Distributed Fiber Optic Sensors (BOTDA) In simple language

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Abstract—Most papers published on fiber opticsand fiber optic sensors are so technical andthe language is extremely complex that nonexperts in the field who wish to appreciatethe basics of how these technologies work arethrown off when they pick a paper or apublication on the subject matter (in thiscase distributed fiber optic sensors) to read.This paper presents to the best of my knowledge insimple language, how distributed fiber optic sensors(BOTDA) work. We simplify the concepts of fiberoptic communication, fiber optic sensors and thenzoom in on distributed fiber optic sensors withBrillouin Optical Time Domain Analysis (BOTDA)sensors as a focus sensor to present the principles,theory and operation of distributed fiber opticsensors and their applications.

Index Terms—Fiber Optic Communication, Fiber OpticSensors, Distributed Sensors, Brillouin Scattering,BOTDA Sensors.

I. INTRODUCTIONPTICAL fiber (cladded) was developedby Corning Glass works in 1970 and the

first fiber optic communication system wasOThis paper was submitted for review on 19th July2015 with support from the Hong Kong PolytechnicUniversity’s Research Office.

Appiah Sarfo George is with the department of Electronic and Information Engineering at the Hong Kong Polytechnic University, Hong Kong email: [email protected]

Prof. Lu Chao is with the department of Electronicand Information Engineering at the Hong Kong Polytechnic University, Hong Kong, email: [email protected]

deployed in 1975 which used GaAssemiconductor lasers, operating at awavelength of 0.8 µm, with a bit rate of45 Mb/s and a 10Km repeater spacing [1].By 1987, InGaAsp fiber optic systems werecommercialized operating at bit rates ofup to 1.7 Gb/s on single mode fiber with50Km repeater spacing. A bit rate of 14Tb/s was realized in 2006 using opticalamplifiers over a single 160km line. Thenext generation of fiber opticcommunication (5th generation) is lookingat reducing the nonlinear effects bycounteracting the negative effects ofdispersion by preserving their pulse-shape, as a result optical solitons arebeing explored [2]. Other developmentsfocus on extending the traditional C bandwavelength window to achieve a largerwavelength range: from the widely used

1.53µm – 1.57µm to a much wider wavelengthrange of 1.30µm – 1.65µm using dry fibers[2].

Low loss fibers have made it possible forseveral industrial deployments of fiber:fiber-to-the-x (FTTX – FTTH, FTTC etc).FTTH stands for Fiber to the home, FTTCrepresents fiber to the cabinet.

Optical Communication is a broad fieldin communications and it is thetransmission of information/messages using

Distributed Fiber Optic Sensors– Brillouin Optical Time DomainAnalysis (BOTDA) Sensor inSimple Language (July 2015)

Appiah Sarfo George, Prof. Lu Chao

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light as the carrier. The transmission canbe through a guided, unguided (e.g FreeSpace Optical Communication) or a blend ofboth unguided and guided media. Fiberoptic communication systems use guidedmedia (optical fiber) to transmitinformation superimposed on light from atransmitter to a receiver. Fig. 1 is ablock diagram of the fiber opticcommunication system; it comprises atransmitter, channel (optical fiber),photo-detector and a receiver. The channelfor fiber optic communication is fiber andit consists of a core with a higherrefractive index, a cladding surroundingthe core with a lower refractive indexcompared to the core and a protectivesheath for physical protection. The corehas a higher refractive index than thecladding to achieve total internalreflection. Fibers are classified into twobroad categories: based on the boundarybetween the cladding and the core; if it’sabrupt then we have a step – index fiber,if the boundary is gradual then it’s agraded – index fiber. There are howevertwo types of fibers based on their modesof propagation; mono-mode / single modefibers and multimode fibers. The modes ofpropagation M depends on the V number in(1). Fig. 2 shows the refractive indexprofile of both classes and types offiber.

