针对长距离水下无线光通信系统，采用蒙特卡罗方法研究了水下光束的传输特性。提出采用平顶光束替代传统高斯光束，以延长通信系统的传输距离。通过对比平顶光束和高斯光束在三类清澈海水中的传输特性，基于脉冲位置调制技术，对两种水下无线光通信系统进行实验仿真。结果表明：在相同条件下，高斯光束和平顶光束的水下无线光通信系统的可靠视频传输距离上限分别为268 m和340 m。在清澈海水中，采用平顶光束可以有效延长水下无线光通信系统的通信距离。该结果为长距离水下无线光通信系统设计提供了参考。

Using the Monte Carlo method, we investigate the transmission characteristics of light beam through seawater for long-reach undersea wireless optical communication system. The traditional Gaussian beam is replaced with flat-topped beam, which can extend the distance of communication system. Then we compare the transmission characteristics of flat-topped beam and Gaussian beam in three kinds of clear seawater, and simulate the two kinds of underwater wireless optical communication systems based on pulse position modulation. The results show that the upper limit of reliable video transmission distance of undersea wireless optical communication system is 268 m for using Gaussian beam and 340 m for using flat-topped beam under the same condition, respectively. The application of flat-topped beam is viable to extend the communication distance of underwater wireless optical communication system in clear seawater. The results provide a reference for the design of long-reach underwater wireless optical communication systems.