Fiber Optical Sensing with Fiber Bragg Gratings

Eisenmann Th. INFAP GmbH, Fürstenrieder Straße 279a, 81377 München [email protected]

Summary areexplosion-proof zones, exhibit strong Anew measurement method is presented uti- magnetic fields or arenot accessible anymore lizing glass fibers with inscribed Fiber Bragg after the sensors have been installed. Gratings (FBG). Strain and temperature – Attenuation 0.1 dB/km only.Thus ameas- changes have direct effects on these gratings. urement lengthofsome 100 km can With this technology and suitable transducers, be realized. however,also parameters like pressure, dislo- cation, vibration, acceleration, humidity and 1. Introduction even chemicals can be monitored. Abroad- Initially conceived as amedium to carry light band or sweeping light source is used and images for medical endoscopic applica- and light with corresponding to tions, optical fibers werelater proposed in the the FBGs is reflected back to adata acquisition mid 1960’sasanadequate information carry- unit (interrogator). Depending on variations of ing medium for telecommunication applica- the parameter to be measured, the grating is tions. At the heart of this technology is the stretched or compressed and as aresult the itself – ahair-thin cylindrical fila- reflected (Bragg) is shifted to ment made of glass that is able to guide light longer or shorter wavelength respectively.This through itself by confining it within regions shift is proportional to the change of the para- having different optical indices of . meters value. The fiber sensor or the trans- Ever since, optical fiber technology has been ducers can be fixed, welded, glued onto or the subject of considerable research and devel- even embedded in the media, which shall be opment. For measurements, optical fibers monitored. The biggest advantages of the eventually found numerous applications in technique are areas, as different as pharmacy and structural – Asingle fiber can carry amultitude of sen- health monitoring. sors, sensing is done simultaneously parallel (multiplexing). In principle, afiber optic sensor contains an – The optically encoded signals of the meas- optical element, whose material properties are ured parameters aretransmitted in the fiber changed by the measured parameter.Light is to the data acquisition unit. Thus, no addi- influenced in its intensity,phase or polarity. tional data line is necessary. Consequently,acharacteristic spectrum results – The sensors have minimum volume with low from the measured value. Many physical and weight, areflexible and easy to install. chemical parameters can be determined by – Measuring with light does not requirepower this. Temperature, pressureand strain can be for the sensors or the fibers, sensing is measured as wells as humidity and appearance immune against electro-magnetic interfer- of gases. FBG sensing is aso-called intrinsic ence (EMI) and intrinsically safe. Consequent- technique, i.e. the fiber properties are ly,measurement can be done in places that changed by external effects and hence, the

79 fiber itself is the senor.Temperatureorstrain fulfilling this condition, the grating is trans- has adirect impact on the glass fiber,asthe parent (Fig. 1). Consequently,incident light properties of light travelling through the fiber travels through the grating structurewith neg- will be altered locally (Krug, 2007). ligible signal variation or attenuation, as the reflected light signal will be very narrow.Thus, 2. Principle an important property of aFiber Bragg Grat- Fiber Bragg Gratings areoptical interference ing is the precise wavelength. filters inscribed into the coreofsingle-mode glass fibers. For generating fiber gratings, the The reflected wavelength is centered at the coreofasingle-mode fiber is exposed to the Bragg wavelength fulfilling the Bragg equation perpendicular impact of aperiodic patternof λrefl =2nΛ,with n being the light coming from e.g. an Excimer and Λ the distance between the gratings. Due laser (UV 248 nm). This impact produces aper- to the temperatureand strain dependence of manent increase of the refractive index corre- n and Λ,also the reflected wavelength is sponding to the periodic patternwith fixed dependent and varies as afunction of tem- spacing, called grating. As aresult, aperiodic peratureand/or strain (Fig. 1). This depen- modulation of the refractive index takes place dence is known and enables the unambiguous with areas of high and low refractive indices, determination of temperatureand strain from reflecting back light of acertain wavelength. the reflected Bragg wavelength. Hereby the The grating strength or amplitude is afunction detected signal is spectrally encoded so that of how long the fiber has been exposed to the transmission losses areofnoconcern. Never- ultraviolet illumination. Aportion of light is theless, if strain and temperaturealterations reflected back at each change of the refractive areexpected to occur simultaneously,it index (Fig. 1). All these signals becomes necessary to use agrating decoupled together result in acoherent large back-scat- from strain for temperaturecompensation. ter of aspecific wavelength. Thereby,the With appropriate transducers many parame- spacing of the FBG is half the reflected wave ters like pressure, deformation, dislocation, length. This is denoted as Bragg condition and vibration etc. can be inferred from the basic Bragg wave length. For all wavelengths not strain or temperaturechanges.

