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, Faculty of Egieerig, Basra Uiversity, Basra, Iraq E-mail: husham_kadhum@yahoo.com Abstract This paper presets a umerical aalysis for the ect of temperature variatios o the strai respose of polymer optical fiber (POF) Bragg gratigs. Results show that the depedece of the Bragg wavelegth (λ B ) upo strai ad temperature variatios for the POF Bragg gratigs is lies withi the rage of.46.47 fm με -1 C -1 compare with.14.15 fm με -1 C -1 for the SOFs Bragg gratigs. Also, results show that the strai respose for the POF Bragg gratigs chaged o average by 1.34 ±.fm με - 1 C -1 ad o average by.36 ±.3fm με -1 C -1 for the silica optical fiber (SOF) Bragg gratigs. The obtaied results are very importat for strai sesor applicatios especially i the eviromets where the temperature chage. Idex Terms Polymer optical fiber (POF); fiber Bragg gratig (FBG); Strai sesitivity; temperature variatios. 1. INTRODUCTION Over the recet years, fiber optics techology has see rapid icrease i the field of optical sesors as well as be used widely i the scietific research ad i various egieerig applicatios [1, ]. Optical fiber sesors, especially fiber Bragg gratigs (FBGs), show a uique features likes immuity to electromagetic (EM) iterferece, low power fluctuatios, small size, highly precisio, its ability for multi-sesig alog sigle fiber ad so o [3]. Thus, FBGs have become the most importat ad widespread ad are beig used icreasigly by egieers, as a result of their ability to perform measuremets uder very tough eviromet coditios, such as highly mechaical vibratios, temperature ad pressure variatios, where other covetioal sesors caot be operated [4]. Due to their uique features, fiber optic strai sesors are of great importace for may applicatios. Where, it combies the ability for high sesitivity ad high dyamic rage. They are of particular importat especially for idustrial applicatios with harsh eviromets such as highly temperature ad pressure [5]. Add to that, they are ot affected by EM fields ad their potetials for moitorig at various locatios i the istallatios [6]. Fused silica, the material used i the maufacture most of the silica optical fibers (SOFs), characterized by a ideal mechaical properties thus makig it suitable for may strai sesig applicatios. However, fused silica have a upper strai limit of ~3-5% ad i geeral their reliability are ot exceed 1% strai ad uder a special procedures. Therefore, covetioal silica fibers based strai gauges caot be used i several applicatios [7]. The uique merits of polymer optical fibers (POFs) such as a egative ad large thermo-optic ect, thereby, high refractive idex tuig by heatig ca be obtaied ad good flexibility makes them much more suitable i high strai applicatios tha their SOFs, especially that FBGs have bee writte ito POFs [3]. May researchers have bee reported o the strai sesitivity of a FBG sesor. However, i all these reported, the strai sesitivity of the Bragg wavelegth is geerally assumed to be idepedet of temperature [8, 9]. However, it is kow that the material parameters that determie the strai sesitivity, the ective strai optic coiciets ad 53
Iraq J. Electrical ad Electroic Egieerig Vol.13 No.1, 17 Poisso s ratio, are themselves temperature depedet [1-1]. Therefore, the results have bee obtaied do ot reflect the true reality of the performace of these sesors. I this paper, we preset for the first time to our kowledge, the ect of temperature variatios o the strai sesitivity of the POFs ad compare the results with that for SOFs. II. TEMPERATURE DEPENDANCE OF FBGs STRAIN RESPONSE Usig FBGs as a fiber sesor is based o the idea that their properties vary depedig to evirometal chages. The amplitude of the fiber idex modulatio, the gratigs period, the optic thermal-strai ects, the desig wavelegth are shifts leads to chage the Bragg coditio [3]. This results i, the reflected ad the trasmitted lights are chaged. By measurig the chage i the spectrum, the value of temperature, strai, ad other parameters are obtaied [13]. The chage i Bragg wavelegth (Δλ) is described by the formula [3, 14] Lg Lg L g T T T (1) I Eq. (1),, Λ, Lg ad T are represets the fiber refractive idex, gratig period, gratig legth ad temperature variatios, respectively. By eglectig the variatio i refractive idex due to a chage i icidet wavelegth ( ) [14], Eq. (1) ca be reduced to Lg Lg L g T Where Lg ad T T T () are the chage i gratig legth due to applied strai ad the chage i ambiet temperature, respectively. The differetial shift i ceter wavelegth of a FBG due to the chage i the applied strai ca be writte as [14] B Lg Lg L g (3) By assumig (3), we get L g L g B Lg Lg Lg z ad substitutig it i Eq. (4) Usig the facts that ad 1 3 z Lg Lg, Eq. (4) rewrite as 3 1 B. L g z Lg Lg (5) The strai-optic ect i a optical fiber results i a chage i the ective refractive idex is give by [14] 1 i I Eq. (6), 3 j1 p. S p ij ij j is the strai-optic tesor ad the strai vector. The strai vector S j (6) S j is for a logitudial strai alog the fiber gratig axis (zaxis) is give by [14] v z S j v z z (7) Where v is Poisso s ratio, ad represets strai i the z-directio. For a typical germaium-silicate optical fiber, p has oly two umerical values, 11 ij مجلد 13 العدد 17 1 p ad p 1 [14]. Thus, Eq. (6) modified to z 54
1 p vp p. (8) i 1 11 Sice L g L g 1 z, the Eq. (5) rewrite as B z 1 11 1 1 p vp p (9) Therefore, the differetial shift i the Bragg wavelegth due to the applied strai give by B B 1 p. Where i Eq. (1) z p (1) is the idex-weighted ective strai-optic coiciet give by [13] p p vp p (11) 1 11 1 The depedece of the Bragg wavelegth ( ) upo strai ( ) ad temperature variatios after takig ito accout the temperature depedece of the ective strai optic coiciets ( ) ad z Poisso s ratio ( v ) is give by [1] ( T ) z B z z ) T, 1 ( T Where, P T) Iraq J. Electrical ad Electroic Egieerig Vol.13 No.1, 17 T p ij (1) p ( T) p ( T) p ( T) v( ) (13) ( 1 11 1 T Equatio (1) represets the modified Bragg wavelegth resultig from logitudial-applied strai ad temperature chages. By usig Eq. (1), the temperature depedece strai sesitivity give be B Accordig to the Eq. (14), the temperature depedece of the FBG strai sesitivity arises from the temperature sesitivity of the strai optic coiciets ad of Poisso s ratio. III. RESULTS ad DISCUSSION I our simulatio, we assumed a sigle-mode fiber with uiform Bragg gratigs. Table I show the typical values of, POFs have bee used i the aalysis. P 11 P 1ad v for SOFs ad PMMA Table I Typical values for the strai-optic tesor ad Poisso s ratio [8, 9, 14] SOF مجلد 13 العدد 17 1 PMMA POF P 11 P 1 v P 11 P 1 v.113.5.16.3.97.35 Figure 1 ad show the ect of temperature variatios o the Bragg wavelegth for SOF ad POF, respectively. It is clear that the respose is liear ad there is o hysteresis ect. Due to the egative temperature coiciet [3], POF has a egative slope of the temperature respose. This is i cotrast to the temperature respose of SOF which has a positive slope. It ca be see also that almost 1 m tuig rage ca be achieved whe the POF is temperature heated up from 5 ο C (it is assumed the referece temperature) to 75 ο C, which is larger tha the few aometers achieved i SOF. Furthermore, i order to icrease the tuig rage for SOF, this requires icreasig the gratig temperature by several hudreds of times tha that for POFs. T z ( T ) B 1 P( T ) (14) 55
Bragg wavelegth (m) Bragg wavelegth (m) Iraq J. Electrical ad Electroic Egieerig Vol.13 No.1, 17 1551 155.8 155.6 155.4 155. Temperature ( ο C) 155 5 4 55 7 85 1 155 1548 1546 1544 154 154 5 4 55 7 85 1 Figure 1 Temperature ect o Bragg wavelegth of Silica Optical Fiber (SOF) ad Polymer Optical Fiber (POF) Figure ad show the ect of strai o the Bragg wavelegth of SOFs ad POFs, respectively. As show, the Bragg wavelegth shift i POF over a strai rage of 1 milli-strai is more tha 15 m. I additio, the strai sesitivity of the POFs is foud to be 1.48 pm/µε. This is early 1. times larger tha the value for the SOFs, which is 1. pm/µε at the desiged wavelegth. This is because the strai sesitivity of POF is more tha of SOF [4, 13]. For example, the Youg s modulus of POF is more tha 3 times smaller ad its break-dow strai is also much larger tha for SOF. Thus, the tuability of POF is higher tha that of SOF [3]. Wavelegth (m) Wavelegth (m) 157 1566 156 1558 1554 155 4 6 8 1 157 1566 156 1558 1554 مجلد 13 العدد 17 1 155 4 6 8 1 Strai (milli-strai) Figure Effect of strai o Bragg wavelegth of Silica Optical Fiber (SOF) ad Polymer Optical Fiber (POF) Figure 3 ad show the depedece of the Bragg wavelegth (λb) upo strai ad temperature variatios for the SOF ad the POF, respectively. Results show that there is a chage i the respose of the FBG at differet temperatures, though the 56
