Laser induced damage on dielectric mirrors and its rapid growth with successive shots have been and continue to be an important barrier to high power laser systems. Here the morphology of mitigation pit is optimized theoretically, and an ultrashort laser is utilized to totally remove damage on both high-reflective (HR) and anti-reflective (AR) coating. At the same time, the substrate is handled carefully and free of laser ablation, which lower the scattering loss and the amount of debris during laser machining process. Then, using R-on-1 test procedure, several mitigated sites with size of 1mm× 1mm are investigated by a Nd:YAG laser system with a flat-top spatial distribution of fully covering the mitigated site. The experimental results show even at the average fluence of 18J/cm2@6ns, there’s no damage initiation on AR coatings and no damage growth on HR coatings. It demonstrates that ultrashort laser machining is an effective and robust way to mitigate laser damage and a promising way to improve dielectric mirror performance of high power laser system in volume production.
A kind of defects on the incident surfaces of fused silica optics are reported having the potential to initiate the damages on the exit surfaces in the final optical assembly in high power lasers. In this light, the new safe criterions for defects on the incident surfaces are proposed to avoid the detrimental modulation effects in downstream.