Research on laser beams focusing and coupling technology of water jet guided laser with high adjustment tolerance

In the traditional water jet guided laser (WJGL) processing technology, a convex lens focused beam is used to couple into the water jet, and the coupling adjustment range is limited within the Rayleigh range of the focused beam, which results in large difficult adjustment. In this paper, combined concave axicon (CCA) and convex axicon (CVA) through pasting the two axicons in the circumferential direction is used to generate non-diffraction laser beam (NDLB) with micron level central spot, and a non-diffraction region (NDR) reaching tens to hundreds of millimeters and wide range of focal depth, realizing arbitrary random coupling in Z direction, which can reduce the difficulties of water jet coupling with laser beam (WJCLB) in WJGL and machining accuracy of single CVA with small cone angle. Numerical calculation, simulation and experimental analysis show that combined CCA and CVA possesses the characteristics of CVA. When the equivalent cone angle is less than 10 degrees and the laser wavelength is 532 nm or 1064 nm, the central spot diameter of the NDR is less than 100 mu m and the focusing angle is less than 10 degrees, which meets the critical condition of total reflection of water jet. In addition, the NDR of NDLB extends nearly 16 mm with an equivalent cone angle of 5 degrees and a wavelength of 532 nm, effectively utilizing the coupling adjustment range of the NDR. The NDLB is successfully coupled into the water jet, and the central spot achieves a stable transmission with coupling efficiency about 92%. A method of adjustable ultra-fast NDLB with a combination of CVA and CCA and phase modulation is proposed constructively in order to better implement the technical scheme of water jet guided NDLB micro-machining with high adjustment tolerance possessing of WJCLB.