Optics Communications 285 (2012) 3466–3470

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Optics Communications

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Temperature self-compensation fiber-optic pressure sensor based on fiber Bragggrating and Fabry–Perot interference multiplexing

Wenhua Wang a,b,⁎, Xinsheng Jiang c, Qingxu Yu b

a School of Science, Guangdong Ocean University, Zhanjiang 524088, Chinab School of Physics and Optoelectronic Engineering, Dalian University of Technology, Dalian 116024, Chinac Fujian Castech Crystals Inc., Fuzhou 350002, China

⁎ Corresponding author at: School of Science, Guangjiang 524088, China.

E-mail address: mrwangwh@yahoo.com.cn (W. Wan

0030-4018/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.optcom.2012.04.001

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 February 2012Received in revised form 2 April 2012Accepted 2 April 2012Available online 13 April 2012

Keywords:Fiber-optic pressure sensorMultiplexed sensorExtrinsic Fabry–Perot interferometerDiaphragmFiber Bragg gratingTemperature–pressure cross sensitivity

A high performance multiplexed fiber-optic sensor consisted of diaphragm-based extrinsic Fabry–Perotinterferometer (DEFPI) and fiber Bragg grating (FBG) is proposed. The novel structure DEFPI fabricatedwith laser heating fusion technique possesses high sensitivity with 5.35 nm/kPa (36.89 nm/psi) and exhibitsultra-low temperature dependence with 0.015 nm/°C. But the ultra-low temperature dependence still resultsin small pressure measurement error of the DEFPI (0.0028 kPa/°C). The designed stainless epoxy-free pack-aging structure guarantees the FBG to be only sensitive to temperature. The temperature information is cre-ated to calibrate the DEFPI's pressure measurement error induced by the temperature dependence, realizingeffectively temperature self-compensation of the multiplexed sensor. The sensitivity of the FBG is 10.5 pm/°C.In addition, the multiplexed sensor is also very easy to realize the pressure and the temperature high-precisehigh-sensitive simultaneous measurement at single point in many harsh environmental areas.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Fiber-optic sensors possess the intrinsic advantages of intrinsic elec-trical passivity, high temperature survivability, corrosion resistance,immune to electromagnetic interference, fast response, high sensitivity,allowing remote and distributed measurement, compact size, lightweight, and convenience of light guiding/detection through opticalfibers, so they are a very attractive sensing option for an extensive vari-ety of environmental monitoring, down-hole monitoring, fuel storagetank, process control, electric power industry, aerospace industry,blast characterization, structural health monitoring, voice communica-tion, chemical and biomedical fields [1–23]. Pressure and temperatureare two important parameters in many applications, and the fiber-optic sensors for measuring the two parameters can be categorizedinto two groups: Fabry–Perot (FP) interferometer sensors and fiberBragg grating (FBG) sensors. FP interferometer sensors are mainly formeasuring pressure, and cover the intrinsic FP interferometer (IFPI)and extrinsic FP interferometer (EFPI) sensors. The sensing cavity ofthe IFPI sensors is between two dielectric mirrors inside the fiber [4],while the sensing cavity of the EFPI sensors is between two cleavedfiber end-faces or between a cleaved fiber end-face and a diaphragm[14]. EFPI sensors are easier to adjust the FP cavity length and oftenhave higher pressure sensitivities than IFPI sensors, while IFPI sensors

dong Ocean University, Zhan-

g).

rights reserved.

are often more sensitive to temperature changes, causing larger tem-perature–pressure cross sensitivity. Especially, diaphragm-based EFPI(DEFPI) sensors are suitable for measuring the dynamic and static pres-sure because of their high sensitivity and high bandwidth, so theybecome the research focus in the pressure measurement applications[10–12,14–16]. However, the temperature–pressure cross sensitivityof the EFPI sensors is still a crucial issue, and reduces significantly theperformance of the sensors. Recently, researchers, especially Prof.Wang's research group, have developed different kinds of approachesto decrease the temperature–pressure cross sensitivity [12,13,16,17],but the residual temperature–pressure cross sensitivity induced bythe air in FP cavity still influences on the performance of the sensors,especially in the high precision measurement or larger temperaturefluctuation applications, and the methods in Refs [16,17] complicatethe fabrication process. In addition, the cost present by Ref [17] isexpensive. Also, EFPI sensors are difficult to be multiplexed in series,so the EFPI pressure sensors are limited to single parameter sensing atsingle position in the practical applications to date. For FBG sensors, themost important applications for measuring temperature and strain/pressure have been developed in the past decades [19–25] since W.W.Morey first reported FBG sensor in 1989 [26]. For FBG pressure sensors,they have some drawbacks such as complex auxiliary structure for pres-sure transfer and higher temperature–pressure cross sensitivity, butfor FBG temperature sensors, they are easily multiplexed in series andpossess simple packaging structure. Therefore, we can integrate theadvantages of EFPI pressure and FBG temperature sensors to realize apressure–temperature multiplexing structure in a single optical fiber.

