As aerospace, microelectronics, communication, and medical fields continue to develop rapidly, the components and parts of the related equipment or systems are becoming increasingly miniaturized and refined. However, traditional laser and mechanical processing methods often encounter challenges, such as low efficiency, significant thermal damage, and limited processing capabilities. Water-guided laser technology offers a promising solution by coupling high-energy lasers with micro-water jets, effectively addressing the thermal damage and stress issues commonly encountered in conventional techniques. This approach has demonstrated remarkable advantages for ultra-precision processing. In this article, we provide a comprehensive review of the advancements in microjet control technology, including improvements in high-pressure coupling devices for enhanced jet stability, dynamics within water chambers, formation mechanisms of microjets, and key aspects of microjet-guided laser coupling. Additionally, we explore how water jets influence processing quality in terms of mechanism, efficiency, precision, and depth. Looking ahead, we discuss the current challenges and future trends in this technology, and offer valuable insights for further research and practical applications.

Traditional machining methods, including mechanical and conventional laser processing, have numerous limitations. Mechanical processing often suffers from problems, such as stress concentration, limited accuracy, and rapid tool wear. Conventional laser processing, particularly continuous and long-pulse laser processing, is a heat-based technology that generates significant thermal defects, such as large heat-affected zones, recast layers, and microcracks. Although short-pulse laser processing can achieve ultrafine processing with minimal heat-affected zones, the high costs of equipment, operation, and maintenance restrict large-scale industrial applications. Therefore, water-jet guided laser technology has emerged as a promising solution. By adjusting the incident angle, the laser can undergo total internal reflection within the water jet, similar to a multimode optical fiber. Stable water?air interfaces are crucial for water-jet-guided laser processing. Various coupling devices, such as Synova, Avonisys, and the design by Cao et al., have been developed. These devices use various methods, such as blowing helium or forming coaxial air curtains, to enhance the water-jet stability and laser coupling (Fig.1). The flow channel structure in the coupling device significantly affects the water-jet quality. Structures such as a 4×4 channel can better adjust turbulent fluctuations and provide a more stable internal flow field for water jets (Fig.2). Cavitation in water-jet-guided laser processing can affect the stability of micro-water jets, and under certain conditions, is beneficial for water-laser coupling. Researchers have studied cavitation control by adjusting nozzle structure parameters (Fig.3). Understanding the interaction process of the laser water jet material through thermal-fluid-solid coupling analysis helps optimize the technology. For example, in the processing of carbon fiber-reinforced composite materials, differences in the thermal properties of the components lead to different removal modes (Fig.4). In addition, different nozzle structures, such as cylindrical capillary, conical, upper conical, and lower conical nozzles, result in different water-jet flow characteristics (Fig.5). Nozzle parameters, such as the length-to-diameter ratio, also affect jet stability and laser-water jet coupling efficiency. In addition, the relationship between nozzle diameter and jet stability has been investigated (Fig.6). Efficient laser-water-jet coupling is essential for improving nozzle life and system stability. Near- and far-field coupling strategies have advantages and disadvantages. Errors in the coupling process can result in laser radiation loss and equipment damage. Researchers have analyzed the influence of various factors on laser coupling and proposed non-diffraction laser-focusing modes to improve the coupling performance (Figs.7, 8, and 9). A stable water beam is crucial for laser conduction. Gas constraint systems, such as a coaxial spiral gas shield formed by argon in some designs, can provide a stable environment for water-jet and laser coupling. The gas and water pressures also affect the coupling length (Fig.10). The processing mechanism of water-jet-guided laser technology is complex owing to the involvement of high-speed water jets, lasers, and processed materials. This includes processes such as material melting, vaporization, and plasma removal, in which a water jet can reduce heat damage. The presence of gas assistance can also affect the processing quality (Fig.11). Furthermore, the gas flow rate, scanning order, and scanning speed affect the processing efficiency of materials, such as SiC_f/SiC composites. Water-jet-guided laser cutting has the advantage of processing small holes with large depth-to-diameter ratios (Fig.12). Unstable water jets can affect processing accuracy, but they can also reduce the heat-affected zone. Methods such as the water-jet laser field adjustment method can improve processing accuracy. Moreover, different processing parameters have different effects on processing accuracy (Fig.13). The water-jet pressure, flow rate, and scouring ability affect the processing depth. Multifocus lenses can improve the processing depth in high-power laser processing. The relationship between the hole diameter, processing depth, and other factors has also been studied (Fig.14).

Microjet control technology for water-jet-guided lasers has achieved substantial progress; however, it still encounters several challenges. The complex interaction mechanisms among lasers, water jets, and substrates, the instability of water jets affected by multiple factors, energy losses, and alignment inaccuracies during laser-water-jet coupling, and the limitations of traditional focusing methods hinder the further development of this technology. The scope of application of water-jet-guided laser technology is expected to expand. In addition to its current applications in aerospace and semiconductors, it has significant potential in fields such as biomedicine and MEMS manufacturing. By delving deeper into the interaction mechanisms among lasers, water jets, and materials, optimizing processing parameters such as laser power, pulse width, scanning speed, and water-jet pressure, and developing advanced control algorithms, we can enhance processing quality and efficiency. The development of finer and longer water jets by optimizing nozzle structures and parameters is also crucial. Additionally, strengthening research on the interactions among lasers, water jets, and workpiece materials, and improving multi-physical field modeling can better predict processing processes and results, providing theoretical support for process optimization. This technology is expected to play an important role in the development of high-precision manufacturing processes.