A Magnetic Field Sensor Based on a Magnetic Fluid-Filled FP-FBG Structure

Transcription

1 s Article A Magnetic Field Sensor Based on a Magnetic Fluid-Filled - Structure Ji Xia 1, Fuyin Wang 1, Hong Luo 1, Qi Wang 2 Shuidong Xiong 1, * 1 Academy Ocean Science Engineering & College Optoelectronic Science Engineering, National University Defense Technology, Changsha , China; blovexiaji@163.com (J.X.); fuyin_wang@126.com (F.W.); luohongrong2000@163.com (H.L.) 2 College Information Science Engineering, Norastern University, Shenyang , China; wangqi@e.neu.edu.cn * Correspondence: nudtxsd@163.com; Tel.: Academic Editor: Vittorio M. N. Passaro Received: 3 March 2016; Accepted: 21 April 2016; Publhed: 29 April 2016 Abstract: Based on charactertic magnetic-controlled refractive index property, in th paper, a magnetic fluid used as a sensitive medium to detect magnetic field in fiber optic Fabry-Perot () cavity. The temperature compensation in fiber Fabry-Perot magnetic demonstrated achieved. The refractive index magnetic fluid varies applied magnetic field external temperature, a cross-sensitivity effect temperature magnetic field occurs in Fabry-Perot magnetic accuracy magnetic field measurements affected by rmal effect. In order to overcome th problem, we propose a modified structure. With a fiber Bragg grating () written in insert fiber end Fabry-Perot cavity, acts as a temperature compensation unit for magnetic field measurement it provides an effective solution to cross-sensitivity effect. The experimental results show that sensitivity magnetic field detection improves from 0.23 nm/mt to 0.53 nm/mt, magnetic field measurement resolution finally reaches 37.7 T. The temperature-compensated - magnetic has obvious advantages small volume high sensitivity, it has a good prospect in applications in power industry national defense technology areas. Keywords: magnetic field ; Fabry-Perot Cavity; magnetic fluid; Fiber Bragg Grating; temperature compensation 1. Introduction Magnetic field s have been widely applied for navigation, vehicle detection, current sensing, spatial geophysical researches. Compared or types magnetic field, all-fiber based s have been extensively investigated owing to ir portability, high geometric adaptability, immunity from electromagnetic interference, long dtance signal transmsion for remote operation, restance to high pressure corrosion. Magnetic fluids (MFs) have numerous interesting optical charactertics [1 3], such as tunable refractive index, tunable transmittance, birefringence rmal lens effects, etc. Generally, components a MF are magnetic particles, a carrier liquid surfactants. When an external magnetic field or rmal effect applied to a MF, n refractive index MF varies changing magnetic field or rmal effect. Various MF-based optical devices have been developed [4 6], such as magnetic fluid modulators, magnetic fluid optical gratings, magnetic fluid optical fiber filters, etc. Recently, various optical fiber structures MFs have been proposed for magnetic field sensing, The employed structures include Fabry-Perot cavity [7,8], polymer optical fiber [9], Sensors 2016, 16, 620; doi: /s

2 [4 6], such as magnetic fluid modulators, magnetic fluid optical gratings, magnetic fluid optical fiber filters, etc. Recently, various optical fiber structures MFs have been proposed for magnetic field Sensors sensing, 2016, The 16, 620 employed structures include Fabry-Perot cavity [7,8], polymer optical fiber [9], 2fiber 10 Bragg grating [10,11] multimode interference [12,13] Michelson interferometer [14] Sagnac interferometer [15,16] photonic crystal fiber (PCF)-based interferometers [17 19]. Homa [7] used fiber Bragg grating [10,11] multimode interference [12,13] Michelson interferometer [14] Sagnac a MF-filled extrinsic interrogated an infrared wavelength spectrometer to detect magnetic interferometer [15,16] photonic crystal fiber (PCF)-based interferometers [17 19]. Homa [7] used a fields. The readily measured magnetic field a range 0.5 mt to 12.0 mt MF-filled extrinsic interrogated an infrared wavelength spectrometer to detect magnetic