Achieving and maintaining performance goals for high-peak-power laser systems is in part related to the ability to manufacture optical components with high laser-induced damage thresholds and maintain this performance during the operation of the system. Many factors affect the laser-induced damage performance of optics, such as material quality, optic design, and cleanliness. Extensive effort has been devoted to increase the intrinsic quality of the materials, including reducing or eliminating defects that can couple laser energy into the material. However, extrinsic defects, such as those arising from contamination in various forms, is of equal or higher importance. Ideally the optic is not subjected to any type of contamination after its manufacturing, although this is practically impossible. Contamination can arise from handling during transport and installation of the optics, which will directly impact its initial performance. In addition, contamination from the operational environment will determine the effective damage threshold of an optic throughout its lifetime. Particles located on the optics surface, especially for reflective optics, can be precursors for laser-induced damage on the optic itself or further downstream of the laser. There is a perception that the laser beam can remove such particles, also referred to as laser cleaning. However, such a "cleaning" process is not totally benign as it often leads to various types of microscale damage or modification of the optic. Once optics become damaged via this mechanism, a number of damage sites will continue to grow in size (commonly referred to as damage growth). In the case of continuous accumulation due to contamination in the operational environment and subsequent laser "cleaning" of particles on the optic, a degradation of its optical quality will follow and the laser-damage threshold will decline.

For short-pulse, high-intensity laser systems, the cost to manufacture and replace optics is large. It is therefore imperative to understand all sources that can lead to performance degradation. With regards to particle contamination, careful handling and clean operational environments are very important issues. However, there still is a background level of particles that optics are exposed to. For example, a class-1000 clean room facility contains about 300 particles per m³ with more than 5-μm radius, 8300 particles with more than 1-μm radius and 35,000 particles with more than 0.5-μm radius. Electrostatic charging of the optics following laser irradiation can increase particle attachment on the optics.

The research group from the Laboratory for Laser Energetics, University of Rochester characterized the particle load and its chemical composition that exists inside the OMEGA EP grating compressor vacuum chamber over a period of about 2 years. The research results are published in High Power Laser Science and Engineering, Volume 11, Issue 3 (B. N. Hoffman, N. Savidis, S. G. Demos. Monitoring and characterization of particle contamination in the pulse compression chamber of the OMEGA EP laser system[J]. High Power Laser Science and Engineering, 2023, 11(3): 03000e39). The characterization was done by placing glass collection substrates inside the vacuum chamber in front of critical optics. Particles were collected for about 3 months per sample before the samples were removed from the chamber and were characterized. Analysis was performed with optical microscopy to evaluate particle density and size, scanning electron microscopy to determine particle morphology and surface texture, and energy dispersive x-ray spectroscopy to identified atomic elements.

The results demonstrate that despite extensive efforts to minimize particle contamination, hundreds of particles per mm² were found on collection samples located in front of critical optics. The majority of particles have diameters ≤4 µm and are comprised of metals, glass, and carbonous materials. The results indicate that the particle distributions change between operational periods. The working hypothesis is that particles are comprised mostly of target materials that migrated into the grating compressor chamber. Although understanding of the mechanisms for particle generation and transport remains uncertain, the hypothesis is that this particle load represents a risk for contaminating the surfaces of high-value optics located inside the chamber, including the compression gratings and deformable mirrors, and therefore affect the optical elements laser-damage resistance and overall operational lifetime.

Representative SEM images of stainless-steel particles collected in the OMEGA EP pulse compressor vacuum chamber.

Photograph of the interior of the OMEGA EP pulse compressor vacuum chamber adjacent to the diffraction grating assembly consisting of three separate grating tiles.