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In this article, we present a new multiplexed sensor based onDEFPI and FBG, a novel structure DEFPI fiber-optic sensor and a com-pact stainless packaging structure. The DEFPI sensor with a vent holefabricated with laser heating fusion technique is used to measurepressure, while the FBG sensor is for measuring temperature, andthe obtained temperature is employed to compensate the pressuremeasurement error induced by temperature–pressure cross sensitiv-ity, achieving temperature self-compensation of the DEFPI/FBGmulti-plexed sensor. In addition, the designed sensor can also realize thesimultaneous measurement of the pressure and temperature withhigh sensitivity/precision, and reduce considerably the cost of thesensor system.

Fig. 2. (a). Illustration of laser heating fusion between diaphragm and ferrule, and the insetin the right is the top viewmicroscopephotographof awelded ferrule; (b). fabricated lead-inoptical fiber of a multiplexed sensor; (c). schematic for heating fusion bonding between thefiber and the ferrule, the right image is the welded DEFPI sensor head.

2. Fabrication and principle of DEFPI/FBG multiplexed sensor

2.1. Fabrication of DEFPI/FBG multiplexed sensor

Fig. 1 illustrates the schematic diagram of a DEFPI/FBG multi-plexed sensor. To implement the multiplexed sensor head, firstly, anultra-thin fused silica diaphragm (30-μm-thickness) was directlywelded onto the end face of the fused silica ferrule with the horn-shape cup by CO2 laser heating fusion technique, as shown inFig. 2(a). The right image is the top view for the microscope photo-graph of the welded ferrule. Secondly, a piece of FBG was cleavedand fusion spliced at both ends with two pieces of conventional singlemode fiber (SMF), then the right SMF was cleaved to a length of 2 cm,as depicted in Fig. 2(b). The right and left SMFs, the FBG consist of thelead-in optical fiber of the DEFPI/FBG multiplexed sensor. Finally, thecleaved right SMF was inserted in the welded ferrule and bondedwith the inner side wall of the ferrule via CO2 laser, as illustrated inFig. 2(c). It can be seen from Fig. 2(c) that the gap between thefiber and the ferrule inner sidewall isn't hermetically sealed afterheating fusion bonding, reserving a vent hole. In this scheme, owingto the Fressnel reflection of the right SMF tip surface and the insidesurface of the diaphragm, the air-gap between them can behave as aFP cavity. This fabrication process with laser heating fusion techniqueis relatively environment-friendly because no chemical processes areinvolved, and the epoxy-free bonding makes the DEFPI sensor to bereliable and endure very high temperature. In addition, the fabrica-tion is simple, clean and inexpensive.

For the bare multiplexed sensor head, we designed a particularepoxy-free packaging structure, making FBG to being only sensitiveto temperature. Fig. 3 illustrates the packaging structure. An Epoxy-free stainless pressure isolation structure is designed for isolatingthe DEFPI from the FBG and signal transmitting SMF. The SMF wasbonded with a silica ferule by laser heating fusion technique, thenthe ferrule and the DEFPI sensor head were sealed with a pressuremetal ring to the package structure, making the FBG to fix loosely inthe 316 stainless tube. The leaving vent hole during the heating fusionbonding process together with stainless tube ensured the air in the FPcavity to connect to the ambient environment.

Fig. 1. Basic structure of a DEFP

2.2. Principle of DEFPI/FBG multiplexed sensor

FBGs are extensively employed to measure temperature in manyfields, their Bragg wavelength, λB, is determined by the Bragg law

λB ¼ 2neff ð1Þ

where neff is the effective refractive index, and Λ is the index modula-tion period (grating period). Since the refractive index and indexmodulation period change with temperature variation, the λB is sen-sitive to temperature. The λB-shift is mainly contributed by thechanges of temperature inducing refractive index and grating periodin our case. The λB-shift, ΔλB, with temperature change, ΔT, is givenby

λB ¼ 2∂neff

∂T þ neff∂Λ∂T

!⋅ΔT ð2Þ

As a result, temperature variation can be simply acquired with FBGby means of measuring the reflection λB-shift of the FBG. The reflec-tion λB can be directly read from the wavelength interrogator in apeak search algorithm.

