Temperature insensitive magnetic field sensor based

Temperature insensitive magnetic field sensor based on an etched TCFMI cascaded with a FBG Guofeng Yan*a, Liang Zhanga, and Sailing Hea Centre for Optical and Electromagnetic Research, State Key Laboratory of Modern Optical Instrumentation, Zhejiang Provincial Key Laboratory for Sensing Technologies, Zhejiang University, Hangzhou 310058, China a

ABSTRACT In this paper, a dual-parameter measurement scheme based on an etched thin core fiber modal interferometer (TCMI) cascaded with a fiber Bragg grating (FBG) is proposed and experimentally demonstrated for simultaneous measurement of magnetic field and temperature. The magnetic fluid cladding surrounding the TCFMI was used as a magnetic field-to-refractive index transducer. To depress the temperature influence on the performance of such sensors, an FBG was inscribed in the leading SMF of the TCFMI. Experimental results show that, the reflection of the FBG has a magnetic field intensity and temperature sensitivities of -0.017 dB/Oe and 0.133 dB/℃, respectively, while the Bragg wavelength of the FBG only has a temperature sensitivity of 13.23 pm/℃. By monitoring the reflection wavelength and intensity of the Bragg mode, the intensity of the magnetic field and the temperature variance can be measured, which enables magnetic field sensing under strict temperature environments. Meanwhile the reflective sensing probe is more compact and practical for applications in hard-to-reach conditions. Keywords: Fiber-optic magnetic field sensor, etched thin core fiber modal interferometer, temperature cross-sensitivity, fiber Bragg grating

INTRODUCTION As a stable colloidal suspension, magnetic fluid (MF) has excellent magnetic-optical properties such as refractive index (RI) tunability [1], birefringence effect [2, 3] and Faraday effect [4]. In the past few decades, MF-based fiber-optic magnetic field sensors have attracted much attention of researchers and been widely developed with various optical fiber measurement configurations. The common magnetic flied sensing principle is based on the tunable refractive index effect. For example, MFs were employed as an outer cladding material of an etched fiber Bragg grating (FBG) [5], a tilted FBG [6], a long period grating [7], a collapsed PCF based Mach-Zehnder interferometer (MZI) [8], and a single modemultimode-single mode (SMS) fiber structure [9]. Due to the relative small tunable range of the refractive index of MF under external magnetic field (normally < 0.01RIU ), the sensitivities of these MF-based magnetic field sensors are no more than several pm/Oe. The relative small birefringence change of the MF film also could be magnified and measured by a properly modified Sagnac interferometer with relatively high sensitivity of up to 592.8 pm/Oe [10, 11]. However, such structures always need long PCF sections or high birefringence fibers, which increase the cost significantly. The transmission readout type and the bulky free-space-optics requiring expert alignment and servicing limit the possibility for practical applications [10,11]. In addition, cross-sensitivity between magnetic field and temperature is generally an important issue to be solved [12, 13]. All these put forward the request to a sensing structure to be with high magnetic field sensitivity, low MF absorption loss and low temperature cross-sensitivity. In this paper, we present a sensing scheme based on an etched TCFMI cascaded with an FBG for simultaneous magnetic field and temperature monitoring. The TCFMI was firstly optimized by chemical etching to highly improve its magnetic field sensitivity. An FBG was cascaded with the TCFMI to obtain the magnetic field and the temperature variance information by way of monitoring the reflection and the Bragg wavelength. Due to the different responses of the reflection and Bragg wavelength to temperature and magnetic field, by using the sensing matrix method, the simultaneous measurement of temperature and magnetic field intensity can be realized.

