介绍了一种基于相位调制干涉仪解调光链路的新型光纤长度测量系统。推导了光链路增益理论模型，分析了光纤干涉仪的两臂光程差和增益自由频谱范围（FSR）的关系，得出利用长度已标定光纤来计算待测光纤长度的方法。该测量方法和传统的光时域反射（OTDR）方法相比，无测量盲区且可以将测量精度由米量级提高到毫米量级；与光频域反射法（OFDR）和光相干域反射法（OCDR）相比，对光源的相干性要求显著降低。通过理论分析和实验验证，采用现有的67 GHz安捷伦网络分析仪可以测试的光纤最小相对长度为3 mm，在相对光纤长度100 m的情况下，测量误差小于0.05 mm。该系统的原理及结构都很简单，是一种具有实用价值的光纤长度测量方法。

Based on phase modulation optical link, a novel fiber length measurement system is proposed. The theory model of gain of optical link is derived. By analyzing the relationship between the free spectral range of gain of the optical link and the optical path difference of the fiber interferometer, the fiber length is acquired. Compared with the traditional time domain reflectometry (OTDR), this system has no dead zone, and improves the precision from meter to millimeter. Compared with optical frequency domain reflectometry (OFDR) and optical coherence domain reflectometry (OCDR), this system does not need costly and high-performance laser to ensure coherence. The feasibility of the system is analyzed in theory, and is verified by experiments. It is shown that the smallest measurable relative length of fiber is 3 mm with the existing 67 GHz vector network analyzer, and the error can be controlled within 0.05 mm when the relative length of fiber is 100 m. This system is characterized by simple structure, easy operation and higher practical value.