Journal of Scienti fic & Industrial Research Vol. 60, October 200 I, pp 779-785

Experimental Feasibility Study of Fibre Optic Extrinsic Fabry-Perot Interferometric Sensor for Civil Structures and Other Applications

S C Jain , Nahar Singh, J K Chhabra and A K Aggarwal *

Coherent Optics Di vision, Central Scientifi c Instruments Organi sati on , Sector 30, Chandi garh 160 020

Received: 05 March 200 I; accepted : 09 Jul y 200 I

An experimental bench set up of a fibre optic Extrinsic Fabry-Perot Interferometer (EFPI) sensor was reali sed and its feasibilit y demonstrated in an open confi gurati on. The basic interferometric coupling between the sensing and reference retl ections from a Fabry Perot (FP) air cavity reali sed between two fibre ends was so achieved that the sensor ex hi bits extreme sensitivity to ex ternal dt st~rbances. The study reports about the sali ent features, scope, operat ing principle, and details of the experimenta l set-up of a ltbre opttc EFPI. The results of qualitative studies undertaken fo r sensing vibrations due to external dis turbances under different situations and its further refi nement/modificati ons for applications in hea lth monitoring of structures are also di scussed.

Introduction

In the realm of fibre optics, after optical fibre

communication, it is the area of fibre optic sensors which is growing rapidly and making a revolutionary impact in

various app li cation sec tors such as indu stry, c ivil

structures, novel materia ls, medicines, aerospace, power generation, tran sporta tion , military and sc ientifi c

research . As different phys ica l perturbation s affect

optical power travelling in a fibre in different ways, unique signature can be obtained which allow abso lute

or differential measurement of the incident di sturbance.

Optical fibre sensors have thus been configured to detect

and measure different phys ical phenomena such as, strain , pressure, temperature, acceleration, magnetic, and

e lec tric field s. Fibre -b ased sensors hav e found

applications in industry for process contro l but more recentl y their important application has e me rged for health moni toring of advanced c ivil struc tures t-x. The

conventi onal methods used for the quantitative analys is and visua l in specti on of c ivil structures, however, are expens ive, often unreliable, and ti me consuming. Th us, there is a growing need for the development o f efficient ,

reliable and low-cost sens ing instrumentati on th at can be used in multiplexed and dist ri buted configurati ons nnd cnn nlso be integrntcd with structures and mnteria ls

· Author for corres pondence. E-mail: aka Ca.~c,c~io. rcn.nic . in ,

Fax No: 0172-65726 7/ 657ml2

for their lifetime henlth monitoring. Optical fibre sensors and large-scale fibre sensor systems, in particular, are

a ttractive candidates for sens in g in material s and structures. Fibres are small , light weight, and fl ex ible

and they can operate over a wide range of environmental

conditions, and can be configured to respond to many

physical observables. As optical fibres are manu fact ured from dielectric mate rial s the signal s tran smitted through

them are immune to EMI, and fibre sensor ne twork s are

not plagued by ground loops. Additi onally, optical fibres

may be interrogated simulta neous ly us in g several method s to allow both multi-measurand sensin a and

b

distributed sensing capabi lity on a single fibre . The large

dynamic bandwidth , geometric versatility, and ease of

e mbedment make o ptical fibre senso rs parti cul a rl y a ttractive for th e in situ hea lth monito rin a of c iv il

b

structures. Fibre o ptic se ns o rs are be in g used fo r quantitati ve, non-des tructive monitorin g of ad vanced

materi a ls and struc tures nnd their deployment for the

meas ureme nt o f inte rnal mate rial changes durin g fabri cation , the in-service lifetime measurement o r stra in , te mperature, vibra ti on, and the eve ntu a l de tec ti on o f

damage or property deg radati o n has been rece ntl y demonstrated <J - tr ••

Th e prin c iple o f ope ra ti o n o f a spec ifi c senso r, depends on the modulation method be ing used . The most sens iti ve meth ods ex ploit the modul ati on of the phase

780 J SCIIND RES YOLoO OCTOBER 200 1

of li ght propagating through the fibre and interfering the sensing beam with an unaffected reference beam to g ive ri se to constructive and des tructive interference signal s. While phase-modulated sensors are among th e most sens iti ve, th e ir impl eme ntati on requires com pl ex stabilisation methods. Severa l types of hi gh-sensiti vity pha se -modulated senso rs , lik e Mac h-Ze hnd e r. Michelson, Fabry Perot, dual mode and Bragg grating interferometric optical fibre sensors, are graduall y being developed for end-user applicati ons.

