Ship wake is the disturbance of water flow caused by the propeller when the ship is sailing, drawing air into the water to form bubbles. In addition, the cavitation of the propeller itself also generates a large number of bubbles around the propeller, creating a wake belt full of bubbles in the stern of the ship. The optical characteristics of the wake differ from those of the surrounding water environment. By studying the laser characteristics of the ship wake, we can further judge the characteristics of the ship’s trajectory and speed in the sea and then realize the precise guidance and damage attack of underwater vehicles such as torpedoes. The bubbles in the ship wake have the characteristics of sparsity, discreteness, small scale, and low numerical density. The scale and number density are related to the ship speed, propeller speed, and distance from the measurement area to the ship. The laser detection method of ship wake must have the ability to sense bubbles with a large dynamic diameter range of 10-1000 μm, and it must adapt to different water environments.

In this study, the Monte Carlo simulation method is used to simulate the backscattering of wake bubbles with diameter of 40-500 μmin pure seawater, clean seawater, coastal seawater, and port seawater. To address strong water scattering interference in close range and low signal-to-noise ratio of weak signal detection in underwater laser wake detection, an anti-saturation and gain control signal processing method is studied. By adjusting the bias voltage to change the photocurrent gain of the avalanche photodiode, the APD (avalanche photo diode) gain can be controlled. An underwater microbubble laser measurement and analysis system that can suppress the strong scattering interference of near-field water is designed, and the matching adjustment between the APD receiving gain and the signal strength under different water environments can be realized.

The backscattering law of wake bubbles in different seawater qualities is investigated. The echo amplitude of laser backscattering of wake bubbles in pure seawater is the strongest, and the amplitude of laser backscattering echo of port seawater is the weakest. With the increase in the water quality attenuation coefficient, the laser backscattering echo amplitude of wake bubbles gradually decreases (Fig. 3). With the increase in the distance, the backward laser echo of wake bubble gradually moves backward, and the amplitude gradually decreases (Fig. 5). When the bubble number density is below 10⁹ m^-3, the bubble echo amplitudes of pure, clean, and coastal seawater decrease with the increase in the bubble size. In the port seawater environment, the backward echo of bubbles always maintains consistency with the water signal. In the port seawater environment, when the bubble layer thickness is less than 1.8 m, the laser detection system faces difficulty in detecting the bubble echo. When the bubble layer thickness is greater than 1.8 m, the laser detection system can detect the bubble echo (Fig. 6). By designing an underwater micro bubble laser test and analysis system that can suppress the strong scattering interference of near-field water body, the detection of large-scale wake bubbles in different water environments can be realized (Figs. 10, 14, 15, 16).

The simulation results show that the echo amplitude of laser backscattering of wake bubbles in pure seawater is the strongest and the amplitude of laser backscattering echo in port seawater is the weakest. With the increase in the water quality attenuation coefficient, the laser backscattering echo amplitude of wake bubbles decreases gradually. With the increase in the distance, the laser backscattering echo delay of wake bubbles increases and the amplitude decreases gradually. When the bubble number density is below 109m-3, with the increase in the bubble size, the bubble echo amplitude of pure, clean, and coastal seawater decreases in order, and the echo amplitude of port seawater maintains consistency with the water signal. In the port seawater environment, the laser detection system faces difficulty in detecting the bubble echo when the bubble layer thickness is less than 1.8 m, while the system can detect the bubble echo when the thickness is greater than 1.8 m. In the indoor laboratory environment, when the APD gain is 10 dB, the detection signal-to-noise ratio is the highest. In the anechoic pool environment, the bubble echo changes more evidently than the water background echo, effectively detecting the simulated bubbles with diameter of 20-30 μmand 100-300 μm. In the lake environment, the bubble echo changes more evidently than the water background echo, effectively detecting the bubbles in the wake of actual ships.