Figure1:Principle of Fiber Bragg Gratings

80 Figure2:Wavelength Division Multi- plexing (WDM)

Typically,the fractional wavelength change in windows of 50 to 100 or even 160 nm) and the the peak Bragg wavelength is in the order of ability to simultaneously interrogate many 10–12 pm/°C. Strain shifts the Bragg wave- fibers, each with dozens of sensors. length by physically increasing or decreasing the grating spacing by mechanical strain and Afurther increase in the number of sensors by changes in the refractive index due to the per fiber that can be interrogated is possible strain optic effect. For axial loads, the wave- using time division multiplexing techniques length change is typically 1.2 pm per micro (TDM) in combination with WDM. In this strain (µε)or12nmfor 1% strain. Assuming approach, the spectrum of the source is used such performance, an accuracy and resolution multiple times to scan separate groups of FBGs of 1 µs, 0.1 °Cor1µmincase of deforma- in time. If ashort duration pulse of light from tion measurements can be achieved. the source is launched into this system, the reflections from the FBGs at every point in the One of the key characteristics and biggest array will returnatthe detector at increasing- advantages of this measurement method is ly later times depending on how farther away multiplexing, i.e. the answer of numerous dif- they aretothe detector itself. If the detector ferent FBGs can be achieved with one fiber is synchronized and time-gated, it is possible only by inscribing various FBGs with different to selectively interrogate agiven FBG array in spacing and wavelengths in aserial configura- time for agiven wavelength window.Draw- tion. Each reflected wavelength corresponds back of this way to interrogate sensors is the to adistinct FBG. This is called wavelength limited scanning frequency,which would limit division multiplexing (WDM). In this configura- the response of the system to dynamic signals tion each FBG is assigned agiven »slice« of and transients. FBGs can be spaced no closer the input broad-band light spectrum. Caution than 1meter for the even the best TDM sys- has to be exercised, however,toavoid over- tems (MICRON OPTICS, 2005). lapping of distinct FBG’sspectra. Another drawback of the TDM technique, Today,most popular WDM interrogators (spec- when combined with Bragg gratings, is that of trum analyzer and data acquisition units) use cross-talk (Morey,etal., 1991). Thereare two fast sweeping as light source instead of a sources for cross-talk: multiple reflections and broadband source. The advantages arelonger spectral shadowing. Multiple reflection cross- range (due to higher source power) and greater talks arise from the delay introduced into a sensor capacity (due to the wider wavelength reflected light signal upstream that has under-

81 gone multiple reflections during its travel and implemented at the expense of additional has effectively overlapped in time with the components such as fiber couplers, delay lines reflected signal of agrating downstream. The and stronger reflectivity FBGs. If morethan effect is proportional to the grating’sreflectiv- one sensor fiber is used with one data acqui- ity and can be minimized using low reflectivi- sition unit, an optical switch triggers the rela- ty gratings (<5%). In fact, the number of grat- tive fiber. ing elements that can be interrogated under a given signal-to-noise ratio will depend on the 3. System components amount of cross talk. Hence, the pairing of An FBG optical fiber sensing system usually low-reflectivity gratings with high sensitivity consists of the following components (see also detection will be essential to interrogate large Fig. 3&4): arrays of gratings. The spectral shadowing cross-talk is the distortion introduced in the a. Interrogator: Light source and data acquisi- reflected spectrum of adownstream grating tion unit – spectrum analyzer (mono-static resulting from the double pass of the incom- arrangement, i.e. light source and detector ing light through an upstream grating. The arearranged adjacent in one case) for light above deleterious effect can be eliminated by transmission and reception of the optical staggering the grating arrays to avoid spec- signals and conversion into digital data. trum overlaps by having arrays branching off b. Fiber Sensor: The Fiber Sensor is aglass instead of serially connected. To reduce time fiber with inscribed sensors (Fiber Bragg overlaps and cross-talk the time interval Grating) and fiber plug (FC/APC). These between gratings can be increased by adding fibers areusually coated with polyimide, passive delay lines. These measures can be acrylate or preferably with ORMOCER®,an

Figure3:Interrogator (model si730 from MICRON OPTICS)

