# Laser Transmission Characteristics of Seawater for Underwater Wireless Optical Communication
## 3 Results and Discussion
### Beam Spreading  
Figure 6 shows the variation in the beam diameter on the receiving plane under different water properties and attenuation lengths. The beam diameter is twice the offset at which the laser power decays to 1% of the power at the center, calculated from beam distributions. With increasing attenuation length, the beam diameters in the various waters increase with distinct patterns. The beam diameter in clear water remained relatively stable, increasing from approximately 0.08 m at an attenuation length of 1 to 0.10 m at an attenuation length of 25. A small beam diameter is primarily due to the significant decline in laser power in clear water. In contrast, the changes in beam diameter are much more pronounced in coastal and turbid harbor waters, in which noticeable beam spreading occurs. In coastal water, the beam diameter is about 0.19 m at an attenuation length of 1 and increases gradually to 0.60 m at an attenuation length of 15. Then, it expands dramatically to 83.8 m when the attenuation length reaches 35. Similarly, in turbid harbor water, the beam diameter begins at about 0.18 m at an attenuation length of 1, increases to 8.9 m at an attenuation length of 15, and then rises to 25.3 m at an attenuation length of 35. It is important to note that laser energy attenuates more slowly in turbid harbor water due to high scattering ability, resulting in less significant beam spreading than in coastal water and a smaller final beam diameter in the harbor.  
Figure 7 shows the laser beam distributions under different attenuation lengths in clear water, coastal water, and turbid harbor water, which provide a more detailed overview of the power attenuation process for the different offsets away from the beam center. Overall, at small attenuation lengths, the power rapidly decayed with increasing offset, but as the attenuation length increased, the power slowly declined. After reaching a certain attenuation length, the power loss remained uniform within the offset range of approximately 1 to 10 m in coastal and turbid waters. The higher \(\omega_{0}\) is, the smaller the attenuation length. For example, the power loss remained constant at an offset of less than 1 m, with a corresponding attenuation length of 30 in coastal water and approximately 20 in turbid harbor water.  
The analysis of underwater characteristics indicates that laser beam spreading significantly affects the receiving gain. For a given attenuation length, the characteristics of beam spreading differ among three typical types of water. In strongly scattering water bodies (e.g., coastal and turbid harbor waters), the beam spreading is noticeable, and the  
Figure 6: Variations in the beam diameter with attenuation length in **(a)** clear water, **(b)** coastal water, and **(c)** turbid harbor water.  
laser power remains consistent over a wide range of offsets at long attenuation lengths. In contrast, in weakly scattering water (e.g., clear water), the beam spreading is relatively inapparent, and the laser power decreases rapidly with increasing offset. Therefore, it is essential to ensure proper alignment between the transmitter and receiver to minimize power loss in clear water. Alignment is the requisite for high receiving gain at short attenuation lengths in coastal and turbid harbor waters, unnecessary at long attenuation lengths because linear misalignment of the receiver may not result in significant power loss at long attenuation lengths. It suggests the need to consider the type of water, transmission distance, and offset on the receiving plane when estimating receiving gain.