Magnetic field sensor based on asymmetric optical fiber taper and magnetic fluid

Sensors and Actuators A 211 (2014) 55–59

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Magnetic field sensor based on asymmetric optical fiber taper and magnetic fluid Ming Deng a,∗ , Danhui Liu a , Decai Li b a b

Key Laboratory of Optoelectronic Technology and Systems (Education Ministry of China), Chongqing University, Chongqing 400044, China School o f Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China

a r t i c l e

i n f o

Article history: Received 7 October 2013 Received in revised form 12 February 2014 Accepted 12 February 2014 Available online 6 March 2014 Keywords: Fiber-optic sensors Optical fiber core–cladding–mode interferometer Magnetic fluid Measurement of magnetic field strength

a b s t r a c t We report an optical fiber magnetic field sensor by merging the advantages of magnetic fluid and a core–cladding–mode interferometer which is directly fabricated on a standard single-mode fiber by using an arc fusion splicing machine. The sensing performances of the sensors are controllable by designing the parameters of the asymmetric-tapered structure. Experimental results show that the sensor with axial offset of 168 ␮m and taper waist diameter of 45 ␮m not only has good optical properties but also a relatively high magnetic-field sensitivity of ∼162.06 pm/mT ranging from 0 to 21.4 mT. The proposed sensors would find potential applications in weak magnetic sensing fields. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Measurements of magnetic field are important in many fields, such as medicine, electric power, and military. Fiber-optic magnetic field sensors have been widely used in many applications due to their advantages of high resolution, ultimate precision, compact structure, and resistance to corrosion [1–7]. In these sensors, magnetic functional material is an important element since it interacts with the light parameters such as intensity, phase or polarization. Magnetic fluid is an attractive magneto-optical material which possesses various outstanding magneto-optic effects including Faraday Effect, thermal lens effect, tunable refractive index, linear dichroism and birefringence, and so on [8–11]. By utilizing the tunable refractive index of magnetic fluid, a number of versatile magnetic field sensors have been developed, for instance, combining an optical fiber device (i.e. an etched FBG, a tilted FBG, a microfiber Michelson interferometer or a Fabry-Perot interferometer) with magnetic fluid and the wavelength shift or the extinction ration was used to measure the external magnetic field strength [12–15]. Due to the low interaction between the core mode/lowerorder cladding mode and magnetic fluid, such sensors are relatively less sensitive to magnetic field strength and the highest sensitivity is ∼74.9 pm/mT. A magnetic field sensor which is configured

∗ Corresponding author. Tel.: +86 2365106917. E-mail address: [email protected] (M. Deng). http://dx.doi.org/10.1016/j.sna.2014.02.014 0924-4247/© 2014 Elsevier B.V. All rights reserved.

as a Sagnac interferometer structure with a magnetic fluid film has been proposed and the sensitivity is ∼167 pm/mT [16]. Gao reported a long-period fiber grating within D-shaped fiber using magnetic fluid for magnetic-field detection and the sensitivity is ∼176 pm/mT [17]. Moreover, a magnetic field sensor with sensitivity of 242 pm/mT is proposed, which is composed by 23.5 cm polarization-maintaining photonic crystal fiber whose air holes have been injected with magnetic nanofluid [18]. In this paper, we proposed and experimentally demonstrated an optical fiber magnetic field sensor based on a core–cladding–mode interferometer (CCMI) incorporating with magnetic fluid. The CCMI consists of an asymmetric taper which is directly fabricated on a standard single-mode fiber by using an arc fusion splicing machine. Experimental results show that the sensor with axial offset of 168 ␮m and taper waist diameter of 45 ␮m has sensitivity of ∼162.06 pm/mT in the magnetic field range of 0–21.4 mT, which is higher than that of some existing magnetic field sensors mentioned above. 2. Sensor fabrication and operating principle The schematic diagram of the proposed CCMI is shown in Fig. 1, which is fabricated on a standard single-mode fiber (Corning: SMF28e) by using a Furukawa S176 arc fusion splicing machine. The input optical beam is split into two optical paths at the upper taper, along the core and the cladding of SMF, respectively. After transmitting a short distance, they recombine and interfere at the

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Fig. 1. Schematic diagram of the CCMI.

down taper, leading to a smooth and regular interference fringe. The transmission spectrum can be mathematically described by:



I = Icore + Icladding,m + 2 ˚m =

2neff L 

Icore Icladding,m cos(˚m )

