【作者】 王潇；

【导师】 胡永明；

【作者基本信息】 国防科学技术大学 ， 光学工程， 2009， 硕士

【摘要】 法拉第旋镜FRM (Faraday Rotator Mirror)法是抑制干涉型光纤传感系统偏振诱导信号衰落的一种重要方案,它在理论上可以完全消除偏振衰落,但是由于所选FRM的旋转角度并非精确的45°,系统中会出现残余偏振诱导相位噪声(简称:偏振噪声)。当偏振噪声与目标信号频带重合时二者无法区分,在这种情况下,深入进行偏振噪声和相关抑制方法的研究有特别重要的意义。本文用琼斯矩阵建立了基于FRM的干涉型光纤传感系统偏振噪声的模型,分析出影响偏振噪声的三种主要因素:FRM的旋转角度偏差,入射光偏振态调制度,干涉仪两臂光纤双折射。根据这些因素提出了三种抑制偏振噪声的方法:1、尽可能选择角度偏差接近0的FRM,这种方法最直接有效;2、在传输光纤周围加一圈缓冲层以减小外界环境对光纤的直接作用,减小偏振态调制度;3、在干涉仪两臂各加一个PZT,对其施加直流调制以改变光纤双折射,从而减小偏振噪声。计算出当所选FRM为最大旋转角度偏差1°(由产品指标得到)时,系统在偏振态调制度为1.84 rad时可能出现最大偏振噪声,其幅度为0.0815 rad (-26.6 dB)。为了调制入射光偏振态并研究由此给干涉系统带来的偏振噪声,设计了一种M-Z型光纤偏振态调制器PSM(Polarization State Modulator),测量了PSM的稳定性,结果表明在测试时间内PSM的波动小于0.8%。搭建了基于FRM的平衡干涉型光纤传感实验系统,用PSM对入射光进行频率为1 kHz、不同调制度的偏振态调制,通过PGC外调制解调出由此产生的偏振噪声,结果表明测量值与理论值吻合较好,证明了所建偏振噪声模型的正确性.扭曲Michelson干涉仪两臂光纤,测量了干涉仪内光纤双折射对偏振噪声的影响,结果表明扭动光纤得到的偏振噪声比扭曲前减小了约12 dB ,证明了通过调整干涉仪双折射以消除或抑制偏振噪声的可行性。最后指出,实验中设计的偏振噪声测试系统还可用于检测传感探头的光学偏振噪声特性。用PSM对入射光施加相同的偏振态调制度,测试各探头由此产生的偏振噪声,选出偏振噪声较小的探头,从而保证各阵列的低偏振噪声特性。更多还原

【Abstract】 The method based on Faraday Rotator Mirror(FRM) is one of the most important schemes to eliminate polarization induced fading(PIF) in interferometric fiber-optic sensing system, which in theory can completely eliminate PIF, but due to the rotation angle of the selected FRM is not exactly 45°, the system will appear in the residual polarization-induced phase noise(PIN). When the frequency bands of the residual PIN and the target signal overlap, they can not be distinguished from earch other. In this case, in-depth studies on the characteristics of residual PIN and the associated noise suppression methods seem to be especially important.In this dissertation, the residual PIN model of the fiber optic interferometric system based on FRM has been setup, and the three main factors affecting the PIN have been analysed : the ratation angle deviation of FRM, the state of polarization (SOP) of the input light and the birefringence of the optical fiber of the interferometer. Based on those factors three kinds of ways have been proposed to restrain the PIN:1、try the best to select FRMs with the rotation angle deviation close to 0, this approach is the most direct and effective; 2、wrap the lead-fiber with cushion materials to minish direct effect by external environment so as to decrease the degree of polariztion modulation. 3、add a PZT to each interferometer beam and drive the PZT with direct current to decrease PIN. The greatest noise with its amplitude of 0.0815 rad(-26.6 dB) possibly existing in the system when the degree of the poarizaion modulation equals to 1.84 rad is obtained.In order to modulate the SOP of input light and investigate the PIN induced by it, a M-Z type optic fiber polarization state modulator(PSM) is put forward and its stability is measured with the result that its fluctuation is lower than 0.8%. Experimental sensing system of balance fiber optic interferomter based on FRM has been set up, and the PIN induced by the input light with different SOP demodulated by PSM at 1 kHz is measured through PGC, results indicate that experiment values are in good accordance with the ones of theory, which demonstrates the correctness of the PIN model. The PINs are measured as well by twisting the optic fiber of the interferometer, which directly changes the birefringence of the fiber, and results show that a decrease by about 12 dB on PIN is obtained by twisting the optic fiber, and the feasibility to eliminate or suppress the residual PIN by adjudting the birefringence of the optic fiber is proved.In the end, it is pointed that the system testing PIN can also be used to check the characteristics of optical PIN of the sensor. Modulate the SOP of the input light by the same degree, and test the PIN for every sensor to be used in big scale system, then select the ones with low PIN to ensure low-PIN characteristics of the whole system.更多还原