fields. corresponding sensitivities in 0.3 to 2.3 nm/mt range. In 2014, Lv [20] proposed a novel fiberoptic magnetic field, which was composed an extrinsic fiber Fabry-Perot interferometer The readily measured magnetic field a range 0.5 mt to 12.0 mt corresponding sensitivities in 0.3 to 2.3 nm/mt range. In 2014, Lv [20] proposed a novel fiber-optic magnetic a magnetic fluid. Preliminary experiments illustrated that magnetic field measurement sensitivity field, which was composed an extrinsic fiber Fabry-Perot interferometer a magnetic fluid. was nm/gs measurement resolution was better than 0.5 Gs in range from 0 to 400 Preliminary experiments illustrated that magnetic field measurement sensitivity was nm/gs Gs. Their work however did not take rmal effect MF into consideration. The refractive measurement resolution was better than 0.5 Gs in range from 0 to 400 Gs. Their work index MF behaves differently under varying magnetic field temperature conditions, which however did not take rmal effect MF into consideration. The refractive index causes a cross-sensitivity effect from temperature magnetic field exting in MF. The MF behaves differently under varying magnetic field temperature conditions, which causes a measurement systems mentioned above don t take impact operating temperature on cross-sensitivity effect from temperature magnetic field exting in MF. The measurement charactertic refractive index into account, which makes resulting magnetic field measurements systems mentioned above don t take impact operating temperature on charactertic unstable inaccurate. In th work, a modified probe based on multiplex - refractive index into account, which makes resulting magnetic field measurements unstable structure put forward for temperature compensation, which solves cross-sensitivity effect inaccurate. In th work, a modified probe based on multiplex - structure put temperature magnetic field in MF. The written on insert fiber end forward for temperature compensation, which solves cross-sensitivity effect temperature cavity, it sensitive to temperature variation but insensitive to magnetic field changes. The magnetic field in MF. The written on insert fiber end cavity, it resulting - structure experimentally demonstrated for high resolution magnetic field sensitive to temperature variation but insensitive to magnetic field changes. The resulting - detection. structure experimentally demonstrated for high resolution magnetic field detection Principles Principles - - Sensor Sensor Filled Filled Magnetic Magnetic Fluid Fluid The The multiplexing multiplexing structure structure based based on on - - shown shown in in Figure Figure 1, 1, where where written written on on insert insert fiber fiber end end cavity. cavity. When When a broadb a broadb Iin Iin transmits transmits through through,, near near Bragg Bragg reflection reflection wavelength wavelength I1. I1. Then Then transmsion transmsion I2 I2 reaches reaches cavity. cavity. In In a low-reflectivity low-reflectivity cavity, cavity, interference interference spectrum spectrum I3 I 3 generated generated transmitted transmitted back back through through once once again. again. Finally, Finally, Iout I out from from - - combined combined second second transmsion transmsion I4 I 4 former former I 1, I1, output output - - Iout I out can can be be obtained obtained as as below: below: 2 Iout I1 I4 Iin [ f (1 f ) I out 1 ` 4 I in r f ` p1 f q 2 ff f P ] (1) F P s (1) 2 2 where, f reflection coefficient it defined as f R exp[ ( B ) where, f reflection coefficient it defined as f R expr pλ λ B q 2 / c ], R,,c {c 2 s, R c are Bragg peak reflectivity, center- wavelength, bwidth are Bragg peak reflectivity, center- wavelength, bwidth peaks. Accordingly, ff P 2 r [1 cos(4 L/ )] reflection coefficient peaks. Accordingly, f F P 2r r1 ` cosp4πl{λ ` πqs reflection coefficient cavity, cavity, r reflectivity r reflectivity fiber end, fiber L end, length L length cavity. cavity. Figure 1. The structure a -. Meanwhile, resonant peak wavelengths should be set in a proper measurement wavelength-range from input both m can be dtinguhed through one anor. In order to maintain interference fringes peaks at a same

