【作者】 顾海栋；

【导师】 张广玉； 董惠娟；

【作者基本信息】 哈尔滨工业大学 ， 机械电子工程， 2013， 博士

【摘要】 基于分布式光纤布里渊频移的传感器不仅具有一般光纤传感器特有的防爆、耐候等特点，而且可以实现数十公里的分布式多参数测量，较适用于井下工作要求。该技术在温度和形变测量方面，已经成功应用于某些大型工业结构和建筑的健康监测，然而，在液体压力检测方面却鲜有报道，更未见用于井下压力参数的直接测量。因此，本文以油田重要地质参数——压力监测为背景，研究基于分布式光纤布里渊频移的液体压力检测技术。在分析了分布式光纤布里渊频移测量原理和方法的基础上，本文首先构建了基于光纤受激布里渊散射（SBS）的布里渊光时域分析（BOTDA）实验平台。在不同脉冲宽度入射光下，对该实验平台所测得的布里渊散信号进行洛仑兹拟合优度评估；并与商用布里渊光时域反射计（BOTDR）作了关于散射信号线宽的比较分析，从而验证了该实验平台比BOTDR具有更高的布里渊频移测量精度。该实验平台为本文后续工作的主要实验载体。基于BOTDA技术，本文对两组不同裸光纤进行了布里渊频移的液体压力响应实验。实验表明，布里渊频移与光纤所受到的压力存在负线性关系，对于本文实验所用的G652标准单模裸光纤，其比例系数约为-0.742MHz/MPa。并且，该结果与基于块状硅玻璃的理论推导结果基本一致。此实验和分析验证了光纤布里渊频移在压力作用下的线性敏感特性，为利用基于分布式光纤布里渊频移的传感器进行液体压力检测奠定了基础。为了跟一般情况下光纤不受径向应力时的布里渊频移与光纤应变关系进行比较和统一，本文还设计了一复合力学实验，用于解耦压力导致的光纤轴向应变和径向应变，从而获得了布里渊频移同时与光纤轴向应变和径向应变关系模型的数学表达式。实验结果显示，光纤布里渊频移是这两种应变复合作用的结果，并与这两种应变都成线性关系，但它们的比例系数却存在很大差异，其中轴向应变系数实验值为0.053MHz/μ，径向应变系数实验值为0.029MHz/μ。此实验结论扩展了光纤布里渊频移关于应变的传感理论，并为利用不同光纤护套实现不同压力灵敏度提供理论基础。基于上述布里渊频移与光纤轴向和径向应变的关系模型，本文进而通过数值分析和实验验证方法，量化分析了典型双层聚合物护套结构光纤的外层护套各参数对布里渊频移的温度灵敏度和压力灵敏度影响。分析表明：这两种灵敏度都能通过聚合物护套得以大大增强，其中，温度灵敏度与护套泊松比关系不大，而随着护套的弹性模量、厚度、热膨胀系数增加而增大；然而，压力灵敏度却随着护套弹性模量和泊松比增大而减弱，只能随着护套厚度增加而增强。这些分析为后续利用不同护套结构设计不同温度和压力灵敏度的分布式布里渊传感器提供依据和参考。虽然布里渊频移对压力敏感，但同时也对温度敏感，即压力和温度交叉敏感，而且在现场应用中又难以保持恒温环境。于是，本文利用上述不同护套结构的光纤具有不同压力和温度灵敏度系数的特点，设计了基于双通道分布式光纤布里渊频移的传感器，解决了压力和温度的交叉敏感问题，并实现了压力和温度的同时测量。而且，本文还对该类双通道布里渊传感器的测量误差进行了理论分析和设计质量评估，并得到了基于误差分析的设计准则。以本文所构建的双通道分布式光纤布里渊传感器为例，其理论压力精度和温度精度分别为0.256MPa和0.284°C，此结果对于30MPa左右的井下压力已达到高于1%的精度，而且通过护套的优化设计还可以实现更高精度。最后，本文在井下压力监测的实用化方面做了相关研究工作。不仅根据井下地层分层分块的特点，设计了井下准分布式光纤布里渊压力传感器方案；还根据井下空间狭长的特点，围绕传感探头的小型化做了相关实验和研究：一方面，对于光纤缠绕式传感探头，本文通过实验研究了光纤缠绕对背向布里渊散射信号强度的影响，其结果显示，对于普通单模光纤，为避免布里渊散射信号的强度受到明显影响，光纤缠绕的直径应大于30mm，即所设计的保护壳体的内径应大于30mm；另一方面，对于非缠绕式传感探头，提出了错位布里渊频移法以优化传感器在局部位置的最小可测量长度，使作为传感探头的光纤最短长度可由传统的1m缩短到30cm以下，并且提高被测量（温度或光纤应变）较小时的频谱拟合精度和测量精度。更多还原