Fig. 1. Block Diagram of an optical communicationsystem [3]

Fig. 2. Refractive Index Profile of various classesand modes of fiber. [4]

The number of modes (M) increases when theV number is greater than 2.405. The Vnumber depends on the core-claddingcharacteristics as shown below.

V=ωac √(n12−n2

2)=2πaλ

N.A. (1)

Where ω=2πf, n1,n2 are the refractiveindices of the core and claddingrespectively ‘a’ is the radius of thecore, and N.A. is the numericalaperture which is a dimensionless quantitythat describes how much light can beallowed into the core of the fiber.

As illustrated by Fig. 3. For V < 2.405we have a single or mono-mode propagation,multimode propagation occurs for V > 2.405

Fig. 3. V – number and the number of modes ofpropagation, also represents solutions of Maxwell’s equations.

Multi-mode fibers are used for short

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distance (up to about 1km) communicationsbecause they suffer a lot of lightdispersion, single mode fibers are mostlyused for long haul communications(>4000km).

II. FIBER OPTIC SENSORSA sensor is an electrical or optical

transducer that detects or senses somephysical or natural characteristic(s) ofits proximal or distant environs. Itdetects changes in the environs andprovides a proportional output. A fiberoptic sensor (also called optical fibersensor) uses optical fiber as a sensingelement, or as a medium for sendingsignals from a remote sensor to a receiver[5]. In simple terms: fiber optic sensorsare used to measure/sense anything whichchanges the way light travels through thefiber or changes the properties of thelight travelling through or leaving thefiber. Fiber optic sensors have wideapplications and they can measuretemperature, strain, pressure,displacement, acceleration, flow rate,vibrations, chemical concentrations,electrical and magnetic fields as well asrotation rates etc.

There are several advantages in usingfiber optic sensors in many industrialapplications, these include but notlimited to: harsh environment capability(electromagnetic interference, hightemperature, high voltage, high pressureetc.), light weight and miniature size,high sensitivity and bandwidth, long rangeapplications and multiplexed/distributedmeasurements. Most traditional sensorsfall short to these advantages and thatmakes fiber optic sensors the optimalchoice for various industrial sensorapplications.

Optical sensors are classified into twobroad categories: intrinsic sensors (wherelight is modulated inside the fiber by theenvironment) or an extrinsic sensor (where

the light is modulated outside the opticalfiber).

Fig. 4. Extrinsic and Intrinsic Fiber Optic Sensors[6]

The photon field that a fiber opticsensor must vary or modulate informsanother means of classifying fiber opticsensors (FOS) as well as the design of thesensor. The electromagnetic (light) waveequation and the variation of the variousparameters (optical modulation mechanism)informs the classifications: Intensitymodulation, wavelength modulation,frequency modulation, phase modulation andpolarization modulation [7] are achievedby varying one or more of the parametersin (2).

E(t)=Ecos (ωt+∅(t))(2)

Where E is the amplitude/intensity, ω=2πfdefines the frequency, and ∅(t)definesthe phase.To sense or measure a parameter ofinterest, the optical sensor design mustallow this external source to vary one ofthe terms in (2). We can vary theamplitude E(t) (or intensity), asintensity-modulating sensors does, one canmodulate the frequency or wavelength ofthe light, as do frequency or wavelengthmodulating sensors (or simply colorsensors) [8]. Another type of amplitude

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modulation involves modifying thepolarization characteristics of the lightfield, yielding a polarization-basedsensor [8].

Modulating the phase of the light fieldmay prove difficult because the term∅(t) resides within the cosine term, aninverse cosine must therefore be performedto allow the entire phase angle (frequencyand phase) to be available [8]. Once thecosine term is removed, the frequencyvariable is negated, leaving only thephase term, from which the parameter ofinterest is deduced [8]. Such a techniquerequires some form of frequency trackingor a frequency canceling technique such asthose used in heterodyne applications [8].

