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Abstract
A kind of compact all-fiber-optic vector magnetic sensor is proposed and demonstrated. The sensor consists of a side-polished-fiber (SPF)-integrated with singlemode-no core-singlemode (SNS) fiber structure. A section of side-polished fiber breaks the axially symmetry of the composite structure. The as-fabricated sensor supports vector sensing and has a magnetic field strength sensitivity of up to −2370 pm/mT over 2-6 mT range. The physical mechanism is that the modal interference is strongly influenced by the refractive index (RI) near the side-polished surface. The advantages of the proposed sensor lie in low cost, simple structure and easy manufacture, which make it attractive in the field of magnetic field vector sensing.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Magnetic field as a fundamentally physical quantity has both magnitude and direction. Magnetic field sensors have been studied extensively due to its importance in industry, military, etc [1]. Recently, magnetic fluid (MF) as a stable colloidal system formed by magnetic nanoparticles coated with surfactant has been widely investigated in optical field, which is assigned to its unique optical properties, especially tunable refractive index (RI) and tunable birefringence [2]. Since MF has good compatibility with fiber optic devices, many MF-based optical fiber magnetic field sensors have been proposed [1–7]. Among them, the mostly employed structures and mechanisms are singlemode-no core-singlemode (SNS) fiber structure [1,4,8], fiber Fabry-Perot (FP) [9,10], fiber Sagnac interferometer [11,12], microfiber knot [13,14], fiber surface plasma resonance (SPR) [15,16]. Compared with other structures, SNS fiber structure has received much attention due to its simplicity.
In general, the sensing principle of SNS fiber structure is modal interference within the no-core fiber (NCF) [17]. Its sensitivity depends on the intensity of evanescent field within the interference region. In previous studies, most sensors can only sense the strength of the magnetic field. Recently, some structures with the ability of response to magnetic field direction have been fabricated [13,15,18]. Among them, side-polished-fiber (SPF) is widely used as a sophisticated scheme. In this work, a MF integrated side-polished SNS fiber structure (SP-SNS) has been proposed for vector magnetic field sensing with high strength sensitivity in the meantime. To the best of our knowledge, this is the first time to realize magnetic field vector sensing by using side-polished structure combined with modal interference effect.
2. Sensor structure and sensing principle
The schematic diagram of the proposed sensor is shown in Fig. 1(a). The sensing part is composed of SP-SNS fiber structure with sandwiched NCF of 2.5 cm length. The SPF is coated with MF and the RI of MF is close to that of the NCF. The NCF and SMF have a diameter of 125 µm, which are provided by Yangtze Optical Fiber and Cable Joint Stock. The diameter of SMF core is 9 µm. In our experiments, the room temperature is maintained at 25 °C. The pristine oil-based MF (EXP08103, Ferrotec, Chiba, Japan) is used to mix with ethyl oleate to get MFs with different RIs. The utilized MF has a RI of 1.45703, which is measured by a refractometer (A670, Hanno, Jinan, China). The side-polished part occurs in the middle of the NCF and is made by a homemade wheel polishing system. The side-polished defect has a parabola-like profile, which is 4.8 mm in length and about 39 µm in depth in the center. Details of the polished defect is shown in Fig. 1(b). UV glue is used to seal the ends of the capillaries to prevent MF leakage or evaporation. During package, the sensing fiber is located at the center of the capillary tube and cannot be bent or distorted. The transmission spectra of the unpolished SNS, the SP-SNS in air and the SP-SNS in MF are shown in the Fig. 1(c). Compared with the unpolished SNS fiber structure, the transmittance of SP-SNS structure in air is lower. After being coated with MF in the capillary tube, the spectrum changes obviously and a deep transmission dip appears around 1310 nm.
Fig. 1. (a) Schematic diagram of the proposed sensing structure, (b) microscope picture and profile of the side polished fiber, (c) transmission spectra of the unpolished SNS, the SP-SNS structure in air and MF, respectively.
The transmission spectrum of an unpolished SNS structure can be expressed as [17]
At zero magnetic field, the magnetic nanoparticles distribute uniformly around the fiber as shown in Fig. 2(a). Therefore, the MF is optically isotropic. When magnetic field is applied to the sensing structure, the RI of MF will change with the external magnetic field. In general, neff increases with magnetic field, which will cause the shift of peaks and dips of the SNS structure. From a microscopic point of view, the magnetic nanochain-clusters will distribute around the fiber anisotropically as shown in Fig. 2(b). So, the change in RI is different at different regions (i.e. different directions) [19,20]. There is almost no nanochain distribution near Region A (see Fig. 2(b)). Thus, the RI of this region has a reduction of Δn1. On the contrary, the nanochain largely converges in the tangential direction, viz. Region B (see Fig. 2(b)), which leads to RI increase of Δn2. The RI changes gradually between these two regions.
Fig. 2. Schematic diagram of the distribution of the magnetic nanochain-clusters (a) without magnetic field (b) with applying magnetic field.
Obviously, for a fiber structure with axisymmetric cross-section, when magnetic field is applied in any direction perpendicular to the fiber axis, the transmission spectra will always be the same. But for SPF, different directions cause different changes in effective RI around the side polished defect, which affects the position of the interference dips. Based on this characteristic, the vector magnetic field sensing can be realized.
