The Use of Bragg Gratings in the Core and Cladding of Optical Fibres for Accurate Strain Sensing

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Sesors & Trasducers ISSN 176-5479 008 by IFSA http://www.sesorsportal.com The Use of Bragg Gratigs i the Core ad Claddig of Optical Fibres for Accurate Strai Sesig Ia G. PLATT ad Ia M.

1 Sesors & Trasducers ISSN by IFSA The Use of Bragg Gratigs i the Core ad Claddig of Optical Fibres for Accurate Strai Sesig Ia G. PLATT ad Ia M. WOODHEAD Licol Vetures, PO Box 133, Licol, Christchurch 7640, New Zealad, Tel.: , fax: platti@lvl.co.z, Received: 15 October 007 /Accepted: 0 February 008 /Published: 15 April 008 Abstract: Optical Fibre Sesors (OFS) have may advatages over others type of sesors for measurig strai or micro-displacemet. This paper itroduces a ew cofiguratio of Bragg gratigs withi a optical fibre to improve strai measuremet resolutio ad accuracy. We describe the geometry, together with the research directio curretly beig udertake to produce a commercially viable micro-displacemet sesor suitable for a umber of architectural ad egieerig applicatio. Copyright 008 IFSA. Keywords: Optical fibre, Strai, Bragg, Micro-displacemet 1. Itroductio The ability to perform distributed strai measuremets over a structural beam is of cosiderable importace i assessig both the safety ad maiteace requiremets of most civil egieerig structures. There is a icreasig awareess that efficiet Structural Health Moitorig (SHM) ca substatially decrease the maiteace expeditures of may structures ad alleviate delays caused by uecessary or uexpected repairs. As a example of this, the structural health moitorig of bridges has become a sigificat issue over the last decade. I the Uited States aloe there are some 591,707 major bridges, early a third of which are classified as deficiet or fuctioally obsolete [1]. The aual budget for maiteace is aroud US$4 billio, though it is estimated that approximately US$13.5 billio aually is required to repair ad maitai all bridges to a acceptable level. This huge expediture has placed cosiderable pressure o admiistrative bodies to put ito place advaced bridge maagemet programs, icludig real time structural moitorig ad assessmet istrumetatio []. 333

2 The importace of SHM has attracted the attetio of those workig i the emergig field of Optical Fibre Sesors (OFS) ad may advaces have bee made i techiques to eable such measuremets (e.g. [3]). Much of this iterest has bee prompted by the relative iertess of optical fibres to evirometal factors (e.g. moisture) ad their low iterferece to the beam s structural itegrity (i.e. small cross sectio) makig them ideal cadidates for the desired distributed measuremet process. Strai ad loadig measuremets most ofte use devices firmly fixed to the sample to measure its micro-displacemets uder the actio of some force. Bragg gratigs etched withi optical fibres do this by idicatig the chage i gratig spacig as the fibre is stretched alog with the sample (e.g. [4]). The practical applicatio of such techology however has a umber of drawbacks, icludig; 1) the respose due to icreased gratig spacig is similar to that imposed by chages i temperature so the two effects are difficult to resolve, ) fixig the fibre withi or o the sample so that the fibre actually measures its true displacemet is also difficult (while maitaiig its major advatage of havig a small cross sectio), 3) the strai iduced deformatio of alog the fibre is ot uiform ad this effect is difficult to allow for [5]. It is the purpose of this paper to outlie work curretly uder way to develop a robust ad commercially viable Bragg gratig strai sesor that ca be used i a variety of measuremet scearios, while mitigatig some of the major disadvatages.. Bragg Sesors Bragg gratig techology is oe of the most popular choices for OFS strai measuremet sesors due to their simple maufacture (direct etchig i the fibre core) ad relatively strog reflected sigal stregth. Other OFS types, such as Fabry-Perot iterferometers, are less popular sice sigal detectio ad resolutio ca be problematic requirig expesive equipmet ad specialist operators. Fig. 1 shows the typical arragemet used i curret strai (micro-displacemet) sesors. Whe a force is applied to the eds of the fibre sesor, the Bragg lie spacig is icreased ad this results i a frequecy shift of the backscattered peak. Claddig F F Bragg Gratig Core Amp litud e Frequec y Frequec y shift betwee ic idet (laser sourc e) ad reflected sigal is proportioal strai + tem perature Fig. 1. Basic cocept behid a Bragg gratig strai sesor. The frequecy of the Bragg scattered compoet withi a optical pulse is give by: λ B λ G =, (1) 334

