Diffractive optical element designed by use of an irregular etching-depth sequence

In fabricating a diffractive optical element the ratio of the etching depth between the (n - 1)th and the nth mask is usually 1/2. We found that the diffraction efficiency of a diffractive optical element can be improved by as much as 7.8% if the above ratio (1/2) is not kept constant. For achieving this improvement the difference between the desired and the actual diffraction pattern is also used as an objective function for phase quantization.