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Fiber-optic magnetic field sensor using magnetic fluid as the cladding
Sensors and Actuators A 236 (2015) 67–72
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Fiber-optic magnetic field sensor using magnetic fluid as the cladding Longfeng Luo, Shengli Pu ∗ , Shaohua Dong, Jiali Tang College of Science, University of Shanghai for Science and Technology, Shanghai 200093, China
a r t i c l e
i n f o
Article history: Received 12 December 2014 Received in revised form 24 September 2015 Accepted 20 October 2015 Available online 27 October 2015 Keywords: Magnetic fluid Magnetic field sensing Mode interference
a b s t r a c t A kind of fiber-optic magnetic field sensor is proposed. The sensing structure is composed of singlemode–multimode–singlemode fiber structure cascaded with core-offset fusion splicing between singlemode fibers. The sensing principle is based on the cladding mode interference. Experimental results indicate that, the magnetic field sensing sensitivities of 65.9 pm/Oe and 0.1185 dB/Oe are obtained for wavelength interrogation and intensity interrogation, respectively. The corresponding measurement range of magnetic field strength is 30–110 Oe. The response of temperature is also investigated. The corresponding temperature sensitivities for the interference dip around 1594 nm is obtained to be −42 pm/◦ C and −0.124 dB/◦ C. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Magnetic fluid (MF) is a kind of stable colloidal system consisting of surfactant-coated magnetic nanoparticles dispersed in a suitable liquid carrier [1]. It possesses both the features of magnetic property of solid magnetic materials and fluidity of liquids. Thus, MF presents versatile magneto-optical properties, such as tunable refractive index (RI) [2], magneto-volume variation [3] and magneto-dielectric anisotropy [4]. Until now, various MF-based optical devices have been proposed, such as modulators [5], optical switches [6], optical gratings [7,8], tunable slow light [9], current sensor [10] and magnetic field sensors [11,12]. Recently, magnetic field measurements gradually become significant in various areas including military, aviation industry, biomedical applications, and vehicle detection [13]. Comparing with other magnetic field sensors, MF-based optical fiber magnetic field sensors have attracted particular interest due to their fiber compatibility, compactness, and high sensitivity [14,15]. Various structures have been employed to realize the sensing function, for example, fiber structures with up-tapered joints [16], singlemode–multimode–singlemode (SMS) fiber structures [17,18], singlemode fiber (SMF) Michelson interferometer [19], photonic crystal fiber (PCF) [20,21], and polarization-maintaining PCF [22]. In this work, a novel magnetic field sensing structure has been designed. The proposed structure consists of a very short section of
∗ Corresponding author. Fax: +86 21 65667144. E-mail addresses: [email protected], [email protected] (S. Pu). http://dx.doi.org/10.1016/j.sna.2015.10.034 0924-4247/© 2015 Elsevier B.V. All rights reserved.
multimode fiber (MMF) spliced between two pieces of SMF and a lateral-offset fusion splicing between the main structure and the lead-out SMF. The fundamental sensing principle is based on the cladding modes interference. The sensitivity is high due to the high sensitivity to external stimuli for the cladding modes. Comparing with other structures, the simple fusion splicing technique is employed with the proposed structure. So it possesses the advantages of compact size, low cost, ease of fabrication and high sensitivity. While the previous structures usually include complicated fabrication process (e.g., tapering [14], corroding [17], microfabrication [19]) or are costly (especially for the PCF based structures).
2. Experiments and principles Fig. 1 shows the schematic diagram of the proposed sensing structure. The lead-in SMF1 is firstly spliced with a piece of MMF. Then, the free end of MMF is carefully cleaved and the length of left MMF segment is 1 mm. The obtained SMF–MMF structure is spliced with a SMF2 and the other end of SMF2 is carefully cleaved. The length of left SMF2 is 43 mm. Finally, the lead-out SMF3 is fusion spliced with the SMF2 with laterally core-offset of 30 m. A microscope with a digital CCD camera is utilized to acquire the microscopic images for the SMF–MMF and lateral-offset joints as shown in the low panel of Fig. 1. The whole sensing structure is positioned in a capillary filled with MF. The inner diameter of the capillary is about 0.9 mm. Both ends of the capillary are sealed with UV glue to avoid MFs leaking or evaporating. The MMF (SFS 105/125Y) utilized is provided by Thorlabs Inc. Its core and cladding diameters are 105 and 125 m, respectively. Through trial and
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where I0 is the intensity of fundamental mode LP01 in SMF1 and N is the total number of excited modes in the cladding of SMF2 . i or j is the serial number representing the excited mode. i and j are the coupling coefficients of LP0i and LP0j modes, respectively. nij is the effective RI difference between the involved interference modes. L is the length of SMF2 . According to Eq. (1), the wavelength corresponding to the interference valley can be expressed by m =
Fig. 1. Schematic of the proposed sensing structure. The low panels from left to right show the microscopic image of the SMF–MMF joint, the sealed sensing structure and the microscopic image of the lateral-offset splicing joint, respectively.
