Research On Optical Fiber Magnetic Field Sensing Technology Based On Magnetic Fluid

Magnetic fluid is a stable colloidal suspension with both fluidity of liquid and strong magnetism of solid.It is a new functional material with special magneto-optical characteristics that can be used as sensing material.In this thesis,magnetic fluid is used as the sensing medium,and its refractive index is modulated by external magnetic field.By combining the magnetic fluid and the fiber-optic interferometer,the detection of the magnetic field is converted into the measurement of the output interference wavelength and intensity of the fiber-optic interferometer,and two optical fiber magnetic field sensors with excellent performance were prepared.Firstly,an M-Z interferometer with singlemode-multimode-singlemode biasing structure is proposed and fabricated.Since the essence mechanism of the optical fiber sensor based on magnetic fluid is refractive index sensing,its ability of sensing refractive index was verified before studying magnetic field sensing.Six sucrose solutions with different concentrations were used to obtain solutions with different refractive indexes.By analyzing the experimental results of the refractive index response of the sensor structure,it was proved that the sensor structure has good refractive index sensing characteristics,which lays a theoretical foundation for further research on the magnetic field sensing characteristics.Secondly,an interference-type optical fiber magnetic field sensor based on magnetic fluid is prepared by taking magnetic fluid as the outer cladding of the sensing structure.The interference mode in the interference structure is modulated by the magnetotropic properties of magnetic fluids,and the corresponding magnetic field variation parameters are demodulated by the wavelength shift of interference valley and the change of transmission intensity in the spectrum.The experiment results show that the wavelength shift sensitivity is 24.4pm/Oe,and the transmission intensity sensitivity is 0.0407 d B/Oe.Compared with similar sensors,the sensor designed in this thesis has high sensitivity with simple structure,which is made on the basis of traditional single-mode and multi-mode fiber,with low cost,and has obvious advantages over those made by special fiber.Finally,in view of the high magneto-optical coefficient of the magnetic fluid and its sensitivity to temperature,this thesis proposed a sensor that simultaneously measured the temperature and magnetic field,which used single-mode fiber with eccentric fusion splicing with traditional SMS structure.Utilizing the magnetotropic properties of magnetic fluids,magnetic field parameters are applied to fiber-optic sensing structures.The traditional SMS interference structure is fused to the core ofthe input single-mode fiber,so that the single-mode fiber core mode and the cladding mode,the multimode fiber fundamental mode and the high-order core mode interfere with each other,forming different interference valleys of sensitivities to temperature and magnetic fields.Two dips with different properties at 1537 nm and1563nm in the interference spectrum were selected for observation.The experimental results showed that the sensitivity of the interference valley wavelength at 1537 nm to the magnetic field was 6.05 pm/Oe,and the temperature sensitivity was 35.6pm/?.The magnetic field sensitivity of the interference valley wavelength at 1563 nm was16.7pm/Oe,and the temperature sensitivity was 20.44pm/?.By setting up the sensitive matrix,the temperature and magnetic field parameters are measured simultaneously,and the cross-sensitive problem of temperature and magnetic field is effectively eliminated.The sensor has good temperature,sensitive magnetic field response and high measurement accuracy.