提高布里渊光时域反射应变仪测量空间分辨力的等效脉冲光拟合法

由于探测光脉冲宽度受到限制,布里渊光时域反射仪(BOTDR)在对光纤上的应变进行分布式测量时,空间分辨力只能达到1 m。针对布里渊光时域反射仪单次采样接收背向布里渊散射信号(BBS)需要一定的时间,提出了基于等效脉冲光的多洛仑兹拟合法以提高其应变测量的空间分辨力。该方法将探测光脉冲在布里渊光时域反射仪完成单次采样所需的时间上进行积分,将积分函数作为等效脉冲光的表达式,再根据等效脉冲光的形状将布里渊光时域反射仪接收到的背向布里渊散射谱(BBS)细分,并对它进行多洛仑兹迭代拟合,准确求得每个细分布里渊散射谱的中心频率,进而利用光纤中布里渊频移与应变的对应关系,得到光纤中与细分布里渊散射谱对应的细分光纤单元上的应变情况。实验结果表明,利用这种方法,可使布里渊光时域反射仪应变测量的空间分辨力提高至0.05 m。     

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。     

环境温度宽范围变化对光纤布里渊频移的影响

搭建了光纤布里渊频移测量系统,实现了色散位移光纤20 ℃到820 ℃布里渊频移的测量,并对测量数据进行拟合. 在环境温度大范围变化时,目前采用的布里渊频移-温度一阶线性关系式对测量数据分析误差较大,针对这个问题,本文从纯石英光纤的结构特点和材料的物性系数出发,在分析光纤的热膨胀系数、光纤密度、折射率、弹性模量和泊松比与宽范围温度关系的基础上,根据布里渊频移与各物性参数的关系式理论推导了宽范围温度变化与布里渊频移之间的二次项关系式. 理论推导结果与实验数据基本符合,验证了理论分析的正确性,从而为光纤宽范围布 关键词： 光纤 高温传感 布里渊散射 布里渊频移     

基于TTDF和CNS算法的多路BOTDR散射谱信息高精度分析研究

传统的BOTDR光纤传感系统采用单路传感光纤实现对信息特征的测量,交叉敏感等不可控因素会使散射谱拟合精度较低,信息分析偏差较大。设计了一种同时对多路传感信息进行检测的BOTDR系统。针对基于布里渊光时域反射(BOTDR) 的多路传感散射谱高精度特征提取的要求,提出了一种三次数据融合(TTDF)与布谷鸟牛顿搜索(CNS)相结合的散射谱信息分析方法。该方法利用TTDF对信息数据的融合能力,根据狄克逊准则和格拉布斯准则,剔除了异常值的影响,减小了传感信号的误差;采用布谷鸟牛顿搜索算法进行频谱拟合,不仅通过布谷鸟的智能搜索能力得到全局最优解,而且以该最优解作为牛顿算法的初值进行局部寻优,保证了频谱拟合的精度,提高了布里渊散射谱信息分析的准确度。在温度信息散射谱线性权重比为1∶9的情况下,分析了不同线宽散射谱信息的提取。采用该方法进行多路数据融合的方差约为0.003 0,散射谱的中心频率约为11.213 GHz,温度误差小于0.15 K。理论分析和仿真结果表明,将此方法用于基于布里渊光时域反射的多路分布式光纤传感系统,可有效提高多路传感信号的准确度和布里渊散射谱信息分析的精确度。     

少模光纤的不同模式布里渊散射特性

少模光纤可以传输有限的正交模式,具有模间干涉小、模式易于控制等优点.基于少模光纤的布里渊散射传感器能够有效地减小多参量测量过程中的交叉敏感,实现多物理量的测量.本文基于波动光学理论对阶跃型少模光纤各个模式布里渊散射谱的频移、线宽、峰值增益等参数进行了分析.首先对少模光纤进行了模式分析,其次分析了少模光纤不同模式的频移、线宽、增益的数学模型,以及不同模式叠加的布里渊散射谱频移、线宽、增益.结果表明:少模光纤各模式布里渊散射谱的频移在10.19—10.23 GHz范围内,且随模式阶数的减小而增加;各模式布里渊散射谱的线宽在32—34 MHz,且随模式阶数的减小而增加;各模式布里渊散射增益谱相对幅度因为模式阶数的减小而增大.少模光纤中各个模式布里渊增益谱和多模式联运布里渊增益谱均符合洛伦兹曲线分布,多模式联运导致布里渊增益谱线宽展宽,且多模式联运布里渊增益谱相对振幅一般小于单个模式的布里渊相对振幅.     

