A study on laser multi-focus separation technology of thick KDP crystal

KDP crystal is an important electro-optic material in various laser systems, and belongs to very difficult-to-cut material, especially for thick crystals. In this study, an innovative separation method—laser multi-focus separation technology (LMFS) for thick KDP crystal has been developed by skillfully combining femtosecond laser and LMFS optical system of fiber laser for the first time. In this way, the uniformities of temperature and thermal stress distributions along crystal thickness and the utilization efficiency of the laser energy are greatly improved. A penetration crack along crystal thickness could be formed and its propagation direction could be controlled to achieve safe and high-quality separation. The separating thickness of LMFS (50 mm) is at least 4 times thicker than that of existing laser separating technologies, and the separating efficiency of LMFS (200 µm/s) is at least 20 times faster than that of traditional mechanical methods. The generation mechanism of multi-focus was expounded by optical analysis and design, and verified by an established LMFS optical system. A numerical simulation was also established to analyze the dynamic distributions of temperature and thermal stress of KDP crystal generated by the LMFS optical system, and explore the separation mechanism of LMFS. In addition, a window of appropriate processing parameters was predicted by a series of numerical simulations to successfully separate a thick KDP crystal with the thickness of 50 mm. The quality of the separated sidewall surface was clean and flat (roughness of 10.857 nm, flatness of 3.5389 µm) without any contamination, subsurface damage and edge fragmentation. The experimental results proved the feasibility of LMFS, and are in good agreement with the theoretical analysis.