应变梯度对布里渊光时域反射计测量精度的影响

基于脉冲抽运的布里渊光时域反射计技术具有米数量级的空间分辩力。为了分析在其空间分辨力范围内的应变梯度增加对布里渊谱测量的影响,建立了后向布里渊散射谱与应变梯度的函数关系。数值分析结果表明应变梯度增大,不仅使布里渊频移随之线性增大,也会使后向布里渊散射谱峰值非线性降低,谱峰变得平坦。空间分辨力10m的100με／m应变梯度相对于零应变梯度,引起的布里渊频移测量误差增加2．04倍。传感光纤固定于悬臂梁上以模拟不同应变梯度,采用对频宽小于1MHz的激光脉冲调制和基于相干平衡检测的布里渊光时域反射计系统,实验测得了沿传感光纤的后向布里渊散射谱数据和应变分布。布里渊谱数据经最小二乘法拟合后,得到的不同应亦梯度的谱曲线变化与理论分析相吻合。     

基于微波电光调制的布里渊光时域分析传感器

针对布里渊光时域分析分布式传感原理和受激布里渊散射的特点,应用微波电光调制分布反馈式半导体激光器产生频移可调的探测光,和传感光纤中相反方向传输的脉冲激励光进行受激布里渊散射作用,当探测光和激励光的频率差在布里渊频移附近时,频移探测光和激励光产生受激布里渊散射,通过改变探测光的频移值,检测探测光功率信号,可得到沿光纤各处的布里渊频移,再利用布里渊频移和应变（或温度）的关系,计算得到沿光纤分布的传感量。设计了基于微波电光调制的布里渊光时域分析传感器实验系统,实现了25km的分布式温度传感,达到5m的空间分辨力和3℃的温度分辨力。     

一个新型的基于全光纤Mach-Zehnder干涉仪BOTDR系统

报道了新型的分布式传感测量布里渊光时域反射(BOTDR)系统.布里渊散射频移和强度均依赖于温度和应变,因此,BOTDR利用光纤中的自发布里渊散射作为测量信号可以实现分布式温度和应变测量.在BOTDR中,光源采用窄谱半导体激光器,并由声光调制器调制成脉冲光,经掺铒光纤放大器放大后,注入测试光纤以产生自发布里渊散射.利用双通Mach-Zehnder干涉仪分离光纤背向散射中的自发布里渊散射与瑞利散射信号,实现了自发布里渊散射的直接检测.实验结果表明基于全光纤Mach-Zehnder干涉仪BOTDR方案是可行的.     

一种新的光纤布里渊传感散射谱拟合方法

讨论了应用于光纤分布式布里渊传感散射谱最优化拟合的Levenberg-Marquardt(L-M)非线性最小二乘算法。分析并推导了一种可较好地反映入纤脉冲宽度越来越窄所引起的光纤分布式布里渊传感散射谱形状变化的Pseu-do-Voigt分析拟合模型。由调制脉冲光和调制器泄漏连续光共同作用所产生的光纤分布式布里渊传感散射谱应为Lorentzian型谱和Gaussian型谱的线性权重组合。Gaussian型函数部分可视为Lorentzian型函数谱的近似修正处理。基于L-M算法对光纤布里渊传感散射谱采样数据进行了曲线拟合和最优化参数估计,所得到的结果与理论分析情况吻合。     

光脉冲编码对基于拉曼放大的布里渊光时域分析系统的影响

报道了光脉冲编码对基于拉曼放大的布里渊光时域分析系统(BOTDA)的影响. 实验表明,光脉冲编码能够在保证较高空间分辨率的同时减小布里渊频移的不确定性. 实验实现了49.6 km传感距离, 在整段传感光纤上温度分辨率1 ℃,空间分辨率2.5 m. 本文实验测量并分析了系统的信噪比和光功率分布特性. 关键词： 分布式光纤传感 受激拉曼散射 布里渊增益 布里渊光时域分析系统(BOTDA)     

结合布里渊光时域分析和光时域反射计的分布式光纤传感器

对于普通单模光纤的布里渊散射光,其频移是温度或应变的函数,因此通过检测布里渊频移可得到沿光纤分布的温度或应变。但布里渊散射分布式光纤传感器有光时域分析和光时域反射计两种结构,两者具有不同的性能特点和使用场合,在实际传感应用中需要加以选择,用以实现有效的长距离光纤传感。如何在一个系统中融合这两种分布式光纤传感技术,其关键是产生传感所需的频移参考光或探测光。采用光纤激光器作为单一光源,基于微波电光调制产生频移参考光或探测光,并应用正交偏振控制来抑制偏振相关信号衰弱。在同一系统中实现了25 km普通单模光纤的布里渊光时域分析和光反射计,在5 m空间分辨率下达到3℃的温度分辨率。     

