Optical Fiber Magnetic Field Sensor Based On Surface Plasmon Resonance

Magnetic field is a fundamental physical parameter in the earth,and the related magnetic field sensing technology can provide important technology support for many applications,such as navigation,earthquake warning,medical diagnosis and industrial production.Most of conventional magnetic field sensors are electrical sensors,which still present some drawbacks,such as the requirement for an additional temperature control system,large size,high cost,high power consumption andweak anti-electromagnetic interference.Besides,they are heavily influenced by the inherent magnetic field of the environment,which can add additional noise to such sensing devices.Optical fiber mangetic field sensors have advantages compared to the aforementioned electrical sensors.In paricular,they have small size,strong anti-electromagnetic interference ability,remote sensing capability,easy integration and multiplexing.These excellent sensing properties make optical fiber sensor not only overcomes the shortcomings of most existing electrical sensors,but can also be networked to meet demand of a wide range environmental magnetic field detection.Therefore,the importance of developing new optical fiber magnetic field sensors cannot be overstated.Considering the importance of magnetic field detection and research on related optical fiber magnetic field sensors,and problems of the limited sensitivity,temperature cross-sensitivity,ill-conditioned matrix and sensitivity nonlinearity in demodulation of optical fiber magnetic field sensors based on magneto-refractive effect need to be further solved.This paper designs and analyzes photonic crystal fiber（PCF）magnetic field sensors based on surface plasmon resonance（SPR）by the finite element method,characteristics and the sensing performance of which are also optimized to meet the demands of practical applications.The main contents are as follows:（1）Considering the field distribution of the surface plasmon polariton（SPP）mode,a high sensitivity D-shaped SPR-PCF magnetic field sensor is designed and investigated,the side polishing surface of the proposed sensor is coated with the gold film and a sensing channel is designed on the lower of surface and filled with magnetic fluid（MF）to achieve the magnetic field response.This paper systematically investigates the effect of structure parameters on sensing performance based on coupling strength and dispersion relationship between the SPP mode and the core mode.Finally,the designed magnetic field sensor achieves a high refractive index sensitivity of 59714.3 nm/RIU,a magnetic field sensitivity of 21750 pm/Oe,and a temperature cross-sensitivity of only-100 pm/°C.Compared with optical fiber magnetic field sensors reported previously,the magnetic field sensitivity in this paper is nearly two orders of magnitude higher and it can initially achieve n T magnitude magnetic field resolution and testing capability.（2）Based on orbital angular momentum characteristics of the SPP mode,a magnetic field and temperature dual-parameter sensor based on multi-order SPR-PCF is proposed,which can be applied in detecting the magnetic field and temperature.In this sensor,the left sensing channel is coated with the gold film and filled with MF to achieve a magnetic field response,the right sensing channel is coated with the gold film and filled with Polydimethylsiloxane（PDMS）to achieve a temperature response.In addition,in order to broaden the response range of excited SPR peaks for the magnetic filed and temperature,coating the right sensing channel with the Ta₂O₅ film to modulate the SPP mode and increase the wavelength spacing of the SPR peaks.This paper also investigates the physical principle of excited higher order SPP modes in the designed sensor.Finally,the proposed sensor achieves dual-parmmeter sensing of magnetic field and temperature and its magnetic field and temperature sensitivities are 65pm/Oe and-2360 pm/°C,respectively.By establishing and demodulating a sensing matrix,the sensor can not only measure the temperature and magnetic field simultaneously,but also solve the temperature cross-sensitivity problem in practical application.（3）Using different dispersion curves of the SPP mode can be produced by different metals,a magnetic field and temperature dual-parameter sensor based on dispersion modulated SPR-PCF is theoretically designed,where the left open loop channel is coated with the gold film and filled with MF for the magnetic field response,the right open loop channel is coated with the silver film and filled with PDMS for the temperature response.The dispersion of the SPP mode is modulated by different metals to achieve high sensitivity sensing of magnetic field and temperature.The principle of a SPR peak responding only to a single physical parameter is vestigated.Finally,the magnetic field sensitivity is 266.7 pm/Oe and the temperature sensitivity is-6000 pm/°C.The sensitivity of the proposed sensor is higher than that of the existing magnetic field and temperature dual-parameter SPR sensors.In addition,this sensor also solves the problems of non-linear changes of sensitivty and ill-conditioned matrix during demodulating matrix.（4）Combining MF and tilted fiber Bragg grating（TFBG）,a MF-TFBG sensor is designed to conduct an experimental study on the magneto-refractive properties of MF.In the parallel magnetic fields,the central wavelength of the sensor red shifts by MF modulation,thus confirming the magneto-refractive properties of MF.And the trend of the refractive index variation of MF is consistent with reported studies.In addition,due to the non-circular symmetric optical field of TFBG,the central wavelength blue shifts by MF modulation in the perpendicular magnetic field,and this phenomenon can explain the anisotropic arrangement of MF to a certain extent.The experiment has implications for further researching on MF properties and designing related magnetic field sensors.