中英文文献翻译-使用了光纤光栅技术的光学水位传感器

使用了光纤光栅技术的光学水位传感器基于光纤光栅(FBG)技术我们开发了一种光学高精度水位传感器。该传感器可用于测量河流,湖泊和污水处理系统的水位。传感头由一个膜片,一个特制的弹簧管和两个光纤光栅组成,一个用于张力测量另外一个用于温度补偿。光纤光栅贴在弹簧管上，由于水位的上升，来自光纤光栅的发射光，使中心波长改变。通过波长检测设备检测中心波长，波长检测设备由一个调谐F-P滤波器组成。我们实现了传感器精度为土0.1%即为土1CM，水位测量范围为10m。几个传感头可以通过一个光纤串联在一起。用一块波长讯问设备可以同时测量不同的水位。1简介：河川管理员具有非常重要的责任,是维护其河设施和监测流域状况。由于河川设施是沿着河流域分散的，管理员要定期检查这些设施。这一工作是非常花费员工的时间和体力的。此外,很难对这些设施进行抽查,以确定其结构完整性,在发生自然灾害,如洪水冲刷,地震干扰。从这个角度出发，日本政府沿着其河川流域建立了光纤网络。利用这种光纤网络近日有效地研制光纤传感器和传感系统,而适用于流域防洪设施的安全管理。光纤传感器及其系统,有以下的优点：低传输损耗使光纤遥感超过几十公里。实时监控是通过传感头连接在一起，测量设备直接使用光纤。传感头为全光通信无源器件,不需要进行电力供应，传感器头已装入。由于是不受电磁干扰的一个光学传感器,它不遭受触电有害影响的,如发生了雷击。水位传感是一个河川最关键的问题,无论是河流管理者和附近居住的立足点都是一个流域设施的维修以及预防自然灾害。我们已开发出光学水位传感器利用光纤光栅(FBG),拥有的优势比上述更多。在本文中,我们详细阐述了光纤光栅作为应变传感器,水位传感应用的业绩,获得了实用领域。2光纤光栅技术及传感应用：光纤光栅的原理FBG是一个沿着给定长度光纤的折射率的调制解调器。图1所示光栅的结构示意图，由于耦合关系向前和向后传播方式,特定波长的光前其折射率取决于调制周期，反映在光栅位置上。入射光反射时它的波长等于布拉格光栅波长九,B定义为下式：九=2nA(1)Beffn—是有芯模有效折射率，A—是光栅周期。其他波长的光传送通过FBGeff及下一个光纤光栅的照射。因此，他是一个可以实现多点传感在一个串联的光纤光栅上。各自有不同的反射波长。光谱的反射光来自光栅，可实现对应力和温度的分别测量，布拉格波长的变化量AX和应变和温度的关系。BA^b二(1-P)t+g.AT(2)尢8B其中P是有效的光弹性系数，£是应变，g是热光系数，AT是温度的变化量。£使用普通的单模光纤通信，他的有效光弹性系数P是0.22，灵敏度在应用轴向£应变和温度补偿1.2分/微应变和10分/°C分别在1.55微米波长范围内。2.2光纤光栅的制作图2显示了光纤光栅的制作工艺，我们使用了聚酰亚胺涂层光纤的光纤光栅作为应变传感器，在边界上覆盖涂层不让其滑动，使外部应变正确地转移到光栅上。用来制造光纤光栅的光纤用经过紫外线的照射处理。提前写光栅，聚酰亚胺涂层光纤受到一掺氢处理，为了获得足够的光致折射率变化量，掺氢过程是在紫外线照射之前完成。聚酰亚胺涂层，部分外用一种化学溶剂溶蚀。KrF准分子激光器的输出波长为248nm,随着纵向扫描通过相位掩膜法fbg刻在光纤核心区域。底片感应折射率调制沿着光纤，即趾剖面可以控制的可编程扫描速度的激光束。我们采用高斯切趾法，以抑制分辨损失的反射光谱。之后写光栅，加聚酰亚胺涂层是为了保护剥离部分。最后，光纤与光纤光栅受到高温（300C）热处理30分钟，为确保长期稳定。3光学水位传感器用光纤光栅结构光学水位传感器图5和图6分别显示了光学水位传感器结构示意图和原理图，传感器是一个压力转换元件。传感头由一个膜片，一个特制的弹簧管和两个光纤光栅组成，一个光栅是用于张力测量，另一个光栅用于温度补偿。这一水位传感器用于测量水压和水位的比例，水压传送到内部有硅油的弹簧管。这是弹簧管将油压转换为弹簧管尖的张力，然后利用玻璃管的高弹性将油压转换为管部顶端的张力并且扩张到FBG,即在顶端和底部串成一串。因此,光纤光栅是拉伸比例随水位上升。光纤光栅是有轻微的预拉伸初期，为使测量水位直线下降，最低水平布拉格波长随温度的变化而变化，也和石英玻璃的遮光系数有关。补偿的温度依赖性，另一种是光栅串联与光栅传感头拉伸测量。传感器在安装温度补偿的光纤光栅是在不受任何压力（如水压力）的基础上的。内在波长漂移A九比例的水位变化的计算：WLA九二A九-K・A九WLtemstemp这里AXt是贴在弹簧管上的光纤光山的波长，其中包含温度的动荡，A九tenstemp是测量过温度补偿了的光纤光栅的波长，K是光纤光栅的温度系数，将这个值置1来校准光纤光栅设备。气压传感器的平衡情况与外部大气压力的空气感应水管，内部含有光纤的电缆有关。光学水位传感器的测量结果图7显示来自光纤光栅传感器测量系统反射光的探测波长。宽带光来自ASE光源发射的光经过FBG反射后经由三个循环点返回的光。通过另一个循环点走向F-P滤波器作为光谱仪器。其他波长的光通过光纤光栅,并传递到下一个光纤光栅,不同波长的光反射也不同。因此，通过将光纤光栅串联，一个光纤传感器就可以实现多点测量水位，这主要是由光纤光栅反射不同波长。光纤使用几个光学开关开关，该系统可连接多传感器获得的测量结果采用单一波长访问设备。终端用户的F-P可调谐滤波器是700个，是传输带宽的一半，最高接近0.1nm,而用波长范围是1530至1570nm左右。调谐滤波器正在驱动压电换能器，50赫兹三角波电压扫描距离已广泛应用于F-P滤波器标准。为改善波长准确性，我们采用了温控光栅作为参考，这表明有固定波长低于±1时波动，而这些数据是平均值的10倍以上，以减少随机噪声。结果如图8所示。当平均超过10倍，波长波动可以抑制小于5pm。我们设计的最大应变适用于在10米的水压，FBG拉伸测量0.3%左右。它相当于一个3.6nm的中心波长的光纤光栅。为了核实准确的水位测量，我们测量了波长漂移的10倍。运用伪水压力传感器膜片空气稳压器，图9显示应用压力表测量压力和应用光纤光栅传感器测量波长的对应关系。结果表明，水压力有良好的线性关系，且每个测量点的误差小于3pm/cmH2O,我们衡量在0,20和40°C，这些特定的温度范围内这种传感器的特性。结果如图10所示，纵坐标表示水位测量的误差计算所得的线性拟合线，测量结果依赖于压力和波长两者的变化。在每个温度条件下，其误差为每点不到±1cm。我们证实了这种传感器的重复性和耐久性，适用于测量水位在0到3米反复变化。图11的结果显示，即使在100倍的压力下，测量结果也接近真实值，测量误差在±1cm以内。我们观察了传感器因温度波动或金属疲劳要素导致结果的漂移和蠕变。光学水位传感器在河川中的实用性能从1999年8月至2001年3月我们在Kakehashi河流和Kanazawa工作办公室内进行了传感器系统的实地测试。通过这次实地测试我们证实了传感器和测量设备的稳定性。他们操作超过一年半河流水位测量精度达到土1cm，无致命的系统误差。