ARTICLE IN PRESS. Optik 119 (2008) A novel fiber Bragg grating high-temperature sensor $

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1 Optik 119 (2008) Optik Optics A novel fiber Bragg grating high-temperature sensor $ Yage Zhan a,, Shaolin Xue a, Qinyu Yang a, Shiqing Xiang b, Hong He b, Rude Zhu b a Department of Applied Physics, College of Science, Donghua University, Shanghai , China b Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai , China Received 25 May 2006; received in revised form 21 February 2007; accepted 25 February 2007 Abstract A novel fiber Bragg grating (FBG) sensor for the measurement of high temperature is proposed and experimentally demonstrated. The interrogation system of the sensor system is simple, low cost but effective. The sensor head is comprised of one FBG and two metal rods. The lengths of the rods are different from each other. The coefficients of thermal expansion of the rods are also different from each other. The FBG will be strained by the sensor head when the temperature to be measured changes. The temperature is measured basis of the wavelength shifts of the FBG induced by strain. A dynamic range of C and a resolution of 1 1C have been obtained by the sensor system. The experiment results agree with theoretical analyses. r 2007 Elsevier GmbH. All rights reserved. Keywords: Sensor; Fiber Bragg grating; High temperature 1. Introduction Reliable high-temperature sensors are important and indispensable in some fields, such as in some structure health monitoring and material processing, electrical transformer, petroleum pipeline and so on [1,2]. Traditional electrical high-temperature sensors have some disadvantages, including low reliability, large temperature fluctuation and latent danger of fire accident. Optical fiber Bragg grating (FBG) sensors have numerous advantages over traditional electrical sensors, such as immunity to electromagnetic interference, higher stability and sensitivity, more easiness of multiplex, being competent for application in harsh environments, smart structures and on-site measurements [3,4]. FBG $ Project supported by Science and Technology Committee of Shanghai (Grant No ). Corresponding author. address: ygzhan@yahoo.com.cn (Y. Zhan). sensors are the most appropriate sensors for monitoring applications in the fields mentioned above. But common FBG sensors cannot used directly as high-temperatures sensor because they will be decayed when its temperature higher than 200 1C and will be destroyed when its temperature higher than 350 1C [5,6]. Until now, only a very few kind of technologies on FBG high-temperature measurement have been researched [7,8]. Brambilla et al. have researched the high-temperature measurement characteristics of FBGs that with special dopants (such as Sn and/or Na 2 O). They discovered that these FBGs exhibit unusual oscillations in reflectivity [9]. These methods are not suited for high-temperature measurement. This paper proposes a novel kind of FBG hightemperature sensor. The novel sensor is very suited for high-temperature object, especially for high-temperature object in usual temperature atmosphere. The experimental results and the characteristic of the sensor system are also described. The sensor is based on a novel FBG /$ - see front matter r 2007 Elsevier GmbH. All rights reserved. doi: /j.ijleo

2 536 ARTICLE IN PRESS Y. Zhan et al. / Optik 119 (2008) sensor head and a fiber long period grating (LPG) as a linear edge filter for interrogation. The novel sensor head has been designed, prepared and used in hightemperature measurement experiments successfully. A dynamic range of C and a resolution of 1 1Chave been experimentally achieved. Experimental results agree with theoretical analyses. 