A novel fiber Bragg grating high-temperature sensor

ARTICLE IN PRESS Optik Optik 119 (2008) 535–539 Optics A novel fiber Bragg grating high-temperature sensor$ Yage Zhana,Ã, Shaolin Xuea, Qinyu Yanga, Shiqing Xiangb, Hong Heb, Rude Zhub a b Department of Applied Physics, College of Science, Donghua University, Shanghai 201620, China Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Received 25 May 2006; received in revised form 21 February 2007; accepted 25 February 200
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  Optics Optik Optik Optik 119 (2008) 535–539 A novel fiber Bragg grating high-temperature sensor $ Yage Zhan a, à , 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 201620, China b Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, 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 experimentallydemonstrated. The interrogation system of the sensor system is simple, low cost but effective. The sensor head iscomprised 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 thetemperature to be measured changes. The temperature is measured basis of the wavelength shifts of the FBG inducedby strain. A dynamic range of 0–800 1 C and a resolution of 1 1 C have been obtained by the sensor system. Theexperiment 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 andindispensable in some fields, such as in some structurehealth monitoring and material processing, electricaltransformer, petroleum pipeline and so on[1,2]. Tradi-tional electrical high-temperature sensors have somedisadvantages, including low reliability, large tempera-ture fluctuation and latent danger of fire accident.Optical fiber Bragg grating (FBG) sensors have numer-ous advantages over traditional electrical sensors, suchas immunity to electromagnetic interference, higherstability and sensitivity, more easiness of multiplex,being competent for application in harsh environments,‘‘smart structures’’ and on-site measurements[3,4]. FBGsensors are the most appropriate sensors for monitoringapplications in the fields mentioned above. But commonFBG sensors cannot used directly as high-temperaturessensor because they will be decayed when its tempera-ture higher than 200 1 C and will be destroyed when itstemperature higher than 350 1 C[5,6]. Until now, only avery few kind of technologies on FBG high-temperaturemeasurement have been researched[7,8]. Brambilla et al.have researched the high-temperature measurementcharacteristics of FBGs that with special dopants(such as Sn and/or Na 2 O). They discovered that theseFBGs exhibit unusual oscillations in reflectivity[9].These methods are not suited for high-temperaturemeasurement.This paper proposes a novel kind of FBG high-temperature sensor. The novel sensor is very suited forhigh-temperature object, especially for high-temperatureobject in usual temperature atmosphere. The experi-mental results and the characteristic of the sensor systemare also described. The sensor is based on a novel FBG ARTICLE IN PRESS$-see front matter r 2007 Elsevier GmbH. All rights reserved.doi:10.1016/j.ijleo.2007.02.010 $ Project supported by Science and Technology Committee of Shanghai (Grant No. 011661081). à Corresponding author. E-mail address: (Y. Zhan).  sensor head and a fiber long period grating (LPG) as alinear edge filter for interrogation. The novel sensorhead has been designed, prepared and used in high-temperature measurement experiments successfully. Adynamic range of 0–800 1 C and a resolution of 1 1 C havebeen experimentally achieved. Experimental resultsagree with theoretical analyses. 2. Theoretical analyses 2.1. Principle of sensor head Sensor head is very crucial in sensor system. Butcommon FBG cannot be used as high-temperaturesensor head directly. We have designed a novelhigh-temperature FBG sensor head. The sensor headis mainly comprised of a FBG and two metalrods, as shown inFig. 1. The two metal rods havedifferent length and different coefficient of ther-mal-expansion (CTE). The lengths of the two metalrods are L 1 and L 2 , respectively. The CTEs of thetwo metal rods are a 1 and a 2 , respectively. The rodsare fixed into one adiabatic plate. In order that thereis not transverse thermal radiation, the two metalrods have been protected by adiabatic cylinder1 andadiabatic cylinder2, respectively. The left ends of thetwo 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 isprotected by the adiabatic cylinder3 in order thatthe FBG is not be modulated by the environ-mental temperature and the thermal radiation of theadiabatic plate.The sensing ends (see also inFig. 