## 3 Results
### Experimental Setup  
On the basis of simulation, the selected water tank made of high transparency glass is 1.5 m in length, 0.42 m in width and 0.4 m in water depth. The single pulse energy is 20 uJ, the beam is expanded into the water tank and then travels 1 m to hit targets. The reflected beam returns to the detector, which consists of an image intensifier and a complementary metal oxide semiconductor (CMOS) camera with a maximum resolution of 1024 \(\times\) 1024 and a frame frequency of 1000. The image intensifier has a gate width of 3 ns and includes a double-layer micro channel plate (MCP) to enable the maximum gain. However, the image intensifier has a power limit on the internal power supply to protect MCP and the phosphor screen, so once the detection light intensity exceeds the preset threshold, the image enhancement effect is invalid. Considering this issue, there was about 20 cm of spatial separation between the ICMOS and the laser to minimize the overlap between them and to ensure that the image intensifier was not affected by the reflected light from the glass pool. After being triggered by the signal generator, the synchronous output signal obtained by the Si-PIN detector inside the laser was used as the external trigger of the signal generator, and the delay of the timing sequence provided by the generator to the intensifier was adjusted based on it. When the reflected signal from the target was about to reach the detector, the gate of the intensifier was opened and the camera exposed to record information afterwards. However, the gate was closed for the rest of the time and the camera could not work, which overcame the strong backscattering in the water. The specific parameters were consistent with those in simulation and the experimental setup with timing diagram of synchronization signals is depicted as Figure 5.  
Figure 4: (**a**–**c**) Original and simulation images of the “Diver” target under AL = 0.42 and 1.42; (**d**–**f**) original and simulation images of the “Double Slit” target under AL = 0.42 and 1.42. Considering the insoluble properties of calcium carbonate, a pump was applied until the medium was uniform and two targets placed in the designated position were sequentially detected; the results are shown in Figure 6. As AL increased, the energy returning to the detector decreased due to absorption and scattering; it is displayed in the images that the overall gray value became smaller, more noises appeared around the edges and became blurred. The same phenomenon as the simulation was also observed in the experimental results. Taking the image at AL of 0.42 as the reference, we calculated the image quality evaluation indicators of the unoptimized and optimized simulation and experimental results with AL of 1.42, including structural similarity (SSIM), mean structural similarity (MSSIM) and peak signal-to-noise ratio (PSNR). The statistical results are shown in Table 1.  
Figure 5: Experiment setup of underwater imaging with timing diagram of synchronization signals. Figure 6: Experimental images of different targets. (**a**,**b**) Results of the “Diver” target under AL = 0.42 and 1.42; (**c**,**d**) results of the “Double Slit” target under AL = 0.42 and 1.42. It can be seen that the optimized results not only outperformed the unoptimized simulation results on SSIM and PSNR, but also the optimized simulation results were closer to the experimental results. At the same time, it is also intuitively seen from the figures that the increase in the AL led to obvious degradation of the image quality. Notably, since the Gaussian filter matrix is used to smooth and denoise the image in the process of solving the MSSIM index, the SSIM was directly solved on the original image. It led to the MSSIM index of the processed distorted image being closer to 1 than the SSIM index, that was closer to the original image.