Module for zooming in extended depth of focus in digital holographic microscopy

Abstract

Microscopes with continuous magnification and a quick zoom reaction time are much needed for imaging of biological specimen. Here, we are proposing a module consisting of two electrically tunable lenses (ETL) to facilitate the digital holographic microscopy with a continuous variable zoom/magnification within the extended depth of focus of the microscope objective. The proposed module provides a large working distance for the movement of the sample as well as the elimination of break-off while switching objectives. The major benefit is that the image plane remains stationary throughout the zooming range within the extended focal depth. The range of both ETL currents for the continuous zoom operation is discussed. Post experimental calculations, over a 3.5 mm expanded focal depth, the zoom precision is determined to be 12 µm with a response time of ∼7 ms. The zoom ratio has been found to be in the range -28.75X to -122.5X experimentally. In the expanded depth of focus, the zoom ratios at different focus planes are presented with simulation and experimental results. Moreover, the quantitative information of micro samples at deep depths with high zoom ratios without sacrificing resolution over a field of view of 1.77° is provided. The results are provided for human red blood cells and a standard USAF chart present at different depths to demonstrate the feasibility of utilizing the proposed configuration for zooming into extended focal depths of objective for optically transparent samples.