The Research On Weak-magnetic Field Sensor Based On Chirp Fiber Grating Fabry-Perot Cavity

Optical fiber weak magnetic field sensor has attracted considerable attention because of their potential applications in national defense and industry. In the current research of the magnetic field sensor,the main difficulty is that the miniaturization of the sensor structure and high sensitivity performance are carried out simultaneously.This paper presents a new FPI (Fabry-Perot interferometer) weak magnetic sensitive structure composition of broadband chirped fiber grating, composed all fiber magnetic devices with the giant magnetostrictive material. Based on reflectance spectrum characteristic, the weak magnetic field is measured by using phase demodulation. This system has the advantages of compact structure,stable and high measurement precision etc. The structure of a system is applicable for the construction of large capacity magnetic field detection array. This thesis focuses on the research of the weak magnetic field sensor with all fiber grating F-P cavity, the main contents are as follows:1. The coupled mode equations of chirped fiber gratings are derived, and the transmission theory model of chirped gratings is established by using the piecewise transfer matrix. The factors that impact broadband optical fiber F-P cavity transmission and reflectance characteristics are simulated and analyzed. The simulation results show that the longer the grating is or the larger the chirp coefficient is, the wider the reflection spectrum is and the denser the interference fringes are formed and the higher the phase sensitivity of the interferometer is. The longer the cavity length is, the smaller the distance of the interference fringes is and the more interference fringes are formed and the higher the accuracy of the interferometer is.2. Based on the theory of coherent rotation, the variation of magnetostrictive material in magnetic field is derived. The magnetostriction of rare earth giant magnetostrictive material was measured by Michelson interferometer. The experimental results are in good agreement with the theory. The magnetic transducer is designed and constructed. The phase sensitivity of the fiber optic magnetic field sensor system is calculated by analyzing the performance of the magnetic transducer and FPI. The calculation results show that the phase sensitivity of the sensor system is 1.56 rad/?T in H = 10 ?T, and the phase sensitivity is 1.96 rad/?T in H = 10 ?T.3. The F-P cavity phase demodulation algorithm based on fringe counting method is researched. The magnetic field generated by a solenoid was measured by using the optical fiber F-P cavity magnetic field sensor the sensor. The experimental results show that the phase sensitivity of the magnetic sensor system is 1.57 rad/?T, the average measurement error of the system is 1.2 ?T, the measurement error variance is 0.6 ?T.The measurement error can be attributed to the coupling loss of the magnetic transducer and the inhomogeneity of the magnetic field measured.4. The F-P cavity phase demodulation algorithm based on Fourier transformation is studied. The sampling point in frequency domain is corrected by using the the three spline interpolation and the subscript value of the fundamental frequency obtained by the Fourier transformation is corrected by frequency Gauss interpolation method, the precision of magnetic field measurement is improved. The results of magnetic field detection experiments show that the phase sensitivity of the system is 1.62 rad/?T, and the average measurement error of the system is 0.43 ?T, and the measurement error variance is 0.3 ?T.