Another important classifications offiber is based on changes in refractiveindices of the core and/or cladding, themain classes are, fiber bragg grating(FBG) sensors, and distributed fibersensors. Bragg gratings are made byilluminating the core of a suitableoptical fiber with a spatially-varyingpattern of intense UV laser light [9] thisfilters out light or wavelength byreflecting them away and allowing otherlight colors or wavelengths to travelthrough the fiber. Short-wavelength (<300nm) UV photons have sufficient energy tobreak the highly stable silicon-oxygenbonds, damaging the structure of the fiberand increasingits refractive index slightly [9]. Aperiodic spatial variation in theintensity of UV light, caused by theinterference of two coherent beams or amask placed over the fiber, gives rise toa corresponding periodic variation in therefractive index of the fiber [9]. One ofthe well-known applications of FBG sensorsis “fiber optic smart structures” whereFBG sensors are embedded into thestructure to monitor the structure’sstrain distribution [10]. For an in depthunderstanding of FBGs see other materials.

III. DISTRIBUTED FIBER OPTIC SENSORSOther fiber optic sensors do not use

fiber Bragg gratings as sensors, insteadthe fiber itself is used as the sensor, asthe name implies, distributed fibersensors use the fiber to continuallymonitor changes in its environs. Mostsensors detect a physical quantity (e.gtemperature, pressure) at a point orseveral points and thus do not detect allchanges in the physical quantity beingmonitored in the environment that thesensor is found. Distributed Sensingsolutions enable a continuous distributedmeasurement along the length of a sensingfiber. With distributed fiber sensing, youcan measure the strain or temperature ofyour test article at not just onelocation, or a few key locations, but athundreds of locations with a single fiberoptic sensor [11].  This unique technologyenables strain or temperature measurementsat every point along a simple opticalfiber with gage lengths as small as afew millimeters [11].  Instead of needingtwo to three wires per sensing point, onlyone optical connection gives you access tohundreds of sensing points. [11].Distributed fiber optic sensors offersolutions for improved and reliable, yetaffordable monitoring in large and complexstructures. The qualitative differencebetween the monitoring performed usingdiscrete and distributed sensors is thefollowing: discrete sensors monitor strainor average strain at discrete points,while the distributed sensors are capableof one-dimensional (linear) strain fieldsmonitoring [12]. See Fig. 5.

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Fig. 5. Simple diagram of distributed fiber opticsensing compared with other forms of fiber opticsensing [14].

Distributed fiber optic sensors can belaid along the whole length of a structure(e.g. a pipe, columns of buildings etc.)and in this manner each cross-section ofthe structure is practically instrumented[12]. The sensor is sensitive at eachpoint of its length and it provides fordirect monitoring, avoiding the use ofsophisticated algorithms [12]. Distributedfiber optic sensor technologies havereached market maturity, and can beapplied in several industries worldwide asseen in Fig. 5.

Fig. 6. Distributed Fiber Optic SensorsApplications [13].

The fact that distributed sensors are

used to monitor a physical quantity leadsus to another means of classifying fiberoptic sensors as shown Fig. 7 we regroupfiber optic sensors based on the length ofsensors and functionality.

Fig. 7. Classifying FOS based on gauge length andfunctional principle; ε=strain, T= temperature[12].

For an in-depth reading on the short andlong gauge sensors see other text.

IV. DISTRIBUTED FIBER OPTIC SENSORS – BRIEF ONTHEORY AND PRINCIPLES OF OPERATION There are three major principles on whichdistributed fiber optic sensing operate:Rayleigh scattering effect, Brillouinscattering effect, and Raman scatteringeffect, each technique is based on therelation between the measured parameters,i.e., strain and/or temperature, andencoding parameter, i.e., the change inoptical properties of the scattered light[12]. Rayleigh scattering effect is basedon the shifts in the local Rayleighbackscatter pattern which depends onstrain temperature [12]. This allows forthe strain measurements to be compensatedfor temperature. Major characteristics ofthis system are high resolution ofmonitored parameters and short spatialresolution but the maximum optimal lengthof sensor is limited to about 70 m (230feet) [12]. Thus, this system is suitablefor monitoring of localized strain changesover relatively short distances [12].

