用平面波计算拍频条纹间距时，因激光器中存在衍射效应，且实际光强分布为高斯型，故给光电探测器光窗尺寸计算带来很大误差(40%)，导致系统不能正常工作。为此，以高斯光束为基础，利用光电探测器测得2路信号的相位差，通过数值计算得到2束光拍频条纹间距，由此计算得到理想相位差光电探测器的尺寸。最后，用实验验证了该设计方法的可行性，为RLG光电探测器光窗尺寸的设计提供了一种简单准确的方法。

Because there is diffraction effect in laser and the laser intensity distribution is in Gauss form,a big error is introduced in the calculation of optical window size for the photodetector (PD) when the spacing between the beat frequency fringes is computed with plane wave. Two steps are applied to the design. First,the phase difference of the two signals is detected by a PD. Then,based on Gauss beam,the spacing between the beat frequency fringes of the two beams is calculated to get the ideal size of the optical window. The method was verified with an experiment.