3 Sensors 2016, 16, Meanwhile, resonant peak wavelengths should be set in a proper Sensors measurement 2016, 16, 620 wavelength-range from input both m can be dtinguhed through 3 10 one anor. In order to maintain interference fringes peaks at a same order magnitude, reflectivity should be set to close to that low fitness. order magnitude, reflectivity should be set to close to that low fitness. Figure 2 shows spectrum simulation -. Figure shows spectrum simulation -. Reflectance / mw T=20 C T=30 C T=40 C T=50 C Wavelength / nm Figure 2. Simulation output spectrum under different temperatures (H = 0). In simulation, length cavity cavity set setas asl L = = μm, µm, center- wavelength wavelength at at room room temperature C C λ B B nmnm a reflectivity 4%. The incident 1 mw a spectrum range 1525 nm 1565 nm. The simulated spectrum - dplayed clearly combination two spectra at a same spectral resolution spectra moves through one anor out interference. As temperature increases from 20 C C to 50 C, C, interference spectrum shifts towards short wavelength direction, however, center-reflecting wavelength moves moves towards towards along along long long wavelength direction. direction. In Figure In Figure 2, 2, amount amount movement movement resonant resonant peaks peaks less than less than that that interference interference fringes. fringes. With reference to research results Chen et et al. al. [21], [21], under a constant a temperature value value T, T, relationship between refractive index index MF MF magnetic field field as below: as below: 1 n MF nmf pn ( s n s n 0 qrcothpα )[coth( H Hc ) 1 q ] n αph H c q s ` n 0 (2) 0 0 T ( H Hc ) where, Hc critical value applied magnetic field (H ( Hą Hc), nn0 0 refractive index MF critical magnetic field ns s saturated refractive index MF, α a fitting coefficient. Generally, parameters ns, n s, n0, n 0, α, Hc for a certain kind magnetic fluid film are regarded as constants. Therefore, at a certain experimental temperature T, re only one refractive index for MFnMF n MF for for a a certain magnetic field field H. The H. The refractive refractive index index MF nmf ntested MF tested under under different different magnetic magnetic field field temperature conditions, experimental results are used for magnetic field detection temperature compensation. In probe a - structure, written on insert fiber end cavity. Being inspired by by concept that that central central wavelength shifts shifts are applied are applied to detect to detect external external varying varying temperature, temperature, central wavelength central wavelength shift shift can be defined can by be Equation defined (3): by Equation (3): λ B λ B pβ Tn ` β Tl q T (3) B B Tn Tl T (3) where, β Tn, β Tl are rmal-optical effect coefficient 8.0 ˆ 10 6 / C rmal expansion effect where, βtn, βtl are rmal-optical effect coefficient / C rmal expansion effect coefficient 0.55 ˆ 10 6 / C in. T variation value operating temperature. It coefficient / C in. T variation value operating temperature. It noted that central wavelength shift not sensitive to magnetic field applied on noted that central wavelength shift not sensitive to magnetic field applied on -, which used for measurement magnetic field temperature compensation. -, which used for measurement magnetic field temperature compensation. Due to cross-sensitivity effect temperature magnetic field in MF, charactertic magnetic fluid refractive index are defined as follows: n MF α Hn H ` α Tn T (4)

