# Research on Laser Dual-Mode Fusion Detection Method of Ship Wake Bubbles
## 1 Introduction  
Underwater laser detection is a technology that uses laser to search, track, detect, locate, and identify underwater targets. It has the characteristics of high precision, high robustness, and rapid response. It has great application value in marine remote-sensing surveying and mapping, underwater target detection, and tracking. Ship wake detection technology based on laser guidance is a typical application of underwater laser detection technology [1]. The wake of the ship is a dynamic air curtain belt formed at the tail of the ship when the ship is sailing on the water surface due to the cavitation of the propeller, the breaking of the wave, and the involvement of a large amount of air in the water line of the ship [2, 3]. The theoretical geometric optics of the scattering characteristics of bubbles in water is in good agreement with the Mie scattering theory. Using the Mie scattering effect of the ship wake bubbles on the laser, the detection of the ship wake can be realized, so as to invert the trajectory and velocity characteristics of the ship in the ocean [4, 5]. An underwater vehicle equipped with the laser detection system can realize the tracking and striking of the ship. However, the current laser detection system has problems and bottlenecks such as large dynamic changes in the received signal, poor stability of the received signal, and low SBR of the received signal [6, 7]. At present, in the wake bubble detection method, the intensity detection based on laser forward scattering and the backward radar detection aremostly used. In the wake signal processing, the intensity modulation detection mode is mostly used. The experimental scene is mostly concentrated on the laboratory simulation environment, without considering the environmental interference such as scattering and multi-target occlusion. The model application scene is limited, and the signal processing method has low adaptability to external field experiments. Therefore, there is an urgent need for a laser detection method that combines multiple detection modes to meet the adaptive performance of ship wake laser detection in different hydrological conditions, sea areas, seasons, hydrology, and other external environments, and to achieve high sensitivity, strong anti-interference, large dynamic echo, and high reliability of underwater weapon wake laser detection and tracking. In the field of laser detection of ships' wakes, domestic and foreign scholars have carried out simulation detection and other related research in the laboratory for the intensity detection and image measurement of simulated bubbles in ships' wakes. For example, [PERSON] et al. [8] studied the bubble size distribution and bubble rising law in the bubble curtain of a simulated ship wake using high-speed photography technology in the laboratory environment. [PERSON] et al. used holographic imaging technology to detect the wake, and studied the forward scattering characteristics of wake bubbles. Units such as the Huazhong University of Science and Technology, the Naval Engineering University, the Shanghai Institute of Optics and Mechanics, and the Chinese Academy of Sciences have used pulsed laser to detect ships' wakes, and carried out research on the high-power pulsed laser detection of wake bubbles. [PERSON] et al. [9] studied the laser backscattering characteristics of a bubble curtain using the Monte Carlo simulation method, and effective scattering signals from the wake bubbles of a real ship were detected at 11.5 s.  
To enhance the reliability of the ship wake detection system and enable the identification of bubble size, number density, and thickness in ships' wakes, this paper proposes a dual-mode laser fusion detection method for ships' wakes. The wake bubble area is illuminated with green light, and the light scattered back by the bubble is received by the APD receiving system and the imaging CMOS detection receiving system. Through high-precision timing control, synchronized signal acquisition between the APD and CMOS detection modules is achieved.