The transmission distance, alignment requirement and stability of underwater wireless optical communication (UWOC) systems are usually difficult to trade off. In this paper, to ensure that the optical spot at the receiver end has sufficient optical power and uniform power distribution, the transmitter of a UWOC system is designed based on a single-color light-emitting diode (LED) array with a beam shaping system. The lens group-based beam shaping scheme not only extends the transmission distance, but still maintains the easy-alignment nature and stability of the LED-based UWOC link. At the receiver, a diversity reception scheme with an array of photomultiplier tubes (PMTs), and a high-gain scheme combining a PMT with an optical antenna, are adopted and compared. To alleviate the inter-symbol interference (ISI) and compensate for the nonlinear distortion of the signal, hardware pre-equalization and nonlinear post-equalization based on Volterra are adopted at the transmitter and receiver, respectively. A UWOC transmission with a distance of 50 m and a data rate of 300 Mbps is realized, which is the highest rate-distance product realized by an ordinary monochromatic LED-based transmitter ever reported. The stability and practicability of the proposed UWOC system with relaxed alignment requirement are verified in laboratory and swimming pool, as well as in a real harsh sea environment.

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.