Accuracy enhancement in Brillouin scattering distributed temperature sensor based on Hilbert transform

The scattering optical signal of OTDR based Brillouin scattering distributed sensor is very weak and have a frequency width of several decades megahertz, so it is hard to perform the traditional analogue coherent demodulation. A novel optical coherent detection based on Hilbert transform is presented here. In detail, Brillouin backscattering light is coherent detected with the reference light, which is modulated by microwave electric optical modulator to produce frequency-adjustable light, then the detected photocurrent signal is demodulated by digital signal processing based on Hilbert transform, and at last the distributed sensing signal with better S/N ratio is gained, which can enhance the performance of the sensor. The simulation and experimental results of the detection method are given.     

分布式光纤温度压力传感器设计

为了实现油田井下温度压力的全分布式测量,提出了一种基于光纤散射原理的分布式温度压力测量方法。该方法通过对普通光纤进行封装设计,制作成传感光纤。由于光纤传感器周围流体的温度和压力会对传感光纤内的散射光产生调制作用,通过光纤解调仪解调出光纤拉曼散射参数和布里渊散射频移就能够实现温度和压力的实时在线测量。实验结果表明:设计的分布式光纤温度压力传感器可以实现的温度测量分辨率为0.1 ℃,压力测量分辨率为0.07 MPa。基本满足油田井下温度压力测量的全分布式、实时在线、可靠性高、精度高、抗干扰能力强等要求。     

Brillouin fiber optic current sensor

A novel reciprocal Brillouin fiber optic current sensor has been developed and tested. The sensor is based on two counter-propagating Brillouin lasers circulating in a fiber ring cavity. It shows a sensitivity of 126 Hz/A that is very close to the maximum sensitivity obtained by using fibers without linear birefringence. The limitations due to the Kerr effect and optical feedbacks in the system are also investigated.     

基于布里渊散射的结构应力光纤测试技术研究

本文通过分析一定功率的脉冲光射入光纤中的布里渊散射规律,介绍了分布式光纤测量测量轴向应力的基本原理。制作实验装置,测量单独的应变模型,分析散射回来的波形图,初步了解应力在光纤布里渊散射波谱上的图像特征。将应力作用于光纤的不同位置,对比它们与无应力作用诗所得到的散射波形之间的图像差异,研究其对光脉冲在光纤传输过程中的影响规律。结果表明,应力的作用大小,作用位置的不同都会对脉冲光的传播造成影响,主要在于影响布里渊散射的斯托克斯光和反斯托克斯光。本次研究所得可以为分布式光纤测量提供参考,为分布式光纤在测量微型形变的应用中有一定的促进作用。本次研究的创新点在于使用滑轮的方法,解决同等应力在分布光纤的不同位置作用效果。     

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.     

布里渊背向散射光谱数据拟合算法设计与实现

针对光纤某些点处布里渊背向散射光谱的测量数据出现多峰的情况,提出采用区间划分和基于最小二乘的非线性回归方法对采集到的布里渊背散光谱进行处理.该方法不仅可以拟合单峰布里渊背散光功率数据,而且可以自动识别多峰数据并分别对几个峰进行拟合,使得计算结果能更准确反应光纤沿线的应变情况.实验结果证实了算法的有效性和可行性.     

布里渊动态光栅原理及其在光纤传感中的应用

自从2007年布里渊动态光栅被首次提出用于实现光存储以来,该技术得到了国际上的广泛关注和研究.布里渊动态光栅本质上是由相干声波场激发的折射率光栅,一般情况下两束抽运光(频率差等于光纤的布里渊频移)以相同的偏振态从光纤两端注入到光纤中,通过受激布里渊散射效应激发出相干声波场,即形成布里渊动态光栅.光纤布里渊动态光栅因具有全光产生、参数灵活可控的优点,已被广泛研究应用于光纤传感、光纤特性表征、光存储、全光信号处理、微波光子学和高精度光谱分析等.本文分析布里渊动态光栅产生和探测原理,重点探讨在高性能分布式光纤传感上的应用,这些应用包括高灵敏度温度和应变分布式传感、温度和应变同时解调、分布式横向压力传感、分布式静压力(气压或液压)传感、高空间分辨率分布式传感和高精度光谱分析.     

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     

光纤中的瞬态SBS过程的数值分析与探讨

 通过数值方法得到了光纤中的瞬态SBS过程中耦合波和声场的强度分布图，从而分析了光纤中瞬态SBS的发生过程以及光纤长度、泵浦脉宽等因素对SBS反射率及阈值的影响，据此提出了一种新的光纤相位共轭器。     

组合型光纤相位共轭器的数值模拟

 组合光纤是一种结合了小口径光纤SBS阈值低和大口径光纤损伤阈值高的特点而形成的新型光纤相位共轭器。通过建立数学模型，数值计算了组合光纤中SBS形成的过程，并对其特性参数进行了一定的理论分析。结果表明组合光纤不但可以大幅度降低SBS阈值、获得高反射率和扩大动态范围，而且通过优化其几何尺寸可以抑制Stokes波形的波动现象并显著压缩泵浦光脉冲。     

波分复用串联的布里渊散射分布式光纤传感器

在一定分辨率和响应时间要求下，应用波分复用技术来串联布里渊散射分布式光纤传感器，解决了同时进行传感和通信存在的信号干扰.具体是采用波分复用来隔离传感光信号与通信光信号，同时应用光相干外差接收来分离传感与通信之间的电信号频谱.实现了两台布里渊散射分布式光纤传感器之间的串联，单台传感器的传感距离为25 km，串联后达到50 km.     