82 Figure4:Different types of transducers: Spot welded strain sensor,screw-in temperaturesensor,embedded concrete strain sensor

inorganic-organic hybrid polymer (trade- Once the sensor is attached by e.g. sticking, mark of the Fraunhofer-Gesellschaft zur welding or embedding it to the place or into/ Foerderung der angewandten Forschung onto the structureinthe field, it is very impor- e.V.Munich/Germany). ORMOCER offers a tant to protect the fiber from damage. This moredurable and homogenous coating can be challenging if the point, wherethe than the so called recoated fibers. measurement is taken will be quite distant c. Transducer (some examples aredepicted in from the interrogator.Fortunately,anumber Fig. 4) for the parameter to be measured, of excellent cabling methods for optical fibers e.g. weldable or embedded strain or tem- exist, such as Kevlar wrap or metallic shrouds peraturesensors, screw-in temperaturesen- for optical fiber. sor,accelerometer etc. d. Extension fibers: Commercially available As strain measurement is of high significance glass fiber cables for realizing large dis- for applying FBG sensing in rail tracks and rail tances between sensors and data acquisi- networks and also for earthquake prone tion units (up to several km). regions and structures, moregenerally for e. Data evaluation software: e.g. LabVIEW structural health monitoring, emphasis shall be (National Instruments) based softwarefor put on static and dynamic strain sensing in the read-out and analyzing data as well as spe- following applications chapter. cific adaptation for applications and of the graphic user interface (GUI). Long-term Static Strain Sensing Long term static strain testing is very easy to Usually application-oriented systems including accomplish with fiber Bragg gratings due to commissioning areconfigured with above their inherent self-referencing (MICRON components. This guarantees that measure- OPTICS, 2005). Self-referencing is the ability to ments can be done exactly adapted to fit the determine astarting point. Each Bragg grating requirements for laboratory or field purposes. has an associated zerostrain wavelength. Ini- tially the unstressed fiber Bragg grating has a 4. Applications specific center wavelength. However,when it Fiber Bragg gratings areoften used for either is attached to astructure, this process will alter strain or temperaturesensing, especially where the shape of the FBG. Therefore, the zero environments areharsh (e.g. high-EMI, high- strain point must be reevaluated after mount- temperatureorhighly corrosive conditions). As ing. The internal forces in the glue used for mentioned, it is also possible to use fiber mounting will cause strain within the grating. Bragg gratings to sense other parameters such Hence, after the fiber Bragg grating is as pressure, vibration, acceleration, dislocation attached to the structure, it needs to come to by using an additional transducer instead of asteady state and then the zeropoint before using the Fiber Bragg grating itself. the measurements is determined. With the

83 grating mounted to the structureinquestion, tion during the ebb and flow of tides or the the initial wavelength is determined. With a reaction of high-rise buildings to wind. set amount of time passed, one can returnto the structure, reattach and refer back to the Dynamic Strain Sensing initial wavelength. That would give the deter- In addition to the very low frequency modes mination of the strain compared to the initial that structures may have, they may also have condition at that time. higher frequency modes due to the effects of wind and tide. Dynamic strain testing appropri- This is in contrast with electrical strain gauges ate to the 25 Hz region can also be performed and other types of instruments werechanges on transportation vehicles such as automobiles, in the sensor itself would have to be con- trains, and airplanes. Such measurements can stantly monitored, whether it is aresistance also be used to diminish fatigue effects on vehi- change, acapacitance change or other electri- cle structures. Fatigue is an example of how cal change. With an electrical strain gauge, fiber Bragg gratings can be used for Dynamic the ability to disconnect your monitoring Strain Testing in addition to long-term Static instrumentation does not exist as is with a Strain Testing. Herenot only the momentary fiber Bragg measurement arrangement. The vibrations undergone by the structures areof reason for this is that with electrical strain significance, but also the effects of these vibra- gauges you have to balance out the gauge tions on the long-term strain of the metallic sur- each time you connect your resistance strain faces. Fiber Bragg gratings can similarly be gauge to it. This ability to returntothe fiber attached to industrial machinery to determine Bragg grating after long time such as months the frequency and amplitude of stress vibra- or even years constitutes another big advan- tions. The vibration signatureofeither piece tage of the method (MICRON OPTICS, 2005). can then be used in afeedback loop to be sure that the correct amount of load is applied at all An example is strain testing over time in num- times during the machining process (MICRON ber of bridges. If sufficient budget exists, one OPTICS, 2005). set of instrumentation per bridge could be installed. However,if100 bridges need super- For static and dynamic strain as well as tem- vision, it is not economical to purchase 100 peraturemeasurements, many application times the equipment. Instead one set of instru- areas can be found like mentation would be adequate to test each of these 100 bridges on an e.g. monthly cycle. It – Structural health monitoring (SHM) would be much moreefficient to attach – Construction/plant construction numerous fiber Bragg grating sensors through- – Geotechnical engineering, e.g. landslides, out those bridges, attach the instrumentation slopes, pipeline, dam and tunnel moni- to them on aperiodic basis and conduct all the toring, building inventory in earthquake testing within very reasonable time. Besides threatened areas bridges, other examples of using fiber Bragg – Infrastructure, e.g. roads and rails gratings for long-term static strain monitoring – Aerospace would be rails, buildings, tunnels and struc- – Renewable energies like blade supervision tures in high earthquake prone areas. Most for wind turbines earthquakes and other earth tremors arelow – Composite or metallic structures frequency events. Fiber Bragg gratings can be – Transformers and power grids attached to structures and monitor the vibra- – Pharmacy tions during such events. Other applications to – General research monitor low frequency events would be the connection of fiber Bragg gratings to piers and other shorestructures to determine their vibra-