4 Sensors 2016, 16, where n MF change magnetic fluid refractive index, α Hn, α Tn are magnetic field sensitive coefficient temperature sensitive coefficient magnetic fluid, respectively. Considering rmal expansion effect optical fiber, central wavelength shift in output spectrum - magnetic given by Equation (5): λ m λ m rα Hn H ` pα Tn ` α Tl q Ts (5) where, λ m central wavelength shift - magnetic, α Tl optical fiber rmal expansion effect coefficient. Combining Equations (3) (5), a relationship matrix between output wavelength shifts two under-test parameters obtained in Equation (6). Equation (7) achieves measurement temperature magnetic field through calculating matrix in Equation (6): «λ m λ B ff «α Hn λ m pα Tn ` α Tl q λ m 0 pβ Tn ` β Tl q λ B ff «H T ff (6) «H T ff «α Hn λ m pα Tn ` α Tl q λ m 0 pβ Tn ` β Tl q λ B ff 1 «λ m λ B ff (7) Obviously, when spectrum drifts temperature detected by are measured in an experimental system, n change magnetic field H can be obtained by matrix in Equation (7). According to Equation (7), MF-filled - can achieve simultaneous measurement magnetic field temperature, temperature detected in real-time can be used for compensation magnetic field measurement. Finally, cross-sensitivity effect temperature magnetic field in magnetic fluid eliminated. 3. Experiment System Analys The magnetic field measurement system setup shown in Figure 3. The output from a 1550 nm DFB injected into optical fiber circulator via port1, it transmits into MF-filled - for magnetic field temperature detection. The programmable DC controls power output electric current to generate a stable magnetic field through a set coils. When applied magnetic field varies, refractive index MF changes signal modulated. The passes through port 2 to port 3 it detected by spectrometer (AQ6370 OSA, YOKOGAWA, Tokyo, Japan, a wavelength resolution 0.02 nm). The MF-filled - located in a temperature control oven (FLYSET Temperature Controller, Shenzhen Dragon Born Hot Runner Technology Co., LTD, Shenzhen, China, a temperature resolution 1 C) for tests at different stable temperatures, rmometer put close to head to detect operating temperature. In addition, a Gauss meter also located near head to measure varying magnetic fields. The temperature variation detected by rmometer magnetic field variation detected by Gauss meter are taken to compare measurement results MF-filled -. As shown in inset Figure 3, MF-filled - has a length 10 mm a refractive index R = 4% ( grating wavelength 1550 nm). The length MF filled in cavity fabricated 32 µm. First, fiber end written a inserts into capillary tube; after fiber aligned capillary it observed under a microscope; MF (EMG605, Ferrotec USA Corporation, Bedford, NH, USA) a volume content C = 1.8% infiltrated into capillary tube. Finally, fiber inserted into capillary tube sealed UV glue under ultraviolet irradiation for 24 h.

5 Sensors 2016, 16, Sensors 2016, 16, Sensors 2016, 16, Figure Experimental setup configuration for for - sensing system. Inset: cross-section schematic Figure diagram 3. Experimental setup sensing sensing configuration head. head. for - sensing system. Inset: cross-section schematic diagram sensing head. To illustrate operation, optical fiber end face reflection method based on Fresnel To illustrate operation, optical fiber end face reflection method based on Fresnel reflection principle performed to measure charactertics refractive index MF under reflection principle performed to measure charactertics refractive index MF under different magnetic fields temperatures, as shown as shown in Figure in Figure 3. The 3. refractive The refractive index index MF under MF different magnetic fields temperatures, as shown in Figure 3. The refractive index MF parallel under parallel magnetic magnetic fields behaves fields behaves as shown as in Figure 4a, which 4a, which shows shows that nthat MF = nmf under parallel magnetic fields behaves as shown in Figure 4a, which shows that = out nmf = out any applied any applied any magnetic applied magnetic magnetic field. field. field Refractive index Refractive index Refractive index Magnetic field/t Magnetic (a) field/t Refractive index (a) Temperature/ (b) Figure 4. (a) Magnetic fluid refractive index variation Temperature/ versus transverse magnetic field; (b) Magnetic Figure 4. (a) Magnetic fluid refractive index variation versus transverse magnetic field; (b) Magnetic fluid refractive index variation versus temperature. fluid refractive index variation versus temperature. (b) Figure 4. (a) Magnetic fluid refractive index variation versus transverse magnetic field; (b) Magnetic fluid refractive index variation versus temperature.