Fig. 2. Experimental setup for investigating the magnetic field sensing properties of the proposed structure.
error, the 1 mm length of MMF (utilized in the current work) is sufficient to expand the mode field efficiently and cladding modes will be excited within the cladding of SMF2 . The SMF (G.652D LWPF) is provided by Beijing CFYC Communication Technology Co. Ltd. The water-based Fe3 O4 MFs provided by Beijing Sunrise Ferrofluid Technological Co. Ltd. are utilized in our experiments. The density and saturation magnetization of the MF are 1.18 g/cm3 (25 ◦ C) and ∼200 Oe, respectively. The diameter of the magnetic nanoparticles within the MFs is around 10 nm. The experimental setup for investigating the magnetic field sensing properties is shown in Fig. 2. Light from the broadband amplified spontaneous emission source (ASE, wavelength ranging from 1525 to 1610 nm) is coupled into the sensing structure. The transmission spectrum is recorded and analyzed by the optical spectrum analyzer (OSA, AQ6370C). The resolution of the OSA is set at 0.02 nm for all experiments. The sensing structure is placed between two poles of an electromagnet, which generates a uniform magnetic field with nonuniformity of less than 0.1% within the sensing region. The strength of the magnetic field is adjusted by tuning the magnitude of the supply current. The magnetic field direction is perpendicular to the optical fiber axis. During our experiments, the ambient temperature is kept at 25 ◦ C. When light from SMF1 approaches the MMF, the fundamental core mode LP01 will spread out widely and then the highorder cladding modes LP0j will be excited within the cladding of SMF2 . These high-order cladding modes will interfere with each other. The interference results will be coupled into the core of SMF3 . Because the lateral-offset distance between SMF2 and SMF3 (30 m) is much larger than the SMF core diameter (9 m), there is no overlap between the core mode fields of SMF2 and SMF3 . The interference between the core mode and cladding mode cannot occur within the SMF3 core. SMF3 only receives and transmits the interference happening between cladding modes with different orders. The transmission spectrum of the whole structure can be expressed as [12,15,19,23]
I () =
N i=1
2i × I0 () +
N
i × j × I0 () × cos 2nij
i= / j=1
L
(1)
2nij L (2m + 1)
(2)
where m is the interference order. If the order of stimulated cladding mode is relatively high, the surrounding refractive index (SRI) variation can exert a considerable influence on the mode field distribution. In this case, the variation trend of effective RI is mainly related with the variation trend of mode field fraction inside the fiber. The radius of mode field will increase with SRI. Then, the fraction of mode field inside the fiber will decrease with the increase of SRI. This will result in the decrease of effective RI with the increment of SRI. If the order of stimulated cladding mode is relatively low, the mode field distribution is hardly affected by the variation of SRI. The variation trend of effective RI is mainly related with the variation trend of SRI. This will result in the increase of effective RI with the increment of SRI. Moreover, the SRI variation can affect the transmission loss of the proposed structure according to Eq. (1). Considering the magnetically tunable RI of MF, the valley wavelength and transmission loss may be sensitive to the external magnetic field when using MF as the cladding of the structure. Thus, magnetic field measurement can be realized by detecting wavelength shift and transmission loss of the interference spectrum. On the other hand, if the lateral-offset is relatively small, the interference probably happens between core and low-order cladding modes. Then, the effective RI difference between the involved interference modes is relatively insensitive to the ambient stimuli. So the sensitivity is reduced. 3. Results and discussion Before proceeding with investigating the magnetic field sensing properties, the response and applicability of our sensing structures in a wide range of RI variation will be characterized firstly. The glycerol–water solutions with different mass fractions are utilized. Their RIs are given in Ref. [14]. The transmission spectra of the sensing structure surrounded with glycerol–water solutions of different concentrations are shown in Fig. 3. Three distinct interference valleys are observed in the wavelength range of 1525–1610 nm. Fig. 3 indicates that all the interference valleys shift to long wavelength side with the RI (viz. concentration) increase of surrounding liquids. The red shift of wavelength valley implies that nij increases with SRI. Thus, the experimental phenomena may be assigned to the interference between low-order and high-order cladding modes. According to the inset of Fig. 3, the maximum sensitivity of RI measurement is achieved to be 433 nm/RIU when RI approaching 1.44. To investigate the magnetic field sensing properties, MF is used as the cladding of the proposed structure. Fig. 4 shows the typical transmission spectra of the sensing structure immersed in MF at different external magnetic fields. It is clear from Fig. 4 that the wavelength and intensity of the interference dip increase with the magnetic field. The explicit results are plotted in Fig. 5. Figs. 4 and 5 show that the dip wavelength shifts monotonically from 1593.5 to 1599.5 nm when magnetic field changes from 0 to 110 Oe. For magnetic field beyond 110 Oe, the interference valley becomes unobvious. Fig. 5 implies that the dip wavelength is almost linearly dependent of the magnetic field at moderate strengthen (30–110 Oe). The corresponding sensitivity is obtained to be 65.9 pm/Oe. Besides,
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Fig. 3. Transmission spectra of the proposed sensing structure surrounded with glycerol–water solutions of different concentrations. The inset shows the corresponding dip wavelength as a function of RI of the surrounding liquids.
Fig. 4. Transmission spectra of the proposed sensing structure immersed in MF at different magnetic field strengths.
