CN108845387B - 一种能同时测量海水温度盐度压力的楔型微孔光纤光栅 - Google Patents
一种能同时测量海水温度盐度压力的楔型微孔光纤光栅 Download PDFInfo
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Abstract
本发明提供了一种能同时测量海水温度盐度压力的楔型微孔光纤光栅，所述微孔光纤内为围绕纤芯对称分布多个微孔结构；其中一个被精确破坏外壁的微孔结构形成楔形结构；楔形结构的开角等于微孔内角θ1＝45°～55°；楔形结构及微孔表面的镀有Au膜，其由化学镀制法完成，形成传感所需的SPR传感区，膜厚在各处保持一致，适合的Au膜厚度D1＝20～40nm；所述的微孔结构在镀膜后用高热光系数的敏感材料(PDMS)填充；楔形结构沿纤芯轴向方向最靠近纤芯处，通过采用光纤刻写技术形成能够在纤芯处产生周期性折射率调制的栅区。该结构较好地解决三种参数交叉敏感问题，实现高灵敏度测量。其高集成、一体化的设计有强的稳定性，具有很大的传感应用潜力。
Description
技术领域
本发明属于微型光电子器件设计技术领域，涉及一种能同时测量海水温度盐度压力的楔型微孔光纤光栅一体化结构，主要面向于海水中的多参数高灵敏度的光纤检测技术开发，为海洋环境勘测，军事国防等领域服务。
背景技术
近年来，由于对海洋关键资源开发和长期环境勘测的技术需求不断提高，对实时高灵敏度的传感器的研制使用也提出了更高的要求。光纤传感技术在海洋探测中具有独特的优势：全光纤传感系统可沿任意路径远距离传输光信号，信息容量大，数据传输准确率高，且避免了传感器水下漏电的问题。此外它的应用成本低、寿命长、免维护，且对温度、应变、盐度等多个参量敏感，在众多原理的检测技术中脱颖而出得到广泛关注。
在保持高灵敏度的同时实现更多的参数检测同时解耦交叉敏感问题已经成为研究热点。在2004年葡萄牙学着利用串联的FBG结构实现了温度(10pm/℃)和盐度(1.28pm/‰)的双参数同时测量(文献1.Pereira,Dionisio A.,O.Fraz？O,andJ.L.Santos."Fiber Bragg grating sensing system for simultaneous measurementof salinity and temperature."Optical Engineering 43.2(2004):299-304.)。其率先提出了使用传递矩阵方式解决参数交叉敏感问题。该结构具有良好的线性度，但是检测灵敏度过低限制了使用领域。2011年澳大利亚学着使用法布里-珀罗(Fabry-Pérot)干涉原理设计的结构取得了温度敏感度(29pm/℃)和盐度敏感度(48pm/‰)(文献2.Nguyen,LinhViet,M.Vasiliev,and K.Alameh."Three-Wave Fiber Fabry–Pérot Interferometer forSimultaneous Measurement of Temperature and Water Salinity of Seawater."IEEEPhotonics Technology Letters23.7(2011):450-452.)。在灵敏度提升的同时，该原理下输出的周期性信号难以提取特征值，使引入新参数和解调都存在瓶颈。此外，近年来微纳光纤结构由于其高灵敏度引起了研究者的关注。2016年王等人使用微型定向耦合的微纳光纤结构实现了盐度(1.13nm/‰)和温度(0.93nm/℃)的高灵敏度检测(文献3.Wang,Shanshan,et al."High-Sensitivity Salinity and Temperature Sensing in Seawater Based ona Microfiber Directional Coupler."IEEE Photonics Journal 8.4(2016):1-9.)