Based on Magnetic-Fluid-Clad Fiber-Optic Structure With Up-Tapered Joints Volume 6, Number 4, August 2014 Shengli Pu Shaohua Dong DOI: 10.1109/JPHOT.2014.2332476 1943-0655 Ó 2014 IEEE
Based on Magnetic-Fluid-Clad Fiber-Optic Structure With Up-Tapered Joints Shengli Pu and Shaohua Dong College of Science, University of Shanghai for Science and Technology, Shanghai 200093, China DOI: 10.1109/JPHOT.2014.2332476 1943-0655 Ó 2014 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. Manuscript received May 8, 2014; revised June 16, 2014; accepted June 18, 2014. Date of publication June 24, 2014; date of current version July 3, 2014. This work was supported in part by the Shanghai Natural Science Fund under Grant 13ZR1427400 and in part by the Innovation Fund Project for Graduate Student of Shanghai under Grant JWCXSL1302. Corresponding author: S. Pu (e-mail: shlpu@usst.edu.cn). Abstract: A kind of magnetic field sensor based on mode interference effect is proposed. The sensing arm consists of two special up-tapered joints formed on the traditional singlemode fiber by fusion tapering technique. Magnetic fluid is used as the cladding of the structure. The interference valley wavelength or the transmission loss of the sensing structure is sensitive to the external magnetic field, which is utilized for magnetic field sensing. The sensitivity of shift of interference valley wavelength with magnetic field can be up to 325.3 pm/mt in the range of 0 16 mt. The variation of transmission loss at valley wavelength with magnetic field has a sensitivity of 0.2121 db/mt in the range of 2 30 mt. Index Terms: Magnetic field sensing, magnetic fluids, fiber-optic sensor, mode interference, up-tapered joints. 1. Introduction Optical fiber sensors have attracted a great deal of attention over the past decades due to their particular characteristics, such as anti-interference, compactness, and high sensitivity. Recently, Mescia et al. presented a review work [1] introducing abundant fiber sensing structures, such as fiber Bragg grating (FBG), long period gratings (LPGs), up-tapered structure and so on. On the other hand, magnetic fluid (MF) is an attractive material, which is a kind of stable colloidal system consisting of surfactant-coated magnetic nanoparticles dispersed in a suitable liquid carrier. It possesses both the features of magnetic property of solid magnetic materials and fluidity of liquids. So far, versatile properties have been demonstrated (e.g., tunable refractive index (RI) [2], [3], magneto-volume variation [4] and magneto-dielectric anisotropy [5]). Many potential optical and sensing applications based on MFs have been proposed, such as optical switches [6], optical gratings [7], [8], tunable optical capacitors [9], modulators [10], [11], tunable slow-light [12], and magnetic field sensors [2] [4], [13] [21]. Due to the fluidity of MFs, they can be easily integrated with fibers. MF-based fiber-optic magnetic field sensors are attracting continuing research interests. The schemes of this kind of sensors include immersing microfiber knot inside the MFs [18], injecting MFs into the air holes of the photonic crystal fiber (PCF) [19] [21], and using MFs as the claddings of the special fiber or fiber structures (e.g., S-tapered fiber [13], singlemode-multimode-singlemode (SMS) fiber structures [3], [14], [15], PCF [16] and
Fig. 1. Schematic diagram of the proposed sensing structure. The upper panel shows the microscopic image of the up-tapered joint. singlemode fiber (SMF) Michelson interferometer [17]). The sensing methods of these sensors can be classified into two types. One is the wavelength-shift-type (WST) based on the wavelength of peak/valley shift with the magnetic field. The other is the transmission-loss-variationtype (TLVT) based on the transmission power change with the magnetic field. For the popular SMS-based fiber structure, several authors have achieved relatively good magnetic field sensing performance. For example, Wang et al. obtained a sensitivity of 168.6 pm/mt (WST) in the range of 12 32.5 mt [3], Chen et al. achieved a sensitivity of 905 pm/mt (WST) in the range of 4 10 mt [14], and Lin et al. acquired a sensitivity of 0.1939 db/mt (TLVT) in the range of 10 25 mt [15]. In addition, the S-tapered fiber structure presented by Miao et al. also has good sensitivities of 560 pm/mt (WST) and 1.3056 db/mt (TLVT) in the range of 2.5 20 mt [13]. In this work, a novel fiber-optic sensing structure using MFs as the cladding is fabricated. The sensing structure is made up of two up-tapered joints fabricated on the conventional SMF through in-line fusion tapering technique. The fundamental sensing principle of our proposed structure is that the high-order modes will be excited in the SMF cladding with the help of the up-tapered joint, which will interfere with the fundamental mode. The effective RIs and energy leakage of these interference modes could be influenced by the external environment, so sensing purpose is implemented. 