As the area of structurally embedded optica l fibre sensors is rapi dly evo lving, it is important to reduce the number of leads required and to eliminate the sens iti vity of va ri ous leads . The Fabry-Perot optica l fibre se nsor prov ides a system in wh ich a single optical fibre acts as both the lead-in and lead-out fibre. Under a situation of appl ied loads, the abi lity to meas ure the internal strain is important to the health moni toring of materials and structures. Such meas urements have traditi onall y been acco mplished throu gh th e use of surfac e mount ed res ista nce strain rosette gages. However, these gages are limited to ex ternal applications and the Fabry Pe rot fibre op ti c strain sensor could pro vide a viab le alternative to them for assessment of an internal stra in state.

Two types of Fabry- Perot interfero meters are being exploited for fibre optic sensing app lications namely. the intrinsic and ex trinsic types . The intrinsic Fa bry-Perot sensor differs from its ex trin sic counterpart in that the cavity, where the sensing and reference signa l re rl ec tions occur, is created in a glass sub-section of the optical fibre instead of in air. All phase- modu lated senso rs. except fo r th e ex trin s ic Fabry- Pe rot, when embedded, are affected by transverse strai ns on the sensing reg ion of the optical fibre. Transverse strain s in in trins ic optical fibre sensors induce a refractive index change in the core, which changes the effective opti ca l path length . causing erroneo us st ra in read in gs . Besides thi s, du e to th e ex istence of a semi-reflective sp li ce. th e senso r is susceptible to failure at low applied stress leve ls. Also the exposed mirror ends in an intrinsi c Fabry Perot are subjected to end effec ts that cause e rrors in locai measurements when gage lengths arc not sufficiently long. Quanti tati ve studi es have demonst rated that the results obtain ed from intrins ic dev ices may de viate s ignificant ly if the strai n fie ld is not known be fore hand and this limits the use of intrinsic dev ices, except in spec iri c c ircumstances. On the other hand. EFPI sensor otTers the advantage of simple construction. single-ended

operati on, high reso lution and accuracy. and low costs . The EFPI output is not affected by trans verse strains and onl y monitors axial strain components, since the cavi ty consists of only an air gap rormed between two fib re end faces. The st rain measurements by EFPI are also not affected by surface shears 17

•2

' .

In order to monitor hea lth of structures and material s. it is im portant to know the fu ll three-dimensional stare of strain to quantitatively a~sess the damage state of the materials. Embedded Fabry Perot fib re optic sensors could prov ide accurate and reprod ucible measurements of stra in within a material containing comp lex damage states and offer a too l to int errogate th e hea lth of structures and an in strument to study the basic ph ys ica l behav iour. Three-dimens iona l strain measurement inside a structure would fac ilitate enhanced understanding and de tec ti on o r co mp os ite failure modes (s uch as de­lam ination), sensing for adapti ve structura l control. and structu ra l health mon itoring systems for c ivil structures. The latter applicati on could in vo lve remote monit oring or structural int eg rit y durin g and after an earthquake. Three-d imensional strain se n.-i ng in .- ide a structure is not poss ible with conve nti ona l stra in sensing tec hnique suc h as e lec tr ica l strain gages that require a surface attac hment, while a fib re opti c EFPI Fabry Perot sensur oilers the unique ability to be suspended v, ithin the matri x portion of the composi te system. Since one must contain a sensor in the small est surface area to make a co nstant strain assumpti on reasonable . a small size sensor makes possib le the acceptance of thi s approxi1nation. The EFPI sensor can be manufactured with a very sma ll gage length and meets the require ment of esse ntiall y providing a point meas urement of th e st r<I in. Embedcl i ng or FP se nsor allows fo r the determinati on of strains ex perienced by the matri x materi al that is intern al to an inhomogeneous system. These embedded sensors can be subseq uen tl y used for in-situ characterisation of· the host materi al or st ru ct ure.

Optical fibre sensors arc increasingl y bei ng used to mo nito r strain. te mpe rature . v ibrat io n, and oth e r environmental effects in materi als and st ructures . Strain and remperalll rc may be measured empl oy in g various fibre sensing methods and recent deve lopment s in that catego ry ha ve bee n EFP I dev ices and in - li ne fibre grat in gs . EFP I dev ices, in pa rti cu lar, are be in g ex tensi vely in v<..:s ti gated to a wide range of measurement problems in co mpos ites , metal , and c ivil stru ctures. Bes ides thi s. EFPI based sensi ng co nfi gurati ons could

JAIN el a/.: FIBRE OPTIC EXTRINSI C FABRY-PEROT INTERFEROMETRIC SENSOR 7XI

Input

Output

/ Adhtulve

Gogo Length (L)

Figure !- Schema ti c of a fibre opti c extr in s ic Fab ry Perot

int erferomet ri c sensor

find other appl ications24' 2x such as, measurement of blast

pressure, load on prope ll er blades of an ice breaker, pressure in internal combusti on engines, and monitorin g of vehicu lar traffic.

