Research On Optical Fiber Magnetic Field Sensing Based On Sangac Interference Structure

Magnetic field sensing has important applications in many fields,including military equipment,industrial manufacturing,geophysical exploration and biomedical sensing.Driven by military,national defense,and economic development needs,magnetic field sensing technology now requires high precision,small size,low cost,large detection range,and strong anti-interference ability.The traditional magnetic field sensing technology is mainly based on fluxgate,Hall effect and other electrical effect,but most of these sensing technologies are difficult to meet the needs of new applications.The various types of optical fiber magnetic field sensing technology have many advantages,such as high sensitivity,easy manufacturing,low cost,lightweight and compact system,and strong anti-interference capability.In magnetic field detection and other fields,they have shown good application potential.Based on this,the paper addresses the application needs of magnetic field sensing technology by specifically conducting the following research work:(1)For the measurement of weak alternating magnetic field,a magneto-strictive optical fiber magnetic field sensing system is proposed which is based on Sagnac interference structure.The magnetic sensitive unit consists of a magneto-strictive material rod,optical fiber wrapped around it and a pair of Neodyfeb permanent magnets.The magnetic sensitive unit is connected to the Sagnac interference structure,and the magnetic field information is calculated by detecting the non-reciprocal phase difference.The sensing mechanism of the system was deduced theoretically and analyzed by simulation.The results show that the sensitivity is influenced by both the enhancement of non-reciprocal phase difference in Sagnac interference structure and the suppression of eddy current effect on the high-frequency magnetic field.(2)The performance of the proposed system is tested.The experimental system is built and the simulation results are verified,and the system’s magnetic field sensitivity was improved by lengthening the delay fiber ring.The experimental results show that the measurement range is 0-274.05 μT(rms),3 d B bandwidth of the system is 100-2500 Hz.At 500 Hz,the sensitivity of the system is up to 1.34 m V/μT(rms),and the minimum detectable magnetic field is 25 n T/√Hz(rms).The system has the advantages of large bandwidth,wide range,linear operation and high sensitivity.(3)For the measurement of direct current magnetic field,we proposed an all-fiber vector magnetic field sensing scheme.The system consists of Sagnac interferometric structure,which uses the polarization-maintaining fiber ring as a sensitive unit for magnetic field intensity and direction measurement.When the magnetic field is applied to the polarization-maintaining fiber ring,the circular birefringence caused by Faraday effect will produce phase difference related to the magnetic field intensity and direction,and the theoretical model of the magnetic field phase difference is established by Jones matrix.The experimental results show that the system could achieve magnetic field direction measurement,the direction error is only ± 0.4°.Besides,the high sensitivity is realized,which reaches 0.507 m V/m T.(4)The environmental adaptability of the proposed system is studied.Because the system can simultaneously use two orthogonal polarization states for magnetic field sensing,the measurement errors caused by environmental temperature,vibration can be suppressed by the difference calculation of polarization states.We analyzed theoretically and experimentally the influence of environmental temperature,vibration on system.The experimental results show that the measurements of the fast axis and the slow axis change equally with the environmental temperature and vibration,and the suppression effect of temperature and vibration error still exists even under the changing environmental conditions.In this experiment,the system can achieve good inhibition of temperature variation between 30 ℃ and 50 ℃,and also achieve good inhibition of environmental vibration within 1 g,achieving environmental anti-interference.