Generation of an extended depth of focus using diffractive micro-lenses with binary structures in the non-paraxial domain

Diffractive micro-lenses (DMLs) with binary structures are analyzed and designed to generate an extended depth of focus (DOF) based on rigorous electromagnetic theory in the non-paraxial domain. The DOF can be extended by setting a positive preset DOF and by setting a lower height for the binary profile of the DML, respectively. Compared with the paraxial approximation (scalar theory), the focal characteristics of the designed DMLs, such as the actual DOF, the ratio between the DOF of the designed DML and the DOF of the conventional DML, and the spot size of the central lobe at the actual focal plane, for different f-numbers and for different heights of the binary profile, have been studied by using the rigorous vector method (FDTD method). Numerical and graphic results show that the designed DMLs indeed have an extended DOF while maintaining high transverse resolution by modulating the distribution and height of the binary structures of a DML's profile. The phenomenon that the variation of the height of the patterns can improve the DOF is also given an initial interpretation.