Different from usually-used bulk magnetostrictive materials, magnetostrictive TbDyFe thin films were firstly proposed as sensing materials for fiber-optic magnetic field sensing characterization. By magnetron sputtering process, TbDyFe thin films were deposited on etched side circle of a fiber Bragg Grating (FBG) as sensing element. There exists more than 45pm change of FBG wavelength when magnet field increase up to 50 mT. The response to magnetic field is reversible, and could be applicable for magnetic and current sensing.
©2009 Optical Society of America
Great interest in giant magnetostrictive thin films has grown over the past few years due to their potential as powerful transducer for realization of microactuators, meanwhile they can be easily scaled down to micro- or nano-structure. Much effort has been concentrated on the development of magnetostrictive fiber sensors, where strain in a magnetostrictive material resulting from an applied magnetic field is transferred to an optical fiber [1–3]. In principle, a fiber grating can detect magnetic field by Faraday effect, this mechanism is ideal since no external transducer is required. However, its sensitivity is too low for practical application. Properties like electric or magnetic fields can be detected by fiber grating using magnetostirictive transducers . Optical fiber magnetic/current field sensors have been proposed for decades. The measurement of small changes in optical path length can be accomplished using phase-modulating Mach-Zehnder interferometer . It is demonstrated that a nickel coated optical fiber shows sensitive to magnetic field . Sensitivity can be further improved by using proper composition of metallic glasses. It has also been demonstrated that optical fiber jacketing with a magnetostrictive material is more efficient compared to simply gluing a fiber to a flat strip . A disadvantage of using metallic glass jacket is that metallic and metallic glass materials exhibit saturation of magnetostriction at comparatively low magnetic fields, therefore this sensing element may be undesirably sensitive to small current field fluctuation . A magnetostrive sensor using Terfenol-D and Ni65Cu33Fe2 epoxy-bonded with FBG fiber is proposed for DC-current and temperature discrimination , however, this method is based on bulk magnetostrictive materials in size of 6 × 25 mm, which is problematic for miniature application.
Fiber optical sensors offer several significant advantages over conventional electrical sensors. They show no interference with electromagnetic radiation, so they can function in many hostile environments where conventional sensors would fail [10,11]. TbDyFe is one of the most promising magnetostrictive materials, especially TbDyFe has giant magnetostrictive coefficient [12,13]. It will expand when putting in magnetic field, which has a linear and reverse response to strength of magnetic field. If these magnetostrictive materials are coated onto a FBG fiber, the elongation of magnetostrictive film due to magnetic field will result in a change of grating period in FBG fiber, and therefore generate a shift of FBG central wavelength. In this way the drift of FBG wavelength to magnetic field response is correlated. Using TbDyFe magnetostrictive material as sensing element has been proposed, however most of them use bulk TbDyFe gluing to a fiber. Up to our knowledge, there is no report concerning single TbDyFe coating as sensing element. The associated problem is that magnetostrictive response relies on the size of materials used; response with thin film could be too weak for application compared to bulk materials. Improvement of sensitivity would be a key issue for magnetostictive thin film materials. Exchange magnetostriction in giant magnetostorictive hard-magnetic layer and soft-magnetic high magnetization layer has been reported , this could be also an effective way to improve magnetostrictive response, and therefore to enhance magnetic field sensitivity.
In this paper, we firstly propose the idea of depositing magnetostrictive TbDyFe thin films on cladding-etched FBGs. From the principle of mechanics, at the same stress, the strain is proportional to the mass that affected; an improvement of sensitivity by magentostrictive material in magnetic field with less mass of fiber due to etching process could be anticipated. Therefore, in order to improve sensitivity, fiber grating sensors with the same TbDyFe layer thickness, but different cladding thickness after etching process are also prepared and characterized. Meanwhile, magenetostrictive multilayer is also investigated with the interest to improve sensitivity of magnetic field response.
Single mode FBG fiber was dipped into mixed HF solvent to remove cladding layer. The HF solvent was diluted by propanone and de-ioned water with 1:1:10 in volume. The etching process was carried out at room temperature; fiber diameter was measured with optical microscopy after the etching process. An average etching speed of 20 μm/h was concluded, to prepare the fiber with different thickness, we stopped the process with different total etching time, finally etched fiber with outer cladding diameter of 85 μm, 105μm are prepared. The etched FBG length is about 15 mm.
Thin films of giant magnetostrictive material TbDyFe were coated on the etched single mode optical fiber. A BESTECH sputtering system was used to prepare these thin films. The system is specially designed for optical fiber coating, and equips with DC and RF sputtering sources. It has turbo pump and allow a basic vacuum pressure down to 10−9 mbar. 3 inch TbDyFe targets are installed to RF sources, the pre-treated optical fibers are located at a position with substrate-target distance at 150mm. meanwhile two 10 × 10mm Si pieces are used as monitoring sample in order to characterize the deposited layer later. Deposition power for TbDyFe targets are 150 W, which responds to a deposition rate about 0.08nm/s. TbDyFe single layer with the same thickness of about 0.8 μm were sputtered on fiber with different etched cladding diameter (85 μm, 105 μm and 125 μm) as described before. For comparison FeNi layer and TbDyFe/FeNi multilayer are also deposited on non-etched FBG fibers.
