Low-frequency fiber optic magnetic field sensors

The detection of a magnetic field by a magnetostrictive-type fiber optic magnetic field sensor (FOMS) essentially involves measuring the longitudinal strain produced in the optical fiber to which a magnetostrictive material has been bonded.1 The performance of such a FOMS is critically dependent on the coupling efficiency between the magnetostrictive material and the optical fiber. The sensing elements are commonly (a) an unjacketed fiber bonded to a metallic glass ribbon or wire,2-5 (b) an optical fiber wrapped around a metallic glass cylinder,2 and (c) an optical fiber coated with a magnetostrictive film.6,7

We present a fiber optic magnetic field sensor conceived for magnetic resonance imaging (MRI) applications. The sensor is based on the integration of fiber optic strain sensors (fiber Bragg gratings—FBGs) with a sensing material (Terfenol-D). The response of an FBG integrated with a block of Terfenol-D was preliminarily investigated by taking into account the dependence of the Terfenol-D magnetostrictive response on both the longitudinal and transversal magnetic fields, with different preloads. Based on the performed characterizations, a triaxial magnetic field sensor was designed, characterized, and fabricated. An algorithm enabling the demodulation of the magnetic field from the readout of the three FBGs was also implemented, by taking into account the interdependence among the different sensor responses. Experimental results demonstrate the ability of the triaxial sensor to measure the magnetic field. Performance assessment and critical analysis are reported as well, elucidating both the abilities and limitations of the implemented sensing configuration. Finally, as proof of principle, a sensing system constituted of 20 triaxial sensors has been fabricated and used to map the magnetic field strength distribution in an MRI diagnostic centre.

A novel magnetic field sensing system based on surface plasma resonance (SPR) optical fiber sensor and filled with magnetic fluid (MF) is proposed and demonstrated for the first time. In the magnetic field SPR optical fiber sensor, SPR is excited by Ag as metallic material and MF is filled into the capillary sealed with epoxy glue, which utilizes the tunable refractive index (RI) of MF, and the transmission spectrum will change with different magnetic field intensities. The magnetic-optic effect of MF and the high RI sensitivity of optical fiber SPR sensor are utilized to enhance the sensitivity of the novel magnetic field sensor significantly. In the experiment, the performances of the magnetic field sensing system are tested by applying different measured magnetic fields. The final results indicated that a sensitivity of 303 pm/Gs is achieved. Compared with other optical fiber magnetic field sensors, the advantages of the proposed sensor in this paper are simple structure, small in size, easy to make, low cost, high sensitivity, and anti-interference.

This paper proposes a novel fibre optic magnetic field sensor based on core-offset bending structure cascaded with Fiber Bragg Grating (FBG). The sensor is fabricated by immersing a sensing structure, consisting of a single-mode fiber (SMF) with a lateral core-offset of 4 µm spliced to a single bending-ring fiber (SBF) with a bending radius of 4.5 mm and cascaded with FBG, in magnetic fluid (MF). The core-offset facilitates coupling of a portion of the core mode into the cladding, thereby enhancing the excitation of cladding modes and improving interference effects. Moreover, the cascaded FBG serves as a temperature compensation, which effectively separates temperature-induced and magnetic-field-induced variations. Experimental analysis shows that the magnetic field sensitivity of the sensor reaches 0.5117 nm/mT within the range of 0 to 27 mT, representing a 3.95-fold improvement over previously reported sensitivities. The sensor is characterized by its simple fabrication, low cost and high sensitivity.

A fiber optic magnetic field sensor is proposed and experimentally demonstrated. Pristine and Co doped ZnO nanorods of different Co concentrations (5, 10, 15 and 20 at%) were synthesized using a hydrothermal method. The synthesized nanorods were subjected to various characterization methods like X-ray diffraction (XRD), optical absorption, scanning electron microscopy, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, vibrating sample magnetometry and X-ray photoelectron spectroscopy (XPS). XRD and XPS analysis confirms that the Co ions were successfully incorporated into the Zn site of the wurtzite ZnO lattice without altering the structure. The pristine and Co doped ZnO nanorods showed remarkable changes in the M–H loop where the diamagnetic behavior of ZnO changes to paramagnetic when doped with Co. The sensor structure is composed of cladding modified fiber coated with Co doped ZnO nanorods as a sensing material. The modified cladding is proportionally sensitive to the ambient magnetic field because of the magneto-optic effect. Experimental results revealed that the sensor has an operating magnetic field range from 17 mT to 180 mT and shows a maximum sensitivity of ∼18% for 15 at% Co doped ZnO nanorods. The proposed magnetic field sensor would be attractive due to its low cost fabrication, simplicity of the sensor head preparation, high sensitivity and reproducibility.

