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Damage precursor mitigation towards enhanced laser resistance of ultraviolet antireflective coatings
In the paper, results on coating activities using the IBS process are discussed in respect to two aspects. One is the adaption of the IBS to strongly curved surfaces like lenses exemplified by the FM lenses for the Copernicus Sentinel-4 mission. The other is a study to increase the laser-induced damage threshold for future LIDAR systems. This is accomplished by eliminating nano-scaled particles that act as damage precursors during laser radiation.
Comparison of ALD and IBS HfO2/Al2O3 anti-reflection coatings for the harmonics of the Nd: YAG laser
The electric field distribution within a thin film stack was optimized to avoid peak intensities in critical layers using refractive index engineering and/or layer thickness grading. Femtosecond laser mirrors and dichroics for 780 nm and 390 nm were designed, realized and characterized. Here we present LIDT measurements of electric field optimized mirrors and dichroics, which are almost a factor of three higher compared to standard coating designs. At 780 nm a LIDT of 1.49 J/cm2 has been achieved and at 390 nm 0.58 J/cm2. With the exception of Al2O3, all investigated coating materials show a proportional dependence of the LIDT with electric field maximum, as expected by theory. For Al2O3 based systems the electrical field penetrates deep into the layer stack, a high number of interfaces are involved and interface effects probably limit the achievable LIDT. A similar effect was observed for rugate designs. To exclude such interface effects from the LIDT measurement, a special AR design was developed, which is practically equal for all high index materials. Here a LIDT above substrate damage threshold of 1.7 J/cm2 was achieved.
On the basis of the presented measurement facility, the essential properties of bare flat optics in respect of their polishing state, roughness level, state of cleaning and defect distribution can be investigated with the TS system in a nondestructive way. The homogeneity of the whole surface of an optical component can be tested with a defined lateral resolution. The knowledge of the inhomogeneity is an important indication for the quality evaluation of optical components. We present the TS result and the calculated defect density distributions of selected components, which are handled by different cleaning procedures. Also, additional effects in TS and T will be outlined and compared with spectral photometric measurement.
Laser-induced damage and nonlinear absorption of ultrashort laser pulses in the bulk of fused silica