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.