采用声光调制器设计了一种透射式外差干涉椭偏测量系统。实验测量了单层透明氧化铟锡(ITO)膜，膜厚和折射率测量误差分别达8nm和7%。采用琼斯矢量法分析了金属反射镜引起的光束椭偏化对测量结果的影响。从光学系统中移出被测样品得到的标定数据，可以消除金属反射镜本身退偏效应的影响，但无法消除退偏效应和方位角误差共同作用所引入的椭偏参数测量误差。计算结果表明，退偏效应和方位角误差共同作用引入的膜厚测量误差可达4 nm左右，该误差与薄膜参数无关，与方位角误差近似成线性关系。

A transmission ellipsometer with the configuration of heterodyne interferometer and two acousto-optical modulators was investigated. A single layer of indium tin oxide on a glass substrate was measured, and the measurement errors of the sample thickness and refractive index range up to 8 nm and 7%, respectively. The influence of the elliptic polarization due to metal-coated mirrors on measurement accuracy was analyzed with Jones vector method. The error only produced by the depolarization effect of metal-coated mirrors can be eliminated just by removing the sample out of the configuration. However, the calibration does not work to the error resulted from the misorientation and the depolarization effect. The calculation shows the error of sample thickness measurement is 4 nm, and the error is approximately linear with the orientation error and independent of the film parameters.