Rotationally symmetric wavefront coding for extended depth of focus with annular phase mask

Wavefront coding (WFC) with a cubic phase mask (CPM) for extended depth of focus (EDOF) has been studied since 1995. However, the asymmetric surface shape of a CPM has several disadvantages, such as necessity of a square aperture, directional artifacts in images, rotational adjustment during assembly, defocus-dependent lateral image displacement, and difficulty in fabrication. To overcome these disadvantages, we propose an annular phase mask (APM) with a rotationally symmetric form. An APM adds the phase of the parabolic profile in plural annuluses and phase steps between annuluses to the penetrating light. We confirmed an EDOF effect comparable to that of a CPM in our APM design by simulation. The depth of focus of a WFC system with the APM was estimated to be about 6.8 times larger than that of a conventional system. The effect observed in the experiments using a fabricated APM is consistent with the simulation result. (C) 2015 The Japan Society of Applied Physics