Study on laser multi-focus separation technology for thick KDP crystal

KDP crystal is an important electro-optic material in various laser system, and belongs to very difficult-to-cut material, especially for thick crystals. In this study, a laser multi-focus separation technology (LMFS) of thick KDP crystal has been developed by skillfully combining femtosecond laser and LMFS optical system. In this way, the uniformity 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 technology, and the separating efficiency of LMFS (200 mm/s) is at least 20 times faster than that of traditional mechanical method. The generation mechanism of multi-focus was expounded by optical analysis and design, and verified by an established LMFS optical system. A thick KDP crystal with the thickness of 50 mm was separated successfully by LMFS. The quality of the separated sidewall surface was clean and flat (roughness of 10.857 nm, flatness of 3.5389 mm) without any contamination, underground damage and edge fragmentation. The experimental results proved the feasibility of LMFS, and are in good agreement with the theoretical analysis.