Researchers from Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences (CAS), have proposed a method of full-field differential interference contrast imaging based on ancient Greek-ladder sieves. The related work was published in Optics and Lasers in Engineering on June 11, 2022.

The traditional DIC technique shows the two-dimensional distribution of the optical path gradient observed in a certain shear direction. Based on the research team’s previous works, ancient Greek-ladder sieves with small optical path difference has been successfully constructed and used to realize full-field differential interference contrast imaging because of the effect of radial-shearing interference.

Fig. 1 shows the experimental setup of full-field differential interference contrast imaging. Fig. 2 indicates the experimental results of phase-only resolution test target under the illumination at a wavelength of 532nm. The edge of the observation object can be seen with a clear relief structure. In this image, group 5 element 4 can be resolved clearly, which responds to the spatial resolution of 11um. This method can be used for monitoring the damage dots and providing security protection in the high power laser system.

Ancient Greek-ladder sieves is a kind of diffractive optical element and can be used to generate structure light array. High-efficiency vortex grating with phase modulation can be obtained by coding. The interference happens between the vortex array and the Walsh field (Fig. 3) and then decoded through convolutional neural network (Fig. 4), which can simultaneously determine the topological charge, chirality and the initial angle. The predict accuracy is more than ninety-five percent. The related work was published in Applied Optics on September 7, 2022.

The proposed method has great possibilities to realize feature recognition of 2D array structure light.

Fig. 1. Experimental setup for full-field differential interference contrast imaging. (Image by SIOM)

Fig. 2. Experimental results of phase-only USAF resolution test target. (Image by SIOM)

Fig. 3. Interferogram between the vortex array and Walsh field. (Image by SIOM)

Fig. 4. Flow chart of convolutional neural network. (Image by SIOM)