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1 Cryogenis 49 (2009) Contents lists ville t SieneDiret Cryogenis journl homepge: Design prmeter evlution of metl reote Fier Brgg Grting sensors for mesurement of ryogeni temperture or stress in superonuting evies R. Rjinikumr,,, *, M. Süßer, K.G. Nrynkhekr, G. Krieg, M.D. Atrey Institute of Tehnil Physis, Forshungszentrum Krlsruhe, P.O. Box 3640, D Krlsruhe, Germny Deprtment of Mehnil Engineering, Inin Institute of Tehnology Bomy, Mumi, Ini Sensor System Tehnology, University of Applie Sienes, P.O. Box 2440, D Krlsruhe, Germny Veermt Jiji Tehnologil Institute, Mumi, Ini rtile info strt Artile history: Reeive 27 My 2008 Reeive in revise form 7 Jnury 2009 Aepte 13 Jnury 2009 Keywors: D. Temperture sensors C. Therml stress A. Metl otings D. Sensitivity A. Superonuting evie There re plenty of omplex physil phenomen whih remin to e stuie n verifie experimentlly for uiling n optimize superonuting mgnet. The min prolem for experimentl vlitions is ue to the unvilility of suitle sensors. This pper proposes Fier Brgg Grtings (FBG) sensor for this purpose whih llows ess to the lol temperture/stress stte. To mesure the low temperture (20 K), FBG n e reote with mterils hving high therml expnsion oeffiient (HTCE). This n inue therml stress for temperture hnge, whih in turn inreses the sensitivity of the sensor. The performne of suh sensors hs een experimentlly stuie n reporte in erlier pper [Rjinikumr R, Suesser M, Nrynkhekr KG, Krieg G, Atrey MD. Performne evlution of metlli ote Fier Brgg Grting sensors for sensing ryogeni temperture. Cryogenis 2008;48:142 7]. This pper ims t evlution n etermintion of ifferent esign prmeters like oting mterils, oting thikness, grting perio n the grting length for esign of etter performne FBG sensor for low temperture/stress mesurements. Ó 2009 Elsevier Lt. All rights reserve. 1. Introution * Corresponing uthor. Aress: Institute of Tehnil Physis, Forshungszentrum Krlsruhe, P.O. Box 3640, D Krlsruhe, Germny. Tel.: ; fx: E-mil ress: r-k.rmlingm@itp.fzk.e (R. Rjinikumr). The ility to provie rel time informtion for super onuting (SC) mgnets is n importnt ignosti proess for effetive esign, onstrution n protetion of the SC mgnet. There re mny prmeters like urrent ensity, mgneti flux, ritil temperture, stress, efine n optimise SC mgnet esign. Knowing the temperture n stress istriution insie the SC mgnets t speifi lotion oul help the mgnet esigner to ientify the ext lotion of the hotspot generte n the iretion of the propgtion of the hotspot. The stility mrgin, the hot spot temperture, the mximum pressure in Cle in Conuit Conutors (CICC), the helium explosion from the CICC les n the hoop n therml stress istriutions in the onutor ross-setion re few prolems worth to e investigte in more etil. Bse on this informtion, the mgnet esigner n improve the esign y moifying the mteril, routing the ooling hnnel in the require ple n so on. A onsierle theoretil effort ws eite to the unerstning of the omplex physil properties ssoite with SC mgnets ut the experimentl vlition is missing. The min prolem for experimentl vlitions is ue to the unvilility of the suitle sensors. Eletril strin guge (ESG), ioes n mny other onventionl eletro-mehnil sensor systems re onsiere. Unfortuntely, these sensors re suseptile to the eletromgneti interferene (EMI). In ition, the numer of sensors require for long SC les will e more to get the signl istriution. Also, the risk involve in introuing eletril wires insie the SC mgnets mkes them unsuitle nite for the neee mesurement system. Use of FBG sensors is very ppeling for sensing the low temperture n stress in super onuting mgnets euse of their miniture size n the possiility of hving mny sensors in single fier. Even though the FBG sensors re more suitle nite to stuy the ove mentione phenomen, the min rwk of these sensors re their low intrinsi therml sensitivity t low tempertures. This is ue to very low therml expnsion oeffiient of glss fier roun 120 K. In ft, the intrinsi temperture sensitivity of FBG sensors is proportionl to the therml expnsion oeffiient of the optil fier onstitutive mteril sili, whih reues s the temperture ereses. Approhing ryogeni tempertures, temperture hnges lower thn few egrees Kelvin nnot e resolve, for they o not use n ppreile shift of the wvelength iffrte y re FBG sensor (grting elements written on ore of the fier n left reote) /$ - see front mtter Ó 2009 Elsevier Lt. All rights reserve. oi: /j.ryogenis