Raman scattering is due to the non-linearinteraction between light traveling

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through a fiber and silica. When highintensity light signal is directed intothe fiber, two frequency-shiftedcomponents called respectively RamanStokes and Raman anti-Stokes appear in theback-scattered spectrum [15]. The relativeintensity of these two components dependson the local temperature of the fiber[15]. If the light signal is pulsed andthe back-scattered intensity is recordedas a function of the round-trip time, itbecomes possible to obtain a temperatureprofile along the fiber [15].

Brillouin scattering, named after LéonBrillouin, occurs when light, transmittedby a transparent carrier interacts withthat carrier's time-&-space-periodicvariations in refractive index [16],the index of refraction of a transparentmaterial changes under deformation(compression-distension or shear-skewing)[16]. Raman and Brillouin systems workwith an Optical Time Domain Reflectometer(OTDR), in which the monitoring unittransmits a short light pulse and uses thetime of flight of the back – scatteredlight to determine the location of thereflection [17].

Rayleigh scattering, too, can beconsidered to be due to fluctuation in thedensity, composition and orientation ofmolecules, and hence of refraction index,in small volumes of matter (particularlyin gases or liquids). The difference isthat Rayleigh scattering considers onlyrandom and incoherent thermalfluctuations, in contrast withthe correlated, periodic fluctuations (phonons) thatcause the Brillouin scattering [16].

Brillouin scattering is caused by aninteraction between light and latticephonon modes [18]. Raman scattering iscaused by an interaction between lightand molecular vibrations [18]. The keydifference is that phonon modes area collective, long-rangephenomenon involving billions or more

atoms, whereas molecular vibrations arelocalized vibrations of a single molecule,which typically only has 2 to 20 atoms[18]. Brillouin scattering denominatesthe scattering of photons from low-frequency phonons, while for Ramanscattering photons are scattered byinteraction with vibrational androtational transitions in the bondsbetween first-order neighboring atoms.Therefore, the two techniques provide verydifferent information about thesample: Raman spectroscopy is used todetermine the chemical composition andmolecular structure, while Brillouinscattering measures properties on a largerscale – such as the elastic behavior [16].Experimentally, the frequency shifts inBrillouin scattering are detected withan interferometer, while Raman setup canbe based on either interferometer ordispersive (grating) spectrometer [16].

Fig. 8. Schematic spectrum of scattered lightresulting from three scattering processes in opticalfibers [19].

A. Stimulated Brillouin Scattering Brillouin scattering can occur

spontaneously even at low optical powers,reflecting the thermally generated phononfield. For higher optical powers, therecan be a stimulated effect, where theoptical fields substantially contribute tothe phonon population. For intense beamslike laser light travelling in a mediumsuch as an optical fiber, the variationsin the electric field of the beam itselfmay produce acoustic vibrations in the

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mediumthrough electrostriction or radiationpressure [16]. The beam may undergoBrillouin scattering from thesevibrations, usually in opposite directionto the incoming beam, a phenomenon knownas stimulated Brillouin scattering (SBS)[16]. For liquids and gases, typicalfrequency shifts are of the order of 1–10 GHz (wavelength shifts of ~1–10 pm for visible light). StimulatedBrillouin scattering is one effect bywhich optical phase conjugation can takeplace [16].

Classically, the thermally generateddensity fluctuations of a material mediumare involved in the scattering of light[20]. The density fluctuations culminatein the compression and rarefaction ofregions within the medium, and may beconsidered to consist of two components,the propagating component and the non-propagating component [20]. When lightwave is directed into the medium,scattering from the non-propagatingcomponent gives the central Rayleigh lineand scattering from the propagatingcomponent gives the Brillouin lines, seeFig. 8. The propagating component ofdensity fluctuations acts as a sound waveof high frequency. The damping of such awave in the material medium is responsiblefor finite width in Brillouin lines whilenon-zero lifetime of the non-propagatingcomponent produces width in Rayleigh lines[20].

