提出一种基于相位调制和样品摆动的1/4波片相位延迟量测量方法。准直激光束依次通过起偏器、相位调制器、待测1/4波片和检偏器由光电探测器所接收，光信号被转换成电信号后经过放大、滤波以进行数据处理。利用待测1/4波片的摆动可以计算得到与快轴方位角无关的归一化二次谐波分量，在无需知道1/4波片快轴方位角的情况下得到其相位延迟量。实验中对一块石英1/4波片进行了测量，实验结果与现有快轴方向确定条件下的光弹调制测量方法的测量结果一致，某一快轴方位角上多次测量的重复性为0.13°，不同快轴方位角上多次测量的再现性为0.17°。

A method based on phase modulator is presented to measure retardation of the quarter-wave plate by swinging it without the information of its fast axis direction. In this method, collimated laser beam goes through a polarizer, a phase modulator, the quarter-wave plate to be measured, and an analyzer, and finally incident upon the detector. Its electric signal is amplified and filtered for data processing. The retardation can be attained by swinging the quarter-wave plate, without the precondition of the knowledge of fast axis direction. In experiments, a quartz quarter-wave plate is measured, and its repetitiveness is 0.13° and the reproducibility is 0.17°. Our result is consistent with the one got by the method on the photoelastic modulator when the azimuth of the fast axis is 0°.