Mitigating damage precursors to improve the laser resistance of ion beam sputtered antireflective coatings
International Conference on Space Optics (ICSO)
29. März - 02. April
Satellite missions to study the atmosphere are becoming increasingly important due to the climate change. The Aeolus satellite for example provides much needed information to improve weather forecasting as part of ESA’s Earth Explorer missions. One key part of the payload is the high power ultraviolet laser system. A major risk for the success of this space mission is the combination of laser induced contamination and damage resistance of the ultraviolet coatings within the laser system. Damage precursors located in the dielectric coating are heated by the intense laser beam and possibly initiating film damage. In order to increase the sensitivity and accuracy of future space-based LIDAR systems, higher pulse energies of the transmitter require improved optical components in order to avoid both damage and contamination growth due to the intense laser radiation. Within the scope of the work presented here, sets of improved optics were manufactured and tested using several defect mitigation strategies during layer deposition of an ion beam sputtered antireflection coating for 355 nm. The first approach to avoid the damage precursors in the film is based on a secondary ion source. The goal is to remove the particles that settle on the surface of the growing film. In addition, this resputtering process can be used to reduce stoichiometric defects in the growing coating material. This was achieved by integrating a radio frequency pumped ion source into the coating system pointing towards the substrates. A second strategy is to avoid the damage precursors in the ultraviolet wavelength range by laser conditioning of the thin films. A laser beam at application wavelength (in this case 355nm) scans the deposited layer on the sample during deposition still in vacuum. The aim is to evaporate precursors near the surface before they are being covered by the layer system with an application oriented energy density, wavelength, and pulse duration. Resulting irregularities will be compensated by the growth of the remaining layers. Significant improvements were achieved during layer growth as the test coatings were applied to super-polished fused silica substrates. In order to demonstrate the enhancements in laser induced damage resistance, the samples were raster scanned at an energy density of 25 J/cm2. Dark field microscopy was used to identify the laser-induced damage of the antireflection coatings. The improvement in the susceptibility level to laser induced contamination was also investigated. The results were then compared with optics that had not undergone any defect mitigation treatment. This work has been carried under the ESA contract AO 1-8683/16/NL/BJ activity with name “Particle mitigation in high power laser optics”.