A fiber optic magnetic field sensor node designed for seabed-mounted surveillance arrays, incorporating three orthogonally mounted magnetic field sensors and a microelectomechancial systems inclinometer, is described. Each sensor comprises a current carrying nonmagnetic bridge, which experiences the Lorentz force generated in a magnetic field. A metalized fiber laser attached to the bridge measures the induced strain. The operating principle and fundamental performance of the sensor as well as relevant performance parameters are described. Measured data illustrating the sensor and node performance in laboratory tests are presented. Less than 2 nanoTesla/Hz1/2 resolution at frequencies from 0.002 to 10 Hz is achieved with only 75 mA-rms of current. The sensor node is separated from the interrogation electronics with a 1-km fiber optic cable (extendable to 10 km) and qualified to an operating depth of 100 m. Methods to further improve the magnetometer performance are also discussed.

Due to the vast area of application and reliability, fiber optic magnetic field sensors have been the subject of several studies, however, some of these application areas are submitted to temperature variations, which can hinder the sensors in monitoring the magnetic field. With this panorama, this work analyzes through computational modeling a fiber optical magnetic field sensor, using the magneto-optical Faraday effect and observing temperature effects in the sensor response. For modeling, a numerical model built in COMSOL Multiphysics is used. The results show a value for cross-sensitivity of 3.27 mT/°C in a non-optimized configuration of the sensor and of 2.47 mT/°C for an optimized configuration. A methodology for optimizing the sensor to operate in a certain temperature range, 55 to 75 °C, is also discussed. The results presented in this work show that the temperature is an important factor to be considered to improve the selectivity and to obtain the correct sensitivity of the sensor.

Research on fiber-optic magnetic field sensor based on surface plasmon resonance

A distributed optical fiber magnetic field sensor based on a polarization-sensitive optical frequency domain reflectometer (POFDR) is proposed. It extracts the accumulated Faraday rotation by combining the Stokes vectors and the backward Mueller matrices from the measured states of polarization (SOPs) and obtains the magnetic field component. This method avoids adjusting the input polarization during the magnetic field sensing process. It overcomes the drawback of the conventional POFDR scheme, which requires at least two sets of different input SOPs for each sensing. Finally, the aforementioned effectiveness has been experimentally verified by using a single-mode sensing fiber. The results show that the sensor has good repeatability and linearity. The measurement error of the magnetic field sensor is 19.4 mT. The measured magnetic field variations agree with the applied ones with similarities higher than 0.98.

This work presents results of analysis of the optical fiber magnetic field sensor with external conversion fabricated in the Department of Optoelectronics of the Silesian University of Technology. Such sensor can be used as optical fiber magnetic field sensors or as the optical fiber current sensors. This study is focused on possibility of using a wide range of optical wavelengths in order to extend the measuring range of this sensor.

We present a novel distributed feedback fiber laser (DFB-FL) fiber laser sensor, which can measure static magnetic field through sweep acoustic wave. A fiber laser is bonded on the giant magnetostrictive material Terfenol-D (Tb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> Dy <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.92</sub> ) tube oriented along the longitudinal direction. When acoustic wave of different frequency is added to the fiber laser, it has the maximum response at its resonant frequency, which will vary with the external magnetic field. The measurement sensitivity of the sensor is 0.39 Hz/mT to the magnetic field. The experimental results also show that the sensor exhibits excellent linearity and directivity in response to static magnetic field.

This paper is devoted to the description of a new type of fiber optic magnetic field sensor based on spectral modulation encoding techniques. This device uses a Faraday magneto-optic element as transducer and converts the rotation of the plane of polarization of the light induced by the magnetic field into a small change in the large optical path difference provided by a birefringent coding plate. The use of a broadband source and of an interferometric detector, with an optical path difference matched to the sensor one, allows to reach high resolution and perfect down-lead insensitivity. A dual-wavelength arrangement is proposed to determine the temperature and magnetic fied values with the same probe.