3 where λ B is the scattered Bragg wavelegth, λ G is the gratig wavelegth ad is the refractive idex of the fibre core. The chage i the Bragg wavelegth (δλ B ) is give by [6]: 1 d δλ 1 B λ G ( 1 ( 11 1) ) T P υ P P δε α δ, = dt () where: P 11 ad P 1 are compoets of the fibre strai tesor; δε is the applied logitudial strai; ν is Poisso s ratio; α is the coefficiet of thermal expasio ad T is the temperature. So a spectral shift is also geerated by chages i the fibre temperature with the overall shift beig a combiatio of the two effects. The coefficiet of thermal expasio, α is very small ad for silica fibre: so that equatio () becomes: ( P υ 1 ( P + 11 P 1) ) 0. δλ 1 d B λ G 0.78 δε T dt δ = + (3) I practice it is difficult to resolve the δε ad δt compoet cotributios of the spectral peak shift, δλ B. Attempts have bee made to combie two fibres, oe uder strai while a eighbourig oe is shielded from the strai, i a effort to calibrate the temperature respose. This approach works to some degree, but that fact that two fibres have to be excited i exactly the same maer for the small differeces to be measured, meas that a cosiderable degree of accuracy i relative fibre placemet ad detector sophisticatio is required. Other methods usig several wavelegth gratigs ad/or fibre dispersio properties have met with eve less success, agai because of the small differeces ivolved. As well as temperature effects, other practical cosideratios eed to be addressed whe costructig a Bragg gratig OFS. Table 1 lists some of these. Here iterrogatio techiques (icludig multiplexig) are ot icluded, with the focus beig upo the accuracy ad reliability of the sesor itself. Table 1. Some of the major problems facig the use of Bragg gratigs i strai measuremet sesors. Sesitive to a Temperature Localized effects Bodig to the sub stratum Small dyamic rage The Bragg gratig respose will also be depedet o temperature. It is difficult to uwrap this from strai ad this is the major iaccuracy ivolved. Localized pichig aroud the gratig will cause it to be distorted ad thus give a false idicatio of the distributed strai (i.e. strai over some loger legth) To measure strai the fibre jacket eeds to be firmly boded to the material. I exteral placemets particularly, the small amout of movemet to be measured may reside i fixigs ad glue etc. ad ot the fibre. The rage of measurable strai is quite small (from zero to about 1000 µstrai 000 µstrai) so peak detectio ad thus resolutio are critical. 335

4 3. Dual Bragg Gratigs Sesors & Trasducers Joural, Vol. 90, Special Issue, April 008, pp Whe cosiderig a recofiguratio of the Bragg gratig system to achieve optimal measuremet performace some importat poits are worth otig: Strai is give by L/L where L is the origial legth ad L is the chage i legth due to stress. Thus to measure strai requires oly the measuremet of chage i distace betwee allotted poits. Bragg diffractio ca be iduced by iscribed lies i either the core or claddig of the fibre. The temperature sesitivity of the fibre is mostly (95%) due to the chage i refractive idex with temperature. A similar percetage chage will occur withi both the core ad claddig material. Sice a proportio of the boud mode travels parallel to the core axis withi the claddig material via evaescet propagatio (ad this ca be icreased by a variety of methods), a gratig withi this regio may also act as a good scatterer. By mechaically detachig the claddig from the core so that each is free to move idepedetly two sets of adjacet gratigs may be costructed (Fig. ). Fig.. Dual Bragg Gratigs i core ad claddig. The figure below shows a possible arragemet of claddig gratigs to take advatage of a verier geometry. The optical properties betwee the two ca be strictly maitaied by applyig liquid resis that are commoly used for other fibre optic matchig purposes. Such a fibre, with Bragg gratigs etched o both the core ad claddig sectios, will allow two ew geometries to be exploited for micro measuremet: 1. The claddig is uder stress while the core is ot. This meas that both Bragg gratigs will be uder the same temperature ad optical coditios, but with oly oe uder stress. The curret problems of providig a referece idetical to the measuremet sesor will be mitigated by this applicatio.. Neither the core or claddig is uder stress, so that the Bragg gratigs do ot measure the micro shift by a icreased gratig spacig. Istead the relative displacemet of the two is measured by a 336

