Multimode fiber optic Bragg grating sensors for
strain and temperature measurements using correlation signal processing methods have been developed. Two multimode Bragg grating sensors were fabricated in 62/125 m graded-index silica
multimode fiber; the first sensor was produced by the holographic method and the second sensor by the phase mask technique. The sensors have signal reflectivity of approximately 35% at peak
wavelengths of 835 nm and 859 nm, respectively.
Strain testing of both sensors has been done from 0 to 1000 με and the temperature testing from 40 to 80°C. Strain and temperature sensitivity values are 0.55 pm/με and 6 pm/°C, respectively.
The sensors are being applied in a power-by-light hydraulic valve monitoring system

Jackson, PR

Jones, BE

Lim, J

Yang, QP

IEEE Transactions on Instrumentation and Measurement

English

Brunel University Research Archive

622 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 51, NO. 4, AUGUST 2002Strain and Temperature Sensors UsingMultimode Optical Fiber Bragg Gratings andCorrelation Signal ProcessingJirapong Lim, Qingping Yang, Barry E. Jones, and Phillip R. JacksonAbstract—Multimode fiber optic Bragg grating sensors forstrain and temperature measurements using correlation signalprocessing methods have been developed. Two multimode Bragggrating sensors were fabricated in 62/125 m graded-index silicamultimode fiber; the first sensor was produced by the holographicmethod and the second sensor by the phase mask technique. Thesensors have signal reflectivity of approximately 35% at peakwavelengths of 835 nm and 859 nm, respectively.Strain testing of both sensors has been done from 0 to 1000and the temperature testing from 40 to 80 C. Strain and temper-ature sensitivity values are 0.55 pm and 6 pm C, respectively.The sensors are being applied in a power-by-light hydraulic valvemonitoring system.Index Terms—Bragg grating, correlation, hydraulic valve, mul-timode, optical fiber, optically-powered, strain, temperature.I. INTRODUCTIONDURING the past decade, single-mode optical fiber Bragggratings (SM-FBGs) have been used for strain and temper-ature sensing in many applications such as structural monitoring,down-hole and hazardous environments. Recent research workhas been reported on a differential-pressure (DP) flow sensorusing SM-FBGs for hydraulic valve monitoring applications[1]. However, using SM-FBGs in the application of an opti-cally-powered hydraulic valve monitoring system is not reallyappropriate [2], [3]. This is because the system must utilizeonly one optical fiber for both optical power transmission andfor sensing. The optical fiber transmits power from a lasersource into the optically powered hydraulic valve at the remoteend of the fiber. It is also used for feeding the valve operationinformation back to the valve monitoring system via the samefiber, as shown in Fig. 1; the optical fiber link might be 4km in length. The other reason is that it is difficult to launchhigh optical power into the single-mode optical fiber due toits small core diameter. Therefore, a multimode optical fiberBragg grating (MM-FBG) is more suitable due to its largercore diameter. A particular advantage of this configuration isthat no optical fiber coupler is required at the valve end, andthis makes the system less complex and minimizes opticalpower loss.Manuscript received May 29, 2001; revised May 2, 2002. This work was sup-ported in part by a Ph.D. scholarship from The Royal Thai Government and byFlight Refuelling Ltd., Dorset, U.K.J. Lim is with the King Mongkut’s Institute of Technology, North Bangkok,Thailand.Q. Yang and B. E. Jones are with the Brunel Centre for ManufacturingMetrology, Brunel University, Uxbridge, Middlesex, U.K.P. R. Jackson is with Flight Refuelling Ltd., Wimborne, Dorset, U.K.Digital Object Identifier 10.1109/TIM.2002.802253Fig. 1. Optically powered hydraulic valve system.Fig. 2. Experimental setup.The early use of MM-FBGs was reported by Wanser et al. [4],[5]. Microbending was used to disturb the mode population inan optical fiber through mode coupling before passing the lightto the MM-FBG. A similar approach using the bending effecton a transmission spectrum of the MM-FBG for displacementsensing was introduced by Mizunami et al. [6], [7]. However,these sensing techniques require mechanical parts and are there-fore more complicated than conventional SM-FBGs. The use ofMM-FBGs for both strain and temperature sensing by using thesame method as used with a Bragg grating in a single-mode fiberis also possible [8], [11].Changes in optical signals obtained from a multimode opticalfiber can be caused by changes in both strain and temperature.Significant variation in these signals can be noticed from the am-plitude-wavelength signature by number of peaks, peak valuesand signal distortions, but it is difficult to pinpoint the amountof wavelength shift that is proportional to strain or temperaturechanges.In this paper, a method of using correlation signal processingto quantify