Study of modulation property to incident laser by surface micro-defects on KH ₂ PO ₄ crystal

Abstract

KH2PO4 crystal is a crucial optical component of inertial confinement fusion. Modulation of an incident laser by surface micro-defects will induce the growth of surface damage, which largely restricts the enhancement of the laser induced damage threshold. The modulation of an incident laser by using different kinds of surface defects are simulated by employing the three-dimensional finite-difference time-domain method. The results indicate that after the modulation of surface defects, the light intensity distribution inside the crystal is badly distorted, with the light intensity enhanced symmetrically. The relations between modulation properties and defect geometries (e.g., width, morphology, and depth of defects) are quite different for different defects. The modulation action is most obvious when the width of surface defects reaches 1.064 μm. For defects with smooth morphology, such as spherical pits, the degree of modulation is the smallest and the light intensity distribution seems relatively uniform. The degree of modulation increases rapidly with the increase of the depth of surface defects and becomes stable when the depth reaches a critical value. The critical depth is 1.064 μm for cuboid pits and radial cracks, while for ellipsoidal pits the value depends on both the width and the length of the defects.

Role of tool marks inside spherical mitigation pit fabricated by micro-milling on repairing quality of damaged KH2PO4 crystal

“…The modulation to incident laser light by surface features can potentially produce light intensification inside optical components, which is closely associated with the localized energy deposition and increased nonlinear absorption of intense light 23 24 25 29 . These negative effects introduced by tool marks would largely lower the laser damage resistance of repaired crystal surfaces.…”

“…2b , the effect of tool mark density is also taken into account to describe the interactions between neighboring tool marks. It has been widely known that the modulation property to incident light by rear-surface features always differs from that by front-surface structures 13 25 26 27 , hence, the effects of tool marks on front- and rear-surface mitigation pits are both simulated in this work. As shown in Fig.…”

“…The dimensions of these tool marks are generally comparable to the laser wavelength with several micrometers in width and sub-micrometers in depth. Surface defects with these specific sizes can largely modulate the incident laser and correspondingly lower the laser damage resistance of high-precision optical surfaces 24 25 26 27 . Hence, making a thorough investigation on the effect of tool marks on laser damage resistance can not only provide instructive guidance for the close supervision of tool wear in the practical repair process of KDP crystal, but also contribute to the full evaluation of the repaired crystal surfaces.…”

Laser induced damage due to scratches in the surface of nonlinear optical crystals KH2PO4 (KDP)

“…The simulation domain was rectangular and gridded with a uniform grid size. The grid size was δ = 0.01 μm, which was less than λ/12 to weaken the effect of numerical dispersion caused by differencing in 2D FDTD and consequently guarantee the accuracy of the simulation [11]. The perfect matched layer (PML) boundary condition was utilized along the z axis, and periodic boundary condition along the x axis [20].…”

“…Thus it is of great importance to carry out investigations into the damage characteristics, in particular, the effect of scratches on laser-induced damage of KDP. Chen et al have studied the potential effects of micro cracks on the laser induced damage of KDP [10,11]. Using finite-difference time-domain (FDTD) algorithm, they find that light intensity of incident laser can be intensified in the vicinity of microcracks and the intensification is related to many factors, including depth, width and shape and the angle of microcacks relative to crystal orientation as well as the wavelength of incident laser.…”