V N Ư Joum al o f Science, M athcm atics - P hysics 23 (2007) 237-242 Investigation of the embedded íìber bragg grating temperature sensor T n T h i T a m 1’*, D ang Q u o c T ru n g 2, T n A n h V u 2, L e H u u M in h 2, D o N g o e C h u n g D e p a rtm e n t o f E n g in ee rin g P hysics a n d N a n o -T e c h n o lo g y , C ollege o f T echnology, VN U 144 X uan Thuy, C au Giay, Hanoi, Vietnam In stiĩute o f M a teria ls S c itn c e , Vietnam A ca d em y o f S cien c e a n d T echnology H o a n g Q uoc Viet, C au Giay, H anoi, Vietnam R eceived N o v em b er 2007; receiveđ in revised form 27 D ecem ber 2007 A b s tra c t A n cm bedded íib e r B ragg grating (F B G ) sensor, vvhich fabricated in IM S , for tem peraturc sensing w as proposed T he tem perature responses o f the in-fiber B ragg gratings (F B G s) have becn investigated It w as found that these responses ranged from 10.6 pm/°c to 12.0 pm/°c T he tem perature scn sitiv ity o f the FBG s w as 0.2°c T he strain responses rem ained tem perature indepenđent o v cr a tem perature range o f 20 - 180 ’c T h e results o b tain ed are in agreem ent, w ithin the expcrim ental error, w ith predictions based on m aterial param eters A lthough the tem perature response is n onlinear over the tem perature range 20 ° c to 180 ° c , in practicaỉ system s the tem peraturc response o f the sensor m ay be assum ed to be linear w ith in the tếm perature range 20 °c to 80 °c K ey\vords: F ibcr B ragg G tin g (F B G ), sensor, strain, tem perature sensor In tro đ u c tio n T h e F ib e r B gg G tin g (F B G ) firs t presented b y H il l et aỉ [1 ] in 1978 is a stru ctu re on an o p tic a l íìb e r in w h ic h the re íra c tiv e in d e x o f a íìb e r core is p e rio d ic a lly m o d u la te d vvith a p itc h A (F ig u re 1) T h e in d ex m o d u la tio n is in d u ce d b y e xpo sin g the íìb e r to an in te ríe re n ce p a tte m fo rm e d betw een in te rĩe rin g u ltra v io le t ( U V ) beam s o f lig h t b y d ou b le -be a m o r phase m a sk te ch n iq u e T h e center w a ve le n g th o f the re A e c tio n ẢB is nam ed “ B g g w a ve le n g th ” T h e B g g w a v e le n g th is \ = w here n is ỉ n \ (1 ) thc e ffe c tiv e re ữ a c tiv c in d e x o f the íìb e r core T he F B G fo rm s a d is trib u te d re A e cto r, a c tin g as a naưovv band channcl d ro p p in g spectral íĩlte r in tra nsm issio n, and as a narrovv band re íle c tio n Tilter T he ty p ic a l b a n d w id th o f the r e ík c te d lig h t is about 0.1 n m to 0.3 nm T h e re íle c te d B g g w a ve le n g th is depenđent u po n the p itc h o f the g tin g , and is se nsitive to strain and tem pe ture a cco rd in g to the re la tio n sh ip s: (2) • Corresponding author E-mail: drtranthitam@gmail.com; ktlaser@ims.vast.ac.vn 237 Tran Thi Tam et al / VNU Journal o f Science, Mathematics - Physics 23 (2007) 237-242 238 vvhere and A X bt are the strain and tem perature in duced B g g vvavelength sh ifts , Xu is th c B gg \va vcle n g th , pa is the p h o to c la s tic c o e ffíc ie n t o f the fib e r, a is the c o e ffic ie n t o f th e rm a l expansion Ẹ, is and the th e n n o -o p tic c o e ffĩc ie n t A change in the e ffe c tiv e re íra c tiv e index a n d /o r the g tin g p c rio d w ill cause a s h ift o f the B g g w a ve le n g th Sensing p rin c ip le is based on tha t s h iít o f the B gg w a v e le n g th in response to a p p lie d stra in , tem perature o r pressure changes F B G sensor is one o f the most a từ a ctive o p tic a l íìb e r sensors because Fig Structure o f the F iber B ragg G rating o f s im p lic ity in