5 adoptig a Verier scale. This method uwraps strai from temperature, but also has the added advatage that whe fixig the sesor to the sample, o allowace has to be made for stress withi the fittigs; the sesor becomes a purely stress free measurer of micro-displacemet. The chage i gratig spacig is thus solely due to temperature variatio. This also overcomes the relatively limited strai rage of the OFS. These two approaches have a commo startig poit, the detachmet of the core-claddig iterface, ad a sigificat part of future research will be devoted to this phase. Of the two methods the detached Verier geometry, while beig the most difficult to realize, offers the greatest advatage for a strai sesor. Such a system ca solve (or at least alleviate) most of the techical difficulties highlighted i Table 1. As a rough, but reasoable guide, equatio (3) may be rearraged to give for costat strai: givig: 1 d = dt δ, (4) δλ T B λ G 1 δλ B λ δt B 1 d = = dt 6 o C, (5) The umerical value assiged to equatio (5) is give by [6] ad [7]. If is take as the core refractive idex ad d/dt is cosidered the same for the two media the it ca be show that the effect of temperature i gratig separatio betwee the core ad claddig is aroud o C, well below the expected system respose of about Sice the strai is a measure oly of the chage i legth there is o eed for the fibre to be stretched i the maer of covetioal techiques. This meas that it is possible to shield the fibre from localized effects without effectig the strai measuremet (see Fig. 3). The same argumet leads to a reductio i the effect ay bodig material may have i the measuremet of actual strai as well as those problems caused by the o-homogeeous logitudial strai field. There are also some practical limitatios o the type of optical fibre that ca be used i the dual Bragg gratig sesor as specified here ad i [8]. If the sesor is ideed realized by the detachmet of the core ad claddig a simple step refractive idex profile will eed to be used. The ormalized frequecy, V, for a circular waveguide is give by: where: ρ is the diameter of the core; λ is the propagatio wavelegth; cl is refractive idex of the claddig. π ρ V = ( ) 1/ cl, (6) λ For sigle mode operatio it ca be show (e.g. [9]) that V <.405. The amout of power flowig i the core ca be estimated by the Mode Field Diameter (MDF) ad this is a fuctio of V ad the core radius ρ. For V = the MDF ca be used to show [6] that approximately 85% of power flows withi the core (15% i the claddig) ad this core value icreases quickly as V becomes larger. The power propagatig withi the claddig must be sufficiet for reflectio discrimiatio ad so requires V <. 337

6 This limits the fibre waveguide to sigle mode propagatio, with a small ormalized idex differece,, (see equatio (8)) ad small core radius (aroud 10 µm). To provide the maximum umber of combiatios, the claddig gratigs will eed to be distributed i aular sectios radially aroud the core. The umber of possible sectios will deped upo the resultat reflectio amplitudes of costructive ad destructive iterferece over the active regio. This together with the miimum umber of aliged etched lies ad proportio of power that ca be ijected ito claddig modes required for detectable scatterig will eed to be modelled carefully i the early stages. The modellig compoet should be a relatively straight forward process, but costructig the physical system will of course preset may optical egieerig costraits icludig all those usually associated with optical systems such as diffractio ad etedue etc. as well as some uique to fibre propagatio such as mode couplig. Claddig Achor (with detached Claddig) Fibre Achor Gratig Verier Claddig Gap Fig. 3. Proposed housig for the Verier geometry type sesor. 4. Curret Research Curret work i costructig the two types of sesors described i the last sectio is focused upo mitigatig agaist a umber of operatioal problems, both observed ad perceived. The most obvious difficulty to be solved is the detachmet of the core from the claddig i a way that allows idepedet movemet without compromisig the itegrity of the waveguide performace. It is likely that lubricats of the correct refractive idex exist that ca be used for this purpose, though other solutios or mitigatig procedures also eed to be explored. From equatio (1) it ca be see that the refractive idex of the medium will effect the Bragg spectral respose. Typical step refractive idex fibres have a ormalized idex differece ( ) betwee So usig the lower limit: co cl co =, (8) = cl , (9) co Usig equatio (1) for costat λ G ad a 1300 m laser source gives the chage i the Bragg reflected wavelegth betwee the core ad claddig of: 338