接下来，我们安装了这一系统配有三个传感器，在Uono和aburuma河，shinanogawa工作办公室，hokuriku区域发展于2001年11月。系统图及水位测量的结果显示，分别如图12和图13所示：测量的同时，利用传统的电热水位仪表进行测量，二者安装在同一个测量点。两种类型的传感器水位测量数据大致是相符的，二者的不同的是光学水位传感器的测量误差小于±1cm。通过实地测量我们证实了光学水位传感器在液位测量上的优势，而且它可以远程和实时监控，在传感点上不需要电源供电。结论们开发的这种全光学的水位传感器，其测量精度具有±0.1%，水位测量范围为10m。目前已经在很多位置建立这种传感器系统，如河川流域、湖泊和污水处理系统，在那里持续工作，用来测量其实际水位。传感器技术到目前为止，我们考虑到电磁辐射的自然特性，其来源和作用是物体和材料之间的接触。据指出，感觉到的大部分的辐射或放射目标，通过空气直到它由传感器监测。传感器的组成和它的运行是重要的学科范围。它涉及的太多了以至于不能用一种方法讲解。虽然如此，一些基本的概要在这能得到阐述。传感器技术全面整体回顾。由日本的遥感协会开发，而且在互联网上找到这个镜象站点。一些传感器的有用的连接和它的应用在这个叫做的NASA的地方包括了。我们指出许多读者现在使用复杂的传感器，这些传感器使用的是下述的技术:数码相机;更多是在这页底部的常用的传感器。很多遥感器都被设计来测量光子。传感器运行的潜在的基本原则主要集中在一个重要的元件上即探测器。这就是光电效应的概念（是爱因斯坦首先详细地解释了，并且赢得了他的诺贝尔奖。他的发现是，在光子物理学发展上的关键性的一步。这点简单得说，当一些适当的光敏材料一块服从光子射线时将有负电子的放射。电子可能在板材中流动，被收集，然后算作是信号。一个关键点是：产生的电流的幅度（单位时间内产生光电的数量）与光强度成一定比例。因此，电流的变化可以用来测量光子的变化（数量；强度），光子在指定的间隔时间触击扳子。被发布的光电子的动能随冲击辐射的频率（或波长）而变化。但是，不同的材料接受光电效应释放的电子有不同的波长间隔时间；每个都有一个现象开始的门限波长并且有它停止的一个更长的波长。现在，有了建立在多数遥感器的操作的基础上的原则，让我们总结传感器类型（典型）几种主要想法在这两张图上：一是几组传感器的一种功能处理，被作为三角图，壁角成员由主要参量的测量来确定；光谱；空间；强度。从这张宏伟的名单，我们将集中讨论光学机械电子。辐射计和扫描器，在别处把照相机影片系统和主动雷达留为考虑，主题是在讲解和热量系统的描述对一个最小值（见第9部分的进一步处理）。上面的小组包括我们主要考虑及早在这个部分的地球物理的传感器。传感器（能量导致辐射被接受来自一个外部来源，即太阳）和积极得（能量引起了从传感器系统内部，放光向外，并且分数返回被测量）。传感器可能是非想象的（测量辐射被接受从所有点在感觉的目标，报告结果作为电子信号强度或某一其它定量属性，譬如发光）或想象；因为在目标辐射与具体点有关，，最终结果是影象［图片］或光栅显示［如同平行线｛地平线｝在电视屏幕。辐射计是一个一般用为定量地测量EM辐射在EM光谱的在某一间隔时间的一种仪器。当辐射是从狭窄的光谱带且包括可看见光，光度计可能被替代。如果传感器包括一个组分，譬如棱镜或衍射滤栅，可能打破辐射延伸在光谱分离波长和驱散（或分离）它们的部份在不同的角度对探测器，这称分光仪。一种类型的分光仪（被用在实验室为化工分析）通过多种波长辐射到一个裂缝再生产裂缝依照被排行在各种各样的间距在影片板材的一个分散的媒介。很多空气/空间传感器都是光谱声音仪。能瞬间地测量辐射立即来自整个场面的传感器叫做构筑的系统，眼睛，照相机和电视光导摄象管属于这个小组。被构筑场面的大小由定义视野的开口和光学系统，或FOV确定。如果场面是点由点感觉的（等效与小范围在内）在有限计时连续线之内，这个测量方式组成扫描系统。多数非照相机传感器扫描操作从移动的平台图象场面开始。行动的进一步下来分支，想象传感器的光学设定为可能是图象飞机或光学飞机聚焦（取决于光子光芒）依照被显示聚合的地方，在这个例证中能看出来。在这个分类的其它属性是传感器经营在非扫描或扫描方式的地方。这是可能有几个意思相当棘手的对期限扫描暗示行动横跨场面的，在间隔时间并且非扫描提到传感器被修理在现场或目标利益照原样感觉在非常片刻内。影片照相机被刚性地拿着是非扫描的设备，当快门被打开夺取光几乎是瞬间的描设，然后关闭。但当照相机并且/或者目标可能是静态的（不移动）但是传感器清扫横跨感觉的场面，可能扫描因为传感器被设计使它的探测器系统地行动既使他们推进横跨目标。这是您也许栓了入您的计算机的扫描器的情况。这里它的平板车平台（图片被安置）的框和玻璃表面被留在原地；扫描可能被执行通过投入了图片或纸文件在一转动的鼓上（两个方向:圆圈和在鼓轴方向的转移）,在扫描照明上是一条固定的射线。或者，另外两个相关的例子：摄像机包含光击中那光子的敏感表面而产生电子被去除在连续的光导摄象管（线每英尺是电视性能）能有措施被固定或可能旋转对打扫在场面（自身空间扫描操作）的英寸并且可能及时扫描当它继续监测场面的时候。一个数字相机包括有包含被释放他们的光子导致的电子在连续的转变成变化的电压信号的X-Y一些探测器。放电的发生是由系统地扫描探测器引起的。相机本身能被固定或移动。所有这要义（在某种程度上明显）是，期限扫描可能向整个传感器的运动被申请和，在它的更加共同的意思,对一个或更多组分在检测系统或移动轻的汇聚，场面观察用具或光或辐射探测器逐个读导致信号的过程。多数扫描器两个宽广的类别由期限〃光学机械〃和〃光学电子〃定义,由区别参加扫描场面和后者由有感觉的辐射移动直接地通过光学线性或列阵探测器。另一个属性遥感传感器，而不是表现在分类涉及到模式中的那些遵循一些向前发展的轨道（简称为轨道或航道）搜集他们的资料。这样做，据说能监测道路通过在区域道路的对面;这就是带宽。遥感探测器阵列通常小于整个场面的带宽,这比整个场面带宽通常狭窄的光被通过外径，包含天文望远镜全视场角（普通望远镜）。OpticalWater-LevelSensorsusingFiberBragg