A CuNiCe-O nanocomposite was fabricated on the Cu40Ni20Al10Ce26Pt3Ru1 metallic glass (MG) ribbon surface by dealloying. The influences of Ni and dealloying time on the morphology and EOR performance were analyzed. The results suggest that the catalytic activity and stability of the dealloyed MG ribbon could be significantly enhanced owing to the alloying of Ni to the Cu60Al10Ce26Pt3Ru1 MG ribbon precursor. The activated D-Cu40Ni20Al10Ce26Pt3Ru1 ribbon obtained at an etching time of 3 h had a better electrochemical ethanol oxidation reaction (EOR) performance than other dealloyed samples due to the formation of abundant active sites and the presence of defects within the CuNiCe-O composite.

Fiber-optic magnetic field sensing is an important method of magnetic field monitoring, which is essential for the safety of civil infrastructures, especially for power plant. We theoretically and experimentally demonstrated an optical fiber magnetic field sensor based on a single-mode-multimode-single-mode (SMS) structure immersed into the magnetic fluid (MF). The length of multimode section fiber is determined based on the self-image effect through the simulation. Due to variation characteristics of the refractive index and absorption coefficient of MF under different magnetic fields, an effective method to improve the sensitivity of SMS fiber structure is realized based on the intensity modulation method. This sensor shows a high sensitivity up to 0.097 dB/Oe and a high modulation depth up to 78% in a relatively linear range, for the no-core fiber (NCF) with the diameter of 125 μm and length of 59.8 mm as the multimode section. This optical fiber sensor possesses advantages of low cost, ease of fabrication, high sensitivity, simple structure, and compact size, with great potential applications in measuring the magnetic field.

In many fields, such as aerospace and marine environmental monitoring, magnetic field measurement is an important link. In recent years, optical fiber magnetic field sensor has received much attention because of its advantages such as small size, electromagnetic immunity, resistance to erosion and capability of remote sensing. In that case, magnetic fluid as a kind of medium between photons and magnetic field is widely used in optical fiber magnetic field sensors. Moreover, in the process of magnetic field measurement, disturbance introduced by temperature fluctuation always happens and brings uncertainty to the sensor. Temperature is also an important parameter in production process and needs to be measured. Therefore, designing a high-sensitive optical fiber sensor for simultaneously measuring magnetic field and temperature is a valuable work. In this paper, we present a high-sensitive hollow core fiber (HCF) interferometer for simultaneously measuring magnetic field and temperature. A segment of HCF filled with alcohol is inserted into single mode fiber (SMF) with 50 m offset at two splicing joints to guide light into the wall of HCF. And then this SMF-HCF-SMF structure is packaged by a capillary tube with full magnetic fluid (MF) inside it. Since the modal field area is large enough, the silica wall can support a series of guiding modes among which modal interference occurs and the interference spectrum can be recorded by an optical spectrum analyzer. Besides thermo-optic effect and thermal expansion effect of silica itself, the RI variations caused by thermo-optic effect of alcohol and MF as well as the magneto-optic effect of MF can also cause the phase difference of the guiding modes to change, thereby rendering interference dips movable. Thus, the sensitivity of temperature or magnetic field is higher than those given in some other previous studies. In addition, it is calculated that the effective RI sensitivities of guiding modes for inside and outside liquid are different because of the peculiar non-circular symmetry structure of HCF. So there is a possibility to find two dips in interference spectrum, which are formed with different modes and have various sensitivities to the variations of temperature and magnetic field. Finally, a sensitivity matrix can be built to demodulate those two parameters simultaneously. Experimental results show that within 20-58℃, the temperature sensitivities are 112 pm/℃ and 468 pm/℃ for dip1 and dip 2 whose magnetic field sensitivities are 37 pm/Oe and 82 pm/Oe within 0-169 Oe, respectively. The proposed sensor possesses high sensitivity and good mechanical strength, and can effectively eliminate the cross disturbances between temperature and magnetic field.