Comparative study of wavefront coding imaging with rotational and non-rotational symmetric phase masks

Abstract

Wavefront coding enables conventional optical imaging systems to operate over an extended depth of field/focus by modifying the light field using a specially designed phase mask. Different phase masks can be used to alter the transmitted wavefront of the optical system which may result in different performances in terms of the capability of the depth-of-focus extension, aberration suppression and the process of imaging acquirement. In this paper, we present a comparative study on the performances of two major different categories of the phase mask, i.e., rotational symmetric and asymmetric type phase masks. Three different types of phase masks that are of cubic, quartic, and logarithmic phase profile are investigated. Fabrication and metrology of a cubic mask is conducted and a full cycle of imaging process including the image coding and decoding is performed.Keywords: wavefront coding; depth of focus; rotational symmetric and asymmetric; phase mask INTRODUCTIONWavefront coding imaging technology(WFC) was first proposed to extend the depth of focus of an optical system by Dowsky and Cathey in 1995[1], and has attracted much attentions since then [2][3][4]. The technique is based on the modification of the wavefront (or wavefront coding) by means of a suitable phase mask (such as a cubic phase mask) placed at the aperture stop of the system. The major function of the WFC is to modify the point spread function (PSF) of the system in such a way that it becomes invariant over a range of distances around the image plane. The final image with diffraction-limited quality is obtained by using digital filtering process from the coded images.Phase mask is a key element in the WFC system and plays a crucial role in terms of imaging quality, capability of extension of the focus depth, and the process of the imaging with the additional possibility of aberration suppressing of the system. In the past of ten years, a variety of phase masks were proposed for the purpose of improving imaging quality, increasing the capability of extension of focus depth and also for the easy fabrication of the phase mask itself, which includes a cubic phase mask[1], a quartic phase mask[5-6], a logarithmic mask [7], an exponential phase mask [8], and a high-order polynomial phase mask[9,10], etc. These different types of phase masks can be classified into two major categories, i.e., rotational symmetric

The influence of phase mask position upon wavefront coded system: doublet imaging system

“…Secondly, the wavefront coded optical system is effectively becoming defocus invariant. Research has shown that the wavefront coding technique with CPM can extend much larger DOF than other phase masks such as the quartic phase mask and the logarithmic phase mask 7 . This is a great advantage on improving functionality of the existing system by using CPM.…”