Research On Key Technology Of Depolarized Fiber Optic Gyroscope

Technology of polarization maintaining is the main method to resolve the problems of signal attenuation and nonreciprocal polarization error in fiber optic gyroscope (FOG). Now, FOG with polarization maintaining technology has reached the level of navigation-class and precision-class, however, there are many difficulties to be solved in engineering application, such as high cost not avail to batch production, high bent loss not avail to small volume, weak ability of radiation resistance not avail to space applications, and so on.Depolarization technology overcomes the weakness of polarization maintaining technology, and application of depolarization technology in FOG promotes FOG's development in the direction of low-cost, miniaturization and space application. Compared with polarization-maintaining FOG, the depolarized FOG is structured more complex and with apparent errors, such as polarization state error and magnetic field error. In order to enable the depolarized FOG reaching navigation class function, this paper set the light polarization state error, the magnetic field error, system design and data processing technology of the depolarized FOG as the key technologies, and main works done in this paper are as the follows:1. The influence exerted on Sagnac effect by optic loop shape and optic wave-guide media is analyzed. The method for measuring Sagnac phase with digital closed-loop fiber optic gyroscope is introduced and some related theories including the coherence of light wave, the reciprocity of light wave, and the polarization state of light wave are described in detail. Based on above theories, the influence that spectral characteristics of light source had on coherence of light wave in FOG is compared by using the power spectrum density. According to the light polarization state of the depolarized FOG, the structure and port reciprocity of Lyot depolarizer are investigated. 2. The stress birefringence of the fiber-optic ring is derived by analyzing the stress characteristics of the fiber-optic ring, and the stress birefringence model of the fiber-optic ring is built, which is the base to quantify fiber coil's stress birefringence and to optimize the structural parameters of the Lyot Depolarizer. With Stokes parameters, Poincare Sphere and the Jones matrix, the state of optical polarization in depolarized FOG is analyzed, the propagation process of depolarized optical wave is described and the non-reciprocity polarization phase error model for the depolarized FOG is set up. Based on this model, the inhibitory methods about polarization phase error of the depolarized FOG is studied and the power and wavelength stability are researched, which are the theoretical base to propose solutions.3. Magnetic field bias phase error model for the depolarized FOG is set up by using Jones matrix. Relationship between some influence factors of the depolarized FOG and the magnetic field phase error is theoretical researched, such as fiber length and fiber torsion efficiency. Based on above theoretical analysis, the radial magnetic field error model for depolarized FOG is built according to the structure of optical fiber ring. The fiber-optic ring's transmission matrixes at clockwise and counterclockwise directions are calculated by using micro-unit, and the mathematical description about the radial magnetic field phase shift of the depolarized FOG is concluded. Based on fiber-optic ring's axial structure, the axial magnetic field error model by using the helix angle expansion method is build, and the axial magnetic field phase shift for the depolarized FOG is quantified. The factors which have effect on the radial magnetic field error and axial magnetic field error of the depolarized FOG are analyzed based on the quantitative results above mentioned, and detail measures to reduce the sensitivity of the depolarized FOG in magnetic field environment are proposed.4. The structure of depolarized FOG is explored systematically by considering optic structure and electric system. Measures are adopted to optimize the optical path system, such as, Lyot depolarizer's structure and the structural design of the depolarized FOG. Measures are adopted to optimize signal processing system, such as, closed-loop signal processing style, clock, power supply and electromagnetic compatibility (EMC). Catch the whole error source and to increase system stability.5. The output data of the depolarized FOG contains a great deal of time-varying and non-statistical regularity noise. To solve this problem, the adaptive filtering algorithm to deal with the output data is introduced, fixed step size LMS filter and variable step-size LMS filter are constructed, which have the functions of forming variable fitting model and identification parameters. The capacity of parameter adjustment, computational speed and other indexes in the process of data filtering between the two types of filters are compared. The data filtered by the two filtering algorithms are analyzed by using Allan variance, and the filtering effect is evaluated.