者
黄秋茗
陈映恺
刘鑫煜
陈理平
高波
付林林
李拥政
郭林峰
徐小敏
Huang Qiuming;Chen Yingkai;Liu Xinyu;Chen Liping;Gao Bo;Fu Linlin;Li Yongzheng;Guo Linfeng;Xu Xiaomin(School of Physics and Optoelectronic Engineering,Nanjing University of Information Science&Technology,Nanjing 210044,Jiangsu,China;Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean,Nanjing 210044,Jiangsu,China;China Railway(Shanghai)Investment Group Co.,Ltd.,Shanghai 200126,China;Nanjing Metro Construction Co.,Ltd.,Nanjing 210019,Jiangsu,China;China Railway Tunnel Group No.2 Co.,Ltd.,Langfang 065200,Hebei,China;China Railway No.3 Group East China Construction Co.,Ltd.,Nanjing 211153,Jiangsu,China;Department of Engineering,University of Cambridge,Cambridge CB21PZ,United Kingdom)
出处
《光学学报》
EI
CAS
CSCD
北大核心
2023年第14期75-84,共10页
Acta Optica Sinica
基金
国家自然科学基金(62175113)
江苏省重点研发计划(BE2022076)。
关键词
光纤光学
布里渊光时域反射
短时傅里叶变换
寻极大值法
定位
空间分辨率
fiber optics
Brillouin optical time domain reflectometry
shorttime Fourier transform
maximumseeking method
positioning
spatial resolution