I/FBG multiplexed sensor.

Fig. 3. Epoxy-free stainless steel packaging structure for the DEFPI/FBG multiplexed sensor.

3468 W. Wang et al. / Optics Communications 285 (2012) 3466–3470

According to the FP cavity reflection interference spectrum, the FPcavity length, L, is calculated by a cross-correlation algorithm [27].Because of the detailed description in the literature [27] of ourgroup, we give a simple depiction of the algorithm here. After filteringthe direct current term, the optical intensity of the FP cavity interfer-ence spectrum is expressed as

I λð Þ ¼ 2Is λð Þ⋅γ cos4πLλ

þ π� �

ð3Þ

with Is(λ) the spectrum intensity distribution of the optical source, γthe contrast of the interference fringe, and π the additional phaseof the half-wave loss. The absolute FP cavity length, L, can be deter-mined by calculating the cross-correlation coefficient. The cross-correlation coefficient of a given L is evaluated by

C Lð Þ ¼XNi¼1

x nð Þ cos 4πLλn

þ π� �

ð4Þ

where x(n) is the spectrum intensity data sequence acquired by theinterrogator, λ(n) is the wavelength corresponding to element x(n),and N is the total number of the intensity data sequence. For a givenseries of acquired data, the effective FP cavity length is correspondingto the L value where the cross-correlation coefficient C(L) is maximal.

For the DEFPI fiber sensor, the diaphragmwill deflect as a result ofthe applied pressure. For a clamped rigidly round diaphragm, the out-of-plane deflection of the diaphragm center Y under the applied pres-sure is expressed as [28]

Y ¼3 1−μ2� �

P

16Eh3a2−r2� �2 ð5Þ

where μ and E are the Poisson's ratio and Young's modulus of dia-phragm material respectively, h is the diaphragm thickness, a is theeffective radius of the diaphragm, and r is the radial distance to thediaphragm center. The FP cavity length will vary with the deflectionof the diaphragm when the environment pressure is applied to theDEFPI fiber sensor. Therefore, the interference fringes of the FP

Fig. 4. Schematic diagram for the multiplexed sensor experiment setup.

interferometer are modulated by the applied pressure, which willbe for the demodulation of the FP cavity length. In this way, we canrealize the measurement of the environment pressure.

3. Experiment setup and results

Fig. 4 shows the schematic illustration of the experiment setup.Light from a wavelength swept laser built in the sm125 Optical Sens-ing Interrogator (Micron Optics Inc.) was launched into the lead-inoptical fiber and propagated along the fiber, which reached in turnthe FBG and the DEFPI, and firstly reflected at the Bragg wavelengthof the FBG. Then the transmission light of the FBG was partially(about 4%) reflected by the fiber tip surface and the inside surfaceof the diaphragm because of Fressnel reflection at the glass–air inter-face, respectively. The beam reflected by the diaphragm coupled intothe lead-in optical fiber and propagated back along the lead-in fiber,producing interference fringes with the beam reflected by the fibertip surface. These reflection signals were detected by the photodiodebuilt in the sm125 interrogator and displayed on the PC. A data process-ing program developed on the LABVIEW platform based on the cross-correlation algorithm with a demodulation resolution of 0.2 nm wasused to interrogate the reflective spectrumof the compound signal gen-erated by the FP interferometer and the FBG reflection. The reflectioncompound spectrum of the DEFPI/FBG multiplexed sensor is depictedin Fig. 5, where the strong narrowband overlap spectrum is the FBGreflection peak and the sinusoidal spectrum is the interference signalof the DEFPI.

Firstly, the DEFPI/FBG multiplexed sensor was evaluated for itspressure performance. As described above, the DEFPI was designedfor measuring pressure while the FBG was only for measure temper-ature in our scheme. According to the Eq. (5), the sensitivity of theDEFPI sensor is 5.77 nm/kPa (39.78 nm/psi). Fig. 6 shows the pressureresponse characteristic of the DEFPI at room temperature, and theinset is an enlarge figure of the dashed rectangle. The DEFPI pressure

Fig. 5. Illustration of the compound spectrum of the multiplexed sensor.

Fig. 6. Pressure response characteristic of the DEFPI pressure sensor. Fig. 7. Temperature dependence of the FP cavity length.

Fig. 8. Temperature sensing characteristic of the FBG sensor.

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sensitivity is 5.35 nm/kPa (36.89 nm/psi), and the pressure resolutionof the sensor system based on the cross-correlation algorithm isapproximately 37 Pa (0.005 psi).

