1. Introduction
The rapid development of aerospace, semiconductor, medical, and energy industries has significantly increased the performance requirements for critical components, thereby driving continuous innovation in cutting technologies and processing equipment. Compared with conventional mechanical cutting, laser cutting offers remarkable advantages in terms of precision, efficiency, and environmental compatibility. These advantages include non-contact material removal without mechanical stress, broad material adaptability for flexible manufacturing, and high processing efficiency enabled by programmable control, making laser cutting suitable for large-area and high-precision applications.
According to pulse duration, laser sources can be categorized into continuous-wave lasers, long-pulse lasers, short-pulse lasers, and ultrashort-pulse lasers. Continuous-wave and long-pulse lasers provide high processing speeds but typically induce extensive heat-affected zones (HAZs) and recast layers. Ultrashort-pulse lasers, such as femtosecond lasers, can theoretically achieve “cold processing” by directly transforming materials into plasma; however, their material removal efficiency remains limited, especially for large-scale industrial applications. Nanosecond pulsed lasers offer lower cost and higher ablation efficiency, but they are fundamentally thermal processes and often result in typical thermal defects, including microcracks and recast layers. Even femtosecond laser processing may exhibit non-negligible thermal effects under high repetition rates and high energy densities.
To overcome the intrinsic thermal limitations of dry laser processing, researchers have introduced water-assisted laser technologies. Among them, water-jet guided laser (WJGL) processing represents a unique hybrid technique that integrates laser energy delivery with a high-speed water jet. The fundamental concept was first proposed in the early 1990s, followed by systematic development and commercialization by Synova, leading to the emergence of laser microjet (LMJ) systems. Today, WJGL has been successfully applied to cutting, drilling, and grooving of metals, brittle crystalline materials, diamonds, ceramics, and composite materials.
This paper presents a comprehensive review of WJGL cutting technology, including its working principles, laser–water coupling mechanisms, material removal processes, and energy transmission behavior. Recent application progress in metals, brittle crystals, and composite materials is critically discussed. Technical challenges and future development trends are also analyzed to provide systematic guidance for both fundamental research and industrial implementation of WJGL technology.
2. Water-Jet Guided Laser Processing Technology
Water-jet guided laser processing combines the advantages of laser machining and high-speed water jets, offering distinctive benefits compared with conventional dry laser cutting. In WJGL, the water jet replaces auxiliary gas flows and serves simultaneously as a laser waveguide, cooling medium, and debris removal mechanism. As long as the laser wavelength is absorbable by the target material, WJGL can process ultra-hard, brittle, or thermally sensitive materials regardless of electrical conductivity.
Unlike dry laser processing, a substantial portion of laser energy in WJGL is dissipated within the water jet rather than directly inside the workpiece. The water jet continuously cools the kerf edges between laser pulses, effectively suppressing thermal accumulation, residual stress, and HAZ formation. Moreover, the high kinetic energy density of the water jet enables efficient removal of molten material, producing smooth cut walls free of burrs, redeposited debris, and cavities.
The mechanical force exerted by the water jet on the workpiece surface is extremely small (typically below 0.1 N), significantly lower than that encountered in conventional laser processing. As a result, WJGL is essentially a non-contact process with minimal mechanical damage. Additional advantages include extended working distance, large depth of focus, high aspect ratio cutting capability, and fine kerf widths typically ranging from 25 to 150 µm.
3. Formation Principle of Water-Jet Guided Laser
3.1 System Configuration and Optical Guiding Mechanism
WJGL relies on the refractive index difference between water and air to guide laser energy via total internal reflection at the water–air interface, analogous to optical fiber transmission. When a laser beam is injected into a stable micro water jet at an angle smaller than the critical angle for total internal reflection, the laser propagates along the water column with minimal divergence until it reaches the workpiece surface.
A typical WJGL system consists of four main subsystems: a laser and optical module, a high-pressure water supply unit, a protective gas module, and a coupling head. Ultra-pure water is pressurized (5–80 MPa) and expelled through a micro-nozzle with diameters ranging from 10 to 200 µm, forming a stable, hair-like water jet. The nozzle is commonly fabricated from sapphire, ruby, or diamond to resist wear and thermal damage. The laser beam is focused precisely at the nozzle entrance through optical windows and lenses, ensuring efficient coupling into the water jet.
3.2 Laser–Water Coupling Mechanism
Efficient coupling of the focused laser beam into the micro water jet is a critical requirement for WJGL. First, the laser spot diameter must be smaller than the nozzle orifice to prevent energy loss and nozzle damage. Second, the angular distribution of the focused beam must satisfy the condition for total internal reflection at the water–air interface.