。高的敏感性带来了高的检测分辨力。但是该类结构的机械结构稳定性和重复性无法保证，同时在原理上无法解决多参数的较大的交叉敏感问题。目前表面等离子体共振(SPR)传感技术由于具有灵敏度高，制备简便，结构稳定等特点而受到广泛关注。2016年墨西哥学者使用金属膜结合温敏材料实现了折射率和温度的同时检测(文献4.Velázquez-González,JesúsSalvador,et al."Simultaneous measurement of refractive index and temperatureusing a SPR-based fiber optic sensor."Sensors&Actuators B Chemical 242(2016).)。传统的SPR传感结构经常需要通过刻蚀，拉锥等技术来增敏。
压力压强由于作用方式的特殊性经常对其他被测参量产生较大的误差干扰。将压力检测引入敏感结构实现多参数的同时检测一直是当前研究的难点。2012年，黄军等人利用FBG的增敏设计提出了具有较高应力灵敏度(1.57pm/KPa)的传感结构，同时去除了温度的干扰(文献5.Huang,Jun,et al."A diaphragm-type fiber Bragg grating pressuresensor with temperature compensation."Measurement Journal of theInternational Measurement Confederation 46.3(2013):1041-1046.)。但是该结构仅能实现单参数的检测。2016年,耿优福等利用光子晶体光纤级联FBG的结构解调实现了温度和压力的同时测量(文献6.Geng,Youfu."High-birefringence photonic crystal fiberMichelson interferometer with cascaded fiber Bragg grating for pressure andtemperature discrimination."Optical Engineering55.9(2016):090508.)。其中压力灵敏度可达3.65nm/MPa，同时由矩阵较好地消除了交叉敏感。当前在光纤结构上设计同时实现温度、盐度和压力的检测结构仍然有许多限制和待解决问题。2016年，葡萄牙学者结合模间干涉，拉锥和光纤光栅结构实现了三个参数的同时检测(文献7.Oliveira,Ricardo,etal."Simultaneous measurement of strain,temperature and refractive index basedon multimode interference,fiber tapering and fiber Bragg gratings."Measurement Science&Technology 27.7(2016):075107.)。该结构的采用了长距离下的级联设计，未能现在光纤尺寸上的结构一体化和高集成，在封装和实际应用时面临极大困难。
发明内容
本发明的目的在于实现多参数一体化的海水温度盐度压力探头的设计，提出一种结构新颖、性能优良、易于制备的可产生双折射效应的楔型微孔光纤光栅结构用以实现高性能的温度、盐度和压力的同时检测与解耦。
为了达到上述目的，本发明设计一种能产生双折射效应的楔型微孔光纤光栅一体化结构。提出以光纤SPR效应和光纤光栅原理为基础的三参数检测传感思路。建立基于该结构的SPR双折射效应和FBG应力双折射理论模型。通过结构参数的设计优化和引入传输解调矩阵实现高灵敏度的温度、盐度和压力三参数同时检测与解耦。
具体技术方案为：
