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Abstract
Fiber-optic magnetic field sensors based on magnetic fluid (MF) is encountering with thermal effects and demand for vectorization for several years. A common solution is to use axially processed fiber cascaded with fiber Bragg grating (FBG). However, the length of such sensors is usually in centimeter-level, which restricts the sensing applications in narrow space and gradient field cases. In this work, we present an ultracompact reflection-type dual-channel sensor for vector magnetic field (Channel 1, referred as CH1) and temperature (Channel 2, referred as CH2) monitoring, which is composed of a pair of gold-plated wedge-shaped multimode fiber (MMF) tip and gold-plated multimode-no-core fiber (MNF) tip. The surface plasmon resonance (SPR) effect was adopted. The two sensor probes are coated with magnetic-field-sensitive MF and temperature-sensitive polydimethylsiloxane (PDMS), respectively. The issue of vector magnetic field and temperature cross-sensitivity is tactfully resolved. Importantly, the proposed sensing probes are ultracompact and the spatial resolution is extremely small (615 µm for CH1 based on wedge-shaped fiber tip and 2 mm for CH2 based on MNF), which is very helpful for narrow space and gradient magnetic field detection. The obtained magnetic field intensity sensitivities are 1.10 nm/mT (90° direction) and –0.26 nm/mT (0° direction), and temperature sensitivity is –3.12 nm/°C.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Magnetic field sensors are critical in various fields including magnetic shielding, biomagnetism, and automated manufacturing. The technologies for realizing magnetic sensing mainly include search coil, optically pumped, nuclear precession, superconducting quantum interference device (SQUID), Hall-effect, magneto-optic, and fiber-optic [1,2]. Fiber-optic magnetic field sensors have been investigated extensively, which possesses the merits of easy fabrication, low cost, and easy embedment into various sensing structures. However, the weak Faraday effect of silica results in large volume of traditional schemes. To overcome the challenges of compactness and miniaturization requirements in practical applications, such as the detection of high gradient fields at the openings of the magnetic field shielding of fiber-optic gyroscope [3,4], an improved scheme of fiber-optic magnetic field sensors based on magnetic fluid (MF) is proposed [5–7]. In addition, as a magnetic field sensitive material, MF is composed of surfactant-coated magnetic nanoparticles (MNPs) and suitable carrier liquid. MNPs rotate and interact with each other under an external magnetic field, which results in RI change of MF with magnetic field [8]. This change depends on both magnetic field intensity and direction [9].
Most of the fiber-optic sensors are based on the fiber evanescent field penetrating into the analytes or external functional material. Therefore, various functional materials are employed as the cladding or filler of the sensing structure, e.g. Ziting Lin et al. proposed a low-loss vector magnetic field sensor based on MF and a C-type fiber interferometer in 2021 [10]. Similarly, a variety of MF-cladded/filled fiber interferometers [11–20], fiber surface plasmon resonators [21–23], and fiber gratings [9,24–26] have been proposed and investigated for magnetic field sensing. These sensors are characterized by compactness, low cost, high sensitivity and easy fabrication. Recently, fiber-optic vector magnetic field sensors based on non-centrosymmetric optical fields and magneto-induced non-uniform distribution of MNPs around fibers [14] have been demonstrated. For example, Tuan Guo et al. and Yongxi Li et al. proposed vector magnetic field sensors based on MF-coated tilted fiber Bragg grating (FBG) [9] and asymmetric fiber geometry [27] in 2016 and 2019, respectively. However, all the reported MF-based fiber-optic vector magnetic field sensors operate at room temperature. The temperature cross-sensitivity due to thermal effect of MF is not considered. In order to resolve this issue, temperature calibration or compensation techniques are necessary. A widely employed method is to cascade the sensing structure with FBG on the order of a few centimeters in length [28–32], but the sensitivity is relatively low and it will disable the sensing abilities in narrow space and gradient fields. In 2020, Yong Zhao et al proposed a reflective magnetic field sensor scheme based on Fabry-Perot interferometer, whose sensitive length is only 79.5 µm [33]. But due to the symmetrical configuration of Fabry-Perot resonators, the “vector” function is disabled.