1) touch the objectwhose temperature to be measured. When temperatureto be measured is changed, the two metal rods will havedifferent elongation, which will make L change (thedistance between the two adiabatic rods) and the FBGbe strained. The temperature is measured basis of wavelength shifts of the FBG.The adiabatic cylinders are effective. The transversethermal 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 endto the other end. For briefness, the length change of  L isgiven by D L 1 ¼ X nj  ¼ 1 l  1 j  D T  1 j  a 1 j  ð j  ¼ 1 ; 2 ; . . . ; n Þ , (1) D L 2 ¼ X nj  ¼ 1 l  2 j  D T  2 j  a 2 j  ð j  ¼ 1 ; 2 ; . . . ; n Þ , (2) D L ¼ D L 1 À D L 2 , (3)where D L 1 and D L 2 are the elongations of the two metalrods, respectively. D L is the length change of  L , namelythe elongation of FBG section of fiber. l  ij  , D T  ij  and a ij  ( i  ¼ 1, 2) are the length, average temperature andaverage CTE of the j  th subsection of the metal rod.The corresponding wavelength shift D l B of the FBG isexpressed by[3,10] D l B ¼ l B 1 À p e À Á  ¼ l B 1 À p e À Á D LL ¼ l B 1 À p e À Á D L 1 À D L 2 L , ð 4 Þ where p e ¼Àð 1 = Þð D n eff  = n eff  Þ¼ð n 2eff  = 2 Þ½ p 12 À n ð p 11 þ p 12 ފ is the effective photo-elastic coefficient of the glass fiberwith Possion ratio n . P 11 and P  12 are the photo-elasticcoefficients of fiber. n eff  is the effective refractive indexof the guide mode in the fiber. For a typical fused silicafiber, p e ¼ 0.22.The two metal rods of the sensor head are made froman 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 anddetermined numerically by a 1 ¼ 15 : 78250 þ 0 : 02796  T  À 2 : 4085  10 À 5 À  T  2 Á  10 À 6 , a 2 ¼ 10 : 99550 þ 0 : 00994  T  À 5 : 5421  10 À 5 À  T  2 Á  10 À 6 . ð 5 Þ 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 havebeen theoretically simulated with suppositions of both L 1 ¼ 20cm, L 2 ¼ 18cm and L 1 ¼ 18cm, L 2 ¼ 20cm inthe range of 0–500 1 C. The simulation results are showninFig. 2. Similarly, the simulation results in the range of and 0–1000 1 C are shown inFig. 3.If  L 1 ¼ 20cm and L 2 ¼ 18cm, the peak wavelength of the FBG shifts almost linearly with temperature in therange of 0–800 1 C. When the temperature ascends from0 to 800 1 C, it shifts 6.80nm. Generally, 6.8nmwavelength shift will not induce the FBG worse orbroken.The sensitivity of the sensor system is enhanced whenthe metal rod with larger CTE is longer than the metalrod with smaller CTE, which can be confirmed by that ARTICLE IN PRESS Adiabatic rod1ABFBGLAdiabatic plateAdiabatic cylinder1Metal rod2(L 2   α 2 )Metal rod1(L 1   α 1 )Adiabatic rod2Adiabatic cylinder2Adiabatic cylinder3ObjectSensing ends.. Fig. 1. Schematic diagram of sensor head structure. Y. Zhan et al. / Optik 119 (2008) 535–539536  the slope of curve (a) is larger than the slope of curve (b)inFigs. 2 and 3. So all the experiments are implementedin the conditions of  L 1 ¼ 20cm and L 2 ¼ 18cm. 2.2. Interrogation principle Wavelength interrogation technology is very impor-tant for FBG sensor system. In our high-temperaturesensor system, an LPG is used as a linear response edgefilter to convert wavelength into intensity encodedinformation for interrogation. The principle of usingan LPG to interrogate an FBG temperature sensor isbased on the temperature related optical intensitymeasurement.Fig. 4shows the schematic reflectionand transmission spectra of the FBG and the LPG usedin the experiments. The LPG is used as a linear responseedge filter because the useful spectrum region of theLPG is shown to be nearly linear over a sufficiently widerange[9]. If an interrogation system is arrangedaccording as the way shown inFig. 8, light from thebroadband source (BBS) will be modulated by the LPGand then illuminates the FBG via a 2  2 coupler. Afterbeing LPG modulated, the light has a section of available linear spectrum. The reflected light from theFBG is detected by the photo-detectors (PD) and willchange with the Bragg wavelength shift of FBG.Therefore, the filtering mechanism of the LPG yields alinear relationship between the wavelength shift of FBGand the PD detected light intensity. 