System Design And Spectrum Characteristic Analysis Of A Closed-loop RFOG Based On Sine And Bipolar Serrodyne Modulations

The resonator fiber optic gyro?RFOG?is an inertial angular velocity sensor utilizing the Sagnac effect,and it is still in the stage of laboratory research so far.Using two LiNbO₃ phase modulators,this dissertation proposes a closed-loop RFOG system based on the digital sine and bipolar serrodyne phase modulations.For the closed-loop system,the spectrum characteristics of the two loops are analyzed,and a scheme to suppress the two loops' spectrum overlapping induced backscatter noise is proposed.The main research of this dissertation is as follows:?1?Using two LiNbO₃ phase modulators,a closed-loop RFOG system based on the digital sine and bipolar serrodyne phase modulations is proposed and realized.In this dissertation,digital sine phase modulation and bipolar serrodyne phase modulation are respectively applied to the two loops of the closed-loop RFOG system,which is realized by the two LiNbO₃ phase modulators,and the system is optimized.Compared with the previous closed-loop schemes,this scheme could avoid the increase of the number of the phase modulator and the corresponding hardware circuits for loading modulation signal,which is conducive to the miniaturization of the system;meanwhile,because only one loop is modulated by the serrodyne wave,the noise induced by the serrodyne modulation could be suppressed as possible.?2?Aimming at the closed-loop RFOG system proposed in this dissertation,the characteristics of the bipolar serrodyne phase modulation spectrum is analyzed when the RFOG is under the static condition,and the scheme of the static modulation spectrum optimization is proposed.The spectrum overlapping between the loops in a closed-loop system will introduce the backscatter noise.The spectrum characteristics of the two loops are analyzed under the static condition in this dissertation.According to the spectrum characteristics and the gyro output signal test result under different modulation parameters,a spectrum optimization scheme under the static condition is proposed,it is concluded that in order to avoid the spectrum overlapping between the two loops,the difference between the sine modulation frequency and the static initial serrodyne modulation reset frequency should be properly increased,and the sine modulation frequency is required to be close to an even multiple of the serrodyne modulation frequency.After using the spectrum optimization scheme,a bias stability around 7.1 deg/h based on the Allan deviation analysis is obtained under the static condition.?3?Based on the static spectrum study result,the spectrum overlapping characteristics of the proposed closed-loop RFOG is analyzed under the dynamic condition,and a dynamic spectrum optimization algorithm is proposed.This algorithm is realized by switching the static initial reset frequency of the bipolar serrodyne wave at a crucial rotation rate to always keep an enough frequency difference between the main frequency components of the light in the two loops,which can avoid the serious spectrum overlapping as possible.Experimental test result shows that when this algorithm is used,in the range of the rotation rates corresponding to serious spectrum overlapping,the decreased ratio of the standard deviation for gyro output signal can reach to about 70.5%.The above research results could provide a reference for the miniaturization of the closed-loop RFOG system and the further research of spectrum characteristics.