Principle of EFPI

The EFP I sensor is an interferometric sen sor in which the de tec ted inte ns it y is modul ated by th e measurand. The schematic depicting the principle of an EFPI is shown in Figure I. The EFP I system cons ists of a sing le mode laser diode, which illuminates a Fabry­Perot cavity through a fused hi-conica l tapered coupler. The cavity is formed between an input/output sin gle mode fibre and a reflectin g single mode or multim ode fibre . The far end of the multi mode fibre is shattered so that reflections from the far end do not add to detector noise. The input fibre and the re fl ec tin g fibre are aligned in a silica capillary and fix ed to the inside of the tube with an epoxy resin . The di stance betwee n the input/ output and re fl ecto r attachment points in th e sili ca capillary tube defines the gage length 'L' of the EFPI. The gage length could be set du ring fabrication to be seve ral millimeters or less. Fo r uncoated fibre end s. a 4 per cent Fresnel re fl ect ion resu lt s at both ends of the cavity. The first re fl ec ti on, R, ca ll ed th e refe ren ce reflect ion, is independent of the appli ed perturbati on. The second reflec tion , R

2, termed the sens ing re fl ecti on.

is dependent on the length of the cavity 's' which. in turn , is modul ated by the app lied perturbati on. Operati on of the EFPl revolves around the air cav ity of the EFPI. The interference between the two signals i-; observed at one outpu t of the 3dB coupler as si nu soidal variations in opticn l intensity in response to micro-disp lacements in the nir gap ca\'i ty and the sensor signa l thu s provides a relative measurement of strain induced in the sensor head . The optica l signal in FP sensors is dependent , only on the opt ical path length change, thereby producing a lead insensitive opti cal fibre sensor. The air gap acts as a low­finesse FP cavity and the effect of multip le reflec ti ons

taking place between the fibre ends cou ld be shown to be neg li g ibl e and th e system works as a two-beam interfe rom e te r. Th e inte rfere nce of the tw o-wave interferometer can be evaluated in terms of plane wave approximations , assuming that a coherent plane wa ve is detecred at the output of the sensor 1

'1•

This wave can be represented by,

U, (x. z.,t) =A ; exp (j <p, ); i = 1.2 , ... ( I )

where the variabl e A is a func ti on of the tran sve rse I

coo rdinate x and the distan ce tra ve led :::1

an d the subscripts i= I ,2 stand for th e re fe rence and sens in g re fl ec tions , res pectively. Assuming A ,=A. the sens ing reflec tion coe ffici ent A

2 can be approx imated by the

simplified relation ,

A, = A [to I jo +2s tan (arc s in·1 N/\) }, .. . (2)

11·hcre ·o ' is the fibre core radius. ' t ' is the transmissi on co-effi c ient of the air-g lass interface (typically =0.98). ' s' is the end separation, and ' . A' is the numerical aperture of the s ingle-mode fibre, given by (n /-n _,' ) 1 1~:

n1

and n ! are th e re fracti ve indi ces o f the core and cladding, respective ly. The observed intensity at the detector is a superposi ti on of the two amplitudes and is g ive n by,

I =A" [I+ 2to cos (4 n /}c)lj o +2s tan (arc sin 1 NA)) dr l '

+ t 2o 2 flo +2s tan (arc sitr 1 N!\ ))2]. . .. (3)

where it is assumed that <p1= o and (p

2= 2s (2 n I )c) anJ

), is the wavelength of operation in free space.

The simplifi ed loss re lati o n g ive n by Eq. (3) prov ides the basic understanding ab0ut th e operati on of the EFPI sensor. Va riati ons in the separation 's' b ' tween the end faces of the fibres, al igned in a silica capillary. serving as a support tube. produce modu lation uf the outpttt si gnal curren t. This modulation is sinusoid al with a gradually decreasing amp I itude, as ' s' increases. which is also expected, since the relati ve intensity of the sens ing re fl ect ion starts falling re lative to referen ce re fl ectio n.

Strain E is determined as the rat io betv,;een the end face d isp lacement measured . e .g .. t0.s and the gage le ngth 'L '.