Beyond a particular threshold power of alight beam in a fiber, stimulatedBrillouin scattering usually reflect mostof the power of an incident beam. Thisinvolves a high nonlinear optical gain forthe backward-reflected wave: an initiallyweak counter-propagating wave at theappropriate optical frequency can bestrongly amplified. Here, the two counter-propagating waves generate atraveling refractive index grating; the

higher the reflected power, the strongerthe index grating and the higher theeffective reflectivity [21]. VB

The frequency of the incident beam isslightly higher than that of the reflectedbeam; the difference in frequency vB

corresponds to the frequency of emittedphonons. This is the Brillouin frequency shift,it is set by a phase-matching requirement[21]. For pure backward Brillouinscattering, the Brillouin shift can becalculated from the refractive index n(for Brillouin scattering in fiber, theeffective refractive index must be used),the acoustic velocity va   , and thevacuum wavelength λ:

vB=2nva

λ    (3)

Brillouin scattering occurs essentiallyonly in backward direction inoptical fibers. However, weak forwardBrillouin scattering is also likely due toeffects of the acoustic waveguide. TheBrillouin frequency shift depends on thematerial composition and to some extentthe temperature and pressure of themedium. This dependency is what isexploited for distributed BrillouinOptical Domain Analysis (BOTDA/R, BOFDA,and BOCDA) fiber optic sensors. See othertext for Brillouin OpticalCorrelation/frequency Domain Analysis(BOC/FDA) sensors.

B. BOTDA SENSORSA commonly used distributed sensing

technique is the the Brillouin Optical

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time Domain Analysis (BOTDA), it wasinitially proposed to measure opticalfiber attenuation in 1989 but later usedfor distributed sensing in 1990 [26].BOTDA rides on stimulated Brillouinscattering (SBS), where two counter-propagating light beams, usually a pulsedpump and a probe continuous wave, interactalong a sensing optical fiber. At regulartime intervals, the probe wave at somelocation may be amplified by the travelingpump pulse, depending on the frequencydifference between these two light waves[22]. By recording the optical frequencyof either wave with respect to the other,the narrow (30MHz) Brillouin Gain Spectrum(BGS) is recovered, and the frequencydifference, gauged by the position of thepeak gain, can be converted to temperatureor strain along the sensing fiber [22].

In a more detailed description, thesensing principle is based on the factthat the frequency difference, at whichthe maximum amplification of the Stokeswave occurs, known as Brillouin frequencyshift (BFS), varies depending on themechanical and thermal states of the fiber[23]. In particular, the BFS increaseswith both temperature and strain. Spatialresolution, i.e. the ability to measuredeformation and temperature changes in adistributed way, can be achieved throughusing a pulsed pump beam: through this,the interaction occurs along successiveregions of the fiber as the pump pulsetravels down the sensing cable [23]. Byrecording the intensity of the Stokesradiation as a function of time, theBrillouin gain can be traced in eachregion. Measuring the Brillouin gain as afunction of time and frequency allows theentire profile of Brillouin shift alongthe fiber to be obtained, which in turncan be translated to strain/deformation ortemperature through the use of appropriatecalibration coefficients [23].

BOTDA is a double ended access to the asource and detection system for long

sensing lengths combined with high strainand temperature resolution for up to 31miles (50 km) without signal regeneration[24].

The major factors that control thesensing speed of a BOTDA setup are [22]:

1) Flight Time: the repetition rate of the pump

pulses should not exceed 1

Troundtrip .

1Troundtrip

=2LVg

(4)

Where Vgis the group velocity speed oflight traveling within the fiber and L isthe length of fiber.

2) Averaging Navg : Over ten to thousandspump pulses is required for a satisfactorysignal to noise ratio (SNR), especiallyover long fibers.

3) Scanning granularity: in order toaccurately map the Brillouin gain withinthe required dynamic range ofstrain/temperature variations Vfreqranging from 100 to 200 differentfrequencies should be probed.

