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In this work, 532-nm high-reflection (HR) coatings have been deposited at different deposition temperatures by electron-beam evaporation technology. The spectral performance, e-field distribution, surface roughness, stoichiometry, as well as the laser resistance of the prepared 532-nm HR coatings are investigated. Experimental results indicate that the LIDT of the 532-nm HR coatings can be greatly influenced by deposition temperature. A relatively high deposition temperature benefits the crystallization and oxidation, and improves the LIDT of the 532-nm HR coatings. In addition, the SiO2 overcoat layer is also demonstrated to be effective in suppressing the delamination damage morphology and improving the LIDT of the 532-nm HR coatings.
A vacuum chamber was designed to study the risk of laser-induced contamination (LIC) on optical payloads integrated on spaceflight missions. In this context, tests were performed with a nanosecond pulsed laser at 355 nm on fused silica substrates under toluene exposure with multiple laser irradiation. Specific experimental procedures are described in order to obtain repeatable results. Finally, series of tests were performed to investigate the onset of the LIC deposition process and its evolution over time. A slight antireflective effect is consistently observed at the onset of the deposition process. We suggest that this is an indication that the LIC deposition process in our experimental conditions starts with a nucleation layer consisting of small dense islands of deposit.
Laser damage measurements with multiple pulses at constant fluence (S-on-1 measurements) are of high practical importance for design and validation of high-power photonic instruments. Using nanosecond lasers, it was recognized long ago that single-pulse laser damage is linked to fabrication-related defects. Models describing the laser damage probability as the probability of encounter between the high fluence region of the laser beam and the fabrication-related defects are thus widely used. Nanosecond S-on-1 tests often reveal the “fatigue effect,” i.e., a decrease of the laser damage threshold with increasing pulse number. Most authors attribute this effect to cumulative material modifications operated by the incubation pulses. We discuss the different situations that are observed upon nanosecond S-on-1 measurements that are reported in literature and speak in particular about the defects involved in the laser damage mechanism. These defects may be fabrication related or laser induced, stable or evolutive, cumulative or of short lifetime.
In view of the fact that the weak laser damage resistance of HfO2 / SiO2 coatings at 355 nm hinders the observation of the fatigue effect, nanosecond single and multiple pulse laser damage studies on Al2O3 / SiO2 high-reflective coatings were performed at 355 nm. Relative to that at the long wavelength, the fatigue effect at 355 nm is very weak and complicated. The damage probability curves and the evolution of the laser-induced damage threshold under multiple irradiations reveal that the fatigue effect is affected by both laser fluence and shot number. As the laser fluence or number of shots increases, the fatigue effect becomes more apparent. The damage morphologies induced by single and multiple irradiations both manifest as micrometer-scale pits without plasma scalding around, with the characteristics of a high defect density and high absorption coefficient. In particular, the accumulation damage mechanism at 355 nm may be reflected not only in the newly created defects but also in the modification of the coating material around the damage precursors. Thus, the coatings at 355 nm “seem to” have no damage growth threshold, no matter what the laser fluence is; once damage occurs, the damage site will grow sharply under subsequent pulses finally resulting in catastrophic damage.
High intensity nanosecond laser pulses induce damage on uncoated optics and antireflection (AR) coated optics with thin film boundaries. Structured surfaces with AR performance have shown higher damage resistance than conventional AR coatings. This work explores laser-induced damage threshold (LIDT) of random antireflective surface structures (rARSS), covering interfaces of planar fused silica substrates, using a 7 ± 1 nanosecond duration, single damaging pulse at 1064-nm wavelength. The rARSS treated windows were optimized for AR at 1064 nm, and transmittance of each treated interface was increased by 3.2% per ISO 13697. Incident fluence was controlled near LIDT for both entry and exit sides. Multiple locations were tested for each fluence setting in accordance with ISO 21254 1-on-1 LIDT testing. Double-sided and single-sided rARSS, orientation with both entrance-side and exit-side treatments, were tested to determine effects random structures have in damage induced by field enhancement on the exit side. Results show that rARSS on a single side of an optic have higher resistance to onset of laser damage when located at entry (structured) side. All tests resulting in damage of polished and single-sided rARSS oriented on exit side were ballistic on beam exit facet, showing surface cracks. Results from rARSS located on beam entry side (both single- and double-side treated) showed nonballistic damage and densification of rARSS features from localized melting or reflow, capping the rARSS but still preserving effective-gradient index and rARSS transmission enhancement. This suggests that the localized melting of the rARSS optic surface does not constitute damage optically, or at least not catastrophically within the prebulk damage regime. Therefore, it is possible to foresee the onset of true damage within an optical system via tracking transmission drop, and swap optics out before catastrophic system failure. The drop in transmission acts as an early warning system, making these rARSS optics an invaluable tool in high-energy laser development.
A simplified absolute calibration for the photothermal laser-induced deflection technique is introduced and applied to characterize different nonlinear optical crystals at common diode pumped solid state laser wavelengths. In particular, intensity-dependent absorption measurements at 355 nm are performed to investigate the nonlinear absorption in lithium triborate crystals. From the results, it is verified that at least to a certain extent the nonlinear absorption is related to impurities or defects, i.e., to sequential three-photon absorption in addition to a potential intrinsic three-photon absorption process.