6 Sensors 2016, 16, When H less than 20 mt, nmf increases gradually while variation very little. With H increases Whenfrom H 20 lessmt than to mt, mt, n MF increases corresponding gradually while variation very little. With nmf increases from to high Hsensitivity, increaseswhich from 20 plays mtan to important 60 mt, role corresponding determining n MF increases effective from operating magnetic to field range high sensitivity, in experiment. plays Although an important H continues role into determining increase from 60 effective mt, operating magnetic nmf does not change obviously, in which experiment. indicates that Although H continues increase from 60 mt, n MF does not change obviously, nmf has reached its saturation value under strong magnetic field. Therefore, which MF indicates has an effective that n MF working has reached magnetic its saturation field range value in under experiment, strong magnetic range field. Therefore, magnetic field MF H has set anfrom effective 0 mt working to 40 mt. magnetic When field direction range in magnetic experiment, field transverse range to magnetic direction field Hincident set from, 0 mt to 40 mt. polarizability When direction MF increases magnetic fieldincrease transverse tomagnetic direction field (magneto-electric incident, effect). The change polarizability MF increases increase magnetic field nmf similar to performance MF polarizability, it (magneto-electric dependent on effect). counter-direction The change between n MF similar applied to magnetic performance field MF polarizability, transmitting through dependent MF. on When counter-direction magnetic field between set as 0 applied T, refractive magneticindex field MF shown transmitting in Figure through 4(b). As MF. temperature When varies magnetic from field set as 0 T, MF shown in Figure 4b. As varies from 0 C 0 C to 70 C, to 70 C, refractive index MF decreases linearly a temperature sensitivity RIU/ C. refractive When index MF simultaneously MF decreasessubjected linearly to an a temperature operating magnetic sensitivity field a changing RIU/ C. temperature, When MF simultaneously refractive index subjected to an MF operating behaves magnetic according field to both a changing m, making temperature, it hard to determine refractive index refractive index MF behaves MF according as it responds to both to m, making changing it hard magnetic to determine field or changing refractive temperature index MF asth it responds will lead to to an inaccurate changing magnetic field detection. or changing Hence, temperature it necessary th to take willeffective lead to an measures inaccurate to compensate magnetic field detection. operating Hence, temperature it necessary in magnetic to take field effective measurement. measures With to compensate PW*@work temperature operating temperature in experiment in magnetic system field measurement. With PW*@work temperature in experiment system set to 25 C, set to 25 C, - filled MF located in a temperature control oven between a set - coils. The magnetic filled field MF direction located in transverse a temperature to that control oven incident between. aas set coils. applied The magnetic magnetic field fieldcontrolled directionby transverse programmable to that DC power incident increases. As from 0 applied mt to magnetic mt, field normalized controlled by interference programmable spectrum DC power - increases fromshift 0 mt towards mt, long normalized wavelength interference direction (near spectrum wavelength nm) as shift shown towards in Figure 5. long wavelength direction (near wavelength 1550 nm) as shown in Figure 5. Normalized Intensity mT 5.05mT 8.32mT 12.07mT 18.09mT 22.01mT 27.03mT Wavelength(nm) Figure 5. Spectrograms under a magnetic field range 0~30 mt. Figure 5. Spectrograms under magnetic field range 0~30 mt. Figure 6 gives - test results under two magnetic fields. At room temperature (25 C) C) it it can be be obtained that that interference fringes fringes move move magnetic magnetic field field variation variation but but peak peak unchanged. unchanged. The result in Figure 6 in good agreement conclusion that written in insert end cavity insensitive to magnetic field changes. The tests ( test was repeated three times) for a MF-based - under different magnetic fields are shown in Figure 7, where curve fitting relationship between resonance peak peak drifts drifts magnetic magnetic field field indicates indicates MF-based based - has a magnetic has a magnetic field sensitivity field sensitivity 0.34 nm/mt 0.34 nm/mt a repeatabilitya repeatability error error R 2 = 1.5%. R 2 = - 1.5%. The interference spectrum MF-filled - which shifts under different temperature behaves as seen in Figures 8 9.

