Reflective Magnetic Field Sensor Based on Magneto-Optical Crystal and Fiber Bragg Grating
Objective The power system is a key hub in China's power transmission network,and includes many important power equipment such as transformers,distribution cables,and motors.The safety and stability of power equipment is an important guarantee for the reliable operation of the power system.Smart grid sensor technology as a key part of the power Internet of Things continuously develops and gradually covers a variety of applications in power industry perception scenes.Traditional magnetic field sensors are susceptible to electromagnetic interference and magnetic saturation,small dynamic measurement range,slow dynamic response,and insufficient insulation,which cannot meet the demand for real-time accurate monitoring of magnetic field in the emerging smart grid.Therefore,the continuous improvement and development of magnetic field sensing and measurement technology matching the smart grid is crucial for realizing the intelligent control of the power grid and ensuring the safe and stable operation of the power grid.The magneto-optical(MO)crystal fiber-optic magnetic field sensor features small size,sound insulation,and high sensitivity compared with other fiber optic sensors,and the Verdet constant of the MO crystal can be improved by adding rare-earth elements.Thus,it has broad engineering application prospects,but the temperature sensitivity will result in the crosstalk between the magnetic field and temperature.Methods We theoretically analyze the nonlinear effects of temperature on the magnetic field measurement of the sensor and design a reflective integrated fiber optic magnetic field sensor based on the common optical path structure of rare-earth MO crystals and the fiber Bragg grating(FBG)to solve the above problems.In the sensor,Bi∶YIG is selected as the magnetic field sensing element,and FBGs are an alternative to conventional mirrors with their high-reflectivity and temperature-sensitive characteristics.The theoretical sensing model of the structure is built by adopting the Jones matrix to support the magnetic field-temperature dual-parameter sensing.The linearly polarized light passes through the MO crystal to undergo the Faraday effect and then passes through it again after reflection through the FBG.Meanwhile,according to the non-reciprocal nature of the Faraday effect,the linearly polarized light undergoes a secondary deflection and then enters into the dual-optical probe via the polarisation beamsplitter to be demodulated.The temperature-magnetic field two-parameter demodulation method is proposed to employ the two-optical path demodulation method to obtain the Faraday rotation angle by difference and division and thus deduce the magnetic field information.Additionally,according to the two-optical path demodulation principle,the sum of the two-optical probe's detection results is the total reflected light intensity,and the FBG will produce a thermophotoretric effect by the influence of the temperature.This results in the wavelength shift of the center of the FBG and the shift can be demodulated by the edge demodulation method,with the temperature information deduced.Finally,the temperature compensation method is put forward to achieve accurate magnetic field measurement under the sensor de-temperature interference.Results and Discussions A new sensor structure employing MO crystal as the magnetic field sensing unit and FBG as the reflection device and temperature compensation unit is proposed,with the size of the integrated sensor being only 20 mm×4.2 mm(Fig.7).It is confirmed by finite element simulation that the composite structure can support the dual sensing unit for fast and real-time temperature response,which avoids magnetic field overshooting or hysteretic correction under the changing temperature(Fig.8).Meanwhile,the sensor temperature is tested and analyzed,and the experiment proves that after magnetic field calibration,the linear correlation coefficient of the sensor reaches 0.9995 under the direct current(DC)magnetic field input of 0.02‒30 mT,with the sensitivity of 0.034 rad/mT(Fig.10).The signal shows excellent dynamic response performance under the sinusoidal input of 50 Hz‒10 kHz,and there is no obvious aberration with apparent delay phenomena.The amplitude gain fluctuation and phase angle deviation are kept within 0.15 dB and 7.69° respectively(Fig.11).In the temperature characterization experiments,there is a relative error of about 32.77%in the Faraday rotation angle of the system output at an ambient temperature of 80℃compared to that at an ambient temperature of 20℃when a 30 mT DC magnetic field is applied(Fig.12).Additionally,the sensor performance is stable under an alternating current(AC)magnetic field in the temperature range of 20‒80℃after compensation(Fig.13).The comparison test of sensor reflection performance shows that the designed sensor has been substantially improved compared with the transmission sensor sensitivity of 0.018 rad/mT(Fig.15).Finally,it is confirmed that the sensor design in sensitivity,temperature compensation ability,and volume size is better than other MO crystal-type sensors(Table 1).Conclusions The temperature characteristic test of the sensor proves that the designed temperature compensation method can reduce the influence of ambient temperature on the magnetic field sensing performance to a certain extent.Meanwhile,this sensor has a certain temperature compensation function,and the system is not affected by fluctuations in the light source.The sensitivity of the reflective sensor structure in this study is approximately twice as much as that of the transmissive structure,and the sensor with better sensing performance features high integration,simple system structure,simple wiring,small size,and high sensitivity.Thus,the proposed sensor is applicable to the measurement of weak magnetic field in the internal environment of narrow electrical equipment such as the gap between high and low voltages inside the electric motor or transformer.Additionally,it can satisfy the special scenarios of high electric field and wide range fluctuation of temperature and can meet the needs of temperature and magnetic field sensing under special scenarios with high electric field and wide temperature fluctuations,with practical engineering significance.
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