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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 1, JANUARY 1, 2015
Magnetic Field Sensor Based on Photonic Crystal Fiber Taper Coated With Ferrofluid Yong Zhao, Di Wu, and Ri-Qing Lv
Abstract— A novel and compact magnetic field sensor based on a tapered photonic crystal fiber (PCF) coated with ferrofluid is proposed. It consists of a section of tapered PCF, which is spliced between two single-mode fibers with a waist diameter of 24 µm. The ferrofluid is filled in the capillary to coat the PCF taper. Experimentally, the refractive index (RI) of the ferrofluid increased under increasing magnetic field intensity (H) with a sensitivity of 4 × 10−5 RIU/Gs and the RI sensitivity in the evanescent field is 401 nm/RIU. Therefore, the interference spectrum is shifted as the change of H ranged from 100 to 600 Gs with a sensitivity of 16.04 pm/Gs and resolution of 0.62 Gs. The proposed magnetic field sensor is attractive due to its compact size, low cost, and immunity to electromagnetic interference beyond what conventional magnetic field sensors can offer. Index Terms— Photonic crystal fiber taper, ferrofluid, Mach–Zehnder interferometer, magnetic field sensor.
I. I NTRODUCTION
M
AGNETIC field detection is important in many fields, such as military, industrial and electric power transmission. Fiber-optic devices have attracted a variety of research interests due to their outstanding advantages over conventional electromagnetism sensors, including small size, high sensitivity and immunity to electromagnetic interference. Ferrofluids are stable colloidal suspensions of sub-domain magnetic nanoparticles dispersed inside a liquid carrier [1]. As a new kind of functional medium, it has versatile magnetooptic properties, for example, birefringence, Faraday effect, field-dependent transmission, tunable refractive index and so on. The refractive index of the ferrofluid was found to increase linearly with increasing magnetic field intensity, enabling it to be used in the magnetic field sensor with optical read-out [2]. Recently, ferrofluid has been exploited in the design of various magnetic field sensors. The combining forms include Manuscript received July 26, 2014; revised August 24, 2014; accepted September 24, 2014. Date of publication September 26, 2014; date of current version December 4, 2014. This work was supported in part by the National Natural Science Foundation of China under Grant 61273059 and Grant 61203206, in part by the Fundamental Research Funds for the Central Universities under Grant N130104002 and Grant N130604006, and in part by the State Key Laboratory of Synthetical Automation for Process Industries under Grant 2013ZCX09. (Corresponding author: Yong Zhao.) Y. Zhao is with the College of Information Science and Engineering, Northeastern University, Shenyang 110819, China, and also with the State Key Laboratory of Integrated Automation of Process Industry Technology, Research Center of National Metallurgial Automation, Shenyang 110819, China (e-mail:
[email protected]). D. Wu and R.-Q. Lv are with the College of Information Science and Engineering, Northeastern University, Shenyang 110819, China (e-mail:
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2014.2360531
the following three: ferrofluid firm, the fill material and the cladding. The first form is used as a ferrofluid firm, which is inserted in Sagnac loop [3], [4]. However, these sensors always have complicated structures and large volume. The second form is used as fill material, which is filled in the Fabry–Pérot (FP) cavity [5], [6] and the air holes of PCF [7]–[9]. Although the sensitivities in the filling case are really high, the complicated fabrication process also makes them hard to mass production. One problem is how to realize a low-loss and high-strength splice between PCF (filled with ferrofluid) and SMF. The last form is using the ferrofluid as the cladding of tapered or etched fibers. The commonly used combination is tapered fiber and offset [10], [11] or we can change the shape of the taper [12] and the fiber type of the taper. In this letter, a magnetic field sensor based on ferrofluid and a tapered PCF spliced between two SMFs is proposed and experimentally demonstrated. When light propagates through the tapered region, part of core modes are coupled to the cladding modes or leak off to the environment and propagate as cladding modes or radiation modes. Experimental results show that the interference spectrum is shifted as the change of the applied magnetic field intensity with a sensitivity of 16.04pm/Gs ranged from 100 Gs to 600Gs and a resolution of 0.62Gs. II. O PERATION P RINCIPLE OF THE S ENSOR A. Properties of the Ferrofluid Ferrofluids are stable colloidal suspensions of magnetic nanoparticles dispersed inside a liquid carrier. When there is magnetic field, magnetic nanoparticles will form a chain structure along to the field direction and hence we can employ its property of controllable refractive index. The relationship between the ferrofluid refractive index and the magnetic field intensity was studied in our previous work [2]. As shown in Fig. 1, the RI of the ferrofluid changes with the rapidly increasing and then decreasing magnetic field. The two curves of the increasing and decreasing process were almost coincide which proved a perfect repeatable performance. In lower magnetic field intensity range, the change of the ferrofluid refractive index is very slow due to the magnetic field response time. With the magnetic field intensity increasing further, saying larger than 600Gs, the RI of the ferrofluid tends to be saturate gradually because of the magnetic saturation effect. Here, a parameter k R I −H is defined to measure the changing in ferrofluid RI under magnetic field.