variation of wavelength shift caused by alteringstrain or temperature is reported. Advantages of using thismethod include minimizing the effect of variation in signalamplitude and shape of the MM-FBG amplitude wavelengthspectrum, and reducing the effect of noise. Tests have been0018-9456/02$17.00 © 2002 IEEELIM et al.: STRAIN AND TEMPERATURE SENSORS 623(a)(b)Fig. 3. Signal processing scheme for (a) strain test and (b) temperature test.performed to compare the characteristics of the two MM-FBGsensors for axial strain measurement and for the temperaturemeasurement. The effect of temperature on strain sensing hasalso been investigated.II. BASIC SCHEMETwo MM-FBGs fabricated in 62.5/125 m graded-indexsilica multimode fiber have been tested using the same experi-mental setup (Fig. 2). The first MM-FBG was fabricated by theholographic technique without buffer coating, while the secondMM-FBG sensor was fabricated by the phase mask techniquewith acrylate buffer recoating. The reflectivity of the first andsecond MM-FBGs is approximately 35% at peak wavelengthof 835 and 859 nm, respectively. In the experimental setup, oneend of the MM-FBG was connected to one of the output fibersof a 50:50, 2 2 multimode optical fiber coupler. The otherend of the MM-FBG and the output end of the coupler wereplaced in index matching gel. At the input end of the coupler,one optical fiber was connected to a light source and the otheroptical fiber fed a sensing signal to an optical spectrum analyzer(Anritsu MS99A). The spectrum of the reflected signal froma Bragg grating in a multimode optical fiber is very differentfor different light sources [7]. For example, using a halogenlamp high multimode excitation occurs, and this results in moremultiple peaks and a wider spectrum of reflected signal thanoccurs for the lower order mode excitation light sources suchas laser diode, LED, and super-luminescent diode (SLD). Forgraded-index MM-FBGs, the propagation constant for the thprincipal mode can be approximated by the equation [7](1)where is the propagation constant at the phase-matchingcondition , the grating period, the Bragg wave-length, refractive index of the core, normalized frequency, the fiber core radius, the fibernumerical aperture, and the maximum relative index differ-ence between core and cladding of the fiber. To achieve a lowcomplexity of MM-FBG signal, the SLD with a single-modeoptical fiber pigtail, providing 0.5 mW of light power at wave-length 849 nm, was selected for the experiments.To investigate the strain characteristic, axial strain was ap-plied to the fiber using a fiber fixture/drawer. The axial strainwas varied from 0 to 1000 by adjusting an accurate microm-eter. The strain characteristic for the first-order mode of theMM-FBG is similar to that in a conventional SM-FBG strain624 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 51, NO. 4, AUGUST 2002(a) (b)Fig. 4. The multimode FBG spectrum of (a) the first FBG and (b) the second FBG, with zero strain at room temperature.sensor [8]. The Bragg grating resonance peak wavelength shiftmay be expressed by the Bragg equation [8]–[11](2)where is the Bragg grating wavelength shift, the appliedstrain, the effective photoelastic coefficient 0.22 , andthe Bragg grating center wavelength.The thermally induced shift in the FBG is caused by varia-tion in the period of the grating and by variations in the reflec-tive index of the core and cladding. Temperature dependence ofthe MM-FBGs can be estimated from a complex mathematicalmodel [6](3)where is the refractive index of the core, the refractiveindex of the cladding, the fiber core radius, the gratingperiod, and the FBG wavelength. The for silicacore and for silica cladding of the fiber have a value1 10 C. The thermal expansion coefficienthas a small value of 0.55 10 . A temperature test has beenperformed by placing the MM-FBG in a temperature-controlledchamber; the temperature was varied between 40 and 80 C.For the signal-processing scheme, a personal computer (PC)with data-acquisition software has been employed to control andcorrect data from the optical spectrum analyzer obtained via anIEEE-488 data bus. The signal acquisition takes about 2 s, andthe autocorrelation and cross-correlation signal processing usedto analyze the data from the MM-FBG can be done in less than1 s by using a special signal-processing feature in the software.The cross-correlation between two data sequences eachcontaining data points can be written as [12](4)where is the first data sequence and the seconddata sequence. The result of correlation is solely dependent onthe number of data points (400 points for the spectrum analyzerused).Data processing for the strain measurement is performed byprocessing the MM-FBG spectrum (scanned by the optical spec-trum analyzer), initially using zero strain, to provide a referencepattern . The raw reference data is kept in the memory of thePC and is processed by the autocorrelation method [Fig. 3(a)].The output from the auto-correlation is