Fig. 5 indicates that the intensity at dip wavelength increases monotonically from −65.9 to −54.7 dB when H changes from 0 to 130 Oe. Similar to the dip wavelength shift, a good linear relationship between the intensity variation and magnetic field is obtained for magnetic field ranging from 30 to 110 Oe. The corresponding sensitivity is 0.1185 dB/Oe. The obtained sensitivities of the proposed structure are the same order of magnitude for the optimized SMS (90.5 pm/Oe @40–100 Oe) [2] and SMF taper (56 pm/Oe@20–200 Oe) structures [14], 3 times higher than that of MMF–SMF–MMF structure (21.5 pm/Oe @0–120 Oe) [24], 4 times larger than that of nonoptimized SMS structure (16.7 pm/Oe@0–325 Oe) [17], about 10 times larger than that using MF as the cladding of SMF-based
Michelson interferometer (6.5 pm/Oe @0–2000 Oe) [19], and about 28 times higher than that using MF as the cladding of PCF (2.4 pm/Oe @0–200 Oe) [12]. The achieved sensitivity of the as-fabricated structure is lower than that of microfiber taper structure (171 pm/Oe@20–70 Oe) [25], but the complicated and sophisticated microfabrication technique is necessary for realizing microfiber taper structure. In addition, we would like to point out the sensing performance of the proposed structure can be enhanced further through optimizing the design, such as changing the thickness of MF film around MMF and the radius of SMF2 [2]. To investigate the hysteresis of magnetic field senor, repeated experiments are conducted when applying magnetic field at ascending and descending orders for several cycles. The
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Fig. 5. Wavelength and intensity of the interference dip as functions of magnetic field.
Fig. 6. Wavelength and intensity of the interference dip at different magnetic field strengths when applying magnetic field at ascending and descending orders.
measurement interval between each data is 5 min. Hysteresis effect between ascending and descending magnetic fields is observed for every cycle. The typical results are plotted in Fig. 6. Therefore, interrogating the dip wavelength and intensity simultaneously is necessary for sensing the magnetic field solely. Otherwise, additional calibration method is needed, such as simultaneous measurements using two devices with different sensitivities. This hysteresis effect is mainly assigned to the viscosity of MF, magnetization (demagnetization) and agglomeration (deaggregation) of magnetic nanoparticles within the MF, which can be reduced by
decreasing the sweep rate of magnetic field or using thinner MF film with lower viscosity. To investigate the temperature response of the sensor, the experiments are conducted at several ambient temperatures. The wavelength shift and transmission loss of the interference valleys at different temperatures are shown in Fig. 7. The temperature sensitivities are obtained to be −42 pm/◦ C and −0.124 dB/◦ C for the wavelength and transmission loss interrogation, respectively. It is well-known that the RI of MF is temperature-dependent. Therefore, the effective RI difference nij between the involved interference modes will change with ambient temperature. This will lead to the
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Fig. 7. Wavelength and intensity of the interference dip as functions of temperature.
dip wavelength shift and corresponding dip intensity variation with temperature according to Eqs. (1) and (2). The temperature sensitivities are comparable to those of magnetic field sensing. These are contributed to the relatively large thermo-optic coefficient of MF (about −2.4 × 10−4 /K), which is two orders of magnitude larger than that of pure silica. Therefore, the cross-sensitivity of temperature may be worth further considering for certain applications. To solve the cross-sensitivity of temperature and magnetic field, the two-parameter matrix method is proved to be effective [26]. According to our experimental results, a well-conditioned system of two equations can be given in a matrix form as follows
H
=
T
1 K,H KI,T − K,T KI,H
KI,T
−K,T
−KI,H
K,H
I
(3)
where H and T are the variations of magnetic field intensity and ambient temperature, respectively. K,H and K,T are the sensitivities of resonant wavelength shift with respect to magnetic field and temperature, respectively. KI,H and KI,T are the sensitivities of interference valley transmission with respect to magnetic field and temperature, respectively. The values of K,H , K,T , KI,H and KI,T for the as-fabricated structure have been obtained from the above-mentioned experiments. So, Eq. (3) can be explicitly written as
H T
1 = −3.1946
−0.124
42
−0.1185
65.9
I
(4)
Using Eq. (4), the as-fabricated sensing device can be employed to measure magnetic field intensity accurately with dynamic temperature compensation. 4. Conclusion In summary, a kind of optical fiber magnetic field sensor based on MF-clad fiber structure has been proposed and investigated. Through cascading the SMS structure with a SMF core-offset fusion splicing, the interference between cladding modes happens. Magnetic field sensing is achieved by monitoring wavelength shift or