一种能同时测量海水温度盐度压力的楔型微孔光纤光栅，所述微孔光纤内为围绕纤芯对称分布多个微孔结构；其中一个被精确破坏外壁的微孔结构形成楔形结构；楔形结构的开角等于微孔内角θ1＝45°～55°；楔形结构及微孔表面的镀有Au膜，其由化学镀制法完成，形成传感所需的SPR传感区，膜厚在各处保持一致，适合的Au膜厚度D1＝20～40nm；所述的微孔结构在镀膜后用高热光系数的敏感材料(PDMS)填充，其折射率：nPDMS＝1.42；楔形结构沿纤芯轴向方向最靠近纤芯处，通过采用光纤刻写技术形成能够在纤芯处产生周期性折射率调制的栅区。
进一步地，由于微孔结构的尺寸大小影响着SPR信号的双折射效应和受力情况。为保证敏感结构取得最大的灵敏度和线性度，微孔结构尺寸设有一个适宜的范围。微孔结构的尺寸为：微孔窄端距中心点间距D2＝3～7μm；孔端的径向间距D3＝35～55μm。窄端圆角R1＝6μm一般取正常加工尺寸。对称圆角取R2＝10μm。
进一步地，所述栅区的有效折射率n1＝1.48～1.52，栅距Λ＝0.51～0.53μm，栅区加工直径小于等于D2。
进一步地，上述楔型微孔光纤光栅的基底材质采用纯SiO2，n0＝1.45。楔型微孔光纤光栅的结构外半径适应通用光纤传感系统，r＝62.5μm。对楔形结构的精准加工工艺采用飞秒激光加工方式进行处理。加工流程包括：预处理，准直，刻蚀切割，清洁处理。为了保证结构产生高性能的双折射同时保证结构机械性能的稳定性。
进一步地，上述SPR传感区在敏感波段400nm～1000nm处的有效折射率：nAu＝0.3～1.6(不同波长处有对应不同的有效折射率)。
进一步地，考虑光信号损耗和应力分布，加工的栅区长度取L＝1mm。
从上述技术方案可以看出，本发明具有以下有益效果：
1)本发明中，在对称的微孔结构中引入楔形缺陷使整个波导结构产生非对称性。由此打破了宽谱光在光纤内传播的简并模式。使原本单一的特征信号分离成可分别提取的双折射信号。相比于传统单一结构，天然地形成多个特征波长，有利于双参数和多参数的测量和解调。
2)本发明中，设计了金属膜的SPR效应配合楔形微孔填充结构。其中X偏振态下其特征波长主要受缺陷处的盐度变化调制，对温度变化基本不敏感；Y偏振主要受微孔内填充的高热光系数材料的温度变化调制，对盐度变化基本不敏感。可以较好地解决温度和盐度之间的交叉敏感问题。
3)本发明中，引入光纤光栅结构，使光波导在长波长处有一个新的FBG反射峰信号。利用整体结构在机械非对称性产生应力双折射效应。单一的反射峰可分裂成两个偏振态下的独立信号。通过提取两个信号的差值可以得到远超传统光栅的应力灵敏度，同时可以基本消除盐度和温度的影响
4)本发明中，抛弃了传统光纤多参数测量的级联结构设计思路，提供了一种全光纤一体化结构设计。将多参数同时测量的敏感元件压缩至光纤μm级尺度内，真正实现了“单点”检测的目标，提高测量可信度，并有利于探头结构的封装和集成。三个参数的检测灵敏度均大大超过传统原理，可实现高精度的测量。三参数交叉敏感性被基本消除，相比于传统结构更有利于解调，有很大的实用化潜力。
附图说明
图1为楔形微孔光纤光栅的截面示意图，其中①表示微孔光纤拉制后的SiO2基底，②表示镀制的Au金属膜，③表示微孔内填充的热敏材料PDMS，④表示微孔光纤的纤芯(光波导主传光区域)，⑤表示楔形缺陷即海水环境；
图2为楔形微孔光纤光栅的径向示意图；其中⑥为纤芯处引入光栅的区域；
图3为在SPR效应激发下X，Y偏振态的双折射效应图；
图4(a)为盐度环境下X,Y偏振SPR波长的特性曲线图；图4(b)为温度环境下X,Y偏振SPR波长的特性曲线图；
图5(a)为应力影响下光栅信号发生双折射的传输光谱图；图5(b)为压力环境下FBG特征波长差的特性曲线图；
具体实施方式
为使本发明的目的、技术方案和优势更加清晰地表述。将结合结构设计思路并参照附图，对本发明的原理、具体结构参数以及性能特性作进一步的详细说明。
实施例一：