2. Theoretical analyses 2.1. Principle of sensor head is very crucial in sensor system. But common FBG cannot be used as high-temperature sensor head directly. We have designed a novel high-temperature FBG sensor head. The sensor head is mainly comprised of a FBG and two metal rods, as shown in Fig. 1. The two metal rods have different length and different coefficient of thermal-expansion (CTE). The lengths of the two metal rods are L 1 and L 2, respectively. The CTEs of the two metal rods are a 1 and a 2, respectively. The rods are fixed into one adiabatic plate. In order that there is not transverse thermal radiation, the two metal rods have been protected by adiabatic cylinder1 and adiabatic cylinder2, respectively. The left ends of the two metal rods connect two adiabatic rods, respectively. The FBG is pre-strained and glued to the end surface of the adiabatic rods on points A and B. The FBG is protected by the adiabatic cylinder3 in order that the FBG is not be modulated by the environmental temperature and the thermal radiation of the adiabatic plate. The sensing ends (see also in Fig. 1) touch the object whose temperature to be measured. When temperature to be measured is changed, the two metal rods will have different elongation, which will make L change (the distance between the two adiabatic rods) and the FBG be strained. The temperature is measured basis of wavelength shifts of the FBG. The adiabatic cylinders are effective. The transverse thermal radiation of the metal rods is negligible. When the rods are in heat balance, the temperature of each metal rod reduces linearly from whose sensing end Adiabatic cylinder3 FBG Metal rod2(l A B 2 α 2 ).. Adiabatic plate Adiabatic rod1 L Adiabatic rod2 Adiabatic cylinder1 Adiabatic cylinder2 Metal rod1(l 1 α 1 ) Object Sensing ends Fig. 1. Schematic diagram of sensor head structure. to the other end. For briefness, the length change of L is given by DL 1 ¼ Xn l 1j DT 1j a 1j ðj ¼ 1; 2;...; nþ, (1) j¼1 DL 2 ¼ Xn j¼1 l 2j DT 2j a 2j ðj ¼ 1; 2;...; nþ, (2) DL ¼ DL 1 DL 2, (3) where DL 1 and DL 2 are the elongations of the two metal rods, respectively. DL is the length change of L, namely the elongation of FBG section of fiber. l ij, DT ij and a ij (i ¼ 1, 2) are the length, average temperature and average CTE of the jth subsection of the metal rod. The corresponding wavelength shift Dl B of the FBG is expressed by [3,10] Dl B ¼ l B 1 p e ¼ lb 1 p e DL L ¼ l DL 1 DL 2 B 1 p e, ð4þ L where p e ¼ ð1=þðdn eff =n eff Þ¼ðn 2 eff =2Þ½p 12 nðp 11 þ p 12 ÞŠ is the effective photo-elastic coefficient of the glass fiber with Possion ratio n. P 11 and P 12 are the photo-elastic coefficients of fiber. n eff is the effective refractive index of the guide mode in the fiber. For a typical fused silica fiber, p e ¼ The two metal rods of the sensor head are made from an H62 brass rod and a 45# carbon steel rod, respectively. The CTEs of the two metal rods are a 1 and a 2, respectively. a 1 and a 2 have been measured and determined numerically by a 1 ¼ 15:78250 þ 0:02796 T 2: T , a 2 ¼ 10:99550 þ 0:00994 T 5: T ð5þ In the same temperature range, a 1 is larger than a 2. The lengths of the two rods are L 1 and L 2, respectively. The curve of the wavelength change of the FBG have been theoretically simulated with suppositions of both L 1 ¼ 20 cm, L 2 ¼ 18 cm and L 1 ¼ 18 cm, L 2 ¼ 20 cm in the range of C. The simulation results are shown in Fig. 2. Similarly, the simulation results in the range of and C are shown in Fig. 3. If L 1 ¼ 20 cm and L 2 ¼ 18 cm, the peak wavelength of the FBG shifts almost linearly with temperature in the range of C. When the temperature ascends from 0 to 800 1C, it shifts 6.80 nm. Generally, 6.8 nm wavelength shift will not induce the FBG worse or broken. The sensitivity of the sensor system is enhanced when the metal rod with larger CTE is longer than the metal rod with smaller CTE, which can be confirmed by that

3 Y. Zhan et al. / Optik 119 (2008) FBG LPG Intensity (db) Wavelength (nm) Fig. 2. The temperature wavelength response of the sensor FBG in the range of C. Fig. 4. Schematic spectra of the FBG temperature sensor and the LPG employed as edge filter. Interrogation system : Broadband source; : Index matched gel Fig. 5. Schematic diagram of the sensor system. Fig. 3. The temperature wavelength response of the sensor FBG in the range of C. the slope of curve (a) is larger than the slope of curve (b) in Figs. 2 and 3. So all the experiments are implemented in the conditions of L 1 ¼ 20 cm and L 2 ¼ 18 cm Interrogation principle Wavelength interrogation technology is very important for FBG sensor system. In our high-temperature sensor system, an LPG is used as a linear response edge filter to convert wavelength into intensity encoded information for interrogation. The principle of using an LPG to interrogate an FBG temperature sensor is based on the temperature related optical intensity