intensity variation of the interference valley. The magnetic field sensing sensitivities of 65.9 pm/Oe and 0.1185 dB/Oe are obtained for magnetic field strength ranging from 30 to 110 Oe. The temperature effect of the sensing structure is also investigated. The temperature sensitivity can reach −42 pm/◦ C and −0.124 dB/◦ C. The proposed sensing structure has the advantages of low-cost, compactness, and high sensitivity. Acknowledgments This research is supported by Natural Science Foundation of Shanghai (Grant no. 13ZR1427400), Shanghai Key Laboratory of Specialty Fiber Optics and Optical Access Networks (Grant no. SKLSFO2014-05) and Hujiang Foundation of China (Grant no. B14004). References [1] R.E. Rosensweig, Ferrohydrodynamics, Cambridge University Press, New York, 1985. [2] Y. Chen, Q. Han, T. Liu, X. Lan, H. Xiao, Optical fiber magnetic field sensor based on single-mode-multimode-single-mode structure and magnetic fluid, Opt. Lett. 38 (2013) 3999–4001. [3] S. Dong, S. Pu, J. Huang, Magnetic field sensing based on magneto-volume variation of magnetic fluids investigated by air-gap Fabry-Pérot fiber interferometers, Appl. Phys. Lett. 103 (2013) 111907. [4] P.M. Agruzov, I.V. Pleshakov, E.E. Bibik, A.V. Shamray, Magneto-optic effects in silica core microstructured fibers with a ferrofluidic cladding, Appl. Phys. Lett. 104 (2014) 071108. [5] H.E. Horng, J.J. Chieh, Y.H. Chao, S.Y. Yang, C.Y. Hong, H.C. Yang, Designing optical-fiber modulators by using magnetic fluids, Opt. Lett. 30 (2005) 543–545. [6] S. Xia, J. Wang, Z. Lu, F. Zhang, Birefringence and magneto-optical properties in oleic acid coated Fe3 O4 nanoparticles: application for optical switch, Int. J. Nanosci. 10 (2011) 515–520. [7] A. Candiani, W. Margulis, C. Sterner, M. Konstantaki, S. Pissadakis, Phase-shifted Bragg microstructured optical fiber gratings utilizing infiltrated ferrofluids, Opt. Lett. 36 (2011) 2548–2550. [8] S. Pu, X. Chen, L. Chen, W. Liao, Y. Chen, Y. Xia, Tunable magnetic fluid grating by applying a magnetic field, Appl. Phys. Lett. 87 (2005) 021901. [9] S. Pu, S. Dong, J. Huang, Tunable slow light based on magnetic-fluid-infiltrated photonic crystal waveguides, J. Opt. 16 (2014) 045102. [10] L. Li, Q. Han, T. Liu, Y. Chen, R. Zhang, Reflective all-fiber current sensor based on magnetic fluids, Rev. Sci. Instrum. 85 (2014) 083107.
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[11] J. Zheng, X.Y. Dong, P. Zu, L.Y. Shao, C.C. Chan, Y. Cui, P.P. Shum, Magnetic field sensor using tilted fiber grating interacting with magnetic fluid, Opt. Express 21 (2013) 17863–17868. [12] P. Zu, C.C. Chan, W.S. Lew, L.M. Hu, Y.X. Jin, H.F. Liew, L.H. Chen, W.C. Wong, X.Y. Dong, Temperature-insensitive magnetic field sensor based on nanoparticle magnetic fluid and photonic crystal fiber, IEEE Photonics J. 4 (2012) 491–498. [13] J. Lenz, A.S. Edelstein, Magnetic sensors and their applications, IEEE Sens. J. 6 (2006) 631–649. [14] Y. Miao, J. Wu, W. Lin, K. Zhang, Y. Yuan, B. Song, H. Zhang, B. Liu, J. Yao, Magnetic field tunability of optical microfiber taper integrated with ferrofluid, Opt. Express 21 (2013) 29914–29920. [15] S. Dong, S. Pu, H. Wang, Magnetic field sensing based on magnetic-fluid-clad fiber-optic structure with taper-like and lateral-offset fusion splicing, Opt. Express 22 (2014) 19108–19116. [16] S. Pu, S. Dong, Magnetic field sensing based on magnetic-fluid-clad fiber-optic structure with up-tapered joints, IEEE Photonics J. 6 (2014) 1–6. [17] H. Wang, S. Pu, N. Wang, S. Dong, J. Huang, Magnetic field sensing based on singlemode–multimode–singlemode fiber structures using magnetic fluids as cladding, Opt. Lett. 38 (2013) 3765–3768. [18] R. Zhang, T. Liu, Q. Han, Y. Chen, L. Li, U-bent single-mode–multimode–single-mode fiber optic magnetic field sensor based on magnetic fluid, Appl. Phys. Express 7 (2014) 072501. [19] M. Deng, X. Sun, M. Han, D. Li, Compact magnetic-field sensor based on optical microfiber Michelson interferometer and Fe3 O4 nanofluid, Appl. Opt. 52 (2013) 734–741. [20] P. Zu, C.C. Chan, T.X. Gong, Y.X. Jin, W.C. Wong, X.Y. Dong, Magneto-optical fiber sensor based on bandgap effect of photonic crystal fiber infiltrated with magnetic fluid, Appl. Phys. Lett. 101 (2012) 241118. [21] A. Candiani, A. Argyros, S.G. Leon-Saval, R. Lwin, S. Selleri, S. Pissadakis, A loss-based, magnetic field sensor implemented in a ferrofluid infiltrated microstructured polymer optical fiber, Appl. Phys. Lett. 104 (2014) 111106. [22] H.V. Thakur, S.M. Nalawade, S. Gupta, R. Kitture, S.N. Kale, Photonic crystal fiber injected with Fe3 O4 nanofluid for magnetic field detection, Appl. Phys. Lett. 99 (2011) 161101. [23] J. Zhang, S. Peng, A compact SMS refractometer based on HF corrosion scheme, Photonics and Optoelectronic (SOPO), 2010 Symposium on, IEEE (2010) 1–4. [24] J. Tang, S. Pu, S. Dong, L. Luo, Magnetic field sensing based on magnetic-fluid-clad multimode–singlemode–multimode fiber structures, Sensors 14 (2014) 19086–19094. [25] Z. Liu, H. Sun, Y. Yu, X. Zhang, C. Chen, C. Zhu, A. Meng, S. Jing, An optical microfiber taper magnetic field sensor with temperature compensation, IEEE Sens. J. 15 (2015) 4853–4856. [26] J. Tang, S. Pu, L. Luo, S. Dong, Simultaneous measurement of magnetic field and temperature based on magnetic fluid-clad long period fiber grating, J. Eur. Opt. Soc.-Rapid Publ. 10 (2015) 15025.
