A fast autofocus method based on virtual differential optical path in digital holography: Theory and applications

Typical autofocusing approaches have been proposed for digital holography (DH) to ascertain the peaks or valleys of critical functions. However, the sensitivity near the peaks or valleys is very low, which increases the difficulty of determining the best focusing position. This paper proposes a novel and fast focus plane detection method based on a virtual differential optical path, in which two virtual differential detectors are employed along the direction of the optical axis. One of the virtual detectors is located before the real detector and the other is behind it. This feature allows users to obtain two ordinary critical curves and a differential autofocusing curve (DAC). To determine the best focusing plane, the zero-crossing point of DAC is utilised. Mathematical models of the proposed method are developed, and numerical simulations and optical experiments are demonstrated to verify its feasibility. Results of industrial and biomedical objects demonstrate that the proposed method has good sensitivity and computational efficiency compared to traditional methods. To the best of our knowledge, this is the first time that the virtual differential optical path has been exploited for DH.