2 R. Rjinikumr et l. / Cryogenis 49 (2009) In orer to enhne the sensitivity of FBG sensors t low temperture, it hs een propose [2 5] to eme/reot the re FBG sensor with ifferent HTCE polymer of Teflon, PMMA, rylte n Ormoer mterils. But the polymer reote FBG sensors re not sensitive enough elow 77 K n in ition the sensors exhiit non-repetility hrteristi [6]. To improve the sensor sensitivity n its performne hrteristis t low temperture, metls re onsiere for reoting the FBG sensors in present work. This pper reports the evlution n etermintion of the esign prmeters of metl reote FBG sensors for etter performne t low temperture. 2. Theoretil kgroun The Brgg refletion wvelength k B of n FBG is given s [8] k B ¼ 2Kn eff ð1þ where the Brgg grting wvelength, k B, is the free spe enter wvelength of the input light tht will e k reflete from the Brgg grting, n eff is the effetive refrtive inex of the fier ore t the free spe enter wvelength, n K is the grting sping. The FBG is sensitive to strin ue to liner expnsion ffeting the grting perio n hnge in refrtive inex from the photo-elsti effet. Similrly, it is lso sensitive to temperture ue to therml expnsion n thermo-opti effet of fier mteril. By monitoring the Brgg wvelength shift, Dk B given y Eq. (2), the temperture n strin n e mesure [8]: Dk B ¼ 2nKðf1 ðn 2 =2Þ½P 12 mðp 11 þ P 12 ÞŠg þ½ þðn=tþ=nšdt where P ij is Pokel oeffiient, is therml expnsion oeffiient, m is poisson rtio, e is strin, n T is temperture. 3. Sensor esign The FBG sensor n e frite on stnr single moe fier. The grtings re written on the ore of the fier fter removing the protetive sheth y mehnil stripping n removing ling y hemil wsh. The grtings re insrie on the fier y pssing the lser light through phse msk [9]. This lters the inex of refrtion in the photosensitive fier permnently. After ð2þ friting the grting elements in the fier, the FBG sensor hs to e reote with selete HTCE metl to enhne the sensor sensitivity n its performne hrteristis t low temperture. Therefore, the seletion of right mteril for reoting is foremost importnt tsk for the esign of high sensitive FBG sensor. Determintion of the reoting thikness is the next tsk of the filure free sensor esign. Lrger metl reoting thikness ffets the mesurement spee ue to therml inerti n thinner metl reoting les to rks n hene ffets to the life time of the sensor. Hene right seletion of oting thikness not only improves the sensor life time ut lso ensures etter sensor performne. Fier losses, irefringene effets n e reue y proper seletion of operting Brgg wvelength n sensor grting length. The ury of the mesurement n the wvelength ivision multiplexing (WDM) pility of the FBG sensor n e improve y hoosing the sensor grting length refully. The evlution n etermintion of the ove isusse sensor esign prmeters re elt in etil in the following setions Determintion of reoting mterils It is very ler from the ove isussions tht the mterils with HTCE n imprt high therml stress for temperture hnges, whih in turn n inrese the FBG sensor sensitivity. Metls like luminium, opper, le n inium re foun to hve HTCE t ryogeni temperture, whih is shown in Fig. 1 [7]. Hene these metls re tken into the onsiertion Determintion of reoting tehnology The reoting tehnology shoul e rightly selete for oting the fier with require metls n for require oting thiknesses. Mny tehnologies like ipping, sting, physil vpour eposition (PVD) n hemil vpour eposition (CVD) re ville for the metl oting of re fier. Seletion of n pproprite metho is influene y the prmeters suh s proess temperture n the thikness of the oting mteril. The proess temperture shoul e lower thn the melting point of the re fier s n inrese proess temperture my mge the sensing elements in FBG. The ipping proess nnot e use, s the sensor requires uniform oting throughout the sensing element. Dipping proess oes not gurntee uniform oting thikness n the formtion (1/L)(L/T)/K(x 10-5) Copper Aluminium Le Inium Glss fier Temperture (K) Fig. 1. Therml expnsion oeffiient of mterils [6] (1/L)(L/T)/K(x10-5)

