# Detection of Underwater Targets Using Polarization Laser Assisted Echo Detection Technique
## 4 Results and Analysis
### Effect of Different Objects on the Degree of Polarization under the Same Environment  
The data collection conditions of the air experiment are as follows: different objects are placed in the air at a distance of 11 m to measure the polarization information of reflected light. Measurements are recorded for each object at each location. Use Equation (9) to calculate the degree of polarization of reflected light. The experimental data are shown in Table 2.  
When testing in the air, placing different targets on the platform, the laser scans one target after another and records the data in order to more clearly show the degree of polarization of an iron sheet, ceramic tile, plastic objects in the air DOP, the above-mentioned data were made into the broken line diagram of Figure 8, and the average degree of polarization of iron sheet was 0.564333333, ceramic tile was 1.0063333, and plastic was 0.4822222. The degree of polarization of ceramic tile is close to 1, but the degree of polarization of iron plate and plastic is small, being about 0.56 and 0.48, respectively, and can be divided according to this part of the material target.  
Figure 7: Energy contrast diagram of four probes, (**a**) is a plan and (**b**) is a three-dimensional diagram.  
The degree of polarization is as follows: ceramic tile DOP > Iron Sheet DOP > plastic DOP. The degree of polarization of reflected light will be higher for man-made targets due to the smooth surface, while the degree of polarization of ceramic tiles is the highest for both man-made targets. It shows that the degree of polarization is closely related to the material and the surface smoothness of the object.  
The data collection conditions for the in-water experiment were as follows: different objects were placed in the water at distances of 3.5 m, 5.5 m, and 7.5 m, respectively, to measure the polarization information of reflected light, and the objects were selected in turn from three kinds of objects: iron, tile, and plastic. Measurement data were recorded for each object at each location. The polarization of the reflected light was calculated using Equation (9). The specific experimental data are shown in Table 3.  
In the water test, in order to more clearly show the polarization of iron, tile and plastic three objects in the water DOP, the above data made the line graph in Figure 9. From the data analysis, the average polarization of iron in 3.5 m is 0.9655, in 5.5 m is 0.724, in 7.5 m is 0.3625, tile in 3.5 m. With the increase of water depth, the polarization of the object gradually decreases.  
The relationship between the magnitude of polarization is iron DOP > tile DOP > plastic DOP. The polarization of the same object at different water depths will also change. It can be seen that the polarization of reflected light will be higher for man-made targets due to the smooth surface, while the polarization of iron is the highest for the same man-made targets, indicating that the polarization is indeed closely related to the target material and the flatness of the object surface, so it is possible to distinguish different objects according to their reflected light The polarization of the object can therefore be distinguished from different targets according to the polarization of the reflected light. In the multiple measurements of the data, it can be seen that the calculated polarization has deviation, and reviewing the experimental method, the error should be caused by the fluctuation of the water body and other stray noise, and the accuracy of the detection system and the consistency of the measurement still need to be improved.  
Figure 8: A comparison of the degree of polarization of different objects in the air.