Fiber optic distributed sensor for hydrocarbon leak localization based on transmission/reflection measurement

A novel type of distributed fiber optic sensor for petroleum hydrocarbon leak detection and localization based on the measurement of transmitted and Rayleigh backscattered power is presented. The sensor structure includes a sensitive polymer, which reversibly expands in hydrocarbon presence and induces the bending losses in the fiber. The location of the loss region is determined from unique relationships between normalized transmitted and Rayleigh backscattered powers for different positions of the disturbance along the test fiber. The localization of a strong disturbance with an estimated accuracy of ±1 m along a 2.844 km-length single-mode fiber was demonstrated.     

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.     

A novel temperature and strain sensing method based on high-birefringence fiber loop mirror

A novel temperature and strain sensor based on a fiber loop mirror (FLM) consisting of high-birefringence polarization-maintaining fibers is proposed. The output spectrum was theoretically deduced based on Jones matrix. Two equal-length high-birefringence fibers (HBFs) with different birefringence indices were inserted into the FLM. The temperature and strain sensing system was built. The sensitivity coefficients of temperature and strain were calibrated respectively. The double parameters measurement of temperature and strain was realized. High resolution of 2.38 μ? has been achieved for strain sensing. Meanwhile, the temperature resolution is 0.016 °C.     

A low-threshold multiwavelength Brillouin fiber laser with double-frequency spacing based on a small-core fiber

We demonstrate multiwavelength Brillouin fiber lasers (MWBFLs) with double-frequency spacing based on a small-core fiber (SCF) and a standard single-mode fiber (SMF), which have core diameters of 5 and 8.8 μm, respectively. Experimental results show that the SCF-based MWBFL exhibits a higher laser output power and a lower pump threshold. The output powers of the SCF-based MWBFL are > 1.4 times those of the SMF-based MWBFL. Moreover, the threshold power required to generate each channel of the SCF-based MWBFL is 59% that of the SMF-based MWBFL. When the same pump power of 180 mW is injected, the number of laser channels generated for the SCF-based MWBFL is 13, which is twice that generated for the SMF-based MWBFL. In addition, the SCF-based MWBFL exhibits good wavelength tunability from 1535 to 1565 nm and temporal stability over an hour.     

A new method for measuring the threshold of stimulated Brillouin scattering

A new method for measuring the threshold of stimulated Brillouin scattering (SBS) based on the generation location of a Stokes beam is proposed for the first time to our knowledge. The length of the medium cell is selected to be longer than the free gain length of pump pulses in the Brillouin medium. The reflected light from a certain mirror in front of the medium cell is chosen as the reference beam, and the SBS threshold is measured by the "jump" of the delay between the Stokes beam and the reference beam. An 8-ns Q-switched single-longitudinal-mode pulse is used as the pump and the typical SBS medium FC-72 is selected as the nonlinear medium in our experiment. The SBS threshold intensity is measured to be 173-178 mW/cm 2 , which is consistent with existing results measured with the transmitted energy limiting method.     

布里渊散射谱参数提取问题的混合优化算法研究与应用

提出了一种将Particle Swarm Optimization(PSO)算法和Levenberg-Marquardt(LM)算法按概率混合优化的新算法,将其用于Pseudo-Voigt型布里渊散射谱以提高其拟合度和频移提取精度。新算法以PSO算法为主框架,首先用PSO算法进行全局搜索,在优化一定次数后每次优化随机产生一个概率rand(0,1),若rand(0,1)小于或等于事先设定的概率P,则把PSO算法得到的最优解作为LM算法的初值进行局部深度搜索,将LM算法得到的最优解替换先前PSO算法的最优解,继续使用PSO算法进行全局搜索;若rand(0,1)大于P则仍然用PSO算法搜索,等待下一次优化产生随机概率rand(0,1)进行判断,交替使用两种算法直至获得理想的全局最优解。仿真分析和实验表明,新算法克服了单一算法的缺点,提高了布里渊散射谱的拟合度和频移提取的精度,充分证明了新算法的实用性和可行性。     

微小位移光纤传感器谐波分析解调法研究

本文从理论上阐述了微小位移光纤传感器的一种分频比较法解调方案,并利用全光纤干涉光路进行了实验,结果表明采用这种解调方法可提高光纤传感器的测试精度、稳定性和动态范围。     

A method to measure the Brillouin frequency shift of the medium through Brillouin amplification ratio

In order to measure the Brillouin frequency shift (BFS) of the medium, this paper proposes a method using mixtures in a two-cell stimulated Brillouin scattering system, which uses a medium to be measured as amplifier medium and a mixture medium as generator medium. The seed light from the generator gains effective amplification in the amplifier and the amplification ratio changes with the mixing fraction. Only when the BFS of the mixture medium is equal to that of the medium in the amplifier does the seed light obtain the maximum amplification ratio. The method has the advantage of independence of the wavelength of the incident light.     

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。