提出了一种基于波长调制的低双折射光纤拍长测试方法。测试方法采用波长调制的方式进行拍长测试，应用相位检测技术提高信号精度，采用相位补偿手段保证较高检测灵敏度。测试光路包括放大自发辐射(ASE)光源、法布里-珀罗可调谐滤波器、两个起偏器、待测光纤及相位补偿片。通过对线保偏光纤和普通单模光纤进行重复实验，对测试装置进行了实验验证。本方法结构简单，对器件性能要求不高；对光纤长度及结构无特殊要求，适用范围广；拍长测量范围达到20 m，测试误差小于1%。

A measurementof beat-length in low-birefringence fibers based on wavelength modulation is shown. It measures the beat- length by wavelength modulation. Phase detection is chosen to reduce the error from environment influence. Phase compensation plate isused to ensure the testing system always sensitive to the change of phase.The optical path consistes of an amplified spontaneous emission (ASE) light source, a fiber Fabry-Perot tunable filter, two linear polarizes, the measured fiber and a phase compensation plate. The testing system is set up and tested in reality.The system performes well in the repeated test. It is simple and has no strict demand on these equipments. There is no limitation on the configuration and the length of the testing fiber in the measurement.It is reliable for fibers whose beat- length is up to 20 m. The beat- length precision of the measurement is less than 1%.