7 λ = B δλ B co δ co m =, (10) where δ co = From equatio (3) the Bragg wavelegth shift due to a strai of 1000 µstrai (i.e. L/L = 0.1%), at costat temperature is aroud 1X 10-9 m. Clearly, eve at the lowest values of the impact of the differet refractive idices is sigificat whe cosiderig the Verier costructio ad this will eed to be compesated for. Oe way of achievig this could be by creatig differet wavelegth gratigs λ G, for the opposig Verier compoets; though the required resolutio may be difficult to achieve. Also challegig is the 3-Dimesioal aligmet of the gratigs to provide adequate precisio ad coverage ad will take some skill ad udoubtedly icrease the legth of the fibre over which a sigle measuremet ca be take. For both geometries, mode couplig of the forward propagatig paths is likely to be strog whe a gratig mismatch occurs (i.e. whe the gratigs are ot aliged). This meas that icorporatio of more tha oe sesor per legth of fibre may require differet wavelegth regimes, somethig already experimeted with by others. Sice ot all of the fibre claddig will be detached from the core there will be a regio of discotiuity i the claddig. This is ot i fact a uusual occurrece i commercial systems ad there are a umber of materials available to take the place of the claddig gaps. The trick of course will be to fid oe that does ot alter the mechaical properties of the sesor (i.e. relative movemet) while still guidig the required modes. This will i also require developmet of specialized housig to allow simple placemet ad efficiet operatio of the sesor (Fig. 3). 5. Cocludig Remarks The overall research objective is to produce a optical fibre sesor that uambiguously measures micro-displacemet ad which ca be moitored usig commercially available equipmet. The system desig will have i mid the further developmet of iterpretive software ad distributive sesig applicatios, some of which will require adaptatio to specific applicatios. This paper has described a ew arragemet of Bragg Gratigs withi optical fibres that has the capability of satisfyig these objectives. Sice the work is i its iitial stages we have outlied here the proposed geometry of the sesor together with the curret directio of research to mitigate a umber of practical desig problems. Refereces [1]. America Society of Civil Egieers Press Release, visited Jue 006. []. Trasport Research Board A3C06 Committee o Structures ad Maiteace 003, visited Jue 006. [3]. Measures, R. M., Structural Moitorig with Fibre Optic Techology, Academic Press, New York, 001. [4]. Hill, K. O., Meltz, G., Fiber Bragg gratig techology fudametals ad overview, J. Lightwave Tech., V15, 8, 1997, pp [5]. Botsis, J., Humbert, L., Colpo, F. ad Giaccari, P., Embedded fiber Bragg gratig sesor for iteral strai measuremets i polymetric materials, Opt. ad Laser Eg., V43, 3-5, 005, pp [6]. Lopez-Higuera, J. M, Hadbook of Optical Fibre Sesig Techology, Wiley, 00. [7]. Hobbs, Buildig Electro-Optical Systems, Wiley, 000. [8]. Platt, I. G., Woodhead, I. M., A New Geometry For Bragg Gratigs I Optical Fibres to Measure Strai, Proceedigs of the d Iteratioal Coferece o Sesig Techology,

Effect of Temperature Variations on Strain Response of Polymer Bragg Grating Optical Fibers

Iraq J. Electrical ad Electroic Egieerig Vol.13 No.1, 17 مجلد 13 العدد 17 1 Effect of Temperature Variatios o Strai Respose of Polymer Bragg Gratig Optical Fibers Hisham K. Hisham Electrical Egieerig Departmet,