Biographies
Longfeng Luo received his B. S. degree in Applied Physics from University of Shanghai for Science and Technology, Shanghai, China, in 2013, where he is currently working toward to his M. S. degree. His research interests are in the areas of magneto-optics, fiber-optic sensors and optical communications.
Shengli Pu was born in 1978. He received his Ph. D. degree in 2006 from Shanghai Jiao Tong University. Now, he is a professor at College of Science, University of Shanghai for Science and Technology. He is a Visiting Associate Professor at Cornell University during August 2012-August 2013. He has (co-)authored over 70 peer-reviewed papers in Physics and Optics. His research interests focus on advanced photonic materials and devices, especially the novel optical properties and photonic applications of magnetic fluids/ferrofluids.
Shaohua Dong received his B. S. degree in Optical Information Science and Technology from Hubei University of Automotive Technology, Hubei, China, in 2012. He is currently pursuing the Master’s degree at College of Science, University of Shanghai for Science and Technology, Shanghai, China. His research interests are in the areas of optical advanced materials and photonic devices, fiber-optic sensors and optical communications.
Jiali Tang received her B. S. degree in materials physics from Huaiyin Normal University, Jiangsu, China, in 2013. She is currently pursuing the Master’s degree at College of Science, University of Shanghai for Science and Technology, Shanghai, China. Her research interests are in the areas of magneto-optics, fiber-optic sensors and optical communications.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Fiber-optic magnetic field sensor using magnetic fluid as the cladding Longfeng Luo, Shengli Pu ∗ , Shaohua Dong, Jiali Tang College of Science, University of Shanghai for Science and Technology, Shanghai 200093, China
a r t i c l e
i n f o
Article history: Received 12 December 2014 Received in revised form 24 September 2015 Accepted 20 October 2015 Available online 27 October 2015 Keywords: Magnetic fluid Magnetic field sensing Mode interference
a b s t r a c t A kind of fiber-optic magnetic field sensor is proposed. The sensing structure is composed of singlemode–multimode–singlemode fiber structure cascaded with core-offset fusion splicing between singlemode fibers. The sensing principle is based on the cladding mode interference. Experimental results indicate that, the magnetic field sensing sensitivities of 65.9 pm/Oe and 0.1185 dB/Oe are obtained for wavelength interrogation and intensity interrogation, respectively. The corresponding measurement range of magnetic field strength is 30–110 Oe. The response of temperature is also investigated. The corresponding temperature sensitivities for the interference dip around 1594 nm is obtained to be −42 pm/◦ C and −0.124 dB/◦ C. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Magnetic fluid (MF) is a kind of stable colloidal system consisting of surfactant-coated magnetic nanoparticles dispersed in a suitable liquid carrier [1]. It possesses both the features of magnetic property of solid magnetic materials and fluidity of liquids. Thus, MF presents versatile magneto-optical properties, such as tunable refractive index (RI) [2], magneto-volume variation [3] and magneto-dielectric anisotropy [4]. Until now, various MF-based optical devices have been proposed, such as modulators [5], optical switches [6], optical gratings [7,8], tunable slow light [9], current sensor [10] and magnetic field sensors [11,12]. Recently, magnetic field measurements gradually become significant in various areas including military, aviation industry, biomedical applications, and vehicle detection [13]. Comparing with other magnetic field sensors, MF-based optical fiber magnetic field sensors have attracted particular interest due to their fiber compatibility, compactness, and high sensitivity [14,15]. Various structures have been employed to realize the sensing function, for example, fiber structures with up-tapered joints [16], singlemode–multimode–singlemode (SMS) fiber structures [17,18], singlemode fiber (SMF) Michelson interferometer [19], photonic crystal fiber (PCF) [20,21], and polarization-maintaining PCF [22]. In this work, a novel magnetic field sensing structure has been designed. The proposed structure consists of a very short section of
∗ Corresponding author. Fax: +86 21 65667144. E-mail addresses: [email protected], [email protected] (S. Pu). http://dx.doi.org/10.1016/j.sna.2015.10.034 0924-4247/© 2015 Elsevier B.V. All rights reserved.
multimode fiber (MMF) spliced between two pieces of SMF and a lateral-offset fusion splicing between the main structure and the lead-out SMF. The fundamental sensing principle is based on the cladding modes interference. The sensitivity is high due to the high sensitivity to external stimuli for the cladding modes. Comparing with other structures, the simple fusion splicing technique is employed with the proposed structure. So it possesses the advantages of compact size, low cost, ease of fabrication and high sensitivity. While the previous structures usually include complicated fabrication process (e.g., tapering [14], corroding [17], microfabrication [19]) or are costly (especially for the PCF based structures).