used for the correlationstrain reference , and this is also put into the PC memory.Then, scanning data is processed for a particular value of ap-plied strain. This strain data is processed by doing thecross-correlation with the reference data . The result of thecross-correlation is used to compare with the correlationstrain reference . The distance between maximum pointsof each result, (where isthe data index number point at the maximum value of a correla-tion result) can be converted into the MM-FBG wavelength shiftinformation. The information from the temperature-dependenttest has been investigated using the same method [Fig. 3(b)].The reference signal pattern, , for the temperature test wasset at a temperature of 30 C.III. RESULTSThe reflected spectra of both MM-FBGs with zero strainat room temperature are shown in Fig. 4(a), the holographicMM-FBG, and (b), the phase-mask MM-FBG. Six propagationmodes can be implied by noticing the number of peaks of theraw signal (six peaks). The first mode of each MM-FBG canbe identified by the highest peak value, at the wavelengths835.5 and 859.1 nm for the first and second MM-FBGs,respectively. The spectra wavelength width is approximately2 nm. The signal obtained from each MM-FBG includes noise,but provided this is not greater than the largest signal peak, thewavelength shift will be determined by the correlation process.Typical computer screen outputs for correlation signal pro-cessing are shown in Fig. 5. Strain measurements at constantLIM et al.: STRAIN AND TEMPERATURE SENSORS 625(a) (b)Fig. 5. Computer screen outputs: (a) raw reference data and raw scanning data and (b) correlation output.(a) (b)Fig. 6. Multimode FBG spectrum wavelength shift with axial strain, at room temperature of both MM-FBGs: (a) the first grating and (b) the second grating.(a) (b)Fig. 7. Multimode FBG spectrum wavelength shift with temperature, at zero strain, for (a) the first grating and (b) the second grating.ambient temperature were made as shown in Fig. 5(a). The re-lationships between MM-FBG wavelength shift and axial strainat room temperature for both MM-FBGs are shown in Fig. 6.As can be seen, there is a reasonably linear relationship. In theexperiment, the strain was altered and the reflection of the spec-trum measured. The strain coefficient was estimated to be 0.6pm . This value is comparable with that obtained for conven-tional Bragg gratings in single-mode fibers. The experimentalstrain sensitivity for the MM-FBG is 0.55 pm , which is con-sistent with the estimated value. The repeatability for each strainmeasurement point was 0.02 nm wavelength shift (determinedby the spectrum analyzer). Hysteresis was small [Fig. 6(b)].Fig. 7 illustrates the relationship between Bragg gratingspectrum wavelength shift and temperature for both of theMM-FBGs. In this experiment, the temperature was variedfrom 40 to 80 C at constant strain and signal processing and626 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 51, NO. 4, AUGUST 2002Fig. 8. Strain test at various temperatures.measurement made as shown in Fig. 3(b). The average temper-ature sensitivity (between 40 and 80 C) of the first FBG is6.8 pm C, which is compatible with the predicted value of 7pm C, but the relationship is nonlinear. The repeatability foreach temperature measurement point was found to be withina wavelength shift of 0.03 nm range. For the second FBG,the average temperature sensitivity is approximately 6 pm C,which is lower than the predicted value. This may be caused bythe buffer coating that could reinforce the fiber and affect thefiber expansion characteristic.The MM-FBG strain sensitivity to temperature has been in-vestigated by using the second MM-FBG. Axial strain was ap-plied to the fiber using a fiber fixture/drawer that was located ina temperature-controlled chamber. The axial strain was variedfrom 0 to 1000 by adjusting the micrometer and the temper-ature was varied from 20 to 80 C. The results are shown inFig. 8, with an average temperature coefficient of approximately7 pm C, agreeing with the value from the previous test at zerostrain.IV. DISCUSSION AND CONCLUSIONSThe method of signal processing using correlation has themajor advantage of providing an output for the MM-FBG sensorthat is not critically dependent on the power level of the signal,as presented in Fig. 5. A fairly linear characteristic has beenobtained for tensile strain from 0 to 1000 . The strain sensi-tivity from the experimental result is in agreement with the es-timated value obtained from the SM-FBG mathematical model.The two MM-FBGs, one fabricated by holographic and oneby phase mask methods, provided similar strain characteristics,and there was little effect caused by the fiber buffer coating.The temperature characteristic is somewhat nonlinear due to thechanging with temperature of many parameters, such as fiber di-ameter, MM-FBG grating pitch, and reflective index differencebetween core and cladding of the fiber. However, the averagetemperature sensitivity of