本发明提出了一种能产生种能产生高双折射效应的楔型微孔光纤光栅一体化结构。如图1所示为楔形微孔光纤光栅的截面图。首先通过堆积拉制技术制备微孔光纤(n0＝1.45)，其外径r＝62.5μm。通过微加工技术引入楔形缺形成非对称的双折射结构，楔形角θ1＝50°。在该结构中，在微孔表面和缺陷处引入金属膜：D1＝30nm。微孔中填充PDMS(nPDMS＝1.42)具有高的负热光系数，改善了结构的温敏和机械性能。微孔和楔形缺陷的相关尺寸有：D2＝5μm，R1＝6μm，D3＝45μm，R2＝10μm。
在图2中给出微孔光纤光栅的径向图。通过紫外曝光或飞秒激光加工在波导纤芯处引入折射率调制型光纤光栅结构。将该原理下产生FBG反射峰信号调制至与SPR信号不重叠波段。光栅结构参数选取一般固定为通用的参数：栅区有效折射率n1＝1.5，栅距Λ＝0.52μm，栅区加工直径r1＝3μm。加工的栅区长度取L＝1mm，光栅周期数大于1000。
首先利用有限元分析软件(COMSOL)计算在该结构下SPR双折射效应特性。其中相互垂直的偏振态X,Y的损失强度与对应的波长关系有：
αloss(dB/m)＝4π/λ·Im[neff] (1)
式中，λ为光波长；Im[neff]为在该模式下对应的有效折射率虚部。
基于以上原理图3给出了在金属SPR效应激发下X,Y偏振态的特征损失光谱。不同的偏振态在相同波长下的激发状态有明显的不同。两偏振态在损失峰最大强度和特征波长也产生分别。优化的参数选择下波长差可达Δλpeak＝30nm。非常有利于峰值的分别快速提取和计算。
图4(a)给出盐度调制时X,Y偏振态的SPR特征波长特性曲线。优化的结构设计提高了响应线性度。其中X偏振态盐度调制下有超高的灵敏度：1.402nm/‰。Y偏振态则盐度灵敏度较低。图4(b)给出温度调制时X,Y偏振态的SPR特征波长特性曲线。可以看到X偏振态对温度变化基本不敏感；Y偏振主要受微孔内填充材料的温度调制得到超高的灵敏度-7.609nm/℃。通过受力分析可得应力变化对该SPR特征波长移动没有影响。
在楔形光纤结构中引入光调制栅结构，需确定应力特性与特征波长变化的对应关系。温度和压力对三个坐标方向上的应力分布均有影响。直接提取应力偏振态下的某一个单峰值会产生极大的交叉敏感现象，不利于多参数的解调。提取两个偏振态下的FBG反射峰差值将很大地抵消径向上的应力挤压，由此极大地减小温度场的应力变化影响。
式中，n0为栅区有效折射率，p11和p12为光纤的弹光系数，ν1为泊松比，E1为杨氏模量，σx和σy分别为纤芯处X，Y方向的应力大小。由于该应力差主要由外部压力决定，这有效地消除了交叉敏感。
图5(a)给出了在压力环境调制下光纤光栅信号发生应力双折射的传输光谱图。从图中可知在1MPa外部压力影响下，原有简并的单峰FBG信号分裂成可清晰提取的两个双折射信号。图5(b)给出了大的压力跨度下FBG特征谱线差值与压力变化的关系。其在0～10MPa范围内取得了良好的线性度，以及远超不同光栅结构的灵敏度：-1.709nm/MPa。
引入传输矩阵建立被测参量与波长特征提取量之间的关系：
矩阵的单位分别为：℃/nm，‰/nm，MPa/nm。以上结构证明该楔形光纤光栅一体化结构可在微小尺度实现三参数的同时测量和解调任务。
实施例二：
本发明提出了一种能产生种能产生高双折射效应的楔型微孔光纤光栅一体化结构。如图1所示为楔形微孔光纤光栅的截面图。首先通过堆积拉制技术制备微孔光纤(n0＝1.45)，其外径r＝62.5μm。通过微加工技术引入楔形缺形成非对称的双折射结构，楔形角θ1＝55°。在该结构中，在微孔表面和缺陷处引入金属膜：D1＝20nm。微孔中填充PDMS(nPDMS＝1.42)具有高的负热光系数，改善了结构的温敏和机械性能。微孔和楔形缺陷的相关尺寸有：D2＝3μm，R1＝6μm，D3＝35μm，R2＝10μm。
在图2中给出微孔光纤光栅的径向图。通过紫外曝光或飞秒激光加工在波导纤芯处引入折射率调制型光纤光栅结构。光栅结构参数选取参照实施例1。
