Effect of image curvatures in digital holographic microscopy

Abstract

This work presents a novel physical mean scheme for correcting the image curvatures introduced by a micro-objective (MO) lens in digital holographic microscopy (DHM). The image curvature due to the quadratic phase term is commonly referred to as defocus aberration. An offset lens is also developed to compensate for the defocus aberration of an MO, allowing us to remove the image curvatures. The proposed scheme is characterized by the separation of the MO and offset lens, ensuring that their foci are common. Analysis results of the optical operator method indicate that the inverted and magnified object wavefront, having passed through the modified microscopic configuration, no longer carries and contains the wavefront curvature due to MO. The proposed scheme is implemented based on arbitrary two-step phase-shift interferometry (PSI), yet with an unknown phase step. Hence, the numerical algorithm can subtract and eliminate the undesired terms, i.e. zero-order and twin-image terms, which inevitably arise from holographic recording. Additionally, the effect of the image curvatures are compared and demonstrated without and with the proposed on-axis setup. Experimental results indicate that the proposed scheme can remove the image curvatures from the defocus aberration due to MO, ultimately enhancing the phase contrast reconstructed image.