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ZHAO et al.: MAGNETIC FIELD SENSOR BASED ON PCF TAPER
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Fig. 3.
Fig. 1. The relationships between ferrofluid refractive index and the increasing and decreasing process of magnetic field.
Schematic diagram of the MZI structure based on tapered PCF.
The schematic diagram of the MZI structure based on tapered PCF is shown in Fig. 3. After tapering the PCF, a strong evanescent field was formed near the tapered region and made the susceptible to external RI variations. When light travels from the SMF to the tapered PCF, the fundamental mode of SMF begins to diffract. In the first collapsed region, part of core modes are coupled to the cladding modes or leak off to the environment and propagate as cladding modes or radiation modes. Therefore, the interference spectrum can be expressed using the following two-beam optical interference equation [14]: 2πn L ef f (1) I = I1 + I2 + 2 I1 I2 cos λ where, I is the intensity of the total interference signal, I1 and I2 are the intensities of the core and cladding modes, respectively. n e f f is the difference between the effective refractive indices of core and cladding modes:
Fig. 2. (a) The cross section of the PCF used in the experiment. (b) Microscopic image of the SMF-PCF splicing area. (c) The tapered PCF drawn by the fusion splicer.
× 10−5 RIU/Gs
Therefore, a higher sensitivity of k R I −H = 4 was acquired by performing linear fitting the curves in the range of 100Gs to 600Gs than it was 3 × 10−5 RIU/Gs from 0Gs to 650Gs. A water-based ferrofluid (EMG507, Ferrotec) with a particle volume concentration of 1.8% was used in our experiment. The nanoparticles in the ferrofluid are Fe3 O4 , and their nominal sizes are about 10nm [13]. Considering of the temperature effect on the ferrofluid, the experiment was carried out under the room temperature of 28.0 °C. B. Sensor Structure and Principle For the fabrication of the PCF interferometer, the PCF (LMA-8, NKT Photonics) was spliced between two SMFs (SMF-28). The cross section of the PCF used in the experiment is shown in Fig. 2(a) and the air holes of the PCF around the ends were totally collapsed after the fusion splicing, as shown in Fig. 2(b). Next, the PCF in the middle section were drawn into a taper via a fusion splicer (Fitel, S178). Note that the polymer coating of the PCF should be stripped off before drawing. We changed the program’s main parameters (Z_PULL_TIME, Z_PULL_DIST, ARC_POWER and ARC_TIME) to study the effect on the performance of PCF taper. After several trials, the PCF taper with a waist diameter of 24μm and a taper length of 804 μm is shown in Fig. 2(c). By drawing a section of PCF into a taper, a very simple all-fiber MZI can be implemented.
