摘要
陀螺是用来测量角速度的惯性传感器,是导航和制导系统的基础部件。基于Sagnac效应的谐振式光学陀螺(Resonator Optic Gyroscope, ROG)是实现高精度陀螺微小型化和集成化的重要技术途径,相对于传统的干涉式光学陀螺来说,可以实现更小的体积甚至片上集成,相对于微机械陀螺来说,更抗振动,有更高的精度和稳定性。
高相干性光源和光学谐振腔,是谐振式光学陀螺中非常重要的关键元件。论文对上述两种关键元件非理想特性及对陀螺性能的影响进行了理论和实验研究。论文主要工作和创新点包括：
(1)在谐振式光学陀螺系统中提出并深入分析一个重要的噪声因素——激光器频率噪声通过交叉调制效应将对系统输出精度造成严重的影响,称之为交调噪声。分析结果表明,系统调制解调频率偶倍频尤其是2倍频附近的激光器频率噪声将在系统中引起交调噪声。对交调噪声进行了不同调制系数、系统参数、谐振腔结构下完整的理论分析。对于100kHz量级的激光器白噪声线宽,当使用最佳载波抑制比调制系数2.405来抑制瑞利背向散射噪声时,会产生10-3rad/s量级的角速度误差,这比系统极限灵敏度高约4个数量级。
(2)利用相位调制器施加正弦波相位调制和白噪声信号,建立了模拟激光器频率噪声的实验系统,对激光器频率噪声产生的交调噪声大小进行了具体测试,进一步验证了建立的交调噪声理论；结合实际RFOG系统,将四种不同白噪声PSD特性的激光器应用于实际RFOG系统,在RFOG输出端,成功观察到了交调噪声影响大小,测试结果与理论值较为一致。
(3)对光波导谐振腔谐振曲线不对称产生原因进行了分析和比较,包括背向散射、光克尔效应、偏振波动和耦合器正交模式损耗差异；结果表明,耦合器正交模式损耗差异是影响目前光波导谐振腔芯片产生谐振曲线不对称性的最主要因素；在使用正弦波相位调制来检测谐振腔谐振频率时,当谐振曲线具有不对称性时,两路光调制频率的差异会在陀螺开环输出中产生非零的偏置,不对称程度越大,调制频率的差值越大,开环偏置也越大；采用透射式谐振腔则能够完全抑制由单一耦合器正交模损耗差异导致的谐振曲线不对称性现象。
综上所述,激光器频率噪声在谐振式光学陀螺系统中产生的交调噪声进行分析、计算和实验测试。通过对谐振腔谐振曲线不对称性现象的分析和比较,发现在光波导谐振腔中,耦合器正交模损耗差异是影响谐振曲线不对称最重要的因素。从而为今后激光器和谐振腔的选择或优化提供理论和实验依据。
Gyroscope is the basic sensor to detect the inertial angular velocity in the Navigation and guidance system. The resonator optic gyro (ROG) based on Sagnac effect is an important technological approach to achieve miniaturization and integration of high performance gyroscope. Compared with Interferometric Fiber Optic Gyro (IFOG), ROG has smaller volume and has the potential of integrated in a single chip. Compared with MEMS gyroscope, ROG is resistant to vibration and has better accuracy and stability.
Highly coherent laser source and the optical resonance cavity are very critical components in a ROG. This paper conduct theoretical and experimental research on how these two key component affect the ROG performance. The main or innovational work of this thesis including:
(1) The intermodulation noise in the ROG system is proposed and deeply analyzed. The intermodulation noise is produced by the laser frequency noise at the even multiples of the modulation frequency due to an intermodulation effect, this error will seriously limit the random noise performance of the resonator optic gyroscope. A systematic theoreitcal analysis of the intermodulation noise is conducted with different modulation index, system parameter, resonance cavity and laser source. When a modulation index of2.405is used to suppress the Rayleigh baskscattering noise, the intermodulation noise induced by a100kHz white noise linewidth laser source in a RFOG(resonator fiber optic gyro) system is in the10-3rad/s order, which is4orders of magnitude higher than the shot noise limit.
(2) The intermodulation noise amplitude are measured by applying sin signal and white noise signal on a phase modulator to simulate the laser frequency noise. The intermodulation theory are further verified. Four lasers with different frequency noise are used as the laser source of RFOG system, the intermodulaion noise induced by laser frequency noise are observed, experimental results fits the theory well.
(3) The resonance curve asymmetry in a RIOG(resonator integrated optic gyro) is analyzed. Four possible sources including Rayleigh backscattering polarization effect and normal mode loss difference of the coupler are compared. The normal mode loss difference of the coupler is the dominant source for the resonance curve asymmetry observed in the RIOG. When sin wave phase modulation is used to detect the resonant frequency, if the resonance curve isasymmetic, the modulation frequency difference in the modulation frequency in the two lightwaves will cause output bias in the RIOG system. Larger asymmetry and modulation frequency difference means larger output bias. Using the transmission-type resonanct cavity can eliminate the resonance asymmetry induced only by the normal mode loss difference of the coupler.
In summary, the intermodulation noise induced by the laser frequency noise in a ROG system is analyzed, calculated and verified. Coupler normal mode loss difference is found to be the dominant source for the resonance curve asymmetry observed in RIOG. This will provide theoretical and experimental base for selection and optimization of the laser source and the resonant cavity.
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