4) Optical frequency: switching speed of thesweep mechanism requires a finite time,depends on the actual implementations: onthe order of milliseconds or longer,inclusive of stabilization [22]. Thethird factor is determined by the expectedstrain/temperature resolution. The firsttwo factors are dominant for long (tens ofkilometers) sensing fibers, resulting inlong acquisition times on the order ofminutes [22]. On the other hand, whendealing with a relatively short (less than1km) fiber, the fourth factor, frequencyswitching speed, becomes dominant [22].

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The Brillouin frequency shift can give thetemperature or strain information as thefollowing equation [24].

v (ε,T )=v (0,0 )+C1ε+C2T(4)Where ε and T are the strainand temperature, respectively,C1and C2 represent the strain andtemperature coefficients, respectively,known to be about 0.05MHz/micronstrain and 1.2 MHz/°C for conventional singlemode optical fibers used at the 1500nmwavelength range for opticalcommunications [24].

Fig. 8. Basic configuration for BOTDA: (a), (b)and (c) show the waveform of optical power atdetector (Pd), acquired when the frequency offsetbetween the two lasers is tuned to the Brillouinfrequency shift νB of fiber coils 1, 2 and 3, placedat temperatures T1, T2 and T3, respectively [23].

Fig. 9. Basic operation/principle and graphs ofBOTDA [25].

V. CONCLUSIONThis paper to the best of my knowledgepresents in simple

language a basic understanding of fiberoptics, fiber optic communications, fiberoptic sensors, distributed fiber opticsensors and explains in detail yet insimple language how an example of thedistributed fiber optic sensor (BOTDA)operates. It will bridge the gap betweennon experts and experts in the field fiberoptic sensors and any individual who readsthis paper will understand how distributedfiber optic sensors work.

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[6] International School of Photonics at Cochin University of Scienceand Technology India. 2015. [Online]. Available:http://photonics.cusat.edu/Research_Fiber%20Sensors.html

[7] Miao Yu, (2008, Feb). Fiber Optic Sensor Technology,Department Of Mechanical Engineering, University of Maryland,USA. [Online]. Available:http://www.sem.org/pdf/fiber_optic_sensor_technology.pdf

[8] Peter Fuhr, (2001, Dec). Introduction to Fiber-OpticSensing, San Jose University. Available:http://archives.sensorsmag.com/articles/0502/fiber/main.shtml

[9] Dr. Crispin Doyle, (2003), Fiber Bragg Grating Sensors,an Introduction to Bragg Gratings and Interrogation Techniques.[Online]. Available: http://smartfibres.com/Attachments/Smart%20Fibres%20Technology%20Introduction.pdf

[10]Y. J. Rao, (1998), Fiber Bragg Grating Sensors: Principlesand Applications. [Online]. Available: http://link.springer.com/chapter/10.1007%2F978-1-4615-5787-6_11

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[15]Daniele Inaudi, Branko Glisic, (2006, July).Distributed Fiber optic Strain and Temperature Sensing forStructural Health Monitoring. [Online]. Available:http://www.roctest.com/content/download/888/7117/file/c147.pdf

[16]Wikipedia Contributors. (2015, July). BrillouinScattering. [Online]. Available:https://en.wikipedia.org/w/index.php?title=Brillouin_scattering&oldid=671250847

[17]Dirk Samiec, (2011, June). Distributed fiberoptic temperature and strain measurement withextremely high spatial resolution. PolytechGmbH, Waldbronn Germany. [Online]. Available:http://lunainc.com/wp-content/uploads/2013/04/photonik_intl_2012_010.pdf

[18]Stack Exchange, (2014, April), Raman vs.Brillouin Scattering. [Online]. Available:http://physics.stackexchange.com/questions/109814/raman-vs-brillouin-scattering

[19]Weiwen Zou, Xin Long and Jianping Chen, (2015,Feb). Brillouin Scattering in Optical Fibers and its Applicationsto Distributed Sensors. [Online]. Available:http://www.intechopen.com/books/advances-in-optical-fiber-technology-fundamental-optical-phenomena-and-applications/brillouin-scattering-in-optical-fibers-and-its-application-to-distributed-sensors