magitude of this chage is such that its ect would oly be sigificat over large temperature or strai rages. Also, results idicate that the depedece of the Bragg wavelegth (λb) upo strai ad temperature variatios for the POF Bragg gratigs is lies withi the rage of.14.15 fm με -1 C -1 ad i the rage of.46.47 fm με -1 C -1 for the SOF ad the POF, respectively. Iraq J. Electrical ad Electroic Egieerig Vol.13 No.1, 17 15 1 9 6 3 15 1 4 6 8 1 9 6 3 15 1 4 6 8 1 9 6 3 5 o C 5 o C 75 o C 4 6 8 1 Applied Strai (µɛ) 15 1 9 6 3 15 1 5 o C 4 6 8 1 9 6 3 15 1 5 o C 4 6 8 1 9 6 3 75 o C 4 6 8 1 Applied Strai (µɛ) Figure 3 Temperature Effect o Strai Respose of Silica Optical Fiber ad Polymer Optical Fiber Figure 4 ad show the temperature depedece of the strai respose for POF Bragg gratigs ad SOF Bragg gratigs, respectively. As we have metioed, the temperature depedece of the strai respose is arises from the temperaturestrai optic coiciet ad the Poisso s ratio depedece [13, 14]. Results show good liearity over the rage of temperature used. Also, Results show that the strai respose for the POF Bragg gratigs chaged o average by 1.34 ±.fm με - 1 C -1 ad o average by.36 ±.3fm με -1 C -1 for the SOF Bragg gratigs. Strai respose (pm / μstrai) Strai respose (pm / μstrai) 1.5 1.4 1.34 1.6 1.18 1.1 5 4 55 7 85 1.8.776.75.78.74 مجلد 13 العدد 17 1.68 5 4 55 7 85 1 Temperature ( ο C) Figure 4 Effect of temperature variatios o strai respose for silica optical fiber (SOF) ad polymer optical fiber (POF) 57
Iraq J. Electrical ad Electroic Egieerig Vol.13 No.1, 17 V. CONCLUSION The ect of temperature variatios o the strai respose of polymer optical fiber (POF) Bragg gratigs has bee ivestigated for the first time successfully. It was foud that the depedece of the Bragg wavelegth (λb) upo strai ad temperature variatios for the POF Bragg gratigs is lies withi the rage of.46.47 fm με -1 C -1 compare with.14.15 fm με -1 C -1 for the SOFs Bragg gratigs. Also, results show that the strai respose for the POF Bragg gratigs chaged o average by 1.34 ±.fm με -1 C -1 ad o average by.36 ±.3fm με -1 C -1 for the SOF Bragg gratigs. The obtaied results provide a good idea for the temperature strai sesor applicatios. REFERENCES [1]Hisham K. Hisham, Desig Methodology for Reducig RIN Level i DFB Lasers, Iraq J. Electrical ad Electroic Egieerig, Vol. 1, pp. 7-13, 16. []Hisham K. Hisham, Tur_O Time Reductio i VCSELs by optimizig laser parameters, Iraq J. Electrical ad Electroic Egieerig, Vol. 1, 16. [3]Hisham K. Hisham, Numerical Aalysis of Thermal Depedece of the Spectral Respose of Polymer Optical Fiber Bragg Gratigs, Iraq J. Electrical ad Electroic Egieerig, Vol.1, pp.85-95, 16. [4] T. Habisreuther, T. Elsma, A. Graf, ad M. A. Schmidt, High-Temperature Strai Sesig Usig Sapphire Fibers With Iscribed First- Order Bragg Gratigs, IEEE Photo. Joural, Vol. 8, 16. [5] Accessed Feb. 4, 16. [Olie]. Available: http://www.fbgs.com/applicatios/strai-sesig. [6] P. Moyo, J. M. W. Browjoh, R. Suresh ad S. C. Tji, Developmet of fiber Bragg gratig sesors for moitorig civil ifrastructure, Eg. Struct., vol. 7, pp. 188 1834, 5. [7] M. Silva-Lopez, A. Feder, W.N. MacPherso, J.S. Barto, J.D.C. Joes, D. Zhao, D.J. Webb ad L. Zhag, I. Beio, Strai ad temperature sesitivity of a sigle-mode polymer مجلد 13 العدد 17 1 optical fibre, 17th Ite. Cof. o Optical Fibre Sesors, Proceedigs of SPIE Vol. 5855, (SPIE, Belligham, WA, 5). [8] Yu, J. M., Tao, X. M., ad Tam, H. Y. Tras-4- stilbeemethaoldoped photosesitive polymer fibers ad gratigs. Opt. Lett. 4; 9: 156 158. [9] Keiser, G. (). Optical Fiber Commuicatios. 3 rd ed. McGraw-Hill, Bosto, USA. [1] Scholze, H. (1991). Glass; Nature, Structure, ad Properties, Spriger-Verlag, New York, USA. [11]Sirkis, J. S. Uified approach to phase-straitemperature models for smart structure iterferometric optical fiber sesors: part 1, developmet. Opt. Eg. 1993; 3: 75-761. [1]Kersey, A. D., Davis, M. A., Patrick, H. J., Blac, M. L., Koo, K. P., Askis, C. G., Putam, M. A., ad Friebele, E. J. Fiber gratig sesors. J Lightwave Techol. 1997; 15: 144 146. [13]I. Haggmark, Fiber Bragg Gratigs i Temperature ad Strai Sesors, B.Sc thesis, Royal Istitute of Techology, 14. [14]A. Othoos, K. Kalli, Fiber Bragg Gratig- Fudametals ad Applicatios i Telecommuicatios ad Sesig, Artech House, Bosto, 1999. 58