【Abstract】 The sensors which based on the Brillouin frequency shift (BFS) distributedalong the optical fibers have advantages of explosion-proof and weatherability, aswell as distributed sensing along dozens of miles. And they are comparativelysuitable for downhole applications. This technology has been widely employed inhealth mornitoring for many engineering infrastructures and architectures bypossessing the ability of sensing temperature and strain. However, there are fewreports on hydrostatic pressure measuring by this technology, and also it has notbeen introduced to pressure monitoring in downhole. Therefore, this work studieshydrostatic pressure detection technology based on the BFS distributed along theoptical fibers with the background of mornitoring an important geologic paremeter,i.e. pressure, for oil well.After studing the basic principle and method of measuring the BFS distributedalong the optical fibers, this paper sets up an experimental arrangement of Brillouinoptical time domain analysis (BOTDA) based on stimulate Brillouin scattering(SBS). The brillouin spectrums stimulated by different pulse-width are evaluated bygoodness of Lorentz fitting, and the full width at half maximum (FWHM) arecompared with a commercially available Brillouin optical time domain reflectory(BOTDR). And thus it is validated that this experimental arrangement has highprecision for BFS measurement. This experimental arrangement is the mainworkbench for the following work.Based on the BOTDA technology, this study carries out a trial on the responseof the BFS to pressure along two different bare fibers. The experimental resultshows that the BFS has a linear relation with the applied pressure, and thepropotional coefficient is-0.742MHz/MPa for the standard (G652) single modefibers (SMF). What’s more, this result has a good agreement with the theoreticalanalysis according to bulk silica glass. This experiment and analysis offers afoundation for pressure sensing based on the distributed FBS along optical fibers.To compare with the exist relation between the BFS and strain while the fiberssuffers no radial stress, this research designs a mechanical experiment to decouplethe axial strain and radial strain due to pressure along the fiber. This experiment gains the mathematical expression model of the relation between the BFS and bothaxial and radial strains. And the result shows that the BFS in fiber can be caused byboth strains, and has linear relations with the both strains. Yet, the propotionalcoefficients are different; the axial strain coefficient is0.053MHZ/μ, whereas theradial strain coefficient is0.029MHz/μ. This conclusion extends the relationbetween the BFS and strain, and can be a theretical basis for enhancing pressuresensitivity of the BFS by different coatings.Based on the relation model between the BFS and both axial and radial strains,it have been studied with theoretical and experimental anylysis that the influences ofthe fiber coatings on both the pressure sensitivity and temperature sensitivity of BFSwithin typical double coated fibers. The analysis shows that both the sensitivitiescan be enhanced by polymer coatings. The temperature sensitivity of the BFS growswith the Young’s modulus, thickness and thermal expansion coefficient of the outercoating and has neglegt relation with Poisson’s ratio of the coating. For pressuresensitivity, it decreases with the increasement of Youn’s modulus and Poisson’s ratioof the outer coating, and it can be enhanced by the increasement of the coatingthickness. This analysis can be a reference for designing senors with differenttemperature/pressure sensitivities by utilizing different fiber coatings.Although the BFS is sensitivitive to pressure, it is also sensitivitive totemperature. That is to say, the BFS has cross sensitivity to pressure and temperature.And it is hard to keep the temperature constant at field. Accordingly, this workdesigns a dual-path sensor based on the BFS distributed along optical fibers bymaking use of different coated fibers with different pressure/temperaturesensitivities. Thus it resolves the issue of cross-sensitivity between pressure andtemperature and can measure pressure and temperature simultaneously. Moreover, itevaluats the design quality of the sensor by error analysis, and gains the designprinciple to reduce error. For the dual-path sensor designed in this paper, itsprecision can reach theoretically up to0.256MPa and0.284°C, respectively. Thusthe precision of the pressure can be higher than1%of the pressure in downhole(more then30MPa), and it can be further enhanced by designing the configuration ofcoatings.Finally, this work has studied some practicalized researches for pressuredetection in downhole. It not only designs a quasi-distributed fiber Brillouin sensor for downhole pressure sensing, but also studies the small-size design for sensingheads since the space is nallow in downhole. One hand, it studies the effect of thefiber winding on the intensity of the Brillouin spectrum. To avoiding abviousweakening, the winding diameter should be larger than30mm for commensingle-mode fiber. On the other hand, it develops a technique for high spatialresolution locally at the sensing sections by dislocating the BFS. And the techniquecan enhance the frequency accuracy when the measurand (temperature or strain) issmall and the length is also smaller than the tradional spatial resolutions. It can thusreduce the spatial resolution from1m to30cm or less.更多还原