measurement. Fig. 4 shows the schematic reflection and transmission spectra of the FBG and the LPG used in the experiments. The LPG is used as a linear response edge filter because the useful spectrum region of the LPG is shown to be nearly linear over a sufficiently wide range [9]. If an interrogation system is arranged according as the way shown in Fig. 8, light from the broadband source () will be modulated by the LPG and then illuminates the FBG via a 2 2 coupler. After being LPG modulated, the light has a section of available linear spectrum. The reflected light from the FBG is detected by the photo-detectors (PD) and will change with the Bragg wavelength shift of FBG. Therefore, the filtering mechanism of the LPG yields a linear relationship between the wavelength shift of FBG and the PD detected light intensity. 3. Experiments and results Fig. 5 shows the schematic diagram of the experimental setup. The sensor head is made in accordance with Fig. 1. A brass (H62) rod is used as the longer metal rod with a larger CTE and a carbon steel (45#) rod is used as the shorter metal rod with smaller CTE.

4 538 ARTICLE IN PRESS Y. Zhan et al. / Optik 119 (2008) Light from the illuminates the FBG through a coupler. The lengths of the brass rod and the carbon steel rod are 20 and 18 cm, respectively. The CTE of the two metal rods are a 1 and a 2, respectively, same as function (5). The Bragg wavelength of the FBG is nm after it is glued on the adiabatic rods. The reflected light from the FBG is detected by the wavelength interrogation system through the same coupler. The other end of the coupler immerses in index matching gel (). In experiments, the temperature of the sensing end is controlled by a stove. The temperature of the stove can be modulated by step of 0.1 1C in the range of C with an accuracy of 0.2 1C. Three series of experiments have been down Primary experiments To prove elementary performance of the sensor system, first series of experiment has been done. An optical spectrum analyzer (OSA) has been used for interrogating the wavelength of the sensor FBG. The experimental setup is shown in Fig. 6. Limited by the characteristics of present stove, experiments are implemented in the temperature range of C. The experimental result is shown in Fig. 7. Wavelength (nm) OSA Fig. 6. Schematic diagram of the experimental setup (I) Data of the experiment Fitted curve of the data The curve in Fig. 7 is accordant with curve (a) in Fig. 2 in the range of C. It can be deduced that the sensor can measure the temperature in the range of C. The FBG had 4.25 nm wavelength shifts when the temperature changed from 0 to 500 1C. The theoretical value is 4.31 nm. The relative error is 1.4%. All the experimental data in Fig. 7 can be fitted by a slight second-order polynomial function. The function can be expressed as l ¼ 1549:9006 þ 0:0060 T þð5: ÞT 2, (6) where l is the wavelength of FBG and T the temperature to be measured. There is a linear response when the temperature to be measured is higher than 100 1C. The function can be expressed as l ¼ 1549:2677 þ 0:0097 T. (7) The error of the slope value is and the standard deviation of the fit is Profited from the good demodulation system, a resolution of 1 1C is obtained Farther experiments In order to make the whole sensor simple, low cost and effective, an LPG has been used as a linear filter for interrogation [11]. The experimental setup is shown in Fig. 8. Second and third series of experiments have been done with the LPG interrogation technology. In second series of experiments, the output power of the was set at three different work points for the three sub-series of experiments to explore the stability and repeatability of the sensor system. The results are shown in Fig. 9. In third series of experiments, the usual interferences (such as heat convection in surroundings) are attached, to explore the sensor system s ability of anti-interference. The results are shown in Fig. 10. From Fig. 9, it is obvious that the sensor system has good stability and repeatability. From Fig. 10, it is certified that the sensor system has better anti-interference ability when the temperature to be measured is LPG PD Temperature ( C) Fig. 7. The results of the experiment. DAC & Signal processing Fig. 8. Schematic diagram of the experimental setup (II).