2. Experiments and principles Fig. 1 shows the schematic diagram of the proposed sensing structure. The lead-in SMF1 is firstly spliced with a piece of MMF. Then, the free end of MMF is carefully cleaved and the length of left MMF segment is 1 mm. The obtained SMF–MMF structure is spliced with a SMF2 and the other end of SMF2 is carefully cleaved. The length of left SMF2 is 43 mm. Finally, the lead-out SMF3 is fusion spliced with the SMF2 with laterally core-offset of 30 m. A microscope with a digital CCD camera is utilized to acquire the microscopic images for the SMF–MMF and lateral-offset joints as shown in the low panel of Fig. 1. The whole sensing structure is positioned in a capillary filled with MF. The inner diameter of the capillary is about 0.9 mm. Both ends of the capillary are sealed with UV glue to avoid MFs leaking or evaporating. The MMF (SFS 105/125Y) utilized is provided by Thorlabs Inc. Its core and cladding diameters are 105 and 125 m, respectively. Through trial and
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L. Luo et al. / Sensors and Actuators A 236 (2015) 67–72
where I0 is the intensity of fundamental mode LP01 in SMF1 and N is the total number of excited modes in the cladding of SMF2 . i or j is the serial number representing the excited mode. i and j are the coupling coefficients of LP0i and LP0j modes, respectively. nij is the effective RI difference between the involved interference modes. L is the length of SMF2 . According to Eq. (1), the wavelength corresponding to the interference valley can be expressed by m =
Fig. 1. Schematic of the proposed sensing structure. The low panels from left to right show the microscopic image of the SMF–MMF joint, the sealed sensing structure and the microscopic image of the lateral-offset splicing joint, respectively.
Fig. 2. Experimental setup for investigating the magnetic field sensing properties of the proposed structure.
error, the 1 mm length of MMF (utilized in the current work) is sufficient to expand the mode field efficiently and cladding modes will be excited within the cladding of SMF2 . The SMF (G.652D LWPF) is provided by Beijing CFYC Communication Technology Co. Ltd. The water-based Fe3 O4 MFs provided by Beijing Sunrise Ferrofluid Technological Co. Ltd. are utilized in our experiments. The density and saturation magnetization of the MF are 1.18 g/cm3 (25 ◦ C) and ∼200 Oe, respectively. The diameter of the magnetic nanoparticles within the MFs is around 10 nm. The experimental setup for investigating the magnetic field sensing properties is shown in Fig. 2. Light from the broadband amplified spontaneous emission source (ASE, wavelength ranging from 1525 to 1610 nm) is coupled into the sensing structure. The transmission spectrum is recorded and analyzed by the optical spectrum analyzer (OSA, AQ6370C). The resolution of the OSA is set at 0.02 nm for all experiments. The sensing structure is placed between two poles of an electromagnet, which generates a uniform magnetic field with nonuniformity of less than 0.1% within the sensing region. The strength of the magnetic field is adjusted by tuning the magnitude of the supply current. The magnetic field direction is perpendicular to the optical fiber axis. During our experiments, the ambient temperature is kept at 25 ◦ C. When light from SMF1 approaches the MMF, the fundamental core mode LP01 will spread out widely and then the highorder cladding modes LP0j will be excited within the cladding of SMF2 . These high-order cladding modes will interfere with each other. The interference results will be coupled into the core of SMF3 . Because the lateral-offset distance between SMF2 and SMF3 (30 m) is much larger than the SMF core diameter (9 m), there is no overlap between the core mode fields of SMF2 and SMF3 . The interference between the core mode and cladding mode cannot occur within the SMF3 core. SMF3 only receives and transmits the interference happening between cladding modes with different orders. The transmission spectrum of the whole structure can be expressed as [12,15,19,23]
I () =
N i=1
2i × I0 () +
N
i × j × I0 () × cos 2nij
i= / j=1
L
(1)
2nij L (2m + 1)
(2)
where m is the interference order. If the order of stimulated cladding mode is relatively high, the surrounding refractive index (SRI) variation can exert a considerable influence on the mode field distribution. In this case, the variation trend of effective RI is mainly related with the variation trend of mode field fraction inside the fiber. The radius of mode field will increase with SRI. Then, the fraction of mode field inside the fiber will decrease with the increase of SRI. This will result in the decrease of effective RI with the increment of SRI. If the order of stimulated cladding mode is relatively low, the mode field distribution is hardly affected by the variation of SRI. The variation trend of effective RI is mainly related with the variation trend of SRI. This will result in the increase of effective RI with the increment of SRI. Moreover, the SRI variation can affect the transmission loss of the proposed structure according to Eq. (1). Considering the magnetically tunable RI of MF, the valley wavelength and transmission loss may be sensitive to the external magnetic field when using MF as the cladding of the structure. Thus, magnetic field measurement can be realized by detecting wavelength shift and transmission loss of the interference spectrum. On the other hand, if the lateral-offset is relatively small, the interference probably happens between core and low-order cladding modes. Then, the effective RI difference between the involved interference modes is relatively insensitive to the ambient stimuli. So the sensitivity is reduced. 3. Results and discussion Before proceeding with investigating the magnetic field sensing properties, the response and applicability of our sensing structures in a wide range of RI variation will be characterized firstly. The glycerol–water solutions with different mass fractions are utilized. Their RIs are given in Ref. [14]. The transmission spectra of the sensing structure surrounded with glycerol–water solutions of different concentrations are shown in Fig. 3. Three distinct interference valleys are observed in the wavelength range of 1525–1610 nm. Fig. 3 indicates that all the interference valleys shift to long wavelength side with the RI (viz. concentration) increase of surrounding liquids. The red shift of wavelength valley implies that nij increases with SRI. Thus, the experimental phenomena may be assigned to the interference between low-order and high-order cladding modes. According to the inset of Fig. 3, the maximum sensitivity of RI measurement is achieved to be 433 nm/RIU when RI approaching 1.44. To investigate the magnetic field sensing properties, MF is used as the cladding of the proposed structure. Fig. 4 shows the typical transmission spectra of the sensing structure immersed in MF at different external magnetic fields. It is clear from Fig. 4 that the wavelength and intensity of the interference dip increase with the magnetic field. The explicit results are plotted in Fig. 5. Figs. 4 and 5 show that the dip wavelength shifts monotonically from 1593.5 to 1599.5 nm when magnetic field changes from 0 to 110 Oe. For magnetic field beyond 110 Oe, the interference valley becomes unobvious. Fig. 5 implies that the dip wavelength is almost linearly dependent of the magnetic field at moderate strengthen (30–110 Oe). The corresponding sensitivity is obtained to be 65.9 pm/Oe. Besides,
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Fig. 3. Transmission spectra of the proposed sensing structure surrounded with glycerol–water solutions of different concentrations. The inset shows the corresponding dip wavelength as a function of RI of the surrounding liquids.