3. Experimental details
The schematic diagram of the experimental setup is shown in Fig. 3. It consists of a supercontinuum broadband light source (SBS, Wuhan Yangtze Soton Laser Co., Ltd., Wuhan), an electromagnet (East Changing Technologies, Inc.), the sensing structure and an optical spectral analyzer (OSA, Yokogawa AQ6370C). The magnetic field strength is adjusted by changing the magnitude of the supply current and calibrated by a gauss meter. The sensor is fixed on a rotation stage. By rotating the stage, the orientation of the magnetic field relative to the sensor (side polished facet) can be adjusted.
4. Results and discussion
To characterize the response of the structure to magnetic field direction, the magnetic field strength was fixed at 5 mT while rotating the sensor from 0° to 360° with a step of 10°. Herein, 0° represents the position that the direction of magnetic field is parallel to the flat surface of SP-NCF. The measured transmission spectra are shown in Fig. 4. Figures 4(a)–4(d) displays that interference dip around 1310 nm red-shifts and blue-shifts alternately with the change of magnetic field direction. Figure 4(e) shows the magnetic-field-orientation dependent dip wavelength position in polar coordinate system, which obviously shows the remarkable direction dependent feature of the structure. It can be seen from Fig. 4(e) that the symmetry axis of the pattern is not strictly at 0°, which may be due to the inaccuracy in marking the direction of side-polished surface. On the other hand, the side-polished surface may be not smooth enough, and then nanochain movement may be affected by the rough surface. Besides, low magnetic field and high viscosity of ethyl oleate make hysteresis effect much obvious.
Fig. 4. Spectral response around 1310 nm of the sensing structure at different magnetic field orientations (a) 0° to 90°, (b) 100° to 180°, (c)190° to 270°, (d) 280° to 360°, (e) magnetic-field-orientation dependent dip wavelength in polar coordinate system. The applied magnetic field is fixed at 5 mT.
To investigate the response of the structure to magnetic field strength, the direction of magnetic field is fixed at 0° or 90°, and then the strength of magnetic field is gradually increased. When the direction of magnetic field is parallel to the flat surface of SP-NCF (i.e. at 0° position), the measured spectra are shown in Fig. 5(a) and 5(b). On the whole, variation of the spectra is tiny. The dip wavelength around 1310 nm (see Fig. 5(b)) first shifts towards long wavelength when the magnetic field increases from 0 to 1 mT. Then, blue-shift occurs for the magnetic field increasing from 2 to 6 mT. The dip depth is reduced about 6 dB. At 90° position, the measured spectra are shown in Figs. 5(c) and 5(d). Compared with the situation at 0° position, the variation of spectra is more obvious (see Fig. 5(c)). The direction of dip wavelength shift is very similar to that at 0° position, but the extent of wavelength shift is much larger and the depth is greatly increased about 9 dB (see Fig. 5(d)). The wavelength shift with magnetic field for both cases is plotted in Fig. 5(e). Figure 5(e) shows that the as-fabricated sensor has a sensitivity of −1410 pm/mT at 0° position and −2370 pm/mT at 90° position within the magnetic field intensity range of 2-6 mT. For both cases, the dip wavelength shifts in the same direction. As the side-polished area is about 4.8 mm long, the remaining NCF is about 2.5 cm long. Then, the major part of the structure is similar for both cases. Thus, the dip wavelength shift has the same direction for both cases.
Fig. 5. Transmission spectra at different magnetic field strengths for the 0° case (a) and (b), 90° case (c) and (d) and the dip wavelength as a function of magnetic field (e).
For the SP-SNS structure at 0° position, the RI will increase at the side-polished defect when the nanoparticles converge at the corresponding region, which will lead to the suppression of interference effect (when comparing with the unpolished SNS structure). Therefore, the spectrum is relatively smooth and the dip around 1310 nm wavelength is much shallow. However, when the direction of magnetic field is perpendicular to the flat surface of the SPF (i.e. 90° position), the RI at the side-polished surface will become lower (see Fig. 2(b)). Then, the light leakage is weakened and the modal interference effect within NCF is enhanced. As a result, the dip depth around 1310 nm wavelength is enhanced as well. In addition, the location of the interference dip is also regulated by RI at the defect, that is the physical mechanism accounting for vector sensing. It can be speculated that this vector property may gradually increase with the length and depth of the side-polished region within a certain range.
For comparison, Table 1 lists the sensing performance of various optical fiber magnetic field sensors. For the SNS structures, the obtained magnetic field sensitivities are much high [4,8]. But they can not implement vector sensing. Other structures for obtaining high sensitivity are much complex [21–23]. For example, excessively tilted fiber grating (Ex-TFG) needs to be made with phase mask [22], and SPR is realized by depositing metal (gold, silver, etc.) film onto a fiber substrate [15,23]. Our proposed vector sensing structure has a high sensitivity with easy fabrication.
Table 1. Sensing performance of various optical fiber magnetic field sensors.
5. Conclusions
In conclusion, a SP-SNS structure is designed for measurement of magnetic field strength and direction. The distribution of nanoparticles at the side-polished defect inhibit or restore the modal interference of the original unpolished SNS structure. The sensor supports vector magnetic field sensing, and the sensitivity to magnetic field strength can reach as high as −2370 pm/mT. The proposed sensing structure has the advantages of low cost, simple structure, and easy fabrication. Furthermore, optimization of the structure and the hysteresis effect of high concentration MF worth to be further investigated. In particular, it is recommended to replace ethyl oleate with n-dodecane due to its low viscosity and low RI.
Funding
National Natural Science Foundation of China (61675132); Shanghai “Shuguang Program” (16SG40); Shanghai Talent Development Fund (201529); Shanghai Key Laboratory of Specialty Fiber Optics and Optical Access Networks (SKLSFO2014-05).
Disclosures
The authors declare no conflicts of interest.
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