首先计算金属SPR效应激发下X,Y偏振态的特征损失光谱同实施例1。两偏振态在损失峰最大强度和特征波长也产生分别。优化的参数选择下波长差可达Δλpeak＝15nm。有利于峰值的分别快速提取和计算。
计算盐度调制时X,Y偏振态的SPR特征波长特性同实施例1。其中X偏振态盐度调制下有灵敏度：0.537nm/‰。Y偏振态则盐度灵敏度较低。图4(b)给出温度调制时X,Y偏振态的SPR特征波长特性曲线。可以看到X偏振态对温度变化基本不敏感；Y偏振主要受微孔内填充材料的温度调制得到超高的灵敏度-2.208nm/℃。通过受力分析可得应力变化对该SPR特征波长移动没有影响。
计算压力环境调制下光纤光栅信号发生应力双折射的传输特性同实例1。在0～10MPa范围内取得有较为线性灵敏度：-0.412nm/MPa。
引入传输矩阵建立被测参量与波长特征提取量之间的关系：
矩阵的单位分别为：℃/nm，‰/nm，MPa/nm。以上结构证明该楔形光纤光栅一体化结构可在微小尺度实现三参数的同时测量和解调任务。
实施例三：
本发明提出了一种能产生种能产生高双折射效应的楔型微孔光纤光栅一体化结构。如图1所示为楔形微孔光纤光栅的截面图。首先通过堆积拉制技术制备微孔光纤(n0＝1.45)，其外径r＝62.5μm。通过微加工技术引入楔形缺形成非对称的双折射结构，楔形角θ1＝45°。在该结构中，在微孔表面和缺陷处引入金属膜：D1＝40nm。微孔中填充PDMS(nPDMS＝1.42)。微孔和楔形缺陷的相关尺寸有：D2＝7μm，R1＝6μm，D3＝55μm，R2＝10μm。
在图2中给出微孔光纤光栅的径向图。通过紫外曝光或飞秒激光加工在波导纤芯处引入折射率调制型光纤光栅结构。光栅结构参数选取参照实施例1,2。
计算金属SPR效应激发下X,Y偏振态的特征损失光谱同实施例1,2。两偏振态在损失峰最大强度和特征波长也产生分别。优化的参数选择下波长差可达Δλpeak＝20nm。有利于峰值的分别快速提取和计算。
计算盐度调制时X,Y偏振态的SPR特征波长特性同实施例1,2。其中X偏振态盐度调制下有灵敏度：1.279nm/‰。Y偏振态则盐度灵敏度较低。图4(b)给出温度调制时X,Y偏振态的SPR特征波长特性曲线。可以看到X偏振态对温度变化基本不敏感；Y偏振主要受微孔内填充材料的温度调制得到超高的灵敏度-4.572nm/℃。通过受力分析可得应力变化对该SPR特征波长移动没有影响。
计算压力环境调制下光纤光栅信号发生应力双折射的传输特性同实例1,2。在0～10MPa范围内取得有较为线性灵敏度：-0.823nm/MPa。
引入传输矩阵建立被测参量与波长特征提取量之间的关系：
矩阵的单位分别为：℃/nm，‰/nm，MPa/nm。以上结构证明该楔形光纤光栅一体化结构可在微小尺度实现三参数的同时测量和解调任务。
Claims (3)
1.一种能同时测量海水温度、盐度、压力的楔型微孔光纤光栅，其特征在于，所述微孔光纤内为围绕纤芯对称分布多个微孔结构；其中一个被精确破坏外壁的微孔结构形成楔形结构；楔形结构的开角等于微孔内角θ1＝45°～55°；楔形结构及微孔表面的镀有Au膜，形成传感所需的表面等离子体共振传感区，膜厚在各处保持一致，适合的Au膜厚度D1＝20～40nm；所述的微孔结构在镀膜后用高热光系数的敏感材料填充；楔形结构沿纤芯轴向方向最靠近纤芯处，通过采用光纤刻写技术形成能够在纤芯处产生周期性折射率调制的栅区。
2.根据权利要求1所述的能同时测量海水温度、盐度、压力的楔型微孔光纤光栅，其特征在于，微孔结构的尺寸为：微孔窄端距微孔光纤的中心点间距D2＝3～7μm；孔端的径向间距D3＝35～55μm。
3.根据权利要求2所述的能同时测量海水温度、盐度、压力的楔型微孔光纤光栅，其特征在于，所述栅区的有效折射率n1＝1.48～1.52，栅距Λ＝0.51～0.53μm，栅区加工直径小于等于D2。
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