3 204 R. Rjinikumr et l. / Cryogenis 49 (2009) oting. Furthermore, sting nnot e use with metl hving melting point higher thn out 700 C, s this woul use the estrution of the FBG. PVD lso proues ppreile therml stresses. Consequently, eletro-eposition is the est suite tehnique for epositing the metl onto the fier, s it n e performe t room temperture, thus voiing therml stresses, n it llows for the eposition of lrge quntity of metls. But fier is n insulting mteril n hene metls nnot e eposite iretly on the sensor using eletro-eposition metho. Therefore FBG sensor is reote y luminium primry lyer n metl seonry lyer. Aluminum n e use s the intermeite lyer. It my e pplie esily to the glss fier y using Al vporiztion tehnique. This luminum lyer will t like onutive lyer for the eletro-eposition of the esire metls Determintion of reoting thikness Fig. 2. Flow hrt for stress lultions. of ules in the oting lyer uring the proess will lso ffet the sensor opertion. Csting gives higher therml stresses ut llows for virtully limitless mount of mteril to e eposite s The thikness of the reoting etermines the mgnitue of therml stress ting on the metl reote FBG sensor. Hene the knowlege of stresses ting in the re fier n in the reoting n help in the right etermintion of reoting thiknesses. The inue stress ue to the primry oting lyer n the seonry oting lyer is lulte y nlyzing simple theoretil moel of the ul lyer metl ote FBG sensors (DMCFBG). This moel is evelope y opting the infinite irulr thin wlle hollow yliner moel with support of Lme s formul n si stress-strin reltionship [10]. Fig. 2 shows the flow igrm of the nlysis whih is self suffiient to explin the vrious steps in the moel. Figs. 3 n 4 show the ril, tngentil, n the xil stress istriute in the primry oting lyer n the seonry oting lyer of the metl reote Fier Brgg Grting sensor re lulte for vrious thiknesses. The supersripts,, n in the following figures enote the stresses t the inner ounry of the primry oting lyer, outer ounry of the primry oting lyer, inner ounry of the seonry oting lyer n outer ounry of the seonry oting lyer, respetively. From Fig. 3, it is seen tht the inrese in the primry oting thikness inreses the stress evelope in the primry oting n s the thikness inreses more thn 125 lm, the inrese in the stress rte is very less. Hene the thikness for the primry oting n e hosen s 125 lm. For seonry oting, the thikness of 625 lm n 0.4 Stress / (Ε α ΔΤ) (= ) Prmeters r 0 =62.5μm r 1 =242.5μm r 2 =1850μm ν 1 =ν 2 =0.4 Ε 1 /Ε = 0.1, Ε 2 /Ε =0.9 ( = ) (= = ) Primry oting lyer thikness (μm) Fig. 3. Effet of thikness of the primry oting lyer on the therml stresses. σ z

4 R. Rjinikumr et l. / Cryogenis 49 (2009) e hosen, fter whih the stress inue is very smll n nerly get sturte. Hene hoosing the thikness more thn 625 lm is merely going to ffet the mesurement spee n mke the sensor ulky Determintion of grting length Grting length of the FBG sensors n ffet the full with hlf mximum (FWHM) of the k reflete light signl, the temperture, the photo-elsti n the thermo-opti oeffiients. The ury of the mesurement, sensitivity of the sensors n the multiplexing pility of the sensors re the funtion of the ove mentione prmeters. Hene it is essentil to stuy the effet of grting length with respet to these prmeters to etermine right grting length of the sensor esign Effet of full with hlf mximum with respet to grting length The full with hlf mximum (FWHM) signl hrteristis of the FBG sensors with grting lengths of 2 mm, 5 mm n 10 mm re theoretilly lulte using lssil ouple moe equtions. The theoretilly lulte FWHM [FWHM(C)] for 2 mm, 5 mm n 10 mm grting lengths re foun to e 0.34 nm, 0.15 nm n 9 nm, respetively. A 2 mm, 5 mm n 10 mm grting length elements re then frite with the enter wvelength of 1535 nm n the FWHM of those sensors re mesure. The mesure FWHM [FWHM(M)] is then ompre with the theoretilly lulte FWHM [FWHM(C)], shown in Fig. 5. From the lultions it is oserve tht the FWHM of the FBG sensors ereses exponentilly with the inrese in the grting length Effet of photo-elsti oeffiient with respet to grting length The effet of the photo-elsti oeffiient with respet to vrious grting length hs lso een experimentlly teste. In the experimentl setup, two ens of n optil fier with Brgg grting re fixe on oth sies to fixer. The strin is pplie to the optil fier using pneumti ontroller. The reflete spetr re mesure t pressure equl to the strin of 5%. From the (= = ) Stress / (Ε α ΔΤ) (= ) (= ) Seonry oting lyer thikness (μm) Prmeters r 0 =62.5μm r 1 =242.5μm r 2 =1850μm ν 1 = ν 2 =0.4 Ε 1 /Ε = 0.1, Ε 2 /Ε =0.9 Fig. 4. Effet of oting thikness of the seonry oting lyer on the therml stresses FBG(M) 2mm FBG(M) 5mm FBG(M)10mm FBG(C) 2mm FBG(C) 5mm FBG(C)10mm Loss (B) Wvelength(nm) Fig. 5. Clulte n mesure reflete wvelength of the 2 mm, 5 mm n 10 mm grting length.