3. Experiments and results Fig. 5shows the schematic diagram of the experi-mental setup. The sensor head is made in accordancewithFig. 1. A brass (H62) rod is used as the longermetal rod with a larger CTE and a carbon steel (45#)rod is used as the shorter metal rod with smaller CTE. ARTICLE IN PRESS Fig. 2. The temperature–wavelength response of the sensorFBG in the range of 0–500 1 C. Fig. 3. The temperature–wavelength response of the sensorFBG in the range of 0–1000 1 C. 1545 1550 1555 1560-24-20-16-12-8-4 Intensity (dB) Wavelength (nm)FBGLPG Fig. 4. Schematic spectra of the FBG temperature sensor andthe LPG employed as edge filter. BBS: Broadband source; IMG: Index matched gelBBSCouplerInterrogation systemIMGSensor head Fig. 5. Schematic diagram of the sensor system. Y. Zhan et al. / Optik 119 (2008) 535–539 537  Light from the BBS illuminates the FBG through acoupler. The lengths of the brass rod and the carbonsteel rod are 20 and 18cm, respectively. The CTE of thetwo metal rods are a 1 and a 2 , respectively, same asfunction (5). The Bragg wavelength of the FBG is1549.96nm after it is glued on the adiabatic rods. Thereflected light from the FBG is detected by thewavelength interrogation system through the samecoupler. The other end of the coupler immerses in indexmatching gel (IMG).In experiments, the temperature of the sensing end iscontrolled by a stove. The temperature of the stove canbe modulated by step of 0.1 1 C in the range of 0–500 1 Cwith an accuracy of 0.2 1 C. Three series of experimentshave been down. 3.1. Primary experiments To prove elementary performance of the sensorsystem, first series of experiment has been done. Anoptical spectrum analyzer (OSA) has been used forinterrogating the wavelength of the sensor FBG. Theexperimental setup is shown inFig. 6.Limited by the characteristics of present stove,experiments are implemented in the temperature rangeof 0–500 1 C. The experimental result is shown inFig. 7.The curve inFig. 7is accordant with curve (a) inFig. 2in the range of 0–500 1 C. It can be deduced thatthe sensor can measure the temperature in the range of 0–800 1 C. The FBG had 4.25nm wavelength shifts whenthe temperature changed from 0 to 500 1 C. Thetheoretical value is 4.31nm. The relative error is 1.4%.All the experimental data inFig. 7can be fitted by aslight second-order polynomial function. The functioncan be expressed as l ¼ 1549 : 9006 þ 0 : 0060  T  þ ð 5 : 2369  10 À 6 Þ Â T  2 ,(6)where l is the wavelength of FBG and T  the temperatureto be measured. There is a linear response when thetemperature to be measured is higher than 100 1 C. Thefunction can be expressed as l ¼ 1549 : 2677 þ 0 : 0097  T  . (7)The error of the slope value is 1.2330  10 À 4 and thestandard deviation of the fit is 0.0458. Profited from thegood demodulation system, a resolution of 1 1 C isobtained. 3.2. Farther experiments In order to make the whole sensor simple, low costand effective, an LPG has been used as a linear filter forinterrogation[11]. The experimental setup is shown inFig. 8.Second and third series of experiments have beendone with the LPG interrogation technology. In secondseries of experiments, the output power of the BBS wasset at three different work points for the three sub-seriesof experiments to explore the stability and repeatabilityof the sensor system. The results are shown inFig. 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-interfer-ence. The results are shown inFig. 10.FromFig. 9, it is obvious that the sensor system hasgood stability and repeatability. FromFig. 10, it iscertified that the sensor system has better anti-inter-ference ability when the temperature to be measured is ARTICLE IN PRESS BBSCouplerOSAIMGSensor head Fig. 6. Schematic diagram of the experimental setup (I). 0 100 200 300 400 500 600155015511552155315541555 Wavelength (nm) Temperature ( ° C)Data of the experimentFitted curve of the data Fig. 7. The results of the experiment. BBSIMGSensor headPDDAC & Signal processingLPGCoupler Fig. 8. Schematic diagram of the experimental setup (II). Y. Zhan et al. / Optik 119 (2008) 535–539538
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