# Research on Laser Dual-Mode Fusion Detection Method of Ship Wake Bubbles
## 3 Simulation of Ship's Wake Bubble Laser Dual-Mode Detection Based on Monte Carlo
### Simulation of Laser Backscattering Detection of Ship's Wake  
Based on the distribution of the ship's wake and the characteristics of bubble target, the characteristics of laser dual-mode fusion detection with different bubble size, bubble number density, and bubble layer thickness are analyzed. When the number of photons is higher, the simulation effect is better. However, due to the performance limitation of the computer, when the number of photons is set to \(10^{7}\), the simulation efficiency is high and the effect is better. The laser transmission distance of 532 nm is set to be far, and the water scattering is smaller than other wavelengths. Setting the water attenuation coefficient to 0.151 m\({}^{-1}\) can approximately simulate the offshore seawater quality [23]. (1) Laser backscattering characteristics of bubbles with different number densities  
Firstly, the simulation conditions are set as follows: the bubble distance is 5 m, the bubble size is 80 \(\upmu\)m, the bubble layer thickness is 0.3 m, and the bubble number densities are \(10^{6}\), \(10^{7}\), \(10^{8}\), \(10^{9}\) m\({}^{-3}\), respectively. The change in laser backscattering when the bubble number density changes is analyzed. The simulation results are shown in Figure 4a, and the change trend in the peak signal after signal processing is shown in Figure 4b. It can be found from Figure 4a that when the bubble number density increases, the peak amplitude of the laser backscatter echo signal increases continuously, but the change range is small; additionally, the pulse width remains basically unchanged. After signal processing, it is found that when the number density changes, the peak photon number changes within 1-17, and the change range is small. Its peak change trend first rises rapidly and then tends to be gentle. (2) The change in laser backscattering of bubbles with different thickness  
The bubble field in a ship's wake is a constantly changing process, and the echo characteristics of the bubble layer at different distances are different. Therefore, studying the thickness characteristics of ship wake bubbles is of great significance. The bubble layer thickness is set to be 0.1, 0.3, 0.5, 0.7, and 0.9 m, and the bubble number density is \(10^{8}\) m\({}^{-3}\), with other conditions remaining unchanged. The change in laser backscattering when the thickness changes is analyzed. The simulation results are shown in Figure 5a, and the change trend of the peak signal after signal processing is shown in Figure 5b. It can be found from Figure 5a that when the bubble thickness continues to increase, the peak amplitude of the laser backscatter echo signal continues to increase, the amplitude change is more obvious, and its pulse width also varies greatly. After signal processing, it is found that when the thickness changes, the peak photon number changes within 250-400, and the change range is large. The overall change is stronger than the change in the bubble number density. The change trend of the peak value is linear as a whole. Figure 4: The backscattering echo characteristics under different number densities: (**a**) the change diagram of backscattering signal; (**b**) peak signal changes. (3) The variation in laser backscattering of bubbles with different scales  
The bubble size is set to 40, 60, 800, 300, and 400 \(\upmu\)m, and the thickness of the bubble layer is 0.3 m. Other conditions remain unchanged. The change in laser backscattering when the bubble size changes is analyzed. The simulation results are shown in Figure 6a, and the change trend of the peak signal after signal processing is shown in Figure 6b. From Figure 6a, it can be found that when the bubble size increases, the peak amplitude of the laser backscatter echo signal increases continuously, and the amplitude change is not obvious. After signal processing, it is found that when the bubble size changes, the peak photon number changes within 10-25, and the change range is small.