2. Fabrication and Operating Principle Fig. 1 shows the schematic diagram of the proposed sensing structure. The sensing structure is composed of two up-tapered joints fabricated on the traditional SMF. The up-tapered joint is fabricated through fusion tapering technique with a commercial electric-arc fusion splicer (AV6471). The uncut conventional SMF is utilized for fusion tapering. The arc discharging parameters are set at arc duration of 15, arc current of 30 and Z push distance of 30 (all of them are relative values defined by the AV6471 fusion splicer). After arc discharging, the fiber diameter is locally enlarged due to the Z push, which will form the up-tapered joint (see the upper panel of Fig. 1). In order to obtain a relatively large waist of the joint (for easy excitation of higher order modes), the discharging operation is repeated for 5 times. The final waists of the joints are enlarged to 194 m. The length between the two up-tapered joints is 14 mm, which acts as the sensing arm. When the light from the lead-in SMF approaches the up-tapered joint, the fundamental mode will spread out widely and then high-order cladding modes will be excited within the cladding of the sensing arm. These high-order modes will be coupled into the lead-out SMF through the other up-tapered joint and interfere with the fundamental mode. The transmission spectrum can be analyzed by using a simple two-mode interference model, which has been widely used in this field to qualitatively analyze the fiber mode interference [13], [16], [17], [22]: p I out ðþ ¼I core ðþþi h ðþþ2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi I core ðþi h ðþ cosð2n eff L=Þ (1)
Fig. 2. Schematic diagram of the experimental setup for investigating the magnetic field sensing properties of the proposed structure. where I core and I h are the intensities of the fundamental mode and high-order cladding modes, respectively. n eff is the effective RI difference between the fundamental mode and cladding modes. L is the length of the sensing arm. According to Eq. (1), the wavelength corresponding to the interference valley can be expressed by m ¼ 2n eff L=ð2m þ 1Þ (2) where m is the interference order. When the environmental RI (ERI) changes, the effective RIs of cladding modes will be influenced but the fundamental mode is unaffected. Moreover, higher cladding modes will suffer larger effective RI change due to the larger leakage field outside the fiber. Thus, the valley wavelength of the interference spectrum may shift with the ERI change according to Eq. (2). Besides the mode effective RI, the cladding mode energy leakage will vary with the ERI as well. Therefore, the transmission loss of the sensing structure will change with the ERI change according to Eq. (1). It is well-known that MF has unique properties of magnetic-field-dependent RI [23], [24]. So the valley wavelength and transmission loss may change with magnetic field when MF is used as the cladding of the sensing arm, which could be employed for magnetic field sensing. 3. Experimental Details The experimental setup for investigating the magnetic field sensing properties of the sensing structure is shown in Fig. 2. It consists of a broadband amplified spontaneous emission source (ASE, wavelength ranging from 1525 to 1610 nm), an electromagnet (with magnetic field nonuniformity less than 0.1% within the sample region), the sensing structure and an 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. 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. In our experiments, the oilbased Fe 3 O 4 MF (EXP.08105, 1.87% in volume fraction) provided by Ferrotec Corporation is employed. The diameter of the magnetic nanoparticles is around 10 nm. The MF is diluted with n-dodecane ðc 12 H 26 Þ. The volume ratio of EXP.08105 to n-dodecane is 1 : 3. During our experiments, the ambient temperature is kept at 19 C. 4. Results and Discussion The entire transmission spectra of our proposed sensing structure for magnetic induction intensity ranging from 0 to 40 mt are shown in Fig. 3. There are two distinct interference valleys in the spectral range of the ASE source, which are referred as Valley A and B, respectively. It is clear that Valley A and B shift to long wavelength side with magnetic field. The RI of SMF is constant and larger than that of MF used in our experiments. Hence, most of the mode energy is confined within the fiber and the effective RI variation is mainly related with the change of mode energy within the fiber. As the cladding mode energy inside the sensing arm decreases with the ERI increase, effective RI will decreases with the ERI increase. Then, n eff will increase with the ERI increase, which will lead to the red-shift of the valley wavelength according
Fig. 3. Transmission spectra of the proposed sensor for magnetic induction intensity ranging from 0to40mT. Fig. 4. Wavelength and intensity of Valley A as functions of magnetic induction intensity for the proposed sensing structure. to Eq. (2). Because the MF RI increases with the external magnetic field [23], [24], the valley wavelength will red-shift with the magnetic field. The intensity decrease of the interference valley is directly related with the energy leakage of the involved interference modes. Fig. 4 shows the variation of wavelength and intensity of Valley A with magnetic induction intensity. The wavelength of Valley A increases linearly with the magnetic induction intensity at low field (G 20 mt) and tends to be stable gradually at relatively high field (9 20 mt). The intensity of Valley A has a good linear relationship with the magnetic induction intensity in the range of 0 26 mt. The sensitivities of the wavelength shift and intensity variation of Valley A are 185.1 pm/mt and 0.14 db/mt, respectively. The wavelength and intensity of Valley B as functions of magnetic induction intensity are shown in Fig. 5. Similar characteristics are obtained, but the sensitivities of the wavelength shift and intensity variation to external magnetic field are relatively higher. The sensitivity of wavelength shift of Valley B is 325.3 pm/mt in the range of 0 16 mt, which is 13 times higher than that using MF as the cladding of PCF (23.67 pm/mt) [16], and 5 times larger than that using MF as the cladding of SMF-based Michelson interferometer (64.9 pm/mt) [17]. The sensitivity of intensity variation reaches 0.2121 db/mt in the range of 2 30 mt. The obtained sensitivity is much higher than that of our previous work ( 168.6 pm/mt) [3]. In addition, our previous
Fig. 5. Wavelength and intensity of Valley B as functions of magnetic induction intensity for the proposed sensing structure. Fig. 6. Wavelength and intensity of Valley B as functions of magnetic induction intensity for the proposed sensing structure. sensing system is based on the singlemode-multimode-singlemode structure and needs the complicated corrosion technology, while the structure presented in this work just needs a piece of traditional SMF with in-line simple and repeated arc discharging. In addition, the intensity of the transmission spectra at any specific wavelength varies with the magnetic field (see Fig. 3), which may also be employed for magnetic field sensing. As an example, the intensity at 1570 nm is selected for monitoring. Fig. 6 plots the intensity at 1570 nm as a function of magnetic field. The good linear relationship between the intensity variation and magnetic induction intensity are obtained at low field (0 16 mt) and high field (16 30 mt) regimes, respectively. The corresponding sensitivities are 0.1584 db/mt and 0.0767 db/mt, respectively. For this intensity variation at a fixed wavelength, a relatively simple interrogation system is required. 5. Conclusion In conclusion, a kind of novel and compact magnetic field sensor based on MF and SMF with two up-tapered joints is designed. Both of the interference valley wavelength and transmission loss are highly sensitive to the external magnetic field. The sensitivity of wavelength shift can be
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