Planar diffractive imaging element design

In planar diffractive imaging system, extra-axial imaging elements are frequently used. Since the elements have large off-axial aberrations such as coma, astigmatism, field curvature and distortion, it seems a little difficult to design a practical aberration-free element. In this paper, we reviewed the axial imaging diffractive element design procedure. Referring to designing axial elements, we present a semianalytical approach that enables one to determine the exact surface profile of an extra-axial element based on geometrical optics according to design and aberration-free requirements. The design procedure of the element can be divided into two steps: firstly, to obtain the zone boundaries and then to solve the exact surface profile. Finally, a schematic is given to test and evaluate small size diffractive elements.