Fig. 1. Structural diagram of the coupling device of each research institution. (a) Schematic diagram of the water-guided laser processing device^[35]; (b) 3D structure of the high-pressure coupling device^[36]; (c) Synova's coupling device model^[28]; (d) Avonisys's coupling device model^[28]; (e) coupling device model and its cross-section^[37]; (f) cross-section of the coupling device (1 base; 2 nozzle; 3, 6, 8, 10 O-type rings; 4 main body; 5 water inlet hole; 7 top cover; 9 screw; 11 optical glass; 12 laser beam)^[37]; (g) schematic diagram of water jet guided laser; (h) schematic diagram of total reflection of water jet

Fig. 2. Flow channel structure and flow characteristics of the coupling cavity. (a) Flow channel structures of coupling device; (b) velocity cloud maps of the channel^[36]; (c) velocity cloud maps of cross-sections, from left to right is 2×2 structure, 2×4 structure, 4×4 structure^[40]

Fig. 3. Reattachment behavior of water jets under conditions with and without cavitation at different Reynolds numbers. (a) Reattachment of water jet considering cavitation effect at different Reynolds numbers^[43]; (b) jet phase diagram at different time considering cavitation effect with Reynolds number of 5977^[43]; (c) flow conditions of jet at different time considering cavitation effect with Reynolds number of 4940^[45]; (d) comparison of reattachment lengths with and without cavitation reattachment^[45]

Fig. 4. Thermal-flow-solid coupling analysis of water guided laser. (a) Spatial energy distribution of laser in water jet^[29]; (b) variation of the highest temperature in the processing area^[29]; (c) removal mode of carbon fiber reinforced composite materials^[1]

Fig. 5. Structures of water jet nozzle. (a) Schematic diagram of nozzle structures^[36]; (b) velocity contour plots for nozzles with different length-to-diameter ratio^[36]; (c) schematic diagram of nozzle structures, from left to right is Helmholtz nozzle, organ pipe nozzle, Venturi nozzle^[52]

Fig. 6. Nozzle and water jet shape. (a)(b) Schematic diagram of water jet^[28]; (c) stability length curve of laminar flow and turbulence flow^[23]

Fig. 7. Laser-water jet coupling^[28]. (a) Schematic diagram of laser beam coupling to water jet; (b) near-field coupling of laser-water jet coupling; (c) far-field coupling of laser-water jet coupling; (d) principle of lateral error, vertical spacing error, and angular error

Fig. 8. Laser transmission and coupling. (a) Schematic diagram of laser fiber transmission^[45]; (b) cross-section of light transmission path along meridional ray^[58]; (c) cross-section of light transmission path along oblique ray^[58]; (d) at the 60 mm position of the water jet, distribution of the laser detector and its middle part with the eccentric coupling eccentricities of the laser beam of 20, 30, 40 μm, respectively^[59]; (e) images of the water jet with laser and the damaged nozzle in the experiment, from left to right: when the eccentricity is less than or equal to 20 μm, the nozzle is undamaged and the laser can be transmitted with the water jet; when the eccentricity is 25 μm, the nozzle is damaged, the laminar flow is disrupted, and the laser cannot be transmitted; microscopic view of the damaged nozzle^[59]; (f) simulation analysis of the inclined coupling of the laser beam in a 100-μm water jet^[59]

Fig. 9. Energy distribution characteristics of laser in water jet. (a) Simulation model of water jet guided laser system with different focusing modes^[60]; (b) coupling alignment of water jet and laser, from left to right is aligned coupling, axial deviation, radial deviation, and angular deviation^[61]; (c) electric field distribution at different stages of water jet, from left to right is contraction section, onset of cavitation, development of cavitation, and hydraulic flip^[22]

Fig. 10. Analysis of the effect of gas pressure and water pressure on the characteristics of water jet. (a)(b) Influence of auxiliary gas flow rate on water jet^[63]; (c)(d) water phase streamline and argon phase streamline^[64]; (e)(f) vector distribution of argon phase and ambient air on the reference plane^[64]; (g) water jet morphologies and external stable lengths under different gas atmospheres^[64]

Fig. 11. Schematic diagram of the water-guided laser processing mechanism. (a) Experimental phenomenon of water-guided laser processing^[67]; (b) material removal mechanism of water-guided laser processing^[69]; (c) SEM cross-section of laser cutting SiC_f/SiC composites^[69]; (d) working principle of water-guided laser^[70]; (e) material removal mechanism^[70]; (f) machined cross-section of remelted layer^[70]; (g) schematic diagram of the laser water jet processing process without gas assistance^[64]; (h) schematic diagram of the laser water jet processing process with gas assistance^[64]; (i) magnified cross-sectional morphology of the substrate part processed under atmosphere^[64]; (j) schematic diagram of the influence of water layer on laser focusing; (k) influence of water film thickness on the diameter and depth of the holes drilled by femtosecond laser water-assisted drilling (100 kHz, 521 mW, 500 pulses) ^[71]; (l) SEM image of one scan in water^[71]

Fig. 12. Processing efficiency. (a) Cross-sectional morphology of the hole obtained under different scanning sequences^[29]; (b) morphology of the entry and exit of holes cut in SiC_f/SiC composites by femtosecond laser^[76]; (c) morphology of the entry and exit of holes cut in SiC_f/SiC composites by water-guided laser^[76]; (d) ablation depth and material removal rate at different scanning speeds^[69]

Fig. 13. Processing precision. (a) Data plots after cutting with a 12 W laser power under two different ΔR conditions, error bars represent the standard deviation of the mean value (n=6)^[59]; (b) scanning electron microscope images of silicon at a cutting speed of 10 mm/s and a scanning cycle of 4 times^[79]; (c) scanning electron microscope images of the cross-sections of water-guided laser cutting, traditional laser cutting, and electrical discharge machining (EDM) cutting^[81]; (d) morphology of the hole entrance and exit obtained at a single-pulse energy of 5.0 mJ^[29]

Fig. 14. Processing depth. (a) Variation of the cut depth in water-guided laser cutting with the increase of number of cutting cycles^[81]; (b) influence of laser power and lens on the depth of the groove^[67]; (c) cutting depth and the width of the cut seam varing with the speed of water jet^[1]; (d) relationship between hole depth and number of cycles under different hole diameters^[2]; (e) groove depth under different water pressure (200 bar and 300 bar)^[78]