Since the advent of the laser era, extensive efforts have been devoted toward designing and manufacturing optics with a high laser-induced damage threshold (LIDT). In high-energy, short-pulse laser systems, the optics tend to be large, costly and difficult to replace. As a result, optimizing the manufacturing process, careful handling practices, cleaning procedures and a clean operational environment are recognized as important aspects to keep the optics as clean as possible throughout their lifetime. The principal reason for initiation of laser-induced damage in a ‘pristine’ optic is imperfections associated with the manufacturing process[1–4]. However, contamination due to the handling or the operational environment introduces additional challenges in maintaining the original performance of the optics during their lifetime[5–18]. This problem has been extensively investigated during the past 10 years for the case of nanosecond laser systems[18–24]. The impact of particle contamination in short-pulse laser systems has only recently received attention[25–27]. While most of the optics of concern in nanosecond laser systems are transmissive, optical elements for short-pulse laser systems are typically reflective and are based on metal or multilayer dielectric (MLD) coating designs. In reflective optics, the interference of the incoming laser field with scattered waves originating from particles gives rise to the generation of localized high electric-field intensity (EFI) and leads to laser-induced damage. This behavior was demonstrated in experiments that investigated the interaction of 10-ps and 600-fs pulses (at 1053 nm) with four different types of particle materials (stainless steel, borosilicate glass, low-density polyethylene and polytetrafluoroethylene) that are typically found in the operational environment of large-aperture systems[25,26]. The experiments demonstrated that the interaction of these model contamination particles (having a nominal diameter of 40 μm) with short pulses leads to discrete types of effects.
- 1.Damage on the top layer due to interference between the incoming laser field and scattered light by particles.
- 2.Damage on the optic due to microlensing (for transparent particles).
- 3.Secondary contamination of the optic due to ablation or disintegration of the particle. This secondary contamination leads to new damage or damage growth under subsequent irradiation.
- 4.Plasma-induced etching (scalding)[28,29] of the optic due to the formation of plasma, especially in locations where plasma is partially confined (such as near or under the particle).