Imaging performance of off-axis planar diffractive lenses

The imaging performance of off-axis planar diffractive lenses is described. In particular, the second-order monochromatic aberration terms are derived for object and image planes normal to the respective object-space and image-space base rays for arbitrary orientation of those base rays. It is shown that in general, images will suffer from linear astigmatism. If the system is plane symmetric, then for a given object-plane orientation and magnification there will exist a unique image-plane orientation for which the linear astigmatism disappears. With this constraint we show that the only remaining second-order aberrations are distortion terms and that the third-order aberrations are similar to the third-order aberrations for the axially symmetric case. I describe an off-axis lens that we have fabricated for use as a miniature microscope objective (3 × 0.31 NA) and evaluate its imaging characteristics to illustrate the theoretical predictions.