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BOTDA布里渊背向散射光谱数据的拟合算法

BOTDA布里渊背向散射光谱数据表现为洛伦兹型曲线,对不同的单模光纤和多模光纤会生成不同的波峰个数,单模光纤中的布里渊背向散射光谱表现为单个波峰,采用非线性最小二乘法对单峰进行拟合,采用高斯-牛顿迭代法求最小二乘解。迭代运算的效率取决于初始值的精度,当误差较大时,迭代次数会增加,甚至无法收敛。采用传统法和定积分法分别获取某一初始值。实验结果表明,在满足同等精度的条件下,定积分法能降低迭代次数,对布里渊背向散射光谱的单峰拟合具有较好的效果。     

布里渊散射谱拟合的混合优化算法

基于布里渊光时域分析仪的全分布式光纤传感系统中,光纤沿途的探测信号含有噪声导致被测量的温度或应变信息难以识别,光谱拟合的精确度对传感信息的识别非常重要。在传感系统低信噪比的情况下,提出了一种提取高精度布里渊散射谱特征的拟合方法,利用小波去噪结合莱文伯-马奈特(LM)算法调节权值后向传输(BP)网络对布里渊散射谱进行特征提取。克服了传统BP神经网络易陷入局部极值的缺点,保证求解的精度。数值仿真表明,该方法适合不同权重比、不同线宽和低信噪比以及大测量范围的情况进行光谱拟合,并且在信噪比为10dB的情况下得到拟合度均超过0.96。实验结果表明,该方法适用于多种泵浦功率情况下的布里渊散射谱的特征提取,优于传统BP神经网络算法且具有较高的拟合精度。     

基于布里渊散射光时域分析的温度传感技术研究

为提高布里渊时域分析的传感精度,探寻一种相同条件下传感更为稳定、精确的传感光纤,通过实验得到一种布里渊频移与温度拟合线性度约为1的传感光纤.理论上分析了布里渊散射谱线宽度对温度传感测量精度的影响机制,在布里渊时域分析系统实验中分别以普通单模光纤与非零色散位移光纤(G.655)为传感光纤,测出在一定温度变化范围内标定点的布里渊散射谱线宽度和频移值.实验结果表明,非零色散位移光纤的布里渊散射谱线宽度随温度变化比较稳定,具有较高的传感精度.因此以G.655光纤为传感光纤更利于长距离监测场传感精度的提高.     

布里渊散射光纤传感器的交叉敏感问题

基于布里渊散射的全分布式光纤传感器的布里渊频移同时受应变和温度的影响,无法由单一的布里渊频移解析应变或温度的信息,这种交叉敏感问题制约了传感器的实用化。介绍了目前解决交叉敏感问题的主要方法及工作原理。研究了大有效面积非零色散位移光纤(LEAF)的布里渊散射谱与应变和温度的关系,实验发现布里渊谱具有三个布里渊峰,三个峰的布里渊频移与应变和温度成线性关系,第一个峰和第三个峰的布里渊峰峰值功率的差值与温度无关,但与应变成线性关系。最后基于实验结果提出了一种解决交叉敏感问题的方案,获得了大约130με的应变测量精度和8℃的温度测量精度。     

Hydrodynamical field fluctuations in a non-equilibrium quasi-stationary state due to a temperature gradient : II. Application to light scattering

Within the general theory of hydrodynamical field fluctuations in a system at non-equilibrium due to a temperature gradient, the problem of light scattering on fluctuations in number density is discussed. The effect of the gradient as well as fluctuations in sound velocity and heat conductivity on the Rayleigh-Brillouin spectrum is analyzed. The presence of the temperature gradient and fluctuations modifies the scattered-light spectrum leading to asymmetry in the heights of the Brillouin lines.     

High-gain and low-distortion Brillouin amplification based on pump multi-frequency intensity modulation

Using a pump with a multi-line spectrum to broaden the Brillouin gain bandwidth is an effective way to achieve lowdistortion amplification with high gain. Here, we theoretically and experimentally investigate the generation of a broadband Brillouin gain spectrum based on multi-frequency intensity modulation in an optical fiber. The arbitrary bandwidth of the Brillouin gain spectrum of stimulated Brillouin scattering(SBS) can be obtained as expected. In our experiment, a broadband Brillouin gain spectrum with a bandwidth of about 200 MHz is demonstrated. We also achieve a low-distortion amplification of a weak signal, whose maximum magnification is 65 d B for a-68-dBm input power signal.     

