Spatial frequency spectrum optimization method for design of diffractive optical elements in uniform illumination

A spatial spectrum optimization method for the design of diffractive optical elements (DOE) used in uniform illumination, is presented to improve the design results and optimize efficiency. The spatial frequency spectrum of intensity distribution on the focal plane is analyzed, which shows that the uniformity can be further improved by controlling the low frequency spatial modulations in the focal spot. So the goal and cost functions of the new method are chosen in spatial spectrum domain to determine the searching path, which can decrease the low frequency modulations. By using the new method, a DOE that has multi-step phase distribution is optimized. The result shows that the root-mean-square error of light intensity of the main lobe of the focal spot is 3.3% with smoothing in 20 μm. Compared with existing spatial domain optimization methods, the new spatial spectrum optimization method can effectively improve the practical performance of DOE and save computing time.