E=t0.s !L

t0.s is determi ned from th e numb e r or th e interference fri nges shifted due to the appli ed strain. If 'm' is the number of frin ges shifted, then .

782 J SCII ND RES VOL fiO OCTOBER 200 1

>-1-u; z ~ z 1-

~ I­:> 0

DISPLACEMENT

Figu re 2-Transfer functi on curve of an EFPI sensor

L'ls = m A I 2 or E = 1n A I 2 L

As evident from Eq . (3), the change in the output intensity of the EFPI is non-linear, corresponding to the

magnitude of the pa rameter be ing measured . The typi cal

s inu so ida l output trans fe r- func ti on cu rve fo r a lo w­

finesse 2-beam inte rfero mete r is shown in F igure 2. The middle o f the linear intens ity output reg ion is te rmed the quiescent-point or simply, the Q-point of thesensor. S ince

th e change in output intens ity with d is pl aceme nt is max imum at Q-po int, the sens iti vity of the sensor is the

hi g hes t a t thi s po int. However, for sma ll ex te rn a l

perturbati ons, it is des ired that the operation of the sensor

be limited to the linear region around the Q-point , while for large perturbati ons, complex fringe counting schemes

must be empl oyed . S ince signal fadi ng is a common

problem, it is ad vantageous to have two signals out of phase by 90 degree, so that when one of the signals has

a low sensitivity to the phase shift, and hence to the paramete r be ing measured , the other siuna l has a hi uh 0 0

sensi ti vity. Numerous techniques have been deve loped to s ta bili ze s in g le mode inte r fe ro m e tri c sys te ms,

specif ica lly employ ing frequency and phase modul ators

to generate heterodyne signals or ph ase shi fte rs using piezoelectric devices to modul ate the reference fibre2'l·

33. Moreover, if a change in direction of the app lied strain

or any perturbati on occurs at the max ima or minima of the s inuso ida l transfer funct ion curve, th e system is unab le to detect it. Co nventio na l EFPl senso r is a diffe re ntial one , in that it re qu ires a re fe re nc e measurement from which re lative perturbation chan ges

are ex tracted .

Experimental Procedure

Experimenta l feas ibi li ty study for deve lopment of a fibre optic EFPI sen ~or was undertaken and a be nch set up of the sensor (in an open confi ouration) cx hibit ino 0 0

Ht·Ne L oat r

Microscope XY Z Obtective Posi oner

X-Y Optiecl Pow• Photo Recoder Meter Oete cfor

Index Moe hi ng Liqui d

MM Fiber Po si oners

Figure 3-Schematic of a fibre optic EFPI experimental set up

inte rfe rometri c sensiti vity was rea li sed in the laboratory.

The open configuration here signifies that the fibres were not encapsulated in a silica capill ary. F igure 3 represents

a schematic of the experimenta l set up, showing the basic configuration to in vestigate and rea li se the sensor.

Li ght from a He-Ne laser (632.8n m) was launched into an input port of a sing lemode (7 I 125 f.!m ) fibre fused

b ico ni ca l tape red 3dB 2x2 coup ler u s in g a f ixt ure

co mpri sing a mic roscope objective and a x-y-z fibre pos iti one r. Before launching lig ht into the couple r port, the fibre end was properl y c leaved by means of a fibre

c leaver and inspec ted under a microscope. One output

port of the coup le r was immersed in an index matchi nu liquid (g lycerin ) to prevent back re fl ecti on, while th~ other output po rt was he ld in a prec is ion x-y-z fibre

pos iti o ne r. A Fabry Perot a ir cav ity was rea li sed by prec isely pos iti oning a multimode optica l fibre (50/ 125

f.!m ) ac ting as the re fl ec tor of th e FP cav ity, in c lose prox imity of the input/output s ing le mode fibre. The a ir

gap between the two fibre ends was brought c lose to 50 f.!m by means of a stereo mi croscope (Carl Ze iss) under

40x magnificat ion. T hi s a ir cav ity so rea li sed, worked as a low-finesse FP inte rfe rometer, because higher order multipl e reflect io ns occurrin g in the cav ity do no t

contribute s ignificantl y. The sing le mode fibre end was al igned with respect to the re fl ect ing mu ltimode fibre to achieve the desi red in te rferometric coup li ng between the re fe rence a nd sens in g ref lectio ns. T he resu lt in o . 0

tnte rfe rence s ignal was detec ted at the output o f the coupler using a PIN photocliode and di sp layed through a high sensiti vity optical powermeter (Model AQ-1 11 5, Ando). The e lectr ica l output of the powcrmctc r wa: further feel to a x-y reco rder (Model 7044B, Hew lett Packard) with time base on th e x-ax is to stud y th e inte rfe rometric coupling of the sensor.