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.     

Impact of Brillouin amplification on the spatial resolution of noise-correlated Brillouin optical reflectometry

To obtain high spatial resolution over a long sensing distance in Brillouin optical correlation domain reflectometry(BOCDR), a broad laser spectrum and high pump power are used to improve the signal-to-noise ratio(SNR). In this Letter, we use a noise-modulated laser to study the variation of the Brillouin spectrum bandwidth and its impact on the coherent length of BOCDR quantitatively. The result shows that the best spatial resolution(lowest coherent length) is achieved by the lowest pump power with the highest noise-modulation spectrum. Temperature-induced changes in the Brillouin frequency shift along a 253.1 m fiber are demonstrated with a 19 cm spatial resolution.     

Threshold reduction of stimulated Brillouin scattering in photonic crystal fiber

A simple configuration for the reduction of stimulated Brillouin scattering threshold in photonic crystal fiber is presented. The threshold reduction is contributed by the bidirectional pumping scheme through the reflection of transmitted Brillouin pump back to the gain medium. The pump recycling scheme has greatly reduced the Brillouin threshold of a 200-m long of photonic crystal fiber from 50 to 30 mW of Brillouin pump power.     

Investigation on threshold power of stimulated Brillouin scattering in photonic crystal fiber

We have developed a theoretical approach to simulate stimulated Brillouin scattering (SBS) generation in photonic crystal fiber (PCF). The threshold condition for the SBS occurrence has been derived as a function of fiber parameters and input pump power. A particular emphasis is given to the influence of the input pump power and fiber length on the Brillouin gain in the PCF. To assess threshold pump power accurately, the pump depletion effect has been included by employing the 1% criterion. This simulation can anticipate the Brillouin threshold gain value precisely. The threshold gain varies from 14 to 18 depending on the PCF length.     

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.     

Broadband Brillouin slow light with multiple-longitudinal-mode,tunable pump

We propose and demonstrate broadband Brillouin slow light using a multiple-longitudinal-mode tunable fiber laser as Brillouin pump. A tunable broadband Brillouin pump with a tuning range from 1 520 to 1 555 nm is generated using a fiber ring laser with a semiconductor optical amplifier (SOA) as its gain medium. The pump spectrum consists of a large number of longitudinal modes separated by 6 MHz. The 3-dB bandwidth is about 11.5 GHz, and its fluctuation is less than 100 MHz within the tuning range. An 8-Gb/s data signal can be delayed by up to 83.0 ps (bit error rate < 10 9) at 17-dBm pump power.     

Effect of optical phase on a distributed Brillouin sensor at centimeter spatial resolution

Because of the power imbalance between the two arms of an interferometer in an electro-optic modulator (EOM), the output of the EOM is combined amplitude modulation (AM) and phase modulation (PM) for the probe signal consisting of the pulse and the dc component. Because of this PM, the Brillouin gain-loss spectrum becomes asymmetric. The central Brillouin frequency is shifted from that of an AM pulse. The maximum extinction ratio of the EOM is limited to approximately 29 dB for a power-splitting ratio of 51% to 49%. The asymmetric property induced by PM is not pulse shape dependent; for both Gaussian- and super-Gaussian-shaped pulses the Brillouin loss spectrum is symmetric for AM and asymmetric for combined AM and PM (power imbalance).     

Brillouin power spectrum analysis for partially uniformly strained optical fiber

Due to the restriction of the spatial resolution, about 1 m for current commercially available system, strain distribution measured by Brillouin optical time domain reflectometer (BOTDR) is slightly different from the actual one. In this paper, the equation of the Brillouin power spectrum for partially uniformly strained fiber within the spatial resolution is theoretically derived. Based on the derived results, investigation has been made on the shape characteristics of the superposed Brillouin power spectrum, as well as the dependence of the calculated strain of BOTDR on the actual strain of the fiber. It was found that the difference between the calculated strain and the actual strain depends mainly on the strain value of the fiber and the strained length within the spatial resolution for the given distributed sensing system.