Approach to characterize manufacture tolerances of two-dimensional cross-phase grating

Two-dimensional cross-phase grating (CPG), diffracting the incident wave into (+1; +1), (+1;-1), (-1; -1), and (-1; +1) orders diffraction light, is used as a wavefront shearing element in cross-phase grating lateral shearing interferometer. However, the manufacture errors always exist, and they produce unwanted diffraction orders in the process of CPG manufacture. The appearance of the unwanted orders will increase the background noise of the interferogram and may affect the accuracy of the interferogram analysis. So, it is necessary to analyze the influences of manufacture errors and to determine the manufacture tolerances to remove the effect. A method based on Fourier transform analysis is presented to characterize manufacture tolerances of CPG. In this method, first, the normalized intensity distribution produced by CPG with manufacture errors in the far field is obtained. In addition, relative intensity of other unwanted orders to (+1;-1) order diffraction light of the four replicas is calculated, respectively. According to the relative intensity, the influences due to different types of manufacture errors can be evaluated, and the manufacture tolerances can also be determined. Using the proposed method, the manufacture tolerances of CPG at 632.8 nm are determined. Experimental evaluation of the proposed method using the manufactured CPG at 632.8 nm is also carried out. (C) 2013 Society of Photo-Optical Instrumentation Engineers (SPIE)