Abstract: 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 photo-detector (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.