Research Of High Sensitivity Fiber-optic Magnetic Field Sensing Based On Magnetic Fluid

Magnetic field measurement has a long history.In the 12th century,as the most primitive magnetic field measuring instrument,si'nan was widely used in navigation.Magnetic field measurement plays a vital role in many fields such as navigation,environmental monitoring,biomedicine,aerospace and smart grid.The gradual expansion of the application range has led to the development of magnetic field sensors in the direction of high accuracy,high stability,high resolution,and miniaturization.Compared with the traditional magnetic field sensor,the fiber magnetic field sensor has the advantages of anti-electromagnetic interference,small size and high sensitivity,which further improves the performance of the magnetic field sensor.In recent years,optic-fiber magnetic field sensors based on magnetic fluid have become a research hotspot for scholars at home and abroad.Magnetic fluid is a special magneto-optical nanomaterial.It possesses both the magnetism of solid magnetic material and fluidity of liquid material,and it has remarkable properties of refractive index tunability,faraday effect,birefringence effect,absorbing coefficient tunability,and dichroism effect.The refractive index tunability is one of the most important points,that is,the refractive index of the magnetic fluid can vary with the strength of the applied magnetic field.In this paper,the refractive index tunability of magnetic fluid is combined with fiber-optic sensing structure,and a fiber-optic M-Z magnetic field sensor is constructed by using magnetic fluid as the sensing medium.This paper has mainly completed the following works:(1)The sensing principle and characteristics of fiber M-Z sensing structure were studied,and the numerical simulation was carried out.The relationship between refractive index sensitivity,optical loss,length of sensing structure and offset of sensing structure were analyzed.The simulation results showed that the refractive index sensitivity reached-11775.86nm/RIU.And then,the temperature response characteristics of the M-Z sensing structure were simulated,the temperature sensitivity was 28.57pm/?.(2)A sensing structure fabrication system was built,which can accurately control the length of the M-Z sensing structure.The sensing structure is fabricated based on the parameters obtained by the simulation,and the sensing characteristics were studied.The experimental results showed that the refractive index sensitivity is-15568.96nm/RIU,and the temperature sensitivity is 25.23pm/?.The experimental results were in good agreement with the simulation results.(3)An optical fiber M-Z magnetic field sensor based on magnetic fluid was fabricated and a magnetic field measurement experimental system was built.The maximum magnetic field sensitivity in the directions vertical and parallel to the magnetic field were-4.49nm/Gs and 0.52nm/Gs,respectively.The temperature response of the sensor was measured in the range of 26.69??49.75?,and the sensitivity reaches 2.49nm/?.The experimental results showed that the temperature would affect the magnetic field measurement.(4)Three different solutions to the problem of temperature and magnetic field cross-sensitivity of magnetic fluids are proposed.The first one,using fiber grating,which is temperature sensitive and insensitive to refractive index,to achieve temperature compensation.The second one,the sensor achieve temperature compensation by itself,two interference troughs of the sensor has different sensitivity of temperature and refractive index,establishing a two-parameter demodulation matrix to achieve two-parameter measurement of temperature and magnetic field.The third one,connecting the M-Z magnetic field sensor and M-Z temperature sensor on the fiber grating demodulator simultaneously,two output spectra are obtained at the same time,and then the output spectrum is analyzed and calculated to realize the two-parameter measurement of temperature and magnetic field.In this paper,the sensing characteristics of the fiber-optic M-Z sensing structure and the refractive index tunability of the magnetic fluid are studied,and the two were combined to form an M-Z magnetic field sensor,which had a high magnetic field measurement sensitivity.