Laser-induced thermoacoustic generation has attracted considerable attention due to its low energy density threshold and high controllability of the generated signals. This non-contact acoustic generation method shows great potential in underwater communication, remote sensing, and medical imaging. However, current theoretical models typically employ three-dimensional unbounded wave equations, which do not account for the influence of the water-air interface when the laser spot size increases. In this paper, we aim to deepen the understanding of time-domain waveform generation mechanisms in surface acoustic sources, with a particular focus on the influence of the water-air interface under such conditions. Through a combination of simulation and experimental measurements, we investigate the fundamental principles underlying the generation of time-domain waveforms by laser-induced thermal expansion in surface sources.

In this paper, we first utilize experimentally measured photoacoustic signals from a linear source as a time-domain waveform reference for a three-dimensional unbounded acoustic medium. Based on this, we explore how the water-air interface affects waveform generation in surface acoustic sources. A laser with a horizontal beam width of 40 cm and a wavelength of 1064 nm is employed to generate photoacoustic signals in water. We conduct both simulations and experiments to analyze the resulting time-domain waveforms. Advanced signal analysis techniques are applied to isolate and examine the contributions from the upper and lower expansion surfaces of the acoustic source. In addition, the time interval between the two prominent peaks in the time-domain waveform is compared with the physical thickness of the acoustic source to verify the proposed generation mechanism.

The results show that the time-domain waveform of a surface acoustic source is generated by the superposition of vibrations from its upper and lower expansion surfaces. This mechanism is supported by both simulation and experimental data, which consistently reveal two distinct peaks in the waveform. The time interval between these peaks corresponds to the thickness of the acoustic source, further validating the proposed model. We also highlight the significant role of the water-air interface in shaping the waveform when the laser spot diameter exceeds 2 cm, an effect not captured by traditional models based on unbounded wave equations. In addition, although the energy conversion efficiency of laser-induced photoacoustic signals via thermal expansion remains low (typically between 10^-4 and 10^-9), the findings suggest this limitation may be mitigated through optimized laser parameters and advanced signal processing techniques.

In this paper, we offer a detailed examination of the time-domain waveform generation mechanism in surface acoustic sources induced by laser thermal expansion. The findings confirm that the waveform results from the superposition of vibrations on the upper and lower surfaces of the source, with the interval between peaks matching the source’s thickness. The water-air interface is shown to have a substantial impact on the waveform, especially at larger laser spot sizes. These insights address limitations in existing theoretical models and contribute to the advancement of laser-induced photoacoustic technologies. Future research will focus on improving energy conversion efficiency and exploring practical applications in underwater communication, imaging, and remote sensing.