(1) (2)

where Icore and Icladding,m are the intensities of the core and the mth cladding mode, ˚m is the phase difference between them. neff is the effective refractive index difference between the core and cladding modes, L is the physical length of the interferometer and  is the input wavelength in vacuum. When an arc discharge is applied on an optical fiber, the residual tensile stress or compressive stress formed in the fiber’s fabrication process can be released by local heating and therefore to increase/decrease the refractive index of the fiber; moreover, heating the optical fiber below the transition temperature and then rapidly cooling it will make the fiber’s volume decreased and therefore the refractive index increased. So, appropriate laser energy and loading method can be chosen to fabricate an asymmetric taper in different optical fibers [21]. If arc discharge with high power is applied on the optical fiber, there will be a notch in the fiber and even the optical fiber will be broken off [22], so multiple arc discharges with low power produced by a Furukawa S176 arc was used to fabricate an asymmetric taper in our experiments. An optimized multi-step procedure was used to fabricate an asymmetric taper with offset of 0–175 ␮m. The resulting parameters were as follows: arc power of 165 units, pre-fuse time of 150 ms, arc duration of 2000 ms and z-push distance of 3 ␮m. First of all, the axial offset of the two fiber holders in fusion splicer (e.g. 100 ␮m) was manually adjusted with the help of a pair of cleaved fiber tips. Then, the fiber tips were replaced with a section of SMF without polymer coatings. Finally, multiple arc discharges were applied to SMF to form an asymmetric taper. A high-accuracy optical spectrum analyzer (OSA, Si720, Micron Optics, USA) was used to observe the transmission spectrum of the device with wavelength resolution and precision of 0.25 and 1 pm, respectively. Fig. 2(a) and (b) shows the optical microscopic images of one sample whose axial offset and waist diameter are 168 and 45 ␮m, respectively. The black curve in Fig. 2(c) is the interference spectrum of such a CCMI in air. It can be seen that there is a resonant wavelength in the range of 1520–1570 nm, which is due to a small difference between the effective refractive index of core and that of cladding modes as period of fringe spacing of the interferometer can be 2 /(n L). The fringe visibility of the CCMI is calculated by  =  eff calculated by V = 2 Icore Icladding,m /(Icore + Icladding,m ). For such a structure, large amounts of fundamental mode is coupled to the cladding mode, leading to a difference between the intensities of the core and that of cladding modes, so the fringe visibility is ∼4 dB, as can be seen from Fig. 2(c). If we further increase the axial offset of the asymmetric taper to 170 ␮m, there is no interference spectrum observed in the optical spectrum analyzer due to a great difference between Icore and Icladding,m . The above-mentioned CCMI with axial offset of 168 ␮m was positioned into the center of a glass capillary with an inner diameter of 0.3 and 100 mm length. The capillary was filled with magnetic fluid and sealed with UV glue to prevent the magnetic fluid from flowing out and evaporating.

The magnetic fluid (MF) contains the ferromagnetic nanoparticles, Fe3 O4 , and their nominal diameters are 10 nm. It is a water-based MF with concentration of 3.5%, which is provided by Beijing Jiaotong University (Beijing, China). From Fig. 2(c), we can see that the interference spectrum changed greatly when the CCMI was surrounded by magnetic fluid as compared with that in air. This is mainly caused by the mutation in the optical fiber waveguide. When the CCMI is surrounded by magnetic fluid, the waveguide is composed of fiber core, cladding and magnetic fluid. The effective refractive index of the dominated cladding mode will change as compared with that in air because it is determined by the refractive index of the core and that of the external medium. According to Eq. (2), the phase difference between the core and cladding mode changes, leading to a change of the interference spectrum. When external magnetic field with variable strength is applied on the sensor, the effective refractive index of magnetic fluid varies with the change of magnetic field strength since Fe3 O4 particles in the magnetic fluid agglomerate and form more magnetic columns with the increment of magnetic field strength [15], so does that of the dominated cladding mode. But a weak change cannot bring about a change of the dominated cladding mode interfering with the core mode, an indication of a wavelength shift not a changed spectrum. Therefore, the effective refractive index of the dominated cladding mode will depend on magnetic field-induced surrounding refractive index change, so will the effective refractive index difference between the core and cladding modes. When the phase shift is less than 2, the phase ambiguity issue can be avoided, and in this case, the wavelength shift of the interference fringe can be used to monitor the external magnetic field strength and the initial wavelength is that in red curve in Fig. 2(c). Moreover, we can see that the fringe visibility increased to ∼6 dB due to a decreased intensity difference between the core and that of the dominated cladding mode.

