保偏光纤模式耦合分析及其相干检测

Analysis and coherent detection of mode coupling of polarization maintaining fiber

当保偏光纤存在结构不均匀或者受到外部扰动时,将使其内部传输的一部分偏振光耦合到与其正交的偏振态上去。分析了保偏光纤内偏振光模式耦合的原理,给出了基于白光干涉法的偏振模式耦合检测方法,并用迈克尔逊干涉仪对保偏光纤的偏振模式耦合的耦合强度和耦合点发生的空间位置进行了测试。通过步进电机控制迈克尔逊干涉仪扫描臂的反射镜移动,改变干涉仪两臂之间的光程差,来补偿由于偏振耦合而形成的两偏振光从保偏光纤出射时的光程差,实现了对偏振耦合的测量。为提高检测系统的灵敏度,需提高输出信号的信噪比,对光源发出的光进行高频调制,通过频谱搬移和相干解调,有效地抑制了各种干扰和噪声,使输出信号的峰值信噪比提高了5.1 dB,耦合强度测试最小值也由-57.6 dB提高到-62.7 dB,增强了测试系统检测微弱偏振耦合的能力。

When there are structure imperfections within polarization maintaining fiber（PMF） and external perturbations acting on PMF, the polarization mode light which is propagating in PMF would be partly coupled into the other orthogonal one. The principle of the polarization mode coupling in PMF was analyzed. The detecting method of the mode coupling based on white-light interferometry was presented. Micheison interferometer was used to measure the spatial distribution and coupling intensity of polarization coupling along PMF. The optical path difference （OPD） between two arms of Michelson interferometer, changed by the movement of scanning aim reflector driven by a step motor, compensated the OPD of two orthogonal beams resulted from polarization coupling to realize the measurement of polarization coupling. In order to improve the sensitivity of the detection system, the signal-to-noise ratio of the output signal should be higher. By adding high-frequency modulation to the light emitted from the optical source and coherent demodulation to the signal detected by the photodiode, the disturbance and noise were restrained strongly. As a result, the peak SNR improved about 5.1 dB. The minimum coupling intensity of the system increased from -57.6 dB to -62.7 dB. The result shows that the ability to detect weaker coupling intensity of the system is enhanced.