5 206 R. Rjinikumr et l. / Cryogenis 49 (2009) test results, s shown in Fig. 6, it is onfirme tht FBG sensors hve the sme photo-elsti onstnts regrless of the grting length of the FBG sensor Effet of thermo-opti oeffiient with respet to grting length The thermo-opti oeffiient n e lulte through the mesurement of TEC (therml expnsion oeffiient) of n optil fier, temperture hnge, n wvelength hnge. Chnges in wvelength n temperture re mesure simultneously from 30 K to 300 K with the temperture intervl of 10 K t lower temperture rnge n 40 K t higher temperture rnge. In this stuy, the CTE of n optil fier is inferre from the mteril hnook [11]. The lulte thermo-opti oeffiient is shown in Fig. 7. From room temperture own to 30 K, the thermo-opti oeffiient vries linerly with temperture. This teneny is the sme regrless of the grting lengths Effet of temperture with respet to grting length To investigte the signl hrteristis of FBG sensors influene y the temperture, the FBG sensors with ifferent grting lengths of 2 mm, 5 mm n 10 mm re use. Fig. 8 shows the signl hnges of the reflete spetr very well. During the ool own, the pek signls re split euse of the irefringene inue y the therml stress evelope in the FBG sensor. As the grting length ereses, the irefringene lso ereses. The signl of n FBG sensor with grting length of 2 mm shows no splitting while those of FBG sensors with grting lengths of 5 mm n 10 mm shows split s temperture ereses. Also, it is evient tht the pek split of 10 mm is reltively muh higher thn tht of 5 mm. Hene the length of the grting element for high urte mesurement hs to e 2 mm. If the pplition requires severl FBG sensors tht hve to e multiplexe, then grting length with 10 mm n e hosen whih hs the nrrow spetrl with 3.5 Wvelength shift in nm Grting length 2 mm = Grting length 5 mm = Grting length 10 mm = λ Β = 1535 nm Strin in % Fig. 6. Effet of vrious grting length on the photo-elsti onstnt of the FBG sensors. 9.0x x10-6 Thermo-opti oeffiient (1/K) 7.0x x x x x x x10-6 FBG 2mm FBG 5 mm FBG 10 mm Temperture (K) Fig. 7. Thermo-opti oeffiients with respet to temperture hnge.

