Effect of surface scallop tool marks generated in micro-milling repairing process on the optical performance of potassium dihydrogen phosphate crystal

Micro-milling is the prevailing method to repair laser-induced surface damage on potassium dihydrogen phosphate crystal components applied in high-power laser systems. Scallop tool marks are inevitably generated on repaired surfaces due to the interaction of neighboring tool paths involved in the micro-milling process. In this work, the effect of scallop tool marks on the optical performance of micro-milled crystal surfaces is theoretically and experimentally investigated. The results indicate that surface tool marks could be categorized into four bands having different period lengths (L) according to the levels of induced light intensification: high-frequency (0.1 μm ≤ L ≤ 0.7 μm), high-risk (0.7 μm ≤ L ≤ 1.2 μm), intermediate-frequency (1.2 μm ≤ L ≤ 100 μm), and low-frequency (100 μm ≤ L ≤ 1000 μm) bands. Tool marks in high-risk band play dominant role in degrading the repairing effectiveness that they should be strictly excluded. While for intermediate-frequency band marks, the induced light intensification is low, and the laser damage resistance and transmittance capacity are comparable to those of tool mark-free surface. The experimental results agree well with simulation results, which suggest that machining parameters corresponding to tool marks in intermediate-frequency band should be preferred in the actual micro-milling repairing process.