The setup of magnetic field sensing characterization system is schematically shown in Fig. 1 , A SLED light with maximum power of 85 μW is coupled to two FBG fibers with one coated with TbDyFe single layer or TbDyFe/FeNi multilayer. These two fibers are inserted into spiral coil in parallel as sensing element and compensating element. The diameter size of spiral coil is 10mm, totally there are 1000 turns as designed. A Lake Shore 460 Gauss meter is employed to calibrate the magnetic intensity as shown in Fig. 2 . Magnetic intensity corresponds to 50 mT when DC current rises up to 3000mA. During magnetic field characterization, the reflected wavelength is collected with a BCD-100 FBG demodulator. This demodulator is equipped with a Fiber Fabry-Perot Tunable Filter (FFP-TF) from Micron Optics Inc., USA as core component for wavelength demodulation. Compared to the usually-used optical spectrum analyzer based on scanning grating technology, the fiber Fabry-Perot tunable filter is a real fiber etalon, which preserves the high optical resolution advantage, but avoids extreme alignment, temperature, and vibration sensitivities of bulk optic Fabry-Perot interferometer. The resolution of this FFP-TP demodulator is 1 pm, while the usually-used ADVANTEST Q8347 can only provide spectrum resolution of 0.01 nm. Meanwhile a standard non-coated FBG fiber is used here as reference to compensate wavelength shift due to temperature drift and background noise. The measured data is recorded and sent to computer for further data treatment.
3. Results and discussion
Figure 3 shows drift of FBG wavelength (cladding-etched) during the increase of magnetic field up to 50 mT, since a pure FBG sensor is employed to compensate surrounding effect such as temperature change, the drift of measured FBG wavelength can be solely regarded as effect of magnetic field change due to different magnetic field level. Therefore the correlation of magnetic field and shift of FBG wavelength is maintained. It can be also concluded from Fig. 3 that cladding-etched fiber shows more sensitive to magnetic field change. With the same 0.8 μm TbDyFe coating, the sensitivity response of FBG wavelength shift for non-etched (125μm in diameter), one-hour etched (105μm in diameter) and 2-hour etched (85μm in diameter) are 0.386, 0.563 and 0.950 pm/mT respectively. These results prove correctness of the supposition that the strain is proportional to the mass that affected, an improvement of sensitivity with less mass of fiber due to etching process has been demonstrated. The more the etched cladding layer, the stronger the response.
Since the magnetostrictive TbDyFe coatings on different FBGs are the same, one can compar their improvement of sensitivity. When it is normalized with non-etched FBG, it can be concluded that the sensitivity of 2-hour etched FBG is about 2.5 times higher than the standard non-etched FBG, while it is only 1.4 times higher in case of one-hour etched FBG. In this sense, it is better to etch the outer cladding layer as thin as possible in order to enhance sensitivity to magnetic field. However during thin film deposition, FBG fibers with such thin diameter is prone to break, therefore it is not practically feasible to etch FBG cladding layer too thin. Normally we etched FBG fibers to 85 μm with a length of about 15 mm.
Similarly 1 μm FeNi coating, 1μm TbDyFe and a 0.5μm FeNi/0.5μm TbDyFe multilayer were prepared on standard 125μm single-mode FBGs. Their performances to magnetic field are characterized for comparison under the same condition as shown in Fig. 4 . In case of single layer, TbDyFe coating shows stronger response to magnetic field than FeNi, which can be understood with the fact than magnetostrictive coefficient of TbDyFe is greater than FeNi. Furthermore when compared with the former single TbDyFe coating (0.8μm in thickness) as shown in Fig. 3, a slight improvement of sensitivity with thicker TbDyFe sensitive coating can be observed. The sensitivity response of FBG wavelength shift is 0.523 pm/mT for non-etched FBG with 1 μm TbDyFe coating, which is a little higher than the former one as mentioned in Fig. 3.
However when compared to multilayer, 0.5μm FeNi/0.5μm TbDyFe multilayer shows the strongest sensitivity. The central wavelength shift of FBG coated with the above-mentioned multilayer is nearly 45 pm at the magnetic field level of 40 mT, this shift is only 20 pm for 1 μm TbDyFe single layer, and 11 pm in case of FeNi single layer. This has demonstrated the possibility to improve magnetostrictive response, and therefore to enhance magnetic field sensitivity. However this is not possible for bulk TbDyFe material.
From further review on the experimental results, it can be concluded that the TbDyFe/FeNi multilayer has sensitivity response of 1.08 pm/mT, which is already two times higher than the 1μm TbDyFe single layer. Even compared to the best result achieved by side-etching of FBG cladding layer, the sensitivity with multilayer is still a bit higher. It can be expected that the response sensitivity can be further improved by depositing multilayer on the side-etched FBG.
It should be mentioned that FBG wavelength shift is quite linear in any cases of coating or structure designs. This attributes to the good performance of TbDyFe giant magnetostrictive material, which has a linear and reverse response to strength of magnetic field. This linear response is very promising for sensor application.
Magnetic filed sensors based on magnetostrictive thin films have been proposed in this paper, two kinds of method to improve sensitivity has been demonstrated. Sputtered magnetostrictive TbDyFe coatings are deposited on different cladding-etched FBGs. The sensitivity response of FBG wavelength shift for non-etched (125μm in diameter), one-hour etched (105μm in diameter) and 2-hour etched (85μm in diameter) are 0.386, 0.563 and 0.950 pm/mT respectively. TbDyFe/FeNi multilayer has sensitivity response of 1.08 pm/mT, which is already two times higher than the 1μm TbDyFe single layer. The magnetic field response is linear and reversible. By structure and multilayer design, the proposed work cannot only miniaturize sensor size (thin film instead of bulk material), but also can improve response sensitivity to magnetic field, which are very promising for magnetic sensor application.
This work is finically supported by the Key Project of National Science Foundation of China, NSFC (Number: 60537050) and the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China(Number:708064).
References and Links
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