[20]S. P. Singh, R. Gangwar and N. Singh, (2007).Nonlinear Scattering Effects in Optical Fibers, Progresses inElectromagnetic Research (PIER 74). [Online]. Available:http://www.jpier.org/PIER/pier74/23.07051102.Singh.GS.pdf

[21]RP Photonics Encyclopedia, [Online]. Available:http://www.rp-photonics.com/brillouin_scattering.html

[22]Yair Peled, Avi Motil, and Moshe Tur, (2012,March). Fast Brillouin optical time domainanalysis for dynamic sensing. Optical Society of

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[23]Zeni L, et al., Brillouin optical time-domainanalysis for geotechnical monitoring, Journal ofRock Mechanics and Geotechnical Engineering(2015) [Online]. Available:http://dx.doi.org/10.1016/j.jrmge.2015.01.008

[24] Hong-Hu Zhu, Jian-Hua Yin, Lin Zhang, Wei Jinand Jian-Hua Dong, (2010, Apr), MonitoringInternal Displacements of a Model Dam Using FBGSensing Bars, Advances in Structural Engineering, Vol. 13,No. 2 [Online]. Available: http://multi-science.metapress.com/content/22u762vj0212412r/?p=9f93759384d344b98d004c72cb3f09bd&pi=3

[25] Pipeline Corrosion Monitoring by Fiber Optic Distributed Strain

and Temperature Sensors (DSTS). Lufan Zou and OmurSezerman OZ Optics Limited, Ottawa, Canada.[Online]. Available:http://www.ozoptics.com/ALLNEW_PDF/DSTS_2008_NACE.pdf

[26]Chao Lu et al (2012, December), IEEE PhotonicsJournal Vol. 4 (No. 6) 1-cm-Spatial-Resolution BrillouinOptical Time-Domain Analysis Based on BrightPulse Brillouin Gain and Complementary Code.[Online]. Available:https://www.researchgate.net/publication/260359540_1-cm-Spatial-Resolution_Brillouin_Optical_Time-Domain_Analysis_Based_on_Bright_Pulse_Brillouin_Gain_and_Complementary_Code

Appiah Sarfo George comesfrom Baku in the westernregion of Ghana, he was bornin 1987. He received aBachelor of Science degreein telecommunicationsengineering with a first

class honors from the Kwame NkrumahUniversity of Science and Technology wherehe successfully worked on the project

“Free Space Optical Communications”. From2011 to 2012 he was a Teaching andResearch assistant with the ElectricalEngineering Department of the KwameNkrumah University of Science andTechnology where he worked with SeniorLecturers to tutor a number of coursesincluding Microwave Engineering, NetworkPlanning, Information Theory, Analogue andDigital Communication Systems. He workedas Business Intelligence Specialist withVodafone Ghana from 2012 to 2015.

He is currently a PhD student at theElectronic and Information Engineeringdepartment of the Hong Kong PolytechnicUniversity working on Distributed FiberOptic Sensors. He is a member of theNational Society of Black Engineers andthe Ghana Institute of Engineers.

Prof. Chao Lu (BEng (Tsinghua), PhD(UMIST), MIEEE) obtained his BEng inElectronic Engineering from TsinghuaUniversity, China in 1985, and his MSc andPhD from University of Manchester in 1987and 1990 respectively.

He joined the School of Electrical andElectronic Engineering, NanyangTechnological University, Singapore asLecturer in 1991 and has been an AssociateProfessor since January 1999. From June2002 to December 2005, he was seconded to

the Institute for InfocommResearch, Agency for Science,Technology and Research(A*STAR), Singapore, asProgram Director andDepartment Manager, helpingto establish a research groupin the area of optical

communication and fibre devices.

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Since April 2006, he has been aProfessor in the Department of Electronicand Information Engineering, Hong KongPolytechnic University. His researchinterests are optical communicationsystems and networks, fibre devices foroptical communication and sensor systems.

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