Underwater wireless optical communication technology has higher speed and better security than underwater acoustic communication technology, and it has become a key tool to realize the communication between underwater environment monitoring, underwater wireless sensor networks, marine exploration, ships, and submarines. Since all vortex modes of vortex beams are orthogonal, the multiplexing of the vortex beams can further improve the communication capacity and spectral efficiency. Underwater vortex optical transmission can provide a new way to realize ultra-wideband and high-speed underwater wireless optical information transmission. In this paper, the transmission characteristics of the Laguerre-Gaussian (LG) vortex beam and its two superposition states in underwater turbulence are studied. The underwater turbulence caused by random diffusion of temperature and salinity is simulated by adding water with different temperature and salinity differences. The effects of turbulence generated by different temperature and salinity differences on the beam drift and scintillation index of the Gaussian beam, LG vortex beam, and the two superposition states are investigated. The research results can provide an important reference for the research on the transmission of vortex beams and their superposition states in underwater channels.

In marine media, refractive index fluctuations are controlled by temperature and salinity fluctuations. This paper uses a constant flow pump to add water with a certain temperature and salinity differences to simulate underwater turbulence and studies the influence of underwater turbulence on the light spot. In the experiment on underwater turbulence caused by the temperature difference, this paper first adds 20 ℃ clean water into the water tank, sets up four experimental groups with a temperature difference ranging from 0 to 15 ℃ (an interval of 5 ℃), then heats the clean water to the temperature required in the experiment, and finally pours the water with a specific temperature into the water tank through a water pump. In the experiment on underwater turbulence caused by salinity difference, the paper first adds 20 ℃ clear water into the water tank, sets up four experimental groups with a salinity difference ranging from 0 to 3‰ (an interval of 1‰), and then calculates the quality of edible salt used for the experimental salt water in the four groups. In addition, the paper adds edible salt to a certain amount of clear water to prepare salt water with a specific concentration and then pumps it into 20 ℃ clear water. After the hot water or salt water is added to the water tank through the water pump, the paper records the light intensity image data when the light spot received by the CCD begins to change. In order to reduce the experimental error, each group of experiments continuously measures and records 2000 data and is repeated for many times. The light intensity image received by CCD is grayed, and the gray value of each light intensity image is calculated to reflect the light power, so as to calculate the scintillation index.

Gaussian beam, LG vortex beam with order 0 and topological charge 6, vortex light superposition state 1, and vortex light superposition state 2 all produce different distortions after turbulence caused by temperature and salinity differences. Compared with the other three beams, the LG vortex beam has a slight spot variation (Fig. 3). After the turbulence caused by temperature and salinity differences, the probability of the four beams appearing near the center of the calibration position decreases, while that appearing far away from the center of the calibration position increases. In the same simulated turbulent environment, the distribution degree of the centroid offset of the Gaussian beam from the center of the calibration position is the largest, while that of the LG vortex beam from the center of the calibration position is the smallest, with the centroid offset degree of the two vortex light superposition states falling in the middle (Fig. 4). When the temperature difference or salinity difference is constant, the beam drift variance of Gaussian beam is large, and that of LG vortex beam is small. In addition, the beam drift variance of vortex light superposition state 1 is smaller than that of vortex light superposition state 2 (Fig. 5). When the temperature difference is constant, the scintillation index of the Gaussian beam is larger, and that of the LG vortex beam is smaller. The scintillation index of vortex light superposition state 1 is smaller than that of vortex light superposition state 2. When the temperature difference is 0, 5, and 10 ℃, the scintillation index of the two vortex light superposition states is close to that of the LG vortex beam (Fig. 8). When the salinity difference is constant, the scintillation index of the Gaussian beam is larger, and that of the LG vortex beam is smaller. The scintillation index of vortex light superposition state 1 is smaller than that of vortex light superposition state 2. When the salinity difference is 0 and 1‰, the scintillation index of the two vortex light superposition states is close to that of the LG vortex beam (Fig. 10).

In this paper, the beam drift and scintillation index changes of Gaussian beam, LG vortex beam with order 0 and topological charge 6, vortex light superposition state 1, and vortex light superposition state 2 after underwater turbulence caused by different temperature and salinity differences are experimentally studied. The experimental results show that with the increase in temperature and salinity differences, the turbulence intensity increases, and the beam drift variance and scintillation index of the four beams rise. Compared with those of the other three beams, the beam drift variance and scintillation index of the LG vortex beam are smaller. When the temperature difference or salinity difference is the same, the beam drift variance and scintillation index of vortex light superposition state 1 are smaller than those of vortex light superposition state 2. When the temperature difference is 0 and 5 ℃, the beam drift variance of the two vortex light superposition states is close to that of the LG vortex beam. When the temperature difference is 0, 5, and 10 ℃, the scintillation index of the two vortex light superposition states is close to that of the LG vortex beam. When the salinity difference is 0 and 1‰, the scintillation index of the two vortex light superposition states is close to that of the LG vortex beam. According to the comprehensive analysis, under weak underwater turbulence, the use of vortex optical superposition state communication can improve communication capacity and spectral efficiency. Furthermore, under strong underwater turbulence, the LG vortex beam has better transmission quality.