84 5. Features and Benefits – Attenuation typically totals up to 0.1 dB/km Due to its naturemeasurements generally with only.Thus ameasurement length of some fiber sensors and in particular also with fiber 100 km can be realized. Bragg gratings exhibit anumber of benefits, – Aresulting additional value can be lower over- when used for appropriate applications. The all lifetime cost of the measurement set-up. most prominent of these are: 6. Using FBG for Rail Track Application – The technique is fast, accurate and reliable in For rail network monitoring, especially the challenging environments. benefits – Asingle fiber can carry amultitude of sen- sors, sensing is done parallel (multiplexing). – measurement lengths in the kilometer range – The optically encoded signals from the meas- – discontinuous interrogation of multiple sen- ured parameters aretransmitted in the fiber sors or continuous monitoring to the data acquisitionunit. Thus, no addi- and tionalseparate data line becomes necessary. – easy mounting with simple cabling – Temperaturerange can be from –270 °Cup to +600 °C. areadvantageous. The fiber sensors with suit- – The sensors have minimum volume with low able transducers can easily be attached by weight, areflexible, easy to install and do spot welding to the rails. For static strain mon- typically not requireany maintenance. itoring the technique is superior due to the – Measuring with light does not requirepower fact that multiple locations, wherestrain sen- for the sensors or the fibers, is immune sors areattached to rails can be interrogated against electro-magnetic interference (EMI) with one measurement unit. This interrogator and intrinsically safe. Consequently,meas- just needs to be connected to the fiber sen- urement can be done in places that are sors in an e.g. monthly or bi-monthly sched- explosion-proof zones, exhibit strong mag- ule. Sensors can be placed in rail networks at netic and electric fields or arenot accessible distances of 500 or 1000 m. With dynamic anymoreafter the sensors have been measurements of 100 Hz or above, strain from installed (e.g. if they areembedded in con- trains riding above asensor location can be crete or in material structures like in carbon tracked and compared in long-term monitor- or glass fiber reinforced plastic). ing campaigns. Hereboth strain and accelera-

Figure5:Integrating FBGs in atrack network (schematic)

85 tion can be measured. With these results life due to natural disasters. Especially with embed- time monitoring with early detection of arising ded rail systems, sensors can easily be mount- damages as well as tracking of damage per- ed. Consequently,measurements and monitor- petrators seems feasible. Assessment of dam- ing with bespoke fiber-optical systems can con- ages resulting from catastrophic dislocations tribute to protect vulnerable infrastructures. due to e.g. earthquakes or landslides (track buckling) would be another application, where References measurements can be done remotely and in Krug, F(2007): Messen mit faseroptischen real-time utilizing continuous monitoring of Sensoren, Elektronik messen +testen 2.2007, multiple sensors. Generally,fromthe perspec- pp. 54–58. tive of safe rail traffic, the determination of parameters like track gauge, longitudinal MICRON OPTICS (2005): Optical Fiber Sensors height, superelevation, curvatureand twist are Guide Fundamentals &Applications, 21 pp. to the fore. All these values areinprinciple ascertainable with FBG sensors (personal com- MICRON OPTICS: Personal communication. munication Prof. Quante). Morey,W.W., Dumphy,J.R., and Meltz, G. Atotally different, yet railway related applica- (1991): Multiplexing Fiber Bragg Grating Sen- tion is temperaturemonitoring of bogies like sors, SPIE Proc. Vol. 1586, pp. 216–224. it has been done for the Mass Rapid Transit in Hongkong (personal communication MICRON Quante, F: Personal communication. OPTICS).

7. Conclusion Fiber-optical sensing for measuring physical parameters like strain and temperature, but also pressure, dislocation, vibration and accel- eration is rapidly developing, especially into applications, whereconventional electrical gauging finds its limits. Accordingly,measure- ments based on fiber Bragg gratings (FBG) can successfully be utilized in areas as different as construction and geotechnical engineering or material science, power grids/transformers and pharmaceutical production. Static and dynam- ic processes can be monitored in harsh envi- ronments exhibiting strong electro-magnetic fields, corrosive atmospheres or explosion risks.

For structural health monitoring, fiber-optical sensing is already proven in alarge number of installations worldwide, mainly to investigate bridges, dams, dikes, slopes or tunnels. Due to the versatility of FBG sensors by integrating a large number of them in one fiber cable and multiplexing as well as the very low signal attenuation, even extended rail networks can easily be monitored with respect to stress com- ing from operational influences or damages

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