Fig. 4. Transmission spectra of the proposed sensing structure immersed in MF at different magnetic field strengths.
Fig. 5 indicates that the intensity at dip wavelength increases monotonically from −65.9 to −54.7 dB when H changes from 0 to 130 Oe. Similar to the dip wavelength shift, a good linear relationship between the intensity variation and magnetic field is obtained for magnetic field ranging from 30 to 110 Oe. The corresponding sensitivity is 0.1185 dB/Oe. The obtained sensitivities of the proposed structure are the same order of magnitude for the optimized SMS (90.5 pm/Oe @40–100 Oe) [2] and SMF taper (56 pm/Oe@20–200 Oe) structures [14], 3 times higher than that of MMF–SMF–MMF structure (21.5 pm/Oe @0–120 Oe) [24], 4 times larger than that of nonoptimized SMS structure (16.7 pm/Oe@0–325 Oe) [17], about 10 times larger than that using MF as the cladding of SMF-based
Michelson interferometer (6.5 pm/Oe @0–2000 Oe) [19], and about 28 times higher than that using MF as the cladding of PCF (2.4 pm/Oe @0–200 Oe) [12]. The achieved sensitivity of the as-fabricated structure is lower than that of microfiber taper structure (171 pm/Oe@20–70 Oe) [25], but the complicated and sophisticated microfabrication technique is necessary for realizing microfiber taper structure. In addition, we would like to point out the sensing performance of the proposed structure can be enhanced further through optimizing the design, such as changing the thickness of MF film around MMF and the radius of SMF2 [2]. To investigate the hysteresis of magnetic field senor, repeated experiments are conducted when applying magnetic field at ascending and descending orders for several cycles. The
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Fig. 5. Wavelength and intensity of the interference dip as functions of magnetic field.
Fig. 6. Wavelength and intensity of the interference dip at different magnetic field strengths when applying magnetic field at ascending and descending orders.
measurement interval between each data is 5 min. Hysteresis effect between ascending and descending magnetic fields is observed for every cycle. The typical results are plotted in Fig. 6. Therefore, interrogating the dip wavelength and intensity simultaneously is necessary for sensing the magnetic field solely. Otherwise, additional calibration method is needed, such as simultaneous measurements using two devices with different sensitivities. This hysteresis effect is mainly assigned to the viscosity of MF, magnetization (demagnetization) and agglomeration (deaggregation) of magnetic nanoparticles within the MF, which can be reduced by
decreasing the sweep rate of magnetic field or using thinner MF film with lower viscosity. To investigate the temperature response of the sensor, the experiments are conducted at several ambient temperatures. The wavelength shift and transmission loss of the interference valleys at different temperatures are shown in Fig. 7. The temperature sensitivities are obtained to be −42 pm/◦ C and −0.124 dB/◦ C for the wavelength and transmission loss interrogation, respectively. It is well-known that the RI of MF is temperature-dependent. Therefore, the effective RI difference nij between the involved interference modes will change with ambient temperature. This will lead to the
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Fig. 7. Wavelength and intensity of the interference dip as functions of temperature.