Multifunctional integration of optical fiber tips has been proposed in recent decades to form ultracompact sensing schemes [34]. Moreover, SPR as a measurement technique can detect the tiny RI changes over metal film surface by strong evanescent fields [35], and then temperature sensing based on polydimethylsiloxane (PDMS) coated SPR structures was proposed as well [36]. In 2014, Aoqun Jian et al. produced an RI sensor with a gold-plated wedge-shaped fiber tip [37], in which the Kretschmann configuration was used [38,39]. Besides, Bin Li et al. used different metal films deposited on the sensing surface to generate SPR effect, and further wrapped with MF and PDMS at different metal regions to realize dual-channel sensor for magnetic field intensity and temperature measurement [40].
In this work, an ultracompact dual-channel fiber-optic sensor is proposed, which consists of a pair of SPR sensing probes fabricated on a gold-plated wedge-shaped fiber tip packed by MF (referred as CH1, 615 µm in sensing length) and on the gold-plated multimode-no-core fiber (MNF) tip packed by PDMS (referred as CH2, 2 mm in sensing length), respectively. Non-centrosymmetric evanescent field is generated by the wedge-shaped fiber tip (CH1), enabling vector magnetic field sensing. MNF tip (CH2) is relatively compact and highly sensitive to temperature, which resolves the temperature-vector-magnetic-field cross-sensitivity. The proposed sensor is a typical example for realizing multifunctionally integrated reflection-type SPR sensing probe with high sensitivity.
2. Principle and experiment
Figure 1 shows the schematic of the proposed dual-channel vector magnetic field and temperature sensor with two SPR probes. The reflectance spectrum of the structure will have two SPR attenuation peaks (reflection dips). Due to the refractive index (RI) difference between the two packed materials [41,42], the crosstalk between CH1 (clad with MF) and CH2 (clad with PDMS) can be avoided. PDMS is insensitive to external magnetic fields but more sensitive to temperature than MF. Magnetic field and temperature can be detected by monitoring the shift of dip wavelengths stemming from CH1 and CH2.
2.1 Sensing principle
Figure 2 shows the schematic of the sensing probe configurations. CH1 is fabricated by grinding and polishing a multimode fiber (MMF) tip into a wedge shape (sensing surface with a grinding angle of α and reflecting surface with a grinding angle of β, where 2α + β = 90°, α < 20°, β > 50°), which is then plated with gold film. MF as a magnetic field sensitive material is packed around the tip. This structure will generate a non-centrosymmetric evanescent field and the incident light will excite SPR at the sensing surface. When the light is perpendicularly reflected by the reflective surface, it will impinge on the sensing surface and excite SPR again. However, the SPR cannot be excited at the reflective surface in our scheme. The incident angle with regard to the reflective surface is beyond the range enabling the excitation of SPR. As the incident cut-off angle θc of the employed MMF (NA = 0.23) is around 13.3° [43,44], the maximum angle of the incident light directly impinging on the reflective surface is 90° − β + θc < 53.3°. Nevertheless, the prerequisite for exciting SPR on the reflective surface is the satisfaction of the following equalities [39,44]:
CH2 is spliced by MMF and no-core fiber (NCF). The MNF sensor tip is plated with gold film and then coated with PDMS. The reflectance spectrum of the system is sensitive to the RI changes near the SPR excitation surface (sensing surface for CH1, cylindrical surface for CH2).
Figure 3 shows the spectra of the sensor for three cases: (I) Before being cladded with MF, a single reflection dip occurs at about 900 nm, which stems from the PDMS-coated CH2 (black line); (II) New dip stemming from CH1 occurs at about 650 nm after CH1 is cladded with MF (red line); (III) Before the experiment, the distance (see Fig. 1 for “Connector distance”) between CH2 and the fiber coupler (Angled Physical Contact, APC) is continuously adjusted to tune the signal contrast between CH1 and CH2 until the satisfactory contrast and full width at half maxima (FWHM) are obtained. Once the optimal distance is determined, the connector will be fixed by the flange and further secured with tape to ensure the stability of the experiment (see Fig. 1) to ensure the stability during the experiment. This makes both CH1 and CH2 signal easy to demodulate for practical application (blue line).
