Investigation on Axicon Transformation in Digital Holography for Extending the Depth of Focus in Bio-Microfluidics Applications

Limited depth of focus (DOF) is one main shortage in traditional optical microscopy systems that severely affect simultaneous visualization of objects at different depths in the same field of view. In this paper, we propose a novel method to enhance the DOF in digital holography that consists of applying a numerical axicon transformation to the hologram during the reconstruction process. The idea behind this is to exploit the well-known ability of an axicon lens to create long and narrow focal lines along the optical axis. By this approach, we demonstrate that it is possible to obtain an extended focused image in which objects located at different depths are simultaneously visualized in good focus. First, the proposed method is tested in a case study of three different wires, positioned on different planes and recorded in lensless configuration. A comparison with a common DOF extension approach based on cubic phase function is performed. Finally, experiments of motile cells, flowing in a microfluidic channel and at different depths, are investigated for demonstrating the effectiveness of the proposed approach in bio-microfluidics.