GratingTechnologyABSTRACT:Wedevelopedanopticalhigh-precisionwater-levelsensorsbasedonfiberBragggrating(FBG)technology.Thesensorscanbeappliedtomeasurethewaterlevelsofrivers,lakes,andsewagesystems.Thesensorheadconsistsofadiaphragm,acustomizedBourdontubeandtwoFBGs,onefortensilemeasurementandtheotherfortemperaturecompensation.TheFBGattachedtotheBourdontubeisstrainedasthewaterlevelincreasesandcausesashiftofthecenterwavelengthofthereflectedlightfromtheFBG.ThecenterwavelengthisinturndetectedbythewavelengthinterrogationequipmentcomposedofatunableFabry-Perotfilter.Weachievedthesensoraccuracyof+/0.1%F.S.,i.e.,+/1cmforthe-lefvuelllmewaastuerermentrangeof10m.Severalsensorheadscanbeconnectedinseriesthroughoneopticalfiber,andthewaterlevelatdifferentplacescanbemeasuredsimultaneouslybyusingonepieceofwavelengthinterrogationequipment.(1)INTRODUCTIONRiveradministratorshavetheveryimportantresponsibilitiestomaintaintheirriverfacilitiesandmonitorriverbasinconditions.Sinceriverfacilitiesarewidelydispersedalongariverbasin,itisagreatexpenseoftimeandlaborforthestaffofriveradministratorstogoaroundperiodicallytocheckthesefacilities.Moreover,itisdifficulttoperformspotchecksonthesefacilitiestoascertaintheirstructuralintegrityintheeventofnaturaldisasterssuchasfloods,washouts,andseismicdisturbances.Fromthispointofview,projectstobuilduptheopticalfibernetworksaroundriverbasinshavebeenadvancedbytheJapanesegovernment.Toutilizesuchanopticalfibernetworkeffectively,opticalfibersensorsandsensingsystemshavebeenresearchedanddevelopedrecently,whichareapplicabletothesafetymanagementofriverbasinsandtheirfloodcontrolfacilities.Opticalfibersensorsandtheirsystemshavethefollowingadvantages:Thelowtransmissionlossofopticalfiberenablesremotesensingovertensofkilometers.Real-timemonitoringisachievedbyconnectingsensorheadsdirectlytothemeasuringequipmentusingopticalfiber.Thesensorheadconsistsofall-opticalpassivecomponentsandthereisnoneedforanelectricpowersupplywherethesensorheadsareinstalled.Duetoimmunityfromelectromagnaticinterferenceofanopticalsensor,itdoesnotsufferfromthedeleteriouseffectsofelectricalshock,suchastheoccurrenceoflightningstrikes.Thewaterlevelsensingofariverisamostcriticalmatterforboththeriveradministratorsandtheresidentslivingaroundariverbasinfromthestandpointofthefacilitymaintenanceandthepreventionofnaturaldisasters.Wehavedevelopedopticalwater-levelsensorsusingfiberBragggratings(FBGs)thatpossesstheadvantagesdescribedaboveandafewmore.Inthispaper,wedescribedetailsoftheFBGasastrainsensor,waterlevelsensingapplicationsandtheperformanceresultsobtainedinapracticalfield.(2)FIBERBRAGGGRATINGTECHNOLOGYFORSENSINGAPPLICATIONPrincipleofFBGTheFBGisapermanentperiodicmodulationoftherefractiveindexalongagivenlengthofopticalfiber.Figure1showstheschematicstructureoftheFBG.Duetothecouplingbetweentheforwardandbackwardpropagatingmodes,thespecificwavelengthlightdependingonthemodulationperiodoftherefractiveindexisreflectedatthelocationoftheFBG.TheincominglightisreflectedwhenitswavelengthisequaltotheBraggwavelength九Bofthegrating,definedas(3)XB=2NeffA,(1)whereNeffistheeffectivecorerefractiveindexandAistheperiodoftherefractiveindexmodulation.TheotherwavelengthlightsaretransmittedthroughtheFBGandirradiatethenextFBGs.Thus,itispossibletoachievemulti-pointsensingatoneopticalfiberbyconnectingFBGsinseries,eachofwhichhasadifferentreflectedwavelengthfromtheothers.I£WavelengthReflectionQFBGWavelengthTransmission\I£WavelengthReflectionQFBGWavelengthTransmission\jTonextFBG\⑥neffCladdingGratingdistanceA——+4•才WavelengthFig.1-SchematicstructureofFBG.Thespecificwavelengthlight,dependingonthemodulationperiodoftherefractiveindex,isreflectedfromtheFBG.TheotherwavelengthlightsaretransmittedthroughtheFBGandirradiatethenextFBGs.ThespectrumofthereflectedlightfromtheFBGisshiftedaccordingtotheappliedstrainandtemperature.TheBraggwavelengthchangeAXBinresponsetostrainandtemperatureisgivenby冶(1—P£)•£+了AT,(2)wherePsistheeffectivephoto-elasticcoefficient,£istheappliedstrain,gisthethermo-opticcoefficient,andATisthechangeintemperature.Usingordinarysinglemodefiberfortelecommunication,theeffectivephoto-elasticcoefficientP£is0.22,(3),(4)sothesensitivitytoanappliedaxialstrainandthetemperaturedependenceare1.2pm/micro-strainand10pm/°C,respectivelyin1.55pmwavelengthrange.FabricationofFBGFigure2showstheprocessforfabricatinganFBG.Weuseapolyimide-coatedopticalfiberfortheFBGsasthestrainsensor,whichcausesnoslippageontheborderofthecoatinglayer,sothatexternallyappliedstraincanbepreciselytransferredtotheFBG.TheFBGisfabricatedbytheUVirradiationofthefiber.InadvanceofwritingtheBragggrating,thepolyimide-coatedopticalfiberissubjectedtoahydrogenloadingtreatmenttoenhancetheeffectofphoto-inducedrefractiveindexchange.Thepolyimidelayeristhenpartiallyetchedawaybyusingachemicalsolvent.AsshowninFig.3,theBragggratingisinscribedinthecoreregionoftheopticalfiberbyirradiatingaKrFExcimerlaseremittingat248nmthroughaphasemask,withthescanninginthelongitudinaldirection.Photo-inducedrefractiveindexmodulationalongthefiber,i.e.,apodization

profilecanbecontrolledbytheprogrammablescanningspeedofthelaserbeam.WeadoptedaGaussian-shapeapodizationtosuppressside-lobelossesofthereflectionspectrum.Afterinscribingthegrating,thepolyimidelayerisre-coatedtoprotectthestrippedpart.Finally,theopticalfiberwithaFBGissubjectedtoahigh-temperature(under300U)heattreatmentfor30min.toensurelong-termstability.]Hydrogenloadingtreatment|HillHeattreatment300-•00min.JCDaCoreFBGPhasemask]Removingpolyimidelayer]InscribingBragggrating]]Hydrogenloadingtreatment|HillHeattreatment300-•00min.JCDaCoreFBGPhasemask]Removingpolyimidelayer]InscribingBragggrating]Re-coatingpolyimidelayerMovingmirror-KrFlaserTOpticalfiber-▼/Figure2Figure3Figure4showsthetypicaltransmissionandreflectionspectrumofanFBG.Transmissionrejectionwasabout-10dB,thatis,thereflectionratiowasalmost90%.TheFWHM(Fullwidthat?0右p)flWavelength(nm)?0右p)flWavelength(nm)(b)ReflectionUO這匚2<halfmaximum)ofthereflectionwaslessthan0.2nm*Q0\ii1,5451,5501,555(3)0PTICALWATER-LEVELSENSORUSINGFBG(a).Transmission3.1Structureofopticalwater-levelsensorFigures5and6showtheschematicstructureoftheopticalwater-levelsensorandtheprinciplediagramofthepressureconversionelementofthesensor,respectively.Thesensorheadconsistsofadiaphragm,acustomizedBourdontubeandtwoFBGs,whereoneFBGisusedfortensilemeasurementandtheotherFBGisusedfortemperaturecompensation.Thiswaterlevelsensoristhetypeusedformeasuringwaterpressureinproportiontowaterlevel.ThewaterpressureistransmittedtotheinnersiliconeoilintheBourdontube.ThentheBourdontubeconvertstheoilpressuretothetensileforceatthetipofthetubewithinitselasticlimitandstretchestheFBG,whichisstrungbetweenthetipandthebase.Thus,theFBGisstrainedinproportiontothewaterlevelincrease.TheFBGissetwithslightpre-tensioninitially,toenablewaterlevelmeasurementlinearlyfromthelowestlevel.TheBraggwavelengthvarieswithtemperatureduetothetemperaturedependenceoftherefractivecoefficientofsilicaglass.Tocompensatethetemperaturedependency,anotherFBGisconnectedinserieswiththeFBGfortensilemeasurementatthesensorhead.ThetemperaturecompensationFBGismountedinthesensorcasefreefromanystrainbasedonthewaterpressure.TheintrinsicwavelengthshiftAXW.L.inproportiontothewaterlevelchangeiscalculatedbyA九wl=(3)whereAXtens.isthewavelengthshiftoftheFBGattachedtotheBourdontube,whichincludesthetemperatureturbulence,andAXtemp.isthewavelengthshiftasmeasuredbythetemperaturecompensationFBG.KisthecoefficientofthetemperatureFBG,andwesetthisvalueto1accordingtoadjustthepositionofFBGmounting.Theairpressureinthesensorcaseisbalancedwiththeouteratmosphericpressurethroughtheairinductionpipethatcontainstheopticalfiberapproachcableinside.Measurementresultforopticalwater-levelsensorFigure7showstheschematicdiagramofthemeasurementsystemfordetectingthewavelengthofthereflectedlightfromtheFBGsineachsensor.Thebroad-bandlightfromanASElightsourceislaunchedintotheopticalfiberviaa3-portcirculator,andthereflectedlightfromFBGcomesbackthroughanotherportofthecirculatortowardtheFabry-Perottunablefilterasaspectroscopicdevice.(3)(5)OtherwavelengthlightspassthroughtheFBGandtransmittothenextFBGs,whereadifferentwavelengthlightwillbereflected.Thus,itispossibletoachievethemulti-pointmeasurementofwaterlevelwithoneopticalfiberwhensensorsareconnectedinseries,whicharecomprisedofFBGsreflectingdifferentwavelengthsfromeachother.Switchingtheseveralopticalfibersbyusinganopticalswitch,thesystemcanconnectandobtainthemeasurementresultsofmultiplesensorsbyusingasinglepieceofwavelengthinterrogationequipment.ThefinesseoftheFabry-Perottunablefilteris700,thetransmissionbandwidthathalfmaximumisalmost0.1nm,andtheusedwavelengthrangeis1530to1570nm.Thetunablefilterisbeingdrivenbypiezoelectrictransducer,and50HztriangularwavevoltagehasbeenappliedtoittoscanthedistanceoftheFabry-Perotetalon.

whichindicatedaconstantwavelengthhavinglessthan+/-1pmfluctuation.Andthedatawasaveragedover10timestoreducetherandomnoise.TheresultisshowninFig.8.Whentheaveragingexceeds10times,thewavelengthfluctuationcanbesuppressedlessthan5pm.WedesignedthemaximumstrainapplicabletothetensilemeasuringFBGtobeabout0.3%at10-mwaterpressure,whichcorrespondedtoa3.6-nmcenterwavelengthshiftoftheFBG.Toverifytheaccuracyofthewaterlevelmeasurement,wemeasuredthewavelengthshift10times,applyingpseudo-waterpressuretothesensordiaphragmusingtheairpressurizer.Figure9showsthecorrelationbetweentheappliedpressuremeasuredbyapressuregaugeandthewavelengthmeasuredbythesensorFBG.Theresultshowedthattherewasgoodlinearityoverthefullrangeofthewaterpressure,andtheerrorateachpointwaslessthan3pm/cmH2O.Wemeasuredthischaracteristicat0,20and40U,whichwasthespecifiedtemperaturerangeofthissensor.TheresultsareshowninFig.10.Theordinateofthisgraphisthewaterlevelmeasurementerrorobtainedfromthelinearfittinglinethatwascalculatedfromtheresultofthedependencymeasurementbetweenthepressureandthewavelengthshift.Thereweregoodagreementsateverytemperaturecondition,andtheerrorateachpointwaslessthan+/-1cm.Fig.8*Therestthsofthewavelengthaccuracydependingontheaveragingtimes,I,1(),and100.Over1(1limesaveraging,thewavelengthjluctiiaiioncouldbesuppressedlessihuu5pmEu)£6udp祜*Fig.8*Therestthsofthewavelengthaccuracydependingontheaveragingtimes,I,1(),and100.Over1(1limesaveraging,thewavelengthjluctiiaiioncouldbesuppressedlessihuu5pmEu)£6udp祜*2niosqvFig.9・1herelationshipbetweenthepressuremeasuredbyihepressuregen弊atkithedetectedwawlengihfromthesensorFBG.Hitsshowedexceedinghneuri/ycukltheerrorateachpointwaslessthan3pmcmHX)E04暑垂&£6udleA&AJa~uaG,6ii01002003004005006007008009001.000Pressurizedwaterlevel(cmHiO)Fig.10*i'heresultsofthemeustiremenierrt)rforeachwaterlevelatthelemperaiitrvs,0,20anti40*•Thereweregoodagreemenisateverylenifferuittrecondition,amitheerrorateachpointwaslessthan+■■■'••/cni(UD)6u老E£bJUUJWeconfirmedtherepeatabilityandthedurabilityofthesensorbyapplyingpressurefrom0to3mH2Orepeatedly.TheresultisshowninFig.11.Evenafter10,000timespressurization,themeasuredwaterlevelindicatedthetruevalueattheerrorwithin+/-1cm.Nodriftorcreepconceivablecausedbytemperaturefluctuationormetalfatigueofelementsofthesensorwasobserved.Practicalperformanceofopticalwater-levelsensorinriverWeperformedariverfieldtestofthissensorsystemattheKakehashiRiver,KanazawaWorkOffice,HokurikuRegionalDevelopmentBureaufromAugust1999toMarch2001.Throughthisfieldtest,weconfirmedtherobustnessofthesensorandthemeasuringequipment:theyoperatedoveroneandhalfyearsmeasuringthewaterleveloftheriverwithanaccuracyof+/-1cmwithnofatalsystemerror.Next,weinstalledthesystemequippedwiththreesensorsattheUonoandtheAburumaRiver,ShinanogawaWorkOffice,HokurikuRegionalDevelopmentBureauinNovember2001.ThesystemdiagramandthewaterlevelmeasurementresultsareshowninFigs.12and13,respectively.Duringthismeasurement,waterlevelwasmeasuredsimultaneouslybyaconventionalelectricwater-levelgaugethatwasalreadyinstalledatthesamemeasuringpoint.Thewaterleveldatameasuredbytwotypesofsensorcorrespondedquitewellandthedifferencebetweenthemwaslessthan+/-1cm.Throughthefieldmeasurements,wedemonstratedthatouropticalwater-levelsensorhadsuperioraccuracyinwaterlevelmeasurement,anditenabledremoteandreal-timemonitoringwithouttheneedforanelectricsupplysourceatthesensingpoint.