2015 International Conference on Optical Instruments and Technology: Optical Sensors and Applications, edited by Xuping Zhang, David Erickson, Xudong Fan, Zhongping Chen, Proc. of SPIE Vol. 9620, 96200I · © 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2193536 Proc. of SPIE Vol. 9620 96200I-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/10/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

Sensing Principle

OPT TIMIZATIO ON OF THE E ETECHED D TCFMI

T is show wn in Fig. 1. A section of thin-core fibeer (TCF) is insserted betweenn The configuraation of a claddding-etched TCFMI the standard single s mode fibers f (SMFs) to form an inn-fiber multim modal interferoometer. Etchinng process is used u to reducee the cladding of the TCF too enhance the evanescent waves w of its claadding modess. The profile of the hetero--core interfacee MF and TCF arre ideally alignned, only zeroo order azimutthal modes aree regions are asssumed as taper shape. Assuuming the SM excited due too mode mismaatch interface. As theoreticaal analysis in Ref. R [14], the dip d wavelengtth of the outpu ut transmissionn spectrum satiisfies the phasse condition off the destructive interferencce below,

[

] λL

co (λ ) − neffff (λ,next ) 2π neff cl , j

= (2k + 1)π

(1)

D

,j where λ D is thhe wavelengthh of the spectraal dip, nco R of the core mode, ncl is the effective RI of the j-thh eff is the effective RI eff order claddinng mode, L is the interferom meter length, and k is an in nteger. By moonitoring the ddip wavelengtth shifts of thee output of the structure, the refractive inddex of the surrrounding med dium can be deetermined. Laater the experiimental resultss will demonsttrate that the sensitivity off such a claddding-reduced TCFMI will be much highher than that of previouslyy reported TCF FMI sensor [144].

Figure 1. The schematic diagram of the cladding-etcheed TCFMI senso or.

Since the magnetic m fluidd has a tunablee refractive inddex effect, it iss an ideal maggnetic flied-to--refractive ind dex transducerr. When the exxternal magneetic field direection is perppendicular to the light proopagation axiis in the TCF F, the electricc susceptibilityy ߯ of the MF will decreasee with the increeasing magnettic field [15]. The refractivee index n can n be expressedd as [16]

n = εr = 1 + χ (2) o the MF decrreases with thee rising of the external magn netic field in this t case, whicch will lead to a blue shift of Thus the RI of the dip wavellength in the TCFMI T transm mission spectruum by way off evanescent field. fi Fabricattion Experiments have been caarried out to verify v those annalyses. The TCFMI T sensorrs were madee by using com mmercial TCF F SMF-28). Eacch TCFMI wass packaged wiith a shallow ppolyethylene box b for etchingg (Nu-fern 460--HP) with stanndard SMFs (S process. The hydrofluoric solution (HF)) with concenntration of 40% % was used too chemically eetch the TCF cladding. Thee t contro olling the etchhing time. The microscope images of thee TCF claddinggs can be reduuced to speciffic diameters through etched TCF sections are shhown in Fig. 2.. Fig. 2 (a), (b)) are the TCF section with cladding c diameeters of 28 μm m, and the joinnt between the non-etched n annd etched fiberr sections, resppectively. One can see that the surface coorrugation is diminished d forr reducing trannsmission losss. The etched TCMFIs T weree then packaged in capillariies with MF ffor testing of magnetic m fieldd response. Thee magnetic fluuid (EMG605,, Ferrotec Inc.., with an initial RI of 1.40) was infiltrateed into the cap pillary tube viaa capillary forcce and the bothh ends of the capillary c tube were sealed with w UV glue..

Fig. 2. Opptical microscoppe images of the etched TCF section whose clladding diameteer is 28 μm (a),, and the joint between b the etched TC CF and a standaard SMF (b), resspectively.

Proc. of SPIE Vol. 9620 96200I-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/10/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

