# Detection of Underwater Targets Using Polarization Laser Assisted Echo Detection Technique
## 3 Underwater Target Polarization Detection System and Experimental Scheme
### Energy Test and Polarization Test  
The performance of the energy probe used in the polarization detection equipment was evaluated, including full range calibration, dynamic range, non-linearity and Conformance testing. The test data are shown in Table 1.  
Different targets are placed at different distances to test the energy of the reflected echo at 0\({}^{\circ}\), 45\({}^{\circ}\), 90\({}^{\circ}\)and 135\({}^{\circ}\). The test process is shown in Figure 4, and several sets of test data are selected for Table 2.  
Build an underwater test environment using a seawater tank, as shown in Figure 5. The pure water is injected into the pool, and different targets are placed in the pool at different distances. The energy of echo at 0\({}^{\circ}\), 45\({}^{\circ}\), 90\({}^{\circ}\) and 135\({}^{\circ}\) is measured after the laser passes through the water. The test process is shown in Figure 5, Figure 6 shows the structure of the simulated pool and several sets of test data are selected for Table 3.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Energy Point (J)** & **Probe 1** & **Probe 2** & **Probe 3** & **Probe 4** & **Inconsistency** \\ \hline \(2.93\times 10^{-12}\) & \(3.12\times 10^{-12}\) & \(3.02\times 10^{-12}\) & \(3.12\times 10^{-12}\) & \(2.97\times 10^{-12}\) & \(\pm 2.5\%\) \\ \(5.85\times 10^{-12}\) & \(5.94\times 10^{-12}\) & \(5.81\times 10^{-12}\) & \(6.06\times 10^{-12}\) & \(5.85\times 10^{-12}\) & \(\pm 2.1\%\) \\ \(2.34\times 10^{-11}\) & \(2.35\times 10^{-11}\) & \(2.28\times 10^{-11}\) & \(2.30\times 10^{-11}\) & \(2.25\times 10^{-11}\) & \(\pm 3.1\%\) \\ \(1.42\times 10^{-10}\) & \(1.42\times 10^{-10}\) & \(1.39\times 10^{-10}\) & \(1.43\times 10^{-10}\) & \(1.43\times 10^{-10}\) & \(1.41\times 10^{-10}\) & \(\pm 1.4\%\) \\ \(3.47\times 10^{-10}\) & \(3.54\times 10^{-10}\) & \(3.48\times 10^{-10}\) & \(3.47\times 10^{-10}\) & \(3.40\times 10^{-10}\) & \(\pm 2.0\%\) \\ \(1.28\times 10^{-9}\) & \(1.31\times 10^{-9}\) & \(1.25\times 10^{-9}\) & \(1.28\times 10^{-9}\) & \(1.28\times 10^{-9}\) & \(1.28\times 10^{-9}\) & \(\pm 2.4\%\) \\ \(3.16\times 10^{-9}\) & \(3.17\times 10^{-9}\) & \(3.07\times 10^{-9}\) & \(3.10\times 10^{-9}\) & \(3.03\times 10^{-9}\) & \(\pm 2.3\%\) \\ \(6.77\times 10^{-9}\) & \(7.17\times 10^{-9}\) & \(7.00\times 10^{-9}\) & \(7.16\times 10^{-9}\) & \(7.19\times 10^{-9}\) & \(\pm 1.4\%\) \\ \hline \hline \end{tabular} Measurement inconsistency: measured with four probes at the same energy point, the maximum deviation of the four measurements divided by two times the average of the four measurements.  
\end{table}
Table 1: A summary of the consistency of the four probe measurements.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Serial** & **Distance** & **\(\theta^{\circ}\) (\(\Theta\))** & **45\({}^{\circ}\) (\(\Theta\))** & **10\({}^{\circ}\) (\(\Theta\))** & **135\({}^{\circ}\) (\(\Theta\))** & **1** & **O** & **U** & **DOF** & **App** \\ \hline \(1\) (Jores) & 11.029 & 1.03\(\times 10^{-9}\) & 9.7 \(\times 10^{-10}\) & \(2.48\times 10^{-9}\) & \(3.34\times 10^{-9}\) & \(4.31\times 10^{-9}\) & \(-6.03\times 10^{-10}\) & \(-2.27\times 10^{-9}\) & 0.57 & 3.40 \\ \(2\) (Jores) & 1 Light intensity imaging techniques are generally affected by environmental factors, and in harsh environments, imaging will have a certain degree of difficulty because the light intensity is too weak. While infrared imaging is subject to errors caused by the ambient temperature, polarization imaging technology can perform long-range image acquisition operations in harsh environments and has advantages in suppressing background noise, improving detection distance, detailed feature acquisition, and target camouflage identification.