Overview of advanced optical manufacturing techniques applied in regulating laser damage precursors in nonlinear functional KH<sub>x</sub>D<sub>2-x</sub>PO<sub>4</sub> crystal

Nonlinear KHxD2-xPO4 crystal optics, e.g.second/third-harmonic generators, are components of high-energy/power laser facilities, which deliver and convert 1, 2, and 3 lasers to obtain extreme fusion ignition conditions (high pressures, high temperatures, etc.).A laser facility requires extremely high-precision and defect-free KHxD2-xPO4 optics with meter-sized apertures to control laser beams temporally, spatially, and spectrally, yielding great ultra-precision manufacturing challenges.Meanwhile, when irradiated by intense laser pulses, laser damage precursors (e.g.manufacturing-induced micro-cracks, scratches, and debris) in the optics would spark off laser-induced surface damage and damage growth, which have been the bottleneck problems preventing the promotion of the output energies of these laser facilities.Under this circumstance, a variety of advanced optical manufacturing techniques have been developed to regulate these precursors to improve the laser damage resistance of the optics.However, the damage thresholds (8-9 J/cm 2 ) of these optics are still far below the intrinsic threshold of the KHxD2-xPO4 (147-200 J/cm 2 ).Furthermore, the batch engineering applications of these techniques remains challenged by the meter-sized apertures of the optics and their soft-brittle, easily deliquescent, anisotropic, and temperature-sensitive material properties, among others.This work summarises the development of state-of-the-art advanced manufacturing techniques and their problems applied in regulating laser damage precursors in the functional KHxD2-xPO4 optics.Because of their soft-brittle, deliquescent, anisotropic nature, etc., these crystal optics are difficult to cut, and new damage precursors (i.e.corrosion, debris, tool marks) could be introduced in the manufacturing processes.The challenges and their solutions are emphatically discussed and analysed in this paper.The latest development trends for the manufacture of high-performance KHxD2-xPO4 optics with high laser damage resistance are also explored.This work could provide basis and guidance for the function-oriented high-performance manufacturing of KHxD2-xPO4 optics and other functional optics with similar material properties, advancing the development of high-energy/power laser facilities.