3. Experimental results and discussion The experimental setup for magnetic field measurement is illustrated in Fig. 3, where the overall configuration of the magnetic field sensor was placed in the center of the uniform magnetic field zone and its transmission spectrum was monitored by an optical spectrum analyzer (OSA, Si720, Micron Optics, USA). An external magnetic field with variable strength is generated by a home-made magnetic generator and it is applied perpendicularly to the sensor head. A Gauss meter (HT20, Shanghai Huntoon Magnetic Technology Co., Ltd., China) locates next to the sensor head and is used to calibrate the magnetic field strength. The magnetic-field response of the above-mentioned sensor was investigated and the results are shown in Fig. 4. It can be seen that the intensity of the resonant wavelength significantly decreased with the increment of the magnetic field ranging from 0 to 185.2 mT, which is mainly due to the scattering effect caused by the evanescent field of the cladding modes propagating through the metal nanoparticles on the optical fiber surface since Fe3 O4 particles in the magnetic fluid agglomerate and form more magnetic columns with the magnetic field strength increasing. The

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Fig. 2. The optical microscope image of a fabricated CCMI and its transmission spectra.

Fig. 3. Schematic diagram of the experimental setup for magnetic field measurement.

attenuation in intensity of the cladding mode will lead to a decreased intensity difference between the core and cladding modes, which results in an increment of the fringe visibility of the interference spectrum (e.g. ∼6 dB increasing to ∼24 dB). The relationship between the intensity and the magnetic field is illustrated in Fig. 5. The intensity linearly decreased with the increment of the magnetic field in the range of 0 to 185.2 mT, and the sensitivity is −0.098 dB/mT, as shown in blue curve in Fig. 5. In addition,

we can see that the resonant wavelength shifts to a shorter wavelength ranging from 1555.655 to 1545.84 nm as the strength of the applied magnetic field was increased to185.2 mT. The relationships between the dip wavelengths and the magnetic field strengths were recorded in Fig. 5. When the magnetic field strength was smaller than 21.4 mT, the dip wavelength shifted quickly and linearly as the magnetic field strength increasing. The corresponding sensitivity is as high as ∼162.06 pm/mT, which is much greater than that

-20

-20

(b)

(a) -25 0mT 21.4mT 42.9mT 64.4mT 85.9mT 107.4mT 126.4mT 144.8mT 158.7mT 172.7mT 185.2mT

-30 -35 -40 -45 -50 1520

1530

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Intensity(dBm)

-25

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Increasing magnetic field

-50 1540

1550

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185.2mT 172.7mT 158.7mT 144.8mT 126.4mT 107.4mT 85.9mT 64.4mT 42.9mT 21.4mT 0mT 1520

1530

Decreasing magnetic field 1540

1550

Wavelength(nm)

Fig. 4. Transmission spectra of the sensor with axial offset of 168 ␮m and taper waist diameter of 45 ␮m.

1560

1570

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-25

1552

-30

-2 -35

λ=1545nm

1550 -40 1548

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Wavelength(nm)

1554

0

Lignt intensity(dBm)

wavelength change with increasing magnetic field wavelength change with decreasing magnetic field intensity change with increasing magnetic field intensity change with decreasing magnetic field

1556

-45

1546 1544 0

20

-50 40 60 80 100 120 140 160 180 200 Magnetic field intensity(mT)

-4 -6 -8

168um 142um 113um

-10 0

Fig. 5. The relationship between the wavelength shift and the external magnetic field.

50 100 150 Magnetic field strength(mT)

200

Fig. 7. Wavelength shift as a function of magnetic field for different sensors.

of some existing magnetic field sensors [12–15]. When the magnetic field strength was larger than 158 mT, the dip wavelength shift was gradually saturated. The data can be fit well by the modified Langevin function. The hysteresis effect is defined that the external magnetic field strength is applied to the sensing scheme till the saturated magnetization is achieved, after that, when we decrease the magnetic field strength, the dip wavelength/intensity changed differently with the original curve. This is due to the existence of viscosity for the magnetic fluid and it cannot be eliminated but could be reduced by changing the magnetic field more slowly [19]. Such a sensor can be utilized for dc magnetic-field sensing or in fields that the changing rate of magnetic field is relatively slow (<50 Hz). Compared with the wavelength demodulation method, the hysteresis loop is more open by using the intensity demodulation method and therefore it will lead to a great error. So, the wavelength demodulation method is preferable in real applications. Due to the positive thermal coefficient of optical fiber and the negative thermo-optical coefficient of the fluid, the sensing scheme is insensitive to temperature ranging from 20 to 100 ◦ C. When the temperature variation of measurand is great, such a sensor scheme should be first calibrated, and next, a temperature sensor (i.e. thermometer or FBG) can be used to measure the temperature to compensate the errors caused by temperature variations. We fabricated other two sensors with different parameters by using the mentioned-above fusion parameters, one of them has

(a)