Experim mental Test The sensing performance p w tested unnder different magnetic fields generated by an electromagnet and calibrated was c by a Gauss meter with a resoluution of 0.1 Gs. The transm mission spectraal evolution of o the TCFMII under differeent fabricationn r by using a broadbband light souurce (BBS, 1250-1650 nm) and an opticaal spectrum an nalyzer (OSA A, stages were recorded ANDO, AQ66317), shown in Fig. 3(a). With the inccrease of the magnetic field, the interfeerence dips sh hift to shorterr wavelength causing c the ligght intensity chhange at 15500 nm. In order to estimate thhe sensitivitiess of the etched d TCFMIs, thee dip wavelenggth and transm mission as a fuunction of maggnetic fields are a plotted in Fig. F 3(b), and linear fits aree applied whenn the magnetic field intensityy increases froom 30 to 270 Oe. O The waveelength sensitivvity of the etcched TCFMI is -128 pm/Oee, a 1550 nm is -0.015 dB/Oee, respectivelyy. The experim mental results show that thee while the trannsmission losss sensitivity at intensity andd wavelength interrogation approaches are a both capaable to measurre the magneetic fields and d reducing thee cladding of thhe TCF sectionn, the magnetiic field sensitiivity of the TC CFMI can be highly h improvved (one or two order better)) compared witth the previouusly reported values v [5,9,12].

Fig. 3. (a)) The transmissiion spectral evoolution of the ettched TCFMIs for f different maagnetic fields annd (b) The dip wavelength w and transm mission of the etched e TCFMIs as a function of o magnetic fiellds.

The temperature effect on o the perform mances of etchhed TCFMI ass a magnetic field f sensor haas also been experimentally e y investigated. The packed TCFMI T with was placed inn a tunable ov ven with tem mperature resollution of 0.1℃ ℃. Due to thee T and the RI R change of thhe TCF material and MF caaused by the thhermo-optic coefficient, c thee thermal expannsion of the TCF dip wavelenggth blue-shifts almost linearrly as temperaature increasess from 18.9 ℃ to 38.4 ℃, with a sensitivity of 64 pm m/℃ (see Fig. 4(a)). The transmission loss deccreases linearlly as well with h a sensitivityy of -0.076 dB/℃ (see Fig. 4(b)).Thus thee magnetic fieldd measuremennt error induceed by the tempperature can bee calculated ass ~3 Oe/℃ annd 7 Oe/℃ corrresponding too wavelength and a intensity interrogation, i respectively.. In this aspecct, the temperrature effect oon the perform mances of thee sensor could not be neglectted for accuraate measuremeent especially in low magneetic field casess.

Fig. 4. Thhe dip wavelenggth shift (a) andd transmission looss change (b) of the packagedd TCFMI as a ffunction of temp perature.

Proc. of SPIE Vol. 9620 96200I-3 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/10/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

ETEC CHED TCFM MI CASCA ADED WITH H AN FBG Configuration and Seensing Princiiple In order to eliiminate the tem mperature effe fect, an FBG with w a resonance wavelengthh near 1550 nm m was inscrib bed in the SMF F section of thee TCFMI, as shhown in Fig. 5. 5 For practical applications,, the sensor waas designed ass a sensing pro obe working inn reflection moode. As our previous resultts shown (seee Fig. 4), the transmission loss at 1550 nm is associaated with bothh magnetic fielld and temperaature. Thus, thhe reflection intensity i of th he FBG depennds on both thhe environmen nt temperaturee and the applieed magnetic field. fi The refleection variancce ΔR can be ex xpressed as ΔR = α T ΔT + α H ΔH

(3)

where α T andd α R are the reesponse sensitivities of the reflection to the t temperatuure and magneetic field, resp pectively. ΔT and ΔH are the t temperatuure and magnnetic field chhanges, respeectively. How wever, due to the well co onfined of thee fundamental mode m inside thhe fiber core, the reflectionn wavelength of o the FBG is only affected by temperatu ure. Simply thee reflection wavvelength shiftt Δλ can be wrritten as (4) Δλ = β T ΔT where β T is thhe temperaturre sensitivity of the FBG. Therefore, by y monitoring the reflectionn intensity and wavelengthh, solving the Eq. 3 and 4, wee can obtain thhe temperaturee and magnetiic field inform mation as −1

⎡α T α H ⎤ ⎡ ΔR ⎤ ⎡ ΔT ⎤ ⎥ ⎢ ⎥ ⎢ ⎥ = ⎢β ⎣ ΔH ⎦ ⎣⎢ T 0 ⎦ ⎣ Δλ ⎦

(5)

Fig. 5. Schematic diagram d of the magnetic m field sensing s probe based b on an etchhed TCFMI casscaded with an FBG. F