6 R. Rjinikumr et l. / Cryogenis 49 (2009) x10-6 FBG 5mm FBG 2mm FBG10mm Reflete power (Bm) 2.5x x x x10-6 Spilting 5.0x Wvelength (nm) Fig. 8. Splitting of FBG spetr uring ooling own of vrious grting lengths. Aosulte refrtive inex, n λ = 500 nm λ = 1500 nm λ = 1000 nm λ = 1500 nm λ = 2000 nm λ = 2500 nm Temperture (K) Fig. 9. Chnge of refrtive inex of the sili glss fier with respet to the temperture hnge. response. This provies the possiility of multiplexing severl sensors in single fier Determintion of Brgg wvelength Lower the Brgg wvelength, higher is the refrtive inex. The refrtive inex n lso inrese the sensitivity of the FBG sensors. It is expresse s funtion of wvelength n temperture using Sellmeier moel [12]. From Fig. 9, it is inferre tht the refrtive inex of the sili glss fier ereses with respet to the wvelength hnge. Also, it is oserve tht the hnge in the refrtive inex with respet to the temperture is negligile, in other wors, it n e ssume to e onstnt. From Fig. 9, the esigner n e le to hoose the Brgg wvelength of 500 nm whih oul inrese the sensitivity. But se on Fig. 10 it n e seen tht the ispersion n the losses re high for this wvelength. For temperture hnge, the ispersion is more or less onstnt for fixe wvelength. At shorter wvelengths, the loss inreses ue to Ryleigh sttering. At longer wvelengths, it inreses ue to infrre photon sorption. The thermo-opti oeffiient hnges with respet to wvelength n temperture hnges re shown in Fig. 11. It n e seen tht the thermo-opti oeffiient remins more or less onstnt for fixe temperture with vrying wvelength, ut vries linerly with hnge in temperture for fixe wvelength. From the ove isussion, the Brgg wvelength n e etermine to e roun 1550 nm or 850 nm, where the ispersion n losses re low when ompre to other wvelengths. 4. Conlusion Proper etermintion of the esign prmeters is the key for esigning high sensitive n etter performne FBG sensor for mesuring ryogeni temperture or stresses in superonuting evie. The present nlysis shows ul metl ote FBG sensors with primry n seonry lyer thikness of roun 125 mm n 625 mm, respetively, give etter performne for

7 208 R. Rjinikumr et l. / Cryogenis 49 (2009) λ = 1500 nm λ = 1000 nm λ = 2000 nm -20 λ = 2500 nm Dispersion (1/nm) λ = 500 nm λ = 500 nm -560 Temperture (K) Fig. 10. Chnge of ispersion with respet to hnge in temperture t fixe wvelength. 1.0x10-5 Thermo-opti o-effiient (1/K) 8.0x x x x nm 1000 nm 1500 nm 2000 nm 2500 nm Temperture (K) Fig. 11. Thermo-opti oeffiient hnge with respet to temperture hnge. the sensor. In ition, the grting perio n the grting length of 1500 nm n 2 mm, respetively, give improve ury in mesurement. The FBG sensor for ove mentione speifition is frite n teste suessfully [1]. The FBG sensors with these esigne prmeters oul e use to mesure the thermoynmi prmeter istriution insie the superonuting evies. Applition of the fiers in mgnet evies is expete to e possile either uring the CICC ssemling proess into this struture or uring the mgnet wining proess to the onutor lyers. Referenes [1] Rjinikumr R, Suesser M, Nrynkhekr KG, Krieg G, Atrey MD. Performne evlution of metlli ote Fier Brgg Grting Sensors for sensing ryogeni temperture. Cryogenis 2008;48: [2] Mizunmi T, Ttet H, Kvshim H. High-sensitivity ryogeni fier-brgggrting temperture sensors using Teflon sustrtes. Mesure Si Tehnol 2001;12: [3] Li XC, Prinz F, Seim J. Therml ehvior of metl emee fier Brgg grting sensor. Smrt Mter Strut 2001;10. [4] Alin S, Fu W, Zheng J, Lvris AC, Alin J. A ryogeni temperture sensor using fier Brgg grting. In: Proeeings of ICEC17: ryogenis, setion 16. Bournemouth (UK); p [5] Rjinikumr R, Nyils A, Süsser M, Weiss KP, Nrynkhekr KG, Krieg G. Fier rgg grtings for sensing temperture n stress in superonuting oils. IEEE Trnstion of Applie Superonutivity 2006;16(2) [June]. [6] Rjinikumr R, Nyils A, Süsser M, Nrynkhekr KG. Investigtion of fier Brgg grting sensors with ifferent oting mterils for high sensitivity. In: Proeeings of interntionl ryogeni engineering onferene 2006 ICEC 21, Prgue. p [7] Johnson VJ. Properties of mterils t low temperture (phse 1). Pergmon Press; 1961.

8 R. Rjinikumr et l. / Cryogenis 49 (2009) [8] Othonos A, Klli K. Fier Brgg grtings funmentls n pplition in teleommunitions n sensing. Arteh House Optoeletronis Lirry; [9] Rmn Kshyp. Fier Brgg Grtings. Aemi Press; [10] Timoshenko SP, Gooier JN. Theory of elstiity. 3r e. New York (NY, USA): MGrw-Hill; [11] Rihr PR, Aln FC. Mterils t low tempertures. Amerin Soiety for Metls; 1983, ISBN [12] Wry JH, Neu JT. Refrtive inex of severl glsses s funtion of wvelength n temperture. J Opt So Am 1969;59(6):774 6.