(4)CONCLUSIONWedevelopedanall-opticalwater-levelsensorshavingaccuracycorrespondingto+/-0.1%F.S.,i.e.,+/-1cmforthefullwater-levelmeasurementrangeof10m.Thesesensorsandsystemsarecurrentlysetupatseveralsitessuchasriverbasins,lakesandsewagesystemswheretheycontinuetoworkasdesignedtomeasurethepracticalwaterlevel.REFERENCES(1)E.Udd,“TheEmergenceofFiberOpticSensorTechnology,”FIBEROPTICSENSORS(ed.E.Udd)JohnWiley&Sons,Inc.,NewYork(1991)pp.1-8.(2)A.D.Kersey,“AReviewofRecentDevelopmentsinFiberOpticSensorTechnology,”OpticalFiberTechnol.2,291-317(1996).(3)A.D.Kerseyetal.,“FiberGratingSensors,”J.ofLightwaveTechnol.15,No.8,1442-1463(1997).(4)K.O.Hilletal.,“PhotosensitivityinOpticalFibers,”Annu.Rev.Mater.Sci.23,125-157(1993).(5)A.D.Kersey,T.A.BerkoffandW.W.Morey,“MultiplexedFiberBraggGratingStrain-SensorSystemwithaFiberFabry-PerotWavelengthFilter,”Opt.Lett.18,No.16,1370-1372(1993).SensorTechnologySofar,wehaveconsideredmainlythenatureandcharacteristicsofEMradiationintermsofsourcesandbehaviorwheninteractingwithmaterialsandobjects.Itwasstatedthatthebulkoftheradiationsensediseitherreflectedoremittedfromthetarget,generallythroughairuntilitismonitoredbyasensor.Thesubjectofwhatsensorsconsistofandhowtheyperform(operate)isimportantandwideranging.ItisalsofartooinvolvedtomeritanextendedtreatmentinthisTutorial.However,asynopsisofsomeofthebasicsiswarrantedonthispage.AcomprehensiveoverallreviewofSensorTechnology,developedbytheJapaneseAssociationofRemoteSensing,isfoundontheInternetatthismirrorsite.SomeusefullinkstosensorsandtheirapplicationsisincludedinthisNASAsite.WepointoutherethatmanyreadersofthisTutorialarenowusingasophisticatedsensorthatusessomeofthetechnologydescribedbelow:theDigitalCamera;moreissaidaboutthiseverydaysensornearthebottomofthepage.Mostremotesensinginstruments(sensors)aredesignedtomeasurephotons.Thefundamentalprincipleunderlyingsensoroperationcentersonwhathappensinacriticalcomponent-thedetector.Thisistheconceptofthephotoelectriceffect(forwhichAlbertEinstein,whofirstexplaineditindetail,wonhisNobelPrize[notforRelativitywhichwasamuchgreaterachievement];hisdiscoverywas,however,akeystepinthedevelopmentofquantumphysics).This,simplystated,saysthattherewillbeanemissionofnegativeparticles(electrons)whenanegativelychargedplateofsomeappropriatelight-sensitivematerialissubjectedtoabeamofphotons.Theelectronscanthenbemadetoflowfromtheplate,collected,andcountedasasignal.Akeypoint:Themagnitudeoftheelectriccurrentproduced(numberofphotoelectronsperunittime)isdirectlyproportionaltothelightintensity.Thus,changesintheelectriccurrentcanbeusedtomeasurechangesinthephotons(numbers;intensity)thatstriketheplate(detector)duringagiventimeinterval.Thekineticenergyofthereleasedphotoelectronsvarieswithfrequency(orwavelength)oftheimpingingradiation.But,differentmaterialsundergophotoelectriceffectreleaseofelectronsoverdifferentwavelengthintervals;eachhasathresholdwavelengthatwhichthephenomenonbeginsandalongerwavelengthatwhichitceases.Now,withthisprincipleestablishedasthebasisfortheoperationofmostremotesensors