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 laser light source is used and images for medical endoscopic applica- and light with wavelengths 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 optical fiber itself – ahair-thin cylindrical fila- reflected (Bragg) wavelength 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 refraction. 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 refractive index ultraviolet 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 reflection 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 lasers 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.