Laser propagation within the water jet can be classified into meridional rays and skew rays, depending on their trajectories relative to the jet axis. Two coupling strategies are commonly employed: near-field coupling at the nozzle entrance and far-field coupling into the external water jet. Near-field coupling provides a larger acceptance angle and smaller focal spot but may suffer from thermal disturbances inside the nozzle, whereas far-field coupling mitigates thermal effects at the cost of stricter geometric constraints.
4. Material Removal Mechanism in WJGL Processing
Material removal in WJGL occurs through a cyclic laser–water interaction process. Initially, the high-speed water jet impacts the workpiece surface, forming a thin water film. Laser pulses guided by the water jet deliver energy to the material surface, where the absorbed energy is converted into heat, causing localized melting and vaporization.
The rapid formation of vapor or plasma generates recoil pressure and shock waves, which, together with the mechanical action of the water jet, expel molten material from the kerf and suppress recast layer formation. The surrounding water environment confines the plasma plume and redirects shock waves toward the material, enhancing ablation efficiency. At the end of each laser pulse, vapor bubbles collapse, molten material is flushed away, and the processed zone is rapidly cooled before the next pulse begins. This repetitive heating–cooling cycle enables high-quality machining with minimal thermal damage.
5. Energy Transmission in Water-Jet Guided Laser
High-power laser transmission within a water jet inevitably involves energy loss due to absorption, scattering, and nonlinear optical effects such as Raman scattering. Experimental and numerical studies have shown that laser power attenuation increases with transmission length and laser power. Shorter wavelengths (e.g., 532 nm) generally exhibit higher transmission efficiency in water compared with infrared wavelengths (e.g., 1064 nm).
Multiphysics simulations combining electromagnetics, heat transfer, and fluid dynamics have revealed that increasing beam diameter can reduce divergence and mitigate energy loss caused by violation of total internal reflection conditions. However, comprehensive understanding of high-power laser propagation in water jets remains limited, and further experimental validation and theoretical modeling are required to optimize energy delivery efficiency.
6.1 Metal Materials
WJGL has been widely applied to precision cutting of metals such as stainless steel, aluminum alloys, titanium alloys, and nickel-based superalloys. Compared with conventional laser cutting, WJGL significantly reduces HAZ thickness, recast layers, and surface contamination. Although cutting speeds are generally lower, WJGL produces superior surface integrity, smooth kerf walls, and minimal thermal distortion, which are critical for aerospace and medical applications.
6.2 Brittle Crystalline Materials
Hard and brittle materials, including silicon, sapphire, gallium arsenide, and diamond, are particularly challenging to machine using traditional methods. WJGL enables crack-free, low-chipping cutting with excellent edge quality. In semiconductor wafer dicing and sapphire substrate processing, WJGL has demonstrated high cutting efficiency, smooth sidewalls, and minimal subsurface damage, making it highly suitable for microelectronics and optoelectronics manufacturing.
6.3 Composite Materials
Advanced composites such as CFRP, aluminum matrix composites, and ceramic matrix composites benefit significantly from WJGL processing. The combined laser ablation and water cooling effectively suppress delamination, fiber pull-out, and matrix cracking. Experimental results indicate that WJGL can achieve high aspect ratio cuts with minimal thermal degradation and superior surface quality compared with dry laser or mechanical cutting methods.
7. Technical Challenges and Future Trends
Despite its advantages, WJGL technology faces several challenges. Laser energy attenuation within the water jet limits processing efficiency, especially for high-power applications. Further research into alternative guiding media or optimized water chemistry may help reduce energy loss. Miniaturization of water jets is essential for higher precision but poses challenges in jet stability and coupling efficiency. Additionally, high-precision nozzle fabrication, rapid and accurate laser–water alignment, and standardized process control methodologies remain key areas requiring innovation.
Expanding the applicability of WJGL to ultra-hard materials such as diamond, quartz glass, sapphire, and advanced ceramics also demands systematic optimization of processing parameters and auxiliary techniques.
8. Conclusions and Outlook
This review systematically summarizes the principles, material removal mechanisms, and application progress of water-jet guided laser cutting technology. Owing to its unique laser–water interaction mechanism, WJGL enables high-precision, low-damage machining across a wide range of difficult-to-process materials. Its ability to suppress thermal defects, improve surface integrity, and reduce environmental contamination highlights its strong potential in aerospace, semiconductor manufacturing, and medical device fabrication.
Although challenges related to jet stability, energy transmission efficiency, and equipment complexity remain, ongoing advances in laser technology, fluid control, and system integration are expected to further enhance WJGL performance. With continued collaboration between academia and industry, WJGL is poised to become a mainstream technology in ultra-precision manufacturing, supporting the growing demands of next-generation high-tech industries.
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