,letussummarizeseveralmainideasastosensortypes(classification)inthesetwodiagrams:Thefirstisafunctionaltreatmentofseveralclassesofsensors,plottedasatrianglediagram,inwhichthecornermembersaredeterminedbytheprincipalparametermeasured:Spectral;Spatial;Intensity.Fromthisimposinglist,weshallconcentratethediscussiononoptical-mechanical-electronicradiometersandscanners,leavingthesubjectsofcamera-filmsystemsandactiveradarforconsiderationelsewhereintheTutorialandholdingthedescriptionofthermalsystemstoaminimum(seeSection9forfurthertreatment).ThetopgroupcomprisesmainlythegeophysicalsensorsweconsideredearlierinthisSection.ThetwobroadestclassesofsensorsarePassive(energyleadingtoradiationreceivedcomesfromanexternalsource,e.g.,theSun)andActive(energygeneratedfromwithinthesensorsystem,beamedoutward,andthefractionreturnedismeasured).Sensorscanbenon-imaging(measurestheradiationreceivedfromallpointsinthesensedtarget,integratesthis,andreportstheresultasanelectricalsignalstrengthorsomeotherquantitativeattribute,suchasradiance)orimaging(theelectronsreleasedareusedtoexciteorionizeasubstancelikesilver(Ag)infilmortodriveanimageproducingdevicelikeaTVorcomputermonitororacathoderaytubeoroscilloscopeorabatteryofelectronicdetectors(seefurtherdownthispageforadiscussionofdetectortypes);sincetheradiationisrelatedtospecificpointsinthetarget,theendresultisanimage[picture]orarasterdisplay[asin:theparallellines{horizontal}onaTVscreen).RadiometerisageneraltermforanyinstrumentthatquantitativelymeasurestheEMradiationinsomeintervaloftheEMspectrum.Whentheradiationislightfromthenarrowspectralbandincludingthevisible,thetermphotometercanbesubstituted.Ifthesensorincludesacomponent,suchasaprismordiffractiongrating,thatcanbreakradiationextendingoverapartofthespectrumintodiscretewavelengthsanddisperse(orseparate)thematdifferentanglestodetectors,itiscalledaspectrometer.Onetypeofspectrometer(usedinthelaboratoryforchemicalanalysis)passesmultiwavelengthradiationthroughaslitontoadispersingmediumwhichreproducestheslitaslinesatvariousspacingsonafilmplate.Thetermspectroradiometertendstoimplythatthedispersedradiationisinbandsratherthandiscretewavelengths.Mostair/spacesensorsarespectroradiometers.Sensorsthatinstantaneouslymeasureradiationcomingfromtheentiresceneatoncearecalledframingsystems.Theeye,aphotocamera,andaTVvidiconbelongtothisgroup.Thesizeofthescenethatisframedisdeterminedbytheaperturesandopticsinthesystemthatdefinethefieldofview,orFOV.Ifthesceneissensedpointbypoint(equivalenttosmallareaswithinthescene)alongsuccessivelinesoverafinitetime,thismodeofmeasurementmakesupascanningsystem.Mostnon-camerasensorsoperatingfrommovingplatformsimagethescenebyscanning.Movingfurtherdowntheclassificationtree,theopticalsetupforimagingsensorscanbeimageplaneoropticalplanefocused(dependingonwherethephotonraysareconvergedbyalens),asshowninthisillustration.Anotherattributeinthisclassificationiswhetherthesensoroperatesinanon-scanningorascanningmode.Thisisarathertrickypair