## 3 Calculation of Transmission Distance in the Ideal State  
Based on the experimental architecture above, the transmission distance of 1.5 m, 3 m, and 6 m, respectively, was tested by adjusting the number of mirrors. The transmission losses of the three distances are shown in Table 1, and Equation (1) can be obtained:  
\[P_{in}\;-\;P_{out}\;-\;L=\;L_{water} \tag{1}\]  
From the above equation, one can calculate the transmission loss per meter of signal light in water under different system architectures. Where \(P_{in}\) is the light power measured through the water tank glass after passing through the collimators and polarizers. \(P_{out}\) is the received light power, \(L\) is the total loss caused by the signal light passing through the water tank glass and contacting the reflector. \(L_{water}\) can be obtained by subtracting the initial light power from the receiving light power and then subtracting the loss caused by \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Transmission Distance M (m)** & **1.5** & **3** & **6** \\ \hline
**Input optical power \(P_{in}\) (dBm)** & 4.32 & 4.32 & 4.32 \\ \hline
**Received optical power \(P_{out}\) (dBm)** & 2.94 & 0.60 & \(-\)4.32 \\ \hline
**Total loss caused by glass and reflector L (dB)** & 0.70 & 2.05 & 4.75 \\ \hline
**Optical power loss in water \(L_{water}\) (dB)** & 0.68 & 1.67 & 3.89 \\ \hline
**Optical loss per meter of light transmitted in water Y (dB/m)** & 0.45 & 0.56 & 0.65 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The optical loss at different transmission distances. Figure 1: 6 m unidirectional UWOC system. penetrating glass and reflected by the mirror. \(L_{water}\) is the transmission loss of signal light in water. The transmission loss \(Y\) of signal light per meter can be obtained by dividing the total transmission distance, as shown in Equation (2). \[Y=\frac{L_{water}}{M} \tag{2}\]  
Table 1 shows optical power loss under different distances 1.5, 3 m, and 6 m. The input optical power is set as 4.32 dBm, and the received optical power of 2.94 dBm at 1.5 m distance and 0 dBm at 3 m and \(-\)4.32 dBm at 6 m were obtained by a photodetector. Before water was immersed in the tank, the optical power loss caused by the glass and air without water is measured at 0.70 dB for 1.5 m distance, 2.05 dB for 3 m distance, and 4.75 dB for 6 m distance. The optical power loss caused by the water can be obtained by calculating Equation (1). Hence, the optical power loss caused by the water is 0.68 dB, 1.67 dB, and 3.89 at 1.5 m, 3 m, and 6 m, respectively. Therefore, the experiment result for optical loss per meter is 0.45 dB/m, 0.56 dB/m, 0.65 dB/m for 1.5 m, 3 m, and 6 m, respectively. In summary, the data mentioned are shown in Table 1. In Table 1, the received power was measured at different transmission distances. A constant loss per meter of 0.45, 0.56, and 0.65 dB/m for 1.5, 3.0, and 6.0 m, respectively, were obtained. The loss values are not the same may be due to different reflection times per path by the mirror. It may also be due to the laser beam is increasing divergent after transmission longer distances. Hence, the increasing loss is not linear according to the increasing distance. The initial transmission laser power is determined as the maximum optical power and can be received measured by the optical power meter used in this experiment, 16.99 dBm. The minimum power is \(-\)15.96 dBm for reference with the transmission rate of 1.25 Gbps, and the bit error rate should be no worse than \(10^{-3}\). The loss of signal light through the water tank glass is 0.35 dB. The mirror with 86% reflectivity reflects the signal light, and the loss is 0.65 dB. According to Table 1, the loss of signal light transmitting at 6 m in water is 0.65 dB per meter. The Fresnel lens reflection loss in case of different index glass-air is 4% according to Equation (3). \(X\) is the number of times the mirror reflects the signal light. The signal light's total transmission distance can be calculated by \(X\).