The advancement of national aerospace, automotive manufacturing, biomedical, and related industries has intensified the precision and quality requirements for critical components, including aero-engine heat-resistant parts, integrated circuit semiconductor wafers, and cardiovascular stents. The materials utilized in these components, such as titanium alloys, monocrystalline silicon, and ceramic matrix composites, present significant manufacturing challenges. Traditional machining methods are inadequate for meeting the high-quality processing requirements characterized by minimal thermal damage and microcrack-free surfaces. While long-pulse laser processing offers efficient and cost-effective solutions, it generates substantial thermal defects. Ultrafast laser processing achieves “cold machining” through high-power-density electron stripping for non-thermal material removal but faces efficiency and cost limitations. Water-jet guided laser (WJGL) machining technology combines the efficiency and cost-effectiveness of long-pulse lasers while effectively minimizing thermal damage during processing, achieving results comparable to ultrafast laser techniques.

Significant research advances in the WJGL field include Dr. Richerzhagen from ETH proposing a laser thermal defocusing model in water based on FEM in the 1990s, which analyzed the impact of thermal defocusing on laser processing accuracy and efficiency, laying a solid theoretical foundation. Couty et al. established a model based on multimode fiber theory to predict laser intensity distribution in water jets, which was verified experimentally and offered crucial insights into laser transmission characteristics in water jets. Professor Cheng from Harbin Institute of Technology enhanced water jet stability by designing a coupling device with a single-side high-pressure water inlet and axisymmetric multi-channel layout, reducing hydraulic fluctuations. Zhang et al., via FEM, compared water jet velocity distribution and convergent section length with and without auxiliary gas constraints, proving the gas phase flow's positive role in water jet stability.

This review comprehensively examines WJGL processing technology, focusing three primary aspects: WJGL system operational principles, laser?water-jet coupling transmission mechanisms, and current WJGL technology applications.

In examining WJGL system fundamentals, this paper details the operational mechanism and system configuration. WJGL technology employs total internal reflection of laser within a water-jet to guide laser energy to the material surface through water-jet functioning as a liquid optical fiber, enabling non-contact material machining. The water jet serves primarily as a laser transmission medium and cooling agent rather than a direct processing tool. WJGL technology demonstrates notable advantages over conventional long-pulse laser processing, including reduced thermal damage, increased working distance, improved processing accuracy, and capability for high-aspect-ratio structure fabrication.

Multiple factors influence the coupling efficiency and stability of laser?water-jet transmission. Maintaining the water jet in a “contracted flow” state necessitates optimized nozzle geometry, multi-stage flow channel design, and auxiliary gas confinement to minimize turbulent disturbances and extend stable flow length. Laser wavelength selection (532 nm green light or 1064 nm near-infrared) must account for water’s optical absorption characteristics, supported by beam shaping techniques to minimize energy loss and maximize output power. Coupling transmission must fulfill total reflection conditions, requiring minimal transverse and angular alignment errors to prevent energy leakage and component safety risks. Longitudinal errors demonstrate reduced impact and can be addressed through water-jet dimension adjustment to enhance transmission efficiency.

WJGL’s substantial application potential in processing metallic materials, semiconductors, and advanced composites. However, several challenges remain: (1) Suboptimal coupling efficiency and precision between water jet and laser; (2) Technical difficulties in achieving high-quality, high-aspect-ratio machining of holes, grooves, and edges; (3) Insufficient process optimization for challenging materials such as diamond, sapphire, and ultra-hard ceramics. To address requirements for enhanced quality and mass production, future WJGL technology development may focus on:

(1) Reducing the laser beam diameter enhances the coupling efficiency between water jets and lasers. While larger water-jet diameters can improve coupling efficiency, they potentially compromise processing resolution and quality while increasing energy loss. A reduction in laser beam diameter facilitates adequate laser energy density for material processing. However, this approach encounters technical limitations including constraints in laser beam quality optimization, significant beam divergence, and inadequate manufacturing precision and stability of optical components required for stable small-spot focusing.

(2) Dynamic compensation of laser coupling position requires attention: temperature-induced variations in water affect the refractive index and laser absorption characteristics, resulting in coupling position drift. This phenomenon necessitates comprehensive investigation of temperature?refractive index relationships and development of adaptive control systems, though current limitations exist in sensor accuracy and adjustment mechanism capabilities.

(3) Research on laser energy distribution across the processing surface remains crucial. The processed surface structure influences the spatiotemporal distribution of the water-jet flow field, subsequently affecting the ablation dynamics between the coupled beam and material. Enhanced understanding of laser energy distribution on the processing surface could improve WJGL processing technology control. Present knowledge gaps exist in understanding how processed surface structure affects the spatiotemporal distribution of the water-jet flow field. Moreover, laser power stability and beam shaping technology impact the distribution of laser energy on the processing surface.