布里渊光时域分析传感器的消偏振衰落技术

在布里渊光时域分析(BOTDA)光纤传感器中,由于受激布里渊散射的偏振相关性的影响,激励光和探测光的偏振态随机变化引起的信号起伏严重影响布里渊散射光信号的稳定测量。通过建立与偏振态有关的激励光和探测光的耦合方程,推导出接收探测光光强关于激励光和探测光偏振态的理论公式,并由此提出偏振扩展接收技术。理论分析表明,偏振扩展接收技术可以有效地抑制偏振相关信号衰落。通过对耦合方程进行模拟,验证了理论分析的正确性和偏振扩展接收技术的可行性。最后给出了采用偏振扩展接收的布里渊光时域分析实验结果,与理论分析和模拟具有较好的一致性。     

分布式光纤布里渊散射温度传感实验系统

提出一种基于布里渊光时域反射计法的分布式光纤布里渊散射温度传感系统.光源采用窄谱半导体激光器.声光调制器将光源调制成窄脉冲,并由高增益光纤放大器放大,产生高功率光脉冲信号,提高了自发布里渊散射信号的强度.采用双通马赫-曾德干涉仪将布里渊散射从瑞利散射中分离出来,在一段4.25 km长的光纤上进行分布式温度传感的实验研究.双通马赫-曾德干涉仪对瑞利散射进行了很好的抑制,得到了比较纯净的随温度变化的布里渊散射信号.     

布里渊散射谱相干检测的消偏振衰落技术

在布里渊光时域反射计（BOTDR）的相干检测系统中,为减小偏振失配对布里渊谱测量精度的影响．针对后向布里渊散射光偏振态沿光纤随机变化,而同一记录点在重复采样周期内几乎不变的特点,结合偏振分集和扩展的思想,提出了实现简单而有效的时域偏振分集技术。理论分析表明,不管布里渊散射信号光的偏振态如何随机变化,采用该技术有较稳定的信号光功率与本振光被外差接收,接收信号对偏振态随机变化不敏感。系统实际上可基本不改变原系统体系,只对本振光施以无循环反馈的简单偏振控制。在布里渊时域反射计系统巾实验对比了采用与不采用该技术的结果,表明前者的接收信号功率幅度约是后者的四分之一．抑制幅度起伏效果明显。     

基于宽带光源和自外差检测的布里渊光时域反射温度传感系统

为简化系统结构、减小相干瑞利噪声对系统性能的影响,提出了一种采用宽带光源的瑞利和布里渊散射自外差检测布里渊光时域反射温度传感系统.分析了瑞利和布里渊自外差检测原理,研究了布里渊频移和自外差信号功率与光纤温度和应变的关系.设计并搭建采用宽带光源的自外差检测布里渊光时域反射温度传感系统,获得了常温下沿光纤分布的自外差信号功率谱及不同温度时加温段光纤的功率谱,验证了布里渊频移和自外差信号相对功率变化随温度的线性增加关系.通过实验数据获得的布里渊频移和相对功率变化的温度系数分别为1.07±0.01MHz/℃和(0.37±0.09)%/℃.本文的研究结果为基于瑞利和布里渊自外差检测布里渊光时域反射传感系统的温度和应变同时测量提供了理论和实验依据.     

Distributed Brillouin fiber sensing based on spectrum line fitting and wavelet packet denoising

A novel signal processing method is proposed to improve the spatial resolution, frequency resolution and dynamic characteristics of BOTDR. For the BOTDR system with 50 ns pump pulse, by using spectrum line fitting technology based on Levenberg-Marquardt (LM) algorithm, the spatial resolution is improved from 5 m to 5 cm. Combination of LM fitting algorithm, a large frequency scanning interval is adopted without sacrificing measurement accuracy of the BOTDR system. It reduces the number of sampling points of Brillouin spectrum significantly. So, the fitting speed is improved greatly. This is the first time using a large scan interval to increase the spectrum line fitting speed. To improve the fitting speed, the difference between the reference and measured spectrum is used to estimate the variation of Brillouin frequency shift. The measured amplitude of Brillouin spectrum is used to estimate the width of region strain occurred. Finally, by using wavelet packet denoising technology, the spectra containing noise are fitted successfully.     