Experim mental Results The magneticc field responsse of the sensinng probe was carried out by y an electromaagnet and a Gaauss meter with a resolutionn of 0.1 Gs. Fiigure 6(a) shoows the specttral evolution and the FBG G reflection chhange of the sensing probee for differennt magnetic fields. From the spectra, s we caan see that when the magnettic increased from f 3.0 Oe too 301.6 Oe, th he reflection of the FBG increeased gradually while the Bragg B wavelenngth remained d constant. Thee reflection inccrement can be b attributed too that as the magnetic m field changes, the interference dip d of the TC CFMI shifts, innducing the liight intensity change at thee measured wavvelength. Thee constant reflection waveleength is due to o the well conffine of the corre mode which h is inherentlyy insensitive to the outside reefractive indexx change. Thee reflections of the FBG as a function of m magnetic field d are plotted inn Fig. 6(b), froom which thee sensitivity α H can be calculated as -0.017 dB/Oe by b a linear fitt process. Th he temperaturee response has also been im mplemented byy placing the sensing s probee inside a tunaable oven withh temperaturee resolution of 0.1℃. The reeflection specctra of the sennsing probe under u differen nt temperaturees are shownn in the Fig. 7(a). 7 With thee increment off temperaturee, the Bragg wavelength red-shifts with w a good linearity duee to the therrmo-optic andd thermal-expaansion coefficients of the SMF, S and thee FBG reflecttion increasess resulted from m the temperrature-inducedd interference change c of the packaged p TCF FMI. Figure 7(b) 7 gives the FBG F reflection (blue) and B Bragg wavelen ngth (black) of the sensing probe as a funcction of tempeerature. The sensitivity s a β T also caan be obtainedd as 0.133 dB/℃ and 13.233 α T and meters into Eqq. 5, we can get g pm/℃, respecctively. Puttinng all the param

Proc. of SPIE Vol. 9620 96200I-4 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/10/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

⎡ΔT ⎤ − 13.23⎤ ⎡ΔR ⎤ 1 ⎡0 ⎥⎢ ⎥ ⎢ ⎢ ⎥ = 0 . 2 208 ⎣0.017 0.133 ⎦ ⎣Δλ ⎦ ⎣ΔH ⎦

(6)

It indicates thhat a precise magnetic fielld measuremeent and tempeerature monitooring can be realized by simultaneouslyy detecting the reflection inteensity and wavelength of thhe FBG.

Fig. 6. (a)) The reflectionn spectral evoluution of the senssing probe for different d magneetic fields. (b) T The FBG reflecction of the sensing prrobe as a functiion of magneticc field.

Fig. 7. (a)) The reflection spectral evoluttion of the sensiing probe for diifferent temperaatures. (b) The FBG reflection n (blue) and Bragg wavelength (blackk) of the sensingg probe as a funnction of tempeerature.

CONCLUS SION In conclusionn, a fiber-opticc reflective maagnetic field sensor s based on o concatenatiion of an FBG G with an etcheed TCFMI hass been proposeed for simultanneous measurrement of maggnetic field in ntensity and teemperature. B By using the sensing s matrixx method, the proposed p sensoor could be em mployed for acccurate simultaaneous measuurement of tem mperature and magnetic fieldd intensity, andd thus the tempperature cross--sensitivity isssue could be effectively e resoolved. As a reflective probee, the proposedd sensing structture has manyy advantages, such s as compaactness, ease of o fabrication, practicabilityy in hard-to-reaach places etcc. Such excelleent performannces make ouur sensing sttructure a promising platfform for muulti-parameter measuremennt applications.

ACK KNOWLED DGMENT p suppoorted by the National N High Technology T Research R and Development D P Program (863)) of China (Noo. This work is partially 2012AA0122201).