However, although the materials of the optical fiber, the ferruleand the diaphragm are all the fused silica, the thermal expansioncoefficient of the SMF (5.6×10−7/°C) isn't equal to those of the fer-rule and the diaphragm (5.5×10−7/°C) due to germanium dopingin the optical fiber core. In general, more germanium in the opticalfiber is doped, larger the thermal expansion coefficient of the opticalfiber is. For the EFPI sensors, the difference of the thermal expansioncoefficient between the materials will result in the temperature–pressure cross sensitivity (causing the temperature dependence ofDEFPI) of the DEFPI sensors. Furthermore, if FP cavity isn't hermeticallysealed, the air trapped by FP cavity will induce the larger temperaturedependence owing to the different air pressure at different temperature.

As shown in Fig. 2(c), Ls is the length from the right SMF tip sur-face to the fiber heating fusion bonding point, and L is the length ofthe FP cavity. The total temperature dependence of the DEFPI sensoris given by

ΔLΔT

¼ αh Ls þ Lð Þ þ Y⋅Pe

Te−αsLs ð6Þ

where αh and αs are the thermal expansion coefficient of the fusedsilica ferrule and the SMF respectively, Pe is the pressure of thetrapped air at temperature Te. Actually, the right second term ofthe equation is the primary factor of the temperature dependence.The value of the second term is zero if the air in the FP cavity connectsto the ambient environment. In our scheme, as shown in Fig. 2(c), theair in the FP cavity isn't thoroughly sealed during heating fusionbonding, making it to connect the ambient environment, so the DEFPIsensor have very low temperature dependence. Also, in the DEFPI/FBGmultiplexed sensor, the FBG is used to measure temperature while theDEFPI acts as a pressure sensor head, so the temperature acquired byFBG can be employed to compensate the pressure measurement errorinduced by the low temperature dependence.

Secondly, the temperature performance of the DEFPI/FBG wasinvestigated. The temperature dependence of the DEFPI was tested,as shown in Fig. 4 except the sensor head. The sensor head wasinserted into a temperature control case together with a platinum resis-tance thermometer (Hart Scientific 1502A, ±0.012 °C). The tempera-ture dependence curve of the DEFPI is shown in Fig. 7, which indicatesthe temperature dependence is approximately 0.015 nm/°C. The pres-sure measurement error induced by temperature dependence is about0.0028 kPa/°C (0.0004 psi/°C). The FBG's peak wavelength of the com-pound spectrum shifts only with the temperature in the multiplexedsensor because the designed packaging structure makes the FBG to beinsensitive to pressure, and the reflection peak of the FBG can be easilyobtained according to the compound spectrum, so the temperature can

be independently demodulated. Fig. 8 shows the FBG's temperatureperformance graph indicating the 10.5 pm/°C of the temperature sensi-tivity. Therefore, on the one hand, based on the temperature variationobtained by the FBG, the temperature–pressure cross-sensitivity ofthe DEFPI can considerably reduce via the temperature dependenceof the DEFPI, that is, the DEFPI/FBG multiplexed sensor possesses theself-compensation ability of the temperature, which is very easy toemploy the temperature information caught by the FBG to correct effec-tively the pressure error of the DEFPI. On the other hand, the multi-plexed sensor can realize pressure and temperature high-precisehigh-sensitive simultaneous measurement for a multi-parametersmeasurement at single point.

4. Conclusions

In conclusion, a temperature self-compensation high-sensitivityfiber-optic pressure sensor based on DEFPI/FBG multiplexing hasbeen proposed in order to meet the demand of the high precise pres-sure and temperaturemeasurement in practical applications. Combin-ing advantages of the DEFPI pressure sensor and the FBG temperaturesensor, the multiplexed sensor realizes self-compensation of the tem-perature. Experimental results show that the pressure sensitivity andultra-low temperature dependence of the DEFPI sensor are respec-tively 5.35 nm/kPa (36.89 nm/psi) and 0.015 nm/°C; 0.0028 kPa/°C(0.0004 psi/°C) of the pressure measurement error induced by tem-perature dependence can be effectively corrected by the FBG withthe sensitivity of 10.5 pm/°C. FBG packaged by the designed structureis only sensitive to temperature. Further, the multiplexed sensor canperform high-precise high-sensitive simultaneous measurement ofthe pressure and temperature at single point. In addition, due to theepoxy-free during the fabrication and the packaging, we believe thatthis multiplexing scheme would have great potential application as

3470 W. Wang et al. / Optics Communications 285 (2012) 3466–3470

a high performance and low-cost fiber-optic sensor in many harshenvironmental fields.

