The link alignment in underwater wireless optical communication (UWOC) systems is a knotty problem. The diffractive deep neural network (D 2 NN) has shown great potential in accomplishing tasks all optically these years. In this paper, a 6-layer D 2 NN is proposed to alleviate the link alignment difficulties in UWOC systems. Simulation results demonstrate that the proposed method can focus incident light with tilt angles from 0°to 60°into a 6.25% area of the detection plane with an average focusing efficiency of 93.15%. Extra simulations further reveal that more layers lead to a sustained performance improvement before reaching a bottleneck, and the D 2 NN can achieve large field angle focusing within a certain focusing area. The proposed receiver design, which can be highly integrated with detectors, holds promise to realize reliable link establishment in UWOC systems in the future.

Polarization measurement plays an important role in optical detection, communication, and imaging systems. Compact polarimetry with a wide angle of incident light will break the restrictions of the limited incident angle and improve its practicality. In this paper, a full Stokes vector polarimetry with a wide incident angle of ±20°, based on a two-dimensional metasurface, is proposed. According to the simulation results, the maximum measurement error of the Stokes vector at 20° oblique incidence is 0.09. The light transmittance of the proposed structure is higher than 83%. Moreover, the tilt angle of the incident light can be determined with a calculation error lower than 0.5°, according to the focusing position of the transmitted light on the focal plane. The operating wavelength of the proposed polarimetry is set to 530 nm of green light, which makes it a potential application in visible light communication and underwater optical systems.

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