Proc. of SPIE Vol. 9620 96200I-5 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/10/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

REFERENCES [1] Hong, C. Y., Yang, S. Y., Horng, H. E. and Yang, H. C., “Control parameters for the tunable refractive index of magnetic fluid films,” J. Appl. Phys., 94(6), 3849-3852 (2003). [2] Di, Z., Chen, X., Pu, S., Hu, X. and Xia, Y., “Magnetic-field-induced birefringence and particle agglomeration in magnetic fluids,” Appl. Phys. Lett., 89(21), 211106 (2006). [3] Xu, M. and Ridler, P. J., “Linear dichroism and birefringence effects in magnetic fluids,” J. Appl. Phys., 82(1), 326-332 (1997). [4] Martinez, L., Cecelja, F. and Rakowski, R., “A novel magneto-optic ferrofluid material for sensor applications,” Sensor. Actuat. A-Phys., 123-124, 438-443 (2005). [5] Dai, J., Yang, M., Li, X. Liu, H. and Tong, X., “Magnetic field sensor based on magnetic fluid clad etched fiber Bragg grating,” Opt. Fiber Technol., 17(3), 210-213 (2011). [6] Yang, D., Du, L., Xu, Z., Jiang, Y. Xu, J. Wang, M. Bai, Y. and Wang, H., “Magnetic field sensing based on tilted fiber bragg grating coated with nanoparticle magnetic fluid,” Appl. Phys. Lett., 104(6), 061903 (2014). [7] Konstantaki, M., Candiani, A. and Pissadakis, S., “Optical fibre long period grating spectral actuators utilizing ferrofluids as outclading overlayers,” J. Eur. Opt. Soc.-Rapid 6, 11007, (2011). [8] Zu, P., Chan, C. C., Lew, W. S., Hu, L., Jin, Y., Liew, H. F. Chen, L. H., Wong, W. C. and Dong, X., “Temperature-insensitive magnetic field sensor based on nanoparticle magnetic fluid and photonic crystal fiber,” IEEE Photonics J., 4, 491–498 (2012). [9] Wang, H., Pu, S., Wang, N., Dong, S. and Huang, J., “Magnetic field sensing based on singlemode–multimode–singlemode fiber structures using magnetic fluids as cladding,” Opt. Lett., 38(19), 3765-3768 (2013). [10] Zu, P., Chan, C. C., Lew, W. S., Jin, Y., Zhang, Y., Liew, H. F., Chen, L. H., Wong, W. C. and Dong, X., “Magneto-optical fiber sensor based on magnetic fluid,” Opt. Lett., 37(3), 398-400 (2012). [11] Zu, P., Chan, C. C., Koh, G. W.M, Lew, W. S., Jin, Y., Liew, H. F., Wong, W. C. and Dong, X., “Enhancement of the sensitivity of magneto-optical fiber sensor by magnifying the birefringence of magnetic fluid film with Loyt-Sagnac interferometer,” Sensor. Actuat. B-Chem., 191, 19-23 (2014). [12] Huang, G., Zhou, B., Chen, Z., Jiang, H. and Xing, X., “Magnetic-Field Sensor Utilizing the Ferrofluid and Thin-Core Fiber Modal Interferometer,|” IEEE Sens. J., 15(1), 333-336 (2014). [13] Miao, Y., Zhang, H., Lin, J., Song, B., Zhang, K., Lin, W., Liu, B. and Yao, J., “Simultaneous measurement of temperature and magnetic field based on a long period grating concatenated with multimode fiber,” Appl. Phys. Lett., 106, 132410 (2015). [14] Xia, T., Zhang, A., Gu, B. and Zhu, J., “Fiber-optic refractive-index sensors based on transmissive and reflective thin-core fiber modal interferometers,” Opt. Commun., 283, 2136-2139 (2010). [15] Mailfert, A. J. and Nahounou, B., “Dielectric behaviour of a ferrofluid subjected to a uniform magnetic field. Magnetics,” IEEE T. Magn., 16(2), 254-257 (1980). [16] Liu, T., Chen, X., Di, Z., Zhang, J., Li, X. and Chen, J., “Tunable magneto-optical wavelength filter of long-period fiber grating with magnetic fluids,” Appl. Phys. Lett., 91(12), 121116 (2007).

Proc. of SPIE Vol. 9620 96200I-6 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/10/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx