# Laser Transmission Characteristics of Seawater for Underwater Wireless Optical Communication
## 3 Results and Discussion
### Channel Loss  
To determine the variable features of the laser power with attenuation length at the various offsets, the channel loss (per unit area) was calculated as shown in Figure 8. For comparison, two theoretical laser power attenuation patterns \(\mathrm{e}^{-K_{d}r}\) and \(\mathrm{e}^{-cr}\) are also shown in Figure 8. The corresponding diffuse attenuation coefficients \(K_{\mathrm{d}}\) of the three typical waters are 0.120, 0.189, and 0.493 \(\mathrm{m}^{-1}\), and the beam attenuation coefficients \(c\) are 0.151, 0.399, and 2.19 \(\mathrm{m}^{-1}\), respectively. [PERSON]'s law (\(\mathrm{e}^{-cr}\)) governs the laser power attenuation at the center within a specific range of attenuation length. The pattern of laser power decline with attenuation length in clear water is strikingly identical to [PERSON]'s law, as the black and blue dotted lines overlaid in Figure 8a demonstrate. By contrast, the pattern changes at attenuation lengths of 15 and 5 in coastal and turbid waters (the black and blue dotted lines in Figures 8b and 8c, respectively), indicating that [PERSON]'s law causes underestimation of the power received at the beam center at a large attenuation length [32]. The absolute differences in power loss between simulated results and Beer's Law calculations could be almost 2 dB\(\cdot\)m\({}^{-2}\) and 8 dB\(\cdot\)m\({}^{-2}\) at the attenuation lengths of 1 and 15 in turbid harbor waters. Since the simulations only account for the forward scattering photons, the power losses of simulated results are invariably less than the values calculated from the pattern \(\mathrm{e}^{-K_{d}r}\). For a specific offset, the characteristics of laser channel loss variation with attenuation length differ among water types. However, the absolute differences in power loss at various offsets reduce as the attenuation length increases. In weak scattering clear water, the power loss at the offset of 0 m increases rapidly with increasing attenuation length and the power losses are about 31.4 dB\(\cdot\)m\({}^{-2}\), \(-29.5\) dB\(\cdot\)m\({}^{-2}\), and \(-74.4\) dB\(\cdot\)m\({}^{-2}\) at the attenuation lengths of 1, 15, and 25; the power losses at the offset of 10 m are \(-56.6\) dB\(\cdot\)m\({}^{-2}\), \(-84.8\) dB\(\cdot\)m\({}^{-2}\), and \(-121.2\) dB\(\cdot\)m\({}^{-2}\) at the same attenuation lengths. Thus, the absolute differences in the power between offsets 0 m and 10 m are 88.0 dB\(\cdot\)m\({}^{-2}\), 55.3 dB\(\cdot\)m\({}^{-2}\), and 46.8 dB\(\cdot\)m\({}^{-2}\) at the above attenuation lengths. In strong scattering waters, the power loss changes more slowly after reaching a certain attenuation length. In coastal water, the power losses at the offset of 0 m are 31.3 dB\(\cdot\)m\({}^{-2}\), \(-29.1\) dB\(\cdot\)m\({}^{-2}\), and \(-106.2\) dB\(\cdot\)m\({}^{-2}\) at the attenuation lengths of 1, 15, and 35, respectively; the power losses at the offset of 10 m are \(-63.6\) dB\(\cdot\)m\({}^{-2}\), \(-63.6\) dB\(\cdot\)m\({}^{-2}\)  
Figure 7: Beam distributions for the different attenuation lengths in (**a**) clear water, (**b**) coastal water, and (**c**) turbid harbor water. and \(-\)110.5 dB\(\cdot\)m\({}^{-2}\) at the same attenuation lengths; the absolute differences at these offsets are 94.9 dB\(\cdot\)m\({}^{-2}\), 34.5 dB\(\cdot\)m\({}^{-2}\), and 4.3 dB\(\cdot\)m\({}^{-2}\) at the same attenuation lengths. In turbid harbor water, the power losses at the offset of 0 m are 32.6 dB\(\cdot\)m\({}^{-2}\), \(-\)34.9 dB\(\cdot\)m\({}^{-2}\), and \(-\)58.6 dB\(\cdot\)m\({}^{-2}\) at the attenuation lengths of 1, 20, and 35, respectively; the power losses at the offset of 10 m are \(-\)84.8 dB\(\cdot\)m\({}^{-2}\), \(-\)60.5 dB\(\cdot\)m\({}^{-2}\), and \(-\)71.2 dB\(\cdot\)m\({}^{-2}\) at the same attenuation lengths; the absolute differences at these offsets are 117.4 dB\(\cdot\)m\({}^{-2}\), 25.6 dB\(\cdot\)m\({}^{-2}\), and 12.6 dB\(\cdot\)m\({}^{-2}\) at the same attenuation lengths.