dip wavelength shift and corresponding dip intensity variation with temperature according to Eqs. (1) and (2). The temperature sensitivities are comparable to those of magnetic field sensing. These are contributed to the relatively large thermo-optic coefficient of MF (about −2.4 × 10−4 /K), which is two orders of magnitude larger than that of pure silica. Therefore, the cross-sensitivity of temperature may be worth further considering for certain applications. To solve the cross-sensitivity of temperature and magnetic field, the two-parameter matrix method is proved to be effective [26]. According to our experimental results, a well-conditioned system of two equations can be given in a matrix form as follows
H
=
T
1 K,H KI,T − K,T KI,H
KI,T
−K,T
−KI,H
K,H
I
(3)
where H and T are the variations of magnetic field intensity and ambient temperature, respectively. K,H and K,T are the sensitivities of resonant wavelength shift with respect to magnetic field and temperature, respectively. KI,H and KI,T are the sensitivities of interference valley transmission with respect to magnetic field and temperature, respectively. The values of K,H , K,T , KI,H and KI,T for the as-fabricated structure have been obtained from the above-mentioned experiments. So, Eq. (3) can be explicitly written as
H T
1 = −3.1946
−0.124
42
−0.1185
65.9
I
(4)
Using Eq. (4), the as-fabricated sensing device can be employed to measure magnetic field intensity accurately with dynamic temperature compensation. 4. Conclusion In summary, a kind of optical fiber magnetic field sensor based on MF-clad fiber structure has been proposed and investigated. Through cascading the SMS structure with a SMF core-offset fusion splicing, the interference between cladding modes happens. Magnetic field sensing is achieved by monitoring wavelength shift or
intensity variation of the interference valley. The magnetic field sensing sensitivities of 65.9 pm/Oe and 0.1185 dB/Oe are obtained for magnetic field strength ranging from 30 to 110 Oe. The temperature effect of the sensing structure is also investigated. The temperature sensitivity can reach −42 pm/◦ C and −0.124 dB/◦ C. The proposed sensing structure has the advantages of low-cost, compactness, and high sensitivity. Acknowledgments This research is supported by Natural Science Foundation of Shanghai (Grant no. 13ZR1427400), Shanghai Key Laboratory of Specialty Fiber Optics and Optical Access Networks (Grant no. SKLSFO2014-05) and Hujiang Foundation of China (Grant no. B14004). References [1] R.E. Rosensweig, Ferrohydrodynamics, Cambridge University Press, New York, 1985. [2] Y. Chen, Q. Han, T. Liu, X. Lan, H. Xiao, Optical fiber magnetic field sensor based on single-mode-multimode-single-mode structure and magnetic fluid, Opt. Lett. 38 (2013) 3999–4001. [3] S. Dong, S. Pu, J. Huang, Magnetic field sensing based on magneto-volume variation of magnetic fluids investigated by air-gap Fabry-Pérot fiber interferometers, Appl. Phys. Lett. 103 (2013) 111907. [4] P.M. Agruzov, I.V. Pleshakov, E.E. Bibik, A.V. Shamray, Magneto-optic effects in silica core microstructured fibers with a ferrofluidic cladding, Appl. Phys. Lett. 104 (2014) 071108. [5] H.E. Horng, J.J. Chieh, Y.H. Chao, S.Y. Yang, C.Y. Hong, H.C. Yang, Designing optical-fiber modulators by using magnetic fluids, Opt. Lett. 30 (2005) 543–545. [6] S. Xia, J. Wang, Z. Lu, F. Zhang, Birefringence and magneto-optical properties in oleic acid coated Fe3 O4 nanoparticles: application for optical switch, Int. J. Nanosci. 10 (2011) 515–520. [7] A. Candiani, W. Margulis, C. Sterner, M. Konstantaki, S. Pissadakis, Phase-shifted Bragg microstructured optical fiber gratings utilizing infiltrated ferrofluids, Opt. Lett. 36 (2011) 2548–2550. [8] S. Pu, X. Chen, L. Chen, W. Liao, Y. Chen, Y. Xia, Tunable magnetic fluid grating by applying a magnetic field, Appl. Phys. Lett. 87 (2005) 021901. [9] S. Pu, S. Dong, J. Huang, Tunable slow light based on magnetic-fluid-infiltrated photonic crystal waveguides, J. Opt. 16 (2014) 045102. [10] L. Li, Q. Han, T. Liu, Y. Chen, R. Zhang, Reflective all-fiber current sensor based on magnetic fluids, Rev. Sci. Instrum. 85 (2014) 083107.
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Biographies
Longfeng Luo received his B. S. degree in Applied Physics from University of Shanghai for Science and Technology, Shanghai, China, in 2013, where he is currently working toward to his M. S. degree. His research interests are in the areas of magneto-optics, fiber-optic sensors and optical communications.
Shengli Pu was born in 1978. He received his Ph. D. degree in 2006 from Shanghai Jiao Tong University. Now, he is a professor at College of Science, University of Shanghai for Science and Technology. He is a Visiting Associate Professor at Cornell University during August 2012-August 2013. He has (co-)authored over 70 peer-reviewed papers in Physics and Optics. His research interests focus on advanced photonic materials and devices, especially the novel optical properties and photonic applications of magnetic fluids/ferrofluids.
Shaohua Dong received his B. S. degree in Optical Information Science and Technology from Hubei University of Automotive Technology, Hubei, China, in 2012. He is currently pursuing the Master’s degree at College of Science, University of Shanghai for Science and Technology, Shanghai, China. His research interests are in the areas of optical advanced materials and photonic devices, fiber-optic sensors and optical communications.
Jiali Tang received her B. S. degree in materials physics from Huaiyin Normal University, Jiangsu, China, in 2013. She is currently pursuing the Master’s degree at College of Science, University of Shanghai for Science and Technology, Shanghai, China. Her research interests are in the areas of magneto-optics, fiber-optic sensors and optical communications.