stru cture and s u ita b ility fo r m u ltip le x e d and d is trib u te d /m u ltip o in t sensing app lica tio ns, and to the fa ct that sensing in fo rm a tio n is encoded in an a bso lu te param eter, nam ely thc resonant w avelength T h e m easurand is transduced absolute param eter, e lim in a tin g errors associated to the w a v e lc n g th o f lig h t re íle c te d b y the FB G , an w ith d r ift o f thc zcro p o in t o f the m easurem ent, com m on in strain gauge systcm s in w h ic h the gauge and lcad w irc s sce v a ry in g and d iffe re n tia l therm al environm ents F B G sensors e x h ib it m any o th e r d is tin g u is h in g featurcs such as, sm all size, lig h t mass and hig h re s o lu tio n , s u ita b ility fo r rem ote m easurcm ent It can be used w ith great advantages o ve r e le ctro n ic sensors fo r in situ sensing environ m e n ts o r hazardous e n v iro n m e n t, due to its im m u n ity to m o n ito r or m easurem ent in harsh against e le ctro m a g n e tic íìeld s, h u m id ity , nuclear d ia tio n and the m o st ch e m ica l m a ten a ls [2 ] T h e m ost c o m m o n advantageous a p p lic a tio n o f the F B G is a sm art m o n ito rin g fo r c iv il e ng in e ering , b u ild in g s and b rid g e s; o il, gas, o r e le ctric a l p o w e r generation industries [3 ,4 ] T h e a p p lic a tio n to g eo p hysical e x p lo tio n /m o n ito rin g is one o f the p ro m is in g a p p lica tio n s to take advantages o f fib e r sensors F B G s are se n sitive o n ly to the e n v iro n m e n t experienced along the gauge le n g th o f the sensor, ty p ic a lly o f the o rd c r o f - 10 m m T h e sm all dim ensions o f the fíb e r a llo w the m o n ito rin g o f structures w ith v e ry clo sc clearances T h is paper describes p erío rm an ce o f a p ro to typ e o f the FBG tem perature sensors at tem peratures up to 2 °c T h e system consists o f íib e rs w ith F B G s and the surface parts in c lu d c an o p tic a l source, detectors, o p tic a l co up le rs and an o p tica l spectrum a n a lyze r fo r m o n ito rin g Experiments T he F B G s were fa b rica te d in the In s titu te o f M a te ria ls Science (IM S in H a n o i) w ith re íle c tiv ity o f a p p ro x im a te ly 30% , and had a B gg vvavelengths la y in g fro m 1530 n m to 1570 n m E xpe rim en ta l device consists o f an E rb iu m -d o p e d Super F luorescent Source (S F S ), F B G sensors, a tem perature c o n tro lle d pad o r liq u id bath and tem perature m o n ito r T h e F B G re fle c te d sig n als is fed to an O p tic a l S pectrum A n a ly z e r (O S A ) A G IL E N T 6142B to locate the w a ve ien g th sh iíts A schem atic o f the system is show n in íìg u re B g g w a v e le n g th and m o n ito r 239 Tran Thi Tam et al / VNU Journal o f Science, Mathematics - Physics 23 (2007) 237-242 « T h e SFS consists o f an E rb iu m -d o p e d fìb e r ■» pum ped FBG by vvavelength a 980 d iv is io n nm Laser m u ltip lie r D io d e through ( W D M ) The o u u t fro m a SFS is co u p le d to a F B G sensor by fc/ap c connector T he lig h t re íle cte d fro m the sensors is d ire cte d to O S A th ro u g h nm 1550 o u u t p o rt o f the W D M The re so lu tio n o f the Fig A schem atic o f the experim ent o p tic a l spectrum a na lyze r w as 0.06 nm , and the peak w a ve len g th was d isp la ye d to the o rd er o f 0.001 nm T h e F B G tem pe ture was m o n ito re d b y a P la tinu m 1000 Q re sistive therm o-sensor (P t R T D -E P H Y -M E S S G m b H ) w ith a lim it d e via tio n o f ,1 ° c fo r ° c -ỉ- 0 °c range and ° c fo r a range above 0 °c A glass th