5 Y. Zhan et al. / Optik 119 (2008) Relative intensity PD output Temperature/ C higher than 100 1C. The sensor system has weaker antiinterference ability when the temperature is lower than 100 1C. So our work group is making efforts to improve the anti-interference ability of the sensor system. 4. Conclusion Power1 Power2 Power3 (all with the same ) Fig. 9. Experimental results of the stability and repeatability of the FBG high-temperature sensor. Relative intensity PD output Result with external distubance (power4) Result without external distubance (power5) Temperature/ C Fig. 10. Experimental results of the anti-interference ability of the FBG high-temperature sensor. In conclusion, a novel FBG sensor structure for measurement of high temperature is studied. The sensor is simply, low cost and easy to be implemented. A dynamic range of C and a resolution of 1 1C can be obtained by the sensor system. The experiment results well agree with the theoretical analysis. References [1] J. Leng, A. Asundi, Structural health monitoring of smart composite materials by using EFPI and FBG sensors, Sensor Actuators A 103 (2003) [2] Y. Zhao, Y. Liao, Compensation technology for a novel reflex optical fiber temperature sensor used under offshore oil well, Opt. Commun. 215 (2003) [3] A. Kersey, M. Davis, H. Patrick, M. Leblanc, K. Koo, Fiber grating sensors, J. Lightwave Technol. 15 (1997) [4] B. Lee, Review of present status of optical fiber sensors, Opt. Fiber Technol. 9 (2003) [5] Y. Shen, S. Pal, J. Mandal, T. Sun, K. Grattan, S. Wade, S. Collins, G. Baxter, B. Dussardier, G. Monnom, Investigation of the photosensitivity, temperature sustainability and fluorescence characteristics of several Erdoped photosensitive fibers, Opt. Commun. 237 (2004) [6] S. Baker, H. Rourke, V. Baker, D. Goodchild, Thermal decay of fiber Bragg gratings written in boron and germanium co-doped silica fiber, J. Lightwave Technol. 15 (1997) [7] J. Canning, K. Sommer, M. Englund, Fibre gratings for high temperature sensor applications, Meas. Sci. Technol. 12 (2001) [8] T. Morse, Y. He, F. Luo, An optical fiber sensor for the measurement of elevated temperatures, IEICE Trans. Electron. E83-C (3) (2000) [9] G. Brambilla, V. Pruneri, L. Reekie, C. Contardi, D. Milanese, M. Ferraris, Bragg gratings in ternary SiO 2 : SnO 2 :Na 2 O optical glass fibers, Opti. Lett. 25 (16) (2000) [10] Y. Rao, In-fibre Bragg grating sensors, Meas. Sci. Technol. 8 (1997) [11] Y. Zhan, H. Cai, R. Qu, S. Xiang, Z. Fang, X. Wang, Fiber Bragg grating temperature sensor for multiplexed measurement with high resolution, Opt. Eng. 43 (2004) Yage Zhan was born in 1977, in China. She received the B.S. degree in Physics from the HeNan University in 2000, and the M.S. degree in Optical Engineering form Shanghai Institute of Optics and Fine Mechanics, CAS, in 2002, respectively. She is now working for her Ph.D. Her research interests include fiber grating sensors and demodulation technologies for fiber grating sensors.