In long-distance underwater laser communication, the influence of optical attenuation limits the transmission efficiency of the link and expands the capture range, resulting in a decrease in optical power at the communication receiving end and a decrease in system performance. To achieve both fast chain building and stable communication, a new method has been proposed for optical composite transmission in beacon communication, which utilizes variable beam divergence angle control technology. The beam divergence angle control system controls the horizontal movement of the lens using a gear rotation focusing structure combined with a zoom motor. By changing the spacing between the optical lenses, the beam’s focal length can be adjusted to achieve precise control of the beam divergence angle. We analyze the optimization effect of variable beam divergence angle on underwater laser communication links from the perspectives of communication error rate and capture time. Underwater laser communication experiments are conducted in a swimming pool. Under the condition of water quality attenuation of 0.19 m^-1, the communication distance is 40 m, and the capture time and communication error rate are measured. The actual capture time is 41.5 s, and the communication error rate is 3.62×10^-6. Both simulation and experimental results indicate that the beam divergence angle control system can shorten the capture time to less than 60 s and reduce the bit error rate by two orders of magnitude compared to traditional systems.

To quickly build the link and stabilize the communication, we propose a new method for optical composite transmission technology in variable beam divergence beacon communication. The system is composed of a zoom motor, focusing structure, and lens. The beam divergence angle control system uses a gear rotation focusing structure and a zoom motor to control the horizontal movement of the lens. By changing the spacing of the optical lenses and thereby adjusting the beam focus, it achieves precise control of the beam divergence angle. A large beam divergence angle is used for rapid acquisition and chain building. The larger beam divergence angle helps to shorten the acquisition time. Scanning the uncertain area and capturing the signal must be completed within 60 s. Then, the beam divergence angle is gradually reduced. The process of changing the beam divergence angle from large to small primarily depends on the detection sensitivity of the communication detector and the link margin of the underwater channel. The narrow beam is used to effectively overcome the attenuation of water and achieve stable communication over a longer distance with high bandwidth, thereby improving the stability of the communication link, as verified by simulation and experiment.

According to the simulation results, when using the traditional underwater wireless optical communication (UWOC) system with a fixed beam divergence angle, to meet the high bit error rate (less than 1×10^-3) requirements of space optical communication for data integrity, the system needs a minimum beam divergence angle of 8 mrad. Under these conditions, the acquisition and chain-building time is 133.5 s. Generally, as communication performance improves (i.e., the bit error rate decreases), the beam divergence angle will decrease, but the acquisition and chain-building time will increase (Figs. 4 and 6). However, in the case of beam divergence angle control, as shown in Table 2, when using a large angle for acquisition, the chain-building time is 38.4 s, after which the system switches to a small angle for communication. At this point, the communication bit error rate is 8.32×10^-6. Therefore, in the case of the optimized UWOC system with beam divergence angle regulation, the acquisition and chain-building time is reduced by 95.1 s, and the communication performance improves by two orders of magnitude, while still meeting the communication index requirements. This leads to further improvement in the performance of the UWOC system.

To solve the contradiction between fast chain-building and stable communication in the UWOC system, we propose a transmission technology using communication light and beacon light with a variable beam divergence angle. Through theoretical simulation, the effect of changes is studied in beam divergence angle on capture time and communication performance. The simulation results show that when the bit error rate is less than 1×10^-3, compared with the traditional UWOC system, the optimized beam divergence angle control UWOC system reduces the acquisition and chain-building time by 95.1 s and the communication bit error rate by two orders of magnitude. Through a 40 m pool experiment, it is verified that the acquisition and chain-building time is 41.5 s, and underwater transmission with a bit error rate of about 3.62×10^-6 at 40 m and 1 Mbit/s is successfully realized, which is consistent with the simulation results. The experimental results show that, compared with the traditional system, the optimized beam divergence angle control system can reduce the capture time to less than 60 s and significantly reduce the bit error rate by two orders of magnitude. In conclusion, this technology can lay a theoretical foundation for the application of dynamic long-distance underwater laser communication.