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

The technology of magnetic field measurement has been widely demanded in the fields of power industry,aerospace,defense military and biomedicine.Compared with traditional electrical sensors,optical fiber sensors have the advantages of small size,high sensitivity,corrosion resistance,and electromagnetic interference resistance,making them a hot topic in the field of sensing.As a new type of magnetically sensitive material,nano-magnetic fluid has some good optical characteristics,such as optical transmission characteristics,birefringence effect,etc.,under the action of external magnetic field.Based on the combination of the above two,various optical fiber magnetic field sensor devices based on magnetic fluids can be developed.In this paper,a femtosecond laser micromachining technology is used to prepare an optical fiber magnetic fluid film,and its light transmission characteristics are studied under the action of magnetic field.Then,in order to improve the sensitivity and accuracy of magnetic field measurement,we combined the birefringence effect of magnetic fluid to develop an optical fiber Sagnac interference magnetic field sensor based on magnetic fluid and an optical fiber laser magnetic field sensor based on magnetic fluid.The main contents of this article are as follows:(1)By setting a custom laser scanning path and selecting the optimal processing parameters,we use femtosecond laser technology to etch micro-channels carrying magnetic fluid on single-mode fiber to prepare magnetic fluid film.We theoretically analyzed the microstructure of magnetic fluid with and without external magnetic field and different external magnetic fields.In addition,we also experimentally studied the light transmission characteristics of light incident on the magnetic fluid film perpendicular to the direction of the magnetic field.The experimental results show that when the magnetic field strength gradually increases from 0m T to 40.51 m T,the light intensity change range is about 6.12 d B.Besides,the light intensity change is basically linear between 0?11.34 m T,the optimal sensitivity can reach 0.4818 d B/m T,its linearity is 99.612%.Finally,we also verified the feasibility of the magnetic fluid film as a magnetron tunable optical attenuator.(2)An optical fiber Sagnac interference magnetic field sensor based on magnetic fluid is experimentally studied.We combine the birefringence effect of the magnetic fluid with the Sagnac interferometer,and introduce a certain length of polarization-maintaining fiber to make the transmission spectrum "visible" on the optical spectrum analyzer.We change the birefringence of the magnetic fluid by applying an external magnetic field,so that the transmission spectrum drifts and the magnetic field is detected.The theory proves that the sensitivity of the sensor can be further improved by appropriately shortening the length of the polarization-maintaining fiber or increasing the thickness of the magnetic fluid film.The experimental results show that when the PMF length is 100 cm,the transmission spectrum of the optical fiber magnetic fluid film will drift toward the long wavelength in the magnetic field range of 0?160.07 m T,and the total drift is 0.36 nm.Among them,in the magnetic field range of 0?11.57 m T,the wavelength drift increases linearly with the increase of the magnetic field and the magnetic field sensing sensitivity is 20.72pm/m T.The wavelength will continue to drift until saturation after 11.57 m T,but the trend of change is not linear again.When the PMF length is 55 cm,the sensitivity of the magnetic fluid film in the magnetic field range of 0?11.56 m T is 43.48pm/m T,and the linearity is 98.792%.When the PMF length is 9cm,the sensitivity of the magnetic fluid film in the magnetic field range of 0?11.31 m T is 106.93pm/m T,and the linearity is98.768%.(3)A fiber laser magnetic field sensor based on magnetic fluid is experimentally studied.The birefringence effect of the magnetic fluid is excited by the applied magnetic field to affect the change of the total birefringence in the laser cavity,causing the polarization beat frequency mode to drift.Therefore,the magnetic field can be detected.The experimental results show that in the magnetic field range of 0?10.34 m T,the magnetic field sensitivity reaches 10.016KHz/m T,and the linearity is 98.954%.In addition,due to the high signal-to-noise ratio of the selected polarization beat mode signal,the sensor has a high measurement accuracy.