Impact of laser-contaminant interaction on the performance of the protective capping layer of 1 ω high-reflection mirror coatings.

High dielectric constant multilayer coatings are commonly used on high-reflection mirrors for high-peak-power laser systems because of their high laser-damage resistance. However, surface contaminants often lead to damage upon laser exposure, thus limiting the mirror's lifetime and performance. One plausible approach to improve the overall mirror resistance against laser damage, including that induced by laser-contaminant coupling, is to coat the multilayers with a thin protective capping (absentee) layer on top of the multilayer coatings. An understanding of the underlying mechanism by which laser-particle interaction leads to capping layer damage is important for the rational design and selection of capping materials of high-reflection multilayer coatings. In this paper, we examine the responses of two candidate capping layer materials, made of SiO2 and Al2O3, over silica-hafnia multilayer coatings. These are exposed to a single oblique shot of a 1053 nm laser beam (fluence ∼10 J/cm2, pulse length 14 ns), in the presence of Ti particles on the surface. We find that the two capping layers show markedly different responses to the laser-particle interaction. The Al2O3 cap layer exhibits severe damage, with the capping layer becoming completely delaminated at the particle locations. The SiO2 capping layer, on the other hand, is only mildly modified by a shallow depression. Combining the observations with optical modeling and thermal/mechanical calculations, we argue that a high-temperature thermal field from plasma generated by the laser-particle interaction above a critical fluence is responsible for the surface modification of each capping layer. The great difference in damage behavior is mainly attributed to the large disparity in the thermal expansion coefficient of the two capping materials, with that of Al2O3 layer being about 15 times greater than that of SiO2.

The effect of laser pulse shape and duration on the size at which damage sites initiate and the implications to subsequent repair.

Growth of laser damage on SiO(2) optical components used in high power lasers can be reduced or eliminated by pre-exposure to pulses of a few hundred ps in duration. Such pre-exposure would cause weak locations on the optics surface to self-identify by initiating very small damage sites. The sites which initiate will be only a few microns in diameter and will have a very low probability of growing even without any further treatment. Repairing damage sites when small is important because both laser mitigation and acid etching are very successful in preventing such small sites from growing.

Fearing the unknown: a short version of the Intolerance of Uncertainty Scale.

Intolerance of uncertainty is the tendency of an individual to consider the possibility of a negative event occurring unacceptable, irrespective of the probability of occurrence. It is a key component of worry, state anxiety, and related anxiety pathologies. The 27-item Intolerance of Uncertainty Scale (IUS) was developed to measure intolerance of uncertainty. Previous psychometric analyses of the IUS have suggested both four- and five-factor models. High inter-item correlations, factor instability, and previous theoretical research support the development of a reduced measure. The present study used two undergraduate samples and evaluated a psychometrically stable 12-item two-factor version of the IUS. The reduced measure (IUS-12) retained exemplary internal consistency, while correlating extremely well with the original IUS and related measures of anxiety and worry. The IUS-12 also demonstrated a stable two-factor structure, representing both anxious and avoidance components of intolerance of uncertainty. Directions for future research and potential applications for assessment are discussed.

Modeling of frequency doubling and tripling with measured crystal spatial refractive-index nonuniformities.

Efficient frequency doubling and tripling are critical to the successful operation of inertial confinement fusion laser systems such as the National Ignition Facility currently being constructed at the Lawrence Livermore National Laboratory and the Omega laser at the Laboratory for Laser Energetics. High-frequency conversion efficiency is strongly dependent on attainment of the phase-matching condition. In an ideal converter crystal, one can obtain the phase-matching condition throughout by angle tuning or temperature tuning of the crystal as a whole. In real crystals, imperfections in the crystal structure prohibit the attainment of phase matching at all locations in the crystal. We have modeled frequency doubling and tripling with a quantitative measure of this departure from phase matching in real crystals. This measure is obtained from interferometry of KDP and KD*P crystals at two orthogonal light polarizations.

Adaptive phase compensation in a Raman look-through configuration.

Raman look through is a technique for extending the use of low-power adaptive optics to the control of high-power laser beams. This is done by incorporating a Raman amplifier into an adaptive-optics system. We report the results of a Raman look-through experiment, which is to our knowledge the first demonstration of this technique.