## 1 Introduction  
Plate-like structures are now widely used in practical engineering applications such as boat/submarine hulls, offshore platforms, and underwater sensors. However, when working under water or in corrosive environments, these structures are prone to corrosion. Corrosion evolves into cracks and holes, commonly reducing the service life of the structures and leading to catastrophic consequences. Therefore, it is of great importance to regularly check the integrity of these structures. Many non-destructive testing techniques have been developed to identify damages in metallic structures, such as magnetic particle inspection [1; 2], the magnetic flux leakage technique [3; 4], pressure testing [5], and the ultrasonic guide wave technique [6]. However, these methods have been increasingly challenged in practical applications in real engineering structures, especially in underwater plates [7]. In plate-like structures, Lamb wave is a useful tool for analyzing the integrity of such structures because it has characteristics such as long propagation distance and low attenuation and, most importantly, it has a high sensitivity to defects [8; 9; 10; 11; 12; 13; 14]. Because of these characteristics, Lamb waves also have great potential in defect detection in underwater structures. For traditional methods in defect detection, transducers are widely used to generate and receive Lamb waves in plates [15; 16; 17; 18; 19]. When working in underwater environments, the surrounding water acts as the coupling agent for transducers to couple the generated acoustic waves into plate. Antisymmetric Lamb modes are easily generated by placing the transducers on the surface of structures, and their amplitude can be measured accordingly. This antisymmetric Lamb wave mode can be directly used in practical applications such as the estimation of the plate thickness [20], detection of defects in underwater pipes [5], identification of corrosion damage in submerged metallic structures [6], etc. Meanwhile, due to characteristics such as low attenuation, symmetric modes, especially those at a specific _fd_ range, are frequently used in evaluations of solid plates, such as in hidden defect detection [21], multiple defect detection [22], corrosion monitoring [23], imaging of immersed plates with defects [24], and so on. These symmetric modes of Lamb waves at a specific _fd_ range can also be generated and detected by contact-transducers. However, to obtain proper transfer of energy from the transducers to the structure, the transducers need to be in contact with the surface, or very close to the sample. Moreover, the angle between the transducer and sample needs to be adjusted precisely to generate appropriate Lamb wave mode, which increases the difficulty in realizing this approach. Because of these limitations of contact approaches, some researchers have used the laser ultrasound technique to develop a method for detecting underwater structures. [PERSON] et al. [25] used a pulsed laser to generate Lamb waves, including both symmetric modes and antisymmetric modes, in an aluminum plate immersed in water, and used two immersed transducers for reception. [PERSON] et al. [26] used a similar technique to detect defects in underwater plate. In the study, Lamb waves were generated by a pulsed laser focusing on the surface of plate and were detected by an array of immersed transducers. [PERSON] et al. [27] developed a noncontact laser ultrasonic propagation imaging (UPI) system for damage visualization of liquid-immersed structures using the A\({}_{0}\) mode. When the generating laser irradiates the plate surface, both symmetric modes and antisymmetric modes can be generated, while the symmetric modes normally have small out-of-plane displacement because of the asymmetric excitation (the source is asymmetric with respect to the middle plane of the plate).