clad n e f f = n core e f f − ne f f
(2)
The distance L between the two coupling points corresponds to the physical length of the interferometer and λ is the wavelength. From the (1), it can be seen that the max transmission 2πn
L
ef f = 2mπ (m is an integer). Thus, the will appear when λ transmission spectra exhibit peaks at wavelengths given by:
n e f f L (3) m Once, the RI of the ferrofluid in tapered PCF’s evanescent field changes, then n clad e f f and n e f f will change either. The interference spectrum in m order shift λm can be written as: λm =
n e f f L (n e f f + n)L n L − = (4) m m m where, n is the difference between the RI of the ferrofluid. According to (4), we can find that the transmission spectrum shift is hence a function of n, which, in turn, is a function of the changing of the applied magnetic field intensity. Here, another parameter kλ−R I is defined to measure the changing in interference spectrum shift under different RI in the tapered region. Then the sensor could be worked as a magnetic field sensor and the sensitivity K can be written as: λm =
K = k R I −H · kλ−R I
(5)
III. E XPERIMENTS AND R ESULTS A. Experimental Setup Schematic diagram of the sensor system was shown as Fig. 4(a). A broadband amplified spontaneous emission (ASE)
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 1, JANUARY 1, 2015
Fig. 4. (a) Schematic diagram of experimental setup for magnetic field measurement. (b) Schematic diagram of the details of the ferrofluid coated taper.
Fig. 6. Refractive index sensitivity of the sensor. The inset shows the wavelength dip shifts at 1550 nm.
Fig. 5.
Transmission spectrum of NaCl solution with n = 1.3352.
source with 50nm bandwidth (1520nm-1570nm) was launched into the structure of SMF-Tapered PCF-SMF. Firstly, a glass capillary tube with inner diameter of (125+5)μm was set around the SMF and then moved to the tapered region. As described in Fig. 4(b), the tube was filled with ferrofluid through capillary action and then sealed by UV glue at both sides to avoid the ferrofluid from leaking out. Furthermore, the package of the capillary effectively improves the mechanical strength of the sensor. The coils were used to generate a magnetic field parallel to the fiber axis. The magnetic field intensity was measured by a Gauss meter with a resolution of 0.1Gs. After two beams of light interfered at the second collapsed region, a strong interference spectrum could be observed from the OSA. B. Results and Discussion The magnetic field sensor was made as the description above and then an experiment was carried out to get kλ−R I . By immersing the tapered region into NaCl solutions of different concentrations, the transmission spectrum of NaCl solutions with different refractive indices was observed via OSA. As shown in Fig. 5, there is an interference phenomenon in the sensing structure we proposed.
The inset in Fig. 6 shows the shifts for the chosen wavelength dip centered at 1550nm. It is noted that the power gradually reduces as the interference spectrum’s red-shift. That is because when the solutions’ refractive indices change, the power in the evanescent field of the tapered region will change either. From Fig. 6, it can be seen that with the increasing of the refractive index, the shifts of the wavelength dip show high linearity. The sensor exhibits highly RI sensitivity which is found to be kλ−R I = 51.902nm/RIU. So, the sensitivity of the magnetic field sensor can be calculated theoretically according to (5): K = k R I −H · kλ−R I = 4 × 10−5 RIU Gs × 401nm RIU = 16.04pm/Gs The measurement sensitivity of 16.04pm/Gs was obtained while the resolution is 0.62 Gs at the limit resolution of OSA of 10 pm. In this letter, hysteresis time of ferrofluid is 30min. The reason for the delay time is due to oscillations of the magnetic particles, it will take certain time for them to keep balance under different magnetic field. IV. C ONCLUSION A novel and compact magnetic field sensor based on a tapered PCF coated with ferrofluid was proposed. Based on the theoretical and experimental results, it was proved that the sensitivity of magnetic field intensity was about 16.04pm/Gs and the resolution was 0.62Gs. Compared with other magnetic field sensors, our structure has advantages of compactness, high sensitivity, robustness and easy fabrication. It can be further enhanced by increasing the ferrofluid concentration, increasing the taper length and decreasing its waist diameter. Since the smaller the waist diameter is, the stronger the evanescent field is. In this letter, a fiber fusion splicer was used to taper instead of an optical fiber fused taper machine, since the fiber fusion splicer has quite a few advantages, like simple structure, low cost and short taper length over the traditional fused taper machine. This method can largely decrease hardware complexity.
ZHAO et al.: MAGNETIC FIELD SENSOR BASED ON PCF TAPER
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