Damage resistance of fused silica modified by magnetorheological finishing (MRF) and ion beam etching (IBE)

MRF is capable of producing smooth surface without incurring serious mechanical defects. Thus it is employed to machine fused silica in the hope of reducing mechanical defects on the optical components. The MRF-polished surface was damage-tested with 355nm 8ns pulsed laser and it is found that the laser induced damage threshold was not improved (31.2J/cm² ) even if the surface contains almost no mechanical defects. However, the damage threshold is increased to 45J/cm² after slight HF-solution etching (~1μm material removal). On the other hand, the ion beam etching (IBE) was also investigated to find out the potential effects on the laser damage performance of fused silica. The laser damage threshold of IBE processed fused silica is 26.5J/cm² while the threshold rose to 55J/cm² after slight chemical etching (~1μm material removal) with HF solution. For comparison, the control samples finished with conventional pad polishing process were also tested. The thresholds prior to and following HF wet etching (~1μm material removal) are 29 J/cm² and 42 J/cm² , respectively. From the experimental results, it is clear that slight HF wet etching can enhance the damage resistance of fused silica irrespective of the finishing techniques. Neither MRF nor IBE finished fused silica surface behave better than conventionally polished surface whilst IBE-finished surface appears to have stronger damage resistance after HF etching. HF etching can improve the laser damage threshold by >107% for IBE finished fused silica.