the first MM-FBG, from the experi-mental result is very close to the predicted value. The secondMM-FBG provided slightly lower temperature sensitivity thanthe predicted value due to the effect of the fiber buffer coating.The strain test at various temperatures provided results in agree-ment with the previous results. The multimode Bragg gratingsensor systems described in this paper are being applied in apower-by-light hydraulic valve monitoring system. The correla-tion processing method employed may be utilized for MM-FBGoptical sensing of many other physical parameters such as pres-sure and flow-rate.ACKNOWLEDGMENTThe FBGs were provided by the Department of EEAP, AstonUniversity.REFERENCES[1] J. Lim, Q. P. Yang, B. E. Jones, and P. R. Jackson, “DP-flow sensorusing optical fiber Bragg gratings,” Sens. Actuators A, vol. A2960, pp.1–7, 2001.[2] J. Lim, P. R. Jackson, B. E. Jones, K. F. Hale, and Q. P. Yang, “Anintrinsically safe optically powered hydraulic valve,” in Proc. 7th Int.Conf. Actuators, Bremen, Germany, 2000, pp. 216–219.[3] J. Lim, P. R. Jackson, B. E. Jones, and Q. P. Yang, “Optically pow-ered hydraulic pilot valve using piezoelectric multilayer actuator,” Int.J. Fluid Power, vol. 2, no. 3, pp. 6–11, 2001.[4] K. H. Wanser, K. F. Voss, and A. D. Kersey, “Novel fiber devices and sen-sors based on multimode fiber Bragg gratings,” Proc. SPIE, vol. 2360,pp. 265–268, 1994.[5] K. H. Wanser, “Fiber Devices and Sensors Based on Multimode FiberBragg Gratings,” U.S. Patent 58 482, 1998.[6] T. V. Djambova and T. Mizunami, “Simultaneous sensing of the temper-ature and displacement using a multimode fiber Bragg grating,” Jpn. J.Appl. Phys., vol. 39, pp. 1566–1570, 2000.[7] T. Mizunami, T. V. Djambova, T. Niiho, and S. Gupta, “Bragg gratingsin multimode and few-mode optical fibers,” J. Lightwave Technol., vol.18, no. 2, pp. 230–235, 2000.[8] W. Zhao and R. O. Claus, “Optical fiber grating sensors in multimodefibers,” J. Smart Mater. Structure, vol. 9, pp. 212–214, 2000.[9] A. Othonos and K. Kalli, Fiber Bragg Gratings, Fundamentals and Ap-plications in Telecommunications and Sensing. Boston, MA: ArtechHouse, 1999, pp. 302–337.LIM et al.: STRAIN AND TEMPERATURE SENSORS 627[10] R. Kashyap, Fiber Bragg Gratings. London, U.K.: Academic, 1999,pp. 409–440.[11] R. M. Cazo, O. Lisboa, H. T. Hattori, V. M. Schneider, C. L. Barbosa,R. C. Rabelo, and J. L. S. Ferreira, “Experimental analysis of reflectedmodes in a multimode strained grating,” Microwave Opt. Technol. Lett.,vol. 28, no. 1, pp. 4–8, 2001.[12] E. C. Ifeachor and B. W. Jervis, Digital Signal Processing—A PracticalApproach. New York: Addison-Wesley, 1993, pp. 183–250.Jirapong Lim received the B.Eng. and M.Eng.degrees in production engineering with a major inindustrial automation systems in 1991 and 1996,respectively, from King Mongkut’s Institute ofTechnology, North Bangkok, Thailand. He receivedthe Ph.D. degree in 2001 for work in the field ofpower-by-light and control-by-light of hydraulicvalve systems from the Department of SystemsEngineering, Brunel University, Middlesex, U.K.From 1992 to 1998, he worked in the automationindustries and then joined King Mongkut’s Instituteof Technology as a Lecturer. He currently lectures on industrial measurementand control systems at the same university.Qingping Yang received the diploma in instrumen-tation and measurement from Chengdu AeronauticalPolytechnic, China, in 1983. He was awarded ascholarship to study at Brunel University, Middlesex,U.K., in 1998, where he received the Ph.D. degreein 1992.He was an Assistant Engineer with the AircraftStructural Strength Research Institute, China. Hecurrently lectures on manufacturing measurementand quality engineering. His research interestsinclude advanced sensors, dimensional metrology,quality engineering, virtual instrumentation, and measurement science. He haspublished more than 35 papers in these areas.Barry E. Jones received the D.Sc. degree from theUniversity of Manchester, Manchester, U.K., in 1985.He has been Professor of manufacturing metrologyat Brunel University, Middlesex, U.K., and Directorof the Brunel Centre for Manufacturing Metrology(BCMM), since 1986. He holds fellowships offive professional bodies, is author and editor offive books, and has published over 250 papers andarticles.Prof. Jones received a 1995 Metrology for WorldClass Manufacturing Award as Champions ofMetrology. He is a previous Honorary Editor of the Journal of Physics E:Scientific Instruments (now Measurement Science and Technology) and a pastmember of the U.K. Council for National Academic Awards.Phillip R. Jackson received the B.Eng. degreefrom Edinburgh University, Edinburgh, U.K., andsubsequently received the M.Sc. and Ph.D. degreesin industrial measurement and control systems fromBrunel University, Middlesex, U.K., in 1993 and1996, respectively.He is a Senior Project Engineer at Flight Refu-elling Ltd., Wimborne, U.K., working in the field ofpower-by-light and control-by-light technology forthe aerospace industry.

