提出了一种与快轴方向无关的λ/8波片(EWP)相位延迟量实时测量方法。准直激光束经过圆起偏器(CP)和被测λ/8波片后形成测量光束，测量光束由二维正交光栅(2DOG)分束形成4束衍射子光束，它们经过检偏器阵列(AA)后由光电探测器阵列(PA)接收，利用探测到的4个光强信号可以实时测量被测λ/8波片的相位延迟量，且测量结果不受激光器光强波动的影响。实验中，标称值为45°、精度为λ/300的被测λ/8波片在不同快轴方向上的相位延迟量的测量平均值及其标准差分别为45.1°和0.4°。相位延迟量的测量平均值、标准差与被测λ/8波片的相位延迟量标称值、精度相符，很好地验证了该方法的有效性。

A real-time measurement method for retardation of eighth waveplate (EWP) independent of fast axis is presented. The collimated beam passes through the circular polarizer (CP) and the EWP to form a measuring beam. The beam is split to four diffractive beams by a two-dimensional orthogonal grating (2DOG). They are detected by a photodetector array (PA) after passing through a polarizer array (AA). The four light intensity signals can be used to achieve real-time measurement of retardation of EWP. The result is not affected by the fluctuation of the intensity of the laser. In experiments, an EWP whose nominal value of retardation is 45° with accuracy of λ/300 is measured. The mean and standard deviation of measurement results for retardation in different directions of the fast axis are 45.1° and 0.4°. They coincide with the nominal value and accuracy of the retardation, respectively. The usefulness of this method is verified.