Underwater laser plasma detonation wave propulsion technology has important application value in submarine stealth propulsion, detonation engine, supercavitation weapon systems, green ship manufacturing, and other fields. Different from the space focused laser induced propulsion method, the traditional laser propulsion system focuses on inherent defects such as insufficient energy gathering effect, difficult control of propulsion direction, and large laser loss. A new mode of underwater fiber laser induced plasma detonation wave propulsion is proposed, which achieves control of the laser focal point through the design of microcavity structure morphology coupled with fiber optics and research on the performance of fiber laser propulsion structure. Furthermore, the control of the movement direction of small particles entering and exiting can be achieved, which solves the problem of low energy coupling efficiency of laser plasma in underwater environments and difficulty in effectively applying propulsion force to the surface of the pushed object. By utilizing underwater plasma fiber laser induction to achieve directional propagation of detonation waves, we aim to improve the thrust and impulse coupling coefficient structural performance of laser propulsion. We focus on the morphology and energy conversion characteristics of fiber micro-cavity structures and the study provides theoretical support and guidance for future research on underwater laser induced plasma detonation wave dynamics while achieving particle targeting, fixed-point, and fixed-depth propulsion.

We utilize the theory and method of directional propagation of detonation waves through underwater fiber laser induced plasma, aiming to improve the thrust and impulse coupling coefficient structural performance of laser propulsion. Meanwhile, for the microcavity structural performance of underwater fiber laser propulsion, we combine the analysis of laser plasmon detonation theory to build a microstructure underwater laser plasma propulsion model and numerical simulation scheme. Simulation research is conducted by structural morphology and numerical characteristics. Firstly, to address the issues of low impulse coupling coefficient and divergent effects in underwater laser propulsion without microcavities, we numerically simulate the flow field when the microsphere is propagated by an underwater fiber laser under the same laser energy but different microsphere positions. This illustrates the necessity of adding structural constraints for underwater fiber laser propulsion and objecting the rust and pulse coupling coefficient without microcavities as a reference. Then, based on rectangular microcavities, we simulate the flow field of underwater fiber laser pulse for microcavities with different lengths and diameters while maintaining a constant distance between the microsphere and the laser focus. Finally, we conduct numerical simulations of the performance of open, U-shaped microcavities, double tube microcavities, and structures with blocking forces, aiming to establish a mapping relationship between the morphology of microcavities and the changes in the force and weight coupling coefficients of microspheres through comparative analysis of numerical results.

Numerical simulations yield the force curve and impulse coupling coefficient of the microsphere when the laser focus-to-microsphere distance changes without microcavities. The maximum impulse coupling coefficient is 0.117 dyne/W (Fig. 3), and the flow field situation of underwater fiber laser propulsion at 0.2 μs without micro cavities shows higher pressure in the symmetrical range of 30 to 90 degrees, indicating poor directional laser propulsion effects, which demonstrates the necessity of adding structural constraints (Fig. 4). By adding a rectangular microcavity, the impulse coupling coefficient can be improved to the order of 10³ (Tables 1-2) compared to that without microcavities. However, as the microcavity length increases, the peak thrust decreases, reaching a maximum of about 140 dyne/W (Fig. 7). As the diameter increases, the impulse coupling coefficient increases, but the growth rate slows down (Fig. 9). The changes in force on the microsphere and the impulse coupling coefficient are obtained for U-shaped microcavities, double-tube microcavities, and microcavities with blocking structures. The U-shaped micro cavity increases the total applied force on the microsphere by approximately 0.002 N and improves the impulse coupling coefficient by 20 dyne/W (Fig. 12). When propelled by double-tube microcavities, the microsphere experiences a thrust of approximately 0.103 N and an impulse coupling coefficient of 340 dyne/W (Fig. 14). In the case of microcavities with blocking structures, the peak thrust is approximately 0.067 N, which represents a 44% increase (Fig. 17), while the impulse coupling coefficient is approximately 260 dyne/W, indicating an 86% increase (Table 3).

We propose several microcavity structures to solve the low impulse coupling coefficient and divergent effects of underwater fiber laser propulsion without microcavities. By conducting numerical simulations on the flow fields of different microcavity structures, we obtain the thrust curves and impulse coupling coefficients after adding the respective microcavities. The addition of rectangular microcavities can significantly improve the impulse coupling coefficient of underwater fiber laser propulsion to the order of 10³ and concentrate the effects of laser propulsion. Meanwhile, when the distance between the laser focus and the microsphere remains constant, increasing the length of the microcavity decreases the peak thrust, while increasing the diameter increases the impulse coupling coefficient, albeit at a decreasing rate. Compared to rectangular microcavities, U-shaped microcavities, double-tube microcavities, and microcavities with blocking structures provide greater enhancements in force on the microsphere and impulse coupling coefficients. Among the four types of microcavities, double-tube microcavities show the greatest enhancement effect. The results demonstrate that the addition of microcavities can improve the efficiency of underwater fiber laser propulsion and provide corresponding enhancement effects for the four microcavity structures.