水下拉盖尔-高斯涡旋光束及其叠加态传输特性

Transmission Characteristics of Underwater Laguerre-Gaussian Vortex Beam and Its Superposition States

通过实验研究了拉盖尔-高斯涡旋光束及其叠加态在水下湍流中的传输特性,充分考虑了不同温度差和盐度差的水流扩散产生的湍流对4种光束(高斯光束,阶数为0、拓扑荷数为6的拉盖尔-高斯涡旋光束,阶数为1、拓扑荷数为2与阶数为0、拓扑荷数为6的拉盖尔-高斯涡旋光叠加,阶数为1、拓扑荷数为2与阶数为0、拓扑荷数为10的拉盖尔-高斯涡旋光叠加)传输的影响,并对4种光束的漂移方差和闪烁指数进行深入讨论与分析。实验结果表明:随着湍流强度的增大,4种光束的漂移方差和闪烁指数都增大,相比其他3种光束,拉盖尔-高斯涡旋光束的漂移方差和闪烁指数较小;在较弱的湍流强度下,两种涡旋光叠加态的漂移方差和闪烁指数与拉盖尔-高斯涡旋光束相近。

Objective 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.Methods 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 experimentalgroups with a temperature difference ranging from 0 to 15 ℃ (an interval of 5 ℃ ), then heats the clean water to thetemperature required in the experiment, and finally pours the water with a specific temperature into the water tank througha water pump. In the experiment on underwater turbulence caused by salinity difference, the paper first adds 20 ℃ clearwater into the water tank, sets up four experimental groups with a salinity difference ranging from 0 to 3‰ (an interval of1‰), and then calculates the quality of edible salt used for the experimental salt water in the four groups. In addition, thepaper adds edible salt to a certain amount of clear water to prepare salt water with a specific concentration and then pumpsit into 20 ℃ clear water. After the hot water or salt water is added to the water tank through the water pump, the paperrecords the light intensity image data when the light spot received by the CCD begins to change. In order to reduce theexperimental error, each group of experiments continuously measures and records 2000 data and is repeated for manytimes. The light intensity image received by CCD is grayed, and the gray value of each light intensity image is calculatedto reflect the light power, so as to calculate the scintillation index.Results and Discussions Gaussian beam, LG vortex beam with order 0 and topological charge 6, vortex lightsuperposition state 1, and vortex light superposition state 2 all produce different distortions after turbulence caused bytemperature 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 appearingnear the center of the calibration position decreases, while that appearing far away from the center of the calibrationposition increases. In the same simulated turbulent environment, the distribution degree of the centroid offset of theGaussian beam from the center of the calibration position is the largest, while that of the LG vortex beam from the centerof the calibration position is the smallest, with the centroid offset degree of the two vortex light superposition states fallingin the middle (Fig. 4). When the temperature difference or salinity difference is constant, the beam drift variance ofGaussian beam is large, and that of LG vortex beam is small. In addition, the beam drift variance of vortex lightsuperposition state 1 is smaller than that of vortex light superposition state 2 (Fig. 5). When the temperature difference isconstant, the scintillation index of the Gaussian beam is larger, and that of the LG vortex beam is smaller. Thescintillation index of vortex light superposition state 1 is smaller than that of vortex light superposition state 2. When thetemperature difference is 0, 5, and 10 ℃, the scintillation index of the two vortex light superposition states is close to thatof the LG vortex beam (Fig. 8). When the salinity difference is constant, the scintillation index of the Gaussian beam islarger, and that of the LG vortex beam is smaller. The scintillation index of vortex light superposition state 1 is smallerthan that of vortex light superposition state 2. When the salinity difference is 0 and 1‰, the scintillation index of the twovortex light superposition states is close to that of the LG vortex beam (Fig. 10).Conclusions In this paper, the beam drift and scintillation index changes of Gaussian beam, LG vortex beam with order0 and topological charge 6, vortex light superposition state 1, and vortex light superposition state 2 after underwaterturbulence caused by different temperature and salinity differences are experimentally studied. The experimental resultsshow that with the increase in temperature and salinity differences, the turbulence intensity increases, and the beam driftvariance and scintillation index of the four beams rise. Compared with those of the other three beams, the beam driftvariance and scintillation index of the LG vortex beam are smaller. When the temperature difference or salinity difference isthe same, the beam drift variance and scintillation index of vortex light superposition state 1 are smaller than those ofvortex light superposition state 2. When the temperature difference is 0 and 5 ℃, the beam drift variance of the two vortexlight superposition states is close to that of the LG vortex beam. When the temperature difference is 0, 5, and 10 ℃ , thescintillation index of the two vortex light superposition states is close to that of the LG vortex beam. When the salinitydifference is 0 and 1‰, the scintillation index of the two vortex light superposition states is close to that of the LG vortexbeam. According to the comprehensive analysis, under weak underwater turbulence, the use of vortex opticalsuperposition state communication can improve communication capacity and spectral efficiency. Furthermore, understrong underwater turbulence, the LG vortex beam has better transmission quality.