7 Sensors 2016, 16, Sensors 2016, 16, Sensors 2016, 16, Light Light /nw /nw T, T, 0.03T, T, Figure 6. Spectrograms under a magnetic field range 0~30 mt. Figure 6. Spectrograms under a magnetic field range 0~30 mt Test1 Test1 Test3 Average value curve fitting Test3 Average value curve fitting y= x, r=0.9954, Repeatability error:1.5% y= x, r=0.9954, Repeatability error:1.5% Magnetic field/t Magnetic field/t Figure 7. peak wavelength variation versus magnetic field. Figure 7. peak wavelength variation versus magnetic field. The interference spectrum MF-filled - which shifts under different temperature The Considering interference behaves that as spectrum seen output in Figures spectrum 8 MF-filled 9. MF-filled - - which shifts combined under different temperature spectra behaves as seen in Figures 8 9. ir resonance peak shifts, analys 1533 interference spectrum - under different temperatures can be equally separated 1533 Test into those. As temperature increases, refractive index Test1 Test MF decreases output interference spectrum Average value curve fittingshifts in a shorter wavelength 1531 Test3 direction. When resonance peak moves Average in value curve fitting temperature range 20 C < T < 1531 y= x,r= C, MF-filled has a temperaturerepeatability sensitivity error:0.8% nm/ C a repeatability y= x,r=0.995 error R 2 = 0.8%. As illustrated 1530 in Figure 9, center wavelength Repeatability error:0.8% shifts less than that 1529, has a temperature sensitivity nm/ C a repeatability error 1529 R 2 = 1.2% Finally, when magnetic 1528 field temperature are applied to MF-filled -, 1527 corresponding magnetic field temperature can be calculated from Equation (7) combined 1527 analys Figures 7 9. In1526 system, temperature measurement can be used as a compensation for magnetic field detection 1526 by -Temperature/ filled MF. According to Equation (7), a magnetic field sensitivity 0.34 nm/mt, temperature Temperature/ sensitivity nm/ C in MF-filled Figure 8. peak wavelength shift variation versus temperature. a temperature sensitivity nm/ C in can be obtained: Figure 8. peak wavelength shift variation versus temperature. «ff «ff 1 «ff H 0.34 nm{mt nm{ C λ m (8) T nm{ C λ B

8 Figure 6. Spectrograms under a magnetic field range 0~30 mt Sensors 2016, 16, 620 Test Test1 Average value curve fitting y= x, r=0.9954, Based on sensing charactertic Repeatability matrix error:1.5% in Equation (8) - magnetic, MF-filled - (after compensation) MF-filled (before compensation) are 1545 simultaneously tested in magnetic field range 0.02T < H < 0.06T. First, temperature in head area accurately measured by peak wavelength ; second, wavelength - varying magnetic field modified according to sensing charactertic matrix in Equation (8) magnetic field temperature compensation obtained in Figure 10. Magnetic field/t Figure 10 illustrates fitted curves based on test points. The magnetic field sensitivity after Figure 7. peak wavelength variation versus magnetic field. compensation improved to 0.53 nm/mt comparon magnetic field sensitivity before compensation The interference 0.23 nm/mt. spectrum As wavelength MF-filled - measurement which resolution shifts under OSAdifferent 20 pm, magnetic temperature field measurement behaves seen resolution in Figures 8-9. could reach 37.7 µt Test1 Test3 Average value curve fitting y= x,r=0.995 Repeatability error:0.8% Temperature/ Figure Figure peak peak wavelength shift variation versus versus temperature. Sensors 2016, 16, 620 temperature Test1 Test3 Average value curve fitting y= x, r=0.993 Repeatability error:1.2% Temperature/ Figure Figure wavelength shift variation versus temperature. Sensors 2016, 16, Considering that output spectrum MF-filled - combined spectra 1570 ir resonance peak shifts, analys interference spectrum - under different temperatures can be equally separated into 1565 Before compensation those. As temperature increases, refractive index MF After compensation decreases output 1560 interference spectrum shifts in a shorter wavelength direction. When resonance peak moves in temperature range 20 C < T < C, MF-filled - has a temperature sensitivity nm/ C a repeatability error R 2 = 0.8%. As illustrated 1550 in Figure 9, center wavelength shifts less than that, has a temperature sensitivity nm/ C a repeatability error R = 1.2%. Finally, when magnetic 1540 field temperature are applied to MF-filled -, corresponding magnetic 1535 field temperature can be calculated from Equation (7) combined analys Figures 7 9. In system, Magnetic field/t temperature measurement can be used as a compensation for magnetic field detection by - filled MF. According to Equation Figure Figure 10. (7), a The 