Damage Mechanisms of Mid-infrared Coating Components Induced by 2.1 μm Nanosecond Laser Pulses

Mid-infrared (MIR) lasers have important applications in atmospheric detection, materials processing, and laser medicine. However, the robustness of high-power MIR lasers is constrained by the low laser-induced damage resistance of optical coating components. This study, investigated laser-induced damage mechanisms of high-transmittance (HT) and percent-transmittance (PT) coating components susceptible to optical damage in high-power MIR lasers under 2.1 µm laser irradiation. Both HT and PT components showed the damage induced by significant thermal effects. Different damage morphologies were identified on the PT and HT coating surfaces of 2.1 µm PT components, characterized by dome-shaped bulges and funnel-shaped pits due to nodule defects and nano-absorptive precursors, respectively. The analysis of substrate effects on MIR multiband HT coatings (HT 1.9–2.2 & 3.5–4.8 µm) damage revealed that CaF2-based coatings showed higher susceptibility to mechanical interfacial damage compared to fused silica-based coatings, primarily attributed to higher coefficient of thermal expansion (CTE) of CaF2, which induced significant coating–substrate interfacial stress mismatch. These findings provide critical insights for the fabrication of durable MIR optical coatings, paving the way for advancing high-power MIR lasers with enhanced robustness.