基于相关序列脉冲的布里渊光时域反射测量系统解码方法研究

布里渊光时域反射测量(BOTDR)系统空间分辨率和传感距离存在相互制约的关系.相关序列脉冲编码技术可用于解决这一矛盾,但直接对编码脉冲的布里渊散射采样信号进行解码,会因采样时间间隔小于编码单位脉冲宽度而导致解码信号的失真,从而降低了系统空间分辨率和测量精度.分析了直接互相关解码对BOTDR系统空间分辨率的影响,提出一种...     

Spatial Resolution Enhancement in BOTDR by Spectrum Separation Method

BOTDR (Brillouin optical-fiber time domain reflectometry) is a promising technique for measuring the strain distribution along an optical fiber which can be used for diagnosing civil structures. However, there is a trade-off between the spatial and the frequency resolutions, and its practical spatial resolution is limited to about 1 m; to date the application scope has been restricted. In order to enhance the spatial resolution without decreasing the measurement accuracy, we intentionally introduce two kinds of strain within the spatial resolution length determined by the optical pulse width, which results in Brillouin gain spectrum separation. In this paper, we describe the theoretical analysis of the spectrum separation method together with experimental verification, where a spatial resolution of 0.5 m is demonstrated. We further propose a new parabolic data fitting method especially effective in the spectrum separation method.     

Influence of laser linewidth on performance of Brillouin optical time domain reflectometry

The effects of optical sources with different laser linewidths on Brillouin optical time domain reflectometry (BOTDR) are investigated numerically and experimentally. Simulation results show that the spectral linewidth of spontaneous Brillouin scattering remains almost constant when the laser linewidth is less than 1 MHz at the same pulse width; otherwise, it increases sharply. A comparison between a fiber laser (FL) with 4-kHz linewidth at 3 dB and a distributed feedback (DFB) laser with 3-MHz linewidth is made experimentally. When a constant laser power is launched into the sensing fiber, the fitting linewidths of the beat signals (backscattered Brillouin light and local oscillator (LO)) is about 5 MHz wider for the DFB laser than for the FL and the intensity of the beat signal is about a half. Furthermore, the frequency fluctuation in the long sensing fiber is lower for the FL source, yielding about 2 MHz less than that of the DFB laser, indicating higher temperature/strain resolution. The experimental results are in good agreement with the numerical simulations.     

Optimum Temporal Pulse Shape of Launched Light for Optical Time Domain Reflectometry Type Sensors Using Brillouin Backscattering

We investigated numerically the relationship between the temporal shape of an optical pulse launched into an optical fiber and the power spectrum of the Brillouin backscattered light it produces. We analyzed the measurement error of the peak-power frequency of the Brillouin backscattered light power spectra obtained from the launched light with various pulse shapes. In this investigation and analysis, we characterized the pulse shape by the width, leading-trailing time, and steepness. Regardless of the launched pulse shape, the peak-power frequency-measurement error increases as the pulse width shortens. For identical launched pulse widths, a triangular pulse generates the Brillouin backscattered-light power spectrum with both the narrowest profile and the largest peak power, and consequently provides the minimum error when we measure the peak-power frequency. This shows that a temporally triangular pulse is the best for the launched light.     

毫米级高分辨率的混沌激光分布式光纤测温技术

近年来,随着分布式光纤传感技术在各大基础设施健康监测领域的广泛应用,人们对能够实现毫米量级精准定位和监测技术的需求日益增长.本文提出了一种基于宽线宽混沌激光的高分辨率分布式光纤测温技术.实验通过改变光反馈混沌源的偏振匹配态和反馈强度等外部参数,产生了–3 dB线宽约为7.5 GHz的宽线宽混沌激光,并在300 m传感光纤实现了空间分辨率为7.05 mm的分布式温度测量.同时,为了抑制光源线宽增加造成的布里渊增益谱恶化,在泵浦路中引入了时间门控技术,其中经脉冲调制后的泵浦光峰值功率提高了约9.5 dB,同时脉冲调制使混沌互相关锁定于脉冲持续时间内,从而布里渊增益谱的信号背景噪声比由约2.28 dB提升为4.55 dB,最终实现了空间分辨率为3.12 mm的分布式温度测量.     

基于实时小波变换信号处理的相干检测布里渊光时域反射计

布里渊光时域反射计检测信号较微弱的自发布里渊散射光,其布里渊频移只有11 GHz左右.相干检测是一较好的方法,但其接收信号是一宽带的幅度调制信号,难以直接解调.采用实时小波变换对传感信号进行处理,设计了布里渊光时域反射计传感器实验系统,实现了25 km的分布式温度传感,在5 m的空间分辨率和1 min的响应时间下,达到40 με的应变分辨率.