When the external magnetic field is perpendicular to the fiber axis, the basic principles of vector magnetic field sensing of CH1 are as follows: (1) First, the wedge-shaped SPR sensing probe is only sensitive to the RI change near the SPR excitation surface, viz. the sensing surface; (2) As a kind of nanomaterial, MNPs in MF are dispersed homogeneously in carrier liquid without an external magnetic field; (3) Besides, previous studies verified that MNPs will rotate and interact with each other and then form nanochain-clusters around fiber (see Fig. 4) under an external magnetic field. So, the local RI changes with and is dependent of magnetic field direction and intensity [12,14]; (4) As a result, the SPR attenuation peak wavelength will be modulated by both the intensity and direction of the external magnetic field. Finally, a vector magnetic field sensor is achieved. Meanwhile, PDMS is a temperature-sensitive material [36]. Thus, the SPR attenuation peak wavelength stemming from CH1 and CH2 will be modulated by vector magnetic field [27] and temperature, respectively, viz. dual-channel sensor is realized.
According to the above-mentioned analysis, the shift of reflection dip wavelength λ1 corresponding to CH1 is related to the temperature and magnetic field intensity and direction. The relationship between λ1 and magnetic field direction θ, magnetic field intensity B and temperature T is empirically formulated as:
2.2 Experimental details
The fabrication processes of CH1 and CH2 are illustrated in Fig. 5. First, the reflective surface is grinded with a grinding angle β and then plated with gold film; Second, rotating the optical fiber 180° along the fiber axis and adjusting the grinding angle to α to grind the sensing surface. The previous deposited gold film of this part will be polished off. Then, re-plating it with gold film. An ion sputtering instrument (ETD-900, Vision Precision Instruments) was used to plat the gold film. The gold film on the reflective surface is thick enough to achieve the effect of reflection, while the gold film on the sensing surface is precisely controlled through setting the sputtering the current of 10 mA and sputtering duration of 180 seconds. The thickness of the gold film on the sensing surface is about 42.3 nm, which is measured with an atomic force microscope (Dimention Icon, Bruker). For fabricating CH2, a fusion splicer (FSM-80C+, Fujikura) is employed to splicing NCF and MMF. Then, plating gold film with the same parameters for the sensing surface of CH1. At last, the sensing probe is packed with MF and PDMS, respectively.
Before encapsulation with functional materials, the sensitive area lengths of the proposed sensing scheme are 615 µm (CH1) and 2 mm (CH2), respectively. These will become 2 cm (CH1) and 2 mm (CH2) after packaging. Since the probe of CH2 is wrapped by PDMS, surrounding interference can be effectively isolated. Therefore, both CH1 and CH2 can be encapsulated in a single capillary/container filled MF, which will make the structure more compact.
The experimental setup employed to investigate the vector magnetic field sensing properties is shown in Fig. 6. A tungsten halogen light source (HL-2000, Ocean Insight) and 2×2 fiber coupler are used. The incident light coming from Port 1 is reflected by the CH1 and CH2 sensing probes, which are connected to Port 3 and Port 4, respectively. Finally, the reflected light is received by the spectrometer (USB4000, Ocean Insight). The magnetic field intensity B and direction θ (see Fig. 4 for definition of θ) are adjusted by the electromagnet power supply (KEITHLEY 2260A-30-72, Tektronix) and the rotating platform, respectively. A column oven (LCO 102 LONG, ECOM Ltd., Czech Republic) is used to test the reflection dip wavelength as a function of temperature. The RI of the employed MF and PDMS are 1.341 and 1.401 at room temperature (25 °C).
As shown in the left panel in Fig. 6, during the experiment, CH2 is fixed outside the bare fiber adapter (BFA). This will guarantee the temperature self-compensated vector magnetic field measurement based on dual-channel SPR sensor.
3. Results and discussion
The temperature dependent reflectance spectra of the dual-channel sensor are illustrated in Fig. 7(a). Dip wavelengths from both CH1 and CH2 show temperature dependence and the achieved temperature sensitivity of CH1 and CH2 are –0.17 nm/°C and –3.12 nm/°C (see Fig. 7(b)), respectively.
The reflectance spectra of the as-fabricated device under different magnetic field directions θ and intensities B were measured. Typical results are shown in Fig. 8. Figure 8(a) indicates that the signal from CH1 is greatly dependent on magnetic field and shifts sharply when magnetic field direction approaches 90°, while the signal from CH2 shifts randomly with the magnetic field, which may be assigned to the temperature fluctuations. To explain the large sensitivity of CH1 near 90°, it is speculated that the dip wavelength shift is exponentially related with the magnetic field direction (see Eq. (3)).