-35

-45 -50 -55 -60 1520

-32

0mT 21.6mT 41.9mT 64.8mT 85.5mT 107.1mT 125.8mT 144.3mT 158.6mT 172.2mT 184.2mT 1530

(b)

-30

Intensity(dBm)

Intensity(dBm)

-40

axial offset of 142 ␮m and taper waist diameter of 44 ␮m, the other has axial offset of 113 ␮m and taper waist diameter of 43 ␮m. Both of them were firstly positioned into the center of a glass capillary with an inner diameter of 0.3 and 100 mm length and then were placed in the center of the uniform magnetic field zone. Magnetic-field responses of the sensors were experimentally investigated and the results are shown in Fig. 6. It can be seen that the resonant wavelength changes with magnetic field have a similar trend. Namely, with the increment of the external magnetic field, the resonant wavelength shifts to a shorter wavelength and its intensity decreased, but the fringe visibility increased. For such a structure, the taper length and taper waist are correlation, that is to say, a taper length corresponding to a taper waist diameter [20]. Due to a slight difference in the taper waist of the three sensors, the effect caused by the taper waist can be ignored. The relationships between the wavelength and the external magnetic field for the three sensors are shown in Fig. 7. It can be seen that the magnetic-field sensitivity increased with the increment of the taper’s axial offset, which is because that a higher cladding mode is excited at the upper taper as a larger axial offset is introduced. Therefore, the sensing performances of the sensor are controllable by designing the parameters of the asymmetric-tapered structure.

0mT 21.4mT 42.9mT 64.4mT 85.9mT 107.4mT 126.4mT 144.8mT 158.7mT 172.7mT 185.2mT

-34 -36 -38

1540

1550

Wavelength(nm)

1560

1570

-40 1520

1530

1540

1550

1560

1570

Wavelength(nm)

Fig. 6. Transmission spectra of the sensor with different parameters: (a) the sensor with axial offset of 113 ␮m and taper waist diameter of 43 ␮m; (b) the sensor with axial offset of 142 ␮m and taper waist diameter of 44 ␮m.

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4. Conclusions This paper demonstrated an optical fiber magnetic field sensor by incorporating a CCMI with magnetic fluid. The CCMI is based on an asymmetric taper which is directly fabricated on a standard single-mode fiber by using an arc fusion splicing machine. We experimentally investigated the characteristics of the sensors with different structure parameters including axial offset and taper waist diameters. Experimental results show that the sensing performances of the sensors are controllable by designing the parameters of the asymmetric-tapered structure and the magneticfield sensitivity can be improved by increasing the sensor’s axial offset since a higher cladding mode is excited and interferes with the core mode. The sensor with axial offset of 168 ␮m and taper waist diameter of 45 ␮m has a sensitivity of ∼162.06 pm/mT in the magnetic field range of 0–21.4 mT, which is higher than that of some existing magnetic field sensors. Therefore, the proposed magneto-optical fiber sensor has the advantages of easy fabrication, small size, low cost and high sensitivity, offering potentials in the medicine, electric power, and military fields. Acknowledgments This work is supported by the Natural Science Foundation of China under Grant No. 61107046, the Natural Science Foundation Project of CQ CSTC under Grant No. cstc2012jjA4007 and the Fundamental Research Funds for the Central Universities under Grant No. CDJZR13125501. Assistances and good suggestions of Prof. Tao Zhu in Chongqing University are appreciated.

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Biographies

Ming Deng received the B.S. and M.S. degrees in communication and information engineering from Xi’an University of Science and Technology, Shannxi, China, in 2003 and 2006, respectively. She received the Ph.D. degree in optical engineering from Chongqing University, China, in 2009. Her research focuses on photodynamic effect of nanomaterials, fiber-optic passive and active optical sensors. She is a member of the Optical Society of America.

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Danhui Liu was born in Sichuan, China, in 1991. She received the B.S. degree in measurement and control technology & instrument from Chongqing University, Chongqing, China, in 2013. She is currently working toward the M.S. degree in optical engineering from the Key Laboratory of Optoelectronic Technology and Systems, Ministry of Education, Chongqing University, Chongqing, 400044, China. Her current research interests include fiber optical sensors.

Decai Li received the M.S. degree in Beijing University of Aeronautics and Astronautics, Beijing, China, in 1992. He received the Ph.D. degree in Beijing Jiaotong University, Beijing, China, in 1996. Then he received the M.S. degree in Teaching Science and Engineering in English in the University of Manchester Postgraduate Certificate, Manchester, Britain, in 2005. Now he is a professor of Beijing Jiaotong University and the director of magnetic liquid (magnetic fluid) research center. His research focuses on mechanical and electrical integration of magnetic fluid and nano-magnetic functional materials.