10. magnetic test The comparon test in MF-filled -. field sensitivity magnetic 0.34 nm/mt, field sensitivity temperature in sensitivity MF-filled nm/ C. in MF-filled a temperature sensitivity nm/ C in can be obtained: 4. Conclusions Based on charactertics 1 H refractive nm /mt index nm a MF, / a fiber optic m - magnetic field proposed demonstrated T for magnetic field measurements nm/ temperature (8) B compensation. The key point in magnetic field measurement to overcome cross-sensitivity effect temperature magnetic field in MF. The written on insert fiber cavity Based for on operating sensing temperature charactertic detection. matrix in Due Equation to fact (8) - sensitive magnetic to temperature, MF-filled - (after compensation) MF-filled (before compensation) are

9 Sensors 2016, 16, Conclusions Based on charactertics refractive index a MF, a fiber optic - magnetic field proposed demonstrated for magnetic field measurements temperature compensation. The key point in magnetic field measurement to overcome cross-sensitivity effect temperature magnetic field in MF. The written on insert fiber cavity for operating temperature detection. Due to fact sensitive to temperature variations, operating temperature in sensing area can be measured compensated for magnetic field measurement. Preliminary experimental results show that sensitivity magnetic field measurement could reach 0.53 nm/mt magnetic field measurement resolution could reach 37.7 µt. The - magnetic field probe has advantages simple structure, easy fabrication, anti-corrosion properties, low cost, so on, th would find potential applications in measurement electromagnetic fields. Acknowledgments: Th work was supported by National High Technology Research Development Program China (863 Program) under Grant 2013AA09A412-1, Major Application Basic Research Project NUDT under Grant number ZDYYJCYJ , National Natural Science Foundation China under Grant , Specialized Research Fund for Doctoral Program Higher Education China under Grant Author Contributions: J.X. has developed EF-DCM algorithm, conceived designed MF-filled - experiments drafted paper. And he played an important role in interpreting results. F.W. has performed experiments, contributed experiment materials analys tools, contributed significantly to acquition data, analys interpretation data. Both J.X. F.W. have contributed equally to th work. H.L. S.X. has contributed to conception study, helping perform analys constructive dcussions. Q.W. S.X. has contributed to manuscript preparation, reving it critically for important intellectual detail final approval version to be submitted. Conflicts Interest: The authors declare no conflict interest. References 1. Layeghi, A.; Latifi, H.; Frazao, O. Magnetic Field Sensor Based on Nonadiabatic Tapered Optical Fiber Magnetic Fluid. IEEE Photonics Technol. Lett. 2014, 26, [CrossRef] 2. Huang, Y.; Li, D.; Li, F.; Zhu, Q.; Xie, Y. Transmitted relaxation microstructure evolution ferrluids under gradient magnetic fields. Opt. Commun. 2015, 338, [CrossRef] 3. Agruzov, P.M.; Pleshakov, I.V.; Bibik, E.E.; Stepanov, S.I.; Shamrai, A.V. Transient magneto-optic effects in ferrluid-filled microstructured fibers in pulsed magnetic field. Europhys. Lett. 2015, 111. [CrossRef] 4. Gao, R.; Jiang, Y.; Abdelaziz, S. All-fiber magnetic field s based on magnetic fluid-filled photonic crystal fibers. Opt. Lett. 2013, 38, [CrossRef] [PubMed] 5. Shaohua, D.; Shengli, P.; Haotian, W. Magnetic field sensing based on magnetic-fluid-clad fiber-optic structure taper-like lateral-fset fusion splicing. Opt. Express 2014, 22, Wang, J.; Zhou, C.; Fan, D.; Ou, Y. Fiber optic magnetic field based on magnetic fluid etched Hi-Bi fiber loop mirror. In Proceedings OFS rd International Conference on Optical Fiber Sensors, Cantabria, Spain, 2 6 June Homa, D.; Pickrell, G. Magnetic Sensing Ferrluid Fiber Optic Connectors. Sensors 2014, 14, [CrossRef] [PubMed] 8. Zhao, Y.; Lv, R.-Q.; Ying, Y.; Wang, Q. Hollow-core photonic crystal fiber fabry-perot for magnetic field measurement based on magnetic fluid. Opt. Laser Technol. 2012, 44, [CrossRef] 9. Ciani, A.; Argyros, A.; Leon-Saval, S.G.; Lwin, R.; Selleri, S.; Psadak, S. A loss-based, magnetic field implemented in a ferrluid infiltrated microstructured polymer optical fiber. Appl. Phys. Lett. 2014, 104, Dai, J.; Yang, M.; Li, X.; Liu, H.; Tong, X. Magnetic field based on magnetic fluid clad etched fiber Bragg grating. Opt. Fiber. Technol. 2011, 17, [CrossRef] 11. Childs, P.; Ciani, A.; Psadak, S. Optical Fiber Cladding Ring Magnetic Field Sensor. IEEE Photonics Technol. Lett. 2011, 23, [CrossRef]

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