Fig. 7. Temperature dependent reflectance spectra (a) and dip wavelength (b) of the proposed dual-channel sensor.
Fig. 8. Reflectance spectra for the as-fabricated sensing probe at different magnetic field directions (at 10.5 mT) (a) and magnetic field intensities (at θ = 90°) (b).
To be explicit, the corresponding reflection dip wavelengths for CH1 and CH2 at different magnetic field directions and intensities are extracted and replotted in Fig. 9. Figure 9 obviously displays that the signal of CH1 depends remarkably on the magnetic field direction and intensity. For the magnetic field direction response of CH1, the minimum and maximum sensitivities occur at 0°/180° and 90°/270° (see Fig. 9(a)). The magnetic field intensity sensitivity of CH1 is 1.10 nm/mT at 90° and –0.26 nm/mT at 0°, respectively (see Fig. 9(b)). Considering the temperature and magnetic field intensity sensitivities of CH1 and CH2 (see Fig. 7(b) and Fig. 9(b), respectively), the temperature variation is estimated within ± 1.5°C during the vector magnetic field sensing experiment. The magnetic field measurement error of CH1 are estimated to be ± 0.28 mT (90°/270°) and ± 0.065 mT (0°/180°). Considering the 0.1 nm resolution of the spectrometer (HR4000, Ocean Insight) employed in our experiments, the magnetic field intensity detection limits are 0.38 mT (0°) and 0.09 mT (90°), and the temperature detection limit is 0.03 °C.
Fig. 9. (a) Experimental and calculated dip wavelengths of CH1 as a function of magnetic field direction and appended with dip wavelength of CH2; (b) Dip wavelengths of CH1 as a function of magnetic field intensity.
In addition, the red line in Fig. 9(a) is calculated according to Eq. (3) with fitting coefficient κ = 0.07. The following data are obtained through the data fitting: S1x = 1.10 nm/mT, S1y = – 0.26 nm/mT, B = 10.5 mT, λ10 = 651 nm, T = 25 °C, and S1T = – 0.17 nm/°C, which are in good agreement with the experimental results. Therefore the empirical Eq. (3) is justifiable and gives reliable results. The temperature-compensated fiber-optic vector magnetic field sensor is realized.
According to Eq. (3), vector magnetic field and temperature can be calculated out under specific intensity B by the following matrix:
We would like to point out that the MMF is employed in this work for experiment convenience. Intuitively, thin core fiber like singlemode fiber SMF and few mode fiber (FMF) may be favorable for achieving narrower SPR valley. Our preliminary study displays that the diameter of the fiber core hardly affects the FWHM of the SPR valley, which is shown in Fig. 10.
Furthermore, the actual magnetic field usually has a spatial gradient. If the magnetic field gradient is large in a relatively small space, the detection results of traditional sensors should be the average value within a certain spatial range. Thereby, it is necessary to consider the spatial resolution (sensing length). In this work, the sensing length of CH1 and CH2 are 615 µm and 2 mm, respectively. For comparison, the performance indicators of the relevant sensing configurations are presented in Table 1. Compared with previous works on temperature-compensated (vector) magnetic field sensors, our work not only has temperature-compensation capability, but also is feasible for vectorization detection and compact configuration with high sensitivity. Finally, our preliminary experiments show that the optical fiber surface roughness affects the FWHM of the reflectance spectrum and the resonant wavelength, which is worth investigating further in the future.
Table 1. Sensing performance of related temperature-compensated fiber-optic magnetic field sensors.a
4. Conclusion
In conclusion, a kind of dual-channel SPR temperature-compensated fiber-optic vector magnetic field sensor has been proposed and demonstrated. This configuration overcomes the issue of temperature and vector magnetic field cross-sensitivity, while keeps the merits of compactness and highly sensitivity. It is believed that this sensing structure can promote the development of sensor miniaturization and integration in designing temperature-compensated fiber-optic vector magnetic field sensing based on SPR.
Funding
National Natural Science Foundation of China (62075130, 61675132); Shanghai Shuguang Program (16SG40).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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