Acknowledgments

This work is supported by the Project for Industrial Research Pro-gram in Science and Technology of Zhanjiang, Guangdong Province,China (Grant No. 2011C3108005) and the Project for SpecializedResearch Fund for the Doctoral Program of Higher Education ofChina (Grant No. 201000411100287).

References

[1] O. Kilic, M. Digonnet, G. Kino, O. Solgaard, Measurement Science and Technology18 (2007) 3049.

[2] M. Naci Inci, S.R. Kidd, S. Barton, J.D.C. Jones, Measurement Science and Technology 3(1992) 678.

[3] E. Udd, Fiber Optic Smart Structures, John Wiley & Sons, New York, 1995.[4] S.E.U. Lima, O. Frazao, F.M. Araujo, L.A. Ferreira, V. Miranda, J.L. Santos, Optical

Engineering 48 (2009) 024401.[5] D. Monzon-Hernandez, J. Villatoro, Sensors and Actuators B 115 (2006) 227.[6] Y. Zhang, X.P. Chen, Y.X. Wang, K.L. Cooper, A.B. Wang, Journal of Lightwave

Technology 25 (2007) 1797.[7] K.I. Kang, T.G. Chang, I. Glesk, P.R. Prucnal, Applied Optics 35 (1996) 1484.[8] W.H. Wang, N. Wu, Y. Tian, X.W. Wang, C. Niezrecki, J.L. Chen, Optics Express 17

(2009) 16613.

[9] K. Bohnert, P. Gabus, J. Nehring, H. Brandle, Journal of Lightwave Technology 20(2002) 267.

[10] D.C. Abeysinghe, S. Dasgupta, H.E. Jackson, J.T. Boyd, Journal of Micromechanicsand Microengineering 12 (2002) 229.

[11] M.J. Gander, W.N. MacPherson, J.S. Barton, R.L. Reuben, J.D.C. Jones, R. Stevens,K.S. Chana, S.J. Anderson, T.V. Jones, IEEE Sensors Journal 3 (2003) 102.

[12] J.C. Xu, G. Pickrell, X.W. Wang, W. Peng, K. Cooper, A. Wang, IEEE PhotonicsTechnology Letters 17 (2005) 870.

[13] S.H. Aref, H. Latifi, M.I. Zibaii, M. Afshari, Optics Communications 269 (2007) 322.[14] J.C. Xu, X.W. Wang, K.L. Cooper, A.B. Wang, Optics Letters 30 (2005) 3269.[15] J.H. Yi, E. Lally, A.B. Wang, Y. Xu, IEEE Photonics Technology Letters 23 (2011) 9.[16] J. Xu, G.R. Pickrell, X. Wang, B. Yu, K.L. Cooper, A. Wang, SPIE 5998 (2005) 599809.[17] O.C. Akkaya, O. Kilic, J.F.M. Digonnet, S.G. Kino, O. Solgaard, IEEE Sens. Conf.,

Hawaii, America, 2010, p. 1148.[18] E. Cibula, D. Donlagic, Applied Optics 44 (2005) 2736.[19] H. Noritomo, S. Yasukazu, ISA Transactions 39 (2000) 169.[20] A.D. Kersey, M.A. Davis, H.J. Patrick, M. LeBlanc, K.P. Koo, C.G. Askins, M.A.

Putnam, E.J. Freibele, Journal of Lightwave Technology 15 (1997) 1442.[21] B.A. Tahir, J. Ali, R.A. Rahman, A. Ahmed, Journal of Optoelectronics and Advanced

Materials 11 (2009) 1692.[22] V. Mishra, N. Singh, U. Tiwari, P. Kapur, Sensors and Actuators A 167 (2011) 279.[23] Y.C. Chen, C.C. Hsieh, C.C. Lin, Sensors and Actuators A 167 (2011) 63.[24] Y.G. Zhan, H.W. Cai, R.H. Qu, S.Q. Xiang, Z.J. Fang, X.Z. Wang, Optical Engineering

43 (2004) 2358.[25] H.J. Park, M.H. Song, Sensor 8 (2008) 6769.[26] W.W. Morey, G. Meltz, W.H. Glenn, SPIE 1169 (1989) 98.[27] Z. Jing, Q. Yu, 6th International Symposium on Test and Measurement, Dalian,

China, 2005, p. 3509.[28] M.D. Giovanni, Flat and Corrugated Diaphragm Design Handbook, Mercel Dekker,

New York, 1982.