Investigation on hybrid laser ablation and its application in fused silica damage mitigation.

We present and investigate a hybrid laser-based method of surface shaping for damage mitigation on fused silica surfaces. Damage sites were removed and precisely shaped into an optically-benign cone by a procedure of femtosecond laser ablation with a subsequent CO laser polishing process. The morphology of the cone rim was quantitatively predicted by a numerical model. Since the heat-affected zone (HAZ) of the laser polishing process was effectively confined by the optimization of ablation parameters, the dimensions of the raised rim were reduced by an order of magnitude. The intensity of the on-axis hotspot was positively related to the dimensions of the raised rim, and thus an inapparent downstream intensification was achieved by the rim reduction. Laser-induced damage threshold (LIDT) of the cone was tested to be ∼14 J/cm on the input surface. Therefore, the presented method is appropriate to mitigate damage and also provides a promising approach to manufacturing functional microstructures for high-power applications.