e rm o m e te r w ith gra du a tion o f ° c also was used fo r co m p arison fo r tem perature belovv °c T h e t ip o f the the rm o sensor was assem bled close to the g tin g , so tha t the re a din g tem pe ture in the th e rm o sensor in d icate d the actual tem perature o f the F B G B o th sensors then vvere attached to s m a ll m e tal s trip fo rm e d a probe and placed in inve stiga ted e n viro n m e n t T he expe rim en ts w e re carried o u t in d iffe re n t e nviron m e n ts (a ir, s o lid as w e ll as in liq u id e nviron m e n ts) T h e tem perature set by c o n tro lle d h eating source, had been tuned in sm a ll increm ents and fix e d in about 15 m in to a llo w a c h ie v in g th e rm a l e q u ilib riu m fo r m easuring the sensor’ s ch aracte ristic In the c x p e rim e n t w ith liq u id e n v iro n m e n t, the probe c o n ta in in g the F B G w as im m ersed in to a w a ter bath heated b y an e le ctrica l heater, the F B G tem perature changed fro m - ° c to 0 °c F o r the a ir e n v iro n m e n t w e put the pro be inside sm a ll closed oven, its tem perature w ere changed fro m ° c to 2 ° c A s to s o lid e n v iro n m e n t case w e used copper pad w ith tem perature c o n tro lle d b y P eltie r c o o le r in a range -1 -ỉ- ° c and b y h e a tin g gun w ith m a xim u m tem perature up to 0 ° c R c s u lts a n d d is c u s s io n F ig u re shovvs the th e rm a l s h iít o f the B ragg vvavelength fo r the F B G sensor w ith center w a ve le n g th of 1548.6 ln m at ° c , íitte d b y lin e a r equation W ith the increase o f the FB G tem perature, the Bragg w a v e le n g th s h ifts to the lo n ge r d ire c tio n T h e tem perature s e n s itiv ity is 11.8 p m /° c F o r a ll tested F B G ’ s sensors the observed s e n s itiv ity ranged fro m 10.6 p m /° c to 12.0 p m /° c T h e d e te ctio n s e n s itiv ity o f the tem perature m easurem ent u sin g O S A d e m o d u la tio n is around 0.2 ° c based on w a v e le n g th re s o lu tio n ± lp m o f the device T h e tem perature s e n s itiv ity o f a F B G sensor co u ld be enhanced b y c o m b in in g w ith the w a ve le n g th s h ift under stra in due to th e rm a l expansion and w ith the one due to tem perature induced re íra c tiv e in d e x change T h e F B G was em bedded in the co pp e r base o f thickne ss 0.5 m m The em bedded le n gth was 5 m m T h e copper s trip then had been changed to an A lu m in u m a llo y s trip o f the same 0.5 m m thickness T h e re su lt p lo tte d in íig u re show s the w a ve le n g th s h ift fo r a bar F B G sensor (F B G ) and those w h ic h are em bedđed in C u (F B G ) and A I base (F B G ) o v e r a tem perature range o f ° c to °c w ith center w a ve le n g th o f 1530.4 nm at ° c T h e w a ve le n g th s h ift observed fo r bar F B G is d o m in a te d b y the th e rm o -o p tic c o e ffic ie n t o f the fib e r w ith a c o n trib u tio n the therm al c o n tra c tio n o f the fib e r its e lf, w h ile fo r em bedded FB G s the th e rm a l expansion o f the substrate is d om ina te d o v e r the th e rm a l c o n tra c tio n o f the fíb e r, and the w a ve le n g th s h ift is g o ve m e d b y the sừain induced b y the substrate th e rm a l expansion and th e rm o -o p tic c o e ffic ie n t T he tem perature s e n s itiv ity o f em bedded cases increased to 14.6 p m /° c fo r F B G w ith C u base and 8.2 p m /° c fo r an A lu m in u m a llo y base 240 Tran Thi Tam et aỉ / VNU Journal o f Science, Mathematics - Physics 23 (2007) 237-242 The th e o re tica l w a v e le n g th s h ifts due to XB = 1548.61 @ 25°c 1551 0 th