Comparison of encoding methods for computer-generated diffractive optical elements.

Five modern encoding methods for diffractive optical elements are compared in terms of diffraction efficiency for an f/1 focusing element. It is shown that all five produce similar results for a given value of the minimum feature size delta and number of phase levels N when the grating period is larger than N delta. In contrast, when the grating period is smaller than N delta, the analytic quantization method cannot be employed. When this condition is met, it is shown that the radially symmetric iterative discrete on-axis method can result in diffraction efficiencies as much as 30% higher than those obtained with direct sampling. It is also shown that performance advantages of iterative algorithms improve as the target region in the focal plane increases.