[4] K.H.Wanser,K.F.Voss,andA.D.Kersey,“Novelfiberdevicesandsensors based on multimode fiber Bragg gratings,”

Bragg gratings in multimode and few-mode optical fibers,”

DP-flow sensor using optical fiber Bragg gratings,” Sens.

Experimental analysis of reflected modes in a multimode strained grating,”

Fiber Bragg Gratings, Fundamentals and Applications

Fiber Bragg Gratings, Fundamentals and Applications in Telecommunications and Sensing.

Fiber Bragg Gratings.

Fiber Devices and Sensors Based on Multimode Fiber Bragg Gratings,” U.S.

Fiber Devices and Sensors Based on Multimode Fiber Bragg Gratings,” U.S. Patent 58482,

He has been Professor of manufacturing metrology at Brunel University,

Jones received the D.Sc. degree from the

Jones received the D.Sc. degree from the UniversityofManchester,Manchester,U.K.,in1985. HehasbeenProfessorofmanufacturingmetrology at Brunel University,

Novel fiber devices and sensors based on multimode fiber Bragg gratings,”

Optical fiber grating sensors in multimode fibers,”

Optically powered hydraulic pilot valve using piezoelectric multilayer actuator,” Int.

Qingping Yang received the diploma in instrumentation and measurement from Chengdu Aeronautical Polytechnic, China, in 1983. He was awarded a scholarship to study at

received a 1995 Metrology for World Class Manufacturing Award as Champions of Metrology. He is a previous Honorary Editor of the

received the B.Eng. degree from Edinburgh University, Edinburgh, U.K., and subsequently received the M.Sc. and Ph.D. degrees in industrial measurement and control systems from

Strain and temperature sensors using multimode optical fiber Bragg gratings and correlation signal processing

Strain and temperature sensors using multimode optical fiber Bragg gratings and correlation signal processing

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