e rm a l e x p a n s io n ’ s strain, ca lcu la te d fro m the th e rm a l expansion c o e ffic ie n ts quoted in re íeren ce [5 ] and fro m the w a v e le n g th m easured s h ift due to stra in response o f the F B G are p lo tte d fo r co m p arison in F ig T h e data show s a reasonable agreem ent betw een the th e o re tica l and e x p e rim e n ta l response fo r C u base F o r an A lu m in iu m a llo y base it deviates m ore T h is d is cre p a n cy betw een the m easured F B G data and the th e o re tic a l m ay be a p re d ic tio n re s u lt dependence of of Fig T he w avelength shift o f the F B G sensor tem pcrature T he experim ental data set w as íitted by first ord er equations the th e rm a l e xpansion c o e ffic ie n t o f the a llo y on tem perature T he tem perature s e n s itiv ity of the B g g w a v e le n g th arises fro m the changc in p e rio d associated w ith the th e rm a l e xpansion of th e íìb e r, co u p le d w ith a change in the re ữ a c tiv e in d e x a ris in g fro m the th e rm o -o p tic e ffe ct T he sừ ain B gg s e n s itiv ity of w a v e le n g th the arises fro m the change in p e rio d o f the fib e r, change in d e x in c o u p le d the w ith a re fra c tiv e a ris in g fro m 80 100 T e m p e r a tu r e (°C) the s tra in -o p tic e ffe cts So, the B g g w a v e le n g th e x h ib ite d by FBG2 s h ifts and F B G are d e te rm in e d b y the stra in im p o sed by Fig T he w avelength shift for a b a r F B G sen so r (F B G 1) and those w hich are em bedded in C u (F B G 2) and A! b ase (F B G 3) T he experim ental data sets w ere íitted b y first o rd e r equations the th e rm a l c o n tra c tio n o f the sam ple to vvhich each is attached, and b y the re íra c tiv e in d e x change o f the core, d ete rm ine d b y the th e rm o -o p tic c o e ffic ie n t, w h ile FB G w a ve le n g th s h iíì is g o ve m e d b y therm al stra in o f the fib e r and re íra c tiv e in d e x change o f the core o n ly Som e authors m e n tio n e d about a 241 Tran Thi Tam et / VNU Journal o f Science, Mathematics - Physics 23 (2007) 237-242 th e rm a l dependence o f the stra in s e n s itiv ity o f an F B G , w h ic h w o u ld be a re su lt o f the tem perature dependence o f the sừ ess-op tic c o e ffic ie n t o f s ilic a glass T h e y fo u n d that the strain response changed on average by 0.21 ± 0.03 fm HE- ' C - l o v e r a range o f tem peratures betw een 0 -4 0 "C [ ] H o w e v e r, th is n e a rly n e g lig ib le tem perature dependence has n o t been observed in o u r e xpe rim en ts T h e r e s p o n s e s o f th c F B G to stra in m e a s u r e d at a range o f tem pcratures b etw cen ° c and 18 ° c can be co n sid e re d as tem pe ture independent w ith in the accuracy lim its o f the e x p e rim e n t and equals 5.247 n m /m m o r 1.1 p m /^e It w as observed tha t the tem perature dependence o f the th e rm o -o p tic c o e ffĩc ie n t is lin e a r o v e r the tem pe ture range ° c to 220°c at a w a ve len g th o f 633 n m in u n b u ffe re d sin g le -m o d e fib e r [7 ] H o w e v e r, the m a g n itu d e o f the average th e rm o -o p tic c o e ffic ie n t is k n o w n to decrease at lo n g e r w a v e le n g th s , th e re fo re it is p o ssib le tha t the tem perature dependence o f the th e rm o -o p tic c o e ffíc ie n t w il l also be d iffe re n t at lo n g e r |o FBG2 • Cu_base_Predict ì FBG3 • AI_base_Predict w a ve len g ths m e a n in g fu l *r • r to Thus it is fin d out any n o n lin e a r e ffe c t fo r F B G sensors at ?c ° 1550 nm tem perature suggestcd ỊC25 • !c ư> ỊỊ2.0 í ■ *—> D) c 4?15 •