提出了一种直线式萨尼亚克(Sagnac)干涉仪入侵检测系统。该系统通过传感光纤末端连接法拉第旋转镜(FRM)构成直线式检测结构，可用作各种区域的光纤围栏。实验证实人行走引起的干涉输出是宽频信号，可利用求入侵信号零点频率的方法进行定位。对入侵引起的干涉信号的频谱分析发现，在特定条件下其与內嵌的入侵信号呈倍频关系。因此，提出由干涉信号的零点频率除以2得到入侵信号零点频率的方法，不必再用相位生成载波(PGC)技术对入侵信号进行解调。数值计算证明了该方法的可行性。把噪声信号驱动的压电陶瓷(PZT)调制器串接进传感光纤进行实验室模拟实验。传感光纤长11 km时平均定位误差为170 m，相对误差为1.5%。这表明利用获取零点频率的方法，该系统能较好地定位入侵。

A novel in-line Sagnac interferometric intrusion detection system was presented. The system constructs in-line detection structure by connecting a Faraday rotation mirror (FRM) with the sensing fiber end. The system can be used as a fiber fence in various zones. It has been confirmed experimentally that the output of the interferential system generated by one person′s walking is wideband. Therefore, the location method of seeking the null frequency can also be used for intrusion detection. The analyses of the frequency characteristic of the interference signal show that the frequency is twice as large as that of the inner intrusion signal under certain condition. The frequency doubling relationship can be used to obtain the null frequency of the intrusion signal from that of the system output. This system avoids using complicated phase generated carrier (PGC) techniques to demodulate the intrusion signal. The feasibility of the method has been proved by employing numerical simulation. A PZT modulator driven by a noise signal was added into the sensing fiber to simulate an intrusion in laboratory. An average location error of 170 m for sensing distance of 11 km was obtained, and the relative error is 1.5%. The experiment results show that this system works well in locating the intrusion by the proposed method of seeking the null frequency.