Hes ults and Discussion

i\n cx per in c nt;1l ben ch set up o f a fi h•·e opt ic

ex trinsic Fabry Pero . inte rfe rometri c sen or in an open

JAIN e1 a/.: FIBRE OPTIC EXTRINSIC FABRY-PEROT INTERFEROMETRIC SENSOR 783

Figure 4--Typical photograph of experimental bench set up of the fibre optic EFPI

b

Figure 5-Yibration amplitude record for no-impact situation for : (a) Coupled and (b) Non-coupled EFPI con figuration~

configuration was realised and implemented. Figure 4 shows a photograph of the experimental set up of EFPI sensor. The realised sensor shows the interferometric cou plin g and ex hibits hi gh sensitivity to ex tern a l disturbance. Qualitative studies were ca tTied out, using the reali sed confi gurat ion of EFPI , for monitoring I recording various impact situations/d isturbances. Some of the typical results are presented for a no impact situation , both for coupled (50f1m gap) and uncoupled ( 150f1m gap), as shown in Figure 5. Fi gure 6 shows results for di sturbances in vo lvin g different types of impacts: gentle hand thumping of the work bench, foot thumping of the floor nearby, and random vibrati ons created due to a drillin g machin e operatin g in the neighbourhood. In order to fab ricate a rugged EFPI and its safe bonding in concrete spec imens for measurement of strain, crack growth , and temperature, the rea li sed senso r head needs to be encapsulated into a s ili ca capi ll ary tube using appropriate epox ies. Fabrica ti on of an encapsulated fibre optic EFPI with a gage length of 7mm and air cav ity spac ing of 40f11ll was attempted by

(a)

r~-k __ ~_;L __ ;___ _ __ ~

(b)

(c)

Fig ure 6-Vibrati on amplitud e record for coupl ed EFPI conf"igurati ons: (a) Gentle hand thumping of the work bench, (b) Foot thumping of the fl oor nearby, (c) Random vibrati on created clue to a drilling machine operating in the neighbourho0d

bonding optical fibers to a silica cap ill ary tube ( 150f1m dia , I Omm length ) using araldite. After complete curing of the EFPI, it was bonded using araldite to the surface of a concrete spec imen ( I Omm x I Ocm x 35cm) whi ch was properly cleaned with isopropyl alcohol prior to bonding. The concrete spec imen was then subjected to compressive stress under a load ing machine and output intensity variati ons with increasing contrast as cav ity spacing decreases. were observed. However, these studies need furth er refinements to obta in elaborate quan ti tat ive results wh ich shall be incorporated in our future research pub licati ons. This ru gged configuration serves as an important tool fo r monitorin g st ra in , temperature. displacement, vibration, acoustic emission and pressure for vari ous indu stri al appli cat ions in th e areas of

784 1 SCIIND RES VOL 60 OCTOBER 200 1

structures, material s, internal combustion engines , vehicular traffic control and blast s tudi esD-~1\ etc.

Conclusions

An experimental set up of a fibre optic EFPI sensor, which is primarily a narrow air cavity created between two cleaved fibre end faces, has been realised in an open configurat ion and the basic interferometric coupling between two reflections taking place from the fibre end faces has been achieved and demonstrated. The realised sensor configuration ex hibit s high sensitivity to vibrations and other physical perturbations taking place in its vicinity. This mode of the sensor can be exploited after appropriate modifications and mggedisation for detection and monitoring of traffic and seismic and other disturbances. Further, an encapsulated version of the fibre optic EFPI with a gage length of 7mm and air cavity spacing of 40!lm was fabricated by bonding optical fibres to a silica capillary tube ( IS011m diam, I Omm length) using araldite. The encapsulated EFPI was bonded using araldite to the surface of a concrete specimen (I Omm x I Ocm x 35cm) which was then subjected to compressive stress under a loading machine and output intensity variations with increasing contrast as cavity spacing decreases , were observed . This mgged EFPI can serve as an important sensor for embedding appl ications in stmctures for assessment of internal state of materials , besides various other applications.

Acknowledgement

Authors are grateful to the Dr R P Bajpai, Director CSIO, for permitting to publish this study. They are also thankful to Mr Sushi! Kumar, Mr D P Chhachhia, Mrs Asha Kumar and Mrs Madhu Mehta for he lp and cooperation . The financial assistance provided for this work by the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy, is acknowledged .

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