# Laser Transmission Characteristics of Seawater for Underwater Wireless Optical Communication
## 4 Conclusions  
Based on Monte Carlo ray tracing technology, the underwater laser transmission of a blue-green narrow beam was simulated. With the use of the average AOA, beam spreading, and channel loss, the laser transmission characteristics in three typical waters were analyzed. The differences in the average AOAs at various offsets are more significant in clear water and at short attenuation lengths in coastal and turbid harbor waters because of power attenuation and then decrease considerably at long attenuation lengths in coastal and turbid harbor waters due to multiple scattering. For instance, in clear water, the difference in average AOA between offsets of 0 m and 10 m is approximately 62.3\({}^{\circ}\) at an attenuation length of 1, decreasing to 14.5\({}^{\circ}\) at an attenuation length of 15, and 12.9\({}^{\circ}\) at the attenuation length of 25. In coastal water, the differences in average AOAs are about 74.4\({}^{\circ}\), 21.9\({}^{\circ}\), and 5.8\({}^{\circ}\) at the attenuation lengths of 1, 20, and 35, respectively. In turbid harbor water, the  
Figure 8: Variations in the normalized power loss with the attenuation length at the different offsets in (**a**) clear water, (**b**) coastal water, and (**c**) turbid harbor water (represented with solid lines); two theoretical channel loss models (i.e., \(\mathrm{e}^{-K_{d}r}\) and \(\mathrm{e}^{-\sigma}\)) are drawn in dash-dotted and dashed lines, respectively. differences in average AOAs are 67.2\({}^{\circ}\), 28.3, and 12.2\({}^{\circ}\) at the attenuation lengths of 1, 20, and 35, respectively. The characteristics of beam spreading in different water types are distinct. In clear water, the beam diameter is small, increasing from approximately 0.08 m to 0.10 m when the attenuation length changes from 1 to 25. In coastal water, the beam diameter increases gradually from 0.19 m to 0.60 m at an attenuation length of 1 and 15, then expands dramatically to 83.8 m at an attenuation length of 35. In turbid harbor water, the beam diameter is about 0.18 m at an attenuation length of 1 and 8.9 m at an attenuation length of 15 and then increases to 25.3 m at an attenuation length of 35. The difference in the received power at the various offsets decreases with increasing attenuation length but with distinct patterns. Take the offsets of 0 m and 10 m as examples. In clear water, the absolute difference in the power loss is up to 88.0 dB\(\cdot\)m\({}^{-2}\) when the attenuation length is less than 10 and has no significant change exceeding the attenuation length of 10 with the absolute difference of 55.3 dB\(\cdot\)m\({}^{-2}\) at the attenuation length of 15 and 46.8 dB\(\cdot\)m\({}^{-2}\) at the attenuation length of 25. In coastal water, the absolute difference in the power loss has the maximum value of 94.9 dB\(\cdot\)m\({}^{-2}\) when the attenuation length is less than 10, then rapidly reduces to 34.5 dB\(\cdot\)m\({}^{-2}\) at the attenuation length of 15 and 4.3 dB\(\cdot\)m\({}^{-2}\) at the attenuation length of 35. In turbid harbor water, the maximum absolute difference in the power loss is 117.4 dB\(\cdot\)m\({}^{-2}\) when the attenuation length is less than 15, then gradually reduces to 25.6 dB\(\cdot\)m\({}^{-2}\) at the attenuation length of 20 and 12.6 dB\(\cdot\)m\({}^{-2}\) at the attenuation length of 35. Moreover, the minimum underestimation of power loss by applying Beer's Law could be almost 2 dB\(\cdot\)m\({}^{-2}\) in turbid harbor waters. Based on the above results, lasers exhibit different transmission characteristics in different water types. Therefore, when designing and developing UWOC systems, it is necessary to consider the water type in the application scenario. At short transmission distances, there is still a high receiving gain on the center of the receiving plane using a small FOV receiver, while a large FOV is required to achieve a high receiving gain at the positions away from the center.